Today, technology plays a key role in sewage treatment. Through several aerobic and anaerobic techniques, sewage can be treated for reuse. With advancements in technologies over time, new sewage treatment techniques have been deployed by various urban local bodies (ULBs). These technologies have become more cost-effective, efficient, and require less land, and result in an odour-free process.
In India, the sewage treatment technologies that are deployed can be broadly categorised as conventional and advanced. Conventional technologies are usually simpler and easier to deploy, with less reliance on advanced machinery and equipment. The conventional treatment plants installed in the country usually deploy one or more of eight main technologies.
- Activated sludge process (ASP): This is an aerobic treatment process wherein oxygen is forced into the sewage to develop a biological floc. The floc is used to reduce the organic content in the sewage. The conventional ASP method removes biological oxygen demand (BOD) and total suspended solids, and incorporates biological nitrification and denitrification of the wastewater in the process.
- Extended aeration: This is a modification of the ASP, where long aeration periods are used to promote aerobic digestion of the biological mass by endogenous respiration. It includes the stabilisation of the organic matter under aerobic conditions and the disposal of the gaseous end-products into the air.
- Trickling filter: This technology consists of a basin or tower filled with support media such as stones, plastic or wooden slats. Microorganisms become attached to the media and form a biological layer or fixed film. The film is used to retain and metabolise organic matter in the sewage.
- Karnal technology: This technology involves planting trees on 1 metre wide and 50 cm high ridges. The untreated sewage is disposed of in the furrows where it is regulated to ensure that it is absorbed in the soil within 12-18 hours and there is no standing water left in the trenches. This technique utilises the entire biomass as a natural filter for supplying nutrients to the soil and plants..
- Upflow anaerobic sludge blanket: This is an anaerobic treatment process suitable for soluble waste and that which contains particulate matter. The process does not require any power as there are no mechanical components or external energy requirements. Gas of high calorific value and sludge are generated as by-products of the process.
- Oxidation pond: These ponds are used for simple secondary treatment of sewage effluents. They are used to degrade organic matter in the sewage through heterotrophic bacteria. The process produces cellular material and minerals, which support the growth of algae in the water which further decomposes organic matter by producing oxygen.
- Waste stabilisation ponds: These are shallow man-made basins comprising a single or several series of anaerobic, facultative and maturation ponds. The primary treatment takes place in the anaerobic pond, designed to remove suspended solids and certain soluble elements of the organic matter. During the secondary stage in the facultative pond, most of the remaining BOD is removed through algae and heterotrophic bacteria. The maturation pond is mainly used for tertiary treatment and removal of pathogens and nutrients.
- Aerated lagoons: These are generally in the form of simple earthen basins with an inlet at one end and outlet at the other to enable the sewage to flow through. Aeration is usually provided by mechanical means to stabilise the organic matter in the basin.
Presently in India, ASP is the most commonly deployed sewage treatment technology. Other prominently used methods are the upflow anaerobic sludge blanket technique and oxidation ponds. Extended aeration, trickling filter and karnal technology systems are the least deployed.
In a bid to improve the quality of treated sewage and enhance the efficiency of operations and maintenance, several ULBs have started deploying advanced sewage treatment technologies. The following are the three most commonly used techniques for sewage treatment.
- Sequencing batch reactor (SBR): An SBR is essentially a variant of the conventional ASP. It is a batch treatment process which combines primary settling, aeration, secondary settling and decanting of the treated sewage in a series of sequenced reactions in the same basin on a time-deferred cycle. The technology is capable of removing nitrogen and phosphorous concurrently with BOD, and does not require any separate secondary clarifiers, sludge pumping stations, etc.
- Moving bed bioreactor (MBBR)/Fluidised aerobic bioreactor (FAB): The MBBR technology is also similar to ASP, except that it allows for media suspension in the reactor for microbes to grow. Therefore, it maximises microbe growth within the aeration tank, as compared to the conventional aeration process. The FAB is similar to the MBBR, except that instead of suspended media, the media is kept stationary and fluidised in the aeration tank.
