Extending the city sewer to its new middle and high school buildings would cost Guilford County Schools in Greensboro, N.C., more than $4 million.

Seeking a sustainable onsite system, representatives met with Amber Farrelly, P.E., of B&F Con-sulting in Raleigh, N.C., and Dave Maciolek, P.E., of Worrell Water Technologies in Charlottesville, Va.

Their design, involving horizontal and tidal flow wetlands with a rotary-kiln-expanded slate lightweight aggregate, produced the largest subsurface reuse system in North Carolina. The recycled water irrigates three athletic fields, saving the schools from buying 5 million gallons per year.

Site conditions

Soils are clayey atop loamy sand (saprolite). The water table is 40 feet below grade. Saprolite’s conductivity is 10 to 12 gallons per square foot per day. Its application rate is 0.2 gallons per day per square foot.

System components

Farrelly designed the onsite system and Maciolek designed the wetlands. The system handles 30,600 gpd. Its major components are:

Middle school

• 5,000-gallon concrete grease trap. Concrete tanks from Stay-Right Precast Concrete Inc., Franklinton, N.C.

• 12,000-gallon dual-compartment concrete septic tank with three 12-inch FT1254-36 effluent filters from Orenco Systems Inc., Sutherlin, Ore.

High school

• 8,000-gallon concrete grease trap

• 14,000-gallon dual-compartment concrete septic tank

• 6,000-gallon dual-compartment concrete septic tank with three Orenco 15-inch FT1154-36 effluent filters. Septic tanks are in series.

Advanced treatment

• Two 25,000-gallon single-compartment 10-foot-diameter fiberglass equalization tanks in series with 6-inch flexible coupling bottom connection.

• Fiberglass tanks from Xerxes Corp., Minneapolis, Minn.

• 32- by 96-foot Living Machine tidal flow wetland with three cells from Worrell Water Tech-nologies, Charlottesville, Va.

• 110- by 240-foot Living Machine horizontal subsurface flow wetland. Stalite PermaTill aggregate from Carolina Stalite Co., Salisbury, N.C.

• Model EFB-0102-AB screen and 4000-micron mesh filter basket from Hayward Industrial Products Inc., Clemmons, N.C.

• Effluent collection chamber and lift station with two 1/2-hp 230-volt single-phase effluent pumps.

• Two 25,000-gallon single-compartment, 10-foot-diameter fiberglass effluent tanks in series with 6-inch flexible coupling bottom connections

• 4,475 feet of 4-inch force main

• Two 6.8-acre drainfields. Drain-field #10 has six zones and Drainfield #11 has eight zones. Both have duplicate replacement zones.

• 400,000 feet of 1/2-inch Netafim Bioline tubing: 2 foot on centers at 0.62 gallons per hour for drainfields and 16 inch on centers at 0.33 gallons per hour for athletic fields.

• Radio controls and control panels from Custom Controls, Raleigh, N.C.

System operation

Wastewater from the middle school gravity feeds through a 6-inch PVC sewer and from the high school through an 8-inch sewer to a manhole. An 8-inch pipe carries the waste-water to the equalization (EQ) tanks.

When the tidal cells call for water, pumps send effluent from EQ 2 through a screen and filter basket. It mixes with water from effluent tank 1 before entering the bottom of each independent cell, rises to within 6 inches of the aggregate’s surface, and percolates down to a drain.

Once the drain is full, an electric solenoid valve opens, directing effluent into a 6-inch pipe running through the center of the horizontal cell. Liquid disperses outward over the aggregate to a weir on the return side. The overflow runs to an effluent collection chamber and lift station that pumps it to effluent tank 1.

Pumps in the first compartment of effluent tank 1 send the liquid to the UV treatment area in the second compartment, where it recycles to the front of the tank. Pumps in effluent tank 2 send the water through a filter and flow meter before it goes to the absorption beds and athletic fields. Disinfected water from tank 1 flows into tank 2 as its level falls.

The Stalite PermaTill aggregate in the tidal and horizontal cells has a 97 percent void ratio with 50 percent more surface area than sand or gravel for higher nutrient and TSS removal. With high hydraulic conductivity, it will not clog, yet it retains 8 percent of the moisture for growing plants and microorganisms. “Stalite is the shining star in this system and one reason why it runs so well,” says installer Michael Halas, owner of Spectrum Environ-mental Inc. in Raleigh, N.C.

Installation

Spainhour and Sons Grading of Rural Hall, N.C., excavated, transferred materials, and prepared the subgrade. The crew excavated 75- by 20-foot-wide holes for the tanks in a hillside. “It took four days to dig them because the south face required massive benching,” says Halas. After placing gravel, the crew positioned the deadmen, installed the tanks, and backfilled them with gravel. The tanks were then water tested for 24 hours.

Installing the horizontal cell took seven days. Halas’ crew covered the shaped and compacted native soil with geotextile. Then Landsaver Environmental in Rich-mond, Va., installed the 20-foot-wide lengths of 40-mil HDPE liner and sealed the joints. The liner was then water tested. Workers added another layer of geotextile fabric to protect the liner from 5-foot groupings of various-sized washed stone.

Reinforced concrete footers 18 inches deep supported the cinder block walls surrounding the tidal cell basin. After a mason set the courses and molded in the influent, discharge, and overflow penetrations, Halas’ men applied the decorative stone veneer. They poured concrete floors in the cells, deburred the walls and floor, and covered the surfaces with geotextile fabric and the 40-mil liner.

A concrete conveyor belt system deposited 2 feet of washed stone and 3 feet of 5/16-inch washed Stalite PermaTill aggregate into the cells. The material does not compact and withstands 43,000 pounds per square foot of pressure.

Plants for the wetlands were harvested from the area and propagated on a nearby farm. “They had to winter over so they could be planted at the proper time,” says Halas.

His crew cleared a wooded area for Drainfield #10. Each bed required 25 tractor-trailer loads of sand, with drip tubing installed 3 feet deep.

Spainhour and Sons excavated 4 feet for the athletic fields, then mined saprolite rock from different areas of the site for use as the drip irrigation system’s treatment medium. Each field received 250 tractor-trailer loads of sand that Spainhour and Sons dredged from the Yadkin River 60 miles away, then screened.

Dosing the absorption beds and athletic fields is radio controlled because they are so far from the control room. Water from daily backflushing flows through the gravity sewer to the septic tank. Rainwater from a 308,000-gallon, 86- by 60-foot cast-in-place concrete cistern flushes the lines — and the toilets. Its top is a basketball court. The grass is a verdant, lush carpet on the athletic fields. “Everything works flawlessly,” says Halas.

Maintenance

R. David Hicks LLC Environ-mental Consulting in Jamestown, N.C., operates and maintains the system, and uses a supervisory control and data acquisition (SCADA) system to monitor it. The effluent from the Living Machine has 0 to 0.5mg/l cBOD and TSS. A colder-than-normal winter caused no system shutdowns.