These important steps will maximize system functionality.
In the second part of my four-part series on rainwater harvesting (“Cistern Filtration for Rainwater Harvesting” in the March 2012 issue of pme), I discussed the importance of separating debris from collected rainwater and aerating the water before it enters the cistern.
The central premise, as stated in Part 2, is the overall health of a rainwater harvesting system starts with the cistern. This article will discuss methods engineers employ to design an effective and cost-efficient rainwater harvesting system.
Inside the cistern, there are three practical ways to take advantage of the good, aerobic microbial process and sequester the bad, anaerobic microbial process. First, the water entering the cistern must do so calmly and with a smooth up-flowing pattern.
Second, the cistern must be able to overflow in a skimming action to draw off floating debris such as pollen and dust to allow the free surface of the water to draw in oxygen. Third, when the water is drawn from the cistern for treatment and use, it must be taken from near the free surface and not from the bottom of the cistern.
The screening process discussed in the March article is one means by which entering water is allowed to calm itself. Rainwater collection is a “harvesting” process, not a “hoarding” process. Harvesting collects the low and average storm events over the course of a year.
There is no expectation for the cistern to be inundated by a century storm event. The collection piping, as part of a code-compliant roof drainage system, needs to be sized for the century storm event or that which is prescribed by code, but what is allowed to be collected by the screening process reduces the rate and quantity directed to the cistern while disposing of the excess. This may sound like a missed opportunity to collect more water, but the low frequency and short duration at which such strong rainfall events occurs makes the “missed opportunity” negligible.