Performance evaluation of sub-surface flow constructed wetland systems under variable hydraulic loading rates

Abstract

Wastewater treatment has given an immense attention in the field of pollution control throughout the world. This has become a challenge in developing countries due to the limitations of resources and expertise. Constructed wetlands where water, plants and microorganisms interact to improve the quality of water have been proven to be an effective low-cost wastewater treatment technology in many parts of the world, which does not necessarily require skilled personnel to run the system. However, these systems are not yet widely spread in developing countries due to lack of information. Constructed wetlands can be designed as surface flow or subsurface flow systems, depending on the level of the water column. This study compares the performance of vertical subsurface flow (VSSF) and horizontal subsurface flow (HSSF) constructed wetland systems at laboratory scale at tropical condition. This paper also evaluates the effects of Hydraulic Loading Rate (HLR) on treatment capacity of wastewater parameters such as Five day Biochemical Oxygen Demand (BOD5), Total Suspended Solids (TSS), Nitrate Nitrogen (NO3 —N), Phosphate (PO4 3-), Ammonia Nitrogen (NH4-N ), Fecal Coliforms (FC) and Total Coliforms (TC). Six wetland models of size 1.4 m x 0.5 m x 0.5 m (L x W x H) were constructed and arranged: 1) Two models as VSSF system with plants, 2) Two models as HSSF system with plants, 3) One model as a VSSF control without plants and 4) One model as a HSSF control without plants. An emergent macrophyte specie; cattail (Typha angustifolia), gravel media (size 10 – 20 mm) and synthetic wastewater with average concentrations of BOD5 ; 29.51 ± 4.21 mg/L, NO3 - - N ; 3.22 ± 1.25 mg/L, NH3 - - N ; 15.14 ± 2.65 mg/L, PO4 3- ; 6.78 ± 5.67 mg/L, Fecal Coliform 495.12 * 103 ± 307.12 * 103 counts/100 mL and Total Coliform 915.5 * 103 ± 719.83 * 103 counts/100 mL were used in this study. The HLR was increased from 2.5 – 25 cm/day at 12 days interval during two and a half months period. Sampling was carried out with each HLR from both influent and effluents of each wetland system after 12 days of constant flow rate, and wastewater quality parameters such as the BOD5, TSS, NH4-N, NO3 --N, PO4 3-, pH, Conductivity, FC and TC were measured in all samples. Results show that VSSF systems perform better than horizontal systems, but the treatment performance declines with the increasing HLR in all six wetland models.