A.Jennifer Hause responds: For the most part, there are few problems with septic systems associated with the use of low-flow fixtures. While it’s true the wastewater is more concentrated, the increase in pollutants will likely be insignificant in existing septic systems that were sized for older fixtures ( problems only arise when you get a flow reduction of 20% or more for a given amount of effluent).

More often, the use of low-flow fixtures is a benefit to a septic system because it helps reduce the size of the drain field and reduces "hydraulic overloading" of septic tanks. Excessive amounts of water in the system, typically from laundry, toilet flushing, and bathwater, commonly cause undersized septic tanks to overflow, sending solids and pollutants on to the disposal field. Using low-flow fixtures can offset these heavy amounts of wastewater, so the system operates properly. Under optimal conditions, you want wastewater to stay in a septic tank one to two days. This allows the solids to settle, the scum (including fats and greases) to rise, and pollutants to gradually decompose, allowing a partially clarified effluent to flow on to the drain field.

Any new septic system should be properly sized to handle whatever wastewater loading is expected. At the very least, most states require a minimum septic tank size (ranging from 500 to 1,000 gallons). Some states may require the septic tank to be sized to hold a certain number of gallons per bedrooms (often about 250 gallons per bedroom). Similarly, the drain field must be sized according to on-site soil conditions (based on the results of a percolation test) and the expected waste flow.

Jennifer Hause is a wastewater engineer with the National Small Flows Clearinghouse at the Environmental Services Division of West Virginia University (800/624-8301). Callers may ask to speak with any of the wastewater engineers available, 8 a.m. to 5 p.m. EST.