PILOT PLANT INVESTIGATIONS OF VARIOUS PROCESSES TO ACCOMPLISH BIOLOGICAL PHOSPHORUS REMOVAL FROM WASTEWATER AT RALEIGH'S NEUSE RIVER WASTEWATER TREATMENT PLANT

July 1, 1987 - December 22, 1987

by DONALD E. FRANCISCO STEPHEN R. SHOAF JAMES C. LAMB, III

Department of Environmental Sciences and Engineering University of North Carolina at Chapel Hill

A Report to THE CITY OF RALEIGH AAD

THE NORTH CAROLINA URBAN WATER CONSORTIUM

Administered by: WATER RESOURCES RESEARCH INSTITUTE of THE UNIVERSITY OF NORTH CAROLINA

June 14, 1988

ACKNOWLEDGMENTS This project was supported by a grant from the N. C. Water Resources Research Institute (NCSU Contract No. 63~ 0026) of the University of North Carolina. Funding was provided by the City of Raleigh, N. C. and the N. C. Urban Water Consortium. The N. C. Water Resources Research Institute has its offices on the campus of North Carolina state University and the University was instrumental in facilitating the contractual arrangements for this project. During this pilot project, in kind contributions were provided by the City of Raleigh. These services included laboratory analyses, equipment loan and repair, and assistance with pilot unit operations. In every phase of this study, the assistance provided by the·staff of the Neuse River Wastewater Treatment Plant a~d th~.Department of Public utilities of the city of Raleigh was invaluable. The Orange Water and Sewer Authority of Chapel Hill Carrboro provided the equipment for transportation of the pilot facilities to and from the Raleigh Neuse River treat~ent plant site. without their help, the logistics of this project would have been much more complicated. Carolyn Dunham and Janice Braxton, employees of the University of North Carolina at Chapel Hill, served as full-time technicians on the project. Their duties included sampling, analyses, and trailer operations. Their dedication and diligence were instnL~ental to the success of this project. Facilities and support for this project were provided through the UNC 'Wastewater Research Center and the Department of Environmental Sciences and Engineering of the School of Public Healtrr, University of North Carolina at Chapel Hill.

ii

ABSTRACT

The biological removal of phosphorus from municipal wastewaters is now recognized as an alternative to chemical precipitation under favorable circumstances. Several approaches to enhanced biological phosphorus removal (BPR) have been described in the literature. Research by various individuals has focused on the identification of those parameters responsible for the successful operation of these processes. This investigation explored the feasibility of operating a BPR system at the Raleigh Neuse River wastewater treatment plant. Influent to this 40 MGD plant is composed of domestic, research, commercial, and industrial wastewaters. Four continuous-flow, laboratoryscale pilot plants were operated simultaneously at this site from July 20, 1987 to November 16, 1987. The four pilot plants were used to empiricallY evaluate four BPR proc~sses. These were the Air Prqducts and Chemicals, Inc. itA jO" process, a Modified Biological Phosphorus Removal (MBPR) process, the University of North Carolina (UNC) process, and an experimental process (RALEX) which combined features of several processes. All pilot units received primary clarifier effluent as influent at an average flow rate of 200 mL/minute. The actual pilot influent flow was paced to follow the full-scale plant's hydrograph. The pilot influent averaged 6.4 mg/L total phosphorus, 15.3 mg/L ammonia nitrogen, 25 mgjL total Kjeldahl nitrogen, and 83 mg/L CBOD S " simultaneous pilot operation eliminated some of the variables that would ordinarily interfere '..;i th the comparison of resul ts. This study evaluated system perfo~ance in terms of BOD, ammonia, and phosphorus removal. Comparisons were based on the full-scale plant effluent discharge permit requirements and on the expected perforillance of conventional activated sludge treat~ent. The BOD and ammonia removal of each pilot system was sufficient to suggest that, with some adjust~ents, these systems were capable of satisfying the NPDES discharge pe~it limits for those two parameters. The Modified Biological-Phosphorus Removal (MBPR) system achieved the most consistent ammonia and BOD removal. BPR performance was less conclusive. Of the four pilot systems tested, three were capable of phosphorus removal, but the perfo~ance was erratic. None of the four produced consistent periods of operation resulting in phosphorus concentrations in the final aerated tank of less than 2.0 mgjL. The best (RALEX) was slightly

iii

above the target of 2.0 mg/L. It is recommended that further investigations at a larger scale and with modified operating conditions be undertaken before making a choice of the phosphorus control system for the Neuse River Plant.

iv

TABLE OF CONTENTS

I. II. III. IV.

V.

