Application of horizontal flow constructed wetland and solar driven disinfection technologies for wastewater treatment in India

Water resources in India are under serious threat due to the discharge of wastewater into aquatic ecosystems. With the rapid expansion of cities and domestic water supply, the quantity of domestic wastewater is increasing day by day. As per recent estimates made by the Central Pollution Control Board of India (CPCB), about 70% of total water supplied for domestic use is then disposed of as wastewater (CPCB 2015). Disposal of domestic sewage from cities and towns is the biggest source of pollution of water bodies in India. A large number of rivers are severely polluted due to discharges of domestic sewage. Pressure on fresh water resources can be reduced by the treatment and utilization of domestic sewage for irrigation. According to the CPCB, the total volume of wastewater generated from Class I cities and Class II towns in the country is around 35,558 and 2,696 million litres per day (MLD) respectively, whereas the installed sewage treatment capacity is approximately 11,553 and 233 MLD respectively. During 2015, the estimated volume of sewage generated in the country was 61,754 MLD, compared to the available sewage treatment capacity of 22,963 MLD. About 38,791 MLD of untreated sewage (62% of the total sewage) is therefore discharged directly into nearby water bodies. Thus, there is a huge gap in sewage generation and sewage treatment capacity (CPCB 2016). Evidently there are many problems arising from reduced fresh water availability and increased wastewater generation.

There are many technologies that have been developed to treat wastewater from different sources and to produce an effluent of safe quality. Conventional wastewater treatment has certain serious limitations: there are very high operation and maintenance costs, technically skilled personnel are needed, and there is a high energy demand (Mishra & Tripathi 2009). In India, there are 816 sewage treatment plants (STPs), out of which 522 are operational, and most of the STPs are located in a few major cities and towns along the banks of major rivers (CPCB 2015). Due to improper design, poor maintenance, frequent electricity outages and lack of technical man power, the facilities constructed to treat wastewater do not function properly and remain closed most of the time (CPCB 2015). A performance evaluation of STPs carried out by CPCB in selected cities indicated that almost one third of the STPs had not met prescribed standards in term of various water quality parameters. Wastewater treatment capacity is increasing, yet the total sewage is not treated effectively, and the remaining amount of partially treated sewage is discharged into the natural ecosystems (CPCB 2015).

Constructed natural systems such as wetlands have proven to be a very reliable, cost efficient and long-term solution in many countries. Constructed wetlands (CWs) are manmade, engineered systems that have been designed and constructed to utilize the natural processes of wetland vegetation, soils, and the associated microbial assemblages to assist in treating wastewaters. CWs have been utilized for the treatment of a wide variety of pollutants (Vymazal 2009). They are designed to take advantage of many of the same processes that occur in natural wetlands within a more controlled environment (García et al. 2004). CWs for wastewater treatment may be classified according to the life form of the dominating macrophytes into systems with free-floating, rooted emergent and submerged macrophytes (Brix & Arias 2005). Combined with an appropriate primary treatment and polishing technologies, highly polluted effluent water can also be treated (Huang et al. 2004; Hua et al. 2010). The treated water can be reused in irrigational, gardening, and inhouse applications, directly substituting limited fresh water resources. Even the generation of renewable energy from the wastewater treatment becomes possible.

Studies on CWs in the Indian context are limited and operate mostly on an experimental scale, treating different kinds of wastewater (Juwarkar et al. 1995; Billore et al. 1999, 2001). Some recent studies have been conducted on a relatively larger scale (Rai et al. 2015; Upadhyay et al. 2016). Sub surface wetland systems are about 100 times smaller in size and the hydraulic retention time (HRT) is a third of that of surface flow systems. Considering all the conditions, wetlands may be considered as one of the better options for developing countries. Shorter HRTs generally translate into smaller land requirement. Batch flow systems, with decreased detention time, have been reported to be associated with a lower treatment area and higher pollutant removal efficiency. (Billore et al. 1999). Gravel filtration breaks down impurities, removes odors, and is capable in treating wastewaters of different qualities. Gravel and sand as a media for wastewater treatment in CW is one of the emerging techniques in India. Gravel bed CWs can remove biological oxygen demand (BOD), suspended solids, dissolve solids, ammonia nitrogen, and pathogens from the water in the wetland. The different pollutants get deposited in between its fragments, where they are broken down by microbes that grow on the surface of the gravel. In rural and remote areas of India, where space is not a major limitation, and availability of electricity is still limiting, CWs with gravel bed filters can be an excellent option for wastewater treatment. Under present investigation a small horizontal flow constructed wetland (HFCW) of 35 m² was combined with a solar-driven inline electrolytic disinfect unit to treat and disinfection partially treated sewage for its reutilization as irrigation water.

