Greywater: Synopsis

Conventional sanitary engineering has maintained that "sewage is sewage" whether it be greywater alone or total sewage (grey and blackwater mixed together. There is one reasonable argument for this position: namely, that greywater, if left untreated for a few days, will behave like total sewage. Both will become malodorous (become anaerobic), and both will contain a large number of bacteria. The observation of these common characteristics has given rise to regulations that do not distinguish between the various sources of pollution and which therefore mandate the same treatment for all wastewaters. But the differences between greywater and total sewage are far more important than their similarities, the following document will present an alternative strategy for treating/managing greywater and give the rationale for this approach.


Greywater is specifically washwater. That is, bath, dish, and laundry water excluding toilet wastes and free of garbage-grinder residues.When properly managed, greywater can be a valuable resource which horticultural and agricultural growers as well as home gardeners can benefit from. It can also be valuable to landscape planners, builders, developers and contractors because of the design and landscaping advantages of on-site greywater treatment/management. It is, after all, the same phosphorous, potassium and nitrogen making greywater a source of pollution for lakes, rivers and ground water which are excellent nutrient sources for vegetation when this particular form of wastewater is made available for irrigation.

 Greywater irrigation has long been practiced in areas where water is in short supply. However, proper precautions for its use have not always been observed. This has posed a problem for health officials, who contend that there is no good management method for greywater which both balances user needs with public safety considerations. In fact, options for making safe use of greywater as a source for irrigation are many and diverse. The engineering of these systems is still a relatively young technology; but it is one making rapid progress. It also makes sense from both the environmental and "waste" management points of view. As these systems utilize the nutrient (potential pollutant) content in the effluent, they constitute a real solution to the treatment /management of greywater. "Real solution" here means that these greywater treatment/management systems simply do not generate waste products which, by definition, require disposal.

In the following material, technical as well as practical aspects of greywater irrigation are discussed. The requisite equipment, now commercially available through Clivus Multrum, Inc., is described as is the matter of system-sizing.

Greywater characteristics data cited here are from the most thorough report known on the subject at the present time: "Residential Waste Water" (Hushållsspillvattnet) by Lars Karlgren, Victor Tullander, Torsten Ahl and Eskil Olson. This report was funded by the Swedish National Board for Building Research in 1966 and was published in the magazine Water (Vatten, 3 -67) in March of 1967. Some of this report's diagrams and data are used here as references. The report is based on separated greywater/blackwater plumbing in an multi-apartment complex in Stockholm, and data was collected over a period of 12 weeks. The report is based on about 3500 analyses. Of particular interest is its investigation of the BOD-curve characteristics of greywater. It documents the difference in speed of decay over time between greywater and blackwater.

Cambridge, November 30th 1992 Carl Lindstrom.Copyright©