WATER & ENERGY IN THE URBAN WATER CYCLE Improving Energy Efficiency in Municipal Wastewater Treatment
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Prepared by Nanyang Environment & Water Research Institute (NEWRI) In collaboration with PUB Singapore & Global Water Research Coalition
Acknowledgements This report has drawn on work performed by others before the project team started its work and also as the team performed the study. In addition, the project team had the privilege to consult many colleagues in the industry and academia. These individuals have contributed with their suggestions during the study and comments on the resulting report. We are grateful to the study’s panel of experts and these include: Prof Yehuda Cohen (The Hebrew University, Israel), Prof Anthony Fane (University of New South Wales, Australia), Prof Jurg Keller (University of Queensland, Australia), Prof Staffan Kjelleberg (University of New South Wales, Australia), Dr. Anders Lynggaard-Jensen (DHI, Denmark), Prof Mark van Loosdrecht (Delft University of Technology, The Netherlands), Prof Norbert Matsche (Vienna University of Technology, Austria), Prof Perry McCarthy (Stanford University, USA), Dr. Hansruedi Siegrist (Swiss Federal Institute of Aquatic Science and Technology, Switzerland), Prof Rainer Stegmann (Hamburg University of Technology, Germany), and Prof Peter Wilderer (University of Karlsruhe, Germany). Our deep appreciation goes to the GWRC project collaborators. These include: Dr. Lauren Fellmore (Water Environment Research Foundation, USA), Dr. Jan Hoffman/Dr. Jos Frijns (KWR Watercycle Research Institute, The Netherlands), Dr. Valerie Naidoo (Water Research Commission, South Africa), Mr. Carlos Peregrina (SUEZ-CIRSEE, France), Dr. Wouter Pronk (Swiss Federal Institute of Aquatic Science and Technology, Switzerland), Dr. Cora Uijterlinde (STOWA Foundation for Applied Water Research, The Netherlands), and Mr. Francois Vince (Veolia Water, France). The project team during its performance of the study had consulted the following reports: 1. Cha, Woosuk, Heechui Choi, Jungwoo Kim and In Soo Kim (2003). Evaluation of Wastewater Effluents for Soil Aquifer Treatment in South Korea. Water Reuse Technology Center, Kwangju Institute of Science and Technology. 2. Crawford, G. (2009). Technology Roadmap to Optimize WWTPs in a CarbonConstrained World – Workshop Summary. Water Environment Research Foundation. 3. Gans, N, Shifteh Mobini and Xiaoni Zhang (2007). Mass and Energy Balances at the Gaobeidian Wastewater Treatment Plant in Beijing, China. Master Thesis at Water and Environmental Engineering, Department of Chemical Engineering, Lund University. 4. Hugh Monteith, Youssouf Kalogo, and Nuno Louzeiro, Hydromantis, Inc., (2007). Achieving Stringent Effluent Limits Takes a Lot of Energy! Water Environment Research Foundation. 5. Japanese Municipalities Targeting Energy Self-Sufficiency at Sewage Treatment Plants 2008. 6. Jonasson, M. (2007). Energy Benchmark for Wastewater Treatment Processes – a comparison between Sweden and Austria. Master Thesis, Dept of Env Engineering, U of Innsbruck, Austria. 7. Kasia Chapman, James Newton (2009). Maximising the Value of Biogas. UKWIR Project WW05.
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8. Keller, J. (2008). Paradigm Shift from Wastewater Treatment to Resource Recovery System. Advanced Water Management Centre, University of Queensland, Australia. 9. Keller, J. and S. J., Kenway (2009). Water Heating – By Far the Largest Energy Demand in the Urban Water System. Advanced Water Management Centre, University of Queensland, Australia. 10. Kenway S. J., A Priestley, S Cook, S. Seo, M Inman, A Gregory and M Hall (2008). Energy Use in the provision and consumption of urban water in Australia and New Zealand. CSIRO, Australia ISBN 978 0 643 09616 5 11. van Loosdrecht, M. C. M., kuba, T., van Veldhuizen, H. M., Brabdse, F. A. and Heijnen, J. J. (1997). Environmentally impacts of nutrient removal process: case study. Journal of Environmental Engineering. 12. Mizuta, K. and Shimada, M. (2009). Benchmarking Energy Consumption in Municipal WWTPs in Japan. 13. M/J Industrial Solutions (2003). Municipal Wastewater Treatment Plant – Energy Baseline Study. PG&E, San Francisco, USA. 14. Monteith, H. Youssouf Kalogo and Nuno Louzeiro 2007. Achieving Stringent Effluent Limits Takes A Lot of Energy. Water Environment Federation WEFTEC®07. 15. Pergrina-Cambero, C.A., M. Large, J.M. Audic, L. Monnot, J.M. Pezzoni, M. Lesoille (2009). WWTP Upgrading towards positive Energy Balance: The Case Study of Dijon (France). Environment and CIRSEE. 16. Rogalla F, Abigail Field, Banu Sumner, Jaro Kolarik. Cost Effective Treatment Technologies for Temperate Wastewaters. 17. Science Applications International Corporation (SAIC) (2006). Water and Wastewater – Energy Best Practice Guidebook. State of Wisconsin, Department of Administration, Division of Energy 18. United Kingdom Water Industry Research (2009). UK Case Studies on Energy Efficiency (Current Status) in the Urban Circle. 19. UKWIR (2009). Maximising the Value of Biogas. Summary Report by Mott MacDonald for UKWIR. 20. USEPA (2006). Emerging Technologies for Biosolids Management. EPA 832-R-06-005. 21. USEPA (2008). Ensuring a Sustainable Future: An Energy Management Guidebook for Wastewater and Water Utilities. GS-10F-0337M 22. Water Services Association of Australia and PUB Singapore (2009). Australiasian Energy Efficiency: Report for the GWRC – Energy Efficiency Compendium of Best Practice. 23. Wett B., K. Buchauer, and C. Fimml (2007). Energy self-sufficiency as a feasible concept for wastewater treatment systems. Institute of Infrastructure/Environmental Engineering, University of Innsbruck. Last but not least the project team is much indebted to colleagues at Singapore’s PUB whose unfailing support had made completion of the project possible.
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Executive Summary Within the urban water cycle, the wastewater treatment component is a significant consumer of energy. However, it can potentially be energy neutral or even energy positive with appropriate process optimization and technological improvements. The need to investigate the energy aspect in the urban water cycle was first mooted at the 12th Meeting of the Global Water Research Coalition (GWRC) Board of Directors in Sydney, November 2007. Thereafter Nanyang Environment & Water Research Institute (NEWRI) entered into agreement with PUB of Singapore in 2009 to undertake a project intended to generate a report on municipal wastewater treatment which determines current energy consumption patterns, articulates potential improved energy balances at the predetermined timelines in 2015 and 2030 leading to the anticipated paradigm shift by 2030 wherein an 80% energy reduction is achieved. The project team included NEWRI and PUB members and the resulting report proposes and discusses the variations in unit process configurations and indeed new unit processes which can be considered towards achieving that shift. The project included a review of published scientific literature and GWRC reports, site visits, invited contributions and comments from international researchers and wastewater treatment professionals, and feedback at workshops specifically organized for the project. A survey document was developed and sent out. This provided the framework and indications of the data the project team was attempting to gather from the GWRC community and other contacts. An international discussion platform in the form of an e-collaboration website was set-up to augment the workshops and email communications. The findings suggest possibility of increasing energy reductions from 20 – 50% while still retaining and largely using existing assets. Changes which can be considered include fine bubble air diffusion, dissolved oxygen control, variable frequency drives (VFDs), appropriate plant sizing, utilization level versus design capacity, the type of aerobic process, and anaerobic biogas production. However, from 50 - 80% and beyond, escalating number of changes to existing assets will likely be required leading eventually to a treatment train which can be quite different from the current state-of-the-art. Such changes can include the anaerobic ammonia oxidation (ANAMMOX) which will significantly reduce aeration needs, nutrients (N-P-K) recovery at the beginning of the treatment train instead of at the end, preconcentration of carbon substrates, organic solids conditioning prior to anaerobic degradation, and polymer construction. Discussion at the workshops had identified solids conditioning and the anaerobic process as the most likely unit processes which can yield significant overall system energy performance improvements. These shall likely be followed by nitrogen removal. The report provides discussion (and unit process schematics) to illustrate the escalation in number of changes to the current state-of-the-art plant and leading eventually to the 2030 WWTP. A technology development roadmap is provided to give indication of the interest areas, timeframe, and possible costs.
