Tuesday, 29 April 2014

Things to avoid in a water storage system

There are a few things to watch out for in designing a rain water collection system. These probably include mosquitos, algae, freezing, over flow, first flush, noise, leaks and condensation.
watercache.com gives some great information on first flush devices, and there's more here.

The basic issue here is that between rainfall your roof can accumulate a lot of crap. Some literal, of the avian variety, some more in the engineering sense of the word. So when it does rain the first few millimetres will be dirty. This is likely to be a bigger problem when there are longer dry periods, and is probably affected by local pollution.

Research in Tokyo found the first 1-1.5 cm from the roof is dirty, and it seems a rule of thumb is that you should throw away the first 40 litres per 100 square metres of roof. There is no definitive amount of rain needed to clean the roof, and it has been pointed out that rain often starts as a trickle so if the first-flush strategy is to divert a fixed volume of rain at the beginning of a rainfall, then this may all be drizzle, and when the heavy rain comes and stats washing the roof it will go straight into the tank.

The important point is that keeping the water going into the tank as clean as possible will keep maintenance and cleaning easier and make the system last longer. Filters obviously come into this too, and I should probably have put that in the list.

Also obviously, there must be a strategy for the tank overflowing. Since this is likely to happen when it's raining heavily, the overflow pipe should be as big as the pipes going in, and possibly bigger since the pipes going in could be full of water, but the overflow pipe, at least at the beginning, will be full of air.

Even if the tank doesn't leak, there may be a fair bit of condensation on it in the summer, and there is likely to be a pool of water at the bottom, so getting a well drained area below the tank is a good idea.

There's a good site here from the Australian government and lots of information just in this thread on the Alternative Technology Association. They cover most points.

I'm not sure about noise but if there is a lot of water moving around, it's possible that it could be be dripping, hissing or humming. It's probably going to do most moving while it's raining so this is unlikely to be a serious problem.

Freezing is likely to be a challenge when the average temparature is below zero for at least a couple of weeks of the year, so either the system needs to be drained, insulated or set up so the water can freeze without breaking anything.