下水処理場の水処理工程および放流河川における亜酸化窒素発生量の一体的評価

摘要

In order to make clear the characteristics of the nitrous oxide (N_2O) emission from a sewage treatment process, total amount of the N_2O emission including the direct emission, the indirect emission and the dissolved N_2O emission through the treated water was assessed based on the survey. Three daily surveys were conducted in the A-STP, which is run with Pseudo-AO process, and the river receiving its effluent in Sep.2013, Dec.2013 and Sep.2014，respectively. Based on the survey, the amounts of the N_2O emission were estimated by 868kgCO_2/day in September and 3,897kgCO_2/day in December, respectively. The percentages of each origin in September were as follows: the duct gathering the gases from the grit chamber, the first settling tank and sludge treatment units was 0.3%; the direct emission from aeration tanks was 14.8%; the gasification of the dissolved N_2O in the treated water was 0.9 %; the effluent to the river as dissolved N_2O in treated water was 5.8%; indirect emission based on the IPCC guideline was 78.2%. On the other hand, the percentages of these origins in December were 0.7%，66.4%, 8.9%, 7.1% and 16.9%, respectively. Based on the results, by accounting for the dissolved N_2O in treated water as well as the direct and the indirect emission, the correct amount of N_2O emission can be calculated. Since the dissolved N_2O concentrations in the aeration tanks were high in winter, the controlling of DO concentration in the aeration tanks and SRT with the monitoring of nitrite concentration, might be effective to reduce the N_2O emission, especially in winter.%温室効果ガスである亜酸化窒素(N_2O)について，下水処理場水処理工程のフラックスを正確に算出し，発生量の抑制につなげることを目的として，反応槽から放流先までを一体的に評価した.疑似嫌気好気活性汚泥法を行うA浄化センターと放流先のA川において，2013年の9月と12月，2014年の9月に通日調査を行い，N_2O発生量を算出したところ，CO_2換算で9月が868kgCO_2/d,12月が3,897kgCO_2/dと見積もられた.発生源別の割合は，9月において沈砂池，最初沈殿池および汚泥処理棟由来を合計したガス態N_2O(GN_2O)が0.3％,反応槽由来の直接発生が14.8％,処理水中溶存態N_2O(DN_2O)のガス化が0.9％,河川へのDN_2Oの放流が5_8％,間接発生が78.2％であったのに対して，12月においてはそれぞれ0.7％,66.4％,8.9％,7.1％,16.9％であった.これより，水処理工程由来のN_2O発生量は，反応槽由来の直接発生や間接発生はもとより,処理水中のDN_2O成分の放流後の挙動を考慮して一体的に評価することで,値を正確に算出できると考えられた.また，反応槽においては特に冬季にかけてDN_2O濃度が高くなる傾向にあるため，前駆物質であるNO_2濃度を指標として，冬季においてDO濃度とA-SRTの管理を適切に行うことがN_2Oの抑制に有効であると推察された.