Language is important; it is one of the things that define us as human. As the environmental building industry expands, new systems, processes and products are introduced and new words start to become common currency. I am interested (and often surprised) by the way technical terms become misused, by building professionals as well as the general public.

Rainwater and greywater
Rainwater and greywater (two completely different types of water, with differing requirements for treatment and storage) are increasingly referred to as greywater. So much so I now routinely ask whether the speaker really means greywater, regardless of the conviction with which they state the word. Rainwater is obviously rain that has fallen out of the sky, which in a conventional building is discharged to a surface water sewer, combined sewer, or a soakaway; if it is stored for use back in the building it is still rainwater, until it is used. Once it is used it becomes either foul water (if used to flush WCs or urinals) or waste water if used for washing clothes. If wastewater from a bath, basin or shower is collected for re-use it becomes greywater. If greywater is used to flush WCs it becomes foul water. If it is used in washing machines it becomes waste water (but would not circulate through the greywater recycling system again as waste water from washing machines has too many detergents in it to be considered as suitable for greywater recycling). Simples!

Greywater and “blackwater”
When I first heard the term greywater used for waste water (back in 1997) I naively assumed it to be a reference to the appearance of the water due to the effect of scum formation, and the colour wastewater becomes after it begins to biologically decompose. But then I started to hear the term “blackwater” to describe water from toilets. “Blackwater” categorically does not describe the appearance of foul water either in the sewers or whilst undergoing treatment at a sewage treatment plant. It is an example of using the term black to describe something that has negative connotations, rather than an actual description (such as blackboard). We have a perfectly adequate term to describe water from toilets, which is foul water, and in the 21st Century our language should be smarter than this.

Sewage treatment for direct or indirect re-use
Now the term “blackwater treatment” 1 is being used in the UK to describe the on-site treatment of sewage in an eco town or on an eco development, where the effluent is used back in the development for certain non-potable purposes. Although this is a different process to normal sewage treatment in so far as the sewage effluent is treated beyond normal secondary and tertiary treatment, (and the effluent may meet drinking water standards), the technology is not new, is not confined to just “eco” developments, and has a technical term that describes it perfectly, which is sewage treatment for direct re-use. In the US, home of the term “blackwater”2 , such treatment and distribution schemes, of which there are an increasing number, are now being referred to as simply “water re-use” systems.

  • Direct re-use: the planned and deliberate use of treated sewage. At its most extreme, the sewage effluent is cleaned to potable water standard and injected directly into the mains supplying a town or city. However at present most direct re-use systems clean the sewage effluent so that it is considered fit for purpose for irrigation, WC flushing and urinal flushing, and supply a secondary network of distribution pipework for this water as well as a mains supply network. A separate distribution network is very costly. Therefore it makes economic sense for sewage treatment re-use to be directly into the mains. From 1985 to 1992 the City of Denver, USA, ran a large scale trial of direct re-use and found that water quality parameters were equal to or better than the city’s drinking water. However, public perception about direct re-use for potable water supplies is still mostly negative, and is holding back large scale take-up of this process.
  • Indirect re-use: water that is taken from a river, lake or aquifer that has received sewage or sewage effluent. Much of the water we use in our buildings in the UK could be classified as indirect re-use, i.e. effluent from one town’s sewage treatment plant is discharged into a river, taken out from the same river further downstream, to be cleaned and supplied to the next town. Hence the saying that every glass of water we drink has passed through seven other people’s kidneys first. With planned indirect water re-use the sewage effluent is discharged immediately upstream of the water treatment plant or used to recharge aquifers. Indirect re-use of sewage effluent is beginning to be used far more around the world as water demand increases and the water suppliers need a guaranteed supply.

In the UK the most high profile example of a sewage treatment plant with direct re-use is on the Olympic site. Whilst I have seen some publications refer to it by its technical term I have also seen it referred to as a “blackwater” sewage treatment facility and a “blackwater” treatment system. It is sewage treatment with direct re-use for non potable purposes. It is a highly technical solution; let’s call it by its correct technical name.


  1. The term isn’t even used correctly as “blackwater” treatment plants deal with both foul and waste water (i.e. “blackwater” and greywater).
  2. The first reference I could find to it was in a US patent applied for in 1974 where water was divided into “white water” (drinking water) and “black water” (sewage from the building).

Thanks to Warren Liebold, Director of Metering and Water Conservation for New York City Department of Environmental Protection, for the discussions we had about this topic while I was writing the column.

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What’s in a word?