Right System, Right Place

The nation’s first installation of a sophisticated aerobic treatment system is performing well at a mobile home park in Michigan

Effluent was ponding in the outdated, overused drainfield of a 44-unit mobile home park in Allegan, Mich., and raw sewage flowed into a swamp forest draining into Swan Lake. The owner called maintenance provider Rod Carroll of Wray’s Septic Tank Co. in Allegan to pump the 20,000-gallon septic tank twice a year until health officials gave him an ultimatum to replace the system in three months.

Wray’s won the bid, and Carroll worked with Randy Rapp, P.E., of Exxel Engineering Inc. in Grand Rapids, to design the system. “We had no room for a replacement drainfield, but the septic code allowed us to build one on top of an existing bed,” says Carroll.

“Nevertheless, we needed variances on everything, including the 100-foot water setback. Our site had water lapping against the tapers of the berm.” To treat the effluent, the men agreed on a highly advanced purification tank, called the Fusion treatment system, from Clarus Environmental. The equipment fit easily into a very small space.

The installation was the first large commercial Fusion system in the country. Its performance is still being documented, but the sophisticated aerobic treatment unit is functioning perfectly.

Site conditions

Soils are sandy with the water table 18 inches below grade. The system is on a 178- by 120-foot man-made berm in a swamp forest eight feet below the park.

System components

Rapp designed the system for a hydraulic loading handle of 5,000 gpd. Its major components are:

• Existing 20,000-gallon three-compartment concrete septic tank retrofitted with WW4 effluent filter from Clarus Environmental.

• Clarus Environmental treatment package from ETNA Supply, Grand Rapids. It includes a Tru-Flow effluent splitter box, two 2,400 gpd ZFL Fusion treatment units with 24-inch risers, two N153 high-head 1/2 hp Zoeller effluent pumps, and a Zoeller duplex control panel.

• 1,500-gallon concrete dosing tank with risers to grade from Wray’s Septic Tank Co.

• Two 32- by 78-foot low-pressure drainfields in six zones.

System operation

The 4-inch laterals of east-side homes tie into a 6-inch main discharging to a 10,000-gallon two-compartment septic tank with dosing chamber. Effluent is pumped 800 feet to the existing septic tank and new system on the west side of the park.

West-side homes have identical plumbing, except that wastewater flows to the primary septic tank, then to the 18- by 18- by 12-inch-tall distribution box. When water enters the box, a tray splits the flow equally between the two treatment systems. The tray can be leveled from the surface if the box shifts later due to settling.

Secondary treatment occurs in the Fusion tank’s four chambers. The sedimentation compartment acts as a septic tank. In the anaerobic chamber, microorganisms on a fixed 4-inch spherical-skeleton type media digest nutrients at the same time suspended solids are captured. The media provides a stable environment so that bacterial colonies are unaffected by high-flow events.

The aeration chamber, with an upper aeration section and a lower filter media section, is filled with 3/4-inch-long hollow cylinders. In-coming air keeps them agitated. Microorganisms growing in a thin biofilm on the cylinders remove impurities. Residual suspended solids are captured by the stationary filter media in the chamber’s lower portion.

Because sludge develops rapidly in the third chamber, the filter is backwashed in 5- or 10-minute cycles twice daily.

Air vigorously stirs the media to break up accumulated materials. An airlift pump transfers wastes back to the first chamber for further digestion and to aid in denitrification. The anaerobic and aeration chambers have risers.

The fourth chamber temporarily stores treated water leaving the aeration chamber before it flows to the dosing tank. The pumps run 7.5 minutes, five times per day, sending 511 gallons (23.8 gallons are drained back) to the absorption beds. A Hydrotek 6606 valve in the center of the field splits the flow through a 1.5-inch header to the six zones.

Each 10- by 39-foot zone has five 1.25-inch Schedule 40 PVC laterals 37 feet long on 2-foot centers. The 1/8-inch orifices with shields are spaced three feet apart with every fourth hole down to drain the lines and prevent freezing. Every lateral has a turnout and cleanout. The effluent meets secondary standards of 30 mg/l BOD and TSS.

