Seasonal efficiency of a hybrid sub-surface flow constructed wetland system in treating milking parlor wastewater at northern Hokkaido

Abstract

This paper presents the effects of seasonal variations on the purification and removal efficiencies of a hybrid sub-surface CW system (VF-VF-HF) treating milking parlor wastewater at northern Hokkaido, Japan. VF(a) and VF(b) are gravel beds, each 160m2 in size whereas HF is sand bed and 336m2 in size. Daily mean air temperature at site showed a difference of 16°C between warm (May–October) and cold (November–April) periods. Average influent concentrations for cold and warm periods were: CODCr: 3749 and 4988mgL−1; BOD5: 1637 and 1395mgL−1; TSS: 661 and 862mgL−1; TN: 161 and 194mgL−1; TP: 23.9 and 31.9mgL−1 and total carbon: 1212 and 1715mgL−1. Average purification rates of BOD5, TSS, TP, PO4–P and total carbon fluctuated <2.5% between both periods. However purification rates of NH4–N and Organic-P decreased by 9.5% and 12% respectively during warm period. Average removal rates of TSS and BOD5 were unaffected between both periods, while that of CODCr, TN, total carbon increased by 3–4% during warm period. Hybrid sub-surface CW system achieved purification and removal rates of >95% for TSS and total coliform; >89% for CODCr and BOD5; >76% for TN and >72% for TP during both cold and warm periods.