Dissolved Methane Recovery and Trace Contaminant Fate Following Mainstream Anaerobic Treatment of Municipal Wastewater.

Anaerobic treatment of municipal wastewater with the staged anaerobic fluidized bed membrane bioreactor (SAF-MBR) shows promise to transform secondary wastewater treatment into an energy-positive process. However, the dissolved methane in SAF-MBR effluent needs to be recovered to reach net energy positive. To recover this methane for energy generation, an air stripping system was constructed downstream of a pilot-scale SAF-MBR facility and operated for over 80 days. The process removed 98% of effluent dissolved methane, and with the addition of intermittent disinfection recovered an average of 90% of the dissolved methane. The exit gas from air-stripping comprised 1.5-2.5% methane and could be utilized by blending with biogas produced from primary solids digestion and the SAF-MBR in an on-site combustion process. The direct energy costs for air stripping methane are <1% of the energy recoverable from the dissolved methane, not accounting for siloxane or sulfide scrubbing. Only siloxanes were observed at levels impacting combustion in this study, with 1.6 mg Si/m3 present in the blended biogas and air stripping mixture. The fate of a subset of trace organic contaminants was examined across the air stripping unit to check for aerobic degradation by methanotrophs or other opportunistic aerobes. Only 1,4-dioxane and benzotriazole showed statistically significant removal among 17 compounds screened, with 0.53 ± 0.13 and 0.34 ± 0.15 fraction removal, respectively. Our results indicate that air stripping is an energy efficient and robust technology for dissolved methane removal and onsite utilization for heat and electricity generation from anaerobic treatment of municipal wastewater.

Recovery of Clean Water and Ammonia from Domestic Wastewater: Impacts on Embodied Energy and Greenhouse Gas Emissions.

Treatment of domestic wastewater can recover valuable resources, including clean water, energy, and ammonia. Important metrics for these systems are greenhouse gas (GHG) emissions and embodied energy, both of which are location- and technology-dependent. Here, we determine the embodied energy and GHG emissions resulting from a conventional process train, and we compare them to a nonconventional process train. The conventional train assumes freshwater conveyance from a pristine source that requires energy for pumping (US average of 0.29 kWh/m3), aerobic secondary treatment with N removal as N2, and Haber-Bosch synthesis of ammonia. Overall, we find that this process train has an embodied energy of 1.02 kWh/m3 and a GHG emission of 0.77 kg-CO2eq/m3. We compare these metrics to those of a nonconventional process train that features anaerobic secondary treatment technology followed by further purification of the effluent by reverse osmosis and air stripping for ammonia recovery. This "short-cut" process train reduces embodied energy to 0.88 kWh/m3 and GHG emissions to 0.42 kg-CO2eq/m3, while offsetting demand for ammonia from the Haber-Bosch process and decreasing reliance upon water transported over long distances. Finally, to assess the potential impacts of nonconventional nitrogen removal technology, we compared the embodied energy and GHG emissions resulting from partial nitritation/anammox coupled to anaerobic secondary treatment. The resulting process train enabled a lower embodied energy but increased GHG emissions, largely due to emissions of N2O, a potent greenhouse gas.

Temperate climate energy-positive anaerobic secondary treatment of domestic wastewater at pilot-scale.

