Only 18 months after a small, two-story commercial building opened in October 2017, its 1,000-gpd drainfield failed. The building held offices and retail spaces in Hale‘iwa, Oahu, Hawai‘i.

Samuel Morton of Gecko Enterprises pumped the septic tank every other day for six months, while system designer David Bills, P.E., of Bills Engineering in Honolulu, assessed alternatives. His solution increased the chambered drainfield from 1,200 to 1,700 square feet and its depth from 4 to 8 feet.

In June 2019, Mike Cowper, the original installer and owner of Cowper Construction in Hale‘iwa, saw-cut the asphalt parking lot over the field and rebuilt it. “The expansion used up all the available space, as the first install included an underground stormwater containment system next to the drainfield,” he says.

By 2020, the drainfield had failed again, Morton returned weekly to pump, and for two years Toru Kumagai, P.E. of Laulea Engineering in Honolulu, and Cowper networked for answers. They eventually became aware of a 2010 study by MFH Design Consultants. Conducted for a major coffee franchise, the 170-page report analyzed wastewater from four quick-serve restaurants on septic systems with a high rate of premature failure.

Researchers learned the franchise required brewed coffee to be discarded every 18 minutes to keep the product fresh. Eastern Analytical, an independent laboratory, did the field testing, which showed septic tank effluent BOD averaged 863 mg/L, but reached 1,045 mg/L at the distribution box. Effluent acidity averaged 5.1 pH, low enough to kill beneficial microorganisms (6.5 to 7.5 pH is normal).

In conclusion, the study recommended pretreating the high-strength wastewater with an aerobic treatment unit before discharge. “Everything made sense now because one tenant was a coffee shop,” says Cowper. Kumagai specified a Fuji Clean ATU and White Knight Microbial Inoculator Generators to help rejuvenate and maintain the drainfield instead of replace it.

System components

• Existing 1,500-gallon dual-compartment concrete septic tank (Jensen Precast) with a White Knight WK-200 MIG column (Knight Treatment Systems)
• 1,500-gallon single-compartment IM-1530 pretreatment tank (Infiltrator Water Technologies) with three WK-200 columns and 8-inch Biotube (Orenco Systems)
• HP-200 linear air pump (Hiblow USA)
• 1,000 gpd Model CE10 Fuji Clean, FujiMAC 150RII blower, alarm panel (Fuji Clean USA)
• M-20 “T” vent odor filter (Mega Wolverine)
• MIG control panel (SJE Rhombus)

System operation

Kitchen and sanitary wastewater gravity-flow 20 feet through the existing 4-inch sewer to the septic tank. The MIG column in the tank’s second compartment is inoculated with IOS-500 microorganisms. A 9-inch-diameter fine-bubble membrane diffuser in the base of the column oxygenates and circulates the wastewater to accelerate consumption of organic matter.

Effluent and proprietary bacteria then gravity-flow to the pretreatment tank to mix with microorganisms from the additional MIGs. “By the time effluent enters the Fuji Clean, high-strength BOD and TSS levels have been reduced to residential strength or less, and total nitrogen has been reduced by 50% or greater,” says Noga. The MIGs also seed the ATU with IOS-500 microbes.

The oxygenated, enhanced effluent gravity-flows from the ATU, through the distribution box, and into the approximate 38-by-22-foot-wide drainfield. Offgas from the three tanks vents to a granulated activated charcoal filter in the bottom of a HDPE manhole (Advanced Drainage Systems) filled with wood chips to trap residual odor-laden moisture to provide further air purification.

Installation

After years of dormancy, the biomat in the drainfield had dried up.

In January 2022, Cowper and helper Kris Magaellous blocked off half the parking lot and turned an adjacent vacant property into auxiliary parking for uninterrupted access to the building. “What complicated this install were all the buried utilities, but since I’d done the building’s site work, I sort of knew where everything was,” says Cowper.

The team saw-cut the concrete to expose basalt boulders slammed tightly together by alluvial wash. Cowper used a hydraulic jackhammer on a Komatsu PC88 MR-10 tracked crawler excavator to excavate the tanks’ 20-by-8-by-10-foot-deep hole. “Basalt is harder than granite but fractures easily if hit on the grain,” he says. “Although that happened 80% of the time, we hammered for two weeks on the hole and pipe trenches.”

Tenants and customers had a respite from the noise and vibrations whenever Cowper made the short trip to deposit spoils in his shop yard. He drove a Kenworth T800 dump truck with 15-cubic-foot K&H rock box.

Maintaining gravity fall in the trenches was a nail biter because of the route. After intercepting the waste line at the septic tank’s outlet, the new 4-inch pipe ran to the pretreatment and ATU tanks, then circled back around the tanks to the distribution box. “The depth of the d-box at 5 feet saved us; otherwise, the elevation for the field would have been off and we’d need a pump,” says Cowper.

After bedding the tanks on 6 inches of gravel, the team installed the HDPE manholes (Advanced Drainage Systems) on the pretreatment tank, plumbed both tanks, then backfilled and compacted the rock.

Because this was Cowper’s first experience with MIG units, Kumagai specified that a company representative supervise the installation. Mark Noga, president of Knight Treatment Systems, answered the call. “It was extremely helpful to watch Mark assemble the pieces because he knew the shortcuts,” says Cowper.

The next day Cowper and Magaellous hand-dug 20-feet of shallow trench in a landscaped area from the building to the septic tank for the air line, with three spurs to the pretreatment tank. While an electrician wired the alarms, the team installed the air pump and triplex 2-inch air vent pipes to the odor control filter.

In mid-February, they tied the systems together during a two-hour bypass with Morton pumping the septic tank twice. To restore the parking lot, Cowper and Magaellous laid rebar, poured a 6-inch-deep concrete slab over it, and inserted cast-iron manhole covers above the risers. “We haven’t had to pump once since the system went on-line,” says Cowper. “The MIG columns are doing their job.”

Maintenance

Gecko Enterprises holds the maintenance contract and Cowper replaces the inoculant packets every three months.