Environmental concerns about the release of greenhouse gases to the atmosphere, and issues of global warming and climatic change regularly feature in the media. What is often not referred to in these stories is the role of wastewater treatment. Biological processes could have a major effect on the composition of the atmosphere. Recent studies reveal that detectable quantities of the greenhouse gas dinitrogen oxide, N2O, is produced during both nitrification and denitrification of wastewater. For example, it is estimated thatca. 25 % of the atmospheric dinitrogen oxide in the Netherlands originates from polluted waters and sub-optimal wastewater plants. The greenhouse gas N2O has a long lifetime in the atmosphere and its greenhouse effect is much greater than CO2. Furthermore, it is the dominant source of stratospheric nitric oxide, NO, and therefore could have a significant influence on our climate.

Nitrification and denitrification are the two main steps of recycling of ammonia to dinitrogen (N2) in the nitrogen cycle. Nitrification is traditionally defined as the aerobic oxidation of ammonia to nitrate, NO3, via nitrite, NO2. This is carried out by two groups of autotrophic bacteria (bacteria that require no organic source of carbon); ammonia oxidisers (NH3aNO2), typified by the genusNitrosomonas and nitrite oxidisers (NO2aNO3), typified by the genusNitrobacter. The carbon is provided by carbon dioxide or bicarbonate. Denitrification, however, is an anoxic process carried out by a consortium of heterotroph bacteria able to use N-oxides as an alternative electron acceptor to oxygen.

Conventional systems for the treatment of nitrogen containing wastewater are traditionally based on the principal outlined above, where nitrification is an aerobic process and denitrification is restricted to anoxic conditions. These conditions are normally obtained, either by separation of the nitrifying and denitrifying processes, or, by temporal separation of each step-achieved by switching between aeration and no aeration in the same unit.

Recent studies illustrate that both pure cultures of nitrifying bacteria and nitrifying activated sludge are capable of simultaneous nitrification and denitrification. The principal mechanisms of N2O production by nitrifiers is NO2-reduction to N2O by the ammonia oxidisers, a process called aerobic denitrification. For example, the production of both the greenhouse gases N2O and NO and of N2, intermediate products and products of denitrification, are well documented gases, produced by the autotrophic ammonium oxidiserNitrosomonas. In addition, micro-organisms capable of heterotroph nitrification have been found capable of aerobic denitrification under dissolved oxygen concentrations as high as 7 mg l-1. Furthermore, the most studied aerobic denitrifier Thiosphaera pantotropha, a heterotroph nitrifier (one that requires organic carbon), exhibits aerobic denitrification rates equivalent to approximately 50 % of the rate under anoxic conditions. These findings indicate the existence of micro-organisms capable of complete conversion of ammoniacal pollution to nitrogen in a one step process. In addition, it might explain the nitrogen losses reported from mass balances on nitrifying wastewater treatment plants, and the reported release of the greenhouse gas N2O from nitrification and denitrification process plants.

Laboratory experiments have shown that up to 8 % of the nitrogen can be lost as N2O and NO during nitrification under sub-optimal oxygen concentrations (i.e. under oxygen stress). While aerobic denitrification at relatively high dissolved oxygen concentrations has been reported, most simultaneous nitrification and denitrification occurs mainly at sub-optimal oxygen levels. The ability to undertake aerobic denitrification is believed to give the ammonia oxidisers an advantage over the nitrite oxidisers. Firstly, oxygen will be conserved for the initial ammonium oxidation step. Secondly, a toxic product is removed. Finally, the competition for oxygen is decreased by the removal of the substrate for the nitrite oxidisers.

Despite environmental concerns regarding the release of greenhouse gases to the atmosphere, the development of nitrification/ denitrification processes encouraging aerobic denitrification could be advantageous for the water industry. The main benefit is the decrease in operating costs – lower oxygen requirements for the plant equate to lower energy demands. Furthermore, denitrification will be cheaper as a number of reduction steps are eliminated from the process, enabling more compact process plants to be constructed. Result – cost savings all round.

However, it must be remembered that by running the nitrification process under sub-optimal oxygen conditions, a significant amount of dinitrogen oxide will be produced. This off-gas will require secondary treatment in order to prevent its release to the atmosphere. One option could be to operate the simultaneous nitrification/denitrification step in an enclosed reactor, before passing the resultant gases through the anoxic denitrification step. Such a process could conceivably be combined with off-gas treatment for odour removal – an increasingly important consideration. This is one of the next steps forward in R&D terms.

Dr. Bettina Colliver, School of Water Sciences, Cranfield University.