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It should be noted that while there was general agreement with the information and arguments presented in this report, there were specifics which had drawn disagreement from some members of the community the project team had consulted as the report was developed. Some of these (which may be beyond the scope of this project) are identified as follows: i.
there is need to consider energy with greenhouse gas emissions as these may be two different and opposing drivers for changes to the wastewater treatment development roadmap. An energy-efficient plant may not necessarily be green house gas (GHG)efficient; ii. there is debate if the final destination for sludge should be the incinerator (for direct heat energy recovery) or the anaerobic digester (for energy recovery via biogas). There may, however, be insufficient studies to date to determine which option is preferable; iii. will improvements in solids capture efficiency at the primary settling tank (PST) (primary sludge produces more biogas than secondary sludge) result in carbon limitation in the rest of the system and so inhibit the processes (e.g. biological nutrient removal, BNR) therein? This may be partially resolved by separate collection of urine and faeces; iv. although separate collection of urine and faeces will allow for much reduced aeration requirements and allow possibility of direct ammonia recovery from urine, this issue may be beyond just technological capacity but is determined by social acceptance and willingness of the public to change its behaviour. The preceding can then lead to a debate on centralized versus decentralized treatment, and the infrastructure required to switch from one regime to another.
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Acronyms and Abbreviations AEBC AIWPS AWWT bCOD BNR BOD BOD5 CAS CETP CHP COD DAF DHI-NTU Centre EWAG GWRC HRAPs JWRP KWR MLSS MFC NEWRI NH4-N NTU OD OSEC PEMFC PO4-P PUB R&D R3C SCADA SOFC SS SRT STOWA THP TN TWAS UASB UKWIR USEPA VSD WERF
Advanced Environmental Biotechnology Centre, NEWRI Advanced Integrated Wastewater Pond Systems Advanced Wastewater Treatment Plant Biodegradable Chemical Oxygen Demand Biological Nutrient Removal Biological Oxygen Demand Biological Oxygen Demand in five (5) days Conventional Activated Sludge Common Effluent Treatment Plant Combined Heat and Power Chemical Oxygen Demand Dissolved Air Flotation DHI-NTU Water & Environment Research Centre and Education Hub, NEWRI Swiss Federal Institute of Aquatic Science and Technology Global Water Research Coalition High Rate Algal Ponds Jurong Water Reclamation Plant KWR Watercycle Research Institute Mixed Liquor Suspended Solids Microbial Fuel Cell Nanyang Environment and Water Research Institute, NTU Ammonium Nitrogen Nanyang Technological University, Singapore Oxidation Ditch Overall specific energy consumption Proton Exchange Membrane Fuel Cell Phosphate Phosphorous Public Utilities Board, Singapore Research & Development Residues & Resource Reclamation Centre, NEWRI Supervisory Control and Data Acquisition Solid Oxide Fuel Cell Suspended Solids Sludge Retention Time Dutch acronym for the Foundation for Applied Water Research Thermal Hydrolysis Process Total Nitrogen Thickened Waste Activated Sludge Upflow Anaerobic Sludge Blanket United Kingdom Water Industry Research US Environmental Protection Agency Variable Speed Drives Water Environment Research Foundation
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WQRA
Water Quality Research Australia
WRP
Water Reclamation Plant (in Singapore, the first portion of such a plant will be akin to a WWTP) Water Services Association of Australia Wastewater Treatment Wastewater Treatment Plant
WSAA WWT WWTP
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Contents Acknowledgements .......................................................................................................................... i Executive Summary ....................................................................................................................... iii Acronyms and Abbreviations ......................................................................................................... v Contents…. ................................................................................................................................... vii List of Figures ................................................................................................................................ ix List of Tables .................................................................................................................................. x 1.
Introduction .................................................................................................................. 1
2.
Current Practices around the World ............................................................................. 4
3.
Towards a Mid-Term Target of 50% or Higher Energy Efficiency .......................... 12
4.
Towards 80% Energy Efficiency and Beyond ........................................................... 16
5.
“Assembling” the 2030 WWTP ................................................................................. 24
6.
Technology Development Roadmap.......................................................................... 26
7.
List of References ...................................................................................................... 37
Appendix A: Project Details ......................................................................................................... 44 1.
Information on the Project ......................................................................................... 44 1.1
Project Background .................................................................................................... 44
1.2
Project Team Organization ........................................................................................ 45
2.
Methodology and Approach of Study ........................................................................ 46 2.1
Methodology .............................................................................................................. 46
2.2
Communications ........................................................................................................ 47
2.3
Team Management & Quality Assurance .................................................................. 47
2.4
Research Team Members ........................................................................................... 49
Appendix B: An Overview of the Urban Water Cycle ................................................................. 52 1.
Urban Water Cycle..................................................................................................... 52 1.1
Source Collection and Storage ................................................................................... 53
1.2
Water Treatment ........................................................................................................ 53
1.3
Drinking Water Distribution and Wastewater Collection .......................................... 54
1.4
Wastewater Treatment ............................................................................................... 54
2.
Water Reuse ............................................................................................................... 56 2.1
Water Reclamation and Reuse at Wastewater Treatment Plants ............................... 56
2.2
Desalination ............................................................................................................... 57
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Energy Footprint in Treatment Processes .................................................................. 58
3. 3.1
Pumping System between Processes.......................................................................... 58
3.2
Hydraulic Design of Flow in a Reactor ..................................................................... 58
3.3
Distribution of Flow ................................................................................................... 58
4.
Energy Footprint in Wastewater Treatment Processes .............................................. 59
Appendix C: An Appreciation of Energy Usage and Recovery Possibilities in Municipal Wastewater Treatment ............................................................................................... 60 1.
Background ................................................................................................................ 60
2.
Preliminary Treatment ............................................................................................... 61 2.1
Screening.................................................................................................................... 61
2.2
Grit Chambers ............................................................................................................ 61
2.3
Pumping ..................................................................................................................... 61
3.
Primary Treatment ..................................................................................................... 61
4.
Secondary Treatment ................................................................................................. 61
5.
Sludge Treatment ....................................................................................................... 62
6.
Disinfection ................................................................................................................ 62
7.
Key Energy Use Spots in a Wastewater Treatment Plant .......................................... 62
8.
Energy Reduction Possibilities .................................................................................. 62
9.
Energy Recovery Possibilities ................................................................................... 63
10.
Anaerobic Process for Wastewater Treatment ........................................................... 63
11.
Enhancing Biogas Production .................................................................................... 65
Appendix D: Case Study from Wastewater Treatment Plants in Singapore ................................ 66 1.
Treatment Processes ................................................................................................... 66
2.
