THE USE OF SEQUENCING BATCH ACTIVATED-SLUDGE REACTORS TO DETERMINE NITROGEN BALANCES AND OPTIMUM PERIODS OF PRE-AERATION DENITRIFICATION

Four identical laboratory scale sequencing batch activated sludge plants were used to carry out comparative performance evaluations of nitrification and denitrification, and to obtain an accurate nitrogen balance for the system. In the first instance the plants were run under identical operational conditions to ensure that results were statistically valid. Ten performance parameters were compared and no significant differences at 95% confidence limits were found. A nitrogen mass balance, considering dissolved nitrogen species, waste biomass and denitrification losses during settlement, accounted for 87.4% of known inputs. On the introduction of denitrification periods, nitrate and total nitrogen removal increased with increasing anoxic period up to a maximum of 74% and 71% respectively. The inclusion of anoxic periods reduced total organic carbon (TOC) removal by as much as 19%. These losses are a consequence of maximising nitrate removal when the cycle duration is fixed. This is due to differences in efficiency between aerobic and denitrifying activity. Good linear relationships were shown between % nitrogen removal (r=0.93), effluent nitrate concentration (r=-0.94), T.O.C. removed (r=-0.99) and the ratio of anoxic to aerobic retention times; this was providing the anoxic period was taken to be the period of nitrate removal. These relationships may provide a guide for designing sequencing batch nitrification /denitrification systems. Some enhancement of nitrification was also evident at short denitrification phases.