Application of a Low-Cost Technology to Treat Domestic Sewage and to Improve Fertility of a Barren Lateritic Soil

Abstract

Selecting low-cost, less power-intensive or power-independent, with less mechanization, and efficient alternative technologies for wastewater treatment is essential to improve treatment capacity, especially in developing regions. Widespread application of constructed wetland (CW) is limited due to land cost. Use of locally available lands having infertile, barren soils nearby townships will be economical, because these types of soils are not in use and also cost of transportation of such land is minimum. The shallow soil media will also help in maintaining aerobic conditions, overall improving the performance of CW. Another advantage of use of infertile, barren soil as substrate of CWs can be improvement in fertility of such soil, thereby resulting in overall benefits for the area. Performance of six different aquatic macrophytes—Typha latifolia, Colocasia esculenta, Alternanthera sessilis, Polygonum, Canna indica and Ocimum americanum L—grown in mono-culture and poly-culture for treatment of domestic sewage and retention of major nutrients nitrogen (N) and phosphorus (P) in lateritic soil substrate (barren or infertile) was studied in sixteen laboratory-scale CWs. Fourteen numbers of laboratory-scale CWs, having dimensions 1.0 m × 0.36 m × 0.35 m (L × W × H) for mono-culture and a pair of CWs having dimension 2.5 m × 1.0 m × 0.35 m for poly-culture, were used. In every CW, the depth of lateritic soil substrate was kept shallow at 0.30 m to maintain aerobic conditions. Depth of 0.05 m was kept as free board. CWs were operated in batch mode with sewage loading for six hours to maintain aerobic conditions. Sewage residence time was twenty-four hours, and applied hydraulic loading was 0.028 m3/m2/day. Good removal of BOD and COD was observed for all the CWs. BOD removal of 36.0–80.6% for the CWs with mono-culture of plants and 71.3–79.8% for poly-culture of plants was observed. Similarly, COD removal of 32.2–72.6% for the CWs with mono-culture of plants and 34.0–63.5% for poly-culture of plants was observed. For nutrient removal, best results were obtained for CW operated with T. latifolia with average removals of 69% for total Kjeldahl nitrogen (TKN) and 89% soluble reactive phosphorus (SRP) at plant density of 4 plants/CW. Nutrient removal rate for the plants followed the pattern—T. latifolia > C. indica > A. sessilis > Polygonum > C. esculenta > O. americanum L. > Pistia stratiotes. Highest amount of average mineralizable phosphorus retention in the lateritic soil substrate was 85%, and nitrogen retention was 46.2% for P. stratiotes plant with the density of 4 plants/CW. Plant uptake was the major nutrient removal pathway in the wetland microcosms. Nutrient removal from domestic wastewater in CWs with poly-culture of plants was not greater than that in CWs with mono-culture plants, due to interspecies competition. CWs prove to be a viable option for simultaneous treatment of sewage and enhancement of fertility of barren lateritic lands.