Hi Jonathan, so it depends on ...

Hi Jonathan, so it depends on the housing type being served - more dense housing will have higher peaks than low dense more affluent housing (who will have greater dilution with grey water). Generally though TKN (total kjeldal nitrogen) is between 60 and 85 mg/l in the raw sewage for domestic users.  during the aeration stage the ammonia component is converted to nitrates, while in the anoxic stage the nitrates are converted to nitrogen gas.  The anoxic stage is usually constructed prior to the aeration stage requiring recirculation from the aeration stage back to the anoxic stage, so the design requires larger basins for both the anoxic and aeration stages.  Also the aeration stage must be large enough to allow firstly usage of the readily available carbon source (COD) before nitrification can occur (ammonia to nitrates).  So to achieve nitrification may require an additional 20 to 25% in costs for the aeration stage, and to achieve denitrification the anoxic basin may be about 20% the size of the aeration basin and requires additional low lift pumps and channels for recirculation.  All other components of the WWTW are of course still required.