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Codigestion of Organic Waste

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Commingling of organic matter with wastewater solids in a co-digestion process is an emerging practice that promises substantial benefit to wastewater treatment plants, producers of industrial, agricultural, and commercial organic wastes, and the environment in general. For wastewater processors it promises an increase in biogas production, a reduction in electrical demand through increased cogeneration with the additional biogas, an offset of natural gas purchases through use of locally generated biogas, as well as a potential source of revenue from disposal fees. Further, because co-digestion of industrial waste with wastewater solids may use existing infrastructure if excess capacity exists, implementation may not require significant expenditures for capital improvements. For industrial and commercial producers of organic wastes, co-digestion promises a reliable and cost-effective way to recycle byproducts that are typically either landfilled or pretreated and added to wastewater plant influent. Finally, co-digestion promises global ecological benefits through reduced greenhouse gas production as biogas generated by the co-digestion process is captured and beneficially used instead of being released into the atmosphere at landfills.

Implementation of co-digestion at wastewater plants is increasing. However, implementation can not come at the expense of anaerobic digestion process stability. Anaerobic digestion operation and performance measures are based upon years of empirical data gained from operation of digesters fed solely wastewater solids. These data are not sufficient to be applied to feedstocks composed of a portfolio of other organic wastes. CDM is currently performing applied research to address this existing knowledge gap. The specific objectives of this research are to develop a practical procedure to assess the potential impacts of a particular organic waste in a co-digestion feedstock on an anaerobic digestion system; to identify major obstacles and design solutions to these challenges in order to facilitate the handling, mixing, and feeding of commingled wastes to anaerobic digesters; to develop the empirical data necessary to support digester design and establish operational stability parameters; and finally, to develop an economic model to assess the viability of co-digestion.

The approach to meeting the research objectives is composed of four sequential research steps, the first of which is waste characterization analyses, followed by laboratory analyses, pilot scale operations, and full scale demonstrations. The research is being conducted by a diverse team consisting of municipal agencies, university researchers, and consulting engineers who will develop reproducible test protocols accessible to wastewater plant operators.

Experiments and full-scale installations have previously shown that co-digesting organic wastes such as fat, oil, and grease (FOG), slaughterhouse wastes, and food processing wastes with wastewater solids can increase methane gas production more than 100 percent under certain conditions. Similarly, biodegradability (as measured by volatile solids destruction) of wastewater solids has been shown to be improved by co-digesting with FOG (Kabouris et al, 2007b; Schafer et al, 2008). Results of a literature search on co-digestion research will be summarized in a table of feedstock characteristics, and associated experimental results. Data from full-scale operations will also be presented. Feedstocks being evaluated are varied and include food processing wastes such as corn squeezing and cheese whey, FOG, and others. The variables characterized include total and soluble chemical oxygen demand (COD), total solids (TS%), volatile solids (VS%), total nitrogen, and total phosphate. A food analysis (carbohydrates, fats, protein, and ash) is also being performed on the organic wastes.

This project is part of the Water Environment Research Foundation (WERF) Optimization of Wastewater and Solids Operations (OWSO) Challenge, which had its kickoff meeting at WEFTEC in San Diego, October 2007. Team members have developed the waste characterization protocol and are currently conducting the literature review and lab scale testing. Waste characterization results will be presented along with results of lab scale analysis; these data will show the response of anaerobic microbes to different co-digestion feedstocks. Subsequent phases of the work will present pilot plant results and full scale operations data. This presentation will describe progress to date on the co-digestion literature search and the waste characterization and laboratory testing, which make up the first phase of the OWSO Challenge. The knowledge to be gained from the research is essential to making co-digestion an efficient process that can be successfully implemented by wastewater treatment plants across North America.

Keywords: Co-digestion; FOG; anaerobic digestion; bioenergy; biogas; organic waste

Document Type: Research Article


Publication date: January 1, 2009

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