VI. VII.

INTRODUCTION

1

PROJECT OBJECTIVES AND INVESTIGATIONAL APPROACH

3

BACKGROUND ON BIOLOGICAL PHOSPHORUS REMOVAL

3

INVESTIGATIONAL METHODS A. Selection of Processes B. Experimental Facilities C. Sampling and Analyses D. Operation of the pilot Units

9

DISCUSSION OF RESULTS A. Characterization of the pilot Plant Influent B. Perfonnance of the A2jO pilot Plant C. Performance of the UNC Pilot Plant D. Performance of the MBPR pilot Plant E. Performance of the RALEX pilot Plant F. Supplemental pilot Plant Studies During December 1. Nature of the pilot Plant Influent 2. Perfonnance of the Modified UNC pilot Plant 3. Perfonnance of the Modified RALEX pilot Plant

9

10 14 17 18

20 23 34 46

56 67

69 69 73

SUM¥JffiY, CONCLUSIONS AND RECOMMENDATIONS OF THE PILOT BPR STUDY

75

REFERENCES

82

VIII. APPENDICES A-I B-1 B-2

Terms and Description of Analyses Raleigh Operations Log Notes; Trailer Operations Influent to All BPR Pilot Plants

v

TABLE OF CONTENTS (continued) C-1

C-2 C-3 C-4

C-5

D-1 D-2 D-3 D-4 D-5 E-l E-2 E-3 E-4

E-5 F-l F-2 F-3 F-4

F-5

Raleigh Operations Log Notes; A2 /0 Operations Notes A2jO System operations Log Data from the A2jO Pilot Final Aerated Tank A2jO Pilot Plant Anaerobic, Effluent and Operation Data Data From Samples Collected From Intermediate Tanks of The A2jO System Raleigh Operations Log Notes; UNC Operations Notes UNC System Operations Log· Data from the UNC pilot Plant Final Aerated Tank UNC pilot Plant Anaerobic, Effluent and Operations Data Data From Samples Collected From Intermediate Tanks of The UNC System Raleigh Operations Log Notes; MBPR Operations Notes MBPR System Operations Log Data from the MBPR Pilot Plant Final Aerated Tank MBPR Pilot Plant Anaerobic, Effluent and Operations Data Data From Samples Collected From Intermediate Tanks of The MBPR System Raleigh Operations Log Notes; Experimental BPR Operations Notes Experimental System Operations Leg Data from the RALEX pilot Plant Final Aerated Tank RALEX pilot Plant Anaerobic, Effluent, and Operations Data Data From Samples Collected From Intermediate Tanks of the RALEX System

vi

TABLE OF CONTENTS (continued) G-1 G-2 H-1 H-2 H-3 H-4

H-5

H-6 I-1

I-2 I-3 I-4 I-5 1-6

Raleigh Operations Log Notes: Trailer operations (December) BPR pilot Plant Influent (Neuse River Primary Effluent) Raleigh Operations Log Notes; Modified UNC Pilot System (December) Modified UNC pilot Plant Operations Log Data From the Modified u~C pilot Plant Final Aerated Tank, December, 1987 Modified ONC pilot Plant Anaerobic, Effluent, and' Operations Data, December, '1987 ' Data From Modified ONC pilot ,Plant Intermediate aerated Tanks Data From the Modified UNC Pilot Plant Endogenous Denitrification Tank 1 Raleigh Operations Log Notes; Modified RALEX pilot System (December) Modified RALEX pilot Plant Operations Log Data From the Modified RALEX pilot Plant Final Aerated Tank, December, 1987 Modified RALEX pilot Plant Anaerobic, Effluent, and operations Data, December,' 1987 Data From Modified RALEX Pilot Plant Intermediate aerated Tanks Data From the Modified RALEX pilot Plant Endogenous Denitrification Tank 1

vii

LIST OF FIGURES FLOWSHEETS FOR RALEIGH NEUSE RIVER BPR PILOT STUDIES.

11

1.

FLOWSHEETS (continued)

12

2.

PILOT INFLUENT FLOW VARIATION BASED ON FULL-SCALE INFLUENT FLOW HYDROGRAPH.

19

CBOD S CON~ENTRATION IN THE FINAL AERATED TANK OF THE A /0 PILOT PLANT.

