The Delhi Jal Board (DJB) Karamchari Union has written to the chief minister, saying "this prevalent practice of using untrained labourers needs to stop immediately".
In three separate incidents, nine people have died in a span of four weeks in the national capital.
Taraspal Tomar, president of the union, said that there are over 3,500 DJB employees who have been trained to work in the sewers of the city.
"However, the contractors employ untrained workers who step inside such dangerous places without any knowledge or equipment for merely Rs 100 per day wage, which is fatal. We have asked the chief minister to put a stop to this to end possibility of any more sewer deaths," he said.