Comparison of Energy Efficiencies among WRPs in Singapore ............................... 67
Appendix E: WWTP Practices in Other Regions ......................................................................... 79 Appendix F: Preliminary Energy Balances across the Various Treatment Trains Described in Figures 4-1 and 4-3 to 4-8 .......................................................................................... 87 Appendix G: Survey Form ............................................................................................................ 88 Appendix H: Proceedings of the Workshop ............................................................................... 130 Appendix I: PowerPoint Presentation at GWRC Workshop on 28 June 2010 ........................... 135 Appendix J: GWRC Workshop Outcomes on 26 June 2009 ...................................................... 153
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List of Figures
Figure 1-1 A schematic representation of an urban water cycle .................................................... 1 Figure 1-2 Managers’ concern over energy costs .......................................................................... 3 Figure 2-1 Overall specific energy consumption (OSEC) values (kWh/m3) in various regions ... 5 Figure 3-1 Specific Power Consumption in relation with four secondary treatment methods: OD, CAS without incineration, CAS with incineration and AWWT ............................... 14 Figure 4-1 Typical state-of-the-art wastewater treatment plant process schematic ..................... 16 Figure 4-2 Summary of energy consumption and production at Strass WWTP from 1996 to 2005 (Jonasson, 2007) ........................................................................................................ 17 Figure 4-3 Anaerobic treatment preceding aerobic polishing processes ..................................... 18 Figure 4-4 Nutrients recovery to reduce aeration requirement .................................................... 19 Figure 4-5 Preconcentration to enhance the anaerobic/fermentative process .............................. 20 Figure 4-6 Solids conditioning to enhance biodegradation ......................................................... 21 Figure 4-7 Polymer construction for preconcentration ................................................................ 22 Figure 4-8 The treatment train with water reclamation and devoid of an aerobic process .......... 23
ix
List of Tables
Table 2-1 Summary of specific energy consumption at various treatment stage/unit processes at various locations ........................................................................................................... 7 Table 2-2 Comparison of overall plant specific energy consumption between Jurong (Singapore) and Beijing Gaobeidian (China) ................................................................................... 9 Table 2-3 Current technologies and practice to achieve 20% savings by decreasing energy consumption ................................................................................................................ 10 Table 2-4 Current technologies and practice to achieve 20% savings by increasing energy generation .................................................................................................................... 11 Table 3-1 Summary of energy consumption (kWh/m3) in various biological processes in Canada, Japan, and USA........................................................................................................... 13
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1. Introduction An urban centre typically derives its raw water supply from external sources and these may be located some distance away. These sources may include surface catchments and reservoirs, groundwater and aquifers, and for littoral urban centres – the sea and desalination plants. The raw water is abstracted, transferred to, treated at water treatment plants, and thereafter distributed among the consumers. The distribution of treated water from plant to consumer can be with a network of pipes and pumps to overcome frictional losses and gravity. The water after use would need to be collected as wastewater and conveyed to the wastewater treatment facility. The wastewater is then treated and thereafter may be discharged into receiving waterbodies or as may be the case at some locations - be used for water reclamation. The components of the urban water cycle can be categorized as: 1. Source collection and storage; 2. Water treatment and this may include desalination; 3. Treated water distribution to consumers; 4. Wastewater collection; 5. Wastewater treatment, and; 6. Water reclamation. A schematic representation of an urban water cycle is shown in Figure 1-1 below. External Source (e.g. stormwater) Rainwater Collection Groundwater
Seawater Desalination
Raw water Water Treatment
Reclaimed Water
Legend
Industry
Domestic
Wastewater Treatment Wastewater
Linkages between domestic water system Industrial water component in the cycle
Figure 1-1 A schematic representation of an urban water cycle 1
Conventional water treatment, based on surface water, can typically include the following unit processes – pre-aeration, coagulation/flocculation, clarification, granular media filtration, and disinfection. Energy consumption at plants representative of the preceding can be comparatively low – in the range of 0.05 – 0.47 kWh/m3 of water produced (http://ec.europa.eu/environment/water/quantity/pdf/desalination.pdf). Energy consumed in water distribution and wastewater collection is dependent on factors such as the size of the area served and its topography. Wastewater collection and conveyance to the treatment facility may, however, be more complicated when a combined sewer and stormwater system is used. The quantity of flow in such a system can increase many folds over the dry weather flow during a rain episode. There is then almost always necessity to design for and operate overflow facilities for occasions when flows exceed the capacities of the combined sewer system and treatment facility. A common basis for comparison of energy consumption by water or wastewater conveyance systems is consequently more difficult to establish. While water treatment consumes a comparatively small amount of energy, wastewater treatment is a substantial energy consumer in the urban water cycle. Conventional wastewater treatment facilities, where secondary treatment applies, are typically built around the aerobic process and not unexpectedly aeration consumes the largest share (~50%) of total plant energy requirements. This is followed by sludge treatment (~30%) and pumping within the facility (~15%). The aeration process can be realized with either mechanical or air diffuser systems and the latter may be with either coarse or fine bubble diffusers. To illustrate the magnitude of energy consumption in the aeration process, current state-of-the-art fine bubble diffusers are only capable of transferring 2 – 3 kg O2/kWh while coarse bubble diffusers deliver about 1 kg O2/kWh. If the treatment facility, in addition to removal of BOD (by oxidation of carbonaceous materials in the aerobic process), has to remove ammonia in the wastewater by way of nitrification (another oxidative process) then oxygen requirement (and hence energy requirement) can increase very substantially since 4.57 kg O2/kg N shall be required to oxidize the ammonia nitrogen. There has been growing interest in managing energy consumption at wastewater treatment facilities since the 1980s. This interest had its initiation in the energy crisis of the 1970s which resulted in a changed perspective on energy resources and associated costs. The proportion of wastewater treatment plant operating budgets attributable to energy costs has increased since. Figure 1-2 indicates facility managers in the USA are in the main concerned with energy costs at their respective facilities. Energy consumption at a treatment plant can be affected by factors which include the wastewater’s characteristics, the operation and quality of mechanical equipment (e.g. pumps and air blowers), and the treatment process and technology used. Additionally, wastewater treatment can be an interesting target for better energy efficiency because energy recovery from the wastewater is possible. Wastewater contains carbonaceous components which in the “traditional” treatment approach would need to be removed as pollutants (and typically with an aerobic process). Going forward, wastewater can be considered a source of carbonaceous materials which can be recovered for energy production. It has been argued there can possibly be 1.7 kWh/m3 energy in wastewater (Benedek, 2008).
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Figure 1-2 Managers’ concern over energy costs (http://www.tanfoundation.com.sg/Images/pdf/Renewable-Energy-from-wastewater.pdf)
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2. Current Practices around the World Wastewater treatment targets may differ at various locations and so consequently do wastewater treatment plants. There are plants which are only needed to provide primary treatment (such as those in Sydney, Australia as shown in Figure 2-1). These would understandably have a lower energy consumption (0.118 kWh/m3) compared to plants providing more comprehensive treatment. Given the almost ubiquitous nature of the activated sludge process in wastewater treatment facilities providing treatment to meet more stringent discharge limits, plants using this process have been targeted in the study. The activated sludge process does, nevertheless, include a number of different configurations depending on treatment objectives - such as nitrifying and non-nitrifying. Figure 2-1 provides the overall specific energy consumption values in plants at locations including Australia, Austria, Canada, China, Iran, Japan, Singapore, Sweden, and USA (and showing differences in energy consumption arising from the differences in specifics at the various locations). A nitrification requirement will understandably result in higher energy consumption as in the case of Canada where the non-nitrifying case at 0.305 kWh/m3 compared with 0.405 kWh/m3 of the nitrifying case. A similar condition was noted in the Singapore example where a non-nitrifying plant had consumed 0.45 kWh/m3 while nitrifying plants’ consumption ranged from 0.51 – 0.56 kWh/m3.
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OSEC
kWh/m3 0.7 0.6 0.5 0.4
0.661
0.3 0.475
0.2 0.1
0.3
0.3 0.258 0.32 0.305
0.605
0.604 0.447
0.405 0.265 0.298
0.373
0.118
0
Figure 2-1 Overall specific energy consumption (OSEC) values (kWh/m3) in various regions Note: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Austria: Jonasson (2007) Sweden: Jonasson (2007) Iran: Nouri et al. (2006) China (Beijing): Gans et al. (2007) Japan: Mizuta, K., Shimada, M., (2009) Canada (non-nitrifying): Monteith et al. (2007) Canada (nitrifying): Monteith et al. (2007) Australia (Sydney): Kenway et al. (2008) Australia (Gold Coast): Kenway et al. (2008) Australia (Melbourne): Kenway et al. (2008) USA (Wisconsin, 1-5 MGD inflow): SAIC (2006) USA (Wisconsin, >5 MGD inflow): SAIC (2006) USA (San Francisco, no-nitrifying, >10 MGD inflow): M/J Industrial Solutions (2003) USA (San Francisco, HPO, 20 MGD inflow): M/J Industrial Solutions (2003) USA (San Francisco, HPO, 63 MGD inflow): M/J Industrial Solutions (2003)
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The stringency of discharge limits to be met has impact on energy consumption. Treatment plants faced with stricter emission standards use more energy to satisfy treatment requirements so as to ensure treated effluent is of a quality suitable for discharge. Sweden is such an example. But stricter discharge standards may not necessarily account for the 45% more energy used compared to the Austria case. Wastewater plants in Sweden may have received industrial wastewater and the latter may be more difficult to treat. These plants may also receive larger volumes of more dilute wastewater which will have reduced the specific energy consumption values and hence impact adversely on the comparison. The climate under which the wastewater treatment plants operated did appear (on the basis of the data shown in Figure 2-1) to have impacted on energy consumption such that temperate (colder) conditions could possibly have resulted in lower energy consumption than in tropical (warmer) conditions (compare the specific energy consumption at plants in China, Japan, Canada, and USA against Singapore’s). It is certain that a number of interacting factors have been at play to produce this phenomenon - including possibly gas solubility, water viscosity, and design factors. Japan and China (see Table 2-1) had the lowest specific energy consumption (at 0.148kWh/m3) for aeration requirements. Unfortunately in the case of Japan, the specific aeration technology used could not be determined. In the case of China, more details on aeration at the Gaobeidian plant could be determined. Centrifugal blowers and fine bubble aerators are used. Cooling of the former is with water in the first phase and with air using fans in the second phase. Furthermore, mixers are used to maintain Mixed Liquor Suspended Solids (MLSS) in suspension and an anoxic environment with autotrophic bacteria to oxidize ammonia to nitrate. Location of the mixers and size of the various zones (i.e. anoxic, anaerobic, and aerobic) in the aeration basins were noted to significantly affect energy consumption. Although more information could not be obtained at the time of writing this report, there was suggestion (from the Iran example) it is important to match installed aeration capacity with actual requirements. The impact of a mismatch on specific energy consumption can be significant especially when aeration is on the basis of operating constant blower capacity and the plant is not operating at the design capacity. The Austrian Strass wastewater treatment plant (WWTP) is particularly interesting because at 109% energy generated (Jonasson, 2007) it is energy self-sufficient. The Strass WWTP has a Combined Heat and Power (CHP) arrangement that converts biogas into electrical energy with an efficiency of 38%. In addition, implementation of deammonification in 2004 has decreased the energy consumed by about 12%. A significant energy saving measure done at the plant is the installation of the CHP unit in 2001. Up till 2001 consumption and production of energy increased almost in parallel. The new CHP unit converted biogas into electrical energy with a total power yield of 340kW. Strass WWTP has a biogas production of about 26 l/PE a day, which the CHP unit can convert to electrical energy with an average efficiency of 38%. The deammonification process introduced in 2004 does not require supplementary carbon and the resulting larger quantity of excess sludge increased the methane content from about 59 to 62%.
