The Eastsound Sewer and Water District operates a small (15,000 gpd) recirculating gravel filter system on Orcas Island, Wash. — one of two treatment facilities that serve separate small communities.

When the filter became plugged with sludge after many years through the normal aging process, the district was faced with a costly remediation. Instead, the district staff devised a much simpler solution that cost less than 10 percent of what its engineers had recommended.

 

Simple system

The gravel filter serves the Town of Orcas Village, and the 160,000 gpd activated sludge plant serves the Town of Eastsound, also on Orcas Island. Both treatment systems are septic tank effluent pump (STEP) systems in which primary solids are removed in individual septic tanks owned by residents and maintained by the district.

There are 500 septic tanks in all, amounting to 1,100 residential equivalents. The recirculating gravel filter plant consists of two concrete boxes, each 30 by 50 feet and about 6 feet deep, filled with graded pea gravel about 3.5 mm in diameter with larger support rock on the bottom and drain rock about a foot deep on top.

A 10,000-gallon recirculation tank is pumped to the filters alternately, and the wastewater is distributed over the top. The water flows downward through the gravel and returns to the recirculation tank by gravity.

A floating ball acts as a valve and releases treated effluent to the contact chamber for chlorination and discharge to Puget Sound. The plant has been in operation since 1989 and produces excellent-quality water with BOD and TSS under 5 mg/l.

About two years ago, the effluent quality began to deteriorate suddenly, and within three weeks the plant was in violation of its permit. On examining the filters, the district staff found a layer of sludge about a foot thick clogging the gravel and causing the filters to go anaerobic. It first appeared that the only solution was to replace the gravel in the filters.

 

Seeking alternatives

Because of the district’s location on an island, the cost to transport the new material, replace the gravel and dispose of the old gravel and sludge was about $500,000. Given the size of the plant and the small number of connections (about 60 residential equivalent customers), that cost would have had a major impact on users.

Instead, the staff devised a solution requiring only three days of labor and about $30,000. It involved cleaning the media in place and returning the plant to operation. In the next two years, the plant has again produced excellent effluent consistently.

The solution was to construct a heavy steel cylinder 6 feet tall and 2 feet in diameter, closed on the top except for two valve openings. This assembly was bolted to the bucket of a large excavator, and the cylinder was connected to the district’s septic system vacuum truck. By pushing the cylinder into the gravel and applying vacuum, workers were able to fluff the gravel and draw the sludge out into the truck for disposal.

 

Useful to others?

The method required some experimentation and finesse on the part of the operator to get the gravel to loosen and stir, but once the technique was perfected, the process worked well. The wastewater and sludge removed was processed in the activated sludge treatment plant. Plant influent was used as the wash water for the process, so no discharge occurred during the cleaning.

There are many small gravel filter plants around the country, and they all eventually plug with sludge. The process Eastsound used to clean the gravel and repair the plant will be useful to others in similar situations.

 

About the author

Roy Light is superintendent of the Eastsound (Wash.) Sewer and Water District.