Case study 4 - A revised sewerage scheme in a remote seaside town.

In order to comply with legislative changes, the original outfall pipe had to be replaced. The existing gravity sewage system was brought together to one point and then pumped to a series of "septic" tanks. From the septic tanks the sewage flows by gravity down to the coast road and back into the town before going to sea outfall.

Since the new system had been installed there had been a number of odour complaints of the "rotten egg smell" variety relating both to the site with the septic tanks and the main road into town.

The new system had three pumping stations, two of which had been equipped with chemical dosing systems to avoid septicity. These were apparently both working so the hydrogen sulphide generation needed a complete review.

EnvEnt Ltd. was called in and installed data loggers at the pumping stations to measure the flows. We also spent some time measuring wet samples at the septic tank site.

The problem turned out to be relatively simple - the solution was however rather more complex. The pumping station transferring sewage to the septic tank site had not been equipped for chemical dosing as there was an issue of planning permission. From our modelling of the system it was apparent that this was a mistake: sewage was being retained in this section of pressure main for many hours. However there was also a massive salt water ingress at high tide that was complicating both the flows and the microbiology.

Consequently the sewage flowing into the septic tanks was already holding several mg/l of sulphide in solution. Inside the septic tanks more sulphide was being generated and as a result at their outlet there was over 100 ppm of hydrogen sulphide in air. This then flowed down to the coast road where on entering the gravity sewer more hydrogen sulphide was stripped off resulting in the bad egg smell all along the coast road at every manhole (!).

EnvEnt showed how the septic tanks could be turned into denitrifying units, thereby raising the redox potential, oxidising the incoming sulphide and preventing the formation of more hydrogen sulphide in the septic tanks. A simple dosing system was installed and the problem was resolved within 48 hours.

Long term, the salt water ingress needs to be tackled and a dosing system needs to be installed at the terminal pumping station.