27

NITRATE+NITRITE AND AMMONIA IN THE FINAL AERATED TANK OF THE A2/0 PILOT PLANT.

29

ORTHOPHOSPHORUS CONCEN~TION IN THE FINAL AERATED TANK OF THE A /0 PILOT PLANT.

30

PILOT INFLUENT CBOD5 vs ORTHOPHOSPHORUS CgNCENTRATION IN THE FINAL AERATED TANK OF THE A /0 PILOT PLANT.

31

FOOD:MICROORGANISM RATIO vs ORTHOPHOSPHORUS CgNCENTRATION IN THE FINAL AERATED TANK OF THE A /0 PILOT PLANT.

32

CBOD S CONCENTRATION IN THE FINAL AERATED TANK OF THE UNC PILOT PLANT.

39

NITRATE-NITRITE AND AMMONIA CONCENTRATION IN THE FINAL AERATED TANK OF THE UNC PILOT PLANT.

40

ORTHOPHOSPHORUS CONCENTRATION IN THE FINAL AERATED TANK OF THE UNC PILOT PLANT.

42

PILOT CBOD s LOADING vs. ORTHOPHOSPHORUS CONCENTRATION IN UNC PILOT FINAL AERATED TANK.

43

CBOD S LOADING:MICROORGANISM RATIO vs ORTHOPHOSPHORUS CONCENTRATION IN THE FINAL AERATED TANK.

44

1.

3•

4.

5. 6.

7.

8. 9.

10.

11. 12.

13. 14. 15.

U~C

PILOT

CBOD S CONCENTRATION IN THE FINAL AERATED TANK OF THE MB~R PILOT PLANT.

50

NITRATE-NITRITE AND AMMONIA CONCENTRATION IN THE FINAL AERATED TANK OF THE MBPR PILOT PLANT.

51

ORTHOPHOSPHORUS CONCENTRATION IN THE FINAL AERATED TANK OF THE MBPR PILOT PLANT.

52

viii

16. 17.

18. 19. 20.

21. 22.

23.

PILOT CBOD S LOADING vs. ORTHOPHOSPHORUS CONCENTRATION IN MBPR PILOT FINAL AERATED TANK.

55

CBOD LOADING:MICROORGANISM RATIO vs ORTBoPHOSPHORUS CONCENTRATION IN THE MBPR PILOT FINAL AERATED TANK.

57

CBOD S CONCENTRATION IN THE FINAL AERATED TANK OF THE RALEX PILOT PLANT.

60

NITRATE-NITRITE AND AMMONIA CONCENTRATION IN THE FINAL AERATED TANK OF THE RALEX PILOT PLANT.

.62

ORTHOPHOSPHORUS CONCENTRATION IN THE FINAL AERATED TANK OF THE RALEX PILOT PLANT.

64

PILOT CBOD S LOADING vs. ORTHOPHOSPHORUS CONCENTRATION IN RALEX PILOT FINAL AERATED TANK.

65

CBOD S LOADING:MICROORGANISM RAT~O vs ORTHOPHOSPHORUS CONCENTRATION IN THE RALEX PILOT FINAL AERATED TANK.

66

MODIFIED FLOWSHEETS FOR DECEMBER RALEIGH NEUSE RIVER BPR PILOT STUDIES.

58

ix

LIST OF TABLES

1.

22

2.

CHARACTERISTICS OF PILOT PLANT INFLUENT (NEUSE RIVER PLANT PRIMARY EFFLUENT) SUMMARY OF A2 /0 PROCESS DATA AVERAGED BY MONTH

3.

SUMMARY OF UNC PROCESS DATA AVERAGED BY MONTH

37

4.

SUMMARY OF MBPR PROCESS DATA AVERAGED BY MONTH

48

5.

SUMMARY OF RALEX PROCESS DATA AVERAGED BY MONTH

59

6.

CHARACTERISTICS OF PILOT PLANT INFLUENT (NEUSE RIVER PLANT PRIMARY EFFLUENT)

70

SUMMARY: OF UNC PROCESS DATA AVERAGED FOR DECEMBER

72

7. 8. 9. 10.

