MAXIMIZING ENERGY RECOVERY FROM BREWERY WASTE STREAMS WITH ANAEROBIC MEMBRANE BIOREACTORS AND BIOGAS CONDITIONING. Abstract

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1 MAXIMIZING ENERGY RECOVERY FROM BREWERY WASTE STREAMS WITH ANAEROBIC MEMBRANE BIOREACTORS AND BIOGAS CONDITIONING Tim Rynders, CDM Smith Inc th St. Suite 1100, Denver, CO Phone: Abstract The New Belgium Brewing Company recently evaluated the effectiveness, reliability, and robustness of submerged anaerobic membrane bioreactor (AnMBR) technology at their Fort Collins, Colorado craft brewery. The pilot study drivers were the performance limitations of Upflow Anaerobic Sludge Blanket (UASB) digesters, space constraints and a desire to maximize recovery of energy resources. While effective in removing (65-85%) of soluble COD, UASBs are limited in their ability to handle and remove total suspended solids (TSS) and other brewery waste flows such as trub, spent hops, and spent yeast. Additionally, the chemical oxygen demand (COD) and TSS in the effluent from UASBs are typically above POTW surcharge discharge limits resulting in additional levied surcharge costs or the need for energy and space intensive downstream aerobic treatment. With changes in consumer preferences resulting in a higher percentage of hops being used in the brewing process, UASBs and other conventional brewery pre-treatment systems have struggled to digest their waste streams due to spent hops-related microbial inhibition impacts to the biota 1. This market shift has also forced breweries to haul and dispose of more solids at landfills. New Belgium Brewing sought to find a more dependable and sustainable solution for treatment of brewery waste and implemented a pilot project to assess the effectiveness of the AnMBR technology to treat brewery waste including the high COD spent hops and spent yeast high strength residual streams with high biomethane potential. The pilot evaluation included assessing the biogas quality and biogas quantity generated from the digestion of the various brewery waste flows to identify any quality impacts from increased COD loading from spent hops and spent yeast. The project sought to identify appropriate technology and determine the feasibility of conditioning the recovered biogas with membranes to increase the options for utilization of the recovered carbon source. Energy recovery options evaluated included direct pipeline injection of the highly purified and compressed biomethane ( green gas) and compared these costs with more traditional biogas utilization options such as boiler make-up, combined heat and power (CHP) cogeneration, and microturbine fuel. The relatively small scale of the proposed full size biogas conditioning system would make this project notable in the US, possibly putting the system within grasp of numerous new breweries and other industries using anaerobic digestion as pre-treatment for their process waste flows. Pilot Testing The pilot was installed in August 2014 and was operated for approximately seven months at the Fort Collins-based existing Process Water Treatment Plant (PWTP). The influent feed to the digester was a slipstream connection downstream of the 600 m 3 equalization basin which is directly upstream of the existing acidification basin and UASB digester. The pilot was set up to 1

2 receive effluent from a line feeding the existing Upflow Anaerobic Sludge Blanket (UASB) digester and to achieve a treatment performance consistent with the city s sewer discharge regulations. The project team consisted of staff from New Belgium Brewing, GE Water and CDM Smith. The overall objectives of the membrane portion of the pilot study were to determine the optimum AnMBR design parameters that generate stable membrane and bioprocess performance, as well as to demonstrate performance of the GE ZeeWeed 500 AnMBR system to treat brewery wastewater alone, as well as brewery wastewater blended with spent yeast and spent hops. The Anmbr Process Figure 1 shows the general process flow diagram for the AnMBR system used in this study. This is a modular system with ultrafiltration (UF) membranes located in a separate vessel from the completely mixed anaerobic digester. The system includes: modules, cassettes, and trains. A module is the smallest replaceable filtration unit which consists of hundreds of membrane fibers oriented vertically between molded plastic headers. Modules are joined together to form an operable cassette. Several cassettes are connected to a common permeate header, therein comprising a train. The UF membranes perform the solid-liquid separation with the treated water collected on the inside of the hollow-fiber membrane. The GE AnMBR treatment process has a compact footprint, and is modular to install and operate. Figure 1 General Process Schematic Screened and equalized wastewater is introduced to the digester and the anaerobic sludge is continuously circulated between the digester and the membrane tank, with a recirculation ratio 4 to 6 times higher than the permeate flow. The membranes are immersed in a closed tank, in direct contact with anaerobic sludge. Using a permeate pump, a vacuum is applied to a header connected to the membranes. The vacuum draws the treated water through the hollow fiber ultrafiltration membranes. Biologically generated gas from the digester headspace is compressed and is introduced intermittently to the bottom of the membrane module in the form of large bubbles, producing turbulence that cleans the external surface of the hollow fibers. The gas 2

