French Type Constructed Wetlands for Sewage Treatment: Experiences from the SWINGS Prototype in India

This study aimed to investigate the performance of the French Type Constructed Wetlands (FTCW) at a pilot scale level in real field conditions for sewage treatment in India. The pilot plant consisted of hybrid wetlands in two stages, vertical and horizontal sub-surface flow types. The first stage comprised three compartments, vegetated with three different native plants (phragmites australis, canna indica, and sagittaria), operated in parallel, and sequentially loaded. Each VSSF bed of 12.25 m 2 was fed with raw sewage directly for 3.5 days followed by twice the time resting period. The second stage consisted of one bed of an area of 45m 2 , planted with canna indica. The filter media used in stage 1 was a dual type (gravel and coarse sand) in three layers. The first stage was designed for the loadings of 33.75g BOD/m 2 d and 14.1 gNH4-N/m 2 d with the flexibility to operate at a variable hydraulic loading rate (HLR). Two years of monitoring data after the steady-state condition show variable performances at different hydraulic loading rates of 0.4m/d to 0.97 m/d. Removal efficiencies of COD, BOD 5 , TSS, TN, and TP of the hybrid system at HLR of 0.4m/day were 87.1%, 87.3%, 84.6%, 67.93%, and 69.32% respectively. Doubling the HLR, the efficiency of the system decreased marginally for the same parameters. Out of the three vegetation, the bed planted with phragmites has shown the highest removal efficiency. The study has demonstrated that FTCW can be a suitable proposition as one of the potential methods for sewage treatment in India and similar regions.


Introduction
In Indian sub-tropical climatic conditions, naturebased solutions (NBS) like constructed wetlands technology for sewage treatment are also gaining popularity over the other methods. 1,2,3or small flows, constructed wetlands (CWs) have been a proven, cost-effective, and easy-toimplement method with low O&M costs compared to conventional high-rate treatment systems. 4,5One of the most promising configurations of the CWs is the French Type wetland system for wastewater treatment. 6In contrast to the conventional CWs, the FTCW can treat raw wastewater directly without any primary treatment and is easy to deal with sludge management. 7The FTCW can be either a single-stage or a two-stage hybrid system.In the latter case, the first stage involves a vertical subsurface flow (VSSF) bed receiving raw wastewater directly without any primary treatment whereas the second stage is a horizontal sub-surface flow (HSSF) bed, both filled with media. 8 countries having sub-tropical climatic conditions as prevailing in the major part of northern India, and in similar regions of its sub-continent, wetlands have an advantage over the colder regions.8 The growth of macrophytes and enhanced microbial activities are the main factors due to which CWs offer higher removal efficiencies in these regions.7,14 Literature suggests that FTCWs have been widely adopted for wastewater treatment.9 In the previous studies, and from the results of the full-scale installations that are in operation, the HLR are ranging from 0.37 m/d to 0.45m/d.1,10 The organic loading rates are generally kept in between 150 and 300 g CODm -2 •d- 1 and whereas TKN ranges in between 25 and 30 g.m -2 •d -111, 13 .Another important factor in the good design practices of the FTCW is the selection of the macrophytes.12 In most of the cases, as has been reported in the literature, phragmites australis is the most common specie used in the FTCW.12 In this study, hybrid FTCW was designed at a pilot scale level in the two stages (vertical flow and horizontal flow) with three different plants (phragmites australis, canna indica, and sagittaria) with the objective to investigate its performance in the removal of the pollutants from the sewage in a real field under sub-tropical climatic conditions. Te secondary objective was to investigate the impact of different species on the performances under varying hydraulic loading conditions.

Materials and Methods
The study was carried out at the Aligarh Muslim University (AMU), Aligarh in India  The basic structure of the pilot set-up was constructed in reinforced concrete.The construction and study on this pilot were one of the objectives of the Indo-European SWINGS Research Project within the FP7 Framework program.The pilot set-up consisted of a screen chamber, grit channel, holding tank, and hybrid wetlands in two stages: a vertical subsurface flow with 3 beds in parallel followed by a horizontal sub-surface flow wetland (Figure 2).
The two stages of the French CW system have a net area of around 82 m 2 (36.75 m 2 for the first stage and 45 m 2 for the second stage) designed to be operated at a variable hydraulic loading rate (0.4m/d to 0.97 m/d).The raw sewage was made available from the adjoining and existing sewage treatment plant (STP) of the university campus where this pilot study was conducted.
The raw sewage was tapped from the inlet chamber of the STP to the holding tank of the pilot set-up.
Table 1 shows the dimensions and type of filter media used in the construction of VF and HFCW units and the design parameter details are given in Table 2.The pilot plant was operated and monitored for a period of 2 years for three variable HLRs of 0.4, 0.81, and 0.97m/day at a pulse loading of a single bed.Locally available native plants were used in the study.Each compartment of stage 1 was planted with different species.These were Canna indica, Phragmites australis,and Sagittaria.The wastewater was distributed uniformly over the top of the bed surface through perforated pipes perpendicular to the water flow 10 cm below the surface.Drainage pipes were installed at the bottom of the beds perpendicular to the flow at the other side of the filter.
The purpose of HSSF CW was to polish the treated wastewater from VF beds on a continuous basis. 15his single bed of stage two in this study (HSSF CW) was vegetated with Canna indica.
After the steady-state condition, the monitoring of the plant was carried out over a period of two years.
The study was started in the year 2017 and completed in 2019.The system took about 4-5 months to reach the steady-state condition after the vegetation was planted in the beds.

