Treating sewage through sulfate-reduction/sulfur-oxidation and denitrification
Putting into practice the removal of nitrogenous compounds and phosphorus from sewage includes not only biological phosphorus elimination via biological nitrification/denitrification and anaerobic-aerobic process, but also simultaneously removing combinations of nitrogenous compounds and phosphorus via the “anaerobic-anoxic-oxic process”.
However, there are still problems to be solved: first, biological denitrification reactions need to be performed by using hydrogen donors in raw water through the addition of methanol or organic matter. However, heterotrophic bacterium involvement generates a lot of sludge. Essentially, since sludge is necessarily drawn out when biological phosphorus is eliminated, it remains a problem to be solved.
On the other hand, recent sewage treatments utilizing organic processes involving the reduction of sulfur through oxidation draw wide attention and are very close to having practical applications.
Many researchers both domestically and abroad have long acknowledged activated sludge processes – sulfate reduction via anaerobic biological processes and sulfur oxidation via aerobic biological processes. The theory that sulfate reduction/sulfur oxidation and sulfate reduction/sulfur denitrification processes using bacteria can be effective for sewage with high sulfate concentrations has been well-known, as well as sewage containing nitric acid with a low concentration of organics.
As an example application, Japan has studied dyeing discharged treated water using anaerobic-aerobic bio-filtration equipment and denitrification reactions from chop sticks and shaved timber wood as filling material. These are used for sewage treatment via a carrier filling type biological filtration process utilizing sulfate reducing bacteria.
Consequently, a detailed analysis of microbial populations along with the elucidation of sewage treatment properties clearly revealed that the sulfur oxidation-reduction cycle is based on the coexistence of sulfate reducing bacteria and denitrifying sulfur oxidizing bacteria with no competition.
On the other hand, Hong Kong reported the results of sewage treatment tests containing saline through SANI Process (biological carbon and nitrogen removal process integrated autotrophic denitrification/nitrification using sulfur cycle causing sulfate reduction). These results, in comparison with conventional sewage treatment, show about a 90% suppression of the generation of excess sludge, a 35% reduction in energy consumption and a 36% reduction in CO2 emissions, indicating that this method is ready for practical applications.
Finally, Bangkok, Thailand applied an aerobic bio-filter bed/sulfur denitrification system to sewage treatment – a down-flow type aerobic bio-filter bed hanging sponge in the preceding stage and up-flow type sulfur denitrification bioreactor in the following stage. The results of continuous tests of residential drainage showed a significant reduction effect of 85% for electricity consumption and 82% for sludge generation, compared with the conventional oxidation ditch process.
As mentioned above, the systems using sulfate reducing bacteria and denitrifying sulfur oxidizing bacteria are energy-saving. Future applications are expected for sewage systems that generate less sludge.