仮想的下水処理場における下水汚泥と厨芥の混合消化がエネルギー収支と水処理系にもたらす効果

摘要

Electricity generation from methane gas and return loading from the sludge treatment process were evaluated at a hypothetical wastewater treatment plant receiving wastewater from 350 thousand people when the kitchen waste from the same population was treated together in the anaerobic digestion. Statistics of sewerage works and solid waste treatment in Japan, and a past surveillance of return loading from sludge digestion to wastewater treatment were used in the assessment. The calculation inferred that co-digestion of the sewage sludge and the kitchen waste increased its methane production by 2.8 times in compared with digestion of the sewage sludge alone. Recovered heat from gas engine to generate electricity and heat obtained from combustion of methane were assumed to be used in heating the digester. This thermal balance and electricity production from digestion process were evaluated in cold, moderate and hot climate condition. The calculation shows that the digestion of sludge alone can cover 22 to 28% of electricity consumption of whole treatment plant, whereas the co-digestion covers its 66.5% irrespective of climate condition. The estimation shows that return loading from the co-digestion to wastewater treatment is small in BOD but relatively large in nitrogen and phosphorus.%人口35万人の都市の下水を処理している仮想的下水処理場において，下水汚泥とその都市で発生す る一般廃棄物の厨芥を合わせて嫌気性消化を行った場合について，メタンからの発電量増加と水処理系に対 する返流負荷を評価した・わが国の下水道および廃棄物の処理に関する統計と汚泥消化返流水に関する既報 の調査を評価に用いた・この推定によれば，厨芥と共に消化を行う混合消化は下水汚泥のみを行う単独消化 に比べてメタン発生量が2・8倍となる．ガスエンジンによるメタンガスからの発電と余熱回収，およびメタ ンの燃焼による熱供給の両者によって消化槽を加温した場合の熱収支および発電量を寒冷地，温暖地，およ び暑熱地に対して求めた・単独消化では気候条件によって処理場全体の電力消費の22－28％を，混合消化で は気候条件によらず66・5％をガス発電によりまかなえると計算された．混合消化が水処理系に与える返流 水の影響は，BODに関しては小さいが，窒素とリンに対しては比較的大きいことが推定された．