Anaerobic Membrane Bioreactors for Sustainable Wastewater TreatmentFaculty Sponsors – Lutgarde Raskin, Ph.D., Professor of Environmental Engineering, Steve Skerlos, Ph.D., Associate Professor of Mechanical Engineering,Nancy Love, Ph.D., Professor of Civil and Environmental EngineeringGraduate Research Students – Adam Smith (project supervisor), Katrina Adams and Sherri CookUndergraduate Research Student – Heather Dorer

AbstractDomestic wastewater (DWW) treatment in industrialized countries typically involves using aerobic bio-logical processes, which are energy intensive and produce excessive residuals. This project is part of a collaborative e�ort to develop a more energy-e�cient method for treating DWW using a combination of anaerobic biological processes and membrane �ltration. Anaerobic treatment in comparison to aerobic treatment is less energy intensive because it does not require aeration and produces less re-siduals since the amount of biomass produced by anaerobic metabolism is much less. Anaerobic wastewater treatment relies on microorganisms to perform four major biological processes: hydrolysis, fermentation, acetogenesis, and methanogenesis, the latter of which produces methane, a valuable re-source. Combining anaerobic treatment with membrane �ltration has potential to meet stringent dis-charge limits while improving upon the sustainability of DWW treatment.

This research consists of operating and analyzing the performance of a bench-scale anaerobic mem-brane bioreactor (AnMBR). System performance is monitored through various assays such as sus-pended solids analysis, gas chromatography, chemical oxygen demand (COD), and alkalinity titrations. Data obtained from these experiments provides a quantitative way to view trends, troubleshoot as necessary, and identify correlations in the system. One speci�c chemical assay, COD, determines the oxygen demand of organic pollutants in a sample. Data obtained thus far from this assay suggests po-tential for meeting standards for secondary treatment with this system. Further research is warranted to assess the system’s ability to improve upon the sustainability of conventional DWW treatment. If this system proves to be e�ective, a pilot-scale AnMBR will be developed, the success of which will lead to implementation into a full-scale system.

ObjectivesEvaluate the performance of anaerobic membrane bioreactors to treat low strength wastewater. This in-cludes determining speci�c conditions for optimal performance, including the establishment of a mini-mum organic loading rate and operational temperature in addition to controlling membrane fouling. A model integrating the Anaerobic Digestion Model No. 1 and a membrane model will also be developed. Finally, process sustainability will be evaluated using a life cycle analysis (LCA).

MethodsThe methods used to monitor the status of the bench-scale AnMBR include chemical oxygen demand (COD) analysis, high performance liquid chromatography (HPLC), total suspended solids analysis, titra-tions, and gas chromatography.

Total Suspended Solids Analysis– Determines the mass of organic (volatile suspended solids) and inor-ganic solid matter in the mixed liquor of the bioreactor. • Run pure water through 47 mm diameter glass fiber filter, place in 55 mm aluminum tray and ignite for 20 minutes in 550°C muffle furnace. Weigh filter and tray • Filter known volume of mixed liquor and ignite filter (in tray) in a 104°C oven (removes moisture), and weigh. (Tray + Filter + Biomass) (g) - (Tray + Filter) (g) = Biomass (g) • Place filter (in tray) in 550°C oven (which will burn off the organic matter) and weigh. (Tray + Filter+ Inorganics) (g) - (Tray + Filter) (g) = Inorganics (g) (Biomass - Inorganics) (g) = Organics (g)

Titrations – Determines the alkalinity, volatile fatty acid concentration, and pH reactor samples. • Add .01 M HCl to a known volume of a liquid (specifically influent, mixed liquor, and perme- ate samples) to speci�c endpoints. Volume of liquid needed to reach endpoint is recorded and used in extensive calculations to determine the alkalinity, volatile fatty acid content, and pH of each sample. • Alkalinity: ability of a substance to neutralize an acid. • Volatile Fatty Acid Concentration: cumulative concentration of short-chain fatty acids (carbon chain of six carbons or fewer) that readily volatize at STP • pH= -log[H+], quanti�es hydrogen ion concentration

Gas Chromatography – Used to monitor the concentration of methane in the produced biogas using direct injet gas chromatograph equipped with thermal conductivity detector (quanti�es concentration of gases in a sample). • Make standards of 60 mL gas with 10%, 25%, 50% and 100% by volume. • Use syringe to inject 200 �L of each standard into the injection port of the gas chromatograph, which sends data to a computer, which uses a method to create a calibration curve. • Use syringe to inject 200 �L of a biogas sample into the injection port. The com- puter will compare the volume of gases in this sample to that of the standard samples using the calibration curve.

