The principal effect of the nitrification process is to transform ammonia-nitrogen into nitrate by the use of nitrifying bacteria under aerobic conditions. Denitrification converts nitrate to nitrogen gas by use of denitrifying bacteria, under anoxic conditions. The efficiency of the nitrification process depends on the extent to which organic nitrogen is transformed into ammonia-nitrogen. Chapters 3 and 4 present, in detail, the different factors governing the nitrification and denitrification processes. Nitrification can be carried out in conjunction with secondary treatment (combined oxidation of organic material and nitrification) or as a tertiary treatment (seperate stage nitrification ) see Chapters 5 and 6. In both cases, either attached-growth reactors or suspended-growth processes can be used. Denitrification can also be carried out in either attached or suspended growth reactors. For the denitrification process to be carried out, a carbon source and an anoxic environment are required. Chapter 5 explains the biofilm theory used in the attached-growth technology and shows the application of some of the most frequently used attached-growth processes: trickling filter, rotating biological contactor and submerged filters. The application of the submerged filter is mainly described as a case study on the use of clinoptilolite as a submerged biobed, for the simultaneous nitrification and denitrification processes. Chapter 6 shows the practical use of the activated sludge process.
The overall removal efficiency in a nitrification and denitrification plant ranges from 70 to 95 per cent for tertiary treatment, and down to 10-20 per cent for secondary treatment. The costs of attached-growth biological removal plants are moderate compared with activated sludge plants.
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