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City of Lathrop UV� Proposed Water Filter Backwash Processing Description dI, ltw re�, 12/05/2018 Public health and safety require a continuous and reliable supply of potable water. ThRvy's er treatment process consists of a series of individual systems that must all perform adequately to ensure a water service is safe and uninterrupted. For example,the City's water filters must be backwashed regularly,which also requires the backwash recovery and processing systems to perform as designed. Any part of the water treatment process that does not function properly impedes the delivery of potable water to the public. The City of Lathrop has experienced recurring problems with their backwash solids dewatering process. Backwash solids need to be"dried"prior to transportation to a landfill for disposal. Currently,the City uses a filter press to reduce water in the backwash solids from approximately 2%to 25%,after which it is disposed of in a landfill. The filter press unit does not produce consistent solids, is labor intensive, and requires frequent repairs. Availability of repair parts and factory service are difficult to secure. Problems associated with the solids handling system adversely impact the City's ability to produce potable water,thereby resulting in a threat to public health and safety. The benefits of using drying beds rather than a filter press for solids dewatering include little to no energy use, little to no mechanical processes, and simplistic operations. All of these factors result in a more reliable backwash solids process. According to the EPA, "Air drying processes should be given strong consideration for all small to moderate sized communities, and for larger facilities in and and semi-arid climates when land is available. Sand beds,freeze dewatering, and reed beds can easily produce a sludge cake with 25 to 40 percent solids...". Process Modifications Changing the solids dewatering process will require significant modifications to the current process. Drying beds must be constructed of sufficient size and depth,with leachate collection systems, new piping and pumps are required to convey the waste stream to the beds, and additional conditioning of the backwash water is required to improve the efficiency of the potable source water/solids separation. The solids drying bed process design and operation is directly dependent on the solids concentration of the waste stream. For example, a waste stream with 2%solids requires approximately twice the bed area of a waste stream with 4%solids. This results in a near linear increase in operations cost to remove dried solids and maintain the drying beds. Chemically conditioning the waste stream with coagulants and/or polymers can increase the solids concentration. Further,chemical conditioning can 1 EPA Design Manual, "Dewatering Municipal Wastewater Sludges"