A new alternative for sizing septic systems.

hen a builder, a designer, and a regulator wonder how big the septic system for a house should be, they might approach the question from different points of view. The builder might wonder, “How big does it have to be to last through my warrantee period?” The designer might ask, “How much water are the people who are buying this house likely to use?” The regulator might ask, “If this house is filled to capacity with teenage obsessive-compulsive hand washers, how much water will they use?” These points of view have fought it out in our political process and have shaped current environmental health laws. The result is that houses are not built with septic systems expected to last the life of the structure and much of the design criteria is based on water use averages (rather than worst-case scenarios), with a modest safety margin usually built in for unusual occupancy periods. What this essay will propose is a reasoned approach using statistics and measured data to design for water use flows. This approach will more evenly distribute public health risk and more fairly spread the financial burdens to builders across the different classes of single-family homes. After years of regulatory evolution, most health jurisdictions size septic systems on the basis of 100–150 gallons per bedroom per day. Some builders, trying to minimize septic-system size so as to maximize house size on small lots, have adopted practices of building “dens,” “bonus rooms,” or “sewing rooms” that look suspiciously like bedrooms. This “three-bedroom mansion” phenomenon can be seen in any area where incomes are high and land is scarce. The response of some regulators has been to require that any room that can be used as a bedroom be counted as a bedroom. This regulation is enforced regardless of the likelihood that these rooms will be used as bedrooms. The bedroom-definition dance has stirred up a great deal of frustration and animosity for builders and regulators alike. Just how important is quantifying the number of bedrooms to a good septic-system design? The time has come to see if the linear-gallons-per-bedroom approach does an accurate job of predicting water use. We should, at the same time, find out if quantifying other physical aspects of houses will give us better water use predictions than does simply counting bedrooms. Below is a review of two studies done by Public Health–Seattle & King County and the Washington State Department of Health that can help answer these questions.