Características del envejecimiento a nivel cerebral

The composition of grey water depends on various factors such as the nature of household and personal care products used, the activities and habits of the residents, the source of the grey water and frequency of use (Prathapar et al. 2005; Al-Jayyousi 2003; Jefferson et al. 2000).

Grey water is often characterised in terms of its physical, chemical (organic and inorganic) and biological composition (Morel & Diener 2006).

The main physical characteristic of grey water is its total solids, composed of dissolved, suspended and settle-able solids. The major sources of total solids are from the kitchen sink and dishwasher. These include food particles from the kitchen sink and hair, lint, soil particles and faeces from the washing machine (Eriksson et al. 2002). Levels of total solids that are too high or too low can have an adverse effect on the treatment efficiency of grey water treatment

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processes such as disinfection and filtration. Total solids also affect the turbidity (clarity) of the grey water and can lead to the development of anaerobic conditions due to depletion of oxygen. Anaerobic decomposition of organic matter results in the generation of odorous gases such as hydrogen sulphide and ammonia (Tchobanoglous et al. 2003). Suspended solids carry a significant proportion of organic matter and thus contribute to the majority of the organic load in grey water. Table 2.1 shows grey water generated from the kitchen sink has the highest concentration of total suspended solids (TSS) ranging between 227 and 720 mg/l whilst bathroom grey water consist of a much lower TSS concentration of between 54-200 mg/l. Organic contaminants in grey water are normally composed of a combination of proteins, carbohydrates, oil, fats and greases. Organic contaminants can be sourced from soaps, conditioners, shampoos, perfumes/fragrances, hair dye, household cleaning products, food waste and detergents. The primary contributor of organic contaminants, including oil and grease, are grey water sourced from kitchen sinks and washing machine due to the high content of food waste (Table 2.1) (Almeida 1999). The analysis that can be used to measure levels of organic material and thus the efficiency of treatment process includes biological oxygen demand (BOD), chemical oxygen demand (COD) and total organic carbon (TOC) (Eriksson et al. 2002). BOD is a measure of the dissolved oxygen used by microorganisms to biologically oxidise the organic matter in the water. One of the disadvantages of the BOD test is that it only measures biodegradable organics. Another disadvantage is that the actual period required for exhaustion of soluble organic matter has not been validated. This reduces the usefulness of the test results (Tchobanoglous et al. 2003). The COD test is used to measure the amount of oxygen required to chemically oxidise the organic matter in water. The resulting value is higher than the BOD value because more organic compounds and some inorganic compounds are oxidised by the chemicals in the test than are oxidised in the BOD test (Tchobanoglous et al. 2003). The TOC test is used to determine the amount of organic carbon present in the water. With the TOC test, all organic and inorganic compounds are oxidised using electrochemistry which means the resulting value would be higher than that obtained either from the BOD or COD test methods. The TOC test is often recommended to validate the BOD test; providing a valid relationship

between the two can be established.

Inorganic constituents in grey water include metals, gases and nutrients. Nitrogen in the form of ammonia, nitrate, nitrite and total Kjeldahl nitrogen and phosphorous are nutrients for the biological development of all major micro-organisms in the grey water. These nutrients can also cause significant problems in the environment when discharged into the ecosystem due to

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rapid oxygen depletion. Gasses such as methane, ammonia and hydrogen sulphide are developed as a result of anaerobic decomposition of organic matter present in grey water. These gases give off unpleasant odour and are of concern to public health and safety. Very small traces of metals such as cadmium, cobalt, lead, selenium and cobalt can also be found in grey water, these presumably comes predominantly from components of the plumbing system (Tchobanoglous et al. 2003). Nitrogen and phosphorous compounds are found in faecal contaminants and the main sources of these are from washing machines and dishwashers (Table 2.1). Information on the origins of all metals in grey water is limited, although some metals such as cobalt, selenium and molybdenum are sources from food and can be shed through faeces. Some metals may also be brought about as a result of water softeners used in homes. Chloride is another inorganic constituent present in grey water and is sourced primarily through the use of household cleaning products, it is also present in water softeners and in potable water. Control of biological contaminant is of fundamental importance in the limitation of water borne disease caused by pathogens. Pathogenic organisms can be excreted by animals and humans who are carriers of a particular infectious disease. The class of pathogenic organism that can be found in grey water are bacteria, protozoa, helminths and viruses. Bacteria colonize in the human intestinal tract, one of the most common pathogenic bacteria found in domestic wastewater is Salmonella and in some occasion Clostridium (Tchobanoglous et al. 2003). All

these forms of pathogenic organisms can be sourced from human and animal faeces as well as from soil and insects. The warm temperature often found in grey water is an ideal condition for these organisms to grow. The numbers of pathogenic organisms present in grey water are usually few and it can be quite a challenge to isolate and identify them. For this reason, micro- organisms which are more numerous and are easier to test for are typically used as indicator organisms for the target pathogens. The organisms that have been used as indicators especially for faecal contamination are E.coli, total coliform (TC), faecal coliform (FC), Klebsiella, faecal streptococci, Enterococci, Clostridium perfringens, Pseudomonas aeruginosa and A. hydrophila.

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Table 2.1 - Characteristic of grey water compiled from data obtained from numerous literature

(Sources; Siegrist et al 1976; Christova-Boal et al. 1996; Surendran and Wheatley 1998;

Almeida et al. 1999; Nolde 2000)