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H2O Urban Solutions, Inc. Page 2 Discussion The City continuously seeks to minimize the cost of treating and delivering water. Strong political and legal forces limit the City's authority to increase water rates, so stabilizing water program costs by improving existing operations is a necessity. Solids handling at the LAWTF has proven more costly than originally expected when designed. The labor and energy costs to operate the existing filter press are approximately$65,000 annually, since the process is not fully automated. Alternatives for dewatering solids include other mechanical methods (i.e. belt press, centrifuge), though use of these may not result in significant reductions in cost. Non- mechanical processes (i.e. drying beds) have the greatest potential to minimize solids disposal costs. The advantages and disadvantages of drying beds for solids dewatering (as opposed to the current filter press) are provided below. Advantages Disadvantages • Passive (less labor) • Large Area or "footprint" needed • No energy used for dewatering • Not effective in winter/spring • No moving parts, machinery, or • Requires more time for dewatering regular maintenance • Requires additional chemical use to • No replacement or reconditioning condition solids prior to spreading required Drying beds are generally a preferred solids drying process for most agencies located in the areas where the weather is conducive to solar drying, but often space limitations restrict their use. However,the LAWTF facility has adequate, undeveloped areas for placement of drying beds. The bed size depends on the solids content of the filter backwash, so concentrating the solids by pretreating with lime or other coagulant is necessary for reasonable bed size and cost. Conditioning solids with chemicals is a common practice, and promoted by the U.S. EPA. 1 A disadvantage of drying beds specific to the City is the cost to change its solids handling process. Implementation of dying beds will require capital costs to construct the beds and modify certain existing process (i.e. remove the filter press, re-route piping, modify existing chemical systems, etc.). Though the drying beds are not complex (a bed of sand placed over gravel in a concrete basin), the size of the beds result in a significant capital cost. As previously mentioned, the size of the beds can be significantly reduced if the solids are conditioned prior to placement on the beds. Adding lime to the solids will enhance the dewatering process, and assumed to be part of drying bed facility modifications. The system envisioned for LAWTF would include the sand drying beds,followed by evaporative or evapotranspirative elimination of the drained water (see Figure 1). Water drained from solids placed in the drying beds will be held in concrete lined basins. If the chemical characteristics are acceptable, the water may be diverted to the existing earthen-lined 1 Design Manual, Dewatering Municipal Wastewater Sludges, U.S. EPA,Office of Research and Development.