After anaerobic digestion, the sludge would contain about 35% organic and about 65% inorganic material. The digested sludge contains about 2,5% nitrogen, about 1% phosphorus and about 0,2% potassium. In addition to these so-called macro-nutrients, the sludge also contains the minor nutrients such as calcium, magnesium, iron, sulphur etc. Depending on the source of the waste-water, the sludge would also contain metals such as copper, chromium, nickel, zinc and cadmium.
Certain components of the sludge may be advantageously utilised when the sludge is incorporated into agricultural land. In fact, as a result of the organic content and the presence of the nutrients and the trace elements essential for plant growth, sludge is generally valued as a soil conditioner. However, due to the presence of certain contaminants such as heavy metals, viable pathogenic and other organisms and complex organic compounds, careful consideration must be given to its potentially dangerous and hazardous properties when disposing of waste treatment sludge.
Waste-water sludge is classified into three types; (a) unstable with high odour and fly nuisance potential, contains a high content of pathogenic organisms - primary or raw sludge falls into this grouping, (b) stable with low odour and fly nuisance potential, has a reduced content of pathogenic organisms - humus, waste activated and anaerobically digested sludges fall into this grouping, (c) stable with insignificant odour and fly nuisance potential, contains insignificant numbers of pathogenic organisms - anaerobically digested sludge when preceded or followed by pasteurisation falls into this grouping, (d) as type c but with contents of specified elements below prescribed limits.
The purpose of describing the waste sludge in terms of the above classification and placing certain restrictions on the disposal of waste sludge is to minimise nuisances and the transmission of pathogenic organisms either directly to man or indirectly through the food chain, as well as protecting water resources and the environment from pollution.
As indicated above, waste-water sludge contains nutrients which can be used beneficially to improve the condition of the soil. Sludge helps to break up heavy clay soils and improves the moisture retaining ability of sandy soils. Although the nitrogen, phosphorus and potassium contents of waste sludges are low compared with the inorganic fertilisers, these nutrients are not leached out as quickly as those from inorganic fertilisers.
Sludges derived from domestic waste-water treatment contains trace elements and minor nutrients in relative concentrations that are of the same order as required by most crops. This means that provided reasonable spreading rates are used, one should not experience metal accumulation problems with domestic sludges. However, when industrial effluents are present in the waste-water, significant concentrations of metals and other undesirable constituents may be present in the sludge and it becomes most important to ensure that the spreading rate is such that undesirable concentrations of metals etc, do not accumulate in the soil.
Where waste sludge cannot be disposed as a soil conditioner, it may be desirable or even necessary to co-dispose the sludge in admixture with refuse on a sanitary landfill site. The co-disposal of refuse and secondary waste-water sludge (in particular digested sludge) in sanitary landfills is being practised in many parts of the world, especially in drier areas which have a perennial water deficit.
Although the utilisation of waste sludge to improve soils used for agricultural process provides an attractive means of disposing of the waste sludge, there are many factors to be considered to avoid creating a secondary waste pollution problem that may be very costly and time-consuming to rectify.
Michael Russell Your Independent guide to Waste Treatments [http://waste-treatments.com].
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