| During sewage treatment, a thick residue is obtained, | | | | include filter presses, sand drying beds, centrifuges |
| known as sludge. This is the primary sludge in | | | | and vacuum filters. By using any of these processes, |
| wastewater. It gives off a strong and offensive | | | | the water content is reduced; it is then 50 to 80 |
| odor. Sludge thus obtained can't be disposed of | | | | percent of the sludge. Dry sludge is obtained as a |
| unless it is treated again. In this article, I discuss how | | | | result in the form of sludge cakes. |
| this sludge can be treated and made safe for | | | | To further reduce the volume of this sludge, both |
| disposal. | | | | aerobic and anaerobic digestive processes are |
| Sludge settles at the bottom of the tank during the | | | | employed in decomposing the organic matter in the |
| primary treatment process. This is called Primary | | | | sludge. A benefit of this digestive process is the |
| Sludge. It gives off a strong and offensive odor. | | | | stabilization of the sludge which results in the |
| Secondary treatment, to dispose off sludge, is done | | | | reduction of odor. This sludge is treated again to |
| by making use of the microorganisms left in the | | | | eliminate harmful microorganisms present, through the |
| sludge, in wastewater, after the primary treatment. | | | | use of caustic chemicals. After this treatment, the |
| Hence the Secondary Sludge thus obtained is rich in | | | | sludge residue in both liquid and cake form is used as |
| microorganisms. The sludge in wastewater is then | | | | a fertilizer, and spread in fields; the organic matter |
| treated with an aim to stabilize the sludge contents. | | | | and nutrients present in the sludge are returned to |
| This causes a reduction in odor. This treatment is also | | | | the soil, in theory. But in practice, Environmental |
| aimed at reducing the volume of the sludge. This is | | | | Protection Agencies object to this, as they are |
| achieved by reducing the water content of the | | | | unsure of the quality of sludge as manure. Also, very |
| sludge. Further reduction in volume is achieved by | | | | few markets accept such sludge as manure. |
| encouraging the microorganisms to breakdown the | | | | The treated effluent from the wastewater |
| organic matter present in the sludge. The next step | | | | treatment plant is discharged into receiving water |
| in the treatment of the sludge is to disinfect the | | | | bodies. To protect these water bodies from |
| sludge. This is accomplished by killing the pathogenic | | | | contamination, it is essential to carefully manage the |
| microorganisms left in the sludge. | | | | treatment process. The incoming influent at the |
| Almost 97 percent of the sludge obtained by primary | | | | treatment plant, the process of treatment, and the |
| treatment of sewage is actually water. To reduce | | | | final effluent to be discharged should all be carefully |
| the volume of this sludge, it is allowed to settle. The | | | | monitored and measured by well trained and certified |
| heavier matter settles to the bottom and the | | | | operators. |
| surface liquid is decanted off to obtain the residue. | | | | There are alternative technologies these days to |
| To separate the water from the sludge there are | | | | such sludge production, viz., by digesting the biological |
| several techniques that can be employed. These | | | | sludge. |