Water Management: Greywater and Septic Tanks

Greywater is a form of used household waste water which is not the black water that comes from toilets being flushed – greywater comes from sinks, dishwashers, washing machines and so forth. In some areas, there is also a distinction with ‘dark greywater,’ which comes from washing nappies (sometimes known as ‘diapers’) or similarly soiled items. In the USA, some states have adopted greywater reuse systems that separate the different flows of waste water between grey and black. Some 22 million Americans use 8 million greywater systems but there has never been a documented case of illness caused by the system: it seems reasonable to conclude, therefore, that the system is safe and should be expanded as far as possible.

One area in which efficiency and cost savings can be made through using greywater is in septic tanks. This may not be a very exciting issue on the face of it but many millions of people around the world rely on septic tanks to deal with their household waste (we have two, in fact). At the household level, septic tanks will more quickly fill up when grey and blackwater are combined and require emptying more often, which is more expensive for the householder and requires more resources from the local authority (let us assume, for the sake of argument, that there is no great clamour to open up waste disposal to the private sector). This raises costs all round and makes the storage areas and reprocessing systems work under greater pressure.

There is also the issue of areas in which septic tanks are not suitable, owing ot the ground conditions or other factors. In these cases, reducing the need for sewerage pipes to take blackwater perhaps far away will be a significant saving. Of course, there are new technologies being developed which can assist with localized waste reprocessing, although it seems these are not yet ready for widespread deployment at the household level.

Municipalities also use septic tank technology to store blackwater while preparation for reprocessing takes place. As might be expected, these facilities are much larger and more complex than those in use in household uses but still subject to the same constraints of time, space and other resource use. Employing the greywater system here, therefore, would bring about more savings and efficiency improvements. It also would mean less need for intensive chemical use and improved growth of plants, since the greywater can be used to recharge groundwater in various areas. It also increases the ability of recycling systems to obtain nutrients that would otherwise be lost to the sea.