3 sparging transfers rejected solids away from the membrane surface and was applied at a rate of SCFM/ft 2. Figure 2 shows the GE Water AnMBR Pilot configuration schematic. Figure 2 AnMBR Pilot Sampling and Analysis In order to determine operational performance of the pilot, several parameters were recorded online including: ph, Oxidation-Reduction Potential (ORP), head space pressure, tank level, flow (influent, permeate and RAS mixing), flux, trans membrane pressure, permeability, temperature, temperature corrected permeability, total biogas flow and biogas sparging rate. In addition, a daily log sheet was completed by New Belgium Brewing personnel. To further characterize produced water and sludge quality, each process stream (influent, permeate and wasted sludge) was sampled in the on-site laboratory and analyzed for solids fraction, chemical oxygen demand (COD) fractions, volatile fatty acids (VFA) and alkalinity. Samples were sent to an external lab for biological oxygen demand (BOD), total organic carbon (TOC), fats, oils and greases (FOG), total nitrogen, inorganics (Fe, Ca, Mg, chloride, sulfide, silica, sulfate), metals (As, Cd, Cu, Pb, Hg, Zn, Ni, Se), and biogas (CH 4, CO 2, N 2 and H 2 S) analysis. During startup, the system was seeded with macerated granular anaerobic sludge obtained from New Belgium s full scale UASB reactor. The pilot was fed from an equalization basin before the acidification basin. For all flows tested, the influent to the pilot system satisfied the macronutrients (nitrogen, phosphorus and sulfur) requirement for optimized biological system health in anaerobic treatment, but was deficient in cobalt and nickel micronutrients. The recommended nickel and cobalt requirement is mg/g COD and mg/g COD respectively. Long term operation without providing these macronutrients can lead to an unhealthy biological population that cannot efficiently degrade the organics. This can lead to higher residual COD impacting effluent quality, and higher potential for membrane fouling. Results Digester Performance Phase 1 Equalized Wastewater Figure 3 depicts the influent COD and AnMBR effluent COD generated over the initial startup and steady state operation period while treating equalized wastewater. The influent wastewater COD ranged from 5,000 to 8,600 mg/l. During this phase, the effluent COD remained below 200 mg/l for all readings. 3

