Cistern management for rainwater harvesting
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.
Go with the flow
The flow pattern by which the collected and screened rainwater enters the cistern is another important and sometimes missed step. The inlet pipe should not simply terminate at the cistern wall and allow the water to cascade freely to the bottom of an empty cistern. This causes the sediment layer at the tank bottom to stir up and into suspension. Instead, the inlet pipe must be turned down and extended to the bottom. Then the termination of the inlet pipe should be configured either with a “stilling inlet” or an upturned opening (typically two elbows for a full 180° turn). This creates an up-flow pattern to divert the flow away from the bottom sediment layer. When the cistern water level is low, it allows the water to bubble upward and combine with oxygen from the air.
Despite the first step in screening the collected rainwater ahead of the cistern, some fine sediment always will settle to the bottom. In addition, a biofilm layer will inevitably form on the inside wall of the cistern.
Incidentally, this biofilm is actually beneficial to the overall water quality. It is important for the biofilm to stay where it is, attached to the inner wall. Allowing the calmed, up-flow flow pattern described above assists in that goal. But in addition to sediment that settles at the bottom, some finer particulates tend to enter into the cistern and float. Such particulates include pollen spores and dust.
If allowed to accumulate, the particulate layer will tend to block the beneficial absorption of oxygen for the good aerobic metabolism below the surface. Therefore, it is important that the cistern be allowed to overflow periodically to a skimming overflow outlet, similar to what one would see in a decorative pool. The skimming action tends to pull the floating particulates to the outlet to clear the water surface. This skimming outlet needs to be configured to prevent the entry of vermin from the connected point of disposal (usually the storm sewer) through a trap seal and a flapper valve or ball-check valve. This also prevents the backflow of water in the storm sewer system into the cistern.
The sizing of the cistern should be optimized to achieve the water use reduction percentage target, but not much larger. In fact, over-sizing a cistern to try to maximize the harvesting volume beyond what is actually necessary will greatly increase the residence time of the water in the tank and delay skimming overflows. The better choice is to have the optimal design size to increase the frequency of overflow to get a fresher turnover of water in the cistern to promote a healthier water quality.
It may appear to be counterintuitive to allow both the inlet straining process and the skimming overflow process to dispose of rainwater. The rainwater cistern is being sized and configured as a water supply to substitute for city water, typically as flushing water for toilets or as irrigation, and in some applications as drinking water. As long as the catchment area and cistern are sized to achieve the intended reduction in city water use, taking into account the quantities disposed of by screening and skimming, the system is doing its job. Very seldom do rainwater harvesting systems function as both water-use reduction and stormwater management systems. The rainwater harvesting system can provide some stormwater management in terms of site runoff reduction, but not all. Also, it is not intended for stormwater quality treatment. This needs to be addressed with other low-impact development methods.
Now that rainwater has been collected in the cistern, it is still important to transfer it out properly. The typical plumbing approach to drawing water from a tank is to have a suction pipe at or near the tank bottom or to lower in submersible pumps that draw water from the tank bottom. But in rainwater harvesting, this is the worst way to go about it.
As mentioned above, any cistern is going to develop a sedimentary layer at the bottom and a biofilm on the inner wall. Taking suction from the tank bottom will routinely draw in poor quality water that will end up requiring a higher level of treatment. Not only is the debris a concern, but the water at the tank bottom, being the farthest away from the free surface and therefore the supply of oxygen, is the most anaerobic and will typically have color and odor.
The best quality water, sometimes referred to as the “sweetest” water, is at the top near the free surface. It has the highest oxygen content and the fewest particulates. Therefore, any suction should be configured with a floating strainer or filter with a flexible length of hose and float assembly to allow the inlet to rise up and down with the water level.