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Challenge
The city of Versailles, Ind., operates a surface water treatment plant for their drinking water supply. The water source is a reservoir that has variable organics and solids loads depending on weather conditions. The turbidity is typically between 25 to 100 NTU, but can increase to more than 400 NTU because of rain events and remain there for more than one day. Prior to 2007, they used two conventional packaged treatment units consisting of two-stage flocculation, tube settling and granular media filtration.
During normal turbidity conditions, the units performed to acceptable levels. However, during spiking conditions, performance deteriorated and the filter run times were reduced to unacceptable levels. In addition, the existing package treatment units were showing signs of corrosion, requiring repair or replacement. The city hired the consulting engineering firm of R.E. Curry, Inc. to investigate new treatment options to address these concerns as well as increase treatment capacity.
Initially, the investigation focused on rehabilitating the existing units to minimize building disruption. A new treatment process, the Trident HS design from Siemens Water Technologies , was considered as the best treatment alternative. This design consisted of two-stage clarification and media filtration, followed by ultraviolet (UV) disinfection. The proposed plan would involve removing the interior components of the existing tanks, repairing the tanks and adding new treatment components. This would allow one unit to remain in service while the other was rehabilitated. Once rehabilitation was completed, the remaining tank would be rehabilitated.
The cost of new stainless steel tanks was also compared to rehabilitating the existing steel tanks. The difference for the new tanks was close enough to the original estimate that the city decided to proceed with new stainless steel tanks. The project shifted from rehabilitation to replacement, providing the city new corrosion-resistant tanks for their plant.
The Trident HS system process consists of tube settling with external sludge recirculation for removal of settling solids, followed by upflow adsorption clarification through a bed of buoyant media, and lastly, mixed media filtration for final polishing. All three treatment stages are combined in packaged treatment units. This multi-barrier approach allows the system to dampen variable contaminant loads and increases system net production with a smaller footprint. The smaller footprint, compared to the existing treatment units, allowed the UV reactors to be placed within the bounds of the original equipment pad.
Results
The new treatment system was designed for a flow rate of 575 gal per minute using a two-tank package treatment system. The two-tank system was required as the first new tank was installed adjacent to one of the existing tanks. Once the first unit was fully installed, it was brought online to provide water to the city while the old system was removed. This space was then used for the installation of the second tank. Minimal site painting was required because the new tanks were constructed of stainless steel.
The first unit was placed online in December 2006. Shortly after, the raw water turbidity spiked to more than 425 NTU after a rain event. During this spike, the filtered water turbidity remained less than 0.1 NTU.
To date, the new Trident HS treatment units have provided exceptional treatment results under a variety of water quality conditions. Multi-barrier treatment and stainless steel construction will allow the plant to address current and future regulations for years to come.
