August 20, 2008 | General

Prepping MSW Substrates For Anaerobic Digestion

BioCycle August 2008, Vol. 49, No. 8, p. 54
New facility in Italy and field trials in the U.S. illustrate innovations in preprocessing municipal solid waste prior to the digestion phase.
Nora Goldstein

MANY anaerobic digestion projects are going beyond treating a single substrate such as manure or municipal biosolids – homogenous feedstocks typically high in moisture content when they arrive at the digester’s door. In many cases, these additional substrates are far from homogenous, and require steps to separate organics from nonorganics and transform them into a digestible slurry.
An article in the January 2008 issue of BioCycle (“Green Energy From Food Wastes At Wastewater Treatment Plant”) described a separation system designed by the East Bay Municipal Utility District (EBMUD) in Oakland, California. In preparation for a full-scale project, EBMUD and Norcal Waste Systems have been experimenting with adding organics from the City of San Francisco’s commercial and residential source separation program to a biosolids digester at EBMUD’s wastewater treatment plant (WWTP). The organics are screened through a trommel prior to delivery to the WTTP. Food waste is unloaded into a slurry tank, then passes through a rock trap/grinder to remove heavy materials such as metal and rocks and grind the bigger solids and fibers to make the slurry more consistent. That material is fed into the digester.
Another approach to separating contaminants from incoming MSW is wet separation. One method is to load feedstocks into a hydropulper, where an agitator spins quickly, opening plastic bags to release the MSW and pulps the contents. Plastic bags and other light materials float to the top and are removed; the heavy fraction gets trapped at the bottom of the hydropulper. In some cases, there is additional removal of contaminants via a hydrocyclone.
A new anaerobic digestion facility, built by Rivalta Energy in Rivalta (Mantova), Italy is designed to receive 10,000 metric tons/year of agricultural and municipal feedstocks. The plant, with four 350 kW generators, produces 1 MW of electricity, and is expandable to 2 MW. The plant is located on a dairy farm with 140 cows, and has been running for approximately one year.
In Italy, households don’t have garbage disposals, so organic waste is put in garbage bags and collected periodically. Plastics, glass, metal etc. are supposed to be separated for recycling, however a significant amount of nonorganic items end up in the garbage bags. The Rivalta plant was designed to utilize the household organic waste, restaurant waste, fats, oils, chicken renderings or most anything that will generate biogas.
Rivalta Energy worked with DODA of Buscoldo, Italy to supply all the equipment to preprocess the household refuse. Three compartments of approximately 70 cubic meters each were installed to handle different digester substrates. The compartments are designed to be under vacuum to pull away the odors created. Odorous air is treated through a biofilter.
One compartment accepts dairy manure that typically is combined with silage from storage bunkers and is mixed with an agitator and chopper pump. Another compartment receives the municipal garbage that usually arrives in 30 cubic meter trucks. That compartment is equipped with a bag opener device that slices open the bags, releasing their contents, and a specially designed chopper pump that processes the bagged garbage into a thick slurry. The bag opener is powered by a 22-kW electric motor. Typically, some digested substrate is added to the compartment to assist the mixing process. If available, manure from the adjacent compartment can be used. The pump is made with Hardox Steel, powered by a 75-kW electric motor and automatically controlled and run by a computer.
Mixing time can vary between 15 minutes and one hour depending on the products being mixed. The final homogeneous slurry, between 10 to 15 percent solids, is pumped into the Bio Separator, which works like a cyclone. Knives rotate at a very high speed to create suction. Material is pumped into the bottom of the unit and pulled through with cyclonic action. The liquid fraction goes through a specially designed screen, and the plastic, metal, glass and nonorganics are slung into a dumpster. “Material is being spun so fast that the products are dry when they go into the dumpster,” says Rich Miller of DODA USA.
Slurry from the Bio Separator flows to the third compartment where it is mixed with the dairy manure and other substrates. After blending, the slurry is pumped to the primary cells of the digester. Rivalta Energy decided on a complete mix digester, with two primary and two secondary cells. Each digester cell has four piped inlets where substrate can flow in or out. Flow of material between cells is controlled by a Bio-Box. “The substrate can be pumped from one cell to another for reseeding, back flushing or potential solids removal or additional agitation,” says Miller. This is accomplished with one pump in the Bio-Box and a series of computer controlled valves.
Biogas mixers manufactured by DODA were installed in the digester cells. The mixers provide complete homogenization of the substrates and keep the digesters running at optimal performance. All electric motors are equipped with variable frequency drives and are computer controlled. “These mixers only run periodically and at about 30 percent of their capacity to accomplish proper mixing,” he adds.
