NOWRA vice president Dick Otis, an engineer and onsite expert, makes interesting observations in an article in the spring Onsite Journal, the NOWRA quarterly magazine. He notes that prescribed design flows for onsite systems, based on a certain flow per bedroom, routinely overestimate the actual flow a system will receive.
For example, the prescribed flows usually assume two people per bedroom, but today's household sizes do not square with that assumption. One consequence of oversizing systems is the cost to consumers. Otis cites data presented by Tony Smithson, Director of Environmental Health Services in Lake County, Ill., during a roundtable discussion at the 2009 NOWRA Annual Technical Conference in Milwaukee.
Smithson noted that in Illinois, design flows must be based on 200 gpd per bedroom (100 gpd per person). Yet census data shows that only one in four Illinois homes have four or more occupants.
"Using reasonable assumptions for hydraulic loading rates on Illinois soils, Smithson calculated
that if the state would use a design flow of 120 gpd/bedroom or 60 gpd per person instead of 200 gpd/bedroom, the cumulative savings from all new systems installed each year would be $7,350,000."
But Otis also makes the case that oversizing can lead to ineffective flow equalization and pressure
distribution, and can hinder the ability to adequately remove pollutants such as nitrogen. Otis concludes that presciptive sizing requirements are not likely to go away anytime soon. And so he suggests that designers consider carefully how systems will actually be used.
"Good estimates of expected flow and pollutant concentrations from an existing or proposed system are necessary to determine how to approach the design," Otis says. "In my opinion, it will be most appropriate in most instances to include flow equalization designed for peak flow with the subsequent downstream system components designed for the actual expected average daily flow. Whether the local jurisdiction will allow this solution is another issue.
You can read the entire article at http://www.nowra.org/journal/spring2010journal.pdf. You can make comments by e-mailing to think@nowra.org.
