greenhouse emissions

CALCULATING THE COST OF GAS EMISSIONS FROM WASTEWATER Calculating carbon tax liabilities from wastewater treatment and biosolids stockpiling B Hutton, E Horan, D Rouch Dr Duncan Rouch is the corresponding author. Email: [email protected]

Abstract Under the Federal Government’s proposed carbon tax legislation, a price will be placed on emissions of greenhouse gases including methane (CH4) and nitrous oxide (N2O). These two gases are substantially emitted during wastewater treatment. We outline how to calculate emissions of CH4 and N2O from wastewater and sludge treatment processes. Further, we provide an example calculation for emissions of methane from sludge pan-drying and biosolids stockpile storage, using data on organic degradation from a large metropolitan treatment plant located in Melbourne. A cumulative total of 6.874/t of CO2e per t of biosolids was estimated to be emitted, after one year of pan-drying and three years’ stockpiling of biosolids. This would incur tax of $174.59/t at a carbon price of $25.40/ tCO2e or $378.05 at a price of $55. These figures do not include emissions from the “liquid train” of wastewater treatment, or any N2O emissions.

Introduction On 10 July 2011 the Federal Government announced its plan to introduce a carbon emissions tax. Initially a flat charge of $23 per tonne of emissions will be levied on the top 500 commercial polluters, for example, Macquarie Generation Australia and BlueScope Steel. The tax is designed to increase 2.5 per cent every year for the next three years, reaching $25.40 on July 1, 2014. In July 2015, the tax system will convert to an emissions trading scheme. Thereafter, the price will fluctuate with the market, but a price ceiling will be set at $20 above the expected international carbon price for 2015–2016 (Commonwealth of Australia, 2011). European Union Emissions Trading System carbon credits are forecast to cost on average of $32 (€24) per tonne in 2013, ranging from $29 to $35 per tonne (Chestney, 2011). The upper price limit

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in the Australian carbon trading scheme could, therefore, be approximately $55, rising as the cap is tightened. The “carbon tax” covers not only emissions of carbon dioxide, but other greenhouse gases including methane (CH4) and nitrous oxide (N2O). CO2 emissions generated from waste management are considered to be from biomass sources and, therefore, are not taxed (DCCEE, 2011). CH4 and N2O will, however, attract a high levy: a kilogram of N2O is equivalent to 298 kilograms of carbon dioxide. Methane emissions are currently estimated using a global warming potential of 21 (that is, they trap 21 times as much heat as an equivalent weight of carbon dioxide). A more accurate figure of 25 will be introduced from 2014 (IPCC, 2007). The aim of a carbon trading scheme is to reduce CO2-equivalent emissions from one sector, such as waste, to create a “carbon credit” to sell to companies with power stations, which cannot so easily reduce their own emissions. When a carbon trading scheme is introduced, wastewater and land managers, like farmers, will be able to trade carbon credits. They will need to accurately measure their existing emissions and then reduce them, as many landfill managers have already done. Significant emissions of greenhouse gases occur in wastewater treatment. Methane is emitted in anaerobic or partially anaerobic conditions such as anaerobic lagoons. N2O is emitted during nitrification and denitrification of wastewater. Both gases are emitted from drying ponds and biosolids storage stockpiles. The carbon price will initially apply to facilities or activities that individually produce more than 25,000 tons of CO2-e emissions per year (Commonwealth of Australia, 2011). Will wastewater treatment plants be covered by the tax? Our calculations suggest that initially the larger ones will be taxed. Smaller ones will have longer to act and are likely to benefit financially from carbon credit trading, by reducing their emissions and selling carbon credits. So how do we assess the

wastewater industry’s potential liabilities and opportunities under the scheme?

Calculating Carbon Tax Liabilities for Wastewater Treatment and Biosolids Storage Simplified prototype treatment trains for a metropolitan and a regional wastewater treatment plant are shown in Figure 1. For a metropolitan plant, screened sewage is piped to a primary sedimentation tank. It then undergoes activated sludge treatment. Secondary sedimentation of activated sludge produces a second round of sediment. If an anaerobic digester is available, the sediment is digested. The digested solids are then dewatered, or dried in a drying pan. The biosolids may then be landfilled, subjected to long-term storage or other treatment, and applied to soil. In contrast, for a smaller regional plant, after screening sewerage may undergo biological treatment in a trickling filter, followed by a sequence of three lagoons, of which the first two involve facultative biological treatment of organic material. The wastewater is generally further treated to clarify, denitrify and disinfect it, before being discharged to the environment (for example, to the sea). Greenhouse gases are emitted throughout these treatment trains, where organic materials and nitrogen-containing inorganic compounds meet biological activities, especially under anaerobic conditions.

