Research and Development Grant Interim Report Municipal Anaerobic Digesters as Regional Renewable Energy Facilities

Larry Krom
Manager: Research and Development
Focus on Energy Renewable Energy Program

Daniel H. Zitomer, Ph.D., P.E.
Marquette University
Department of Civil and Environmental Engineering
Milwaukee, WI
Daniel.zitomer@mu.edu

March 2004

INTRODUCTION

Many of Wisconsin’s municipal wastewater treatment facilities use anaerobic digesters to biologically convert municipal wastewater solids to biogas that contains methane. The methane is then used as a renewable energy source to heat the digesters, heat buildings, generate electricity, and run mechanical equipment. The digesters are often very large and can theoretically treat other wastes, such as high-strength waste, to economically produce more methane. Unfortunately, co-digestion of municipal wastewater solids with high-strength waste is not typically practiced in the United States due to lack of information regarding implementation, operation, and costs.

If a practical demonstration project was performed, then more Wisconsin municipalities could introduce existing high-strength wastes to existing anaerobic digesters for renewable energy production. Many of Wisconsin’s existing municipal digesters could then become regional renewable energy facilities that co-digest a mixture of raw materials that were formerly called waste. This will produce more renewable energy. The reader should refer to the project proposal, “Municipal Anaerobic Digesters as Regional Renewable Energy Facilities,” dated October 17, 2002, for additional background information.

This interim report describes testing performed at Marquette University to demonstrate co-digestion of a suite of wastes and municipal wastewater sludge from the South Shore Wastewater Treatment Plant (SSWWTP), Oak Creek, WI. The work is being performed by Dr. Daniel Zitomer (Department of Civil and Environmental Engineering) with the help of Mr. Prasoon Adhikari, graduate student Research Assistant.

METHODS

The work plan consists of preliminary laboratory studies followed by full-scale pilot testing of the co-digestion concept. The preliminary laboratory studies are nearly completed as of March 2004. The full-scale testing is scheduled to be performed in April and May 2004 after seasonal deicing operations are concluded at the Milwaukee airport. Currently, deicing fluid waste from the airport is treated using the anaerobic digesters at the SSWWTP. The infrastructure used to store and pump used deicing fluid to these digesters will also be used to convey high-strength waste to the digesters for pilot testing. Therefore, we must wait until waste deicer is not being produced to use the full-scale equipment.

Laboratory Studies

Four different high-strength wastes were obtained from existing facilities and tested in the Marquette University laboratory. The following wastes were studied:

• Pandlt’s restaurant, food waste, Bayside, WI


• Southeastern Wisconsin Products, yeast waste, Oak Creek, WI


• Lasaffre Yeast Corporation, first pass beer waste, Milwaukee, WI


• Miller Brewing Company, I-house filter waste, Milwaukee, WI

To determine the volume of methane that can potentially be produced per kilogram of the wet wastes, biochemical methane potential tests were performed and bench-scale digesters were operated, as describer below.

BIOCHEMICAL METHANE POTENTIAL (BMP) TESTS: BMP testing has already been concluded for Pandlt’s Restaurant, Lasaffre Yeast Corporation, and Miller Brewing Company wastes. Tests are now being performed for Southeastern Wisconsin Products waste.

The BMP protocol of Owen et al. (1979) is being used to determine the extent to which each of the high strength wastes may be converted to methane by SSWWTP digester sludge microorganisms.

For the Miller Brewing Company and Lasaffre Yeast Corporation wastes, four different concentrations of waste ( 0.5, 1.0, 2.0 and 2.5 grams of COD per liter (gCOD/L)) were fed to 40-mL aliquots of methanogenic biomass in 160-mL serum bottles (see Appendix, Figure 1 for an image of the serum bottles). For the Pandlt’s waste, serum bottles were organically loaded based upon volatile solids (and not COD) because the waste had a very high solids concentration. Nominal loadings of of 2.5, 2.4, 10, 23 and 45 grams of volatile solids per liter (gVS/L) were employed. Also, a control bottle receiving no waste was prepared.

All systems were run in triplicate. Thus, 30 serum bottles were prepared. Bottles were flushed with 30% CO2 /70% N2 gas to establish anaerobic conditions, then sealed with rubber septa. Total biogas production was measured over approximately 30 days using a 100-mL wetted barrel glass syringe, and biogas methane content was determined by gas chromatography.

BENCH-SCALE DIGESTERS: A set of bench-scale anaerobic digesters was operated for each of the high strength wastes tested. Each digester was fed a different volumetric blend of one high strength waste and municipal wastewater solids.

All digesters were 2.5-liter glass vessels containing 2 liters of active anaerobic biomass and 0.5 liters of headspace which was vented to a gas meter to measure the volume of biogas produced. The content of each digester was mixed using a magnetic stirrer and stir bar (see Appendix, Figures 2 and 3 for images of a typical digester).

