Just the Right Recipe

A pretreatment system including microbial inoculator generators provides a solution for a restaurant and apartments on a tiny brookside lot.
Just the Right Recipe
Two of three Fusion purification units sit in the block enclosure. The small space between the building and retaining wall contains the two grease interceptors. The two septic tanks and pump tank are opposite the purification units on the other side of the wall. (Photos courtesy of Onsite Engineering)

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Slow drains and backups at a restaurant and pub in Amenia, N.Y., attracted the attention of regulators. When frequent septic tank pump-outs failed to provide relief, the owner closed shop. The three-story building also had three apartments.

A system upgrade was the only solution. Officials suggested the owner contact Onsite Engineering in Syracuse, which specializes in complex designs. "The location would never meet today's construction standards," says owner Eric Murdock, P.E. "Because it had a 30-seat restaurant and 20-seat bar, we could grandfather it into the regulations."

The building used half the lot, leaving a 62- by 32-foot space for the pretreatment tanks and the drainfield, with the toe 10 feet from a trout stream. "When the Dutchess County Health Department saw my drawings, they said the drainfield was only 20 percent of what was required to handle the design flow," says Murdock. "The prescriptive code structure doesn't acknowledge pretreatment."

A year passed before the state approved his solution, which called for eight tanks including two grease interceptors, septic tanks, two microbial inoculator generators (MIG), three purification units, and gravelless chambers.

Site conditions

Soils are sand and gravel with a percolation rate of 6 to 12 minutes per inch. The 0.19-acre lot has groundwater 30 inches below grade. Cascade Brook, a protected trout stream, defines the west boundary of the property.

System components

Murdock designed the system to handle 2,200 gpd. Onsite Sales and Service in Syracuse (also owned by Murdock) provided the major components. They include:

1,500-gallon concrete grease interceptor (All tanks are traffic-rated and made by Woodard's Concrete Products, Bullville, N.Y.)

1,000-gallon grease interceptor

2,000-gallon, two-compartment septic/recirculation tank with four WW4 effluent filters from Clarus Environmental, a Division of Zoeller

1,500-gallon, two-compartment septic/pretreatment tank, each with a WK-80 White Knight MIG from Knight Treatment Systems

2,000-gallon dose tank with duplex 0.3 hp pumps from Clarus

Three Fusion 800 gpd purification units from Clarus

180 feet of Equalizer 36 chambers from Infiltrator Systems

Duplex time-dosing control panel from Clarus

System operation

Kitchen waste flows by gravity through a 4-inch PVC lateral and through both grease interceptors before entering the septic tank. Wastewater from the public restrooms and apartments flows through a separate 4-inch lateral to the septic tank, discharging to the pretreatment tank. A 4-inch-thick piece of rigid foam insulation caps the laterals for frost protection.

The compartments in the pretreatment tank each have a 27.5-inch-high by 16-inch-diameter high-density polyethylene MIG column inoculated with a bag of ISO 500 bacteria.

The units are fixed above two fine-bubble membrane diffusers.

An air pump (HiBlow USA) introduces 3.5 cubic feet per minute at 2 psi at the bottom of the columns. The steady stream of fine bubbles mixes and distributes the microorganisms throughout the tank, where they digest organic matter. Each column reduces 6 pounds of BOD daily. The offgas from the tank vents through a 4-inch line to a 20-foot-square bark bed that controls odors.

Effluent from the pretreatment tank flows by gravity into the dose tank. Every 20 minutes, alternating pumps lift 40 gallons to a distribution box that splits it four ways. Three outlets drain to the purification units. When the pump shuts off, water in the distribution box runs back through 1.5-inch PVC piping to the dose tank.

The fourth line drains to the septic tank. "For the price of the pipe, we attained 25 percent recirculation using the same energy that lifted the effluent for dispersal," says Murdock. "Another benefit is denitrification, achieved by returning oxygenated water to the septic tank."

The purification units sit in series inside a 24-inch-high retaining wall filled with soil for the drainfield. Each four-chamber fiberglass module constantly recirculates effluent from the third compartment (aeration) to the first (septic). The fourth chamber temporarily stores water before it flows by gravity to the drainfield's main distribution box. This box simultaneously doses distribution boxes at three zones of three 20-foot laterals.

"We installed the distribution network to deal with future situations," says Murdock. "If part of a zone becomes saturated, we can open the D-box and cap off the trenches under stress."


The original system had a 1,500-gallon concrete septic tank, grease interceptor, and concrete leaching galleries installed partially below the water table and under the parking lot. Kect Construction in Patterson, N.Y., decommissioned the components and lot to make room for the new system.

"Access was so tight between the building and drainfield area that we worked backward, digging holes and setting tanks one at a time," says Murdock. "The structure's laid-up stone foundation wasn't in fantastic shape, so we were very cautious not to dig too close to it."

The crew dewatered the 8- to 10-foot-deep excavations using a vacuum line inside a 12-inch slotted pipe in a small pit. Tanks arrived in halves and were set on 8- to 12-inch-deep gravel beds. The grease interceptors went alongside the building, and the septic tanks and pump tank were placed in a row that would abut the head of a retaining wall. The work took four days.

The county health department requires vertical separation to the seasonal high water table, so Murdock designed an elevated bed inside a U-shaped retaining wall with 36-foot header and two legs, 60 and 62 feet long. To support the three-tiered wall made of 2-foot-high by 2-foot-wide blocks each weighing 1,600 pounds, workers installed 48-inch-wide by 12-inch-deep compacted stone footings.

Where the pump, recirculation and air vent lines penetrated the footing, the crew laid a 6-inch pipe to sleeve the lines, then topped them with 4 inches of rigid foam insulation. They built the wall using a 3-ton mini-excavator. "The wall toenails (angles) inward 2 feet to prevent any effluent from escaping," says Murdock. "We also lined the interior faces of the blocks with impermeable membrane to prevent seepage."

The crew constructed a clay berm at the foot of the enclosure near the stream. "The precast blocks were expensive, and we didn't want to use them where they wouldn't be seen," says Murdock. The work took a week.

Using the excavator, the crew distributed 30 inches of soil in the enclosure and set the purification tanks at the head. They were careful not to compact the soil in the drainfield area. After plumbing the tanks, they distributed 12 inches of gravel, sloping it 3:1 into the berm. They placed the chambers, backfilled with another 12 inches of gravel sloped at the toe, and topped the field with 6 inches of topsoil. "That left one tier of the wall below grade and two tiers above it," says Murdock.

Meanwhile, workers ran the vent pipe from the pretreatment tank through a sleeve in the wall, then through two chambers to distribute the vapors evenly in the bark bed. They covered the chambers with 2 inches of bark on which the moisture would condense. The work took another week.

The crew also installed an electric shut-off solenoid valve on the public water supply to the restaurant. "The pump tank has one day's storage capacity," says Murdock. "During power failures, the pump won't run. Stopping water to the restaurant prevents 1,450 gpd from entering the system and causing overflows." The apartments still have water.


Onsite Sales and Service holds the maintenance contract. Twice a year, technicians check the pretreatment and purification units. "It's a low-maintenance, power-efficient system," says Murdock. "The only energy users are the air pump, dosing pump and control panels."


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