Sludge is the residue accumulated in the sewage treatment plants. Sludge is three forms solid, semisolid, or slurry residual material produced as a by-product of wastewater treatment processes. This residue is commonly classified as primary and secondary sludge. Primary sludge is generated from chemical precipitation, sedimentation, and other primary processes, whereas secondary sludge is the activated waste biomass resulting from biological treatments.
The type of sludge produced varies according to the technology used in the sewage treatment plants. Some technology may have only the primary sludge, and they use a filtration process to get the treated water. The primary sludge usually does not have any microbes. The secondary sludge has flocs or consortiums of microbes called activated sludge, which is sent back to the aeration tank from the secondary clarifier or settling tank. This process is known as the activated sludge process, and the residue after this process is very low and easily disposed of. The residue, when dried, looks like mud which can is used as manure for the plants.
Thickening is usually the most commonly used method in most treatment plants. The sludge is left to settle in the bottom and pumped and pressed through the sludge press. They are made into cakes, which is disposed of or sold as fertilisers. The problem with this method is it produces a horrible smell. There are other methods where air bubbles dissolve along with sludge, which thickens the sludge and starts to float, which can be easily removed.
Sludge digestion is the primary biological process that happens in the aeration tank. Where the organic particles are converted into stable substances, digestion occurs only when the right environment can thrive for the micro-organisms. This process reduces the total mass and makes it easy to dewater or dry in the sun. Digestive sludge has no bad smell and can be disposed of quickly; it can be used as manure for the plants once dried.
Digestion happens faster and better in an Aerobic process, and in OxyO, we use the activated sludge process to digest the sludge and remove them. Unlike other treatment plants, we do not remove primary sludge and produce the secondary type of sludge. The quantity is significantly less and is disposed of away easily.
Generally digested sewage sludge in conventional plants is usually dewatered before disposal. Secondary sludge still contains a significant amount of water—often as much as 70 per cent—but, even with that moisture content, sludge can be removed and spread over sludge drying beds.
Sludge-drying beds provide the simplest method of dewatering. A digested sludge slurry is spread on an open bed of sand and allowed to remain until dry. Drying takes place by a combination of evaporation and gravity drainage through the sand, and after that, it is disposed of.
In OxyO sewage treatment plants, the sludge floats in the settling tank, and it can be easily skimmed and dried. Once it is dried, it can be used as a fertiliser for plants in the garden. Also, this type of sludge does not produce any foul smell.