Toxic, fatal STPs no more

Toxic, fatal STPs no more

Safety lapses, procedural loopholes, design faults. Now, a deadly mix of these could be fatal. This is exactly what happened to three workers, asphyxiated while cleaning a sewage treatment plant (STP) in an apartment complex in the city's HSR Layout.

This one act of manual scavenging camouflaged as tank cleaning has exposed the clear lacunae in a system that makes STPs mandatory without a regulatory mechanism. When questions were raised, all that the Karnataka State Pollution Control Board (KSPCB) and Bangalore Water Supply and Sewerage Board (BWSSB) could do was to indulge in a public blame-game.

Eventually, KSPCB did relent and admitted that no guidelines have been framed to operate and monitor these private STPs. The Board did not deem it necessary since its role was largely limited to checking if the treated water quality met the prescribed standards, as Chairman Lakshman put it.

No prescribed technology

Neither has the Board prescribed any specific STP technology that would suit apartments. Caught in a bind over the different options available, apartment owners are forced to accept whatever their plumbing consultants offer. With the Board breathing down their necks to get the STPs installed or face huge penalties, the apartment associations don't have the luxury of research time as well.

The problem is huge. There are over 2,500 decentralised, private STPs in Bengaluru. More than 300 Million Litres per Day (MLD) of sewage are treated in these plants. But, as veteran STP consultant, Ananth Kodavasal points out, over 85% of the STPs are defunct due to wrong technology, incorrect design and poor engineering.

Blind approvals

So why does it go so wrong? Kodavasal explains, "The problem starts when architects and plumbing consultants with little or no experience start designing waste water treatment plants. It gets compounded when the Technical and Advisory Committee of KSPCB, which lacks adequate competence to critically examine these proposals, simply approves all and sundry technology and designs of treatment plants."

Since there is no standard technology prescribed, architects and plumbing consultants offer an array of STP processes. Options under aerobic include extended aeration, Sequential Batch Reactor (SBR), Fluidized Bed Bio Reactor (FBBR), Suspended Aeration Fixed Film (SAFF) and Membrane Bed Bio Reactor (MBBR) besides anaerobic systems such as Septic Tank and Electrolysis.

Conventional STPs

Conventional STPs – which Kodavasal insists is still the most efficient – account for about 50% of private treatment plants in the city. But, despite its problems, the SBR technology is being adopted in a big way. About 40% of the STPs currently use this method, while MBBR makes up less than 10% of the plants.

The accident that killed the workers occurred inside a decant tank, a key component of the SBR method. The tank using this technology occupies 25% more space than the one used in conventional methods as three critical procedures have to be conducted in one place.

The filters fixed outside the decant tank tend to get chocked very frequently. Plumbers then let the treated water bypass the filters, and enter the final tank. Unlike in conventional methods, the sludge is not thickened internally. It is taken out in tankers.

The conventional STP method essentially has a collection tank, an aeration tank and a settling tank, all designed for 24-hour inflow of sewage from the apartments. But this inflow is not uniform. Out of a daily inflow of 100 KL, almost 60% comes in between 6 am and 9 am.

Faults in SBR method

Now, in the SBR method, the sewage is treated in batches. Each batch of about 8 hours can treat only 33 KL. "How does it handle if 60 KL flows in during the three morning peak hours," asks Kodavasal. Since the inflow is continuous, the conventional collection tanks can be much smaller.

As the collection tank in SBR systems is huge, there is maximum potential for odour. Since the aeration and settling is done in one tank in SBR, any lapse in the air blower system could make the process septic. This would mean lack of oxygen in the tank and a high content of toxic methane.

Indications are that the deadly combination of low oxygen content and high methane led to the asphyxiation of the three workers inside the decant tank.

Settling tank loopholes

The settling tank / decant tank plays a critical role in both conventional and SBR methods. In the conventional system, a typical settling tank is hopper-bottomed and has a center-feed well (also known as influent well) to take the incoming sewage and gently release it inside, without causing any disturbance or turbulance. A direct-suction electric pump sucks out the settled sludge in the bottom.

But the SBR settling tank / decant tank has a flat bottom. Ideally, no sludge should be present. The micro organisms settle at the bottom only after a long time.

Clear water can be neatly taken out only with a decanter. To cut costs, many apartments do away with the decanter, affecting the entire treatment process.

Experts say there is a very high chance that the STP tank where the accident occurred had no decanter. This inevitably meant more methane content and the deaths that followed.

 (With inputs from  Poornima Nataraj)

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