Operations to remove sludge from tunnels beneath the tracks appeared to be under way at Bishan MRT station early yesterday morning. On Tuesday, SMRT revealed that maintenance work on the Bishan pump system, which could have prevented flooding that led to a 20-hour disruption on the North-South Line last month, had not been conducted since September last year. Earlier investigations had found that a storm water pit in the tunnel between the Bishan and Braddell stations, which collects rainwater that is then pumped out, may have been close to full before the Oct 7 flooding. Over 250,000 commuters had their journeys disrupted in the incident. Sludge and debris were also found in the lower part of the pit, which could have affected the operation of pumps and float switches.