Disposal of domestic sewage from cities and towns is the biggest source of pollution of water bodies in India . All Class I cities and Class II towns together generate an estimated 29129 MLD sewage (as per population in 2001 census). Against this, installed sewage treatment capacity is only 6190 MLD. There remains a gap of 22939 MLD between sewage generation and installed sewage treatment capacity. In percentage this gap is 78.7 %. Another 1742.6 MLD sewage treatment capacity is under planning or construction stage. If this is also added to existing capacity, even then there is a gap of 21196 MLD (equal to 72.7 %) in sewage treatment capacity.

Sewage generation and treatment in Class I cities and Class II towns (2001 population basis)
City category & population Number of cities Sewage generation, MLD Installed sewage treatment capacity, MLD Capacity gap in cities having STPs, MLD (A) Sewage generation in cities having no STPs, MLD (B) Total capacity gap, MLD�(A+B) Planned treatment capacity, MLD
Class I cities having more than 10 lac population 39 13503 4472 (In 29 cities) 6135 2896 9031 1549
Class I cities having 5 to 10 lac population 32 3836 485 (In 13 cities) 1293 2058 3351 123
Class I cities having 2 to 5 lac population 119 4807 768 (In 34 cities) 804 3235 4039 4
Class I cities having 1 to 2 lac population 224 4018 322 (In 36 cities) 373 3323 3696 32.5
All the above Class I cities together 414** 26164 (100%) 6047(23.1%) (In 112 cities) 8605 (32.9%) 11512 (44%) 20117 (76.9%) 1708.5 (6.5%)
Class II towns having 0.5 to 1 lac population 489** 2965 (100%) 200 (>143*) (4.8%) (In 22 towns) Nil 2822 (95.2%) 2822 (95.2%) 34.1 (1.15%)
All Class I cities and Class II towns 893** 29129 (100%) 6190 (21.3%) 8605 (29.5%) 14334 (49.2%) 22939 (78.7%) 1742.6 (6.0%)

*Estimated sewage of the cities having STPs
**Delhi being taken as one city

CPCB has prepared and published a report 'Status of sewage treatment in India'. This reports analyzes and presents in detail the gap between sewage generation and treatment capacity, the technologies used for sewage treatment in India, performance of 115 sewage treatment plants studied by Central Pollution Control Board with plant-specific technical remarks and also discusses the efficacies of various treatment technologies. The main recommendations of the report are:

