The City of Rawlins, Wyoming, chose a pressure leaf filtration system for its new drinking water plant back in 1984. After 15 years, the plant is still operating with its original stainless steel pressure leaves.
A safe, reliable supply of water is a priority for the citys 10,000 residents, according to Leroy Graham, Chief Operator of Rawlins Water Supply & Treatment. Rawlins obtains its water from four major sources: the Sage Creek springs, Beaver Creek springs, Miller Hill wells, and the North Platte River. Turbidity levels typically range from 0.5 NTU to 5 NTU. Most of the citys water comes from the springs. Because of the chance of Giardia and silt contamination, the N. Platte River and the Miller Hill wells are reserved as emergency resources.
When the plant was being designed, target flow was set at 1 gpm per square foot of filter. Engineering firm CEPI worked with Roclan Construction to make sure that the finished water turbidity was 0.05 to 0.1 NTU.
With the assistance of a consulting-engineering firm in Casper, WY, it was decided to use a pressure leaf filter because of its effectiveness in removing Giardia and its small footprint. Once this decision was made, competitive bids for a pressure leaf filter were solicited and Aqua Care/FSD was selected based on cost.
The drinking water plant was built next to Peaking Reservoir and filters all incoming raw water. The water is then chlorinated and fluoridated and sent to town for domestic use. The plant was initially designed with three Diatomaceous Earth pressure filters capable of filtering 2 mgd each. A fourth filter was added in March 1996, increasing capacity to 8 mgd. Each filter is 6 ft. x 14 ft. long and holds 36 stainless steel leaves.
Although originally designed for a DE system, the Rawlins plant eventually switched to perlite media which is a byproduct of soda ash or Trona milled in Green River, WY. Perlite is less expensive and easier to obtain, Graham said. "We have soda ash plants located about 150 miles from us, and thats where Perlite comes from," he said.
At the time the plant was built, there were only three DE plants in the US, so DE was cost-prohibitive. Perlite works nearly as well and is less dusty, Graham said.
"It does an excellent job," he said.
The plant is operated by a SCADA system - operators are able to go home after an 8-hour shift and leave the system on automatic control, but the operator on call takes a laptop computer home to continue monitoring the system. The filters have to be cleaned about once a week, or 168 hours of filtration. Staff applies new perlite at this time. Every six months they steam clean the filter leaves. The original filter screens are still on the unit from the time of installation.
Midwest Assistance Program tested the Rawlins plant for EPA in March 1998 for the MPA & PSA. The results showed a 4.7 log removal of cryptosporidium. The plants total log credit was 8.5 for disinfection and removal. The EPA Surface Water Treatment Rule requires a minimum of 3-log reduction (99.9 percent) of Giardia and Cryptosporidium for surface water and for well water under the influence of surface water.
Plant operators plan to install in a pretreatment system in a year or so, Graham said. Population growth has forced the city to draw from the well water reservoir and operators have encountered taste and odor problems caused by algae in the reservoir. In anticipation of new rules and regulations focusing on drinking water turbidity, a Kruger Actiflo system is in the design stages for the Rawlins plant.