Results and Discussions
The mean concentrations of the wastewater contaminants are presented in Table 3.
For the domestic wastewater available, the mean of the fluctuations of the inlet concentrations in all three beds over its cyclic loading was observed i n t e r m s o f C O D ( 1 2 5 .5 1 ± 3 1 .9 m g / l ) , BOD 5 (68.58 ± 17.1 mg/l), and TSS (43.48 ± 9 mg/l).The overall efficiency of the French-type CWs system was observed as 87.1%, 87.3%, and 84.6% for COD, BOD 5 , and TSS, respectively.The use of variable filter media such as gravel and sand for the vertical flow CWs also affected the treatment process by enhancing the growth of microorganisms and the development of the sludge layer on top of the beds. 16The removal efficiencies of nutrients like TN and TP were also assessed.The treatment efficiency measured for TN and TP is 67.93% and 69.32% respectively (Fig. 3 (d)).These efficiencies were observed to be comparable to those reported for domestic wastewater, 13 which can be most likely due to the favourable growth conditions for macrophytes and microorganisms in a climatic condition, as was prevailing in the study region.the overall efficiencies of the system. 10However, a significant change was not observed in the removal efficiencies when the HLR varied from 0.81m/day to 0.97m/day.It was observed that there was a decrease in the removal efficiencies of the pollutants with the increase in influent flow rates 10,20,21 (Fig. 3).The removal rates observed for the overall system for COD, BOD  During the commissioning phase, low efficiencies were observed.This can be attributed to less deposition of the sludge layer on top of the filter beds thereby reducing the interaction between the macrophytes and micro-organisms. 11,16,17However, once the steady condition was reached during the course of this study, it was observed that the performance of the FTCW improved with time and this can be attributed to the deposition of the sludge layer on top of the vertical flow beds and its role in the removal process. 11,14,17The study is also in concurrence with the previous studies that mineralization of the sludge layer increases the capacity of the treatment system and the effect of low influent flow increases the contact time between wastewater and microorganisms. 11,17The decrease in second stage efficiency was observed due to the better removal efficiency by the first stage VF beds. 18,20

Conclusions
Two years of monitoring data of the pilot plant based on the hybrid French-type constructed wetlands configuration in real field conditions under the Indian sub-tropical climatic conditions has shown average removal efficiencies (overall) for the parameters COD, BOD 5 , TSS, TN, and TP of 87.1%, 87.3%, 84.6%, 67.93%, and 69.32% respectively.Out of the three VSSF beds with a different plantation, the bed with sp.Phragmites australis has shown the highest removal efficiency at an HLR of 0.4m/d followed by sp.Canna indica and sp.sagittaria.The efficiencies gradually reduced when the HLR was increased from 0.4m/d to 0.81m/day and further to 0.97m/day.This shows that by lowering the hydraulic retention time, the efficiency of the stage 1 system decreases, and ultimately it has an impact on the overall water quality.Once the steady state condition is reached, the quality of the treated effluent after the second stage of treatment was found within the permissible Indian limits for disposal in the surface water.
The findings showed that the French Type CW in a hybrid configuration can be a very promising way to deal with sewage in India or in regions having similar climatic conditions.
. The city (27 o 88'N, 78 o 08'E) is situated in the northern part of India and about 130 km south west of the capital city, Delhi (Figure 1).The climate is sub-tropical with high temperatures (27 o C to 46 o C) in summer and humid and cold (10 o C to 1 o C)during the winter season.The rainy season also lies in between summers and winters with an average annual rainfall of about 590 mm.

Fig. 1 :
Fig. 1: Maps showing the location of the state of Uttar Pradesh and the Aligarh district.(Source:Uttar Pradesh district factbook)

Fig. 2 :Fig. 3 :
Fig. 2: Flow Scheme of the French Type CW Pilot Plant at AMU Aligarh, India