ИСТИНА

The recently proposed DEAMOX process (Kalyuzhnyi et al., 2006) combines the anammox reaction with autotrophic denitrifying conditions using sulphide as an electron donor for the production of nitrite from nitrate within an anaerobic biofilm (reactions 3 and 4, Fig. 1). The concept was experimentally proved (Kalyuzhnyi et al., 2006) using baker’s yeast effluent and the laboratory set-up consisted of 3 reactors. The objective of this paper was to compare a steady state performance of the process under intermittent and continuous supply of nitrified and anaerobic effluents to the DEAMOX reactor under variation of nitrogen loading rates (NLR). UASB and nitrifying reactors performance. Taking into account an auxiliary role of these reactors, only the most important data on their functioning throughout 151 days of experimentation are discussed here. In short, the UASB reactor demonstrated a stable performance in the range of the COD loading rates applied (3.9-9.5 g CODtot/l/d) giving the average total COD removal of 712% and 991% conversion of sulphate into sulphide (by the accompanying process of biological sulphate reduction). The produced effluents contained 22214 mg N/l and 26414mg S/l of ammonia and sulphide, respectively. Thus, the S/N ratio in these effluents (~1.19 mg S/mg N) was more than sufficient to fulfil denitrification requirements for the DEAMOX process (reaction 3, Fig 1), i.e. in ~ 1.5 times higher than the balance required one (0.79 mg S/mg N under 58:42 split of anaerobic effluent between the nitrifying and the DEAMOX reactor). The nitrifying reactor also functioned without problems throughout an entire study showing near 100% nitrification efficiency under ammonia loading rates of 111-350 mg N-NH4/l/d with nitrate as a prevailing product over nitrite. Performance of DEAMOX reactor. Independently on feeding mode and in spite of decrease of HRT from 0.65 to 0.21 days and, hence, increase of the NLR from 300 to 858 mg N/l/d, the reactor demonstrated only a slight trend in deterioration of its performance: the average ammonia, nitrate and total inorganic nitrogen removals (nitrite was completely consumed) decreased by only a few percents. The ratio of consumed N-NOx/consumed N-NH4 was in the range of 1.33-1.52, i.e., quite close to the stoichiometry of metabolic anammox reaction: NH4+ + 1.32NO2- + 0.066HCO3- + 0.13H+à1.02N2 + 0.26NO3- + 0.066CH2O0.5N0.15 + 2.03Н2О (5) This observation indicates that almost all nitrogen removal in the system was accomplished through ammonia oxidation (reaction 4, Fig. 1) and only a minor part - through denitrification of nitrite by sulphide (which was present in excess): NO2- + 0.375HS- + 0.625H+ à 0.5N2 + 0.375SO42 + 0.5H2O (6) Generally, though statistically it is somehow eroded, but the continuous supply of anaerobic and nitrifying effluents to the DEAMOX reactor seems to be a little bit better than the intermittent one in terms of total nitrogen removal (Fig. 2). This was due to a more complete nitrate removal in the former case whereas the ammonia removal was similar for both the feeding regimes applied under the comparable NLR. These data correlate with the increased ratio of consumed N-NOx/consumed N-NH4 accounting for 1.45-1.52 under continuous feeding, i.e., somewhat higher under intermittent feeding (1.33-1.39), witnessing, in turn, about a higher extent of realisation of reaction 6. The possible explanation can be that, during the simultaneous supply of the anaerobic and nitrified effluents to the DEAMOX reactor, there were fewer mass transfer limitations for either nitrate or sulphide around the sludge aggregates. However, this was not a case for anammox reaction apparently suffering from nitrite mass transfer limitations because the consumption of ammonia was far from complete. Such a pattern can be explained by the formation of joint aggregates with the outer layers of sulphide oxidizing denitrifiers and the inner layers of anaerobic ammonia oxidisers. A close location of these bacterial groups with interspecies transfer of nitrite was shown for marine sediments. The similar aggregate structure was found in the environments with a limited oxygen supply, where anammox bacteria were surrounded by aerobic ammonium oxidizers. Thus, the anammox bacteria located inside the aggregates probably not always can outcompete the denitrifiers for nitrite, especially under the negligible concentrations of the latter, as it is observed for both the feeding regimes.