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0 • maintenance requirement, it has a high operating cost. Please note that the thin sludge is caused by the lack of food mass in the incoming raw sewage flow. The boat clarifier amplifies the situation due to its lower solids concentration on the solids accumulation blanket that results in wasting too much effluent liquid. Further, because it wastes much of the thin supernate liquor, the process needs a larger volume digester basin. Staff investigated the idea of adding an additional digester tank. This idea was put on the back burner because of economic reasons, capital costs, as well as future operational costs.- Without additional digester tank volume, staff must send a large volume of waste activated sludge (WAS) back to the treatment train. This means i+- must tmust be added to the raw sewage stream to receive retreatment. This situaticr. has put great stress on the biological process, reduced efficiency of the plant process, and increased operational costs. ANALYSIS The best solution to alleviate this problem is to thicken the MLSS prior to decanting in the digester basin. Staff has looked into three systems to thicken the supernate liquor. The attached engineering report was prepared by the staff and concluded that the Rotary Drum Thickening (RDT) process is the best choice for the County. In 1994, the City of Vero Beach, along with several municipal utilities in the state, pioneered by using the RDT process and were able to reduce their sludge truck hauling costs by approximately 66 - 73 percent. The City of Vero Beach has reduced posts from S1.30,000/year to 535,000/year. The Board of County C3mmissiorers voted to install an RDT system in the South County Wastewater Treatment Plant expansion project to achieve this same goal. (It is resentl•: ,.rider ..cnstructicn. However, due to the nature of this thin supernate liquor, the staff will not recommend proceeding directly into buying the equipment, but the staff ._opcses to pilot test the equipment to assure us of successful cer_` :_:.,ante . Upon the success of the pilot testing, the staff would present the report to the Board with a proposal and recommendation for the direct purchase of the same equipment from Hycor, a company that supplies RDT equipment for the South County Wastewater Treatment Facility. This standardizes the type of equipment and could reduce the spare parts budget in the years to come. The approved cost of the South County Wastewater Treatment Facility for this RDT was $196,000.00 (see attached excerpt of contract). However, staff will negotiate the price with the contractor, Wharton -Smith, for installation; i.e., concrete pad, platform, electrical connections, etc., and the staff will bring a recommendation back to the Board for approval after a negotiated price has been reached. The projected savings in truck hauling, truck maintenance (parts and manpower), treatment cost, and savings in energy consumption costs are over 568,650.00/year. This does not include the cost for biological stress in the process that could impact the system and cause a violation of the operation permit conditions. This yearly savings will pay back the capital construction cost in less than four years. The projected capital construction cost is detailed as follows: 1. Estimated capital construction cost $220,000.00 (includes direct purchase, in-house equipment, and construction) Engineering design, which includes $ 26,000.00 Outside structural design $5,000.00 (upper limit) Soil testing $1,000.00 (upper limit) Electrical design $5,000.00 (upper limit) 3. Permits (FDEP and IRC building permits) $ 4,000.00 4. Contingency $ 20,000.00 Estimated total capital project cost $270,000.00 This project will be funded through the Utilities impact fee fund. January 20, 1998 61 boa 104 PAGE187