Taking the Small Lot Retrofit Challenge

A system using combined treatment and dispersal tubes provides a solution for a cottage on a small lakeside lot.
Taking the Small Lot Retrofit Challenge
Two Wacker Neuson 3-inch dewatering pumps run continuously as Justin Marvin backfills by hand around the 4-inch PVC lateral. (Photos courtesy of Mike Carbonneau)

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Owners of a two-bedroom seasonal lakefront cottage in Maidstone, Vermont, realized their functioning cesspool was leaching untreated wastewater into Maidstone Lake. They searched online for a contractor to replace it and found Mike Carbonneau, owner of Connecticut Valley Design in Littleton, New Hampshire.

“Today’s standards would never allow the lot to be developed,” says Carbonneau. “A drainage ditch was 10 feet from the dry well, which was less than the mandated 25 feet from the ditch.”

In addition, the Vermont Shoreland and Wetland Protection acts had become effective, and both had their own requirements. For grandfathered systems, the agencies allowed designers to submit a “best fix” that located replacement components where they met as many regulatory criteria as possible.

“With so little room to work, the Advanced Enviro-Septic system from Presby Environmental became my linchpin,” says Carbonneau. “It provides secondary treatment without the expense of an advanced treatment unit, and rates a 50 percent reduction in basal area over pipe-and-stone drainfields.”

Despite the ease with which the treatment dispersal tubes assembled, short daylight hours, inclement weather, high groundwater, and 80-mile round-trips to the quarry slowed progress and stressed the team.

Site conditions

Soils are gravelly fine sandy loam with friable granular structure, a percolation rate of 12 minutes per inch, and a seasonal high water table of 15 inches. Maidstone Lake defines the west boundary of the 0.23-acre lot.

System components

Carbonneau designed the system to handle 420 gpd. Major components are:

  • 2,000-gallon combination concrete tank (Calco)
  • STF-110 effluent filter (Sim/Tech Filter)
  • Champion 1/3 hp high-head effluent pump
  • 180 feet of AES treatment dispersal pipes, vented (Presby Environmental)
  • 30 tons of ASTM C-33 washed concrete sand with no more than 2 percent fines passing a No. 200 sieve
  • 112 tons mound sand
  • Control panel with alarm (Sim/Tech Filter)

System operation

Wastewater flows by gravity from the cottage through a 4-inch Schedule 40 PVC pipe to the 1,250-gallon septic chamber 8 feet away. Effluent passes through the filter, then enters the 600-gallon dose tank. The on-demand pump cycles two minutes, sending 54 gpm 33 feet through a 2-inch PVC Schedule 40 force main to the distribution box.

A 4-inch PVC gravity supply line from the distribution box doses six 30-foot-long laterals. Each 12-inch O.D. AES tube has a 10-inch I.D. corrugated, perforated plastic pipe where treatment begins. Ridges on the pipes increase surface area and circulate effluent uninterrupted, cooling it to ground temperature. Skimmers at each perforation help retain grease and suspended solids, protecting the three outer textile layers from clogging.

The white Bio-Accelerator layer develops the biomat, screens more solids, and evenly distributes effluent along the length of the laterals. A mat of coarse green plastic fibers traps more solids and creates a massive suspended growth area for aerobic bacteria. Black geotextile fabric protects the inner layers and provides more surface area for nutrient uptake. As the sand bed wicks liquid from the fabric, it transfers air to the microorganisms. Third-party testing proved the system treats effluent to less than 2 mg/L TSS and CBOD and to 218 CFU/100 mL fecal coliform.

Installation

Parking the Hyundai Robex 140 LCD-7 excavator just off the narrow main road enabled Justin Marvin to excavate to basal grade for the drainfield. Every scoop he loaded into the Mack 10-wheel dump truck required swinging the boom under the main electric power line overhead. The cautious work helped stretch the day to 14 hours, much of it plagued by rain, snow and cold temperatures.

