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Interim Report of Testing of a Membrane Aerated Bioreactor at the Codiga Resource Recovery Center at Stanford – 12/21/2018 Principal Investigator: Prof. Craig Criddle Prepared by: Emir Aksuyek, Sebastien Tilmans 1. Introduction

A Membrane Aerated Biofilm Reactor (MABR) manufactured by Fluence Corporation (Fluence) was installed at Stanford’s Codiga Resource Recovery Center (CR2C) in January 2018. The system was tested with domestic wastewater diverted from Stanford’s sanitary sewer. Treatment performance was evaluated according to two benchmarks: California Title 22 standards for non-potable water reuse (T22) and a Total Nitrogen limit of 10 mg-N/L. T22 Standards are set forth by the state government. TN10 is a benchmark set by Fluence, to evaluate the Nitrogen removal performance of their MABR system. T22 does not regulate Nitrogen species for wastewater reuse. The system was operated over a 12-month period under various operational conditions, environmental conditions and wastewater characteristics. The treatment performance was evaluated for two steady state operation periods within the 12-month period and a period during which the loading of the system was gradually increased to identify loading limits. This interim report discusses observed treatment performance of the system and operational issues encountered during the two steady-state operational periods.

1.1. Codiga Resource Recovery Center (CR2C) CR2C is a research facility located on Stanford University’s campus dedicated to developing and testing of new wastewater treatment and resource recovery technologies. The facility diverts wastewater from Stanford’s sewer lines on Serra Street. The wastewater available at CR2C is collected from housing and some of the dining halls located on the campus. Wastewater from hospital buildings, laboratory buildings, and event structures such as the stadiums is not collected in this main. Thus, the wastewater can be characterized as domestic wastewater, although the concentrations of key wastewater constituents in this wastewater are higher than in typical US regular strength wastewater. A simple flow diagram for CR2C is given in Figure 1. Wastewater is diverted from the sewer main using chopper pumps and proceeds through three treatment processes intended to mimic full-scale pre-, primary, and secondary treatment. First, the water enters a grit removal tank for de-gritting the wastewater. Second, the wastewater flows to a Microscreen unit by gravity. The Microscreen has a belt screen with 300 µm porosity. Third, the wastewater is pumped to an anaerobic secondary treatment unit. Following each stage of treatment, a sidestream of water is diverted to the CR2C test bed area. The test bed area consists of four 8 ft by 20 ft test beds where new recovery technologies can be tested in a controlled environment. Each test bed can “plug and play” into any of the given water qualities, test their process, and discharge effluent to the sewer for downstream treatment at the regional water quality control plant. This configuration allows for controlled testing of processes with minimal risk to public health and the environment. The Fluence MABR unit is placed in one of the test beds described above.

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Figure 1: Simple flow diagram of CR2C

1.2. CR2C Wastewater Characteristics The wastewater available at CR2C presents unique challenges for test bed technologies. It can be classified as a high strength water (Metcalf and Eddy 2014). The causes of the high constituent concentrations in CR2C wastewater are unclear, but Stanford’s aggressive water conservation measures are likely to be an important causal factor. Another challenge with CR2C wastewater is that the ratio between different constituents is different from regular US domestic wastewater. CR2C wastewater is 1.4 times more concentrated in Ultimate Biochemical Oxygen Demand (BODu) whereas it is 3 times more concentrated Total Nitrogen (TN), yielding a lower BODu:TN ratio from what would be expected in regular strength wastewater. The water’s alkalinity is also lower than in typical wastewater. Stanford’s drinking water system is supplied by snowmelt from the Hetch Hetchy reservoir, a granite reservoir providing minimal alkalinity to the water. Finally, considerable fluctuations in wastewater characteristics occur at different times and seasons. CR2C experiences water quality fluctuations due to diurnal, seasonal and holiday changes. Due to the variability of the wastewater characteristics, mean wastewater characteristics are presented for each analysis period in subsequent sections. A range and mean concentrations of constituents observed over the whole duration of the operations is shown in Table 1. The BODu:TN ratio is used as a metric to evaluate carbon available for Nitrogen removal. For regular strength wastewater, this ratio is 8.4, whereas the mean BODu:TN ratio in CR2C wastewater was 4.03.