During the process of electrolysis, naturally present or artificially added chloride produces reactive chlorine species such as free chlorine (Cl₂), HOCl, ClO⁻ and chlorine radicals (Cl, Cl₂⁻), which act as disinfectants (Jeong et al. 2006; Huang et al. 2016). The disinfecting effect of free chlorine is based on unspecific oxidation reactions on cell membranes of the pathogens. During the disinfection, chlorine reacts back to chloride ions. Under ideal conditions there is no overall change in the chemical composition of the water during electrochemical water disinfection (Kraft 2008). Electrochemical disinfection of wastewater has been proven effective for the treatment of water with high or low concentrations of chloride ions (Kim et al. 2013; Huang et al. 2016). For the disinfection of drinking water and other waters with much lower chloride content, even at very low chloride concentrations sufficient chlorine to efficiently disinfect the water can be produced (Huang et al. 2016).

There are several advantages of inline electrolytic disinfectant production: there is no need to transport, store or dose the disinfectants. Inline electrolytic disinfectant production is cost effective and requires less maintenance compared to other disinfection methods. Photovoltaic power supply also allows for the operation of these plants far from the electricity supply grid (Cho et al. 2014). The application of solar energy in the disinfection may be quite useful in developing countries like India, where scarcity in power supply is one of the most important hurdles for the treatment of wastewater (CPCB 2015).

The present study was conducted on the campus of Indira Gandhi National Tribal University, Amarkantak, MP, which is located almost 20 km from the holy town of Amarkantak at 22°.80′N, 81° 75′E. A subsurface horizontal flow gravity driven gravel filter bed with a surface area of 35 m² has been built to treat the effluent from an existing sewage treatment plant. For the treatment of domestic effluent, the effluent was diverted from an existing 200 KLD sewage treatment plant of the university campus. After passing through the gravel the effluent was pumped through a zeolite filter and disinfected by inline electrolytic-produced chlorine (Table 1). The quantity of water passing through the gravel bed is governed by the anodic oxidation (AO) disinfection system having a treatment capacity of up to 10 m³/d. The process is meant to remove microbial contamination from the secondary treated sewage, but additionally has the capacity to further remove organic load. Photos of the plant's construction are given below in Figures 2–4.

The construction of the 3.5 × 10 m large gravel bed started in June 2014 (Figures 2–4). As the treated wastewater passed through the gravel bed by gravity, it had to be placed below the bottom of the inlet ditch. Due to the elevated landscape, this required the removal of a 2 m deep topsoil layer. Only parts of that work could be done by machine. Most excavation work was done by locals, with a three-month break during the monsoon. The gravel of 26 mm to 32 mm was washed, filled into the bed and then leveled. The sketch of the system is given below (Figure 5). The gravel bed was flooded and planted with locally grown plants of Typha and Comelina Bengalensis (Figure 5).

The solar energy driven inline electrolytic chlorine production unit was developed by AUTARCON GmbH, a small to medium enterprise from Kassel, Germany, and was named SuMeWa|SYSTEM® (from Sun Meets Water). For this study the unit had been installed and adjusted for the disinfection of the wastewater. This system was fitted into a small room behind the gravel bed. A 300 Wp solar PV system was installed to provide power supply to the unit. A solar driven pump was installed into the second chamber (SP2) receiving treated water from the gravel bed. The gravel bed-treated water was pumped through a zeolite filter for turbidity removal and an inline-electrolysis cell where it was disinfected. Finally, treated water was stored in the 500 L water storage tank in which oxidation reduction potential (ORP) sensors for water quality monitoring were installed (Figure 5(a) and 5(b)). The system was equipped with a remote online monitoring system, allowing for the remote sensing of system performance and control of water quality over the internet.

