Green Roof Water Recycling System
The Green Roof Water Recycling System (GROW) is an innovative greywater recycling scheme designed by Water Works UK for low-rise, multi-storey, multi-occupancy urban dwellings. Planted roof-top troughs cleanse the greywater and return it via a dedicated pipework system for use in toilet flushing. A full scale trial of the system is underway at student accommodation at Cranfield University.
How it works
Grey water is captured, filtered to remove debris, and stored in an underground sump. It is then pumped up to roof level by a solar-powered pump, where, by gravity, it descends through the network of gravel and aquatic-plant filled troughs.
The water passes through the roots where bacteria cleanse it. After passing through the last of the troughs, the water is disinfected by ultraviolet light and dyed translucent green with a vegetable dye (1:20,000 dilution) to differentiate this ‘green water’ from potable water. The pipework for the GROW system is also differentiated from potable water pipework by its green colour and being physically non-compatible with potable water pipework. The green water is then stored in a holding tank, ready for use as toilet flush water. It takes about seventeen hours for water to pass round the whole system.
Click on the diagram to see an enlarged version
The plants
The plants were chosen for their hardiness, dendritic root network, minimum maintenance requirements and aesthetic value. Plants used included Iris (Iris pseudocorus and versicolour), Common Rush (Juncus effuses) and Water Mint (Mentha aquatica). The plants in the first trough must be relatively tolerant to soap products and small amounts of bleach that users may pour down the sink.
The water
GROW is designed for multi-occupancy buildings so there is a continual supply of greywater and the plants do not dry out. A typical five-trough system would process 1m3 of water a day which is enough green water for 25 - 40 people.
The target water quality for the green water is less than 10 mg/litre suspended solids, less than 10 mg/litre BOD and less than 5 mg/litre Ammonia (1). As well as providing an alternative sustainable water supply, GROW doubles as a living roof-top garden and urban wildlife habitat.
