OVERVIEW
Rainwater Harvesting systems are at their most effective where there is a good balance between the amount of water that can potentially be harvested, and the amount of non-potable water that the building concerned can potentially be used. This balance is prejudiced therefore, whenever what can potentially be used far exceeds what can potentially be harvested; this arises particularly in high occupancy buildings, such as hotels and hostels, where the ratio of roof area to number of occupants is low.
Such buildings, however, also require a higher proportion of its mains-water usage to be expended on bathing and showering than in a domestic dwelling. This results in general in there being a very good match between the water in the building used for bathing/showering, and the water used for toilet-flushing. This can be exploited for water efficiency purposes by processing and reusing bath and shower wastewater as a source on non-wholesome water that can be used for toilet-flushing and other purposes that do not require wholesome water. This saves much of the mains-water that would usually be used on toilet flushing and also reduces the load on the local sewerage system by the same amount.
Greywater is not directly as acceptable for toilet-flushing purposes as harvested rainwater. This is because bath/shower water may contain contaminants making it unsuitable for storage even for short periods. Before storage for re-use, therefore, bath/shower waste water first needs to be filtered to bring it up to an aesthetically acceptable standard, albeit still non-wholesome.
The pre-storage aspects of greywater re-use, compared to harvested rainwater systems, are therefore more complicated and in need of a more robust maintenance regime. This in turn generally makes GWR systems more costly than their RWH counterparts, although by being aligned with patterns of bathing/showering, rather than weather patterns, this can result in them providing a satisfactory capital costs payback period when installed on a commercial scale, again in hotels, hostels, apartment blocks and other buildings with a high shower usage.
As with RWH, all aspects of the design, manufacture, use and maintenance of GWR systems is covered by the relevant British Standard (BS EN 16941-2 ) which includes the requirement of a systems supplier’s risk assessment.
Waste bath and shower water is not directly as acceptable for toilet-flushing purposes as harvested rainwater. This is because bath/shower water may contain contaminants making it unsuitable for storage even for relatively short periods. Before storage for re-use, therefore, bath/shower waste water first needs to be filtered to bring it up to an aesthetically acceptable standard, albeit still non-wholesome.
The pre-storage aspects of greywater re-use, compared to harvested rainwater systems, are therefore more complicated and in need of a more robust maintenance regime. This in turn generally makes greywater recycling systems more costly than their rainwater harvesting counterparts, although by being aligned with patterns of bathing/showering, rather than weather patterns, this can result in them providing a satisfactory capital costs payback period when installed on a commercial scale, again in hotels, hostels, apartment blocks and other buildings with a high shower usage.
As with rainwater harvesting, all aspects of the design, manufacture, use and maintenance of greywater recycling systems is covered by the relevant British Standard (BS EN 16941-2 ) which includes the requirement of a systems supplier’s risk assessment.
The complexity of a greywater system can vary widely, from simple systems that just filter and redirect water for irrigation, to complex ones that include advanced filtration, disinfection, and storage. The choice of system depends on factors like the amount of greywater that can be used and quality and quantity of greywater potentially available. The following section is therefore generic, individual suppliers being able to provide the more precise information that applies to their systems.
HOW SYSTEMS WORK
Greywater recycling systems target so called “light greywater” which is the water coming from baths, showers, and hand-wash basins. Clothes washing machine water can also be used, but it complicates the treatment process and provides a low yield. Kitchen sinks and dishwashers are excluded as water with fat content is difficult to treat. Recycling greywater can greatly reduce mains-water usage, with the step-by-step guide below showing how a typical greywater recycling system operates.
Collection: The first step in the process is the collection of greywater. The greywater sources like bathroom sinks, showers and baths are separated from the other waste-water services and directed into a greywater system.
Treatment: Will usually comprise a train that includes one or more of the following:
- Sedimentation/flotation
- Screening
- Mechanical fine filtration
- Biological or chemical treatment, as well as disinfection
Storage: The treated greywater is then stored in a tank. Because greywater can quickly become stagnant and develop an odour, it’s often recommended to use it within a defined period, after which it is automatically drained or treated again to prevent stagnation if it’s not used within the defined timeframe.
Distribution: From the storage tank, the recycled greywater can be distributed for non-potable uses. This includes toilet flushing, irrigation, and in some cases, laundry.
Overflow: If the storage tank is full or the greywater system is offline for any reason, there should be an overflow system that can direct excess greywater to the normal waste system.
Integration with Main Water Supply: Also, greywater systems need to be integrated with the mains water supply, so that when there isn’t enough greywater, mains water can be introduced into the treated grey water storage tank should it be needed. This is achieved in a similar way to RWH systems.