Last edited 30 Jun 2021

Black water recycling

Water is an increasingly scarce resource. As the population increases, and climate change makes rainwater patterns less predictable, it is becoming more important that we reduce the amount of water we consume and discharge into the sewerage infrastructure. In addition, the treatment of water to make it suitable for drinking and other uses consumes a considerable amount of energy. Treating water to make it suitable for ‘drinking’ just to use it for purposes that do not require this level of treatment, such as watering gardens, washing cars or flushing toilets, is extremely wasteful.

Environmental concerns, utilities bills and the imposition of restrictions such as requirements for sustainable urban drainage systems and hosepipe bans mean that people are increasingly looking to re-use or re-cycle water.

Typically, water is categorised within one of three broad groups, with the degree of contamination increasing, and so the number of suitable uses decreasing and the treatment requirement increasing:

Fresh, potable water or ‘drinking water’ is sometimes referred to as ‘white water’.
Wastewater from showers, baths, washbasins and washing machines that is not considered to be potentially dangerous is referred to as ‘grey water’.
Water from toilets that may be contaminated with hazardous material is referred to as ‘black water’ (or sometimes brown water, foul water, or sewage). Some definitions of black water also include water from kitchen sinks, dishwashers and waste disposal units which may contain food particles.

Despite this contamination, black water can be recycled and re-used. Typically this re-use is for applications such as watering landscape (although generally not crops) or for flushing toilets (Ref Thames Water, Recycled Londoners’ sewage keeping Olympic Park green). It is also possible, although expensive, to make black water suitable for drinking.

Recycling black water can:

Reduce water use.
Conserve energy and reduce utilities bills.
Reduce loads on local sewerage systems.
Contribute to sustainability standards such as LEED.

However, it can be:

Expensive.
Difficult to manage and maintain.
There can be perception and stigma issues.
It requires a separate, clearly identified pipe network.
Long payback times combined with small storage volumes mean it can be less effective than other water-conservation measures.

It is possible to recycle black water at a domestic scale, but this is not common due to the expense involved, despite the fact that many remote domestic properties already have septic tanks to deal with sewerage locally. Generally, the process is only adopted on larger sites, or for multi-home developments.

Treatment is generally by a process of settlement, bacterial break down, filtration, aeration, and chemical treatment. This can include the use of reed beds. For more information see The Green Age, Recycling Domestic Sewage.