Sewage Treatment Plant (STP) Sludge: How much and its journey & consequences
Sewage Treatment Plants (STPs) play an essential role in treating wastewater before it gets released into natural water bodies like rivers and lakes. One of the by-products of this treatment process is sewage sludge, which is basically the solid waste that settles down at the bottom of the treatment tanks. The amount of sludge produced by a Sewage Treatment Plant can vary a lot, depending on the size of the facility and the amount of wastewater it handles. In a small community having a decentralized 100 KLD STP in an apartment, the STP might produce about 2 to 3 ton tonnes of sludge per month.
Desludging tanker contractors can give you an idea of the amount of sludge disposed off each day!
Almost all conventional wastewater recycling methods generate sludge, with the exception of a few that produce very little. Once the wastewater arrives at the STP, it goes through various stages like primary, secondary, and sometimes tertiary treatment. The sludge is generated mainly in the primary and secondary stages. Sludge is generated constantly by bacteria feeding on sewage and converting it to sludge. Sludge settles at the bottom of the tank, reducing its capacity to hold water. STP capacity to treat sewage drops significantly if sludge cleaning is not done every 6 to 8 months. In spite of the fact that almost all STP manufacturers provide some or the other accessory to regularly desludge and dewater the sludge, very few of them are actually used.
Many cities, not just Bengaluru, are grappling with: the improper disposal of sludge from sewage treatment plants (STPs) into stormwater drains or vacant lands, which eventually leads to pollution of local water bodies like lakes. Desludging tanker contractors can give you an idea of the amount of sludge disposed off each day! Sludge can actually be a valuable resource if treated properly, containing nutrients that can be beneficial for agricultural use, for instance. However, due to various constraints like lack of regulations, inadequate infrastructure, and sometimes just plain negligence, this sludge often ends up where it shouldn't.
Desludging contractors usually do this work at night, making hundreds of rounds between the STP and the nearest storm drain or vacant land. Imagine this happening every day all over the city and all this sludgy water entering the lakes. A 4000 litre desludging tanker would cost between ₹1400 and ₹2000 per trip in Bangalore. Depending on how bad the issue is, desludging, let's say a 500 KLD STP, would require 100–200 trips. The water is first emptied by pumping, and the contractor will then lower a couple of people—sometimes up to their necks—into the tank to help dislodge the sludge. Accidents occur often, particularly in the poorly ventilated STPs found in basements.
For the owner of the STP, it is a significant recurring expenditure; however, the desludging contractor and the STP operator are guaranteed a substantial regular income. This is also one of the factors for not utilising the STP's essential desludging apparatus. When untreated fecal sludge finds its way into water bodies, it contaminates them, making the water unsafe for various uses like drinking, agriculture, and recreation. Fecal contamination carries pathogens that can cause a variety of diseases such as cholera, typhoid, and dysentery. The organic load from the sludge depletes oxygen levels in water bodies, affecting aquatic life.
The concept of a Fecal Sludge Treatment Plant (FSTP) emerged as a response to a critical environmental and public health issue: the draining of untreated fecal matter into stormwater drains. Fecal sludge is collected from various points and transported to the treatment facility. It's a dedicated facility designed to treat fecal sludge and septage collected by the desludging tankers. The STPs (without civil) typically cost between ₹ 12 and 15,000/-per KLD, and they have to recycle raw sewage, which is more work than treating sludge. The cost of the FSTP (without civil) is between ₹1,00,000 and ₹2,00,000 per KLD.
The plan was to set up the necessary infrastructure so that the tanker drivers could come and empty their cargo into the FSTP as opposed to a storm water drain. Centralised FSTP infrastructure are a few and located on the outskirts, so if you ask the desludging contractor to dump it in to the nearest FSTP, they will quote four times more. End of story!
Catch 22? No, not really!
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