Installation

Installation began in early April. Carroll’s crew erected a soil erosion fence around the berm, cleared the perimeter of brush, and stripped the topsoil down to the old beds. They inspected the septic tanks for water-tightness, then replaced the baffles and installed 36-inch Orenco risers.

The men mounted an effluent filter in the third compartment of the primary tank, located in the center of the berm with 2,000-square-foot drainfields of 6-inch clay tile and stone on either side.

Michigan’s winter weight restrictions were still in effect. “We got a variance from the county road commissioner and hauled 2,300 cubic yards of sand as fast as possible,” says Carroll. “The spring thaw was days away, and at that point our trucks would break through the asphalt.”

Drivers dumped their loads by backing 10 feet off the park’s main road to the front of the berm. Using a John Deere 650 bulldozer, Carroll pushed 2-foot-deep layers of sand 120 feet to the far end of the system and worked forward. In less than three days, the crew elevated the berm four feet, bringing it level with the road and establishing a 42-inch separation between drainfields.

Hydraulic pressure from the tons of sand on the failed beds pushed fluid back to the septic tanks. Wray’s two 3,600-gallon vacuum trucks pumped them for three days, then as required to keep any discharge from reaching the absorption area. The pumping action of the bulldozer crossing the ground brought up effluent that the trucks dewatered.

“We installed the treatment units 20 feet after the septic tank so we could add something later if needed,” says Carroll. “The 7-foot-deep excavations broke through the existing drainfields, and we had to dewater the holes.” The men backfilled them with 6 inches of pea stone on top of 12 inches of saturated but solid native sand, then set the plastic fiberglass resin units.

“We were two feet below the water table, and the holes filled immediately if we stopped pumping,” says Carroll. “To prevent the treatment tanks from floating, we filled them halfway with water, then laser-leveled them. Sitting flat is crucial to how they operate.” When the tanks were level, workers backfilled with stone up to the mold line, filled the compartments with water, and finished backfilling.

Excavation for the dosing tank uncovered a nasty layer of the old bed, so Carroll installed the tank on six inches of sand before connecting it to the treatment units with 4-inch PVC lines. “Dewatering this hole wasn’t as bad because the tank is two feet shorter than the Fusion units,” he says. Workers backfilled the tank, then mounted the pumps, float poles and control panels.

To minimize impact on the low-pressure-dosed beds, the crew hand-dug 4-inch-deep trenches in 12 inches of washed and leveled 6-A stone, then laid the piping and orifice shields. “Every lateral has a cleanout on both ends, so each drainfield has 60 caps and six observation ports,” says Carroll. “The area looked like a mushroom farm.”

The fields were covered with geotextile fabric, 12 inches of sand, and 4 inches of topsoil. Because of the layout of the beds and cleanout ports on the end, Carroll had to place the topsoil with the excavator. “I’d take a pile off the road, set it on the other side of the cleanouts, move the excavator, then move the pile over again,” he says.

With the May deadline approaching, Carroll discovered that he needed a new electrical service and meter for the control panel. He hired Redfern Enterprises Inc. of Wayland to directional bore 60 feet under the road, while his crew hand-dug 60 feet between the homes to the nearest transformer and ran the conduit. Esper Electric Ltd. of Kalamazoo completed the electrical work.

Maintenance

Wray’s Septic Tank Co. has the three-year maintenance contract. “It’s a low-maintenance system with no media to change or remove,” says Carroll. “But because it’s the first commercial system of its kind and we’re still adjusting backwash, recirculation, and flow rates, the state and Randy want it monitored monthly and effluent samples taken for two years.”

Normal twice-per-year maintenance includes monitoring sludge levels in the anaerobic compartments, checking the aerators and recirculation valves, and opening the air flush valves fully for a minute. The fixed-film media harbors the bacteria during this procedure to ensure uninterrupted treatment.

The technician also checks the diaphragm air pumps, floats, effluent pumps and control panels. He cleans the effluent filter and pumps the septic tanks and treatment system compartments as needed.



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