Here in California, water diversion and low flow toilets have made my town's sewer system much drier. The sludge is not flowing as well, causing smell.
This is a concern many people bring up.
Here are a few thoughts:
1) For the record, this concern should be brought up to anyone proposing reduced flows with Water Sense standards, plumbing code changes, rebates for installing low flow toilets, etc. I feel that greywater gets this question disproportionately
2) We have an aging sewer system that needs attention, and we need to make it work with modern flows, which will use lower flow fixture.
3) New sewer pipe are required to be sized based on outdated flow estimates, resulting in problems with low flows. We need to change flow rate estimation equations to match modern fixtures.
4) I touched on this in my book, Greywater, Green Landscape, here is an excerpt:
"The Bigger Picture
What would happen to the entire municipal sewer system if lots of people reused greywater? Massive sewage clogs? Probably not. In the Study of the Effects of On-Site Greywater Reuse on Municipal Sewer Systems (2011; see Resources) researchers used hydrologic models of the sewer system to identify levels of flows and pollutant loads throughout the day. They found the sewer system operated with large fluctuations in flows, with two major peaks occurring each day (morning and evening). Reusing greywater would have the largest impact on the system during these peak flow times (imagine lots
of morning bathing water going into the yard instead of the sewer). Widespread reuse of greywater would reduce the peak flows but would have almost no impact on the system during the current lowest-flow times of the day (when toilet flushing is the major source of water in the system).
Because the sewer system currently operates at these low-flow times, and these flow rates would not be affected, researchers concluded that widespread reuse of greywater should not increase blockages. In fact, they hypothesized that sewer systems could experience positive effects from greywater reuse with an increase in capacity."
We use 40-50 gallons of water a day and divert about 25-30 gallons of grey water to the garden.
As Laura pointed out they really need to update plumbing code. In addition to minimum pipe diameters that are too large, they have minimum septic tank requirements that are 10 times larger than they need to be for a household like mine. The codes assume that a house with only 2 people in it will flush away 600 to 800 gallons of water a day, that's just nuts.
I may have done the math wrong in my previous post. Septic sizing requires a tank large enough to hold 2 days worth of waste water, but most areas also specify a minimum size tank which is typically 1,000 gallons, although some areas allow 750 gallon minimum. According to the studies I've seen the average American uses around 100 gallons a day, but I suspect a large portion of that is used for watering the lawn, etc.
My wife and I average ~25 gallons each, but that is just in the house. We use probably another 30-35 gallons a day (annual average) but most of that comes from captured rainwater and now grey water. If we were to install a tank based on just the amount of black water we produce, a 55 gallon drum would be big enough. Heck go with 100 gallons just to be sure, but the minimum size allowed in our area is 10 times that big.
I have read that using a tank that is too large can cause problems because the bacteria concentration ends up being too low to properly process the waste.
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