In facilities' quest to reduce water use, several cases have prompted a deeper look at the impacts of water conservation. One case came up in early 2011 in San Francisco, where the city promoted efforts that significantly reduced water consumption. The results were great. But as water use dropped, managers discovered the sewer system did not have enough water to carry waste out of buildings.
Other issues involve waterless urinals. These devices reduce water consumption to essentially nothing, but among the reported problems are waste pipes corroding, cracking, and occluding from build-up of solids. These problems aren't apparent until a drain line backs up or breaks, which sometimes occurs inside a wall.
Is the reduction of water use in facilities causing these problems? While our fixtures have changed quite a bit, our distribution systems — both for water and sanitary — have not. Facilities now feature new piping materials, and technicians use installation methods. But requirements for pipe sizing and gravity-line slope remain largely unchanged. There is a point in sanitary systems at which building system flows have dropped so much that they cannot properly convey waste away from the building.
One study shed light on the ability of sanitary systems to convey waste from buildings in the face of reduced water flows. A November 2012 report by the Plumbing Efficiency Research Coalition, "The Drainline Transport of Solid Waste in Buildings", found that the three most important factors in waste transport were:
Flush volume. Overall, the study found that standard-efficiency — 1.6 gpf — and high-efficiency — 1.28 gpf — toilets functioned without issue. But lower flush volumes, notably 0.8 gpf, did not provide consistent results and have been identified as needing further study.
Toilet paper type. Toilet paper was a significant factor. High-tensile-strength paper — two-ply — is more difficult to convey compared to low-tensile-strength paper — single ply.
Slope. Gravity-flow piping is typically sloped 1 percent or 2 percent by design. In reality, it often is installed with a slope somewhere in between. In testing, a drain line sloped 2 percent performed better than a line sloped 1 percent.
One of the most significant takeaways from this study is that long, horizontal waste lines that do not have sustained flows might be at higher risk for system clogs.
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