The contractor shall remove material from the digester, screen, macerate, and then dewater it using a centrifuge and polymer to a continuous minimum of 24 percent solids. Polymer is to be supplied by the contractor. Digester material shall be screened (mesh on the screen shall be no larger than ½ inch) and then macerated to shred any material that passes through the screen. The contractor shall dewater the digester contents into a watertight container or dump truck supplied by the contractor. The Contractor is also responsible for selection of, and approvals into, appropriate disposal site(s), transport and disposal of the dewatered material and debris. After the disposal of the material, the contractor will provide the WRBP with a copy of the weight slips for each load and/or manifests to confirm compliant disposal of all materials. Dewatering of anaerobically digested sludges may result in high recycled loads to the headworks of the treatment plant which can rapidly impact the treatment process, severely degrade effluent quality, and cause the WRBP to violate its NPDES permit. The recycled flow is high in ammonia and BOD and, depending upon the capture rate of the contractor’s dewatering equipment, recycled flow may also be high in suspended solids. The WRBP has experienced these problems in past contract dewatering projects or when large quantities of supernatants are drawn from the digesters and recycled through the plant. The WRBP’s concerns with the quality of the contractor’s recycled flow are noted to make the contractor aware that the WRBP will be closely monitoring the capture rate of the dewatering process and that, on occasion, it may be necessary for us to request that the contractor take steps to reduce their recycling load to the treatment plant. These steps may include requirements such as operating the dewatering equipment to increase its capture rate or reducing the number of hours each day the contractor processes solids from the digester. Exercise of these steps shall not affect the contract cost or completion date. The contractor shall use a centrifuge to dewater the digester contents using polymer into a watertight container or dump truck supplied by the contractor. Dewatering equipment (centrifuge) shall provide a minimum of 24% solids and at least 95% solids recovery; however, the maximum concentration of Total Suspended Solids (TSS) in the recycle flow will be no greater than 250 mg/l. Once the contractor begins dewatering, samples of the digester’s solids will be taken by the WRBP and tested to verify compliance with the current federal and state biosolids quality standards and the terms of the contract. Samples may also be required by the contractor’s disposal location(s). The material will be collected by the contractor as it is being dewatered by the contractor and transported/disposed of off-site. Only dewatering using centrifuges will be allowed under this contract.
