Motoyuki Mizuochi

Nitrogen removal and N2O emission in a full-scale domestic wastewater treatment plant with intermittent aeration

Nitrous oxide (N2O) is emitted from wastewater treatment processes. It is known as a greenhouse gas that contributes to global warming (over 200 times more per molecule than carbon dioxide) and to the destruction of the ozone layer. It is therefore of great importance to develop technology that can suppress N2O emission. The effects of an anoxic period on N2O emission and nitrogen removal were investigated in an actual domestic wastewater treatment plant. When operated with intermittent aeration, most of the N2O was emitted into the atmosphere during the aerobic period. N2O emission from the intermittent process was estimated to be 0.43–1.89 g N2O person−1 year−1. Maintaining a dissolved oxygen (DO) concentration of over 0.5 mg l−1 during the aerobic period resulted in the complete conversion of the influent NH4-N to NO3-N and a 60-min anoxic period was sufficient for denitrification to be completed. The findings show that an optimum combination of aerobic and anoxic conditions and their suitable control are very important for improving nitrogen removal efficiency and controlling N2O emission.

Emission and control of nitrous oxide from a biological wastewater treatment system with intermittent aeration.

Nitrous oxide (N2O) can be emitted as a by-product of the process of nitrogen removal from wastewater. Two methods of complete denitrification and media application were studied in lab-scale intermittent aeration reactors fed with domestic wastewater to refine methods of controlling the N2O emission rate. A study on cyclic patterns showed that the highest N2O emission rate was at the beginning of the aerobic phase rather than the anoxic phase. This was probably because the nitrifying bacteria had accumulated nitrite nitrogen (NO2-) under low DO conditions. Methanol as an external carbon source was added during the anoxic phase to reduce nitrate nitrogen (NO3-) when denitrification was completed. The N2O emission rates in both the aerobic and anoxic phases were significantly influenced by residual NO3-, increasing monotonically as the concentration of NO3- in the reactor increased. Over 95% of average N2O emissions in both the aerobic and anoxic phases were prevented when methanol was added. The biofilm reactor showed similar patterns to those of the non-biofilm reactor in track behavior, but the former was more effective in the reduction of N2O emissions.

Measurements of N2O and CH4 from the aerated composting of food waste

Emissions of N2O and CH4 from an aerated composting system were investigated using small-scale simulated reactors. The results show relatively high emissions of N2O at the beginning of composting, in proportion to the application amount of food waste. After 2 days, the N2O emission decreased to 0.53 ppmv on average, near to the background level in the atmosphere (0.45 ppmv). The addition of composted cattle manure increased N2O emissions not only at the beginning of composting, but also during the later period and resulted in two peak emission curves. Good correlation was observed between the N2O concentration at the air outlet and NO2- concentration in waste, suggesting a generation pathway for N2O from NO2- to N2O. Methane was only detected in treatments containing composted cattle manure. The high emission of methane illustrates the involvement of anoxic/anaerobic microorganisms with the addition of composted manure. The result suggests the existence of anoxic or anaerobic microsite inside the waste particles even though ventilation was employed during the composting process.

Aerosol formation by the photooxidation of cyclohexene in the presence of nitrogen oxides

Photochemical aerosol formation from cyclohexene (C/sub 6/H/sub 10/) was studied in a ppm concentration range in the presence of nitrogen oxides. It was found that the aerosol was formed by the reaction of C/sub 6/H/sub 10/ with ozone (O/sub 3/), while the products of the C/sub 6/H/sub 10/ + OH reaction had substantially no contribution to the aerosol production. The average carbon-based aerosol yield from the O/sub 3/ reaction was shown to be 18.3 /plus minus/ 3.6% for a C/sub 6/H/sub 10/ initial concentration range of 5-10 ppm. However, the yield was found to decrease nonlinearly as the initial concentration was decreased, and consequently, it was pointed out that, in obtaining the yield in the actual environmental condition, an extrapolation of the result from higher concentration experiments to a lower concentration region should be done with caution. Information about humidity effects on aerosol formation was also obtained. In addition, the accuracy of the rate constant for the reaction of C/sub 6/H/sub 10/ with O/sub 3/ was checked.

Development of a treatment system for molasses wastewater: The effects of cation inhibition on the anaerobic degradation process

This study evaluated the process performance of a novel treatment system consisting of an acidification reactor, an upflow staged sludge bed (USSB) reactor, an upflow anaerobic sludge blanket reactor, and an aerobic trickling filter for the treatment of a high-strength molasses wastewater with a chemical oxygen demand (COD) of up to 120,000mg/L. The USSB operating at 35°C was capable of achieving an organic loading rate of 11kgCOD/m(3) day with a methane recovery of 62.4% at an influent COD of 120,000mg/L. The final effluent COD was 4520mg/L. The system was effective with regard to nitrification and sulfur removal. Fifty percent inhibition of the bacterial activity of the retained sludge by the cations was determined at 8gK/L for sucrose degradation, 16gK/L for sulfate reduction, and 12gK/L or 9gNa/L for acetoclastic methane production. Cation inhibition of anaerobic degradation reduced the process performance of the USSB.