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According to an energy baseline benchmarking project (M/J Industrial Solution, 2003), where actual energy consumption in the secondary treatment process was measured for a variety of plant sizes and processes, secondary treatment energy consumption ranged from a low of 27% to a high of 57% of total plant energy consumption. This is in general conformance with the expected range of 30 to 60%. The biological nitrification/denitrification process may increase total plant energy consumption by 40 to 50 percent. In the aeration process, typical oxygen transfer efficiency for coarse bubble diffusers is in the range of 9 to 13%. While fine bubble aerators are more expensive, they provide an oxygen transfer efficiency of 15 to 40%. In addition, the results confirmed that secondary treatment using attached growth consumes significantly less energy than the suspended growth processes. However, plants using suspended growth with nitrification/denitrification were not significantly more energy intensive than those plants not using nitrification/denitrification. Table 2-1 Summary of specific energy consumption at various treatment stage/unit processes at various locations Energy consumption (kWh/m3)
Austria
Austria (Strass)
Sweden
China (Beijing)
Japan
Iran
Total energy consumption
0.304
0.317
0.475
0.258
0.320
0.300
Preliminary
0.039
Primary
-
0.068
0.136
Aeration
0.212
0.181
0.226
Sludge thickening Sludge dewatering
0.083 0.015 0.148
0.013
0.013 0.001
0.148
0.231
0.100
0.021
0.002 0.040
0.040
0.068
Sludge digester
0.003 0.007
Pumping
0.013
0.028
0.045
-
0.059
0.034
Total energy generation
-
0.346
-
0.081
0.170
0.182
Energy efficiency (%)
-
109
-
31
50
60.67
Note: 1. 2. 3. 4. 5. 6.
Austria: Jonasson (2007) Austria (Strass): Jonasson (2007) Sweden: Jonasson (2007) Iran: Nouri et al. (2006) China (Beijing): Gans et al. (2007) Japan: Mizuta and Shimada (2009)
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In sludge treatment (i.e. thickening, dewatering, and digestion), energy consumption can be as low as 0.012 kWh/m3. China’s Gaobeidian WWTP applies gravity thickening. The digestion process needs heat to provide the appropriate environment for bacteria to degrade the sludge to produce biogas. In a colder climate application like Gaobeidian’s, the mesophilic system is employed and both internal and external heating is necessarily applied. The next step after the stabilization of the sludge is dewatering. To enhance dewatering, a polymer is used as conditioner. This made water separation easier and led to a considerable energy and cost saving for the next steps due to a smaller volume of sludge. The data shown in Table 2-1 indicates WWTPs at a number of locations are already well able to achieve at least 20% savings on energy imported into the plant. Using the Gaobeidian WWTP and the Jurong plant (JWRP) as examples, an attempt was made to identify where energy savings might be derived (see Table 2-2). The influent Chemical Oxygen Demand (COD) concentration at JWRP is approximately 538 mg/l due to the mixing of industrial and domestic wastewaters, while the influent COD concentration at Gaobeidian WWTP is 346 mg/l. The overall energy consumption at JWRP is 0.45 kWh/m3 while that at Gaobeidian is 0.258 kWh/m3. This difference in overall energy consumption, aside from the impact of the influent characteristics, may also have been caused by the differences in technologies used at various unit processes. For instance, Jurong’s preliminary treatment uses the fine screen and horizontal flow grit chamber while Gaobedian uses the fine screen and aerated grit chamber. Jurong’s preliminary treatment required 0.002 kWh/m3 while Gaobeidian required 0.083 kWh/m3 – the difference is likely caused by the aeration for grit removal. Similarly the simpler primary treatment at Jurong (i.e. simple gravity sedimentation) compared to Gaobedian’s higher level of mechanization resulted in 0.0012 kWh/m3 and 0.015 kWh/m3 respectively. Instances where Jurong’s unit processes has higher energy consumption than Gaobeidian’s included digester mixing (0.043 and 0.007 kWh/m3 respectively) and sludge dewatering (0.021 and 0.003 kWh/m3 respectively). Jurong, in addition to the belt filter press, also operates the centrifuge. Jurong (compared to Gaobeidian) has higher energy consumption for aeration (0.169 and 0.148 kWh/m3 respectively) and bearing in mind Jurong does not practise nitrification, the higher energy consumed could have been caused at least in part by the type of aeration device used. Surface aerators are used at Jurong while Gaobeidian uses the blower-fine bubble diffuser-mechanical mixer combination. Type of aeration equipment aside, it should be noted the Jurong WWTP receives pretreated industrial wastewater (about 40% of total flow) and this would result in a higher energy consumption since the COD concentration is higher than that in usual domestic wastewater.
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Table 2-2 Comparison of overall plant specific energy consumption between Jurong (Singapore) and Beijing Gaobeidian (China) Jurong
Beijing Gaobeidian
0.99 million
2.4 million
Flow (m /day)
0.154 million
1 million
BOD5 (mg/l)
329
173
COD (mg/l)
924
346
Total electricity consumption (kWh/m3)
0.45
0.258
Total electricity consumption (kWh/kgCOD)
0.58
0.746
Total electricity consumption (kWh/kgO2D)
0.58
0.443
0.002
0.083
Fine screen-perforated plate, horizontal flow grit chamber
Coarse screen (6), fine screen (4) and grit chamber
0.0012
0.015
Gravity sedimentation
Travelling bridge, mixer, shredder, sludge pump
0.169
0.1484
Surface-fixed and variable speed aerators
Centrifugal blowers, fine bubble aerators, mixer
0.003
0.002
Dissolved air flotation
Gravity thickening
0.021
0.003
Centrifuge, belt filter press
Belt filter press
0.043
0.007
Digester (kWh/m3)
Draft tube system, gas injection system, mesophilic, no external heating
Mesophilic, external heating: Spiral-Plate Heat Exchange
Pump system (kWh/m3)
0.151
-
0.160
0.081
Gas collection, Gensetelectricity generation
Gas holder for collected gas, methane booster compressor (8)
35.9
31
Population served (PE) 3
3
Preliminary (kWh/m )
3
Primary treatment (kWh/m ) Aeration (kWh/m3)
Sludge thickening (kWh/m3) Sludge dewatering (kWh/m3)
Total electricity generation(kWh/m3) Energy efficiency (%)
Note: 1. Singapore (Jurong): Survey data on Singapore WWTPs provided by PUB (2009) 2. China (Beijing): Gans et al. (2007)
9
The information available would suggest plants with positive energy efficiency have largely achieved this by replacing energy imported into the plant with energy generated from within the plant and this can be attributed to the anaerobic digesters and methane produced by these. Aside from this, there are a number of areas where energy can be used more effectively. Table 2-3 and Table 2-4 summarize the technologies used at various locations and where these can possibly contribute to at least 20% savings in energy consumption. Table 2-3 Current technologies and practice to achieve 20% savings by decreasing energy consumption Technology description
Practices in countries
1
Conversion from a coarse bubble aeration system, in a facility that is nitrifying, to a fine bubble system in aeration processes
Canada4, Singapore7 (Kranji & Ulu Pandan), USA6
2
Installation of flow equalization tank in the primary process to maintain constant flow
Singapore7 (Bedok, Kranji & Ulu Pandan)
3
Installation of variable speed drives (VSD) (for pumps & blowers)
USA5&6, Singapore7 (Bedok, Kranji & Jurong), Canada7
4
Schedule pumping to respond to periods of lower hydraulic loading to optimize energy efficiency
China3
5
Replacing standard efficiency motors to high efficiency motors
USA5&6
6
Installation of supervisory control and data acquisition (SCADA) to monitor and adjust power consumption in aeration processes
-
7
Upgrading old equipment
Singapore7 (Bedok), Austria1 (Strass), USA5
8
Installation of belt filter press to replace centrifuges in sludge dewatering processes
China3, Singapore7 (Jurong)
9
Installation of dissolved air flotation (DAF) to replace centrifuge in sludge thickening process
Singapore7 (Jurong, Bedok, & Kranji)
Note: 1. 2. 3. 4. 5. 6. 7.
