In Virginia, Loudoun Water (formerly the Loudoun County Sanitation Authority) approves all specifications and plans for community-based systems in new developments. The developers then deed the systems to Loudoun Water to own and operate. The agency relies heavily on natural wastewater treatment, such as septic tanks, drip irrigation, and recirculating sand filters, which are shown here. Photos: Loudoun Water
The wastewater treatment building at a subdivision in Aldie, Va., is served by a community wastewater system that disperses effluent in a low-pressure distribution system. About half of the effluent is lost to evapo-transpiration from both the grass and the soil. The other half recharges the groundwater approximately half a mile from the closest well. The wastewater treatment system consists of dual-train extended aeration package plants within a building, followed by a deep bed denitrification filter and a low-pressure distribution in aerobic drainfields.

When the subsurface sewage treatment systems at three condominium developments in Brookfield, Conn., repeatedly failed in the mid-1990s, the physical mess created a financial nightmare for both the condominium associations and the town itself.

Because the associations couldn't pay for the repairs that would eliminate illegal discharges, the Connecticut Department of Environmental Protection ordered the town to do it, a massive undertaking that involves:

  • A 32-unit development generating 5,400 gallons/day. Estimated repair cost: $625,000
  • A 41-unit development generating 7,300 gallons/day from two onsite treatment systems. Estimated repair cost: $300,000
  • A 162-unit development generating 20,000 gallons/day. The development's monitoring reports showed nine failures between 1993 and 1996. The Department of Environmental Protection determined that the onsite system wasn't developed properly. Estimated repair cost: $1.2 million.

So began a protracted court battle between Brookfield and the state. The town has lost several appeals and faces up to $100,000 in fines if residents don't approve a $1.9 million referendum this month to connect the condominium complexes to the main gravity sewer system with a new sewer line.

It's a hard lesson learned from an era of unprecedented growth in the housing market, when subdivisions were built faster than the infrastructure necessary to serve them. With traditional wastewater treatment plants facing restrictions on their total maximum daily loads (TMDLs), decentralized systems were an attractive way to accommodate growth.

Subdivisions were built with “neighborhood-scale systems” that typically serve up to several dozen homes. Some estimates place the number of new homes in fast-growing urban areas served by decentralized systems at more than one-third.

Decentralized systems are used largely for two reasons: when the main sewer system is inaccessible (or linking to it would not be cost-efficient) and when centralized systems and treatment plants face capacity limitations. They're also a fraction of the cost to build and maintain a centralized wastewater treatment plant.

“We're seeing these mostly in areas of high urban growth,” says Peter Grose, senior vice president of engineering and consulting firm Fuss & O'Neill. “Occasionally we see them mediating an existing problem, such as a densely populated area with existing septic systems that aren't doing the job at effectively removing nitrogen.”

Onsite wastewater treatment systems use processes similar to traditional wastewater treatment plants, relying on both aerobic methods (such as activated sludge, suspended growth, or attached growth with textile filters) as well as anaerobic methods to remove nitrogen. But unlike most centralized systems, they discharge through leaching fields, spray irrigation systems, and constructed wetlands rather than to a river or lake.

“Decentralized wastewater treatment, reuse, and resource recovery are key elements in a significantly more efficient use of water that also entails a lighter environmental footprint as well as more creative open space and lower energy costs,” says Valerie Nelson, director of the Coalition for Alternative Wastewater Treatment. “Systems that rely on soil-based treatment also replenish groundwater aquifers.”

“Some rival the efficiency of a larger treatment plant in removal of organic matter, solids, and nutrients such as nitrogen and phosphorous,” Grose says.