Sludge in Wastewater - How it is Made Safe For Disposal

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.

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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.

Digestion of Sludge - Optimizing Digestion in Wastewater Treatment

Sludge Digestion Tanks

A sludge digestion tank is an R.C.C. tank of cylindrical shape with a hopper bottom and is covered with a fixed or floating type of roof. The latter makes the digestion of sludge much more effective. The weight of the cover is supported by sludge, and the liquid forced between the tank wall and the side of the cover provides a good seal. The raw sludge is pumped into the tank where it is seeded with digested sludge.

On undergoing anaerobic digestion, gases of decomposition (chiefly Ch4 , CO2) are given out by the anaerobic digester. Sludge gas rises out of the digesting sludge. It moves along the ceiling of the cover and collects in the gas dome. The cover can float on the surface of the sludge between the landing brackets and the overflow pipe. Rollers around the circumference of the cover keep it from binding against the tank wall.

The digested sludge, which settles down to the bottom of the tank is removed under hydrostatic pressure periodically, say, once a week. To maintain optimum temperature, the tank is generally provided with heating coils through which hot water is circulated.

The supernatant liquor i.e., the part of the tank content lying between the scum and the sludge is withdrawn at the optimum level through a number of withdrawal points located at different elevations of the tank. As it is high in BOD and suspended matter contents, it is sent back to the incoming raw sewage for undergoing re-treatment. During the digestion of sludge, the scum formed at the surface gets broken up by the recirculating flow or through mechanical rackers called scum-breakers.

The amount of sludge gas produced during digestion of sludge varies from 0.014 to 0.028 m3 per capita with 0.017 m3 being quite common. The gas produced contains 65 per cent of methane with a calorific value 5400 -- 5850 kcal. m3, 30 per cent of carbon dioxide and balance 5 per cent of nitrogen and other inert gases. It resembles natural gas and may be used as a fuel for cooking. Principal uses however, are for driving gas engines, and for heating sludge to promote quick digestion of sludge.

For a free eBook on a great new wastewater treatment technology and for everything else you want to know about wastewater treatment including digestion of sludge, click http://www.all-about-wastewater-treatment.com.

Richard J. Runion is the President of Geostar Publishing & Services LLC. Rich loves net research & blogging. His new blog on Wastewater Treatment is fast becoming popular, as it is comprehensive and well-researched.