4 Figure 3 Influent and AnMBR Effluent COD Profile Equalized Wastewater The digester solids concentration (MLSS and MLVSS) profile was maintained during the startup and steady state period while treating equalized wastewater. The MLSS in the bioreactor increased steadily from the initial 8,500 mg/l startup condition to 18,000 mg/l during 30 days of operation. Subsequently, sludge wasting was initiated to maintain the MLSS concentration in the digester at 17,000 mg/l. The average MLVSS/MLSS ratio was observed to be 0.62 and this is indicative of some accumulation of inert suspended solids material in the digester which will eventually have an impact on the active volume of the digester unless controlled through sludge wasting. The total volatile fatty acid (VFA) concentration was also monitored as an indicator of the health of the digester and any upset conditions. The alkalinity was measured once. This measurement showed the VFA/alkalinity ratio was well below the target 0.02, indicating well balanced anaerobic process operation. During the testing period, the feed ph was adjusted upstream with magnesium hydroxide to maintain a ph of in the bioreactor. During standby periods when no influent wastewater was fed to the pilot, a small amount magnesium hydroxide was dosed in the bioreactor to maintain the system ph. Phase 2 Equalized Wastewater with Spent yeast Addition The influent wastewater COD ranged from 4,900 to 15,000 mg/l. The intent was to gradually ramp up the spent yeast dosing until the 2.0% (v/v) target was achieved, but due to mechanical limitations the spent yeast was dosed at or above 2.0% (v/v) with no gradual increase. This resulted in an influent COD as high as 15,000 mg/l as shown in Figure 4. The spent yeast dosing was inconsistent throughout much of this phase of operation, which resulted in frequent spikes in effluent COD. This period also included brewery shutdowns for the holiday periods, and some digester acidification occurred during these periods. After each spike in influent COD, 4

5 the biological performance was able to recover. During this phase, the effluent COD remained below 300 mg/l for 62% of samples collected. The COD removal rate remained above 80% during the entire phase. A mass balance of spent yeast for actual brewery operations indicate spent yeast should remain below 1% of feed stock in full scale operation. Figure 4 Influent and Effluent COD Profile Spent Yeast Addition Phase The MLSS in the bioreactor averaged 17,000 mg/l. Sludge wasting was continued to maintain the MLSS concentration in the digester at 17,000 mg/l. During this phase the average MLVSS/MLSS ratio increased to 0.7 from 0.65after two months of operation. Overall, the higher MLVSS/MLSS ratio is indicative of some accumulation of inert suspended solids material in the digester which will have an impact on the active volume of the digester. No changes in membrane performance were observed when the MLSS in the membrane tank was above 20,000 mg/l. Phase 3 Equalized Wastewater with Spent yeast and Spent hops Addition While treating equalized wastewater with spent yeast and spent hops addition, the influent wastewater COD ranged from 6,000 to 21,300 mg/l. During the testing period the effluent COD remained below 300 mg/l for all readings. While treating equalized wastewater with spent yeast and spent hops addition, the MLSS in the bioreactor increased steadily from the initial 8,500 mg/l startup condition to 18,000 mg/l over 30 days of operation. Sludge wasting was decreased to 4% of influent flow and MLSS concentration in the digester remained at 17,000 mg/l. The test results indicated that the MLSS and MLVSS concentrations were showing a similar trend with an average MLVSS/MLSS ratio of This is an indicative of some accumulation of inert 5