Emissions of Greenhouse Gases Methane CH4 (methane) is emitted by sewage in anaerobic or partially anaerobic conditions, for example from sedimentation tanks, anaerobic treatment ponds, lagoons, drying ponds and biosolids storage piles. It is also produced in the anaerobic digester, but in this case it is generally captured and may be used as process fuel.

Nitrous oxide As well as emitting methane, biosolids also emit N2O. The amount depends on the type of wastewater treatment used: a warm

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greenhouse emissions • S  hallow anaerobic lagoon (2 metres): 0.8.

Biosolids Stockpiling: CH4, N2O emitted Activated Sludge: N2O emitted

Anaerobic digester: CH4 collected

Methods Emissions from wastewater treatment and sludge treated on-site are worked out separately then added together.

B. Regional WWTP (Lagoon system) Sludge harvested & dried: CH4, N2O emitted

Screened sewerage input

Recycled water Trickling filter: CH4, N2O emitted

Facultative Lagoon 1: main CH4 emission

Facultative Lagoon 2: less CH4 emission

Detention Lagoon 3: little CH4 emission

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Figure 1. Simplified wastewater treatment trains for producing biosolids for a metropolitan WWTP (A); a regional WWTP with lagoon treatment (B). For lagoon treatment, emissions would serially fall lower across the three lagoons, as most of the sludge would settle in the first lagoon. stagnant lagoon would emit more N2O than cool flowing wastewater (IPCC, 2006a). N2O is produced in conditions of low oxygen by facultative anaerobes (bacteria that can obtain their oxygen either from air or from nitrates, nitrites and sulfates. These are common in human faeces). In fully anaerobic conditions they obtain oxygen by reducing nitrates and nitrites to N2O, and then reducing N2O to N2, which is not a greenhouse gas. In fluctuating lowoxygen levels some of these bacteria do not take the last step, so the potent greenhouse gas N2O is emitted. Therefore, to control emissions of both methane and N2O, wastes should be treated completely anaerobically (with methane capture) or completely aerobically. Alternating between anaerobic and aerobic conditions causes high N2O emissions (Otte et al., 1996). In contrast, fully aerobic treatment (for example, rapidly bubbling aerobic treatment ponds) produces mostly CO2 emissions, which do not need to be estimated.

Calculating Greenhouse Gas Emissions from Wastewater Treatment Methane

Emissions of methane will occur from upstream wastewater treatment processes, such as sedimentation/clarifying and activated sludge, and also from downstream drying pans and lagoons (Figure 1). Methodologies for calculating emissions from wastewater treatment processes are provided in the Department of Climate Change’s National Greenhouse and Energy Reporting (Measurement) Technical Guidelines (DCCEE, 2011), Chapter 5 – Part 5.3, div 5.3.2. Chemical Oxygen Demand (COD) can be measured directly or calculated from the population served by the WWTP, assuming 58.5 kg per person per year. The methane generated in the plant from wastewater and sludge is the total COD treated on-site times an emissions factor (EF) and a methane correction factor (MCF). COD released in effluent, or sludge sent to landfill or other off-site storage, is calculated separately (see Equation 1). 1) CH4gen = [(CODw – CODsl – CODeff) x MCFww x EFwij] + [(CODsl – CODtrl – CODtro) x MCFsl x EFslij]

What is the MCF?

Where:

The methane correction factor (MCF) is a measure of how anaerobic a site is. A totally sealed landfill has MCF = 1 (100% anaerobic). A rapidly bubbling, fully aerated wastewater pool has MCF = 0 (no methane is produced). An MCF of 0.3 means that 30% of the organic material in a pool is degraded anaerobically. The methane correction factors for wastewater treatment are given in a table on page 340 (DCCEE, 2011) for various types of treatment. These are:

• COD factors are: CODw for wastewater, CODsl for sludge, CODeff for effluent, CODtrl for sludge sent to landfill, CODtro for sludge otherwise stored off-site;

• M  anaged aerobic treatment: 0;

• MCFww and MCFsl are the MCFs for wastewater and sludge, respectively; • EFwij and EFslij are the EFs for wastewater and sludge, respectively.

