District Switches to Automated pH Adjustment System
The selection of plastic pumps has played a role in the successful operation of a new, automated pH adjustment system installed by the Harpeth Valley Water Treatment facility in West Nashville, Tenn.
The Harpeth Utilities District is a nonprofit public utility chartered to meet the water and wastewater needs of more than 11,000 water and over 8,000 sewer service customers. It pumps more than 4 billion gallons of clean water and 2 billion gallons of wastewater per year, has 300+ miles of water and 140+ miles of sewer lines in place. Twenty-four wastewater lift stations are in active service. Water from its 24 mgd drinking water facility is wholesaled to seven other utility Districts and Municipalities.
According to Plant Superintendent, Ronnie Pan-nan, in late 1997 the district decided to switch to an automated system that neutralizes with 25 percent sodium hydroxide. The plant had been using a manual system using granular hydrated lime to maintain an alkaline pH. The installation was completed about a year later, and the operation has been up and running trouble-free ever since.
A critical part of the system involves two sealless, magnetically driven thermoplastic pumps which move corrosive caustic from the 12,500 gallon double wall storage tanks to a 500-gallon double wall day tank.
The pumps are tied into a computerized system that reduces operating and reporting time, provides more accurate and timely operational information and records, and materially improves the workplace environment. The use of double wall tanks and double wall piping has eliminated concerns about chemical leakage and made it unnecessary to build and maintain concrete containment basins normally required as a protection against tank leakage.
In selecting the pumping system, the district specified that the pumps be of sealless design, be magnetically coupled, have no steel components in contact with the fluid, and be constructed to withstand the adverse conditions with minimum maintenance.
After an extensive study of available products, the consultant engineers specified a thermoplastic design meeting ANSI process pump specification B73. 1. The district installed 3 hp Vanton Chem-Gard CGM-PY pumps. The 3 inch by 2 inch polypropylene flanged pumps have 6 inch keyed impellers. The pumps feature dual nonmetallic containment can assemblies. A Teflon sleeve and cap isolate the stainless steel shaft from the fluid.
The installation has proven so successful that plans are now under way to expand the facility to 36 MGD. The switch to liquid caustic has resulted in cleaner air at the facility, and happier employees and neighbors.