Pollutant Removal Efficiency of Hybrid Constructed Wetland System for Recycling the Sewage by Utilizing Aquatic Plants

Recycling of wastewater for irrigation has been practiced for centuries throughout the world. Utilization of untreated wastewater for crop irrigation poses substantial risks to public health, not only to the farmers, but also the surrounding communities and the consumers of the crops. Sewage and industrial effluents may have high concentration of several heavy metals such Cd, Ni, Pb and Cr (Narwal et al., 1993). Their continuous disposal on agricultural soils has resulted in accumulation of some of the toxic metals in soil which may pose serious human and animal health. Recycling of wastewater after adequate treatment and its utilization for crops is the need of the hour. Constructed wetlands are one of the engineered systems that have been designed and constructed to utilize the natural processes involving wetland vegetation, soils, and the associated microbial assemblages to assist in treating wastewaters (Vyamazal Jan, 2010). Hybrid constructed wetland systems are derived from original hybrid systems developed by Kathe Seidel at the Max Plank Institute in Krefeld, Germany, comprise of horizontal flow (HF) and vertical flow (VF) arranged in a staged manner. The hybrid constructed wetlands take advantages of each of HF and VF and combine them for improving the quality of reclaimed water. Therefore in the present study, an attempt has been made to evaluate the pollutant removal efficiency of HCWS by utilizing the native aquatic plants for recycling the sewage.

Description of Wetland Systems

The study was carried out in the Department of Environmental Sciences, Tamil Nadu Agricultural University (TNAU), Coimbatore from December 2015 to September 2016. A lab-scale Hybrid Constructed Wetland System (HCWS) was designed with both horizontal and vertical flow treatment (HF-VF) systems.

A total of four cells were installed in the HCWS, all uniform in size (80 × 57 × 42.5 cm) (L × W × H). Sewage was collected from the TNAU sewage collection area near TNAU staff quarters, periodically, and used for experimental purposes.

• The first cell was the screening cell, where a stainless steel filter mesh was used to filter solid materials, and sewage was pumped into the first cell.
• The second cell was the horizontal flow (HF) treatment system. Sewage effluent entered the HF cell through gravity from the screening cell.
• The third cell was the vertical flow (VF) system, where perforated pipes were installed for the vertical movement of sewage from the HF cell. Both HF and VF cells were packed with pre-sterilized media, including pebbles (40mm), gravels (20mm), coarse sand, and soil.
• The fourth cell was the treated water collection cell.

Native aquatic plants Brachiaria humidicola and Typha angustifolia, collected from Muthanakulam tank of Coimbatore district, were used in the horizontal and vertical flow cells, respectively.

A uniform slope of 1% was maintained between the cells for the easy movement of wastewater among them. Hydraulic retention times (HRTs) of 1, 3, and 5 days were maintained throughout the experimental period to evaluate the treatment efficiency of the HCWS.

Sewage samples were analyzed as per standard analytical procedures. Treated wastewater samples from each cell were collected periodically during the following periods:

• December 2015 – February 2016 (SI)
• March 2016 – May 2016 (SII)
• June 2016 – August 2016 (SIII)

Samples were taken at HRT intervals of 1, 3, and 5 days.

Characteristics of sewage effluent The quality of the collected sewage was assessed by analysing its pH, Biological oxygen demand (BOD), chemical oxygen demand (COD), Total Suspended Solids (TSS), Total dissolved solids (TDS) and total coliforms. The mean value of the parameters analysed are presented in the Table 1. In general, the BOD values ranged from 121-209 mg L-1. The highest BOD range of 168-209 mg L-¹ was recorded at S I and the lowest was recorded in SIII period of study. BOD level of the water increases if the dissolved Table 1. Characteristics of sewage used for HCWS experiment oxygen content of the water is very low. Dissolved oxygen levels change and vary according to the time of day, the weather and the temperature (Muigai et al., 2010). An increase in the BOD levels is usually an indication of an influx of some type of organic pollutant in the collected water. The COD value of the sewage effluent recorded the same trend as that of BOD level. Since the level of COD was very high it pointed towards the high loading of both organic and inorganic pollutants. The high BOD and COD values of collected sewage water samples exceeded the prescribed levels of 100 and 250 mg L-1 (CPCB) for irrigation.

Table 1. Characteristics of sewage used for HCWS experiment

Table 2. Average COD removal efficiency (%) ±SE of HCWS

Similar observation was recorded in the COD removal efficiency of the HCWS (Table 2). The COD removal efficiency increased in order of HRT of 5d > 3 d > 1 d. This may be due to the highly dense vegetative growth of Brachiaria humidicola and Typha angustifolia and the root system of these plants provides large surface area for attached microorganisms, thus increasing the potential of degradation of organic matter. Similar findings were reported by Sayadi et al. (2012). The growth of plants depends on the HRT and the media used in the constructed wetland systems. HRT of 3 days was found to be beneficial. This may be due to the maximum time provided for the plant surface for its reaction with wastewater. The results are in line with the findings of Suntud Sirianuntapiboon et al., (2006). Stottmeister et al. (2003) also stated that the growth rate of both Typha sp and aerobic bacteria increased with the increase of soil percentage in the media because the soil component provides good conditions for growth of both Typha sp. and aerobic bacteria. Generally the removal efficiency was higher in the summer season compared to winter. This may be due to the increased vegetative growth and microbial activity at high temperature during summer than winter. The combined effect of HF and VF in the HCWS proved to be significant in the pollutant removal efficiency. Florentina zurita and John R. white (2014) also reported that the hybrid ecological wastewater treatment systems were effective in treating the reclaimed water. 

The results revealed that HCWS could be beneficial in reducing the levels of BOD, COD of the sewage water. The native aquatic plants used in the system viz., Brachiaria humidicola and Typha angustifolia also proved to perform the best in the designed HCWS. The removal efficiencies of BOD, COD were higher with the hydraulic retention time (HRT) of 5 days compared to 1 and 3 day respectively. The combined effect of HF and VF in the removal of pollutants by using aquatic plants was found to be higher during the summer compared to the winter and autumn seasons.