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The management and ultimate disposal of water treatment plant residuals is an important process consideration in surface water treatment plants. Many utilities can no longer return residuals produced as part of the water treatment process to the raw water source. In Mesa, Arizona, a new Solids Handling Facility was constructed at the Val Vista Water Treatment Plant (WTP). The Val Vista WTP is a 220 mgd capacity surface water treatment plant that provides drinking water to the residents of both the Cities of Phoenix and Mesa. The Solids Handling Facility was designed to continuously process a minimum of 225,000 pounds dry solids per day dry and to provide on-site storage of thickened residuals and dewatered cake, if necessary. Processes used as part of the Solids Handling Facility include filter backwash equalization and clarification, residuals thickening, residuals storage, residuals dewatering utilizing centrifuges, and cake conveyance using shafted screw conveyors. Dewatered residual solids are disposed of off-site at a local landfill and recovered water is recycled to the head of the plant.

This paper presents operating data and reviews equipment adjustments that were made during the commissioning of centrifuges, conveyors, and auxiliary equipment at the new Solids Handling Facility. The new Solids Handling Facility construction project was started in 1998 and was completed in 2001. Testing was conducted by the commissioning team which included Val Vista WTP staff, the Engineer, General Contractor, and equipment vendors. Test results were used to optimize the operational parameters of the individual equipment (i.e. polymer dosage), as well as the solids handling processes as a whole. After initial start-up and troubleshooting, the centrifuges and conveyors all performed as per the design specifications. The average cake solids produced by the centrifuges during the optimization portion of the commissioning program was 29.7%, with an average residuals feed solids concentration of 5.0%. During the same period, the average active polymer dose was 5.2 pounds polymer per dry ton solids producing an average solids recovery of 98.6%. Operating data was averaged and plotted trends indicated that:

Cake solids and solids recovery were inversely proportional to the residuals feedrate through the centrifuge.

Cake solids were not significantly affected by polymer dosage. However, polymer dose was the most important factor for controlling solids recovery.

A minimum polymer dose of approximately four pounds active polymer per ton dry solids was important for minimizing “cake splatter” in the loading area, and for optimizing the conveyance characteristics of the cake.

Sudden or large process changes should be avoided minimize degradation in cake and centrate quality.
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Document Type: Research Article

Publication date: 2003-01-01

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