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Table 1: Range and mean influent wastewater concentration observed over the whole testing period

Parameter Unit CR2C Metcalf & Eddy COD

Range Mean Medium Strength

-Total COD mg/L 700-1600 1152 508 -Soluble COD mg/L 200-620 359 - BOD5 mg/L 115-530 281 200 BODu mg/L 168-776 4111 2931 TSS mg/L 180-1650 567 195 VSS mg/L 180-1450 527 152 pH - 7.05-8.65 8.06 - Alkalinity mg CaCO3/L 235-410 336 - Ammonia mg N/L 40-95 66 20 Nitrate mg N/L 0-1 0-1 0 Total Nitrogen mg N/L 75-130 102 35 Total Phosphorus mg P/L 4.4-10.5 8.4 5.6

1.3. California Title 22 (T22) The specific Title 22 target was to produce recycled water suitable for irrigation as stated in California Title 22 Division 4 Chapter 3 Article 3 Section 60304a. Such water must be a “disinfected tertiary recycled water”, defined by the following parameters:

• Filtered water with disinfection providing either a) chlorine CT of 450 milligram-minutes per liter with a modal contact time of at least 90 minutes, or b) removal of 99.999% of MS2 bacteriophage virus or polio virus.

• The median concentration of total coliform does not exceed 2.2 MPN/100mL over 7 analysis days, does not exceed 23 MPN/100mL in more than one sample over any 30-day period, and never exceeds 240 MPN/100mL

Coagulation is not necessary as part of a filtration process provided that filtered effluent turbidity is never above 2 NTU, filter influent turbidity is never greater than 5 NTU for a period of 15 minutes, and filter influent turbidity never exceeds 10 NTU. A deployed water recycling system would need to include a mechanism to add coagulant or divert wastewater if the filter influent exceeded 5 NTU for more than 15 minutes. If coagulation is used, the mean filtered effluent turbidity must be below 2 NTU over a 24 hour period.

1.4. The Unit The pilot scale MABR unit deployed at Stanford was custom built to comply with the specific physical constraints of test bed bays at CR2C. The unit, shown in Figure 2, contains all the components of a full treatment train, configured to fit within a 8 ft x 20 ft test bed bay. The process flow of the unit is shown in Figure 3. The unit influent is screened on the roof of the container in a 1 mm rotary screen. The screened wastewater flows by gravity into the 1st Stage MABR, followed by the 2nd Stage MABR and the clarifier. Each MABR tank contains a spiral wound gas-permeable membrane with a

1 BODu values calculated from BOD5 assuming a rate constant k=0.23

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surface area of 470 m2. Each tank has a volume of 3.3 m3. Coagulant is added between the 2nd Stage and the clarifier. The clarifier effluent is pumped to the top of the container for filtration and chlorination. The filtered and chlorinated tertiary effluent is stored in a 750L (200 gal) tank prior to discharge into the sewer.

Figure 2: 3D render of pilot scale Fluence MABR unit at CR2C

Figure 3: Process flow diagram of Fluence MABR unit at CR2C

2. Methods 2.1. Project Timeline

A timeline of completed activities and pending activities is shown in Table 2. This interim report provides preliminary results of several tests conducted to date. More detailed analysis of the completed tests and of the remaining tests will be provided in a final report in 2019.

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Table 2: Testing plan as of December 2018

Task Description

Task Duration Months

Months 2018 2019

2 4 6 8 10 12 2 4

Install MABR System 1

Initial testing and optimization 3-5

Steady State Operation 5-7

Limit Testing by Stanford (CR2C Water) 7-11

Report Writing and Decommissioning 12

2.2. “Influent+”- Supplementing CR2C wastewater with carbon and alkalinity Due to the low BODu:TN ratio and the low alkalinity of CR2C water, external carbon and alkalinity were added to the water after the rotary screen to raise the BODu:TN ratio and alkalinity levels to a range consistent with medium strength wastewater. This screened wastewater with carbon and alkalinity supplements is referred to as “Influent+” for the remainder of this report.

2.3. Sampling and Analysis The system was sampled 3 times per week on Mondays, Wednesdays and Thursdays. Typically 7 samples were collected daily. The samples and sampling locations on the process train are shown in Figure 4 and analyses done on each sample is summarized in Table 3. A full list of analyses, their measurement frequency and analysis methods is shown in Table 4.

