Faced with a giardia contamination of its two reservoirs, the Auburn Municipal Authority, Schuylkill County, Penn., entered into a consent order and agreement with the Pennsylvania Department of Environmental Resources to meet the filtration standard of the Surface Water Treatment Rule or provide an alternate supply.
After evaluating several alternatives, the water authority serving 940 people selected a diatomaceous earth (DE) system using vacuum leaf DE filters. The system requires no pretreatment of the raw water and is relatively simple to operate.
Auburn’s raw water comes from two spring-fed reservoirs that see average turbidity readings of 0.5 and 0.3 ntu respectively. Storm events can drive the raw water turbidity as high as 6 ntu.
Raw water flows through intake screens in the reservoirs and is piped down to the new filter plant about 30 feet below the intakes. Consumption ranges from an average of 70,000 gpd to a peak of about 100,000 gpd.
The raw water flows directly into two Mer-Made® vacuum leaf DE filters, with each filter having 120 square feet of filter area. Each filter has five leaves, 30 inches by 60 inches, with 24 square feet of filter area per leaf. The filter tanks are of FRP construction and the leaves are plastic with woven plastic filter cloth covers. This is a redundant system, with each filter capable of producing a maximum of 170,000 gpd. The filters may be operated simultaneously at a rate of 340,000 gpd.
At the beginning of a cycle, the filter going into service is precoated with 30 pounds of coarse grade DE (Celite 545). During filtration, body feed DE of the same grade is added whenever the filter is on stream, but not when the filter goes to recycle. Filters will go on and off of recycle as the 200,000 gallon storage tank signals for water. On average, a filter will be on stream for 14 hours per day and on recycle for 10 hours.
With raw water turbidity averaging 0.375 ntu, cycles can average up to 14 days. During periods of peak turbidity, they can be as short as four days. When one filter comes off line, the other is ready to be precoated and go on line.
At the end of each cycle (signaled by a pressure differential of about 12 inches of mercury vacuum) the cake is drained from the filter to a settling basin. The leaves are hosed with clean water and that water also goes to the basin. The basin has a sand and gravel bed and is approximately 12 feet long, nine feet wide and 3.5 feet deep. The spent cake and water enter one end. The water passes through the sand and gravel to a collector pipe system in the bottom of the basin and then into a small clear well. The water is used in a sprinkler system discharging on the grounds.
The spent DE cake collects in the basin. Since average DE consumption including precoat and body feed is only eight to 10 pounds a day, removal of spent DE from the basin is infrequent. When necessary, it is shoveled up and spread on the grounds and wooded areas as a soil conditioner.
The supervisor of the Auburn Water and Sewer Department normally spends two to four hours at the plant in the morning to add DE as needed, change charts, record data, clean feeder pumps and prepare filters for a new cycle, when necessary. He can also check the operating condition of various key components of the system at any time by telephone.
The entire facility was built for slightly less than $250,000. The Auburn Municipal Authority obtain a 20 year loan for construction from the Pennsylvania Infrastructure Investment Authority at 1 percent interest for five years and 1.6 percent for the remaining 15 years.
