Nature’s Course

An innovative rehabilitation of a sand filter enables a high school in central North Carolina to regain regulatory compliance

Effluent discharging into Bear Creek from the above-grade sand filter at Chatham Central High School in Bear Creek, N.C., was noncompliant, averaging 8 mg/l BOD and 7 mg/l ammonia. Fines from the state Division of Water Quality (DWQ) cost the school $120,000 per year.

After two firms failed to solve the problem, the Chatham County Board of Commissioners called C. Halford House, Ph.D., president of Integrated Water Strategies in Apex, N.C., known statewide as an innovator in wastewater treatment.

House consulted with Darrell Cole, the board’s licensed operator, who maintained the system and sampled effluent twice monthly. House and engineer Vic D’Amato ­analyzed the performance data and concluded that the sand filter was too cold and wet.

“It’s a classic challenge to many people, because the required biological processes don’t work well in those conditions,” says House. “We basically made the system drier and warmer.”

The low-energy solution House designed uses expanded slate aggregate and plants to filter effluent. The rejuvenated sand filter brought BOD and ammonia levels into compliance immediately.

Site conditions

Because the repair used existing components, soils and test pits were unnecessary. Bear Creek, 20 feet from the system, frequently overflows, flooding the sand filter.

System components

House worked with the system’s original 10,000 gpd design. Its major components are:

• Existing 1,000-gallon, one-compartment grease tank.

• 2,000-gallon, one-compartment grease tank. Concrete tanks by Stay-Right Precast Concrete Inc.

• Existing 10,000-gallon, two-compartment concrete septic tank.

• Biotube effluent filter from Orenco Systems Inc.

• 3,000-gallon pump tank bottom connected to existing 8,000-gallon primary dosing tank with Myers 1.5 hp ME series duplex pumps and controls from Zoeller Pump Company.

• Venturi aerators from Mazzei Injector Company.

• 42- by 42-foot concrete block primary and secondary sand filters with wetland plants and 5/16-inch expanded slate aggregate from Carolina Stalite Company.

• Existing Sanitron UV (Atlantic Ultraviolet Corp.) disinfection unit from Fluid Dynamics International Ltd.

System operation

A 4-inch high-strength PVC line runs from the kitchen to the grease interceptor, then joins the 4-inch PVC Schedule 40 sewer pipe discharging to the septic tank. Effluent flows by gravity through the filter and into the primary dosing tank.

Alternating pumps in the tank cycle every two hours. A 30-second dose sends 25 gallons to the primary sand filter through a 1.25-inch PVC line. It takes about six hours for the dose to trickle through the layers of aggregate and washed stone, then flow into the second pump tank.

Batch loading to the secondary sand filter is identical to the first. Venturi aerators in the supply manifold to the beds increase the oxygen level. Effluent is pumped 20 feet to the disinfection unit in the maintenance building and discharges into Bear Creek.

“The expanded aggregate has a 97 percent void ratio with 50 percent more surface area than sand or gravel for higher ammonia nitrogen conversion and BOD and TSS removal,” says House. “It won’t clog because of its high-hydraulic conductivity, yet it retains 8 percent of the moisture for growing vegetation.

“As plants grow, they leak a little air and sugar from their roots, which attracts the microbes that transform organic materials and ammonia nitrogen into harmless gases. The aggregate and vegetation also introduce air into the system and help keep it warmer by acting like a solar sink.”

Installation

Over summer vacation, D’Amato and House supervised Cole, Chatham County Schools employees and subcontractors in repairing the system. “It was important that no waste entered the sand filter, and we needed the parking lot for staging materials,” says House. Workers excavated the old tanks to check their integrity. The grease interceptor was cracked and some pipes were clogged, contributing to the system’s problems.

The septic tank, however, passed the watertight test. “This is a critical issue in repairs,” says House. “Too often, installers make the mistake of using a septic tank that works only partially, causing the whole system to malfunction. Replacing questionable septic tanks is usually money well spent.”

The trickiest part of the installation was removing the top two feet of sand and gravel in the two sand filter boxes without damaging the masonry block or ripping the polyethylene liners. It took half a day to excavate 9,000 cubic yards of material.

Distributing the gravel and 24 inches of aggregate was also delicate. The light, fluffy material was installed in 8-inch-deep layers, then compacted 20 percent. “Certified sand with the right size distribution for sand filters is hard to find,” says House. “We therefore use the aggregate routinely because it has a 97 percent void ratio with 50 percent more surface area than sand or gravel.”

The increased porosity reduces the size of the treatment area while achieving higher ammonia nitrogen conversion and BOD and TSS removal. High-hydraulic conductivity prevents clogging, yet retains moisture for growing vegetation.

Workers sleeved 1.25-inch PVC low-pressure distribution lines inside 4-inch PVC pipes to keep the plant roots from plugging the 7/32-inch holes drilled three to five feet apart. The orifices point up, and the pipes slip out for maintenance. Each filter has two rows of 18 laterals, 20 feet long on 2-foot centers with observation ports in the middle.

The pipes were bedded in washed stone and covered with two inches of 1/4-inch pea gravel to distribute weight and keep people from leaving footprints or compacting the aggregate below. Gardeners planting ground cover, flowers and shrubs removed the peat pots and soil around the roots before setting the plants on 24-inch centers between the pipes. The roots, anchored in the aggregate, grow hydroponically.

Plants were a major part of the regulatory conversation, since the established practice requires maintenance entities to remove vegetation growing on sand filters. Although skeptical, the DWQ permitted House to prove that plants were beneficial.

“Plants are important because the system is inactive during summer vacation,” says House. “Instead of the microbes going into starvation mode and their population plummeting, the plants thrive in summer, maintaining the microbial population at a high number and creating a very stable treatment process.”

In winter, the tops of some plants go dormant, but the microbes on the roots flourish in the toasty environment created by the aggregate’s solar gain. The vegetation and aggregate also act as insulation. A proprietary passive aeration system of pipes and gravel ensures ample aerobic conditions.

Workers built a 10- by 10-foot maintenance building to house the disinfection unit and mounted the pump control boxes to an outside wall. A backflow valve prevents floodwaters from entering the 4-inch PVC discharge pipe when Bear Creek overflows. After remediation, the BOD and ammonia levels dropped to 2.5 mg/l and 0.5 mg/l.

Maintenance

Maintenance is minimal. Twice a month, Cole samples the effluent and checks the distribution inspection ports for root problems. He created a tool that slides into the sewer pipe to extract roots, but used it only once in three years. Annually, Cole removes and inspects the distribution pipes.

Raking the surface of the sand filter is unnecessary, and pruning plants is an aesthetic choice. “In 18 years of doing this, we’ve never replaced the media,” says House. “The reason is balance. If we don’t overload the microbes, they turn all the organic materials into carbon dioxide, which plants use in photosynthesis. No organic material remains to clog the media.”



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