Flushed Away! Green Homes and Water Conservation

Will water conservation programs in green homes cause problems with existing sewer systems? Read on…

A story in today’s Tucson, Arizona, Daily Star, about the push-back from sewage authorities against installing lower-water-using toilets in existing homes reminds me once again of the “law of unintended consequences.” Four generations of civil engineers have been taught to design “gravity” sewage conveyance systems that are self-cleaning through water flows, by sloping them appropriately. In built-up areas, the assumptions about water flows haven’t changed much in 50 years, and they all assume profligate water use. Now, people want to install 1.6-gallons per flush toilets in place of 3.5-gallons per flush (pre-1992 standard) toilets, to save water. In the western U.S., roughly west of the 100th meridian, water conservation is assuming critical dimensions, as we enter into what looks like a permanent drought. How ironic that sewage authorities, claiming that the lower water flows will not be enough to flush solids through their system, should be opposing water conservation measures. Interesting also that it may take periodic flush-outs of sewers to keep them “clean,” mitigating much of the benefit of the household water conservation. Just one more proof that “it takes a system to replace a system.” One easy solution going forward is to replace gravity sewers with household pre-treatment (digestion tanks) and pressurized sewers, a solution increasingly favored by local authorities since it’s quicker and cheaper than the traditional “flush without fear” approach. The big opponents are likely to be large consulting civil engineering firms that make tons of fees desiging expensive “gravity” systems.

Posted by on 02/11/2008 at 07:29 AM