The default emission factor for both sludge and wastewater is 5.3 tonnes CO2-e per tonne COD. The MCF may be different for sludge and wastewater if

they are treated differently. As with landfill, methane from wastewater treatment and biosolids storage can be captured and flared or used as fuel, so lowering greenhouse emissions. The method to be applied in these circumstances is as for the solid waste disposal method (DCCEE, 2011, section 5.25).

Nitrous oxide N2O emissions from wastewater treatment are difficult to estimate. Equation 2 provides a rule-of-thumb method for calculation of nitrogen in wastewater. N2O emissions depend on how the effluent is disposed of (DCCEE, 2011, division 5.3.5). 2) Nin = Protein x FracPr x P

Where: • Nin is the amount of protein in the influent wastewater; • Protein is the estimated average amount of protein consumed per person per year, 36kg/ person/y; • FracPr is the fraction of nitrogen in protein, 0.16; • P is the number of people supported by the WWTP. So, for a population of 1000 people,

N = 36 kg x 0.16 x 1000 = 5.76 tonnes in per year.

If the effluent is released to enclosed waters, a default value of 4.9 tonnes of N2O (in CO2-e) is produced per tonne of nitrogen (DCCEE, 2011, divisions 5.3.1 to 5.5.0). So, for a population of 1000, N2O emissions as CO2-e = 28.2 tonnes. These emissions also are likely to occur in drying pans and lagoons. The IPCC (2006b, “Waste” Chapter 6) indicates that there is a wide range of variability for N2O emissions, and it is advisable to measure them directly. In contrast, if the effluent is released to the deep ocean, N2O produced is negligible.

Calculating Greenhouse Gas Emissions from Drying and Stockpiled Biosolids For assessing emissions from pan-drying or from stockpiled biosolids (Figure 1A), methods are available in IPCC (2006a), on which the Australian guidelines are based.

Methane Biosolids stockpiles are considered to be a form of solid waste disposal on land (UNFCCC, 2010b; IPCC, 2006a, Vol 5, table 3.1 p 3.14). Emission factors are: • Managed – anaerobic

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1.0;

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greenhouse emissions • Deep (>5 m waste) and /or high water table

0.8;

• Shallow (1000-fold higher than those from stable areas. For example, consultants GHD (2010) found flux readings of 0.01g of methane/m3/hour on surface areas of a landfill pile, yet one weak spot emitted 62.1g/m3/hour – a factor of 62,000 higher. Therefore, the site must undergo a “walk over” by an independent expert using a portable gas meter, able to detect hydrocarbon gases at a minimum of 10 ppm. Then “hotspots” are mapped and an estimate of total emissions is made. Expert consultants can calculate a site-specific “k factor” to estimate the decay rate. This can also be done from laboratory measurements of volatile solids. The Department of Climate Change and Energy Efficiency has assessed various technical methods for measuring methane emissions, scoring them from “good” to “poor”. This is a useful guide for facility operators and consultants (DCCEE, 2010b, Table 4.6.)

How to Conduct a Waste Storage Site Survey

Example Calculations for Emissions of Methane from Sludge Drying Pans and Biosolids Stockpiles

The IPCC (2006a, Table 3.1) provides values for solid waste disposal at

A large number of samples were taken from sludge drying pans and biosolids

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greenhouse emissions stockpiles at a metropolitan WWTP over a period of a year (Rouch et al., 2011). Stockpiles were tested after one year, two years and three years, to give a “snapshot” of the effect of ageing on the biosolids. Levels of Kjeldahl nitrogen and volatile solids (a marker for degradable organic carbon) were tested by the ALS, accredited by the National Association of Testing Authorities. The results indicated a steady decline in levels of both organic nitrogen and volatile solids, consistent with dissimilation of organic material to gases CO2 or CH4 and of nitrogen to N2 or N2O. Testing of samples indicated that decomposition in the upper layer of the biosolids stockpiles was largely aerobic, therefore emitting mostly CO2. However, anaerobic conditions appeared to occur below about 500mm from the surface and, therefore, would emit methane. The yearly rate of decomposition of organic material is shown in Table 1 (left-hand column). The Australian guidelines for estimating emissions from solid wastes and wastewater treatment are derived from the Intergovernmental Panel on Climate Change (IPCC) values. Australian values from the Department of Climate Change’s Technical Guidelines (DCCEE, 2011) and IPCC (2006) were used for calculating emissions. These give an estimated “methane correction factor” (MCF) for various types of wastewater and sludge treatment. Drying pans sampled were generally more than 2m deep. As the MCF for deep lagoons (>2m) is 0.8, we conservatively assumed MCF = 0.6. Unmanaged deep stockpiles of deposited waste (> 5m) have a default MCF value of 0.8, shallow unmanaged sites an MCF of 0.4, and deep compacted sites have an MCF of 1. We again conservatively assumed an MCF of 0.6 for the stockpiles. (Choosing a higher MCF would produce a higher figure for total emissions.) The loss of volatile solids (VS) in digested sludge from drying pans and also from biosolids piles was known from VS analysis. Because this measured data was available it was not necessary to estimate the amount of degraded decomposable organic carbon (DOCF), or the decomposition rate. We therefore used a modified version of Equation 3, as Equation 5. 5) CH4gen = DOCF x F x 1.336 x MCF x 25