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Figure 4: Influent supplement addition (blue arrow) and sampling locations (pink arrows) indicated on the process flow

diagram

Table 3: List of all analysis carried out for each sample

Analysis Influent Influent+ Stage 1 Stage 2 Secondary Effluent

Tertiary Effluent RAS

COD X X X X BOD X X X TSS/VSS X X X X X X TN X X Ammonia X X X X X Nitrate X X X X TP X X pH X X X X X Alkalinity X X X Coliform X Turbidity X X

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Table 4: List of lab analyses, frequency and methods used

Parameter Type Frequency Methods COD Total and

Soluble 3/week CHEMetrics kits

TSS/VSS - 3/week Filtration Ammonia Soluble 3/week Hach Kits Nitrate Soluble 3/week Hach Kits Total Nitrogen Total and

Soluble 3/week Hach Kits

Total Phosphorus

Soluble 3/week Hach Kits

pH - 3/week pH meter Alkalinity - 3/week Titration Turbidity - 3/week Turbidity meter BOD Total and

Soluble 1/week Standard Method

Coliform - 1/week Colilert Quanti-Tray

2.4. Wastewater flow properties The system hydraulic and organic loading varied throughout the study period according to different testing objectives. Flow and hydraulic loading throughout the study are shown in Figure 5. The hydraulic retention time (HRT) was calculated considering only the volume of the biological units (6 m3). During initial testing of the system, the system was tested with both grit tank effluent and microscreen effluent. Starting May 2018, the system was fed exclusively with grit tank effluent. System performance results are discussed for two periods: 1) Steady state operation at the conclusion of Fluence operating period, and 2) Steady state operation by Stanford at increased loading following a loading limit testing period.

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Figure 5: Hydraulic retention time in the biological treatment units and the flowrate of the system. Note- the reporting periods for

this report are highlighted in blue

2.5. Hydraulic and Organic Loading Limit Testing A loading schedule shown in Table 5 was implemented to increment the flow rate to the system, in an effort to identify maximum loading rates feasible for this system with CR2C wastewater. As a first step, flocculant addition to the system was eliminated to evaluate system performance without this chemical input.

Table 5: Loading schedule and loading rates based on volume of biological units and total membrane surface area

Week Flowrate HRT Volumetric Loading Membrane Area Loading # gpm m3/d hours kgBOD5/m3/d kgTN-N//m3/d kgNH3-N/m3/d gBOD5/m2/d gTN-N/m2/d gNH3-N/m2/d 0 1 5.5 29.9 0.23 0.09 0.06 1.64 0.60 0.39 1 1.25 6.8 23.9 0.29 0.11 0.07 2.03 0.74 0.48 2-3 1.56 8.5 19.1 0.36 0.13 0.09 2.54 0.92 0.60 4 1.95 10.7 15.3 0.46 0.17 0.11 3.20 1.16 0.75 5 2.44 13.3 12.2 0.57 0.21 0.13 3.98 1.44 0.93

One week was allotted between increments in loading with the objective of allowing the system to adjust, but this time allowance proved to be insufficient to allow the system to reveal changes in system performance. After rapid ramp-up to 7.4 lpm (1.96 gpm), the loading rate was held constant while operating parameters were adjusted in an effort to achieve target performance. Specific parameters that were varied included resumption of flocculant addition, mixing aeration frequency and duration. Results from this testing period will be discussed in the final report. Flowrate was subsequently decreased to 5.7 lpm (1.5 gpm) for 1 month and further to 1.3 gpm, where it was allowed to operate at steady state for 1 month.

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3. Results and Discussion 3.1. COD Removal

Data for influent and effluent total COD (TCOD) and soluble COD (sCOD) for the reporting period are shown in Figure 6 and summarized in Table 6. The system exhibited consistent COD removal regardless of operational changes, stressors, wastewater fluctuation, and changes in loading. The minimum COD removal rate was 89%.

Table 6: Influent and Effluent COD concentrations and overall removal efficiency

Unit Influent Effluent

TCOD SCOD TCOD SCOD Max mg/L 1600 620 55 1 Min mg/L 591 204 1 1 Mean mg/L 1060 347 41 31 Standard Deviation mg/L 244 71 17 11 n mg/L 113 114 109 103

Overall Removal Efficiency Range 89-99% Mean 95.80%

Figure 6: Influent and Effluent COD concentrations

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3.2. 1st Steady State (May-July 2018) The 1st Steady State operation period occurred from May 1 to July 31, 2018. The system was operated by a Fluence operator.

