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The Orange County Sanitation District (OCSD) is responsible for treatment of wastewater and biosolids generated by 2.2 million people leaving in central and north Orange County, California. OCSD currently treats the wastewater at two wastewater treatment plants. Approximately 650 tons of digested and dewatered Class B biosolids are generated at these plants every day. By the year 2020, biosolids production is projected to increase by about 35 percent. As part of the master plan development for OCSD's biosolids management facilities, a dynamic economic model was developed to compare and conduct sensitivity analysis for various biosolids processing alternatives, product manufacturing options, and potential product revenues. The objectives of the economic evaluation model was to develop a flexible tool to:

Evaluate the life-cycle costs of entire biosolids management chains, from sludge thickening to product markets

Incorporate the impact of liquid treatment options on solids production and management costs through a dynamic model

The model ties together mass balances with sizing and costing of different biosolids processing and product manufacturing options. The economic model was designed to be flexible and to allow the selection of plant influent flows and loadings, different liquid treatment processes, and biosolids treatment processes. Biosolids treatment process options ranging from thickening of primary and secondary solids, through digester pre-treatment, advanced digestion, recuperative thickening and dewatering options may be selected for each plant. The model calculates the mass balances, capital costs, annual operation and maintenance (O&M) costs and the 20-year presentworth cost for the entire chain of process options. Biosolids product technologies and potential market revenues have been built into the model, so that the entire biosolids management program can be evaluated.

After the model was developed and validated it was used to evaluate in-plant solids handling options for Plants 1 and 2, evaluate combined dewatering options, and compare costs of product technologies and markets. The model was used to evaluate the capacities of existing and proposed equipment under different process configuration scenarios. Figure 1 presents the biosolids management and marketing components included in the model for processing, product manufacturing, and estimating the market revenues. The economic model indicated the best combination for the biosolids management program to be to:

Add primary sludge thickening.

Install new waste activated sludge (WAS) thickening to achieve solids concentration of 6 percent or more.

Explore options to prethicken WAS through selector process, secondary clarifier improvements, and/or prethickening

Install ultrasound for digestion pretreatment.

Continue using mesophilic digestion.

Install high-solids centrifuge dewatering to produce a drier cake.

Consider combining the dewatering facilities for Plant Nos. 1 and 2 (onsite or offsite).

Develop in-county composting and thermal drying for producing value-added products for viable local markets.

Pursue merchant composting, energy production (co-combustion), and organo-mineral product manufacturing to allow participation in viable markets.

Explore benefits and use of emerging technologies for manufacturing of biosolids products such as drying with hot soil and energy fuels (i.e., char).

Selected options are being further developed through pilot testing, siting analysis and negotiation with the potential merchant facilities, review of performance data from similar facilities, and conceptual sizing and layouts. This economic model was crucial in focusing resources for developing the road map for implementation of the biosolids management plan components such as pilot testing, selection of sustainable product markets, analysis and selection of product manufacturing options.

Document Type: Research Article


Publication date: 2004-01-01

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