Case study: Investigation of sewer gases

The problem

Laboratory staff on the 2nd floor of a large University hospital experienced complaints of ill health and odour, with some describing the odour as sewer odour, and some describing it as chemical odour. This clinical laboratory had a large number of fume hoods handling odorous and hazardous substances.

Our approach

A tracer study established that no re-entrainment of laboratory hood exhaust was occurring, but tracer simultaneously placed in a large sewer line located several hundred meters from the lab, below a busy street, was identified in the occupied laboratory.

The study also indicated that the floor with the laboratory, and the laboratory itself, had exhaust volumes exceeding supply volumes, and was the most negatively pressurised floor/room in the building.

Further investigation revealed that heavy equipment operation in areas in a courtyard, where the main sewer outflow from the hospital traversed, had crushed one or more sewer lines. The pathway identified by the tracer study indicated that a below-ground mechanical room, specifically the one serving the laboratory, and adjacent to the sewer outflows, was the route of entry.

The large and powerful fan was able to suck the sewer gases through the soil and through a 20 cm thick concrete wall separating it from the courtyard with the crushed pipes.

The outcome

Modification and re-balancing of the HVAC system was able to recitify the problem.