TREATMENT OF DOMESTIC WASTEWATER BY CONSTRUCTED WETLANDS IN TROPICAL CONDITIONS

Abstract

In view of the alarming urban and industrial growth in the developing countries, especially India, the pollution level in the water, air and soil has increased substantially over the years. Considering the magnitude of investment needed in wastewater treatment, government and private sector have only been able to mobilize funds towards construction of treatment plants employing conventional methods in few large towns, metropolitan areas and industrial complexes. Further, it seems unlikely that financial resources would be available in future also for construction of such plants in smaller towns, villages, smaller residential and industrial complexes. Constructed Wetlands (CW) promises to serve as an ideal alternative for wastewater treatment, which is simpler, more economical and environment-friendly. The concept ofthis technology has been taken from the natural wetlands, which are as old as our earth. Some of the eminent researchers have keenly observed these natural purification processes and tried to use it for the benefit ofmankind. Siedel at Max Planks institute of Plon, Germany conducted the first work investigating wastewater treatment by constructed wetlands. In 1952, she explored the removal of phenols from wastewater. Prof. Kickuth and co-workers popularized this concept in Europe, resulting in nearly 200 municipal and industrial wastewater treatment plants. USA and China have also reportedly taken keen interest in this technology as indicated by a large number ofoperational treatment units in these countries. However in India, despite the apparent suitability of this technology, the research and implementation of the same have not been found encouraging. It has been observed that the aspects related to the operation of the CW system in tropical conditions like degradation kinetics, effect of hydrometeorology on water balance, suitability of locally available vegetation and estimation of parameters like hydraulic retention time, hydraulic and organic loadings etc. to meet the permissible effluent standards need to be studied in detail. In this light, a research study on CW was planned with following objectives: (a) Design and Construction of a Pilot plant (b) Evaluation of variation of hydraulic loading (Hydraulic Retention Time, HRT) on removal efficiency in treatment of raw domestic wastewater and polishing of the treated domestic wastewater. (c) Performance evaluation for treatment (of raw) and polishing (of treated) domestic wastewater at a hydraulic loading corresponding to the permissible effluent disposal standards of Bureau of Indian Standards (BIS). (d) Performance evaluation of a Natural wetland for treatment of domestic wastewater. The site for constructing the pilot wetland unit was located at U. P. Jal Nigam sewage treatment plant near Jagjeetpur at Haridwar (Uttaranchal State). Two wetland beds were constructed in the premises of U. P. Jal Nigam sewage treatment plant. One bed of size 4.88 m (L) x 3.1 m (W) x 0.80m (d) was employed for the treatment of raw domestic wastewater (raw bed) and other bed of size 4.35 m (L) x 2.25 m (W) x 0.40 m (d) was employed for polishing the wastewater treated by the existing activated sludge process treatment unit available at the site (polishing bed). The HRT and hydraulic loading for treating the raw wastewater (in the raw bed) for limiting the effluent BOD5 to 20 mg/1 (as per BIS standards) has been found to be 1.75 days and 132.21 LitW.d (13.22 cm/day) respectively. The BOD5 removal rate constant (Kt) has been observed to be 1.0042 d"1 at 24°C (0.795 d"1 at 20 °C) and the removal has been observed to be governed by the equation: Ce/C0=e-L0042xt (R2 =0.90) Performance of the bed under the study period has been found to be consistent. The average removal of various parameters like TSS, BOD, COD, TKN, TP and Fecal Coliforms has been estimated as 81.8, 82.7, 83.4, 29.7, 23.9 and 99.7 percent respectively. ii Further, the water balance study has shown that about 21.60, 6.47 and 1.16 percent of water has been lost during summer, monsoon and winter seasons respectively through evapo-transpiration. The average ratio of evapo-transpiration to pan evaporation has been estimated as 2.53. The HRT and hydraulic loading for polishing the treated wastewater (in polishing bed) while limiting Faecal Coliforms to aconcentration of 1000 no./100 ml (as per WHO guidelines) has been found to be 1.25 days and 91.95 Lit/m2.d (9.2 cm/day) respectively. The Faecal Coliform removal rate constant (Kft) has been estimated as 2.41day" at 24 C and the removal has been observed to be governed by the equation Fe/F0=e-14118xt (R2 =0.80) The natural wetland of size 40m x 20m x 0.25m near Ganeshpur area of Roorkee town was selected for evaluating its performance. A mixed culture of submerged and emergent vegetation including Phragmites Karka, Typha Latifolia and Metha Aquatica were found to be profusely growing in the wetland. This wetland was observed to remove TSS, BOD5, Total Kjaldhal Nitrogen, Total Phosphorus, and Faecal Coliform to the extent ofaverage 63.00, 48.00, 12.5, 11.99, and 69.50 percent respectively. Onthe basis of the above results, it is concluded that constructed wetlands emerge as a simple, economical and environment-friendly alternative to the studied conventional treatment system for treatment of raw domestic wastewater as well as for polishing of domestic wastewater treated by the existing treatment process. Natural wetland also displays a reasonably good capability ofwastewater purification.