Austria: Jonasson (2007) Iran: Nouri et al. (2006) China (Beijing): Gans et al. (2007) Canada (non-nitrifying): Monteith et al. (2007) USA: SAIC (2006) USA: M/J Industrial Solutions (2003) Singapore: Singapore WPR survey provided by PUB
10
Table 2-4 Current technologies and practice to achieve 20% savings by increasing energy generation Technology description
Practices in countries
Anaerobic digestion of sludge, biogas generation, combined heating and power (CHP)
Note: 1. 2. 3. 4. 5. 6. 7. 8.
Austria: Jonasson (2007) Iran: Nouri et al. (2006) China (Beijing): Gans et al. (2007) Canada (non-nitrifying): Monteith et al. (2007) USA: SAIC (2006) USA: M/J Industrial Solutions (2003) Singapore: Singapore WPR survey provided by PUB Japan: Mizuta and Shimada (2009)
11
Singapore7, China3, Japan8, Iran2, USA5&6, Canada4, Austria1
3. Towards a Mid-Term Target of 50% or Higher Energy Efficiency The following discussion identifies some of the approaches and technologies which can be considered for progression towards 50% and higher energy efficiency. Fine bubble air diffusion: fine air diffusion is substantially more energy efficient than coarse air diffusion. The former provide oxygen transfer efficiency of 15 – 40% compared to the latter’s 9 – 13%. Retrofitting coarse air to fine air diffusion can produce aeration energy savings of 20 – 40%. Dissolved oxygen control: activated sludge processes require mixed liquor dissolved oxygen concentrations of 1.0 – 1.5 mg/L for stable aerobic operation. Dissolved oxygen levels higher than these represent an over-aeration condition and hence is energy wasting. Dissolved oxygen monitors in the aeration basins and aeration devices with adjustable aeration intensities will help reduce energy consumption and this will particularly be so when such data can be applied with suitable mathematical process models. Process modeling in USA has suggested 30 – 60% savings in energy consumption for a conventional treatment plant is possible through better control of the secondary unit treatment process. Process changes which can be beneficial include shortening the SRT (sludge retention time), and replacing aerobic zones with anoxic zones. The impact of SRT on energy consumption may be seen from Table 3-1 where the oxidation ditch configuration consistently had higher energy consumption in Canada, USA (Wisconsin) and Japan when compared against the conventional activated sludge process. The oxidation ditch will typically have a longer SRT compared to the activated sludge process.
12
Table 3-1 Summary of energy consumption (kWh/m3) in various biological processes in Canada, Japan, and USA
Energy consumption (kWh/m3)
Facultative ponds Lagoons Rotating biological contactor Tricking filter
Conventional activated sludge
Oxidation ditch
Canada
0.1-0.4
0.35-0.65
0.45-1.05
1.437 (0-1 MGD inflow) USA (Wisconsin)
0.661 (1-5 MGD inflow)
1.926 (Lagoons) (C6) (Rinzema et al. 1989; Whittmann et al., 1995). Given the necessity for compact systems in the urban setting, dilution is not a viable solution for mitigating inhibition. Instead, antagonistic substances may be needed to counteract the inhibition. While the literature does describe counteracting substances (Hansen et al., 1998), these typically relate to bench-scale experiments and well defined feeds. Translation of literature into successful practice requires consideration of inhibitor and antagonistic substance interaction with other substances in the effluent. Effective and controllable dispersion within the reactor is crucial. Appropriate mixing in industrial scale reactors is difficult to achieve (Masse et al., 2004; Capela, et al., 2009). This project seeks to resolve two major issues faced by the industry - inhibition and inadequate mixing as contributing factors to process poor performance. Solutions so developed are expected to be “scalable”.
30
(D1) Objectives and Outcomes -
-
Screening for appropriate conditioning method/s (heat, sonication, enzymes) with feed characterisation (Skiadas et al., 2005; Recktenwald et al., 2008; Takashima, 2008; Salsabil et al., 2009; Salsabil & Laurent, 2010; Yang & Luo, 2010) External and in-situ enzyme production Enzyme blend Changes in solubilization and biodegradability (Recktenwald et al., 2008; Yang & Luo, 2010)
Duration 1-3 years Budget S$750K @ lab-scale (D2) Objectives and Outcomes -
Sub-system design & engineering Operating protocols 3-phase bioprocess Impact on gas production and quality Impact on degradation kinetics Verifications on solids reduction and reactor capacity Odour issues
Duration 3-5 years Budget S$1.5M @ lab & pilot-scale (D3) Objectives and Outcomes -
-
Recovering heat via CHP (Tassou, 1988; Zupancic & Ros, 2003) Improving heat recovery and utilization efficiency Thermophilic anaerobic process enhancement – mixing, creating or limiting gradients, phase-separation, and process stability and mitigating inhibition (Capela et al., 2009; Huyard et al., 2000; Hansen et al., 1998) Digested sludge quality Alternatives to single stage
Duration 3-8 years Budget S$2M @ lab & pilot-scale
31
(E-1) Carbon Recovery - Process Development: (E-2) Carbon Recovery - Technology Development: Background and Scope Biogas is mainly composed of methane (~60%) and carbon dioxide (~40% CO2) with other trace gases. Biogas (as in its CH4 component) is a valuable fuel source. For effective use of biogas in various applications, it may need to be enriched in terms of methane (e.g. as syngas). This is primarily achieved by carbon dioxide removal. At present there are four possible methods which are used commercially for CO2 removal, i.e., water absorption, polyethylene glycol absorption, carbon molecular sieves, and high pressure membrane separation (http://www.recyclenow.org/Report_IEA_Bioenergy_1MB.pdf). The problem with these technologies is their cost. Low-pressure membrane technology (membrane contactor) is expected to have potential to overcome the disadvantages of the current techniques when it is combined with acid gas absorption processes. This project proposes to integrate novel membranes with biological/biomimetic absorption for CO2 separation from biogas (Kumar et al., 2004, Heubeck et al., 2007). Specifically, the project will develop two novel membranes – a highly hydrophobic microporous hollow fibre membrane and a CO2-selective facilitated transport non-porous membrane for use as contactors for CO2 absorption by microalgae suspensions (Heubeck et al., 2007) and/or amino acid salt solutions (Kumar et al., 2004). This combination is envisaged to synergize the advantages of advanced membrane technology and biological sorption to make biogas upgrading economically feasible with enhanced separation efficiency. In addition, the integration of anaerobic reactor, membrane contactor and the algal photo bio-reactor offers the benefits of improving biogas quality by capturing CO2 efficiently on-site with no need for transport or storage. (E1) Objectives and Outcomes -
CH4-CO2 separation – membrane-based (Atchariyawut et al., 2007) Polymer construction with recovered CO2 – e.g. bioproduction of polysaccharides (Henderson, 2005) Impact of catalysts on polymer construction (Sridhar et al., 2007) Properties of polymers and impact on anaerobic and aerobic
Duration 2-5 years Budget S$1.25M @ lab-scale
32
(E2) Objectives and Outcomes -
Sub-system design & engineering Process stability and verification of yields Impact of polymers on preconcentration performance Impact of polymers on bioprocesses and O&M issues Impact of polymers on membrane
Duration 5-7 years Budget S$1.5M @ pilot-scale
(F-1) Water Reclamation – Without Secondary Treatment Background and Scope It is envisaged the aerobic process can possibly even be removed from the treatment train. This will likely be more viable if there is intention to reclaim water. Functionalized membranes may remove residual pollutants in the dilute stream. These pollutants may, however, differ from those challenging membranes currently used since these would have treated water with oxidized components. Given the likelihood of power generation on-site, there may also be opportunity for heat recovery and the latter may be used for evaporative water reclamation processes (e.g. membrane distillation) (Warczok, 2005). The heat recovered may also be used to heat the anaerobic process. It is likely R&D shall be necessary not only for the water reclamation technologies such as the functionalized membrane system, membrane distillation, and evaporation but also the thermophilic processes associated with application of such processes. Also critical will be an assessment of the energy balance to ensure technologies such as the membrane distillation and evaporation technologies can be viable. (F1) Objectives and Outcomes -
Screening for separation methods – e.g. functionalized membranes, membrane distillation, and enhanced evaporation Characterization and “treatment” of concentrate stream Characterization of product (dilute) stream – trace contaminants Product stream quality
Duration 3-6 years Budget S$1.5-2.5M @ lab-scale
33