26

-SUMMARY OF RALEX PROCESS DATA AVERAGED FOR DECEMBER 74 CONCENTRATION (micromolar) OF SELECTED VOLATILE ACIDS IN FERMENTED PRI¥JffiY SLUDGE

79

RATIO OF THE CONCENTRATION OF SELECTED VOLATILE ACIDS TO TF~T OF ISOBUTYRATE IN FERMENTED PRIMARY SLUDGE FROM TWO TREATMENT PLANTS

80

x

PILOT PLANT INVESTIGATIONS OF VARIOUS PROCESSES TO ACCOMPLISH BIOLOGICAL PHOSPHORUS REMOVAL FROM WASTEWATER AT RALEIGH'S NEUSE RIVER WASTEWATER TREATMENT PLANT

I.

INTRODUCTION

A two year study, to examine a variety of methods for reducing the concentrations of nitrogen and phosphorus in effluents from municipal wastewater treatment plants, was initiated by the North Carolina Urban water Consortium in July 1985. This project was administered by the Water Resources Research Institute of The UniversIty of North carolina, and implemented through the Department of Environmental Sciences and Engineering at the University of North Carolina at Chapel Hill. Partic1pants in this investigation included the cities of Durham and Burlington and the Orange Water and Sewer Authority (OWASA) which serves Chapel Hill and Carrboro. The original project was divided into three phases, and reports on these studies should be available from the water Resources Research Institute of The University of North Carolina. Upon completion of the original project, the City of Raleigh provided funding to continue the project for pilot studies at the Neuse River Wastewater Treatment Plant. Additional funds were made available through the North Carolina Urban Water Consortium. The Neuse River facility was recently upgraded from 30 to 40 MGD capacity. In conjunction with the" increase in capacity, improvements were made to the'on-site process facilities. Included was the addition of grit removal, primary clarification, and an open channel system to handle mixed liquor and return activated sludge flows between the aeration basins and the secondary clarifiers. A second phase of the 'upgrade improved aeration capacity with centralized blowers. A fine bubble diffusion distribution system eliminated the high speed surface aerators in four of the six activated sludge basins. Coarse bubble diffusion was added to the aerobic digesters. A new,

computer assisted, process control system has resulted in improved treatment and energy cost savings. CUrrently, three of six activated sludge aeration basins are in service to accommodate the present plant loading. The plant is operated as an extended aeration facility with an average hydraulic detention time in each of these basins of 18-20 hours. The sludge age is maintained at about 15 days. Detention time in the mixed liquor/return sludge channels is several hours. The channels are mixed by aeration. Secondary effluent undergoes filtration and chlorination prior to discharge into the Neuse River. Waste activated sludge is routed to the aerated digesters. Additional solids wasting from the plant is achieved by pumping mixed. liquor from the aeration basins to the head of the primary clarifier as heeded. This flow averages between 400 and 800 gallons/minut~ daily and has been as high as 1.5 MGD. Following aerobic stabilization and dewatering, the waste sludge is applied to approximately 800 acres of City-owned and leased land as soil conditioner. The Neuse River treatment facility discharges directly into the Neuse River. This watershed recently has been designated as "nutrient sensitive" by the North Carolina Environmental Management Commission. This designation will allow the plant to discharge no more than 2.0 mg/L of Total Phosphorus based on a quarterly mean of once per week composite samples. As a result, the study design for this project placed priority on controlling this nutrient to levels necessary to meet these discharge requirements. Recognizing that nitrogen is also an important nutrient, its reduction in the treatment plant was also considered in the experimental design. The project proposal for investigations in Raleigh called for the operation of several pilot units to evaluate biological phosphorus removal as a means to achieve the effluent phosphorus permit limit. Economically, the recent upgrades to the Neuse River plant mandated consideration of the existing facilities in the design of biological phosphorus removal systems. Four pilot systems were

Page 2

designed to evaluate biological phosphorus removal and were operated on-site from July 20, 1987 to November 16, 1987. Additional pilot studies were conducted during December as follow-up to the original study. This progress report will focus on the pilot investigations. II.

PROJECT OBJECTIVES AND INVESTIGATIONAL APPROACH

The overall objective of this project was to provide the city of Raleigh with information and recommendations on site-specific strategies for effluent nutrient control. Specific objectives for the pilot scale investigations were: 1. To design and construct continuous flow pilot plants suitable for evaluating multiple biological phosphorus removal (BPR) processes. 2. To operate the pilot plants on-site to evaluate the potential and applicabllity of simultaneous BPR and nitrogen control at the Neuse River plant. 3. To formulate site-specific recommendations for nutrient control based on the pilot investigation results. 4. To evaluate the need for possible back-up systems to insure regulatory compliance. 5.