The different stages of the disinfection system are shown in Figure 4. The disinfection system (SuMeWa|SYSTEM) was kept in a small room (6 × 4 × 3 feet) constructed for this purpose. The system has following components:

• 1.

  Solar driven DC pump

• 2.

  Automatic backwashable zeolite filtration cylinder of 27 L capacity

• 3.

  Electrolytic cell (dimension stable titanium electrodes, coated mixed oxides ‘MOX-electrodes’)

• 4.

  Storage tank with level sensors and ORP sensors for water quality control

• 5.

  Flow meter

• 6.

  Control unit equipped with online monitoring system controlling and running all components on solar energy

• 7.

  Solar photovoltaic modules (300 Wp) and solar batteries for back up

• 8.

  Control unit equipped with online monitoring system controlling and running all components on solar energy

Sampling and analysis of water and wastewater was done as per the standard methods prescribed by the American Public Health Association (APHA 2006). The periodic sampling and analysis of raw sewage, partially treated sewage – being the inlet into the gravel bed (SP1), effluent after gravel bed filtration (SP2), treated water generated after zeolite filtration (SP3), and finally disinfected water after electrolysis (SP4) was done. The sampling points are shown in Figure 4. Samples were collected in triplicate and essential parameters (temperature, pH, dissolved oxygen (DO), conductivity) were analyzed on the spot immediately after sampling. The reaming parameters were analyzed in the laboratory.

Physico-chemical examination of raw and treated sewage was done in order asses the quality of sewage produced in IGNTU (Table 2).

The average pH value of the raw sewage was 7.9 and that of secondary treated sewage was 8.3. The average temperature value for raw sewage and secondary treated sewage was 29.0 °C and 29.8 °C respectively. The conductivity ranged between 1,405–2,250 μS/cm for raw sewage and 700–1,420 μS/cm for secondary treated sewage. The chloride value for raw and secondary treated sewage ranged between 56 and 115 mg/l and between 40 and 71 mg/l respectively. The BOD₅ value for raw sewage was in the range of 150 to 349 mg/l and for secondary treated sewage in the range of 30 to 80 mg/l. The chemical oxygen demand (COD) value range for raw sewage and treated sewage was 209–450 mg/l and 70–190 mg/l respectively. The values of treated sewage (inlet into the gravel filter bed) showed that most of the parameters (BOD₅, COD, and nutrients) were above the permissible limits set by Central Pollution Control Board of India. This was mainly due to a lack of continuous power to the existing sewage treatment plant, which many times had to release its effluent without secondary treatment. In this case the gravel bed is a viable option to constantly reduce effluent concentrations.

During the current study, the treatment of primary – in case of power failure at the STP – or secondary treated sewage was done through a CW made of gravel, followed by zeolite filtration and the electrochemical AO disinfection unit. The result of the analysis of the effluent from four different sampling points (Figures 3 and 4) revealed a significant improvement concerning the analyzed parameters (Table 3 and Figures 6–9).

The temperature at the initial stage of treatment (inlet chamber or the first sampling point) was 21.7 °C and at final stage – i.e. after AO treatment – the temperature was 22.9 °C. The pH value at the initial stage of treatment was 8.2 and increased to 8.3 after final treatment. The DO at the initial stage of treatment was 3 mg/l and increased to 11 mg/l after final treatment. The oversaturation can be explained by the production of oxygen as by product of the electrolysis process. The average acidity value at the initial stage of treatment was 90 mg/l and reduced to 6 mg/l after final treatment. The average alkalinity value at the initial stage of treatment and after final treatment was 66 mg/l and 172 mg/l respectively. The decrease in acidity and increase in alcalinitiy may be explained by the water passing through the freshly broken gravel. Hardness value (carbonate) at the initial stage of treatment was 106 mg/l and after final treatment it was 34 mg/l. The average oxidation reduction potential (ORP) value between initial stage and final stage of treatment increased from +210 mV to +431 mV, which is mainly caused by the chlorine produced.

