Hiroki Itokawa

Nitrous oxide production in high-loading biological nitrogen removal process under low COD/N ratio condition

Effects of influent COD/N ratio on N2O emission from a biological nitrogen removal process with intermittent aeration, supplied with high-strength wastewater, were investigated with laboratory-scale bioreactors. Furthermore, the mechanism of N2O production in the bioreactor supplied with low COD/N ratio wastewater was studied using 15N tracer method, measuring of reduction rates in denitrification pathway, and conducting batch experiments under denitrifying condition. In steady-state operation, 20-30% of influent nitrogen was emitted as N2O in the bioreactors with influent COD/N ratio less than 3.5. A 15N tracer study showed that this N2O originated from denitrification in anoxic phase. However, N2O reduction capacity of denitrifiers was always larger than NO3(-)-N or NO2(-)-N reduction capacity. It was suggested that a high N2O emission rate under low COD/N ratio operations was mainly due to endogenous denitrification with NO2(-)-N in the later part of anoxic phase. This NO2(-)-N build-up was attributed to the difference between NO3(-)-N and NO2(-)-N reduction capacities, which was the feature observed only in low COD/N ratio operations.

Novel autotrophic nitrogen removal system using gel entrapment technology

A pilot plant involving a nitritation-anammox process was operated for treating digester supernatant. In the preceding nitritation process, ammonium-oxidizing bacteria were immobilized in gel carriers, and the growth of nitrite-oxidizing bacteria was suppressed by heat-shock treatment. For the following anammox process, in order to maintain the anammox biomass in the reactor, a novel process using anammox bacteria entrapped in gel carriers was also developed. The nitritation performance was stable, and the average nitrogen loading and nitritation rates were 3.0 and 1.7 kg Nm(-3)d(-1), respectively. In the nitritation process, nitrate production was completely suppressed. For the anammox process, the startup time was about two months. Stable nitrogen removal was achieved, and an average nitrogen conversion rate of 5.0 kg Nm(-3)d(-1) was obtained. Since the anammox bacteria were entrapped in gel carriers, stable nitrogen removal performance was attained even at an influent suspended solids concentration of 1500 mg L(-1).

Stability of autotrophic nitrogen removal system under four non-steady operations.

Stable nitrogen removal from the digester supernatant for sludge via the nitritation-anammox process under steady operations of ammonium concentration and flow rate has been often reported. In this study, the effects of four non-steady operations, intentional fluctuations of influent concentration from 890 to 650 mg-N/L and hydraulic load of the 10% increase, temporally shutdown for 3-d and maximum capacity of each reactor, were evaluated in the nitritation-anammox process treating digester supernatant for sludge. No serious effects were observed in the anammox reactor because the aeration-control system in the nitritation reactor responded and controlled the nitritation efficiency satisfactorily against intentional fluctuations and temporally shutdown. Finally, the maximum capacity of each reactor was evaluated, and the nitritation rate was found to be 2.3 kg-N/m(3)/d at a DO of 4.0mg/L, and the nitrogen-conversion rate was 9.0 kg-N/m(3)/d.

Energy reduction of a submerged membrane bioreactor using a polytetrafluoroethylene (PTFE) hollow-fiber membrane

AbstractIn this study, we modified a polytetrafluoroethylene (PTFE) hollow-fiber membrane element used for submerged membrane bioreactors (MBRs) to reduce the energy consumption during MBR processes. The high mechanical strength of the PTFE membrane made it possible to increase the effective length of the membrane fiber from 2 to 3 m. In addition, the packing density was increased by 20% by optimizing the membrane element configuration. These modifications improve the efficiency of membrane cleaning associated with aeration. The target of specific energy consumption was less than 0.4 kWh·m–3 in this study. The continuous operation of a pilot MBR treating real municipal wastewater revealed that the MBR utilizing the modified membrane element can be stably operated under a specific air demand per membrane surface area (SADm) of 0.13 m3·m–2·hr–1 when the dailyaveraged membrane fluxes for the constant flow rate and flow rate fluctuating modes of operation were set to 0.6 and 0.5 m3·m–2·d–1, respectively. The specific energy consumption under these operating conditions was estimated to be less than 0.37 kWh·m–3. These results strongly suggest that operating an MBR equipped with the modified membrane element with a specific energy consumption of less than 0.4 kWh·m–3 is highly possible.