6 suspended solids material in the digester which will have an impact on the active volume of the digester. Spent hops are well documented to exhibit anti-bacterial properties (Behr, Vogel 2010) and were shown to delay the digestion of COD from 3-15 days during bench biomethane potential testing performed in CDM Smith s Bellevue Laboratory. UASB style digesters are unable to break down most of the COD load from spent hops waste due to the reduced ability of a UASB to manage and handle high feed TSS at a low SRT. The AnMBR was able to provide solids retention times of days which was sufficient for breakdown of the spent hops and spent yeast waste streams which led to increased biogas production. The carbonate alkalinity and total volatile fatty acid concentration was monitored during all phases as an indicator of the health of the digester and upset conditions. The VFA/alkalinity ratio remained well below 0.03 indicating a well-balanced anaerobic process operation. During the testing period, the feed ph was adjusted with magnesium hydroxide to maintain a ph of in the bioreactor. During periods when no influent wastewater was fed to the pilot, magnesium hydroxide was dosed in the bioreactor to maintain the system ph. Membrane Performance The membrane performance was closely monitored throughout the pilot study, and the operation flux, transmembrane pressure (TMP) and membrane cleaning employed during each phase of operation were carefully assessed. The membrane performance results were as follows: Phase 1 Equalized Wastewater The system was initially seeded with sludge from New Belgium Brewing Company s existing anaerobic digester. Following this, the system was purged with nitrogen. The filtration was started at a reduced organic loading rate and progressively increased to the target organic loading rate. The system ph was monitored and magnesium hydroxide was dosed to maintain a ph of 6.6 to 6.7. The system had a capability to dose antifoam if deemed necessary. The target net flux (filtrate volume divided membrane area and by time) demonstrated throughout the study was 4.7 gallons per square foot of membrane area per day (gfd). Initially, the filtration was started up at a lower flux of 1.2 gfd and sequentially increased over the next 7 weeks to reach the target 4.7 gfd. The membrane was operated in a relaxation mode to maintain the design flux. Once the design flux was reached, the pilot operated for one month on the equalized wastewater source. The pilot operated at the design organic loading rate for the duration of this phase. Although the ZeeWeed * system has a maximum TMP of 8 psi, the average operational TMP observed during the startup and optimization phase was much lower <0.5 psi. The system was able to successfully operate during this period where the TMP value remained constant with no apparent membrane fouling. Phase 2 Equalized Wastewater with Spent yeast Addition Following the equalized wastewater demonstration, the pilot operated for three months treating equalized wastewater blended with 2% spent yeast. Early in the piloting, one membrane was taken out of service in order to lower the OLR. Due to the failure of a gas sparging valve, the * Trademark of General Electric Company; may be registered in one or more countries 6

7 membrane in service was not receiving adequate sparging. As a result, the TMP began to rise rapidly. The sparging regime was adjusted to compensate for this until the actuator could be replaced. Due to the elevated TMP seen by the membrane, a recovery clean consisting of soaking the membranes in cleaning chemical (concentration 1,000 mg/l for NaOCl and 2,000 mg/l for citric acid) for 6 hours, each was initiated which restored the membrane back to baseline conditions. Following the recovery clean, a second membrane was brought online, and the net operational flux was reduced to 2.4 gfd. The membrane was operated in a relaxation mode to maintain the design flux. The membrane fouling observed was due to a mechanical failure and not because of the biological process. Excluding this, the average operational TMP observed during the pilot demonstration was <1.5 psi. This episode demonstrated the effectiveness of the sparging system to control TMP. Phase 3 Equalized Wastewater with Spent yeast and Spent hops Addition Approximately six months into the evaluation, the pilot began treating equalized wastewater blended with 2.0% (v/v) spent yeast and spent hops. The pilot operated on this influent stream for two months. Due to the nature of the pilot chemical feed piping for very low flow rates, frequent plugging of the spent yeast/spent hops dosing line occurred initially. This was remedied and continuous steady operation was achieved. The net flux demonstrated throughout this phase was 2.4 gfd. Initially the filtration was started up at a lower flux of 0.7 gfd and sequentially increased by 25% biweekly to reach the target 2.4 gfd. The membrane was operated in a relaxation mode to maintain the design flux. The average operational TMP observed during this phase remained well under <1.0 psi as shown in Figure 5. The increase in TMP towards the end of the study may have been a result of inert solids accumulation in the system, as MLSS in the system was allowed to increase to 25,000 mg/l due to decreased sludge wasting activity. Otherwise, the system was able to successfully operate during this phase. Figure 5 TMP and Flux Profile Equalized Wastewater with Spent Yeast & Spent Hops Addition 7

8 Biogas Yields Bench scale analysis of the biomethane potential (BMP) of the various residual waste streams was tested in CDM Smith s Bellevue analytical laboratory. This testing was performed to develop a control for the pilot digester plans and to identify at what level of residual loading should a disruption to the digestion process be anticipated. Figure 8 below shows the biogas production of the control sample (equalized WW only) compared with five spiked samples that have varying levels of spent yeast sludge or spent hops sludge. Figure 6 Biomethane Production Bench Scale Testing Figure 6 indicates that all spiked samples performed above the control sample in terms of gas production per g of COD. In the case of samples spiked with spent hops from 4-10% of the sample volume, there was a significant lag period on the order of days for the biota to breakdown the material. In a traditional UASB digester with solids retention times of less than 3 days, much of this material would not have been converted to biomethane. The AnMBR digester will have solids retention times on the order of days due to the ability to retain the biomass and sludge feed in the reactor with the membrane separation process. These results from bench scale testing indicated that a full scale test with continual residual loadings should be performed. The biogas quality was monitored over the course of the pilot through grab samples. Table 1 below shows the high quality biomethane that was being produced during the pilot study. 8