Study of the characteristics of CH4 and N2O emission and methods of controlling their emission in the soil-trench wastewater treatment process

In recent years, worldwide concern over global warming has been expressed. It has been reported that domestic wastewater and its treatment processes are sources of CH4 and N2O, designated as greenhouse gases, the reduction of which was noted to be extremely important at the Third Conference of the Framework Convention on Climate Change (Conference Of the Parties; COP3). Here we report a study of a field that has been unexplored until now: analytical evaluation of the properties of the emission of CH4 and N2O and methods of restricting their emission in soil-trench wastewater treatment processes, the use of which is spreading, mainly in developing nations. The results have provided the following information. A field fact-finding survey has confirmed that soil trenches emit 9.3-13.9 g CH4 m(-3) and 8.2-12.2 gN2O m(-3) in Japan, and 3.0-4.5 g CH4 m(-3) and 3.3-5.0 g N2O m(-3) in China. The emission properties widely vary according to the structure of the treatment system. The conversion ratio for nitrogen in the wastewater influent to N2O by a soil trench is between 2 and a maximum of 8%, and ranges from a few- to several 10-fold as much as that with the activated sludge method, suggesting that this can be a large source of N2O emission. It has also clearly been shown that the aerobic-anaerobic state inside the treatment system is closely related to its CH4 and N2O emission characteristics. By performing ventilation to maintain the oxidation-reduction potential (ORP) near the trench at an aerobic condition of +200 mV or more, the quantities of CH4 and N2O emitted can be reduced by as much as 50% from the levels without this ventilation, and that this can make a large contribution.

Redetermination of the rate constant for the reaction of hydroxyl radicals with sulfur dioxide.

The rate constant (k/sub 1/) for the reaction of HO radicals with SO/sub 2/ has ben determined by a competitive rate technique with n-C/sub 4/H/sub 10/ used as a reference reactant. The HO radicals were generated from CH/sub 3/ONO, and the rate of decrease in the SO/sub 2/ concentration has been measured in dry air ((H/sub 2/O) < 1 ppm) at 780-820 torr in the presence of NO. A value of k/sub 1/ = (1.22 +/- 0.13) x 10/sup -10/ cm/sup 3/ molecule/sup -1/ s/sup -1/ at 303 +/- 1 K has been obtained supporting a recent estimate of Calvert and Stockwell.

Humidity effects on photochemical aerosol formation in the SO2-NO-C3H6-air system

Abstract In order to investigate the effects of humidity on the gas-phase oxidation of SO 2 in polluted air and on the subsequent aerosol formation process, photoirradiation experiments were carried out by means of a 4-m 3 chamber, in which mixtures containing SO 2 , NO and C 3 H 6 with concentrations in the ppm range were exposed to simulated solar radiation in different relative humidity (r.h.) conditions. The total amount of oxidized SO 2 was quantified from the SO 4 2− yield determined by the chemical analysis of the aerosol product, and a part due to the oxidation by the OH radical was evaluated by estimating the OH concentration from the decay rate of C 3 H 6 . The remaining part was assigned to the oxidation by the Criegee intermediate, as it had a good correlation with the progress of the O 3 + C 3 H 6 reaction. The contributions of the two oxidizing species to the total conversion and the oxidation rate of SO 2 were measured as functions of r.h. As a result, experimental evidence was obtained for the prediction of Calvert and Stockwells (1983, Envir. Sci. Technol . 17 , 428A–443A) simulation that the oxidation due to the Criegee intermediate was retarded by the increase in humidity. The OH contribution, on the other hand, was almost independent of r.h. It was observed consequently that the total oxidized amount of SO 2 considerably decreased as r.h. was higher. The humidity effect on the aerosol formation process was found to be more complicated than the effect on the gas-phase chemistry. The maximum rate of increase in the particle number concentration rose linearly with increasing r.h., but the number concentration itself measured at its maximum or at the end of the irradiation reached a ceiling value around r.h. = 30% and went down for higher r.h. The average panicle size in the final stage of the reaction showed a minimum around the same r.h. at which the number concentration was maximum. The H 2 SO 4 concentration in the mist particles, however, decreased monotonically as r.h. got higher. It was suggested that these different responses against the increase in humidity resulted from the cooperation of several processes such as the H 2 SO 4 monomer formation, the H 2 O condensation, the particle coagulation, etc., which had different dependences on r.h.

Sulfuric acid aerosol formation by the reaction of HO radicals with SO2

In order to investigate sulfuric acid formation by the reaction of HO radicals with SO2, photoirradiation experiments were carried out by means of a 4‐m3 chamber, in which humidified air containing SO2, CH3ONO, n‐C4H10 and NO with concentrations in ppm range was exposed to simulated solar radiation at 303 ± 1K and under atmospheric pressures. The HO radicals were generated by a photolysis of CH3ONO and their concentration was estimated from the decay rate of n‐C4H10 concentration. The amount of SO2 lost by the reaction was evaluated through time integration of the instantaneous reaction rate k 1 [HO] [SO2] by using the rate constant k 1 = 1.22 × 10‐12 cm3 molecule‐1 s‐1 which we reported previously for the HO + SO2 reaction. The yield of sulfuric acid was determined, on the other hand, by the filter collection and the subsequent ion chromatographic analysis of the aerosol product. From the agreement between the yield and the reactant loss, the accuracy of the k 1 value was proved and simultaneously the st...

Protection of biomass from snail overgrazing in a trickling filter using sponge media as a biomass carrier: down-flow hanging sponge system

This study investigated down-flow hanging sponge (DHS) technology as a promising trickling filter (TF) using sponge media as a biomass carrier with an emphasis on protection of the biomass against macrofauna overgrazing. A pilot-scale DHS reactor fed with low-strength municipal sewage was operated under ambient temperature conditions for 1 year at a sewage treatment plant in Bangkok, Thailand. The results showed that snails (macrofauna) were present on the surface of the sponge media, but could not enter into it, because the sponge media with smaller pores physically protected the biomass from the snails. As a result, the sponge media maintained a dense biomass, with an average value of 22.3 gVSS/L sponge (58.1 gTSS/L sponge) on day 370. The snails could graze biomass on the surface of the sponge media. The DHS reactor process performance was also successful. The DHS reactor requires neither chemical treatments nor specific operations such as flooding for snail control. Overall, the results of this study indicate that the DHS reactor is able to protect biomass from snail overgrazing.