3.2.1. Steady State 1 Influent Wastewater Characteristics and supplementation Mean influent wastewater characteristics during the first steady state period are presented in Table 7.

Table 7: Influent characteristics during 1st Steady State Period

Parameter Unit Concentration Metcalf & Eddy COD

Medium Strength

-Total COD mg/L 1076 508 -Soluble COD mg/L 360 - BOD5 mg/L 2812 200 BODu mg/L 4113 2933 TSS mg/L 474 195 VSS mg/L 442 152 pH - 8.26 - Alkalinity mg CaCO3/L 346 - Ammonia mg N/L 68.3 20 Nitrate mg N/L 0-1 0 Total Nitrogen mg N/L 108 35 Total Phosphorus mg P/L 8.6 5.6

The BODu:TN ratio for CR2C wastewater in this period was 3.81. Carbon (glucose) and alkalinity were added to the system to achieve Influent + characteristics shown in Table 8.

Table 8: 1st Steady State Period external carbon and alkalinity added Influent+ concentrations

Parameter Unit Influent Influent+ ∆ Soluble COD mg/L 360 678 89% Soluble BODu mg/L 1962,3 337 72% Alkalinity mg CaCO3/L 346 597 72% pH - 8.26 8.96 +0.71

3.2.2. Steady State 1 System Performance: Nitrogen The system influent and effluent Nitrogen concentrations are shown in Figure 7 and summarized in Table 9. Mean removal rates of 91% and 97% were observed for Total Nitrogen and Ammonia respectively. Mean observed effluent TN concentration during this operating period was 10.2 mg N/L. During this operating period, mechanical

2 Data from the entire operational period was used, due to data availability and issues experienced with the method. 3 BODu values calculated from BOD5 assuming a rate constant k=0.23

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equipment interruptions were reported by Fluence which may have yielded process disruptions affecting system performance on June 6th and June 12th. If the performance disruptions following these mechanical issues are ignored, the mean TN and ammonia removal rates are 92% and 94% respectively. Despite several data points that exceeded TN10, the mean effluent TN concentration is 8.6 mg-N/L.

Figure 7: 1st Steady State Influent, Secondary Effluent, and Tertiary effluent Nitrogen concentrations. Note: Data points

shown in green follow mechanical disruptions and are not representative of typical performance.

Table 9: 1st Steady State Influent and effluent Nitrogen concentrations and overall removal efficiencies

Unit Influent Secondary Effluent Tertiary Effluent

TN NH3 NH3 TN NH3 Max mg N/L 125 79 29.3 28.8 11.1 Min mg N/L 82 49 0 2.2 0.00 Mean mg N/L 108 68 4.15 10.19 2.3 Standard Deviation mg N/L 12 9.05 6.64 7.61 3.55 n mg N/L 21 22 22 20 21

Overall Removal Efficiency

TN NH3 Range 73-98% 85-100% Average 91% 97%

Ammonia concentrations in the final effluent are lower than in the clarifier effluent. This observed reduction in ammonia concentrations may be the result of the formation of chloramines during the disinfection process. The ammonia kits used in this study may not

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measure the ammonia bound in all species of chloramines. Further investigation of this topic will be conducted for the final report.

3.2.3. Steady State 1 Effluent Nitrogen Species Concentrations of the key nitrogen species in the tertiary effluent are shown in Figure 8. In general, nitrate formed the majority of the Nitrogen species in the system final effluent. Nitrate concentrations in the second stage (clarifier influent), and in the secondary effluent (clarifier effluent) are shown in Figure 9. A mean reduction of 2.9 mg NO3-N/L was observed in the clarifier.

Figure 8: 1st Steady State tertiary effluent Nitrogen species Note: Data points shown in green follow mechanical

disruptions and are not representative of typical performance.

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Figure 9: 1st Steady State Period Nitrate removal between 2nd stage and clarifier Note: Data points shown in green follow

mechanical disruptions and are not representative of typical performance.

