Greywater resource : LUSENET : Grassroots Information Coordination Center (GICC) : One Thread

The following site some good info on Greywater.

****************************************************************** Greywater is washwater EXCLUDING toilet wastes [blackwater]. Many sanitary engineers maintain that all wastewaters are alike and require the same treatment. This notion is not supported by the research data currently available. There are very significant distinctions between greywater and toilet residues; for how these wastewaters should be treated /managed and why they should not be mixed together.

Key differences between Greywater and Blackwater: GREYWATER

a)contains far less nitrogen than blackwater: Greywater contains less than one-tenth of the nitrogen contained in blackwater. Nitrogen (as nitrite and nitrate) is one of the most serious and difficult-to-remove pollutants affecting our potential drinking water supply. As greywater contains far less nitrogen, it need not undergo the same treatment process as blackwater or combined black- and greywater; When only greywater is processed and discharged a 90% nitrogen removal credit should be given;

b)contains far fewer pathogens: Medical and public health professionals view blackwater as the most significant source of human pathogens. Organisms adversely affecting human health do not grow outside of the body (unless incubated and with rare exceptions) but are capable of surviving especially if hosted in human feces. Separating greywater from blackwater dramatically reduces the danger posed by such organisms since greywater is free of regular feces input which carry (and may encapsulate) pathogenic organisms;

c)breaks down much faster than blackwater: The organic content typical of greywater decomposes much faster than the content typical of blackwater (see graphs Fig 2. & 3.). The amount of oxygen required for the decomposition of the organic content in greywater during the first 5 days (known as Biological Oxygen Demand over five days or BOD5) constitutes 90% of the total or ultimate oxygen demand (UOD) required for complete decomposition. BOD5 for Blackwater is only 40% of the oxygen required. BOD1 for greywater is around 40% of the ultimate oxygen demand (UOD) and BOD1 for blackwater is only 8% of the UOD]. This means that the decomposing organic matter in blackwater will continue to consume oxygen far longer and further away from the point of discharge than will greywater. This faster rate of stabilization for greywater is advantageous to prevention of water pollution as the organic material in it decomposes quickly in soils during infiltration --and impacts less on the environment since it does not travel far. NOTE:This faster degradation rate has also prompted engineers to observe that the immediate impact from a greywater discharge into a lake, for example, stimulates rapid plant or algea growth in the area adjacent to the discharge point and may therefore appear to be worse or more polluting.

In view of the fact that grey and blackwaters are so different, it behooves us to separate them - more specifically, to keep urine and feces out of water altogether - and to treat/manage them separately for the optimal protection of public health and the environment.

-- K (, April 23, 2001


Also check out the National Small Flows Clearinghouse.


-- j (, April 23, 2001.

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