How much to tip BWSSB sanitation tanker?

It is the third or fourth time in a year that I had to engage with the BWSSB to clear out the sanitary clogging in the main line. It is the responsibility of the board to clean it for free. However, I do understand that the workers might expect something from the complainant every time they fix the problem. What is a reasonable amount to tip? They show up in a group of 5 people including the truck driver.