  • The estimated sewage generation from Class I cities and Class II towns (as per 2001 census) is 29129 MLD, which is expected to be 33212 MLD at present assuming 30% decadal growth in urban population. Against this, there exist STPs having 6190 MLD capacity while another 1743 MLD capacity is being added. Thus, the existing treatment capacity is just 18.6 % of present sewage generation and another 5.2 % capacity is being added. However, the actual capacity utilization of STPs is only 72.2% and as such only 13.5 % of the sewage is treated. This clearly indicates dismal position of sewage treatment, which is the main cause of pollution of rivers and lakes. To improve the water quality of rivers and lakes, there is an urgent need to increase sewage treatment capacity and its optimum utilization.
  • State Governments should realize the problem of pollution of water bodies and pay attention to their liability to set up sewage treatment plants in cities and towns to prevent this pollution. This activity requires to be recognized as one of the most important indicators of overall development of the States. If not realized urgently, this problem is fast going to magnify to an unmanageable level.
  • Utilization of conventionally treated sewage for irrigation of crops not eaten raw is also equally important i) to save fresh water considering our diminishing water resources, ii) to prevent nutrient pollution of our water bodies and iii) to utilize nutrient value of sewage in irrigation. Importance of utilization of treated sewage in irrigation was emphasized in Water (Prevention and Control of Pollution) Act 1974, i.e. more than thirty years back but this issue continues to be largely neglected by State Governments. Therefore, State Governments are required to take up sewage diversion and utilization schemes as an integral part of all sewage treatment schemes. Sewage diversion schemes should adopt at least 25-30 years plan period for design.
  • Considering the widening gap between sewage generation and treatment capacity, state governments are required to prepare a very thoughtful action plan to fill this gap in a minimum time frame. Large cities where pollution problem is more severe, cities/towns responsible for pollution of critically polluted stretches of rivers, and cities/towns polluting environmentally sensitive water bodies will be required to be taken up on priority basis in first phase. Continuous upgrading of capacity with rise in population in cities/town taken in first phase will also be required along with implementation of next phases.
  • Treatment schemes based on primary sedimentation followed by activated-sludge-process with anaerobic sludge digester and sludge drying beds for anaerobic sludge is quit suitable scheme for large cities where land availability is a problem. However, the plant at Vasna, Ahmedabad based on anaerobic-sludge-blanket reactor followed by coagulant aided tertiary sedimentation needs to be studied in detail for assessing its optimal efficiency, as this scheme also require less land and may be suitable for large cities. This scheme is most likely to be operationally economical as compared to the scheme based on activated-sludge-process in vogue. Moreover, better bacteriological quality may be achieved with the help of coagulants in tertiary sedimentation.
  • Operation and maintenance of existing plants and sewage pumping stations is also a very neglected field, as nearly 39% plants are not conforming to the general standards prescribed under the Environmental (Protection) Rules for discharge into streams. STPs are usually run by personals that do not have adequate knowledge of running the STPs and know only operation of pumps and motors. T he operational parameters are not regularly analyzed hence the day-to-day variation in performance is not evaluated at most of the STPs. Thus, there is a need that persons having adequate knowledge and trained to operate the STPs be engaged to manage STPs and an expert be engaged to visit the STPs at least once a month and advice for improvement of its performance. In a number of cities, the existing treatment capacity remains underutilized while a lot of sewage is discharged without treatment in the same city. Auxiliary power back-up facility is required at all the intermediate (IPS) & main pumping stations (MPS) of all the STPs.
  • In treatment schemes employing activated-sludge-process, plant operators must recognize the importance of using Solids Retention Time (SRT) as a plant control parameter because treatment efficiency, sludge production, oxygen requirements and nutrients requirements are all dependent on SRT. Moreover, SRT being the ratio of total suspended solids in the system and that wasted per day, it is most simple to operate plants on the basis of SRT. Operation of a conventional activated-sludge-process near 5 day SRT is recommended, as it will provide sufficient safety factor. If a plant based on conventional activated-sludge-process receives low strength sewage than it was designed for, then operator has a choice to either operate the plant at higher than 5 day SRT, or he may opt for energy saving by operating fewer aerators provided mixing requirements of the plant are still fulfilled. But all this maneuvering requires a basic knowledge of intricacies of aerobic biological treatment, which an operator must be equipped with. It is also necessary to recognize the importance of return flow and waste sludge flow measurement, in addition to influent flow measurement, as without this it is difficult to have proper control on plant operation and it is not possible to use SRT as a plant control parameter.
  • Treatment schemes based on grit removal followed by up-flow-anaerobic-sludge-blanket reactor followed by polishing pond is a suitable technology for all medium and small size cities/towns where required land can be made available. The operation of these plants is somewhat sensitive. Continuous uniform feeding to the plant, proper removal of grit, maintenance of design VSS concentration and VSS/TSS ratio in UASB reactor, cleaning accumulated sludge from polishing pond after a year time, avoiding stagnation of water in ponds to prevent excessive algal growth and providing proper weir length and baffle in the outlet structure of polishing pond, are the most important factors for successful operation of such plants therefore these factors must not be ignored. Biogas generated in reactors must be utilized; if arrangements for utilization of biogas are not available they must be installed immediately.
  • Treatment scheme based on s eries of Waste Stabilization Ponds (WSP) technology is quit rugged, one of the most economical ones and suitable for small towns where sufficient land is easily available. Multiple stage ponds (at least three) with first pond as anaerobic one is the most widely used and suitable configuration. Continuous uniform feeding to the plant, cleaning accumulated sludge from ponds after suitable intervals (preferably less than 6 month for primary anaerobic pond and once a year for subsequent ponds), avoiding stagnation of water in ponds to prevent excessive algal growth, providing proper weir length and baffle in the outlet structure of pond and not allowing hyacinth growth are the most important factors for successful operation of such plants therefore these factors must not be ignored.
  • As mentioned in section 2 and 3 above, the first emphasis should be given to development of 100% treatment capacity up to secondary level of treatment and diversion of treated sewage for its utilization in irrigation of crops not eaten raw. Improvement in bacterial quality of remaining sewage to be used for irrigation of sports fields and public parks or that has to be discharge into water bodies due to unavoidable circumstances is the next area of concern. This will require augmentation of treatment plants with tertiary treatment units, such as coagulant-aided tertiary sedimentation and chlorination etc.
  • Six STPs in Shimla and one STP in Chandigarh have tertiary sedimentation unit after activated sludge process. These plants need to be studied in detail with different combinations of lime and alum dozing and also with additional chlorine dozing to assess efficacy of this scheme in providing coliform reduction to the desired FC level of 2500 MPN/100 mL for utilization of treated sewage in sports field and public parks or where sewage has to discharged in streams providing negligible/insufficient dilution.
  • Considering the urgency of preventing pollution of our water bodies and preserving our precious water resources, sewage treatment and reutilization of treated sewage need to be accorded higher priority.
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