Residents of Ravenswood Beach noticed ponding in their two- tiered drainfield in spring 2005. The 30-year-old cluster system serves 36 homes on Lake Melissa, south of Detroit Lakes, Minn. The homeowners chose to address the problem.
The only apparent solution was to replace the clogged gravel beds and lateral pipes with a new drainfield using chambers, but the price was steep, and no space was available for a drainfield upgrade and expansion. The homeowner association contacted Eugene Hansen, the private civil engineer who had designed the original 8,000-gpd system.
Hansen knew that Mike Metelak, marketing director for SJE-Rhombus Controls in Detroit Lakes, was seeking a site with residential wastewater to evaluate an innovative flat-sheet ceramic membrane bioreactor (MBR) from ItN Nanovation AG in Germany.
“The daily flow was ideal for a test site, and the residents had to resolve their drainfield problem,” says Metelak. “It was the ideal dream marriage.” In a cooperative venture, SJE-Rhombus installed the components and operates the cluster system at no cost while it logs data. This is the first installation using ceramic flat-sheet membranes in North America.
Site conditions
The local soils are sandy with excellent drainage. The drainfield is located on a hill 30 feet above the beach.
System components
Jim Lockrem, electrical engineer from SJE-Rhombus and the project’s technical director, designed the upgraded system to handle 8,000 gpd. Its major components are:
• Existing 2,000-gallon concrete lift station with duplex 3-hp pumps
• Existing 8,500-gallon concrete septic tank with 6,000-gallon settling compartment and 2,500-gallon effluent equalization compartment
• 1/2-hp dosing pump in a screened effluent vault
• 1,800-gallon fiberglass anoxic (denitrification) reactor tank with 1/2-hp submerged vertical mixer and 1/3-hp sludge pump. Fiberglass tanks made by AK Basin (AK Industries Inc.), Plymouth, Ind., purchased through Rep-Rite-Burk, Mendota Heights, Minn.
• 1,800-gallon fiberglass aerobic reactor tank with two 1/3-hp recirculation pumps.
• 1,600-gallon, two-compartment concrete filtration tank with one tower of eight ceramic membrane modules in the first chamber. Modules from ItN Nanovation AG.
• C-MBR Web-based control platform from SJE-Rhombus.
System operation
Wastewater from six year-round and 30 seasonal homes gravity flows from each home to a collection line running alongside the public road into a lift station. It is then pumped 200 yards uphill to the septic tank and cluster treatment site. Two-inch PVC pipes handle pressurized flows; 4-inch PVC pipes handle gravity flows.
In summer, the pump in the septic tank equalization chamber operates on a timed dose sequence, providing about 40 doses per day of about 150 gallons to the anoxic tank. In winter when daily flows are lower, dosing is reduced to about 20 per day at 50 gallons per dose.
The activated sludge process in the aerobic reactor tank reduces BOD and begins converting ammonia to nitrate. A slow-moving vertical mixer in the anoxic tank maintains volatile solids in suspension, while fine-bubble aeration diffusers in the aerobic reactor tank provide oxygen and keep the liquid well mixed.
“Conventional aerobic treatment systems, or extended aeration systems, have an activated sludge concentration of about 2,500 mg/l,” says Metelak. “Membrane bioreactors have 8,000 to 15,000 mg/l. When the concentration exceeds 15,000 mg/l, we manually activate the pump in the anoxic tank and send some waste sludge back to the primary settling compartment to maintain the proper concentration of activated sludge.”
Effluent in the anoxic tank gravity flows to the aerobic tank. About five times per day the daily flow is recirculated between the aerobic tank and anoxic tank for further denitrification. On demand, the second pump in the aerobic tank sends the activated sludge to the filtration tank, which recirculates by gravity flow back to the aerobic tank to avoid a high concentration of activated sludge in the filtration tank.
“The aerobic tank is the hub of the system,” says Metelak. “Recirculating both ways, from aerobic to anoxic and from aerobic to filtration, really simplifies and stabilizes the treatment process.”