(G-1) Residues Treatment Background and Scope The bottleneck of the sludge handling system is the dewatering operation. Advanced sludge treatment (AST) processes have been developed and investigated (Fytili & Zabaniotou, 2008) in order to improve sludge dewatering, and facilitate the ultimate disposal. Different processes using thermal hydrolysis (neutral, acid, alkaline) or chemical oxidation (H2O2, O3, O2) have been proposed in the literature (Neyens et al., 2003). Effective dewatering is likely determined by the morphology of the microbial mass in the biotreatment processes. The vast majority of these micro-organisms live in aggregates such as films, and flocs. A common feature is that the microorganisms are embedded in a matrix of extracellular polymeric substances (EPS) (Neyens et al., 2003). To understand the action of the AST technologies in the sludge dewatering process, the essential role played by the EPS must be assessed and understood. A novel drying process should be developed to provide economic advantages, namely (1) fuel savings due to the increased calorific value of dried material, including a reduction in supplemental fuel to the boiler, (2) reduce or eliminate the requirement for sludge landfilling or landspreading, and (3) reduction in greenhouse gas emissions if biomass combustion is considered net zero for CO2 emissions (Frei et al., 2004). Pyrolysis and gasification are two of the more promising methods for the thermochemical conversion of biomass toward a clean fuel source (Digman et al., 2009). To truly understand and model these processes requires detailed knowledge ranging from structural information of raw biomass, elemental composition, gas-phase reaction kinetics and mechanisms, and product distributions (both desired and undesired). A primary research goal in biomass thermochemical conversion is directed towards the optimization of these processes to reduce the amount of unwanted byproducts (Bahan et al., 2009). (G1) Objectives and Outcomes -
Characterization of anaerobic residues Dewatering properties and enhancing dewatering Drying & consequent calorific values of dried residues Odour management Pyrolysis/gasification
Duration 5-8 years Budget S$2M @ lab & pilot-scale
34
(H-1) The New Overall System -
Process integration, design & engineering the new system Verification of unit process and overall efficiencies Determine interactions among unit processes System stability Cost-benefit analysis
Duration 8-10 years Budget S$2M @ pilot-scale
Summary Year
Program & Program Details
Estimated Budget
0-2
(A-1) Nutrients recovery techniques other than BNR: Nutrients balances / Recovery of N, P, and K / Process development / Potential impacts on downstream bioprocesses.
Lab @ S$250K
2-4
(A-2) Nutrients recovery technology development: Sub-system design and engineering / Recovered material quality enhancement / Efficacy of recovered material / Cost-benefit analysis.
Lab-Pilot @ S$500K
0-3
(B-1) Biogas dissolution & release in aerobic process: CH4 balances in headspace & mixed liquor / Stripping in aerobic process / Mitigation – e.g. anaerobic CH4 oxidation.
Lab @ S$1M
3-5
(B-2) Control of biogas stripping at aerobic processes: Sub-system design & engineering / Impact of environmental and input conditions on CH4 removal process stability / Impact of consequent effluent on downstream bioprocesses.
Lab-Pilot @ S$750K
0-2
(C-1) Preconcentration as primary treatment: Development of preconcentration chemicals – e.g. biopolymers and sorbents / Developing the preconcentration process / Characterization of the concentrate and dilute streams – biodegradability of concentrate stream.
Lab @ S$500K
1-3
(C-2) Combining preconcentration with biodegradation: Investigating membrane properties / Integration of forward osmosis with bioreactor (anaerobic) / Comparative study of FO-anaMBR and UASB / Membrane fouling control.
Lab @ S$1M
3-5
(C-3) Preconcentration and biodegradation or preconcentration with biodegradation: Sub-system design & engineering / Operating protocols / Retrofit protocol / Verification of process performances, stability, and mass balances.
Pilot @ S$1.5M
35
Year
Program & Program Details
Estimated Budget
1-3
(D-1) Feed stream conditioning for enhanced biodegradation and solids reduction: Screening for appropriate method/s (heat, sonication, enzymes) with respect to feed characteristics / External and in-situ enzyme production / Enzyme blend / Changes in solubilization and biodegradability.
Lab @ S$750K
3-5
(D-2) Development of feed stream conditioning process: Sub-system Lab-Pilot @ S$1.5M design & engineering / Operating protocols / 3-phase bioprocess / Impact on gas production and quality / Impact on degradation kinetics / Verifications on solids reduction and reactor capacity / Odour issues.
3-8
(D-3) Enhancing the anaerobic digestion process: Recovering heat via CHP / Improving heat recovery and utilization efficiency / thermophilic anaerobic process enhancement – mixing, creating or limiting gradients, phase-separation, and process stability and mitigating inhibition / Digested sludge quality / Alternatives to single stage process.
Lab-Pilot @ S$2M
2-5
(E-1) Carbon recovery - process development: CH4-CO2 separation – membrane-based / Polymer construction with recovered CO2 – e.g. bioproduction of polysaccharides / Impact of catalysts on polymer construction / Properties of polymers and impact on anaerobic and aerobic processes.
Lab @ S$1.25M
5-7
(E-2) Carbon recovery - technology development: Sub-system design & engineering / Process stability and verification of yields / Impact of polymers on preconcentration performance / Impact of polymers on bioprocesses and O&M issues / Impact of polymers on membrane performance.
Pilot @ S$1.5M
3-6
(F-1) Water reclamation – without secondary treatment: Screening for separation methods – e.g. functionalized membranes, membrane distillation, and enhanced evaporation / Characterization and “treatment” of concentrate stream / Characterization of product (dilute) stream – trace contaminants / Product stream quality enhancement.
Lab @ S$1.5-2.5M
5-8
(G-1) Residues treatment: Characterization of anaerobic residues / Dewatering properties and enhancing dewatering / Drying & consequent calorific values of dried residues / Odour management / Pyrolysis/gasification.
Lab-Pilot @ S$2M
8 - 10
(H-1) The New Overall System: Process integration, design & engineering the new system / Verification of unit process and overall efficiencies / Determine interactions among unit processes / System stability / Cost-benefit analysis.
Pilot @ S$2M
36
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Appendix A: Project Details 1.
Information on the Project
The Public Utilities Board (PUB) of Singapore and Nanyang Environment and Water Research Institute (NEWRI) of Nanyang Technological University (NTU) had entered into an agreement to collaborate on a research project called “Water and Energy in the Urban Water Cycle Improving Energy Efficiency in Municipal Wastewater Treatment”. Mr Harry Seah and Mr Puah Aik Num, Director and Deputy Director of the Technology and Water Quality Office, PUB, had facilitated many discussions and deliberations, leading to the launch of the 12-month project with a kick-off meeting between PUB and the NEWRI team on February 2009. This report describes work performed from February 2009 to March 2010 and had been prepared by the NEWRI team. The report includes the work performed in the design of the questionnaire and its application on Singapore WWTPs. The questionnaire (Survey Form) which was created is included in Appendix G.
1.1
Project Background
While it is necessary to treat wastewater to protect water resources and the environment, doing so requires energy and so incurs a carbon footprint. This has impact on operating costs and climate change, and therefore provides impetus to focus R&D on energy balances within treatment systems, more efficient use of energy in the treatment processes, and the possibility of energy generation from components within the wastewaters. The need to investigate the energy aspect in the urban water cycle was first identified at the 12th Meeting of the GWRC Board of Directors in Sydney, November 2007. A three phase approach to the investigation was then developed at the GWRC Water & Energy workshop in London, February 2008. At the Water & Climate Change Workshop in Singapore, June 2008, specific projects related to energy and climate change were identified. Thereafter in Vienna, September 2008, a review of the energy footprint, reduction and recovery in municipal wastewater treatment plants was identified as the first project to be initiated. Similar studies on the other components in the urban water cycle shall follow. PUB of Singapore had volunteered to lead the first of these projects at the September 2008 meeting and other GWRC members (i.e. WERF, EAWAG, UKWIR, STOWA, KWR, WRC, and WQRA) had indicated their interest to participate in the project.
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The objective of this project is to generate a report on municipal wastewater articulates improved energy balances at various predetermined timelines anticipated paradigm shift by 2030 wherein an 80% reduction is achieved. approach had been formulated by GWRC members to achieve an energy and neutral urban water cycle by 2030:
treatment which leading to the A three phase carbon footprint
(1) To implement the best practices in the present state of art 1; (2) Reduce energy consumption by 20% by optimization and innovation; (3) Further reduction of energy consumption by 80%.
