Clean Water

The quest to have a clean rainwater tank is as old as rainwater collection itself. With the revival of the domestic rainwater tank, new industries have been created offering various accessories designed to minimise any contaminants entering the tank. For the City dweller, collecting rainwater for any purpose other then usage in the household, a clean tank is of secondary importance compared to the household depending on rainwater for everyday usage. Regardless, many devices have been developed and fitted to tanks, some with questionable success. This page will discuss some of the more common accessories on offer. The authority of the author is based on his active involvement in the rainwater harvesting industry for 12 years and his household has depended entirely on rainwater for more than 30 years.

First Flush Devices

Without question, one of the most useful accessories to any rain harvesting system is the first flush device. By far the most common type of first flush devices is “volume controlled”, in other words, a predetermined amount of water is diverted away from the tank before inflow is allowed. Over the counter first flush devices may provide peace of mind but not the expected results. Volume controlled first flush devices have to be tailor made to match a specific roof size. Obviously smaller roofs produce less foul flush than larger roofs. The other crucially important factor is the volume of the first flush. Over the counter devices may collect as little as 10L of foul flush and then allow water to flow into the tank, where in fact the real first flush has not yet commenced. Various scientific studies determined that the first flush, which should be diverted from the storage tank, has to be between 1mm and 2mm of the first rain, depending on your environment (rural compared to industrial). One mm of rain on a 200 square metre roof translates into 200L of first flush (2mm on 200 square metres equals 400L). No domestic first flush is sized to divert this amount of foul flush. Finally, for a volume controlled first flush device to work again after a rainfall it has to “reset” itself. This is achieved, or better, supposed to be achieved, by a self-draining mechanism, which is a small orifice on the bottom of the collecting chamber. By the very nature of the first flush, which is contamination, this orifice is quickly blocked resulting in a partially drained device. It can be argued that no first flush device is better than a partially drained first flush device, which can quickly become a breeding ground for mosquitos and bacteria. Manufacturers of first flush devices are well aware of this problem and the second generation of first flush devices have now a small filter installed above the orifice. Unfortunately, the filter blocks up as fast as the original orifice due to the amount of dust and other contaminants washing down from the roof. If you have a first flush device fitted, unscrew the bottom of your collecting chamber to confirm the amount of dirt collected.

On a more positive note, a homemade device can work perfectly well, with the compromise that the device is not automated, but has to be operated manually. Simply take a container, tank, drum or PVC pipe, that is big enough to collect your first flush; the overflow of this vessel will be connected to your tank inlet. Fit a 25mm ball valve on the bottom of your home made first flush device. After every rainfall, open the valve to drain the foul flush. Yes, it can be as simple as that to have a working first flush device.

Tank Inlet Filters or Inlet Filter Bags

In principal, these devices work well, but only on smaller tanks to a maximum of 5,000L. On larger tanks, these filters cannot cope with the amount of contaminants coming off the roof. In a major rainfall the filter will simply block up after about 5,000L of water filtered, resulting in rainwater spilling over the tank. The filter bag cannot be removed when it is full of water; it is too heavy to be lifted , holding as much as 100L (100Kg) of water.

Leaf Beaters and Eaters

This is certainly one of the more useful devices. Properly fitted leaf beaters will reduce the amount of coarse contaminants carried over to the inlet strainer. Limitation of this device is the secondary screen fitted below the coarse screen mesh. Due to the minute surface area and 0.98mm aperture, this screen blocks up within a very short period of time resulting in water spilling over from the device. This secondary screen can be removed without jeopardising the function of the leaf beater. Unfortunately, leaf beaters without secondary screens are not suitable for wet systems.

Vacuum Overflows

This is not a common attachment to a rainwater tank. The overflow pipe is internally extended close to the bottom of the tank, terminating about 30-50mm above the bottom. The idea is twofold, firstly there is a slight vacuum effect created when the tank overflows, resulting in a very small area around the overflow pipe being actually “vacuumed” clean on the bottom of the tank. The other reason is that the “old” water is drained first from the tank, as it has been taken from the bottom. The term “old” water refers to the supposed anaerobic (oxygen starved) water on the bottom of the tank, compared to the aerobic (oxygen rich) new inflow water. There is no scientific study done to confirm the existing of these zones and is purely a theoretical speculation. In view, that water harvested from springs originated from aqua layers hundreds of years old, there appears little scientific evidence suggesting that water from the lower part of the tank is of any lesser quality than upper layers.

Gutter Mesh

Has some positive features, but there are many different designs and brands available, and some wok better then others. Important features to be considered are if the mesh is permanently attached to the roof and gutter. If this is the case, it is not possible to clean out the gutter of the unavoidable dust and grim seetling on the bottom. This may invite premature corrosion and a breeding ground for bacteria and possibly mosquito larvae. Recent developments in this field recognised this problem and there are now bristle type gutter inserts available, which fit snuggly into the gutter, but can be easily removed to clean out the mud and dirt accumulated on the bottom.

Filter Assemblies

Filters fitted after the pressure pump make a lot of sense and have become very much a standard accessory for any tank. There seems to be some controversy about the required rating of the filter itself. Filters are rated in microns, indicating the size of particles that will be captured (or passed) by the filter. For example, a 5 micron filter will capture anything bigger then 5 microns. Such filters are available down to 1micron and up to 50 microns or even coarser. In principal, the finest filter (1 micron) is the preferred choice but provides a poor service life. Such cartridges may require replacement after a couple of weeks due to saturation. A good compromise would be a 25 micron cartridge, which should last 3 months before replacement is required. For everyday use, this filter provides clean water free of any “floaties”. The next step up would be a twin filter assembly. The first cartridge filters down to 25 microns and the secondary down to 1 micron.

UV Treatment

UV treatment will ensure that most bacteria are killed without the introduction of any chemicals to the water, not cheap, but very effective. UV treatment will not remove “floaties” (visible dirt) from the water. Properly filtered and UV treated tank water will be of superior quality to any reticulated town water supply.

The Future

All discussed accessories address the issue of contamination in the tank and the consequent counter measure. The ideal solution would be a clean tank in the first place, free of any contaminants.

Our customers who select Stainless Steel Tanks over any other tanks, are particularly conscientious of having a clean tank, free of any kind of contamination. Due to frequent requests, we have developed our SEALED TANK SYSTEM, the only system commercially available which ensures your tank remains completely clean at all times. Please visit our web site

www.sealedtanksystem.com for more information.