The average conductivity value of the initial stage was 1,312 μs/cm and after final treatment the average conductivity decreased to 732 μs/cm. The BOD₅ for the initial stage was 48 mg/l and BOD₅ value for final stage decreased to 10 mg/l. The BOD₅ value decreases and it meets the norms prescribed by CPCB. The water quality improves along the treatment train, making the water fit for reuse. Total chlorine value of the different water samples increased in the electrolytic chamber from ∼0 mg/l to 1.1 mg/l in finally treated water. The nitrate and phosphate values also decreased during different stages of treatment. The average value of nitrate in the inlet of the wetland was 12 mg/l, which indicated the nature of sewage as moderate. The wetland worked excellently in removing nitrate, reducing it by almost 84%, along with 77% of phosphate. The values for nitrate removal are similar to the values described by some other workers (Vymazal 2007). In the case of phosphate removal, it is on the higher side compared to some other works (Vymazal 2004). The gravel bed was able to remove 90% of the total coliform of the wastewater and the water quality improves along the treatment train, making the water fit for reuse.

The wetland treated water was collected in a settling chamber of approximately 1,000 liters capacity. The water from this chamber was pumped through the zeolite filter and then through the electrolytic cell where the chlorine was produced by means of inline-electrolysis (Equations (1)–(4)). Chlorine values and simultaneous values of the some bacteria (total coliform, fecal coliform, E. coli, Enterococcus, Clostridium, and Salmonella) were measured to assess the efficiency of the present system for disinfection and results are given in Table 4.

In order to assess the disinfection capacity of the installed system, selected biological parameters were analyzed in the present study. In this context, the Most Probable Number (MPN)/100 ml of Total and fecal coliforms, E. coli, Enterococcus, Clostridium, and Salmonella, were counted (Table 4). The average MPN value for Total coliforms at the initial stage was 1.6 × 106/100 ml and after final stage treatment the MPN value was <20. The average MPN value for fecal coliforms at the initial stage was 30,000/100 ml and after final stage treatment the MPN value was <20. The average MPN/100 ml value for E. coli at the initial stage was 29,000/100 ml and the same after final stage treatment was <20. During the different stages of treatment, the MPN values of Clostridium, Salmonella, and Enterococcus were also removed below the detection limit. After the final treatment, i.e. after electrolytic chlorination, all of the above-mentioned bacteria were found to be either absent or below the detection limit. Thus the studied treatment system has proven to be very efficient in the removal of bacteriological parameters (Figure 9).

The finally treated water was found to be disinfected to levels which allow a safe reuse of the water in process water applications. Parameters like BOD₅, COD, TDS, acidity, and alkalinity were found to be suitable for any uses, such as irrigation, soil enrichment, aquaculture services, cleaning purposes, and process applications. The proposed solution has proven to be feasible for wastewater treatment in remote regions, especially when power availability is a limiting source as it is often the case in rural India. The implementation of the system offers a very promising technique for municipal wastewater treatment without any energy input, requiring minimal operation and maintenance. Such systems may suit small towns and communities, where high-rate system may not be implemented, but land is easily available. Due to improper design, poor maintenance, frequent electricity break-downs and lack of technical man power, the facilities constructed to treat wastewater do not function properly and often remain closed. In contrast, the treatment train presented here can be a feasible option for the treatment of domestic sewage in India. The applied ORP sensors and a SCADA online monitoring system, which broadcasted operational data such as ORP, pump and electrolytic cell conditions, volume of treated water and water level in the storage tank, is of added benefit, providing the option for remote control of the station, and allowing the initiation of preventive measures in case of malfunction.

The study was part of the international collaborative research project ‘SWINGS’ financially supported under Department of Science and Technology (DST), Govt. Of India and the European Union (EU) Collaborative Program within FP7 Framework for Research, Technological Development and Demonstration under grant agreement no. 308502.