Effects of load fluctuations on treatment potential of a hybrid sub-surface flow constructed wetland treating milking parlor waste water

A hybrid sub-surface constructed wetland (CW) system consisting of 2 gravel filled vertical sub-surface (VFa & VFb) beds, each 160 m2 in size planted with Phragmites australis and a sand filled horizontal sub-surface (HF) bed, 336 m2 in size, planted with rice was operated from 2007 to 2010 for treating milking parlor waste water in Hokkaido, Japan. Hybrid CW system received huge fluctuations in average yearly inlet loads for TSS (526.0-1259 mg L−1 & 2.7-9.0 g m−2 d−1), BOD5 (1,080-2,114 mg L−1 & 8.4-14.4 g m−2 d−1), COD(1,962-7,085 mg L−1 & 14.5-50.0 g m−2 d−1), TN (116.0-243.0 mg L−1 & 0.8-1.6 g m−2 d−1), NH4-N (54.0-90.0 mg L−1 & 0.40-0.64 g m−2 d−1), TC (1,022-2,215 mg L−1 & 6.0-15.1 g m−2 d−1), TP (15.3-41.7 mg L−1 & 0.11-0.28 g m−2 d−1) during study period. Average yearly purification and removal rates were least fluctuated for TSS (95.7-99.4%); moderately for BOD5 (86.1-95.7%), COD(87.5-96.1%) and TC (79.5-91.3%); highly for TN (72.6-90.6%), NH4-N (62.9-85.3%) and TP (64.8-87.2%). A sharp decrease in TP purification and removal rates were observed in 2008 due to sharp decrease in influent TP concentration in 2008 compared to 2007. OTR values for VF(a), VF(b), HF bed and total system were observed as 21.7, 19.3, 4.8 and 12.3 g O2 m−2 d−1 respectively. Average k value of hybrid CW system for BOD5, TN, NH4-N and TP during study period were 7.0 ± 1.8, 7.4 ± 3.3, 5.6 ± 4.1 and 4.9 ± 2.0 m yr−1 respectively.Average concentration of TSS, TP, TN and NH4-N in the final effluent for all years were below the discharge limit value of: 150 mg L−1 for TSS; 8 mg L−1 for TP, 60 mg L−1 for TN and NH4-N. However, average BOD5 and COD concentrations could not meet the discharge limit value of 120 mg L−1 during 2007 and 2008.