1.2
Project Team Organization
The project was undertaken by the collaborating parties - NEWRI of NTU and PUB. The NEWRI component was led by Professor Ng Wun Jern and this interacted with the PUB leads, namely Mr Puah Aik Num, Ms Yunita Tan, Dr Elaine Quek and Ms Hilda Lie. The study team comprised the following: Principal Investigator Professor Ng Wun Jern, Executive Director, NEWRI, NTU Team Members Dr Cao Yeshi, PUB Ms Joy Chua, DHI-NTU Centre, NTU (till end 2009) Assoc Professor Liu Yu, AEBC, NTU Dr Maszenan Abdul Majid, AEBC, NTU Ms Le Tuyet Minh, DHI-NTU Centre, NTU Assoc Professor Tan Soon Keat, DHI-NTU Centre, NTU Dr Tao Guihe, PUB Assoc Professor Wang Jing-yuan, R3C, NTU Dr Zhang Dongqing, DHI-NTU Centre, NTU Dr Zhou Yan, NEWRI, NTU Quality Assurance Reviewer Mr Kiran Kekre, PUB 11
Implementation of the current best practices in today’s operations would result in a first easy to make step but which will nevertheless constitute a significant contribution towards achieving the above articulated objectives: it is akin to picking the ‘low hanging fruit’, an action which will help bring every utility onto the same basis for further forward action. GWRC already has 2 on-going projects related to this: (i) Energy Efficiency in the Water Industry: A Compendium of Tools, Best Practices and Case Studies (led by UKWIR), and (ii) Development of the Toolbox of Integrated Performance Evaluation (led by Water Research Foundation – formerly AwwaRF).
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2.
Methodology and Approach of Study
2.1
Methodology
A study of the energy footprint of a domestic wastewater treatment plant would need to include the various key bio-treatment processes, the practices associated with these, and the various stages of technology development and utilization envisaged. It was noted, to a large extent, the treatment process selected is influenced by characteristics of the influent, climate, state of economic development, and culture. A group of countries at various stages of development was selected to provide a representation of the factors just identified. The study proposed the following candidate countries (and WWTPs therein): Australia, Austria, Canada, China, Israel, Singapore, South Africa, Switzerland, The Netherlands, United Kingdom, the United States of America, and a country in South America. The current practice of wastewater treatment at the selected WWTPs was reviewed and assessed. The assistance of PUB and appropriate GWRC members was sought to assist in the gathering of relevant information and, where appropriate, to gain access and to have interactions with the plant operators. In the desktop review, emphasis had been placed on utilization of energy for the treatment processes in the plant. Processes occurring outside of the treatment plant, while requiring some degree of awareness on the part of the project team, had not been included in this study. Similarly, in considerations of the opportunities for energy recovery - the study had restricted itself to opportunities within the plant and from the wastewater or feed streams arising from the wastewater within the plant. This study focused on the following 5 categories: (1) Climate, state of economic development, influent characteristics, and energy footprint of the various treatment processes in a domestic wastewater treatment plant; (2) Assessment and mapping of practice and technology developments in the candidate countries - in terms of energy usage and energy recovery at the time of study; (3) Energy conservation/reduction and potential energy recovery alternatives and emerging technologies - taking into account the various issues related to climate change and the economic and technological capabilities of the candidate countries; (4) Assessment of the potential for energy reduction and developing a technology roadmap, in terms of technology adoption and changes in practices, and technology development that is achievable by 2015 and 2030; and (5) Identification of possible research projects which would be needed to support the practice changes and technology developments.
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The study was based on literature review, gathering relevant information from sources including GWRC members, site visits, and invited contributions from international researchers and wastewater treatment professionals. A collaborative approach had been emphasized since it had been anticipated much of the relevant information might well be proprietary in nature and owners of such information shall have to be persuaded to share the information. All these collaborators in effect become a part of the larger research team and acknowledged as such. The international participants provide inputs on their local conditions, practices, and developments. The research team’s core provided the framework and guidelines which was then available for use by all participants towards developing the assessment report. The research team core conducted a technology scan to source information on developments and evaluated emerging technologies that could possibly be adopted or adapted for wastewater treatment.
2.2
Communications
An international discussion platform in the form of an e-collaboration platform had been setup. International partners were invited to participate in this effort at reviewing the energy footprint of wastewater treatment and evaluating emerging technologies. This e-effort was expected to culminate in an international workshop in Singapore. This workshop would serve to provide a platform for presentation of the key findings and consequent proposals, review of this, and feedback on the material presented. It was anticipated that significant contributions towards development of components of the technology roadmap relevant to specific candidate countries would be made at this workshop. The e-collaboration platform and workshop were to be supplemented with email, telephone, and video-conferences between the core research team and international participants. Such communications would then lead to selected site visits. Communications within the project team core was relatively simple and usually took the form of face-to-face meetings, through video conferencing or other form of electronic communications and target-specific visits. Reporting to PUB (and to GWRC when instructed to do so by PUB) took the form of face-toface presentations and discussions, and reports at quarterly intervals, with updates at the 3rd and 9th month of project execution, a progress report at the 6th month of the project and a final report at the end of the project. This report is then the final report.
2.3
Team Management & Quality Assurance
The core team comprised members from two organizations – NEWRI-NTU and PUB. Three sub-teams each addressing a particular component of the study had their own leaders and the three sub-team leaders reported to the overall team coordinator from NEWRI-NTU. Of the members within the project team, one had been kept “separate” from the rest and he served as the project’s internal quality assurance member. Additionally, a panel of experts, drawn from both the local and international community specializing in wastewater treatment technologies and practices was invited to critique and enhance the report at various stages of the study and so served as an external quality assurance device.
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The panel of experts and international contact persons are listed in Tables A1 and A2 below: Table A1 List of international experts serving as resource persons No.
Name
Country
Expertise
Email address
1
Mark van Loosdrecht
The Netherlands
Biofilm
[email protected] UDelft.NL
2
Norbert Matsche
Austria
Wastewater treatment
[email protected]
3
Hansruedi Siegrist
Switzerland
Wastewater treatment
[email protected]
4
Anthony Fane
Australia
Membrane technology
[email protected] [email protected]
5
Rainer Stegmann
Germany
Energy recovery
[email protected]
6
Anders LynggaardJensen
Denmark
Process Modeling
[email protected]
7
Staffan Kjelleberg
Australia
Microbiology
[email protected]
8
Yehuda Cohen
Israel
Microbiology
[email protected]
9
Jurg Keller
Australia
Microbial fuel cell
[email protected]
10
Peter Wilderer
Germany
Wastewater treatment
[email protected]
11
Perry McCarthy
USA
Wastewater treatment
[email protected]
Table A2 List of international contact persons serving as project collaborators No.
Contact Person
Organisation
Country
Email address
1
Valerie Naidoo
Water Research Commission
South Africa
[email protected]
2
Wouter Pronk
EAWAG
Switzerland
[email protected]
3
Jan Hoffman / Jos Frijns
KWR
The Netherlands
[email protected] /
[email protected]
4
Cora Uijterlinde
STOWA
The Netherlands
[email protected]
5
Lauren Fillmore
WERF
USA
[email protected]
6
Carlos Peregrina
SUEZ-CIRSEE
France
[email protected]
7
François Vince
Veolia Water
France
[email protected]
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Figure A1 shows the general approach of the study. The technology roadmap so developed eventually was intended to serve as a guideline towards achieving the objective of energy reduction in a WWTP via process/system modifications and adoption of new technologies. The project team had proposed to draw on, where appropriate and where the information was available, information/findings already obtained by other GWRC projects to augment the findings of this study.
Identify candidate WWTPs
•
•
WWTP processes
•
Possible Energy reduction measures
Local core team •
Invite international collaborators
Review influent characteristics to WWTP
•
Develop protocol and energy assessment methodology
Best practices on energy reduction
Compilation of review report
Emerging technologies
Industry
International workshops Emerging technologies Develop roadmaps
Reviewer panels
Identify projects Final report Presentation
Establish framework for survey and reporting
Members of GWRC and Quality Assurance Experts
Figure A1 Approach of the study
2.4
Research Team Members
A core team drawn from Nanyang Environment and Water Research Institute (NEWRI) Nanyang Technological University (NTU), and Public Utility Board, Singapore (PUB) undertook the bulk of the work. The team was supported by a group of independent collaborators who served as reviewers as well as niche experts who provided inputs on, for example, specific technologies and on visioning. These reviewers and experts were drawn from academia and the industry – local and international. Aside from the use of tele-conferencing and video-conferencing for discussions, some travel to specific locations for discussions with collaborators was envisaged. To support these, two (2) workshops (one mid-term and one at the end) were organized to garner information and opinions, and reactions to proposals/findings made by the core team. The work, as planned, largely involved desk studies, consultations with colleagues in academia and industry, and literature reviews. The core team also identified a member from among its members (Quality Assurance Reviewer) who then served to ensure internal quality control of the work and report. The research team was organized as shown in Figures A2 and A3.
