Rain drains down under

The torrential rains had proved transformational. Last month’s heavy downpour had enveloped many city streets in multiple layers of water.

Resembling Venetian canals, the roads had turned a dangerously dirty mix of storm water, sewage and open manholes! Disaster lurked at every step.

The city’s underground drainage system lay thoroughly exposed, yet again. Its old, corroded and clogged pipelines leaking, the network had nothing to defend itself from the marauding flood waters. Total collapse was precariously close!

But the danger ran deeper, a reality that came out starkly two days after the rains. Two labourers, in their attempt to repair a damaged pipeline, slipped into a manhole in Nagawara and died instantly. They had inhaled the toxic fumes that billowed out of the manhole.

The reasons are not hard to seek at all. In disrepair for years, abused by both industrial and residential units, the crumbling underground pipeline network today lies in a shambles.

Tons of solid waste particles, electronic waste, animal parts from illegal slaughter places, and even human dead bodies are shoved into the pipes, severely crippling an already overstretched system.

BWSSB’s elaborate network of lateral, sub-main and trunk pipelines were originally designed to carry sewage generated by houses with ground and the first floor. This worked as long as the city’s growth remained in manageable proportions. But it virtually collapsed with the mindboggling, unplanned explosion of human settlements.

Low quality pipes

The dramatic change in land-use patterns, the unbridled mixing of commercial and residential areas have drastically altered the balance. “The generation of waste water has increased tremendously. The low quality sewage pipelines of NP2 category with smaller thickness have proved inadequate to carry this load,” explains former BWSSB Chief Engineer M.N. Thippeswamy.

The additional pressure has invariably triggered crown corrosion of the pipelines. Simply put, corrosion is a process involving bacteria from fecal and other organic waste matter to produce hydrogen sulfide gas, and subsequently sulfuric acid.

This acid attacks concrete and steel in pipelines exposed to waste water, carving out gaping holes. As Thippeswamy explains, “Hydrogen sulfide and sulfuric acid will eat away the top surface of the pipeline, leading to eventual collapse. Pipelines laid 40 to 50 years ago are all faced with this serious crown corrosion problem.”

Vitrified clay pipes have proved long-lasting in developed countries. “Even after 100 years, they work well. By choosing inferior quality pipes, huge amounts of money are being wasted here. And, when are they are not laid properly, it gets even worse. Sewage starts flowing backwards, entering the storm water drains and causing havoc during rains.”

Dictates of gravity

Sewage needs to flow according to the dictates of gravity. This implies that the pipelines should be laid in a practical gradient. But, as frequent leakages at many places prove, this is often not the case. Experts say sewage should flow with a minimum velocity of 0.6 meters per second.

This allows domestic sewage to flow into the lateral lines of about 200 mm diameter, and thereafter get into the submains of 600, 800 or 1,000mm diameter before flowing into the bigger trunk sewers of 900 to 2,000mm diameter.  

Gradient measurements require careful monitoring while the pipes are being laid. This was not always the case when underground lines were laid in the newly added BBMP areas. In one clear case near Jagadeeshnagar, a contractor was publicly pulled up for not getting the record logbooks as earthmovers and excavators lowered the sub-main lines.

Sewage that cannot flow on gravity will either move in the reverse direction or flow out through the manholes. Contamination with drinking water lines is one serious consequence. The frequency of such cases reported from both the core and new BBMP areas point to the systemic faultlines.

Retired Colonel Joseph, a resident of the KR Puram area, had to make several calls and complaints before the threat of contamination subsided to an extent. This, incidentally, was within months of the laying of new UGD lines in the locality.

Treatment plants

Passing through congested residential layouts, the UGD lines eventually release the liquid waste into the city’s 14 Sewage Treatment Plants (STPs). As an executive engineer attached to BWSSB informs, the inflow of storm water during flooding into the manholes dilute the sewage. “So, instead of 20 MLD, the STP will end up getting 40 MLD. When this dilution happens, the food for the microbes in the form of impurities will become less,” he explains.

But this dilution is a lesser constraint compared to the enormous amount of unwanted solid waste that terminates at the STPs. Says the engineer, “People also dump debris of all kind, which get accumulated as silt inside the pipelines. Even industries dump effluents, thermocol sheets and other packaging material. Desilting is done frequently with jetting machines and other equipment in all the 30 subdivisions, but after 15 to 20 days the muck gets right back in.”

The 14 STPs have a combined capacity to treat 728 MLD of sewage. Eleven more plants are in the pipeline to cater to the additional sewage generated by the UGD lines laid in the new BBMP areas. Yet, the inflow to the existing plants is low. Reason: Almost 40 per cent of the sewage gets into the storm water drains (SWDs), which are meant to carry only rain water!

SWD blockages

To make matters worse, many manholes are built right inside the SWDs, blocking the flow of storm water and triggering floods on the roads. Relocating the drains could prove too daunting a task since the SWDs are largely encroached. But if there is any hope that better coordination between BBMP and BWSSB will show a way out, consider this: BBMP desilting operations often damage BWSSB sewers and manholes!

Experts have been wondering why managing UGD lines and SWD, both of which are critically linked, should be left to two different civic agencies. Now, this is a question that has been left unanswered for years!