The filtration tank has two 800-gallon chambers. One holds a tower of eight ceramic membrane modules; the other is a spare. The activated sludge is filtered on a 10-minute cycle. For 9.5 minutes, a 1.5-hp vacuum pump draws permeate water from the activated sludge through the membrane modules, with columns of flat-sheet ceramic plates stacked 6 feet high. The membranes have 344 square feet of surface area with a durable nanotechnology coating used as the filtration layer, providing a 0.20-micron pore opening.
“The E. coli bacteria are about 35 times larger than the pore size and can’t pass through the pore opening,” says Metelak. “The pressure differential on the membranes is 150 to 450 mBar, and the flux rate averages 18 gpd per square foot of membrane surface area.” The biology held back by the membranes gravity flows to the aerobic tank. All three secondary tanks have the same concentration of activated sludge.
During filtration, some permeate is sent to a 35-gallon storage tank and is used to backflush the membranes from the inside out for 30 seconds to complete the 10-minute cycle. The remaining permeate gravity flows from the filtration tank to the distribution box, then to two 75- by 100-foot-long drainfields.
The permeate averages less than 2-mg/l TSS and BOD and less than 10-mg/l total nitrogen, and has 99.9 percent fecal coliform reduction with no pathogens. Besides permeate backflushing, a 2-hp rotary air compressor continually sends coarse air bubbles up from the bottom of the tower and between the plates to scour any buildup on the membrane surface.
Two to four times per month, the membranes receive a 60-minute chemical cleaning with 500-ppm chlorine bleach. The preprogrammed sequence backflushes the modules, rests for four to five minutes, then repeats the cycle eight to 10 times.
While any membrane with a similar pore size will provide the same kind of treatment, the difference is the material. “The significant advantage of ceramic membranes is that you have additional cleaning options that will not damage the ceramic membrane, but may otherwise damage a polymeric membrane,” says Metelak.
“Because they are durable, ceramic membranes can be backflushed more frequently and at higher pressure and are unaffected by chlorine. They can be power sprayed and it won’t damage the membrane surface.” In extreme cleaning situations, such as fouling from grease or an unusual chemical event, the permeate can be heated to 176 degrees F or steam-cleaned.
Installation
SJE-Rhombus hired Richard Vareberg of Vareberg Backhoe Service and Septic in Detroit Lakes to excavate the tank holes. A subcontractor poured the filtration tank. The tanks were buried six feet deep and extend four feet above ground. The SJE-Rhombus team did the plumbing and hookups.
A 10- by 15-foot metal service building houses the control panel, air compressor and scouring blower, permeate pump, flow meters, and monitoring devices. A slanted roof on the back of the building overhangs the filtration tank. The air compressor helps heat the building in winter in addition to a small electric baseboard heater.
“The installation was straightforward,” says Metelak. “What is important to note is the huge footprint reduction. Conventional aerobic treatment units have a 24-hour retention time. With the heavy concentration of activated sludge biology in MBRs, the retention time drops to six to eight hours. So, installers don’t need one gallon of tank size for every gallon of flow. The tanks can be one-third the size.
“The other space-saver is using filtration tanks, instead of the clarifiers used in an ATU or extended aeration plants. The membranes dewater the mixed liquor and retain all the activated sludge biology in the tank. So contractors also have a footprint savings in reduced biological and clarification tankage.”
As a result of the higher level of treatment, ponding in the drainfields disappeared in less than two months, and the biomass within a year. Today, they are completely rehabilitated.
Maintenance
SJE-Rhombus is the service provider. Once a month, staff sample the permeate, check the equipment, and run a TSS test on the activated sludge concentration. If the level is above 15,000 mg/l, they waste some to the primary settling tank. The entire operating process is automated and can be monitored and controlled remotely via a Web-based control platform designed for the application by SJE-Rhombus.