Carbonneau needed a 50-foot-long temporary drive across the drainfield and lawn to reach the combination tank’s excavation site and cesspool beside the house. He planned to protect the field’s receiving soil with 12 inches of concrete sand overlaid with 24 inches of crushed stone to sustain the weight of heavy vehicles.

Then Marvin unearthed a boulder the size of a Volkswagen. “It had to be 6 by 6 by 7 feet wide,” says Carbonneau. “Lifting it was not an option.” Marvin won the lengthy battle to roll the monster with the bucket to the edge of the lot.

With the drive-in service, Schofield’s Septic Tank Service pumped the cesspool. As Carbonneau backfilled it with bank run sand, Marvin excavated the hole for the 12- by 6- by 6.5-foot tank. Such excavations normally displace 12 to 14 cubic yards of material, but the wet sidewalls sloughed continually.

Marvin sloped the excavation until he had a 20- by 20- by 7-foot hole with one side 12 inches from the cottage. Carbonneau hauled 80 cubic yards of spoil offsite in nine loads with the Mack truck. Meanwhile, Marvin bedded the hole with 6 inches of compacted crushed stone. Two Wacker Neuson 3-inch dewatering pumps exhausting to sediment filter bags barely kept up with incoming groundwater.

Vinny Lepine of Calco delivered the tank sections at 1 p.m. Besides joining the halves with mastic sealant, the top of the pump chamber dividing wall was sealed to stop effluent from infiltrating and affecting dose cycles. Mastic was also applied around the risers to reinforce the gaskets and prevent infiltration through the lid.

“I used to set individual septic tanks and pump tanks, but the 6 feet of compacted fill between them often settled enough to break the pipe connection,” says Carbonneau. “Investing in two-compartment tanks eliminated excavating and repairing my mistake.”

To prevent pressure-induced infiltration, Carbonneau prepared to apply an 8-inch-wide butyl rubber wrap around the exterior tank joint. After Marvin suspended a sheet of plywood overhead to keep out snow, Carbonneau dried and heated the concrete using a 100,000 Btu blowtorch with 2-inch head connected to a common propane tank. “As long as everything is warm, the spray adhesive bonds the wrap to the surface in seconds,” he says. It did.

High groundwater also made the tank buoyant. To counteract it, Marvin set two 2- by 2- by 4-foot concrete anchors on both sides of the tank. After placing stainless steel cables over the lid, they fastened the cables to stainless steel rings in the ballast, tightened the cables and backfilled with 80 tons of mound sand. It took 14 hours to excavate and install the tank instead of the usual two to three hours.

The system depends on passive air movement to support aerobic bacteria, but the pump, acting like a gate valve, interrupts the flow. To restore it, Carbonneau installed a 4-inch PVC bypass vent from the dose tank to the distribution box.

“We use convection to draw air from the 4-inch screened low-vent PVC pipe in the drainfield, through the system, and out the roof vent on the house,” he says. “The bypass continues the vent path. It also mitigates hydrogen sulfide corrosion by allowing heavy gases to escape.”

Marvin excavated the 4-foot-deep trench for the force main and bypass line. As groundwater poured in, Carbonneau started the dewatering pumps. “The solvent needs a dry environment to weld the sticks of pipe properly,” he says.

Good weather returned for the drainfield’s installation. Marvin removed the gravel from the temporary drive, then built up 12 inches of concrete sand (80 tons) to create the 32- by 12.5-foot absorption bed. Carbonneau used a DEWALT laser to ensure the bed was level. “A flat surface is critical to achieving a uniform flow down the length of the laterals,” he says.

Assembling the 10-foot-long pipes went quickly. Corrugations on them matched those on 8-inch-wide couplings that locked around the pipes like zip ties. Marvin backfilled the laterals on 18-inch centers with 12 inches of concrete sand, then covered them with 6 more inches (30 tons total) overlaid with 3 inches of loam. Carbonneau seeded the field in spring.

Maintenance

The tank, effluent filter and pump require inspection every two years. Cleaning is necessary when the combined thickness of sludge and scum equals more than one-quarter of the tank’s liquid depth.



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