3.2.4. Nitrogen Loading Rates A major parameter of interest for the system operation is the Nitrogen Loading Rate (NLR) of the system. The loading rate is normalized by membrane surface area. The NLR of each individual reactor is based on the concentrations and flows of ammonia and if measured the TN entering each one. The mean NLR for the 1st Stage is 0.79 g NH3-N/m2/L and 1.25 g TN-N/m2/L. Because ammonia levels decrease after the 1st Stage, the mean NLR for the 2nd Stage is 0.12 g NH3-N/m2/L. The overall TN loading for the whole system, considering total membrane area in both stages, was 0.63 g TN-N/m2/L.

3.2.5. T22 Compliance Total Coliform indicator organisms were not detected throughout this steady state period. All duplicate measurements showed <1 MPN/100 mL of Total Coliform. Effluent turbidity results for the first steady state testing period are shown in Figure 10. Observed tertiary effluent turbidity remained below 2 NTU throughout this period. When Secondary and Tertiary Effluent Turbidity results were compared, it was observed that the average removal efficiency of the filter is 62% percent.

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Figure 10: 1st Steady State Period Tertiary Effluent Turbidity

3.3. 2nd Steady State 3.3.1. Influent Wastewater Characteristics Operations during the 2nd Steady State were managed by the CR2C team with the support of Fluence. The mean wastewater characteristics for this period are shown in Table 10. The mean BODu:TN ratio was 3.9, comparable to the 1st Steady State period.

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Table 10: 2nd Stable Period influent characteristics and comparison

Parameter Unit Concentration Metcalf & Eddy COD

1st Steady

State 2nd Steady

State Medium Strength

-Total COD mg/L 1076 1204 508 -Soluble COD mg/L 360 332 - BOD5 mg/L 2814 2815 200 BODu mg/L 411 411 2936 TSS mg/L 474 1146 195 VSS mg/L 442 1040 152 pH - 8.26 8.18 - Alkalinity mg CaCO3/L 346 336 - Ammonia mg N/L 68.3 65.5 20 Nitrate mg N/L 0-1 0-1 0 Total Nitrogen mg N/L 108 106 35 Total Phosphorus mg P/L 8.6 8.6 5.6

3.3.2. Influent+ Although preparation and dosing of chemicals to produce Influent+ water were consistent with the 1st Steady State period, Influent+ COD and BOD concentrations obtained for the 2nd Steady State Period did not coincide with the previous period. COD and BOD measurements were identified as outliers since a leak in the carbon and alkalinity dosing line was found in early December. The Influent+ concentrations are shown in Table 11.

Table 11: 2nd Stable Period external carbon and alkalinity added Influent+ concentrations

Parameter Unit Influent Influent+ ∆ Soluble COD mg/L 332 323 -3% Soluble BODu mg/L 1967 181 -8% Alkalinity mg CaCO3/L 336 393 17% pH - 8.18 8.46 0.28

3.3.3. Influent and Effluent Nitrogen The temporal variation of influent and effluent nitrogen concentrations is shown in Figure 11. A summary of nitrogen removal performance during this period is shown in Table 12. The mean nitrogen removal efficiencies were 93% and 96% of Total Nitrogen and Ammonia respectively. The mean effluent TN was 7.02 mg-N/L.

4 Data from the whole dataset was used, due to data availability and issues experienced with the method. 5 Data from the whole dataset was used, due to data availability and issues experienced with the method. 6 BOD5 is assumed to be 60% of 7 Data from the whole dataset was used, due to data availability and issues experienced with the method.

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Figure 11: 2nd Steady State Influent and Effluent Nitrogen concentrations

Table 12: 2nd Steady State Influent and Effluent Nitrogen concentrations and overall removal efficiencies

Unit Influent Effluent

TN NH3 TN NH3 Max mg N/L 124 89 12 5.20 Min mg N/L 78 45 3.10 0.00 Mean mg N/L 106 66 7.02 1.10 Standard Deviation mg N/L 15 12 3.56 1.82 n mg N/L 10 10 10 10

Overall Removal Efficiency

TN NH3 Range 87-97% 90-100% Average 93% 96%

3.3.4. Effluent Nitrogen Species Speciation of nitrogen in the tertiary effluent is shown in Error! Reference source not found.. As in the first steady state test, the majority of Nitrogen in the effluent in the form of Nitrate. Almost all ammonia remaining in the secondary effluent was converted to chloramines during disinfection. Nitrate concentrations before and after the clarifier are

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shown in Error! Reference source not found.. Nitrate removal rate in the clarifier is 0.3 mg NO3-N/L less and averages at 2.6 mg NO3-N/L removed in the clarifier.

