Yes, We Have Bananas!

A school for environmental education practices what it preaches with a treatment system that feeds tropical plants before discharging to the soil.
Yes, We Have Bananas!
The second ecological fluidized bed (EFB) tank provides final polishing filtration before effluent discharges to the drainfield.

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Hidden deep in Wisconsin's Northwoods, Conserve School offers semester-long immersion programs for high school juniors in environmental studies and outdoor activities. The school's offerings are designed to deepen students' appreciation for nature, reinforce their commitment to conservation, and inspire them to meaningful action as environmental stewards.

The 1,200-acre wilderness campus near the community of Land O'Lakes includes eight lakes and some 20 miles of wooded trails for all-season use. It also includes a greenhouse filled with plants not otherwise found in Wisconsin – including banana plants and papyrus, used for papermaking in art classes.

The greenhouse actually represents the secondary and tertiary sides of the school's onsite wastewater treatment system. It's part of a closed-loop water system that begins with a well and ends with dispersal of tertiary effluent to the soil. School personnel call the treatment system "the Green Machine"; officially it's a Living Machines system supplied by Worrell Water Technologies.

Site conditions

Conserve School lies within a mixed coniferous and deciduous forest in northern Vilas County, Wis., just south of the Michigan border. Depth to groundwater in the drainfield areas ranges from 8 to 30 feet. Soil is caving dirty sand and gravel from the surface to an 8-foot depth; caving coarse sand from 8 to 17 feet; caving medium sand from 17 to 30 feet; and caving coarse sand and gravel from 30 to 36 feet.

System components

Four 12,000-gallon septic tanks

9,600-gallon anoxic tank for ammonia removal (treatment tanks from US Filter/Siemens Water Technologies)

9,600-gallon closed aerobic tank with mulch filter for odor control

Three 9,700-gallon open aerobic tanks with tropical plants for nutrient removal

12,000-gallon secondary clarifier

Two 4,500-gallon ecological fluidized bed (EFB) polishing tanks

Five drainfields with 27,300 linear feet of drip tubing

Custom control system (Orenco Systems)

System operation

The Green Machine was designed with a capacity of 38,000 gpd. Because the school's enrollment has declined since it converted from a full-time operation to a semester-based program, flow has declined and now averages about 7,000 gpd (2,000 to 3,000 gpd when classes are not in session).

Wastewater from the entire campus (classroom and administration buildings and student living quarters) is pumped by a lift station to the septic tanks for primary treatment. Myers pumps (Pentair) lift the septic tank effluent into the greenhouse treatment system. Wastewater first passes through the anoxic tank and into the closed aerobic tank, which contains a mulch filter to trap odorous gases. The condition of plants on top indicates when the mulch is drying out and needs watering. Dissolved oxygen in this tank is kept above 2.0 mg/L to sustain bacteria that break down organic matter in the wastewater.

Water then flows to the open aerobic tanks (one of which is out of service because of reduced flow), where tropical plants grow hydroponically. The plant roots provide fixed media for attachment of aerobic bacteria that consume waste matter and convert ammonia to nitrate (nitrification). Dissolved oxygen in the tanks is sustained at 2.0 mg/L or above by aeration blowers (Siemens Water Technologies).

After the series of aeration tanks, the wastewater is recirculated through the anoxic tank at roughly four times the rate of influent flow to the system. Here, nitrate is converted to nitrogen gas (denitrification). The control system maintains the anoxic tank's dissolved oxygen at 0.1 to 0.3 mg/L; glycerol solution is fed into the tank automatically as a carbon source for the denitrifying bacteria.

The next step in the process is the secondary clarifier. Settled solids are piped from the clarifier bottom to a holding tank outside the greenhouse. The material is thickened, removed by a septic service tank contractor, and limed for pH adjustment before application to land.

The first EFB tank contains a bed of rocks at the center for filtration. Water is continuously pumped through the rocks with airlift pumps. The rocks are cleaned by monthly backwashing; the backwash is treated the same as septic waste and land applied.

The second EFB tank provides final polishing and yields crystal-clear water that is discharged to the five drainfields, which are used alternately.

The control system uses timers to regulate recirculation to the anoxic tank, nutrient feed to the anoxic tank, return activated sludge volume, aeration tank airflow, and other parameters.

Analytical reports on the treatment system typically show influent BOD at about 140 mg/L, TSS at about 90 mg/L, and total nitrogen (TKN) at about 50 mg/L. Effluent results typically are non-detectable for BOD and TSS and 6 to 8 mg/L for total nitrogen.


Dale Mattson, water treatment specialist, operates and maintains the treatment system essentially full time; he also operates the on-site water treatment plant and assists with grounds maintenance. He continuously monitors and fine-tunes the process, sometimes calling on Steve Ohm, area wastewater engineer for the state Department of Natural Resources, for technical advice and assistance.

Basic maintenance includes pump service, periodic cleaning of the septic tank effluent filters, pumping of the septic tanks twice per year, regular cleaning of the treatment tanks, changing of the aeration diffusers (about every five years), degreasing of the clarifier, and trimming of the tropical plants. "You have to trim them in summer, otherwise they get out of hand," says Mattson. "The banana plants grow fast. They go from nothing to bananas in two years."


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