Improvement of water quality by constructed wetlands used for treatment of septic effluent

Subsurface flow constructed wetlands are gaining importance for on-site treatment of domestic wastewater. Use of plants in such wetlands is common but needed evaluation. Experiments were conducted to evaluate the role of phytoremediation in reduction of indicator bacteria (E. coli) and viruses (MS-2 coliphage) and improvement of septic water quality. Plants were grown in a glasshouse in 5L or 20L buckets filled with washed gravel and septic water. Phytoremediation reduced populations of E. coli and MS-2 coliphage in February, but not in May. In February, phytoremediation reduced populations of E. coli by approximately 1.5-log no. 100mL-1, and MS-2 coliphages by 1.5-log no. mL-1 in comparison to controls. Plants were effective in reducing the microbial populations at 25°C. Cyperus alternifolius, C. isocladus and Typha latifolia were plants of choice for reduction of populations of indicator bacteria and viruses. Phytoremediation effectively reduced biological oxygen demand, NH₄+-N, P, turbidity, fecal coliforms and volatile suspended solids. Largest water loss by evapotranspiration was in treatments with Cyperus and Typha species. For instance, Cyperus and Typha species resulted in water loss of 0.37 and 0.18 cm d-1 respectively in fall '97. In the constructed wetland, plants showing good capability for growth and ease of maintenance were Typha angustifiolia, T. latifolia, Scirpus sp., Thalia sp., and Cyperus alternifolius . Effectiveness of eight constructed wetlands in Texas was evaluated. The effectiveness of constructed wetlands in reducing the BOD of septic water was excellent and generally resulted in reductions of 80–90% which provided for further disinfection by chlorination to reduce populations of fecal coliforms to less than 2 cfu 100 mL-1. Ammonium-N concentrations in the effluent were lower than in the influent. However the reduction was less than 40%. Percentage wise, less of P was removed than Ammonium-N. Wetlands helped in reducing the TSS and VSS by approximately 88 and 83% respectively. The temperature did not alter the effluent BOD values between 10 and 30°C. Constructed wetlands provided an effective method for secondary treatment of domestic wastewater. Phytoremediation helped in improving domestic wastewater quality by reducing BOD5 and populations of microorganisms indicative of fecal contamination.