Recycle Options for Water Treatment Facilities – Full Scale Evaluation of a Novel Residuals Treatment Process

摘要

The Town of Cary, North Carolina is a regional water provider to customers within the townrnlimits of Cary, Morrisville, Apex, the Raleigh-Durham Airport (RDU), the southern portion ofrnResearch Triangle Park (RTP) and the western portion of Wake County. Potable water isrnprovided to these customers from the 40 MGD Cary/Apex Water Treatment Plant (CAWTP).rnAfter evaluating several different residuals handling options including plate settlers, gravityrnsettlers, and membrane treatment, the Town implemented a novel approach to residuals handlingrnthat became operational in 2010. The residuals handling facilities were designed to treatrnresiduals from a 64 MGD surface water treatment plant and include a dual chamber mixedrnequalization basin to receive combined wastes from sedimentation blowdown and filterrnbackwash. Discharge from the mixed equalization basin is pumped via variable frequencyrndriven submersible pumps through dual transmission mains to an integral clarifier/thickenerrnprocess. Thickened solids are dewatered using centrifuges while the treated residuals arerndischarged to a receiving stream with the ability to recycle to the head of the plant at a rate ofrn10% of the raw water flow.rnOperational data from the clarifier/thickener was evaluated to assess the feasibility of recyclingrnthe treated water to either the head of the plant or, with additional treatment, to the finished waterrnclearwell. Key parameters investigated included temperature, turbidity, pH, manganese, %rnsolids, alkalinity, hardness, chlorine residual, dissolved oxygen, color, TOC, iron, aluminum,rnTSS UV254, and coliforms. The impact of this recycle stream on plant performance andrnregulatory compliance will be discussed along with the technical and economic feasibility ofrnindirect and direct recycle. Data relevant to manganese cycling associated with this residualrntreatment process will be presented and discussed in the context of typical manganese cyclingrnexpectations. Material included in this presentation will be beneficial to utility mangers,rnengineers and operational personnel faced with the need to upgrade or expand their residualsrnhandling facilities or are considering recycling of treated residuals to augment water supplies andrnimplement sustainable design and operational practices.