The quantities of dissimilated volatile solids were multiplied by the fraction

of methane in “landfill” gas F (0.5), the MCF (0.6), and a factor of 1.336, as recommended in the Technical Guidelines (DCCEE, 2011), to convert from carbon to methane (this is approximately 16/12 as shown in Equation 3). The oxidation factor in Equation 3 is equal to 1 as there was no cover material, so it can be omitted. The R factor can also be omitted as no methane was recovered. To convert the methane to CO2-e, we used a GWP factor of 25, as this figure will be in use by 2014. To calculate tax liabilities, we then multiplied total tonnes of CO2-e by the two estimated values for the carbon tax of $25.40 and $55 per tonne.

Results Results are summarised in Table 1. From data on VS reduction it was estimated that one tonne of dry solids produced approximately 3.297 tonnes of methane as CO2-equivalent in the first year (including the pan-drying process) and a cumulative total of 6.874 t DS after three years of stockpiling. The cumulative tax incurred would be $174.59 per t DS at a tax of $25.40 per t CO2-e, or $378.05 per t DS at a tax of $55 per t CO2-e. The amount of biosolids from one drying pan at a metropolitan WWTP is about 1,000 t. So the accumulated carbon tax liability per pan, after three years of stockpiling at $25.40 per tonne of carbon emissions, would be $174,590. At $55 it would be $378,050. The output of dried biosolids in southern Australia is approximately 20 tonnes per 1000 people (MWC, 2010). Therefore, at $25.40, the potential tax from methane alone is $3,491.80 per thousand people, and at $55 per tonne it would be $7,561. For a large WWTP servicing millions of people, the tax liability would be tens of millions of dollars. The figures reported here are for methane emissions from sludge drying ponds and biosolids storage only. They do not include emissions from the upstream wastewater processes, or the initial production of the sludge. These also omit values for nitrous oxide emissions. Rouch et al. (2011) found that total Kjeldahl (organic) nitrogen levels diminished significantly during storage, from a mean value of 15% of freshly anaerobically digested sludge, to 0.05% of a sample of three-year old dried biosolid (taken from near the surface) and 0.01% taken at a depth of 500mm. This nitrogen may have dissimilated to a gaseous form, N2 or N2O. Thus nitrous

oxide emissions from stockpiles are likely to be significant and could add as much again to the tax liability. Under carbon trading a reduction in emissions could mean that carbon credits are earned. While various Australian cities and regional areas have a range of practices concerning treatment, storage and use of biosolids, the large scale of the problem (and opportunity) is apparent.

Conclusions: Future Treatment Options to Reduce Carbon Tax Liabilities Clearly the potential carbon tax would be a substantial additional cost on wastewater treatment, so it is important to consider how treatment systems can be altered to reduce greenhouse emissions. One method of reducing emissions is to cover the waste material (eg, sludge or biosolids) and capture the methane. This is already done by landfill operators, who use best-practice management techniques, including gas wells and a vacuum system to capture more than 75% of the methane. Similar methods could be used on-site to reduce emissions from biosolids stockpiles. Another method is treatment in an anaerobic digester. In both cases captured methane can be used for the production of energy. Alternatively, sludge must be very thoroughly aerated, which is difficult with solids. Covered anaerobic treatment also prevents emissions of N2O. Improved treatment options nominated by the UNFCCC (2010) include: • Optimising efficiency of existing anaerobic digesters; • Addition of anaerobic digesters to current small treatment systems (which could include a simple cover-andcollect system); and • Direct application to land. In addition, composting with a simple cover-and-collect system might be a possible improvement, though that would provide difficulties for physical management, such as for turnover. Methods such as tunnel composting may increase N2O emissions (Amlinger et al., 2008). By comparison, collecting greenhouse gas emissions from liquid sludge would appear to be a more practical option. For land application the microbial safety of biosolids produced by revised treatment systems would need to be assessed in line with current regulations.