Conventional aerobic secondary treatment of domestic wastewater is energy intensive. Here we report net energy positive operation of a pilot-scale anaerobic secondary treatment system in a temperate climate, with low levels of volatile solids for disposal (< 0.15 mgVSS/mgCODremoved) and hydraulic residence times as low as 5.3 h. This was accomplished with a second-generation staged anaerobic fluidized membrane bioreactor (SAF-MBR 2.0) consisting of a first-stage anaerobic fluidized bed reactor (AFBR) followed by a second-stage gas-sparged anaerobic membrane bioreactor (AnMBR). In stage 1, fluidized granular activated carbon (GAC) particles harbor methanogenic communities that convert soluble biodegradable COD into methane; in stage 2, submerged membranes produce system effluent (permeate) and retain particulate COD that can be hydrolyzed and/or recycled back to stage 1 for conversion to methane. An energy balance on SAF-MBR 2.0 (excluding energy from anaerobic digestion of primary suspended solids) indicated net energy positive operation (+ 0.11 kWh/m3), with energy recovery from produced methane (0.39 kWh electricity/m3 + 0.64 kWh heat/m3) exceeding energy consumption due to GAC fluidization (0.07 kWh electricity/m3) and gas sparging (0.20 kWh electricity/m3 at an optimal flux of 12.2 L/m2 h). Two factors dominated the operating expenses: energy requirements and recovery cleaning frequency; these factors were in turn affected by flux conditions, membrane fouling rate, and temperature. For optimization of expenses, the frequency of low-cost maintenance cleanings was adjusted to minimize recovery cleanings while maintaining optimal flux with low energy costs. An issue still to be resolved is the occurrence of ultrafine COD in membrane permeate that accounted for much of the total effluent COD.

A comparative pilot-scale evaluation of gas-sparged and granular activated carbon-fluidized anaerobic membrane bioreactors for domestic wastewater treatment.

Two significantly different pilot-scale AnMBRs were used to treat screened domestic wastewater for over one year. Both systems similarly reduced BOD5 and COD by 86-90% within a 13-32 °C temperature range and at comparable COD loading rates of 1.3-1.4 kg-COD m-3 d-1 and membrane fluxes of 7.6-7.9 L m-2 h-1 (LMH). However, the GAC-fluidized AnMBR achieved these results at a 65% shorter hydraulic retention time than the gas-sparged AnMBR. The gas-sparged AnMBR was able to operate at a similar operating permeability with greater reactor concentrations of suspended solids and colloidal organics than the GAC-fluidized AnMBR. Also, the membranes were damaged more in the GAC-fluidized system. To better capture the relative advantages of each system a hybrid AnMBR comprised of a GAC-fluidized bioreactor connected to a separate gas-sparged ultrafiltration membrane system is proposed. This will likely be more effective, efficient, robust, resilient, and cost-effective.

Effects of sodium hypochlorite concentration on the methanogenic activity in an anaerobic fluidized membrane bioreactor.

The effect of membrane cleaning chemical, NaOCl on specific acetoclastic methanogenic activity (SAMA) of biomass in the anaerobic fluidized membrane bioreactors (AFMBRs) was assessed. Granular activated carbon (GAC) was used as a fluidizing media in the AFMBR to provide membrane scouring effect and surface for biofilm attachment. Effect of NaOCl on methane production was negligible for GAC with biofilm (bGAC) samples up to 150 mg NaOCl/g VSS, but was significant for the bulk liquid samples with noticeable lag period over 300 h even at the dosage of 50 mg NaOCl/g VSS. The toxicity of NaOCl on methane production was reduced in samples with virgin GAC (vGAC) by allowing 14 days of biomass buildup period prior to NaOCl addition, although the vGAC alone did not alleviate the toxicity. The results revealed that NaOCl concentrations beyond 100 mg/L within the reactor inhibited methanogenic activity and the effects were more pronounced on suspended biomass than the immobilized biomass on GAC.

Current status of the pilot-scale anaerobic membrane bioreactor treatments of domestic wastewaters: A critical review.

This review presented the performances of the pilot-scale anaerobic membrane bioreactors (AnMBRs) treating domestic wastewater. High COD removal efficiencies and low biosolids productions were achieved at HRTs comparable to conventional aerobic processes under ambient temperatures. The energy demands for fouling control in the pilot-scale AnMBRs ranged from 0.04 to 1.35kWh/m3, which is lower than those of lab-scale AnMBRs and aerobic MBRs. The energy demands for fouling control were in the order of gas sparging>particle sparging>rotating membrane AnMBR. Two major factors affecting the energy demand in gas sparging AnMBRs were specific gas demands (SGDm) and operating flux. The energy potentials in wastewater were significantly affected by the influent sulfate concentrations. Energy balances indicated that five out of nine pilot-scale AnMBRs was energy positive. However, further improvements of the AnMBRs are required to implement the energy positive wastewater treatment process.

