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In recent years there has been growing concern for sewer collection system capacity management (CMOM) and the prevention of sewer system overflows (SSOs). One major cause of reduced capacity that promotes SSOs is lines becoming clogged with fats, oils and grease (FOG) discharged into the sewer. How a municipality or utility district deals with FOG can turn this problem into an opportunity. For this reason, the Orange County Sanitation District (OCSD) is developing a plan to reduce the amount of FOG discharged into the sewage collection system through the development of Best Management Practices and enacting future new FOG control ordinances. As a result of improved FOG collection practices and enforcement of these future new ordinances, the quantity of FOG discharged directly at the OCSD Plant No. 1 may increase. This paper presents findings of a recently completed study that evaluated disposal options for FOG, their impact and benefits to in-plant processes. The result of this study has found two alternatives for FOG disposal and treatment at the OCSD plant 1 that will truly convert an environmental problem into a positive opportunity for the district and its rate payers.

Processing and disposal alternatives for FOG received directly at the OCSD treatment plants are developed and discussed in this paper. Impacts associated with their implementation are also addressed. Each alternative assumes compliance with new FOG ordinances passed as a result of the BMP study. Alternatives that have been developed include:

Base case – status quo (continue current practices of receiving FOG at the plant headworks, removal with primary scum, pumping to anaerobic digesters along with primary sludge)

On-site treatment using anaerobic digestion

On-site collection and off-site recycling at a grease rendering facility

On-site collection and off-site recycling as a bio-fuel

On-site collection and off-site landfill disposal

Six different criteria were established for evaluating each alternative including: economic impacts, maintenance impacts, staffing impacts, process operation impacts, traffic impacts, and environmental impacts. A workshop was held with OCSD staff to determine the relative weight of each criteria and each alternative was rated on a scale of 1 to 5 with 1 being the least desirable and 5 being the most desirable.

The two alternatives that ranked the highest were, off-site recycling as a bio-fuel and on-site treatment using a dedicated anaerobic digester. Off-site recycling as a bio-fuel is an emerging technology that produces a clean-burning bio-diesel, from recycled vegetable oils, fats and grease. On-site treatment of FOG utilizing an existing anaerobic digester would provide an environment conducive to growth of bacteria that would break down FOG. OCSD performed a bench scale pilot test for dedicated digestion of primary scum several years ago that appeared to be a favorable way of treating FOG. The benefit of this alternative is that it would produce methane gas for on-site cogeneration and reduce other maintenance issues associated with current treatment of FOG at the treatment plant.

The conclusion of this paper was to implement on-site treatment of FOG using a dedicated anaerobic digester. Off-site recycling as a bio-fuel is an option that could be implemented in the future when pilot testing of a yellow grease separation system is completed and a market analysis of this bio-fuel is done. This study has shown that OCSD can effectively deal with FOG by turning an environmental problem become a positive opportunity.
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Document Type: Research Article

Publication date: 2003-01-01

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