Figure 12: 2nd Steady State effluent Nitrogen species

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Figure 13: 2nd Steady State Period Nitrate removal between the 2nd Stage and Clarifier

3.3.5. Nitrogen Loading Rates The NLR in the 1st Stage Ammonia Loading Rate is 0.99 g NH3-N/m2/L and 1.6 g TN-N/m2/L. The 2nd Stage Ammonia Loading Rate is 0.20 g NH3-N/m2/L. The overall TN loading for the whole system considering total membrane area in both stages, was 0.80 g TN-N/m2/L.

3.3.6. T22 Compliance Total Coliform indicator organisms were detected only once during this steady state operations, in one of two replicate samples. This single detection of 2 MPN/100 mL was below the 2.2 MPN/100mL limit set forth in the T22 standard. Tertiary effluent turbidity during the 2nd Steady State Period is shown in Figure 14. T22 standards were met apart from two observation points. The first may have been due to continuing system adjustment to the new system loading rate. During tests, the team has observed that the system can take approximately one week to fully stabilize after a change in loading. The second data point was an independent event from the first. Rising sludge was observed in the clarifier on that day and flocs of sludge were observed to be floating on the water surface of a routine Sludge Settling Velocity (SSV) test. Flocs of sludge were observed in the clarifier effluent trough, suggesting an increase of loading to the tertiary filter.

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Figure 14: 2nd Steady State Period Tertiary Eflluent Turbidity

3.4. Factors Influencing Nitrate Removal and Turbidity Unplanned denitrification occurring in the clarifier is likely to have contributed to rising sludge. On several occasions when higher concentrations of nitrate entered the clarifier and clarifier HRT was high, flocs of sludge were observed to rise to the surface of the clarifier. However, this phenomenon did not always impact effluent turbidity. Rising flocs observed in the clarifier did not yield higher observed turbidity values during the 1st Steady State period but seemed to be a key cause of high turbidity on one instance in November. Several environmental factors in the 2nd Steady State Period were different from the 1st Steady State Period. The ambient air temperature and wastewater temperature were lower compared to Spring and Summer values. These lower temperatures may have reduced microbial activity and increase oxygen solubility in the water, reducing denitrification effectiveness. It is possible that the system may achieve higher loading rates in the summer than were observed in the fall. Both due to increased flowrate and slightly different wastewater characteristics, Nitrogen loading rates were higher in the 2nd Steady State period, which yielded increased 1st Stage effluent Nitrogen concentrations. Therefore, the loading rate for the 2nd Stage was also higher. These increased loading rates to the second stage may have provoked increased nitrate loadings to the clarifier and contributed to rising sludge, impacting effluent turbidity.

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The results provided in this report were obtained from grab samples, which only represent a single point in the day. Diurnal fluctuations in influent wastewater quality have been observed. In the absence of composite samples, online probe data will be correlated with laboratory data in the final report to evaluate the diurnal patterns and provide estimates of continuous system performance.

4. Conclusion During the two steady state periods of operation, the system achieved the objectives of mean Total Nitrogen concentrations below 10mg/L and met T22 requirements as measured by Turbidity and Total Coliform in the Tertiary Effluent. This was achieved with a maximum flowrate of 1.3 gpm and an overall TN loading rate of 0.97 g TN-N/m2/L. Treatment capabilities of this unit may be increased by optimizing clarifier operation, optimizing carbon dosage to the system and improving the mixing system of the unit. The final testing phase will dilute CR2C wastewater to achieve influent characteristics more representative of US medium strength wastewater and explore the system’s performance under hydraulic loadings more aligned with the unit’s design specifications.

5. References Downing, L. S.; Nerenberg, R. Total nitrogen removal in a hybrid, membrane-aerated activated sludge process. Water Res. 2008, 42, 3697−3708. Metcalf and Eddy (2014), “Wastewater Engineering: Treatment and Reuse”, 5th Edition, the McGraw-Hill Companies, Inc. Satoh, H.; Ono, H.; Rulin, B.; Kamo, J.; Okabe, S.; Fukushi, K. I. Macroscale and microscale analyses of nitrification and denitrification in biofilms attached on membrane aerated biofilm reactors. Water Res. 2004, 38 (6), 1633–1641.