BOD (BIOLOGICALOXYGEN DEMAND) is the amount of oxygen in water that will be consumed by microorganisms in a certain period of time. The more organic nutrients in the water, the greater the BOD, because there will be more microorganisms feeding on the nutrients and consuming oxygen. BOD is measured by obtaining equal volumes of water from a source to be tested. Each specimen is diluted with a known volume of distilled water which has been thoroughly shaken to ensure oxygen saturation. One specimen is measured for dissolved oxygen; the other is set aside in a dark place for five days, then measured. BOD is determined by subtracting the second reading from the first. BOD5 is a measure of the oxygen depletion after five days. High BOD is an indicator of organic pollution.
COLIFORM BACTERIA - Bacteria occurring naturally in the intestines of warmblooded animals. Most do not cause disease. Drinking water should have less than four coliform bacteria per 100 ml of water. Counts higher than 2,300/100 ml are considered unsafe for swimming, and waters with 10,000/100 ml are unsafe for boating.
CONSTRUCTED WETLAND - A humanmade complex of saturated substrates (such as gravel), with emergent and submergent plants, animal life and water at or near the surface, which simulates natural wetlands for human use and benefit.
HYDROPHYTE - water-loving plant contact with the graywater. Plant material, soil and food scraps can contribute to the total coliform population, but fecal coliforms indicate that fecal material is also entering the water system. This can come from baby diapers, or just from bathing or showering.
More microorganisms may come from shower and bath gray-water than from other graywater sources. Studies have shown that total coliforms and fecal coliforms were approximately ten times greater in bathing water than in laundry water (see Figure 9.2).13
One study showed an average of 215 total coliforms per 100 ml and 107 fecal coliforms per 100 ml in laundry water; 1,810 total coliforms and 1,210 fecal coliforms per 100 ml in bath water; and 18,800,000 colony-forming units of total coliforms per 100 ml in gray-water containing household garbage (such as when a garbage disposal is used).14 Obviously, grinding and dumping food waste down a drain greatly increases the bacterial population of the graywater.
Due to the undigested nature of the organic material in gray-water, microorganisms can grow and reproduce in the water during storage. The numbers of bacteria can actually increase in graywater within the first 48 hours of storage, then remain stable for about 12 days, after which they slowly decline (see Figure 9.1).15
For maximum hygienic safety, follow these simple rules when using a graywater recycling system: don't drink graywater; don't come in physical contact with graywater (and wash promptly if you acci-dently do come in contact with it); don't allow graywater to come in contact with edible portions of food crops; don't allow graywater to pool on the surface of the ground; and don't allow graywater to run off your property.
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