To identify areas for

fu~her

investigation.

This project was not designed to include basic research on the mechanisms of phosphorus and nitrogen control. III. BACXGROUND ON BIOLOGICAL PHOSPHORUS REMOVAL Biological phosphorus removal (BPR) has been widely discussed in the technical literature, and has been a source of controversy in many respects. Initial interest in this subject was regarded by some as scientific curiosity, with little chance of practical application in the wastewater treatment field. After several years, field

Page 3

data began to accumulate indicating that sUbstantial removal of phosphorus actually could be obtained in some full-scale activated sludge plants, triggered by conditions that were not fully understood. The increased interest in BPR during the past decade has resulted in part from greater regulatory pressure for removal of nutrients, especially phosphorus, and partly because of enhancement of knowledge about BPR through research and BPR plant operations in South Africa and this country. A review of the state-of-the-art concluded that BPR often is capable of producing excellent phosphorus removal, but that existing knowledge was not adequate to insure consistent production of effluent concentrations of 1 mg/L or less (Lamb, ~984). The literature" review also revealed that where uncertainties about BPR design and operation can be resolved, this method for removing phosp?orus offers certain advantages over chemical precipitation. First, BPR can minimize or eliminate ~~e need for purchasing chemicals and controlling their feed to the treatment system. Second, and perhaps of even greater importance, BPR could avoid the major increases in sludge quantities and undesirable c~anges in quality associated with uses of large amounts of alum or other precipitating agents. " Third, in plants where design engineers have had the foresight to provide adeqUate flexibility, th~ capital cost for initiating BPR can b~ minor, as contrasted with additional costs for chemical storage and feeding equipment and increased sludge handling expenses when che~ical treat~ent is employed. Finally, implementation 'of BPR in most activated sludge plants is entirely compatible with simultaneous removal of nitrogen through biological nitrification and denitrification, which may be anticipated as a future requirement in many plants. All biological processes are capable of.removing some phosphorus from wastewaters by incorporating that essential nutrient into cell materials, which subsequently are removed from the system as waste sludge. The amount of phosphorus that can be removed in this fashion is

Page 4

determined by the amount of waste sludge produced and the phosphorus content of that sludge. In conventional activated sludge plants treating municipal wastewaters, phosphorus removal by biological action usually is limited to, perhaps, 10-30% of that in the influent to the plant (Anonymous, 1983; page 150). A BPR system is a modified activated sludge process in which environmental conditions are adjusted to favor growth of organisms in the sludge that are capable of concentrating phosphorus in their cells to levels much higher than in organisms normally populating that type of syste~. The removal and separate disposal of waste sludge that is enriched in phosphorus content enhances removal of phosphorus from the wastewater.· .Flow sheets for various BPR processes have been presented and explained in the report identified earlier (Lamb, 1984). All BP~ processes are based .on exposing the activated sludge alternately to anaerobic and aerobic' conditions to "select" for the proper organisms in the system. A simplified interpretation of the mechanisms involved, supported to some degree by past research, is presented in the following paragraphs. The organisms associated with enhanced biological phosphorus removal from wastewater are capable, under anaerobic conditions, of breaking down complex intracellular polyphosphates to meet their energy and metabolic requirements. This activity includes the storage of si::nple organics inside the cell and results in the release of orthophosphates from the cell. Accordingly, the orthophosphate concen~ration in the mixed liquor increases during passage through the anaerobic phase. This ability to function effectively under anaerobic conditions represents an advantage in competing for food supply to the BPR organisms. Subsequently, under aerobic conditions, those same organisms have the capability to store more phosphorus than they need for normal metabolism and building of cell material. In doing so, they take up the phosphorus that had been released during the anaerobic phase, as well as