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greenhouse emissions The Authors

A, Carbon pricing mechanism. www. cleanenergyfuture.gov.au/wp-content/ uploads/2011/07/Consolidated-Final.pdf Chestney N, 2011: Bar cap ups 2013 UN carbon price forecast by 10 percent, Reuters 12/4/2011. www.reuters.com/ article/2011/04/12/us-carbon-barcapidUSTRE73B2JY20110412.

Barbara Hutton is a research student at the Department of Civil, Environmental & Chemical Engineering, RMIT University, supervised by Mr Ed Horan. Ed Horan is the Program Director, Master of Sustainable Practice at the Department of Civil, Environmental & Chemical Engineering, RMIT University. Dr Duncan Rouch (email: duncan.rouch@ rmit.edu.au) is the principal postdoctoral researcher in a Smart Water Fund Project at Biotechnology and Environmental Biology, School of Applied Sciences, RMIT University.

References Amlinger F, Peyr S & Cuhls C, 2008: Greenhouse gas emissions from composting and mechanical biological treatment. Waste Management Resources, 26, pp 54–55. Commonwealth of Australia, 2011: Securing a Clean Energy Future: the Australian Government’s Clean Energy Plan: Appendix

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DCCEE, Department of Climate Change and Energy Efficiency, 2010a: National Greenhouse Accounts (NGA) Factors. July 2010. www. climatechange.gov.au/~/media/publications/ greenhouse-acctg/national-greenhousefactors-july-2010-pdf.pdf DCCEE, Department of Climate Change and Energy Efficiency, 2010b: Review of the NGER (Measurement) Determination, August 2010. DCCEE, Department of Climate Change and Energy Efficiency, 2011: National Greenhouse and Energy Reporting (Measurement) Technical Guidelines June 2011, Chapter 5 – Parts 5.17F and G, and Part 5.3. www.climatechange. gov.au/government/submissions/reporting/~/ media/publications/greenhouse-report/reviewnger-measurement-determination-paper.ashx GHD, October 2010: “Report for Wollert Landfill: Flux Testing and Emissions Estimates”, available from Hanson Landfill Services, Melbourne. IPCC, 2006a: Guidelines for National Greenhouse Gas Inventories, Waste, Vol 5, Ch 3, pp. 3.14. www.ipcc-nggip.iges.or.jp/public/2006gl/pdf/5_ Volume5/V5_3_Ch3_SWDS.pdf

IPCC, 2006b: Guidelines for National Greenhouse Gas Inventories, Waste, Vol 5, Ch 6, pp. 6.13. www.ipcc-nggip.iges.or.jp/public/2006gl/pdf/5_ Volume5/V5_6_Ch6_Wastewater.pdf IPCC, 2007: Changes in Atmospheric Constituents and in Radiative Forcing, pp. 212. www.ipcc. ch/pdf/assessment-report/ar4/wg1/ar4-wg1chapter2.pdf MWC, Melbourne Water Corporation, 2010: Tender documents: Expressions of Interest: Implementation of a research and development project for ...“Beneficial Use” of biosolids states that MWC produces approximately 60,000 tonnes of biosolids from a population of three million people. Otte S, Grobben N, Robertson L, Mike SM, Jetten M & Gijs Kuenen J, 1996: Nitrous Oxide Production by Alcaligenes faecalis under Transient and Dynamic Aerobic and Anaerobic Conditions. Applied and Environmental Microbiology, July 1996, pp. 2421–2426 Vol 62, No 7. Rouch DA, Fleming VA, Pai S, Deighton M, Blackbeard J & Smith SR, 2011: Nitrogen release from air-dried biosolids for fertiliser value. Soil Use and Management, September 2011, 27, pp 294–304. UNFCCC (2010): Indicative simplified baseline and monitoring methodologies for selected small-scale CDM project activity categories. AMS III.H Methane recovery in wastewater treatment. cdm.unfccc.int/filestorage/8RIV5 MZ4AG7YE9UQJ6HSL3NTFXD1C0/EB58_ repan22_AMS-III.H_ver16.pdf

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