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ConclusionsThe overall functionality and success of the AnMBR can only be determined over long-term observa-tion and data collection. Due to the fact that this system has been in operation for slightly over six months, no conclusive statements can be made. However, preliminary results strongly suggest that the bench-scale AnMBR can meet typical EPA effluent discharge limits, the primary limits of concern being biochemical oxygen demand (after 5 days) and the total suspended solids content. Further re-search must be performed to determined if the AnMBR can treat low concentration waste water in a more energy e�cient manner than aerobic MBRs.

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ResultsOne of the primary discharge limits of concern is biochemical oxygen demand of the DWW after five days (BOD5), which is a measure of the amount of oxygen microorganisms in a given mass of solids will consume while decomposing the solids after �ve days (US standard). Chemi-cal oxygen demand (COD) is a measure of the amount of oxygen microorganisms will consume to decompose the solids after a [theoretically] infinite number of days. It is the goal of DWW treatment plants to have the BOD5 or COD output be as low as possible, as it is often discharged into streams, lakes, etc., and high BOD5 or COD levels would deprive plant and animal wildlife of oxygen. From Figure 2 (middle left), we can see that the influent has a relatively high COD, varying around 400-500 mg/L, and peaking at 700 mg/L. The permeate, however, has an aver-age COD of approximately 25 mg/L which indicates that the AnMBR is effectively treating the synthetic wastewater to drastically exceed COD discharge limits.

Another important discharge limit for DWW plants is the total suspended solids content of the treated waste. In Figure 3 (bottom left) we can see that the total suspended solids content of the mixed liquor is around 10,000-15,000 mg/L, while that of the permeate was either zero or negli-gible (which is why the suspended solids analysis was only performed on the permeate a few times, since this can be assumed to be zero or negligible), which indicates that the AnMBR is ef-fectively treating the synthetic wastewater to drastically exceed total suspended solids dis-charge limits. The primary means of suspended solids removal is with micro�ltration using membranes, which are thin, flat sheets of polyethersulfone that have a pore size of 0.2 µm. These prevent any particles larger than 0.2 µm from leaving the reactor.

The primary goal of this research is to e�ectively treat low-strength DWW (as anaerobic treat-ment is typically performed for high-strength wastewaters, where COD>1,000 mg/L) at 35 de-grees Celsius, or mesophilic temperatures, in a more energy e�cient manner than aerobic MBRs. This would take into account the energy needed to run the AnMBR, as well as the energy outpput of the AnMBR, which would come in the form of methane-rich biogas, which can read-ily be combusted for fuel. From Figure 4 (above), it can be observed that the AnMBR will typi-cally produce 0-20 mL/day of biogas, peaking at over 120 mL/day. The percent of the biogas that is methane varies from 35 to 60 percent, peaking at 70 percent methane, which is a signifi-cant percentage of methane for biogas. This information regarding biogas content, however, is not fully understood, as many factors a�ect this value, such as increased solubility of methane at lower temperatures.

Figure 2: Graph of COD (mg/L) values versus the days from the AnMBR startup. It can be observed that the in�uent COD and the bioreactor COD are much higher than that of the permeate.

Figure 3: Graph of TSS and VSS concentrations (mg/L) of the mixed liquor and permeate versus the days from the AnMBR startup. It can be observed that the total suspended solids content of the permeate is

zero or negligible, which is much lower than that of the mixed liquor.

Figure 4: Graph of the biogas production (mL/day) and methane content (%) versus the days from the AnMBR startup. Low methane content is an indication of a disruption in the AnMBR, such as a leak in the

system, which would allow oxygen to interfere with the ideally anaerobic conditions

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Figure 1: Image of the bioreactor.