Recirculation or artificial aeration in vertical flow constructed wetlands: A comparative study for treating high load wastewater
Graphical abstract
Intermittently recirculated and aerated VSSF constructed wetland.
Introduction
In the application of horizontal subsurface flow constructed wetlands (HSSF) or vertical subsurface flow constructed wetlands (VSSF) in the treatment of domestic or industrial wastewater, the land area requirement is the main constraint in some contexts, especially in mountainous areas. The possibility of applying high loads in constructed wetlands, especially to treat peaks in some periods (due to seasonal industries or tourism), would allow for the reduction of the surface area requirement, thus allowing the constructed wetlands to be more widely used in cases with limited space availability (Foladori et al., 2012). However, further research is needed to understand the performances of constructed wetlands under high applied loads with the aim of reducing surface areas and their footprints. Configurations that may include recirculation of wastewater or artificial aeration are currently being tested in research (inter alia Arias et al., 2005, Lavrova and Koumanova, 2010, Hu et al., 2012, Zhu et al., 2013) in order to increase the applied loads or to enhance the treatment performances, especially regarding nitrogen removal.
Recirculation has been proposed in hybrid systems (HSSF + VSSF) to improve nitrogen removal performances, by pumping the nitrified VSSF effluent to a previous HSSF stage aimed at performing the removal of organic matter and supporting denitrification (Tunçsiper, 2009, Ayaz et al., 2012). In other studies, recirculation has been applied in VSSF systems treating pre-settled wastewater, by recirculating a fraction of the nitrified VSSF effluent to the septic tank where denitrification occurs due to biodegradable COD availability (Brix and Johansen, 2004, Brix and Arias, 2005, Önorm B 2505, 2008). Recirculation has been used on a single stage of a classical French VSSF to improve nitrification efficiency and to increase the removal of organics and suspended solids (Prost-Boucle and Molle, 2012). In a 4-stage tidal flow reed bed system (similar to fill-and-drain VSSFs), the final effluent was recirculated to the first stage to treat high strength wastewater (Zhao et al., 2004). A recirculating vertical flow constructed wetland, called RVFCW, has been developed for single houses (Gross et al., 2007, Sklarz et al., 2009), where the wastewater trickles through the bed into a reservoir and then is recirculated back to the top of the bed, with the aim of prolonging the contact time in the system and of enhancing oxygen diffusion during the trickling of wastewater through the bed.
Artificial aeration has been employed in many studies to enhance the removal performances in HSSF systems, which suffer from limited oxygen diffusion (Ouellet-Plamondon et al., 2006, Nivala et al., 2007, Maltais-Landry et al., 2009a, Maltais-Landry et al., 2009b, Zhang et al., 2010, Butterworth et al., 2013, Fan et al., 2013a).
Conversely, very little research focuses on the application of artificial aeration in fill-and-drain VSSF systems because, in these systems, a natural and spontaneous permeation of air is permitted by the sequence of filling and draining (Green et al., 1998). However, when high loads are applied, the amount of oxygen supplied by natural aeration could be insufficient to ensure both the oxidation of organic matter and nitrification due to the high levels of oxygen demand (inter alia Vymazal, 2007, Hu et al., 2012, Fan et al., 2013b). In this case, artificial aeration could enhance the oxygen availability in the VSSF systems (Dong et al., 2012, Tang et al., 2008, Ong et al., 2010, Pan et al., 2012, Fan et al., 2013b). In particular, intermittent aeration has been recently proven to be an effective way of improving not only nitrification but also total nitrogen removal, due to the alternation of aerobic and anaerobic phases (Jia et al., 2010, Fan et al., 2013b) and the formation of microaerobic/anaerobic zones (Dong et al., 2012). Moreover, intermittent aeration could save operational costs in respect to continuous aeration (Liu et al., 2013).
Despite this recent increased interest in recirculated or aerated VSSF systems, this specific topic has never been completely investigated and most of the applications remain at the lab-scale using synthetic wastewater.
In this research, VSSF configurations equipped either with recirculation and/or artificial aeration were investigated and compared at pilot-scale in the treatment of real domestic wastewater at high loads (for both organics and nitrogen) in an attempt to reduce the specific surface area per population equivalent (PE). A saturated layer was formed on the bottom of the VSSF with the intention of prolonging the hydraulic retention time of the wastewater, as the efficiency of pollution removal in constructed wetlands depends greatly upon retention time (Toet et al., 2005). There has been no reported research in the literature comparing recirculated and/or aerated VSSF systems (with a saturated layer on the bottom) for the treatment of real wastewater at high loads.
The recirculated and/or aerated VSSF systems were compared with a conventional down-flow VSSF (designed following local guidelines), monitored in parallel and then used as a control. Local guidelines indicate effluent concentrations of 125 mgCOD/L, 35 mgTSS/L, whilst requirements are not so strict in regard to nitrification and total N removal (nitrification efficiency of 70% or higher and removal percentage of total N around 70% are suitable). These limits can be commonly obtained in a hybrid configuration (VSSF + HSSF), where HSSF has a polishing role.
Section snippets
VSSF pilot plant
The outdoor VSSF pilot plant, located in the Alps (Ranzo, Province of Trento, Italy) at 739 m a.s.l., treated real domestic wastewater. The scheme of the pilot plant has been described previously (Foladori et al., 2012). The raw wastewater passed through a mechanical grid and an Imhoff tank before entering the pilot plant consisting of two parallel lines (Fig. 1):
- (1)
A conventional down-flow VSSF (surface area: 2.25 m2; depth: 0.8 m) designed with a specific surface area of 3.6 m2/PE in compliance with
Comparison of overall performances
Table 2 shows the average values of the influent and effluent wastewater for the recirculated and/or aerated VSSF configurations compared to the control C-VSSF. The concentrations of total COD, sCOD and TSS effluent from the R-VSSF, A-VSSF and AR-VSSF systems were consistently lower than those of the C-VSSF (Table 2). In the recirculated and/or aerated VSSF systems, wastewater was retained in the bed for a longer period during the treatment phase, thus increasing the contact time between
Conclusion
The effectiveness of recirculated and/or aerated VSSF systems operated under high loadings was demonstrated by the increase of the removed loads of COD, TKN and TN and the reduction of the surface area requirement to 1.5 m2/PE. Efficient simultaneous nitrification and denitrification occurred in the intermittently aerated and recirculated VSSF, due to the better mixing in the bed which accelerated oxygen diffusion and enhanced contact between COD and NO3. The higher costs for electromechanical
Acknowledgement
The Authors wish to thank the staff of ADEP for their cooperation in the building and installation of the pilot-plant at Ranzo (TN).
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