Residuals – Reality and Models Compared

Solids handling and disposal in the water treatment industry is a major concern to most utilities. Considerable time, effort, and expense are spent collecting, processing, and disposing solids. Therefore, it is important to be able to accurately predict solids production when designing a new water treatment plant or when considering changes to an existing one. Inaccurate predictions can cost a plant millions of dollars in retrofits to solids handling facilities and/or increased disposal costs.The purpose of this paper is to evaluate the accuracy of two well-known mathematical models for predicting water treatment plant solids production – the American Water Works Association (AWWA) and the Water Research Center (WRc- commonly used in Great Britain) methods. Model results were compared with documentation of residuals handling obtained from the 50-mgd William S. Warner WTP located in Fairfield, CT. The Warner facility treats water from the Hemlocks Reservoir using a dissolved air flotation (DAF) process and filtration. Residuals produced by the DAF process are removed as floated solids by skimmers and collected in a solids collection tank. The collected solids are dewatered using centrifuges. Cake produced by the centrifuges is conveyed to trucks and hauled off-site for disposal. Centrate is discharged to the sewer. Data indicating the quantity of solids produced were collected (1997–2001) and analyzed and a mass-balance was performed to verify the total solids produced. Raw water quality and chemical use data were also obtained from the plant. These data were input into the models and the model output was compared to actual plant data.Different aspects of these methods were analyzed and adjusted accordingly for the Hemlocks Reservoir water quality parameters. The WRc method accounts for the precipitation of organic matter in quantifying the residuals while the AWWA method neglects this contribution.The results show that both theoretical models overestimated solids production for the Warner WTP. Of the two models, the AWWA method produced more accurate results. This could be a function of this water supply and treatment scheme. The applicability of the theoretical models to waters of other types and qualities should be studied further. Examples of different outcomes and guidance regarding when to use which model are presented.