Technical Memorandum Report No. 73
Wesley M. Hirano and Reginald H. F. Young
Household wastewater can be divided into ³blackwater” (toilet wastewater) and “greywater” (non-toilet wastewater). Recently, schemes have been proposed for the segregation of the two streams with subsequent treatment and reuse of the greywater fraction, thereby resulting in a reduction in the amount of water used and wastewater produced, recycling of nutrients, and replenishment of the groundwater. Greywater, while making up about 60 to 650 of the wastewater flow, is weaker in pollutant concentration than blackwater. Greywater contributes much of the BOD5, about half of the suspended solids, little nitrogen, and most of the phosphorus to the total wastewater flow. It also contains low levels of indicator bacteria and so must be handled and treated properly before being reused. Strategies for segregation and management of the blackwater and greywater streams are outlined. Greywater treatment schemes that include anaerobic and aerobic treatment, disinfection or filtration have been proposed and studied by a few investigators; however, substantial data are still lacking. Household greywater reuse systems have been built and found to be simple, reliable, and aesthetically acceptable. A typical system would consist of a settling/storage tank with disinfection, followed bv a filter, and a pump and pressurized tank for distribution to the toilets or for lawn irrigation. Savings of 30 to 40% of the total water flow can be achieved. When reused for irrigation, a number of factors including the type of soil, topography, climate, selection of plants, method of irrigation, and quality of the greywater must be considered. Reuse of greywater for rural or suburban households in Hawaii may prove to be feasible due to the number of failing cesspools, the larger land area available for irrigation, and the rising cost of water. It could even be used in a residential house in a sewered community. At the present time, however, the cost of this system outweighs the benefits or savings achieved. In view of the increasing shortage of water though, a greywater reuse system may be an attractive alternative in the near future and should be investigated further.