Low energy single-staged anaerobic fluidized bed ceramic membrane bioreactor (AFCMBR) for wastewater treatment.

An aluminum dioxide (Al2O3) ceramic membrane was used in a single-stage anaerobic fluidized bed ceramic membrane bioreactor (AFCMBR) for low-strength wastewater treatment. The AFCMBR was operated continuously for 395days at 25°C using a synthetic wastewater having a chemical oxygen demand (COD) averaging 260mg/L. A membrane net flux as high as 14.5-17L/m2h was achieved with only periodic maintenance cleaning, obtained by adding 25mg/L of sodium hypochlorite solution. No adverse effect of the maintenance cleaning on organic removal was observed. An average SCOD in the membrane permeate of 23mg/L was achieved with a 1h hydraulic retention time (HRT). Biosolids production averaged 0.014±0.007gVSS/gCOD removed. The estimated electrical energy required to operate the AFCMBR system was 0.039kWh/m3, which is only about 17% of the electrical energy that could be generated with the methane produced.

Development and application of a procedure for evaluating the long-term integrity of membranes for the anaerobic fluidized membrane bioreactor (AFMBR).

A bench-scale short-term test, developed to predict the long-term integrity of membranes with potential for use in anaerobic fluidized-bed membrane bioreactors, was used to evaluate several commercial hollow-fiber membranes. It was found that membrane performance varied widely, some membranes failing much more rapidly than others. Also found was that larger sizes of the fluidized media, in this case granular activated carbon (GAC), severely affected membrane structural integrity more than did smaller sizes, as did the method used for membrane attachment. Within the limits studied, the GAC packing ratio had only a minor impact. A decrease in membrane permeability that sometimes resulted during the testing and was caused by the deposition of fine GAC particles could be eliminated without membrane damage through simultaneous chemical cleaning and sonication. This new testing procedure should be useful for selecting membranes and reactor operating conditions to better ensure long-term operating performance of anaerobic fluidized-bed membrane bioreactors.

Anaerobic treatment of low-strength wastewater: a comparison between single and staged anaerobic fluidized bed membrane bioreactors.

Performance of a single anaerobic fluidized membrane bioreactor (AFMBR) was compared with that of a staged anaerobic fluidized membrane bioreactor system (SAF-MBR) that consisted of an anaerobic fluidized bed bioreactor (AFBR) followed by an AFMBR. Both systems were fed with an equal COD mixture (200mg/L) of acetate and propionate at 25°C. COD removals of 93-96% were obtained by both systems, independent of the hydraulic retention times (HRT) of 2-4h. Over more than 200d of continuous operation, trans-membrane pressure (TMP) in both systems was less than 0.2bar without significant membrane fouling as a result of the scouring of membrane surfaces by the moving granular activated carbon particles. Results of bulk liquid suspended solids, extracellular polymeric substances (EPS), and soluble microbial products (SMP) analyses also revealed no significant differences between the two systems, indicating the single AFMBR is an effective alternative to the SAF-MBR system.

Pilot-scale temperate-climate treatment of domestic wastewater with a staged anaerobic fluidized membrane bioreactor (SAF-MBR).

A pilot-scale staged anaerobic fluidized membrane bioreactor (SAF-MBR) was operated continuously for 485 days, without chemical cleaning of membranes, treating primary-settled domestic wastewater with wastewater temperature between 8 and 30°C and total hydraulic retention time (HRT) between 4.6 and 6.8h. Average chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals averaged 81% and 85%, respectively, during the first winter at 8-15°C before full acclimation had occurred. However, subsequently when fully acclimated, summer and winter COD removals of 94% and 90% and BOD5 removals of 98% and 90%, respectively, were obtained with average effluent COD never higher than 23 mg/L nor BOD5 higher than 9 mg/L. Operational energy requirement of 0.23 kW h/m(3) could be met with primary and secondary methane production, and could be reduced further through hydraulic change. Biosolids production in all seasons averaged 0.051 g volatile suspended solids per g COD removed.