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Client: Public Utilities Board
International collaborators/contacts • Dr Valerie Naidoo (Water Research Commission, South Africa) •
•
Dr Wouter Pronk (EAWAG, Switzerland) Dr Jan Hoffman / Dr Jos Frijns (KWR, The Netherlands)
•
Dr Cora Uijterlinde (STOWA, The Netherlands)
•
Dr Lauren Fillmore (WERF, USA)
•
•
Principal Investigator • Professor Ng Wun Jern (NEWRI)
Research Team Members • Dr Cao Yeshi (PUB) •
•
Norbert Matsche (Austria)
•
Hansruedi Siegrist (Switzerland)
•
Anthony Fane (Australia)
•
Rainer Stegmann (Germany)
•
Anders LynggaardJensen (Denmark)
•
Staffan Kjelleberg (Australia)
•
Yehuda Cohen (Israel)
•
Assoc Professor Liu Yu (AEBC-NTU)
•
Mr Carlos Peregrina (SUEZ-CIRSEE, France)
Dr Maszenan A. M. (AEBC-NTU)
•
Ms Le Tuyet Minh ( DHI-NTU Centre)
Mr Francois Vince Veolia Water, France)
•
•
Assoc Professor Tan Soon Keat (DHI-NTU Centre)
Jurg Keller (Australia)
•
Peter Wilderer (Germany)
Dr Tao Guihe (PUB)
•
Perry McCarthy (USA)
•
Quality Assurance Reviewers • Kiran Kekre (PUB) •
Ms Joy Chua (DHI-NTU Centre)
International resource persons /experts • Mark van Loosdrecht (The Netherlands)
Ole Larsen (DHI-NTU)
•
Assoc Professor Wang Jing-yuan (R3C-NTU)
•
Dr Zhang Dongqing (DHI-NTU Centre)
•
Dr Zhou Yan (NEWRI)
Figure A2 Project Organisation Chart
50
Project Steering Group (PSG)
Principal Investigator
Quality assurance reviewer
International panel
Research team NEWRI-NTU and PUB (9 core researchers) Industry partners
International collaborators
NEWRI office support
WWTPs current practices
Energy reduction
Coordinator
Coordinator
Emerging technology and roadmap
Coordinator
Research fellows (2) and project officer (1)
Figure A3 Organisation of the study team
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Appendix B: An Overview of the Urban Water Cycle 1.
Urban Water Cycle
An urban water cycle is a subset of the water cycle. An urban centre typically derives its raw water supply from external sources such as water reservoirs located a certain distance away. These reservoirs and their catchments typically occupy an area significantly larger than that of the urban centre in question, and are typically charged by streams within the catchments which are not part of the urban centre. Certain urban centres may include pockets of nature reserves which may then be able to sustain small surface reservoirs. However, these urban reservoirs typically have a storage capacity measured in days or weeks at best. Where it is geologically endowed, groundwater aquifers are another possible source of raw water. Again like the surface reservoir, the aquifer would be charged from water sources outside the urban centre. Deriving fresh water through desalination is a possible option in certain coastal urban centres. Lastly, where it is feasible to do so, there is also the option of reusing water within the system. To illustrate the above, Singapore derives its raw water supply from the following sources: •
Water piped in from surface reservoirs in Malaysia;
•
Rainwater collection in nature reserves and selected catchments within Singapore;
•
Reclaiming water from used water and;
•
Desalination.
In addition to the above, there are examples of groundwater use but these are for small suburban settlements and industry and the quantities so exploited are relatively insignificant. As urbanization increases, it may be possible that stormwater management may go beyond transfer of water away and so reduce risk of flooding but also include collection and treatment and so serve as a source of potable water. In many urban centres, the combined sewer and stormwater system is used, in which case the amount of water ending up at the WWTP and water reclamation plant would increase during the wet season. The urban water cycle can be presented schematically as shown in Figure 1-1 and this may well represent the Singapore example. The solid lines link the domestic water system while the dashed lines represent the industrial water component in the cycle. Figure 1-1 also includes the sources of raw water. The conveyance system is one of gravity flow through open channels, and pipes through a combination of gravity flow and mechanical pumping. The “raw water” groups the collection and water storage system in the form of surface reservoirs and service reservoirs. The treatment processes include within their own system, production tank and water towers.
52
Viewed from the energy and energy efficiency perspective, the urban water cycle can be categorized as follows: 1. Source collection and storage; 2. Water treatment including desalination; 3. Drinking water distribution and wastewater collection; 4. Wastewater treatment, and 5. Water reclamation. The discussion below does not include the energy foot-print associated with the manufacture of the mechanical systems; manufacture fabrication or construction of plants and materials for elements such as pipes; and chemicals for treatment.
1.1
Source Collection and Storage
1.2
Water Treatment
Source collection and storage in an urban environment generally refers to collection of rainwater and storage in surface reservoir(s). In an urban catchment, the collection of water is generally through storm drainage systems, detention ponds, and perhaps landscaping ponds, waterways or lakes. The storm runoff is usually channeled through pipe flow or open channel flow by gravity, following the grade. Excess water is generally allowed to by-pass or discharge out of the system. Where the receiving water body is large and has retained sufficient head to generate electricity, a portion of the potential energy in the water collection system can be recovered. While it need not always be so, in certain urban settings, such energy recovery may be less significant.
Conventional water treatment based on surface water comprises the following parts: -
Aeration
-
Pretreatment based on coagulation/flocculation using chemicals
-
Clarification
-
Sand filtration
-
Disinfection
Electricity (energy) consumption for this process is relatively low – in the range of 0.05-0.47 kWh/m3. The latter value is for energy consumed at the plant. It does not include the energy consumed earlier in the life-cycle – e.g. for manufacturing of process chemicals.
53
1.3
Drinking Water Distribution and Wastewater Collection
Drinking Water Distribution Drinking water is distributed by supplying water at the required head, through direct pumping or pumping to a service reservoir (or elevated water tower) and flowing by gravity into the reticulation system. In some cases, there may be a main trunk line with booster stations and elevated water tanks or towers. The user may choose to tap directly into the reticulation system, or use a ground tank and a local pumping system to lift the water to a roof tank or a water tower. Wastewater Collection System In an urban centre with separate sewer and stormwater systems, used water flow into the sewer by gravity, and enter the stilling basin or sump in a transfer station, from which the used water is lifted to a predetermined elevation (head) and thereafter gravitates to the next transfer station through a sewer pipe. The used water is transferred in relay fashion and eventually arrives at a wastewater treatment plant. Where a combined sewer and stormwater system is used, the quantity of flow in the system could increase many folds when it rains. The challenges lie in the logistics and infra-structure of handling a large quantity of flow during certain periods of the year. There is almost always the necessity for design and operation of overflow when the flow exceeds the capacity of the combined sewer system and the wastewater treatment plant. As the energy consumed in transferring the used water is proportional to efficiency (η), discharge rate (Q) and head (H), therefore, an efficient system would also require pumping systems with better efficiency, less transfer arrangements (optimum transfer operation) , and lower H (optimal selection of reservoir transfer in terms of elevation difference, and a pipe conveyance system with less friction losses, i.e. larger pipe diameter, shorter pipe length, and/or pipes with smoother internal surfaces).
1.4
Wastewater Treatment
The purpose of wastewater treatment is to degrade and remove unwanted compounds and conventionally treatment would mainly use an aerated process and thereafter separate solid particles from water through gravity clarification. The solids or sludge then undergoes sludge treatment. These systems require intensive energy input. This aspect, which is the focus of the proposed study, shall be discussed in greater detail in Appendix C.
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There appears to be a trend of stricter discharge standards being imposed in wastewater discharges. In part, this has arisen from the growing awareness of the deleterious impact of inadequately treated wastewaters have on the environment – e.g. eutrophication of surface waters since the 1980s. The principal quality parameters monitored in this regard include Ammonium Nitrogen (NH4-N), Total Nitrogen (TN), and Phosphate Phosphorous (PO4-P). For example in Europe, EC Directive – 91/271/EEC has called for NH4-N
