Approaches to Assess the Optimum Dissolved Oxygen Released by Macrophytes in the Substratum of Constructed Wetlands

Abstract

Constructed wetlands (CWs) have been successfully employed to treat wastewater in developing countries as they are economical and based on ecologically friendly principles. Wastewater treatment by CW involves chemical, biological and physical processes like precipitation, sedimentation, absorption, adsorption and biodegradation. However, the degradation of organic matter is dependent on oxygen availability in wastewater. The hydrologic environment of CW with water saturated substrate is prevailed by poor oxygen conditions. Therefore, the oxygen added by the macrophytes through their roots to the substrate of CW plays a key role in wastewater treatment. A part of the oxygen produced during the process of photosynthesis is transported through the inter-connected air spaces comprising aerenchyma of these plants to the roots and rhizosphere. The active oxygen transfer capability is one of the main parameters that controls the performance of a CW and is thus considerably dependent on the oxygen availability in the rhizosphere, which may be controlled by the finest combination of macrophytes, climatic factors and microorganisms. In the current study, a specially designed Wireless Sensor Network (WSN) was used for monitoring temperature, light intensity and humidity supplemented with a mix of digital controlling and measuring devices. A precise, continuous and frequent data about humidity, temperature and light intensity combinations was recorded with insignificant fluctuations of 1 - 2 ºC in temperature and 7 to 20 µmol m-2 s-1 in light intensity. The study discovered the significant increase in dissolved oxygen (DO) in the rhizosphere of Typha latifolia and Phragmites australis affected by the optimum combination of temperature and light intensity, i.e., 30 - 35 ºC with 210 µmol.m-2 s-1 and 35 ºC with 140 - 210 µmol m-2 s-1, respectively. The highest DO recorded from the rhizosphere of Typha when exposed to these combinations was 1.67 mg/L whereas in the rhizosphere of Phragmites it was 2.16 mg/L. The exposure to the optimum temperature and light intensity combinations, increased the fresh plant biomass in Typha by 27% and Phragmites 32% whereas the chlorophyll content was enhanced by 13% and 14%, respectively. The Plant Growth Promoting Rhizobacteria (PGPR) enhanced the plant growth and chlorophyll content. The PGPR also had positive influence on the process of photosynthesis and consequently on the release of oxygen from the roots of plants. The PGPR (GRP-C3) inoculated plants were able to add maximum oxygen to their rhizosphere. The highest amount of oxygen recorded from the rhizosphere of Typha and Phragmites was 4.54 mg/L and 2.57 mg/L, respectively. It was observed that the PGPR, GRP-C3 showed the highest phosphate solubilization index (17 mm) and higher ability to produce Indole Acetic Acid (IAA), i.e., 12.5 mg/L. The higher concentration of DO in the substratum of CW significantly reduced the retention time for treating maximum chemical and biological oxygen demand from the wastewater, i.e., 8 days for the Typha vegetated microcosms and 9 days for the Phragmites vegetated microcosms. The non-vegetated microcosms required 17-18 days, however, all vegetated microcosms exposed to unfavorable temperature and light intensity combinations required 10-15 days for achieving maximum COD/BOD removal efficiency. A major feature of variable rate of photosynthesis also revealed a rhythmic and regular pattern of diurnal fluctuation of DO concentration in the rhizosphere of macrophytes during the light and dark period. The highest photosynthetic rates observed in Saururus cernuus and Pistia stratiotes were 5.76 μmol CO2 m-2 s-1 and 3.52 μmol CO2 m-2 s-1 at light intensity level of 140 µmol m-2 s-1 and 210 µmol m-2 s-1, respectively. The Pistia exhibited a significantly higher release of oxygen (5.83 mg/L) from the roots than the Saururus (3.87 mg/L). The stomatal conductance in both plants was observed to be significantly affected by the variable temperatures where a regression between the stomatal conductance and DO in the rhizosphere of Pistia and Saururus was determined.