Lower operational limits to volatile fatty acid degradation with dilute wastewaters in an anaerobic fluidized bed reactor.

A general concern that anaerobic treatment of dilute wastewaters is limited by the inability of methanogenic and related syntrophic organisms to reduce substrate concentrations adequately was evaluated using a 35 °C granular activated carbon-containing laboratory-scale fluidized bed reactor fed an acetate-propionate equal chemical oxygen demand (COD) mixture synthetic wastewater. Contrary to general expectations, effluent acetate and propionate concentrations remained near or below their detection limits of 0.4 mg COD/L with influent COD of 200mg/L, 17 min hydraulic retention time, and organic loading as high as 17 kg COD/m(3)d, or with influent COD values ranging from 45 to 2010 mg COD/L and organic loadings of 4.2-4.5 kg COD/m(3)d. The effluent acetate concentrations in these well-fed systems were at or much below reported threshold limits for starving non-fed cultures, suggesting that a better understanding of threshold values and factors affecting treatment efficiency with anaerobic treatment of dilute wastewaters is needed.

Effects of influent DO/COD ratio on the performance of an anaerobic fluidized bed reactor fed low-strength synthetic wastewater.

The effect of influent DO/COD (dissolved oxygen/chemical oxygen demand) ratio on the performance of an anaerobic fluidized bed reactor (AFBR) containing GAC was studied. A high influent DO concentration was found to have adverse impacts on organic removal efficiency, methane production, and effluent suspended solids (SS) concentration. These problems resulted with a DO/COD ratio of 0.12, but not at a lower ratio of 0.05. At first organic removal appeared satisfactory at the higher DO/COD ratio at a hydraulic retention time of 0.30 h, but soon a rapid growth of oxygen-consuming zoogloeal-like organisms resulted, eventually causing high effluent SS concentrations. The influent DO also had an inhibitory effect, resulting in a long recovery time for adequate methanogenic activity to return after influent DO removal began. With the growing interest in anaerobic treatment of low COD wastewaters, the increased possibility of similar adverse DO effects occurring needs consideration.

Anaerobic fluidized bed membrane bioreactor for wastewater treatment.

Anaerobic membrane bioreactors have potential for energy-efficient treatment of domestic and other wastewaters, membrane fouling being a major hurdle to application. It was found that fouling can be controlled if membranes are placed directly in contact with the granular activated carbon (GAC) in an anaerobic fluidized bed bioreactor (AFMBR) used here for post-treatment of effluent from another anaerobic reactor treating dilute wastewater. A 120-d continuous-feed evaluation was conducted using this two-stage anaerobic treatment system operated at 35 °C and fed a synthetic wastewater with chemical oxygen demand (COD) averaging 513 mg/L. The first-stage was a similar fluidized-bed bioreactor without membranes (AFBR), operated at 2.0-2.8 h hydraulic retention time (HRT), and was followed by the above AFMBR, operating at 2.2 h HRT. Successful membrane cleaning was practiced twice. After the second cleaning and membrane flux set at 10 L/m(2)/h, transmembrane pressure increased linearly from 0.075 to only 0.1 bar during the final 40 d of operation. COD removals were 88% and 87% in the respective reactors and 99% overall, with permeate COD of 7 ± 4 mg/L. Total energy required for fluidization for both reactors combined was 0.058 kWh/m(3), which could be satisfied by using only 30% of the gaseous methane energy produced. That of the AFMBR alone was 0.028 kWh/m(3), which is significantly less than reported for other submerged membrane bioreactors with gas sparging for fouling control.