Page 5

most or all of that in the incoming wastewater. The excess phosphorus is stored within the organisms as complex polyphosphates, in the form of volutin granules. There are some important aspects of the process that should be recognized in designing and operating a BPR system. Extensive phosphorus release occurs only under environmental conditions that favor development of a population that is capable of storing excess phosphorus as polyphosphates and subsequently using it in anaerobic metabolism. That type of population is capable of removing most or all of the dissolved phosphorus from the wastewater under aerobic conditions by storing it within cells. The operating parameter that has received most recognition is the need for maintaining absence of bo~h dissolved oxygen and nitrates in the anaerobic phase. Apparently, as long as oxygen is available to the organisms in e{ther of those forms, the utilization of polyphosphate as an energy source and anaerobic release of orthophosphate by the cells does not occur, and BPR" efficiency decreases. The anaerobic release of phosphorus indicates presence of the desired organisms, as well as environmental conditions favorable to their metabolism. Completely anaerobic conditions may be achieved by endogenous respiration, or as the result of oxidation of organic loading, or a combination of both means. The co~ventional approach to the successful operation of the BPR process requires that organic loading (BOD) in the system must be high enough to utilize the available dissolved oxygen and nitrates. This has led some to conclude that a key parameter in determining BPR efficiency is the BOD loading, sometimes expressed as the BOD/P ratio. BOD loading clearly is important in generating anaerobic conditions, as well as in determining the amount of sludge production, ~hich must bear some relation to the amount of phosphorus that can be removed through sludge:wasting. However, it does not necessarily follow that the relationship of BOD to phosphorus removal can be expressed simply as the BOD/P ratio. To the contrary, data collected in this and other investigations suggest that efficient removal of phosphorus can occur across a range of BOD/P

Page 6

ratios, as long as adequate anaerobic conditions are maintained. Another important factor that has been observed in these studies, as well as by other investigators, is the role of certain short chain organics in enhancing phosphorus removal. The presence of acetate in the anaerobic phase has been identified most often as beneficial to BPR efficiency (Gerber et al. , 1986; Tracy and Flammino, 1985). The exact mechanism is not yet clear, but it is reasonable to view acetate as a factor that favors development of the desired population, probably through its role in metabolic processes of the organisms. Short chain volatile acids can be incorporated into the BPR cells under anaerobic conditions at little or no energy cost (Communication with Dr. Ken Tracy, Air Products and Chemicals, Inc.; Charleston, South Carolina). It should be noted that acetates and other fermentation products can be formed during the anaerobic phase of a BPR process. Accordingly, production of acetate and other fe~entation products can provide another reasonable hypothesis in attempting to explain the importance of anaerobiosis in the BPR process. Successful operation of the process also depends upon maintenance of dissolved oxygen in the aerobic phase of the process. Only under aerobic conditions can the uptake of phosphorus be rapid and complete. That, of course, is necessary because the basic mechanism of removal is through the incorporation of excess phosphorus into"the cells, which subequently are removed from the system for separate disposal. Clearly, there must be a limit to the extent to which phosphorus may be accumulated in cells. If efficient removal is to be maintained, sludge wasting from the system must be adeqUate to remove the phosphorus extracted from the wastewater. Also, the BOD loading and mode of plant operation must result in production of enough sludge to incorporate the phosphorus removed. This suggests that removal of phosphorus may be favored to some extent by high loadings and short sludge ages that produce more waste

Page 7

sludge. It also suggests that high BOD/P ratios might be helpful. It has been noted that very high concentrations of phosphorus (sometimes in excess of 10% of dry solids) can be attained in the sludge from some BPR plants. However, in many systems described in the literature, phosphorus concentrations in the waste sludge do not approach the maximum attainable levels. Finally, the level of phosphorus that can be attained in solids depends on the proportion of organisms in the population that can store excess phosphorus. This indicates that any process modifications that enhance "selection" of the desired organisms could also increase the potential fo~ phos-phorus removal in. the system, if solids production 'is th~ limiting factor.Surprisingly few details .are available about effects of the variables outlined abovei and others as well, on BPR performance. Accordingly, many of the BPR plants that have been placed in operation have been disappointing in performance and the state of knowledge that is available for correcting their deficiencies generally is unsatisfactory. In many instances, persistent failures can be attributed to plant designs that have not used desirable flow sheets for treating the wastewater in question and that also have neglected to include enough flexibility to pe~it meaningful changes in operation after start up. occasional failures of the BPRprocess might be expected, and suggest that a backup to BPR should be considered. If.the BPR process·was consistent in removing phosphorus toa concentration just above or below the discharge pe~it limit, chemical dosage calculations and metering would be a relatively routine task. Isolated spikes of high phosphorus concentration in the effluent could be ign9red, assuming that the effluent concentration, averaged over time, would be low enough to meet the permit requirements. Questions concerning when to use the chemical, the point of addition, and its impact on other processes, sludge production and handling would still remain. However, with inconsistent BPR performance, chemical addition may become a complex issue. Operators,