9 Date Wastewater Stream Table 1 Biogas Quality Oxygen + Argon Carbon Dioxide Hydrogen Sulfide Nitrogen Methane Sample 1 Sample 2 %, v/v %, v/v %, v/v %, v/v mg/m³ mg/m³ 10/29/2014 Wastewater ,100 1,200 1/28/2015 Wastewater + Spent yeast ,200 1,100 3/11/2015 Wastewater + Spent yeast + Spent hops ,200 Pilot results indicated that biogas generation yields were maintained throughout the residual spiking program. A conservative biogas yield value of 0.47 L of biogas/g of COD removed could be expected for full scale operation of the AnMBR system based upon the biogas generated during the pilot testing. If the spent yeast and spent hops were combined in to the brewery waste streams, the COD load would be expected to double on average. This results in potentially doubling the amount of biogas generated for the facility, from ~30 scfm to approximately 60 scfm at projected brewery buildout. Maximizing Energy/Resource Recovery A comparison of available biogas conditioning technologies was performed to identify the best solution for a still relatively small (X<100 scfm) biogas conditioning application. Responses from qualified vendors indicated that water scrubbing and pressure swing adsorption were feasible using the smallest commercially available units, but generally had low overall recoveries of the biomethane and would require costly and space intensive ancillary systems relative to the throughput volume of biogas and biomethane produced. The third biogas conditioning technology is membrane separation. Similar pre-treatment is required for removing H2S and moisture, and a single pass membrane is expected to recovery about 80% of the methane in the raw biogas. This results in a waste gas stream that is ~25% biomethane that will require a flare or thermal oxidizer for destruction and a product gas stream that is typically sufficient for vehicle fueling but not typically concentrated enough for direct pipeline injection. A second pass membrane system (Stage 1 and Stage 2 membranes in series) will produce pipeline quality biomethane but will still require a thermal oxidizer for the waste gas and will lower the overall saleable biomethane production. A third stage membrane system on the reject waste gas from Stage 1 can be used to recover the biomethane and produce a 99+% carbon dioxide waste stream that does not require a thermal oxidizer or flaring. This final approach has been selected for further evaluation due to the perceived project benefits as shown in Figure 7, below. 9

10 Figure 7 Three Stage Membrane Biogas Conditioning Process Flow Diagram (courtesy of DMT Technologies) Conclusions This pilot study was deemed successful because the AnMBR treatment system capacity and performance quality was successfully maintained throughout the demonstration periods treating a range of challenging brewery wastewater streams that the current UASB system was unable to treat continuously. Stable and reliable AnMBR performance was demonstrated throughout the pilot testing period. This was the case for both the design loading operating conditions as well as realistic brewery episodes of fluctuating and higher than average COD and TSS loading conditions. The following conclusions can be drawn from the study results: AnMBR technology is effective at treating brewery process wastewater and spent hops and spent yeast residuals to increase biogas production. Membrane based biogas conditioning may provide cost effective conditioning for industrial digester systems interested in recovering biomethane for pipeline injection. Digestion of carbon rich brewery residual waste streams reduces the carbon footprint of a facility when compared to off-site disposal of the spent solids 10

11 References 1. Behr, J., Vogel, R. Mechanisms of Hop Inhibition Include the Transmembrane Redox Reaction, Applied Environmental Microbiology. Jan 2010, 76 (1) pp