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.