Sulphate-reducing bacteria also work in WWTP

Sulphate-reducing bacteria in a WWTP

Sludge disposal is one of the biggest cost factors at WWTPs. Since sulphate-reducing bacteria form less sludge per waste unit (measured as chemical oxygen consumption), the use of these bacteria can be of benefit. In Hongkong, where sewage water has a lot of sulphate-containing seawater, sulphate-reducing bacteria work well. But would this also be the case in Dutch WWTPs, which receive less sulphate and have to operate at cold temperatures in the winter?

Harnaschpolder pilot

To test this on a pilot scale, KWR, Paques and Delfluent Services BV carried out a test, under winter conditions, at the Harnaschpolder WWTP. The results show that the sulphate-reducing bacteria use different sulphate sources (which are not entirely characterised) from the waste, and that after a stabilisation period of 70 days, most of the organic matter is degraded by sulphate-reducing bacteria, and not by the methanogens which are – or can be – active under anaerobic conditions. This conclusion is based on the fact that sulphide is created during the degradation and that even if, in the beginning, methanogens are also found, they are gradually flushed out over time. Moreover, the reactor keeps working even when the temperature drops from 16°C to 13°C.

Less sludge, lower costs

Activity tests have shown that the conversion rate of sulphate-reducing bacteria increases over time. Nevertheless, the biomass concentration in the reactor drops and stabilises after about 130 days. This indicates a clear relationship with the drop in temperature in the pilot. There is not only less biomass, but it also forms a clear grain structure and is preserved in the reactor under the applied conditions and tested temperatures. Besides sulphate, domestic wastewater contains other sulphur sources, which can serve as a source for sulphate-reducing bacteria. Reducing the temperature from 16°C to 13°C did not lead to any operational problems.

Potential benefits

This research confirms the potential of the use of sulphate-reducing bacteria for the removal of organic matter from domestic wastewater, with the added benefit of reducing the amount of sludge to be disposed of. Other potential benefits relate to the formation of sulphide during the course of the process: this is potentially toxic to pathogens and precipitates heavy metals. A process with sulphate-reducing bacteria also consumes less energy, because less aeration is required.

The pilot with sulphate-reducing sludge in the Harnaschpolder WWTP’s test area.