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Codigestion: How to Make It a Financial “Win-Win” for All Parties
Author: William Krill, Chris Muller
Date: 4/912
Preprint, Residuals and Biosolids 2012, March 25-28, 2012, Raleigh, NC

In recent years, the practice of co-digesting sewage sludge with other high strength organic wastes has become more common, as utilities try recovery energy investments made during treatment and/or more effectively utilize excess digester capacity. There are occasions wherein one agency may accept sludges from another agency to optimize total biosolids costs or utilize excess process capacity. The technical aspects, process and engineering issues, of such activities are usually somewhat straightforward. However, the financial arrangements can be more difficult to identify and define. When evaluating these types of programs the following questions should be asked: 1. For the “accepting” entity: a. Are capital and/or operating dollars being saved overall? b. Are all costs being considered? c. Are the costs being looked at from a total/average or only marginal cost basis? d. Are there any costs which are being ignored? e. If there are cost savings – how are these apportioned to the users of the system? f. Are there any regulations that need to be examined in the context of co-digestion? 2. For the offering party? a. If an industry – they should be able to quickly assess their economics b. If another wastewater agency: i. Are we saving capital and/or operating costs long term? ii. Where should those savings be allocated and which user classes should benefit? Regional economics, regulatory requirements and other factors can influence the overall viability of a co-digestion program for a utility. Therefore this paper focuses on providing a framework for your current or potential project to answer the key question outlined above.