An anaerobic/aerobic/anoxic/aerobic sequencing batch reactor (SBR) was operated with municipal wastewater to investigate the effect of nitrite on biological phosphorus removal (BPR). When nitrite accumulated, aerobic phosphate uptake activity significantly decreased and, in case of hard exposure to nitrite, BPR severely deteriorated. The interesting observation was that the relative anoxic activity of phosphate accumulating organisms (PAOs) increased after nitrite exposure. Moreover batch tests of aerobic phosphate uptake in the presence/absence of nitrite indicated that PAOs with the higher relative anoxic activity are less sensitive to nitrite exposure. From these results, we concluded that BPR is sensitive to nitrite exposure, but BPR containing PAOs with the higher relative anoxic activity is possibly more stable against nitrite than BPR containing PAOs with the lower relative anoxic activity.

Since metabolic pathway and enzymatic clarification of poly-phosphate accumulating organisms (PAOs) are still unclear, biological phosphorus removal (BPR) sometimes become unstable. We have focused on nitrite as one of unknown factors deteriorating BPR performance. And we obtained some findings from previous studies, namely 1) nitrite inhibits phosphate uptake and growth of PAOs, 2) nitrite inhibition persists even after nitrite disappearance, 3) PAOs with the higher relative anoxic activity are less sensitive to nitrite exposure. This study provides the modified Activated Sludge Model No.2d (ASM2d) to express nitrite inhibition properly by incorporating the assumed intracellular reaction product (reaction complex) of nitrite. The model also considers the tolerance mechanism of nitrite inhibition by the denitrifying activity of PAOs. From the results of previous experiments and relevant references, the modified model called "Nitrite-Complex model" incorporates new components, namely nitrite (S(NO(2) )) and reaction complex (X(complex)), to ASM2d. The model incorporation only nitrite inhibition was able to fit with measured phosphate concentration while nitrite exists. Whereas, this model couldn't describe the persistence of nitrite inhibition after nitrite disappeared. However, result of nitrite-complex model incorporating nitrite and reaction complex inhibition, it fits well with measured phosphate concentration not only before nitrite disappeared but also after nitrite disappeared.

It is well known that N 2 O, a greenhouse gas, is produced during biological nitrogen removal in wastewater treatment plants (WWTPs). It has been reported that the amount of N 2 O emitted varies depending on the type of WWTP and even within the same WWTP depending on the operational conditions. Therefore, it is important to consider the conditions that can reduce the amount of N 2 O emission and to investigate the pathway of N 2 O production by studying the N 2 O production potential and the amount of N 2 O emission.In our sealed batch experiments where NO 2 -N was added as the precursor substance for N 2 O production, the amount of N 2 O produced under aerobic conditions increased with higher initial NO 2 -N concentrations and longer reaction times. Consequently, the initial NO 2 -N concentration of 5 mgN·L -1 and a reaction time of 1 h were selected to evaluate the N 2 O production activity. The high N 2 O production activity in an actual WWTP was observed when nitrifi cation was controlled; however, N 2 O production activity decreased when nitrifi cation was accelerated. This was likely to be due to the competing processes of NO 2 -N reduction to N 2 O with NO 2 -N oxidation to NO 3 -N. Furthermore, the operational conditions under which Nitrospira caused complete nitrifi cation inhibited N 2 O emission.

This paper presents the recent attention in scientific studies and development of electrochemical processes. Electrochemical technology has contributed significantly to the purification of water for better human health and aquatic life forms. In this study, we emphasize the developmental trends of electrochemical technologies, their applications, and recent developments in the context of water and wastewater treatments. Recent studies have made great advances in investigating and optimizing advanced electrochemical oxidation processes in treatment of various organic pollutants, reduction of halogenated contaminants, and disinfection of microorganisms. Besides, electrochemical oxidation processes have been combined with other treatment methods to enable their practical application. Excellent electro-catalytic treatment of contaminant and their by-products was achieved through the application of mixed metal oxides (PbO 2 , SnO 2 , Ti/RuO 2 , etc.), Pt, and boron-doped diamond (BDD) electrodes. Several studies have focused on selective removal of trace pollutants in a complex matrix. These studies have shown the possibility of removing target pollutants with relatively low energy consumption. It can be concluded that enhancement of treatment performance of the present technologies will contribute to a wider application of electrochemical processes in water and wastewater treatment.