- Membrane bioreactor (MBR): This combines the process of aeration and secondary clarification in the same tank by taking out the aerated mixed liquid through membranes, instead of settling it in a separate downstream tank. The technology is capable of treating sewage with practically no BOD content and suspended solids. In addition, it operates under low suction and hence requires less power.
As compared to SBR and MBBR/FAB technologies, MBR is less deployed in the country.
Current technology deployment
Most states deploy conventional technologies to treat sewage. India Infrastructure Research tracked 680 sewage treatment plants (STPs), making up about 90 per cent of the total STP capacity across the country (20,736 million litres per day [mld]). Of this, about 80 per cent of the STPs are based on conventional treatment technologies. ASP is the most widely used sewage treatment technique, with about 37 per cent of the assessed plants deploying the technology.
Maharashtra has the highest installed conventional treatment technology, at 4,217.36 mld. However, while states usually deploy both conventional and advanced treatment technologies, the states of West Bengal, Gujarat and Rajasthan have 100 per cent, 99.57 per cent and 98.41 per cent of their STP capacities, respectively, based on conventional technologies.
However, of the total installed capacity in Punjab, advanced treatment technology comprises 54 per cent. This is followed by an advanced technology share in STP capacity of 42 per cent and 36 per cent in Haryana and Uttar Pradesh respectively. Of the advanced technologies, SBR is the most widely deployed.
Besides the aforementioned conventional and advanced treatment technologies, several emerging sewage treatment techniques are being increasingly deployed in the country. These include biological filtration and oxygenated reactor (BIOFOR), high rate activated sludge BIOFOR-F technology, submerged aeration fixed film (SAFF) technology, fixed bed bio-film activated sludge process (FBAS), rim flow sludge suction clarifiers/bio towers, and eco-bio blocks. The use of these technologies provides several key advantages – lower land requirement, easy operation and maintenance, absence of odour and aerosol during the treatment process, etc.
- BIOFOR: This technology involves enhanced primary treatment with the addition of coagulants and flocculants, and a two-stage filtration through biologically active media and with enhanced external aeration.
- High rate activated sludge BIOFOR-F: This highly mechanised technology uses a flow scheme which excludes a primary sedimentation tank. It involves second-stage aeration and rapid sand filtration, digester heating and temperature-controlled anaerobic sludge digestion.
- SAFF: The technique involves two-stage biological oxidation. It essentially comprises a fixed film media with enhanced oxygen supply through submerged aeration. It uses unconventional plastic media. The process has a large biomass and solid retention time leading to higher organic removal.
- FBAS: The process is essentially an ASP, but one in which plant roots are used to provide an area for the bio-film to develop and grow. The aeration system is divided into a series of biological reactors where fixed bio-film is maintained at every stage of the process. Biodegradation of contaminants is done mainly through plant roots and additional textile media which work as bio film carriers.
- Bio-towers: These are essentially clarifiers which have inlets along the rims. The sludge is sucked out from the floor through suction boxed arms instead of the more commonly used scrappers.
- Eco-bio blocks: These are exfoliated bricks of volcanic ash which do not degrade by themselves but allow microbes into their crevices. These microbes carry out aerobic, anaerobic or facultative activity based on the prevailing oxygen or septic conditions. This technology is usually used for “polishing” treated sewage prior to its discharge into waterbodies.
A wide array of treatment technologies are being used in the Indian sewage treatment sector. While conventional technologies, especially ASP, make up the major chunk of STP capacity, advanced treatment techniques are also being increasingly deployed. At the same time, ULBs are not shying away from adopting new and emerging technologies such as BIOFOR and bio- towers to treat sewage. Thus, the sewage treatment market in India presents a huge potential for technology deployment and upgradation.