Digesters were seeded with anaerobic sludge from the SSWWTP. Subsequently, each digester was operated on a daily fill-and-draw basis for at least 140 days. Every morning, the digester’s pH and produced biogas volume were recorded, and 133 mL of digester contents was removed from each unit. Each digester was then fed 133 mL consisting of a unique blend of high-strength waste and municipal wastewater solids as presented in Table 1. The influent was also supplemented with 2.5 g/L NaHCO3 to provide alkalinity. The municipal wastewater solids was a mix of 30% waste activated sludge and 70% primary sludge from the SSWWTP. The solids retention time (SRT) and the hydraulic retention time (HRT) for all systems were 15 days. This SRT was employed because the full scale digesters at the SSWWTP currently operate at an HRT of approximately 15 days.

Table 1: High Strength Waste-Municipal Wastewater
Feed Blends (% by volume)

Every day, the temperature, digester pH, and daily biogas volume produced was recorded. The following parameters were measured 3 times per week by standard methods: influent total solids (TS), influent volatile solids (VS) effluent alkalinity, effluent TS, and effluent TS. The effluent soluble COD and biogas percent methane were measured once a week. In addition, the following were measured three times for each industrial waste: arsenic, cadmium, chromium, lead, mercury, molybdenum, nickel, selenium, zinc, total Kjeldahl nitrogen (TKN), total phosphorous, and potassium. These measurements were preformed because land application of digested biosolids is regulated in Wisconsin, in part, based upon the concentration of these constituents.
FUTURE FULL-SCALE TESTING: The full-scale testing will be performed in April and May 2004 at the SSWWTP. The required piping, pumps, tanks and appurtenances are in place at the plant so that waste aircraft deicer from General Mitchell International Airport (Milwaukee, WI) can be co-digested in the winter. We will use existing infrastructure to extend the positive results of co-digestion to use two high-strength wastes. The two high-strength wastes to be tested in full scale will be identified after the laboratory testing is completed.

RESULTS

Characteristics of the individual wastes were measured and results are presented in Table 2. The wastes in order of highest to lowest COD are as follows: Pandlt’s Food, Southeast Wisconsin Products, Lasaffre Yeast, and Miller Brewing. The Pandlt’s food waste has the highest COD, and would, therefore, theoretically lead to the most methane production of all the wastes investigated.

Table 2: Waste Characteristics

NA = testing has not been completed

Regarding metals concentrations, none of the wastes tested thus far have metals concentrations greater than the Wisconsin Department of Natural Resources high quality limits for biosolids to be land applied. Therefore, none of the wastes tested thus far will limit land application of biosolids based upon metals criteria. The Southeast Wisconsin Products and Pandlt’s food wastes are being tested now for metals content. Results will be available in approximately 4 weeks. Table 3 shows the solids content of the primary and waste activated sludges used from the SSWWTP.

Table 3: Primary and Waste Activated Sludge Solids

Table 4: BMP Results

The three wastes tested were all biotransformed to biogas containing methane. The average maximum specific methane production rate of 100 ml CH4/g VS-day is comparable to a COD removal rate of 0.25 g COD/g VS-day. All the wastes were readily biodegradable and produced biogas with about 67% methane. The biogas from the Pandlt’s Food waste had a somewhat higher methane content than the other wastes tested thus far.

Table 5: Bench-Scale Digester Results

Ten behch-scale digesters were operated in the lab to assess digestion of various blends of municipal wastewater treatment and high-strength waste. Table 5 presents a synopsis of the results. Digester tests for the Lasaffre, Miller, and Southeast Wisconsin Products waste has been completed. Digester testing of the Pandlt’s food waste is now being performed, and should be concluded in approximately 3 weeks.

The results thus far indicate that the Lasaffre and Southeast Wisconsin Products waste can be successfully digested with wastewater solids to generate additional methane. The Miller Brewing waste, however, did not lead to significant methane production due to its relatively low organic strength. In addition, the digester that received 100% Miller waste (and no wastewater solids) did not produce appreciable methane. The poor performance is not because the Miller Brewing Waste is not amenable to anaerobic treatment, but may be because the complete-mix, 15-day solids retention time digester employed is not appropriate for Miller waste treatment under the conditions studied.

DISCUSSION

Preliminary results suggest that the Pandlt’s Food, Lasaffre, and Southeast Wisconsin Products wastes are all potential candidates for codigestion with municipal wastewater solids under standard municipal anaerobic digester operating conditions. In addition, the Pandlt’s Food waste had the highest organic strength and the highest biochemical methane potential at 173 L CH4/Kg waste at 35°C. Right now, Pandlt’s generates approximately 1200 Kg of wet food waste per week. This would result in 207,600 L/week of methane that could be converted to 4.4 kilowatts of electricity, plus additional waste heat. If waste was collected from multiple restaurants, then significantly more energy could be generated. This estimate will be refined after all testing is concluded.

After the next 4 weeks, lab testing will be completed, and full-scale pilot testing of two of the wastes will be performed at the South Shore Wastewater Treatment Plant.

Laboratory and full-scale testing will be completed by May 10, 2004.

REFERENCES

Owen, W. F., Stuckey, D. C., Healy, J. B., Young, L. Y., and McCarty, P.L., 1979. Bioassay for monitoring Biochemical Methane Potential and Anaerobic Toxicity, Wat. Res., vol. 13, 485-492.

APPENDIX

Figure 1: Serum Bottles for BMP Study

Figure 2: Typical Digester, Stirrer, and Gas Meter

Figure 3: Typical Digester