During sewage treatment, a thick residue is obtained, known as sludge. This is the primary sludge in wastewater. It gives off a strong and offensive odor. Sludge thus obtained can't be disposed of unless it is treated again. In this article, I discuss how this sludge can be treated and made safe for disposal.
Sludge settles at the bottom of the tank during the primary treatment process. This is called Primary Sludge. It gives off a strong and offensive odor. Secondary treatment, to dispose off sludge, is done by making use of the microorganisms left in the sludge, in wastewater, after the primary treatment. Hence the Secondary Sludge thus obtained is rich in microorganisms. The sludge in wastewater is then treated with an aim to stabilize the sludge contents. This causes a reduction in odor. This treatment is also aimed at reducing the volume of the sludge. This is achieved by reducing the water content of the sludge. Further reduction in volume is achieved by encouraging the microorganisms to breakdown the organic matter present in the sludge. The next step in the treatment of the sludge is to disinfect the sludge. This is accomplished by killing the pathogenic microorganisms left in the sludge.
Almost 97 percent of the sludge obtained by primary treatment of sewage is actually water. To reduce the volume of this sludge, it is allowed to settle. The heavier matter settles to the bottom and the surface liquid is decanted off to obtain the residue.
To separate the water from the sludge there are several techniques that can be employed. These include filter presses, sand drying beds, centrifuges and vacuum filters. By using any of these processes, the water content is reduced; it is then 50 to 80 percent of the sludge. Dry sludge is obtained as a result in the form of sludge cakes.
To further reduce the volume of this sludge, both aerobic and anaerobic digestive processes are employed in decomposing the organic matter in the sludge. A benefit of this digestive process is the stabilization of the sludge which results in the reduction of odor. This sludge is treated again to eliminate harmful microorganisms present, through the use of caustic chemicals. After this treatment, the sludge residue in both liquid and cake form is used as a fertilizer, and spread in fields; the organic matter and nutrients present in the sludge are returned to the soil, in theory. But in practice, Environmental Protection Agencies object to this, as they are unsure of the quality of sludge as manure. Also, very few markets accept such sludge as manure.
The treated effluent from the wastewater treatment plant is discharged into receiving water bodies. To protect these water bodies from contamination, it is essential to carefully manage the treatment process. The incoming influent at the treatment plant, the process of treatment, and the final effluent to be discharged should all be carefully monitored and measured by well trained and certified operators.
There are alternative technologies these days to such sludge production, viz., by digesting the biological sludge.
Want a FREE eBook on a new wastewater treatment technology? Want to learn everything you need to know about wastewater treatment including how to treat the sludge in wastewater? Click here: http://www.all-about-wastewater-treatment.com
Rod Nash is the President of Geostar Publishing Services LLC. Rod loves net research & blogging. His new blog on Wastewater Treatment is fast becoming popular, as it is comprehensive and well-researched.
Showing posts with label Disposal. Show all posts
Showing posts with label Disposal. Show all posts
Monday, January 31, 2011
Wednesday, November 10, 2010
Utilisation And Disposal Of Digested Sludge
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].
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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