Sewage Treatment Plants (STP) and their Maintenance in Apartment Complexes

You hardly give a second thought to your local riverbank strewn with brown matter or freshwater lake that smells like rotten eggs, because you’d think that the government will make sure the water supply is clean before it reaches our home. Then the same water flows through your kitchen tap and you seem to believe that it looks clean…but is it really? When the Bureau of Indian Standards did a water quality check on India’s 13 big cities, all (except Mumbai) failed the test of water quality. 

‘So what? We don’t drink tap water, we install water purifiers and buy mineral water anyway,’ you’d rationalize, without addressing the real problem at the source again. 

If our rivers and lakes were never polluted to begin with, and if each one of us cleaned our own mess instead of dumping it on the planet and its precious water source, we’d have uninterrupted, clean water, fit for human consumption, straight out of the tap, much like the world’s most developed countries. 

Water management and treatment in metropolitan cities is a dystopian nightmare.The biggest reason for water pollution in India is untreated sewage, an astounding 70-80% of total sewage of a country of 135 billion people. 

According to a report by the Central Pollution Control Board, “In a number of cities, the existing treatment capacity remains underutilized while a lot of sewage is discharged without treatment in the same city…” As per the government inventory, there are less than a thousand sewage treatment plants in India.

Long story short, we need to consider private wastewater treatment options, like a Sewage Treatment Plant or STP.

What is a sewage treatment plant?

Sewage treatment plants process and treat wastewater/sewage, breaking it down into a cleaner ‘effluent’ that can be returned back to nature in a safer, eco-friendly form. They help restore groundwater balance, curb diseases and stop degradation and pollution of the environment. 

In housing societies, the treated water can be used for non potable purposes such as gardening, washing cars, construction, irrigation and toilet flushing.

Cleaner water would minimize fatalities due to water borne diseases, there’d be less negative environmental impact from water pollution and from a much narrower point of view, no house would have to pay for water tankers, because there’d be abundance of groundwater on macro scales and reusable water at community levels.

How does a sewage treatment plant work?

They work in four phases – preliminary, primary, secondary and tertiary treatment.

1. Preliminary treatment

Removes the biggies like plastic bottles, tree branches, rags, wrappers, solid, coarse objects and materials. 

2. Primary treatment

Occurs inside a sedimentation tank which settles to the bottom all the organic and inorganic solids while the grease, oil and lighter solids are moved to the surface using skimmers. This phase is expected to remove at least 60% of solids (mechanically scraped off and directed to sludge treatment) while the remaining water moves to the next stage.

3. Secondary treatment

Entails secondary clarifiers separating biological floc from the liquid with the use of aerobic biological processes (through managed indigenous microorganisms that consume biodegradable soluble contaminants). This stage is focused on intensive cleaning of water (up to 90%) after flotsam and solids are cleared in primary treatment.

4. Tertiary treatment

Is advanced treatment that reduces pathogens, nitrogen, phosphorus and other inorganic compounds by disinfecting the water chemically, through ultraviolet light, or micro filtration, before releasing for reuse. Sludge is treated in digesting tanks with anaerobic bacteria and later used as fertilizer.

STP norms and guidelines by the government

In metro cities like Mumbai, housing projects with more than 20,000 sq metres are given clearance upon the condition that they treat solid waste in-house, making it mandatory for them to have functional and compliant private STPs. However, the installation at design level is the responsibility of the builder who is also required to maintain it for five years after occupancy by residents.

Sewage treatment norms have eased, renewed and scrapped since 2015 until they were made stricter in 2019 eventually. Here’s an easy to understand lowdown.

In 2015, the Central Pollution Control Board had placed strict rules regarding effluent discharge which regulated the amount of pollutants that can be allowed for discharging back into water bodies. In 2017, the norms were relaxed, allowing higher levels of pollutants. Meanwhile a study by IIT Kanpur, IIT Roorkee, NEERI and CPCB yielded that diluted rules worsened the water quality of rivers. In 2019, the NGT (National Green Tribunal) scrapped the eased norms, reintroducing stringent rules with respect to treated effluent. Now new and old STPs across mega and metro cities will have to follow the revised standards mentioned below.

  • BOD (Biochemical Oxygen Demand) level – 10mg/L
  • pH value – 5.5-9.0
  • Total Suspended Solids – 10mg/L
  • Nitrogen – 10mg/L 
  • COD (Chemical Oxygen Demand) – 50
  • Fecal Coliform – 230 per 100 millilitres

Maintenance of STP

Low-budget STPs need frequent emptying and monitoring but even a good one would have to be desludged and serviced at least once a year. After desludging, it should be immediately refilled by water to balance the internal and external pressure. 

Under no circumstances should rainwater enter the STP.  It could flush out bacteria and cause flooding. 

Emptying of primary tanks and soakaways must be done regularly by the society, not the manufacturer’s service staff.

Below components should be checked during maintenance visits:

  • Unobstructed, strong and efficient blower and ventilation
  • Mechanical components replacement if needed
  • Inspection for air filters and pipes inspection
  • Diffuser inspection for bubbles
  • Biomass color and smell check
  • Final effluent quality
  • Damage repair in all chambers
  • Lid seal integrity
  • Replacement of diaphragms and valve boxes

Refer to the manufacturer’s maintenance guidelines booklet for a detailed understanding of every protocol.

STPs require a CFO ( Consent For Operation) from the state pollution boards which have to be renewed every five years. Consent is granted after submitting paperwork, consent fees and a thorough inspection from regional officers of the Board.

The flipside of conventional STPs and eco-friendly options

An aerobic STP generally in vogue has moving parts and needs manual operation, i.e. higher maintenance and opex. It utilizes a lot of electricity for functioning (since aerobic bacterial colonies require constant air) and a trained professional has to be hired for operating it daily. Depending on the KLD (kilo litres per day) capacity, one unit can cost from Rs 8 lakh to Rs 20 lakh and above. Monthly expenses would thus be somewhere over Rs 60,000 for a mid-sized society (or approx. Rs 20 lakh per annum including labour and electricity for a 200 KLD unit). 

Regular STPs are currently being used due to widespread established acceptance, yet as you may have surmised, they are not exactly ‘install and forget’; they’re energy-intensive and demand care. Societies are realizing the need for eco friendly, self-sustainable STPs that do not raise their electricity bill, don’t need monitoring,  add beauty to the aesthetic ambience of the premises, while providing chemical free effluent that is completely harmless to humans and nature. Enter Bio STPs, otherwise known as Green STPs. 

They use Anaerobic Microbial Inoculum (AMI) in which bacteria disintegrate organic waste, converting it into biogas through components like bio-digesters placed underground and reed beds above ground (constructed wetlands which allow bacteria and microorganisms to clean waste water). It produces biogas for cooking as well as safe non potable water for reuse without any chemicals.

It uses no electricity, is low maintenance and pays for itself in 10 years. It is surprisingly low cost and easy to install too. A unit starts from around Rs 6 lakh onwards.

There is also availability of eco friendly STPs that function on biomimicry of cow’s stomach (ruminant digestion). Another highly eco-friendly technology available in India is Soil Bio Technology based STP (CAMUS/SBT developed by IIT Bombay) that uses trickling filter concept, where purification is done through respiration of organisms, plant photosynthesis and mineral weathering.

STPs are your best bet for a sustainable future, not to mention mandatory for larger complexes. Sooner rather than later, it’s prudent to make them a way of life. Let water come full circle and not flow to countless drains and dead-ends.

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About MyGate

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