13-02-2013, 01:08 PM
Sewage Sludge Treatment
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Introduction
Soil amendment is any material such as lime, gypsum, sawdust or synthetic conditioner that is incorporated with the soil so as to improve the soil’s physical qualities, especially its ability to provide nutrition for plants for better productivity Davis and C.R. Wilson (2012) and Brady and Weil (2008). Soil amendments are sometimes referred to as soil conditioner. Soil amendments are divided into two categories, namely: the Organic and Inorganic amendments. Organic amendments, these come from something is /was alive such as manure, compost, biosolids etc. Inorganic amendments, on the other hand, are either mined or man-made with the like of vermiculite and perlite.
Sewage sludge is the settled or solid sewage combined with varying amount of water and dissolved materials, removed from sewage by screening, sedimentation, chemical precipitation or bacterial digestion according to Brady and Wei (2008). If certain standards are met, they are referred to as biosolids. According to Havlin et. Al, (2005) sludge are divided into two namely: class A and class B, where class A is said to be the most rigorous with no site restriction such as lawn, crop and public accessible area application and in contrast class B is less rigorous treatment with some restriction on application such as in crop and animal grazing fields. Sewage contain trace elements which are metals such as arsenic, cadmium, chromium, lead, mercury, nickel and selenium and these contaminants limit the extent to which biosolids can be used.
Sewage sludge treatment
During sewage treatment, microorganisms digest (eat) the sewage, completely breaking down the original organic solids that have been discharged into the sewerage system. This leaves a low solids effluent and a solids component known as biosolids. The water content of the solids is then reduced and this reduction in water content helps to minimize order and the ability of the sludge to attract flies because they are disease transmitting agents. This usually occurs by passing through mechanical processes such as screening, sedimentation, thickening, dewatering, stabilisation and disinfection, and thermal drying. The sludge may undergo one or several treatments. Domestic wastewater will contain both solid and dissolved pollutants including faecal matter, paper, urine, sanitary items, food residues and a variety of other contaminants. Treatment of sewage is essential to ensure that the receiving water into which the effluent is ultimately discharged is not significantly polluted. However, the degree of treatment required will vary according to the type of receiving water. Sewage treatment involves: The removal of solids by physical screening or sedimentation. The resultant products are biosolids.
Composition of sewage sludge
Sludge is composed of by-product collected at different stages of the wastewater treatment process. It contains both compounds of agricultural value (including organic matter, nitrogen, phosphorus and to lesser extent, calcium, sulphur and magnesium), and pollutants which usually consists of heavy metals, organic pollutants and pathogens.
Organic matter content
Organic matter is mainly used for soil improvement. Known benefits of organic matter application to soil are the improvement of the physical properties of soil such as structure or improvement of water holding capacity. Other benefits of sludge application may be the improvement of the soil bearing strength, or the reduction for the potential for surface runoff and water erosion (Adas 2000). Known benefits of organic matter application to soil are the improvement of the physical properties of soil such as structure or improvement of water holding capacity. Major reservoir of plant nutrients
Nitrogen and phosphorus content
Nitrogen is mostly found under organic form in sludge, and to a lesser extent under ammoniac form. Other mineral forms of nitrogen are found as traces. Nitrogen availability is dependent on the type of sludge. Other factors influencing the availability of the nitrogen are extrinsic factors: temperature, humidity, pH and texture of the soil, and condition of land spreading.
Phosphorus is used by the plant for its growth, the rigidity of its cell walls, and for the development of its root system. Phosphorus in sludge is mainly present under mineral form. It has been observed that composted sludge has lower content of phosphorus as compared to non-composted sludge.