Page 8

trying to achieve economical operation, may not be able to adjust the chemical dosage to match the phosphorus concentration. They are then faced with using enough chemical to insure compliance with the discharge permit limit on a continuous basis, negating many of the advantages of BPR. Therefore, to optimize the benefits of operating a BPR process, the inclusion of a chemical precipitation backup should be linked to the selection of the best BPR technology for the site in question. IV •

INVESTIGATIONAL METHODS

Several BPR systems have been described in the literature (Lamb, 1984), and some are patented in the u.s. Some information on full-scale performance of these systems is available, but performance varies from site'to site and over time. Little is known about key BPR plant design variables and methods for predicting their optimum ranges. Therefore, it. was decided that the most productive way to evaluate the BPR potential for the wastewaters in question would be to follow an empirical approach. These pilot studies were designed to simultaneously compare systems that might operate successfully at the Raleigh municipal treatment plant. simUltaneous operation of the pilot units eliminated some variables that could invalidate the comparison of pilot studies conducted at different times. For example, changes in wastewater characteristics, te~perature variations, power outages, differencei in operator techniques, and other factors' would'apply equally to all plants, allowing direct comparisons. Also, simultan~ous collection and analyses of samples eliminated some of 'the variables associated with these activities. This experimental approach is highly appropriate for evaluating the relative desirability of the various processes. A. Selection of Processes The choice of which pilot processes were operated at the Neuse River treatment plant was based on considerations of their potential for success. The systems piloted were

Page 9

actually modifications of conventional activated sludge processes. Modifications to the physical layout and operation of the full-scale plant would be required to incorporate any of these BPR designs. The efficacy of each flow sheet was judged by evaluating the efficiency of phosphorus removal and consistency of treatment, including the removal of BOD, ammonia, and other parameters specified in the NPDES discharge permit for the treatment plant. Initial pilot system flow sheets are presented in Figure 1. The four systems evaluated were the Air Products and Chemicals, Inc. "A 2 jO" process; the University of North Carolina nUNC" process; a Modified Biological Phosphorus Removal "MBPR" process; and an experimental process named "RALEX". ·The reasons· f·or their selection. and the modifications that wer~ made during the course of the investigation will be explained in the discussion of each pilot unit. It should be pointed out that in all of the flow sheets discussed, the term "Aerobic" refers to a tank in which the contents were aerated sufficiently to maintain dissolved oxygen in the mixed liquor at all times. "Anoxic" is used to describe a tank that was mixed but not aerated. Its contents typically would be free of dissolved oxygen but not necessarily free of nitrates. "Anaerobic" tanks were mixed and operated to contain neither dissolved oxygen nor nitrates. B. Exnerimental Facilities .The City of Durham provided a trailer large enough to house five lab-scale pilot units. The trailer was located on-site at the· wastewater treat~ent plant to permit operation of the pilot systems using real wastewater. Equipment installed in the trailer for this project included air'compressors with an air distribution manifold, a heat pump, lights, timers, ventilation and drainage systems, and built-in counters to support the pilot units. The trailer operated as a self-contained unit with connections for the wastewater flow to the pilot units, tap water and electricity.

Page 10

RETURN SLUDGE DENITRIFICATION RECYCLE

lUiIt~ It~ i -~ i -~ ~ -11 t~ I t~ 1t~ !

PRIMARY EFFLUENT MINUTES

30

30

30

30

30

60

60

60

A. AIR PRODUCTS AND CHEMICALS. INC.

J

60

A2/0

tU

AI

l!l CD

PRIMARY EFFLUENT

t-' t-'

RETURN SLUDGE

fERMENTED

~

SLUDGE

MINUTES

120

120

30

90

90

60

90

60

8. UNIVERSITY OF NORTH CAROLINA, UNC

FIGURE 1. FLOWSJlEETS FOR RALEIGH NEUSE RIVER BPR PILOT STUDIES.

RETURN SLUDGE

PRIMARY

u

~

EFFLUENT MINUTES

60

60

60

90

90

90

90

90

90

c. MODIFIED 8IOLOGICAL PHOSPHORUS REMOVAL

90

90

M8PR

tU PI

lQ (1)

~

tv

PRIMARY EFFLUENT RETURN SLUDGE

FERMENTED SLUDGE

~

u .....

. MINUTES

u .....

u .....

m ~ ld -< ~

ffi-