Rainwater harvesting tanks: Do they need cleaning?

Introduction

Rainwater harvesting systems are becoming increasingly popular, and the fact the Freerain's early customers have had their tanks installed since 1999. Many people are considering the long term effects and issues concerning older rainwater harvesting systems.

Cleaning?

The general water quality and effectiveness of a rainwater harvesting system is largely dependent on the size of the storage tank installed. Too large and it may never overflow and turn the water over enough. Too small and it doesn't store enough water to be effective and the dwelling/project is reliant on mains water top-up.

Assuming the storage tank has been sized correctly, then it should overflow a few times a year, assuming the system includes a calming inlet and has a suitable pre-tank filter, then the tanks really shouldn't need cleaning. The heavier particles should be removed by the leaf and grit pre-tank filter, the finest particles should float and be skimmed off during overflow. The particles in suspension eventually get sucked up by the submersible pump and are then removed by the inline strainer/carbon filter or whatever has been included on the system.

So a well designed system is very unlikely to need cleaning out, but there is one big assumption, this assumes that only roof water has been sent to the system. Water from hard-standing should not be sent to the rainwater tank, nor should water from a green roof system. Although newer green roof systems do filter the water very well. The overflow from the storage tank should also be protected from backing up. In other words, a well designed system, being used in a normal way, should NEVER need cleaning.

Germany

In Germany rainwater harvesting is about twenty years ahead of the UK and their experience is exactly as described above. Only poorly designed or mis-used systems require major maintenance.

Roof top rainwater storage, does it work?

If you think of a traditional rainwater harvesting system, in the UK and most of Europe. Then you would probably think of an underground storage tank, a physical filter before storage and a pumping system. We have previously looked at the options for controls and this is broadly two types. The direct pressure system and the header tank system, being fed from the underground tank.

We at Freerain Ltd, are asked quite often about the option of eliminating the underground storage tank and diverting roof water directly into a header tank. On first thought this seems like a good idea, no excavation, no underground tank and maybe even no pump!! All sounds pretty good?

Domestic system, we need storage of around 800-1,000 litres per person to make the rainwater harvesting system effective. So a 4,700 litre system when full would weight around 5 MT!! and take up a considerable amount of space. In order to strengthen the floor, it could be very costly and indeed more than a non-concrete underground tank installation. (as per Freerain tanks). We have also previously discussed the fact that the water should remain dark, cold and not subjected to thermo-cycling (that is rapid heating and cooling). Whereas inside a building it might be. Finally, the overflow from storage would need to be designed to never flood the building at times of heavy rainfall.

Commercial systems are more likely to be specifically designed to take the weight, if underground or above ground outside is not possible. But internal sectional storage tanks do tend to me more expensive and take up valuable space.

Filtering the water prior to the storage tank can sometimes be problematic. A traditional underground system utilises gravity to move the water from the roof, through/across a filter and finally in to a storage tank. The rainwater filter is normally either housed in the tank neck (as with Freerain domestic tanks) or externally for larger commercial systems. So if you want to have a high-level storage tank it must be designed to be able to accommodate any invert drops across the filters and the overflow.

In summary, for domestic projects we wouldn't consider it as a viable option, too much weight or too small amount of water stored. For commercial projects it is always an option, but only if underground or above ground external has been considered and rejected first.