After digestion, material flows to a reception pit, then is pumped to a screw press and a roller press separator (also manufactured by DODA). Separated solids are conveyed to an agitated compost unit. The liquid fraction flows to a storage tank to be used as liquid for the predigestion mixing compartment, or stored for field application. Composted solids are sold or field applied. Research also is being done on converting the solids into biodiesel.
Three other facilities are in development for Rivalta Energy, using the same equipment and technologies. “Our biogas mixers, chopper pumps, and solid and liquid separation and dewatering equipment have been installed in many manure digesters, but the project in Italy debuts our technologies for preprocessing MSW,” says Miller who works in the company’s U.S. office in Minnesota.
Researchers at the University of California (UC) Davis, working with Norcal Waste Systems of San Francisco, are evaluating use of a rotary drum reactor (RDR) for pretreatment and separation of the organics from MSW. The organics are then digested in the Anaerobic Phased Solids (APS) Digester on the UC Davis campus, or in laboratory scale reactors. The scope of the research includes testing different digester organic loading rates of the pretreated MSW to determine the biogas potential and analyzing the characteristics and biogas production potential of organic materials separated from MSW using a RDR process. A more complete article on these research studies will be published in an upcoming issue of BioCycle. Highlights of the findings are reported here.
Rotary drum reactors are typically used to presort and size reduce municipal solid waste prior to composting. There are about ten solid waste composting plants in North America utilizing rotary drums for this application. These plants typically process a mixed MSW stream (see “Mixed MSW Composting In Transition,” November 2007).
Norcal Waste Systems services the City of San Francisco’s Fantastic 3, where households utilize a 3-cart system for single stream recyclables, organics and trash. Norcal and city officials have noted that despite the separate organics cart, there is a significant volume of compostable organics remaining in the trash fraction. The company already had been testing use of anaerobic digestion for separated organics both at East Bay MUD (discussed above) and at the UC Davis digester.
A recent project funded by the California Integrated Waste Management Board focuses on characterizing the organic materials derived from MSW via the RDR process, determining their biogas production potential and analyzing the feasibility of using them as a feedstock for anaerobic digestion. UC Davis researchers, including Ruihong Zhang, Petros Gekas and Baoning Zhu, worked with six MSW composting facilities in the U.S. with rotary drums and analyzed the samples of organic materials separated from the MSW. Their research is summarized as follows. Four of the six plants process mixed MSW and biosolids in the RDR; one facility recently switched from processing MSW, biosolids and paper and cardboard, to just biosolids and the paper fraction. The sixth facility processes MSW, biosolids and yard trimmings. Five of the six drums use aeration. Retention time of MSW in the drum ranged from six hours to as much as three to five days. Feedstocks in the drum are discharged through a screen. Hole sizes range from a low of 25.4 mm to as large as 44.5 mm. Three facilities use screens with 31.8 mm holes.
Samples of the output from the RDR were analyzed for bulk density, moisture and solids content, and nutrient and metals content. The samples were tested in the laboratory using batch anaerobic digesters to determine the rate of digestion and biogas and methane yields of the organic samples. Test conditions in the anaerobic digesters (working volume of 0.5 L) were: initial volatile solids loading of 3 gVS/L; temperature of 55+2°C and digestion time of 20 days.
Primary conclusions were: 1) Organic materials recovered from MSW using the RDR process have 44 to 60 percent moisture content, high organic content, 70 to 89 percent volatile solids in total solids, and 22 to 40 C/N; 2) They are highly digestible with relatively high biogas yields, 533 to 676 ml/gVS or 0.533 to 0.676 m3/kgVS. The biogas contained 58 to 60 percent methane; and 3) Organic materials derived from RDR process are suitable feedstock for anaerobic digesters.
Sidebar p. 56
Substrate Prep, Biogas Potential Will Be Hot Topics At October BioCycle Conference
BIOCYCLE’S 8th Annual Conference on Renewable Energy From Organics Recycling, October 6-8, 2008 in Madison, Wisconsin, includes sessions on topics discussed in the accompanying article. The two projects mentioned – the DODA anaerobic digestion facility processing MSW, and the research being conducted on using rotary drum reactors to sort MSW organics for digestion – will be presented in a session, “Anaerobic Digestion Of Municipal Solid Waste,” on Monday, October 6th. Related sessions include Codigester Economics, Substrate Additions and Feedstock Digestibility, System Analyses, both on Tuesday, October 7. For complete agenda, see pages 7 to 9 of this issue. To register, go to or call 610-967-4135, ext 21.

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