Effects of algal ponds on vertical flow constructed wetlands under different sewage application techniques
Introduction
Constructed wetlands (CWs) are economic and environmental friendly self-adaptive water treatment systems (Vymazal, 2007, Kadlec, 1997). Vertical subsurface flow constructed wetlands (VSSF-CWs) have been applied to treat wastewater and due to their advantages, such as high space utilization efficiency (Brix and Arias, 2005), simple operation (Cui et al., 2012), low construction and maintenance expenses (Stefanakis and Tsihrintzis, 2012, Bruch et al., 2011), VSSF-CWs have been implemented successfully worldwide (Meng et al., 2014, Prochaska et al., 2007). However, CWs showed the relatively low removal efficiency for wastewater contaminants, especially for the contaminants such as nitrogen and phosphates (Brix et al., 2001, Vymazal et al., 1998).
Nitrification and denitrification processes are common nitrogen removal pathways in CWs (Vymazal, 2005). Typical long-term nitrogen removal efficiency in CWs ranges from 35% to 50% (Tang et al., 2009, Verhoeven and Meuleman, 1999). It is difficult to improve nitrogen removal efficiency by CWs from wastewater due to the limited dissolved oxygen (DO) content (Ye et al., 2012a, Nivala et al., 2013) and lack of microbial carbon source (Moussavi et al., 2015, Ding et al., 2012, Ding et al., 2013). Multiple phosphorus removal mechanisms are involved in CWs, such as plant uptake (Greenway and Woolley, 1999), filtering effect of substrates (Vohla et al., 2011), microbial immobilization (Kantawanichkul et al., 2009, Reddy et al., 1999), root bed media retention (Tanner et al., 1998) and precipitation with cations (Mg2+ and Ca2+) in water (Vymazal, 2007, Diaz et al., 1994). Adsorption and biochemical reactions in substrate are the major phosphorus removal processes (Vymazal, 2010) and phosphate removal efficiency is typically less than 50% in most CWs (Verhoeven and Meuleman, 1999). Sorption and precipitation in CWs are saturated and sorption capacity may gradually decrease (Ádám et al., 2006).
Algae can increase DO in high rate algal pond (HRAP) via photosynthesis and has been applied in low-maintenance sewage purification system. Ammonia and phosphate in wastewater may be nutrient sources for algae (Ma et al., 2014). The combination of stabilization ponds and CWs can improve the treatment efficiency of CWs (Powell et al., 2008, Badhe et al., 2014). Algae can not only improve nitrogen removal, but also act as a good biological sink for phosphorus in HRAP (Babu, 2011). However, some harmful substances such as nitrite and sulfides are discharged during algae metabolism process. These harmful substances can be removed by CWs. The nitrification-denitrification process in CWs is an efficient way to remove nitrite (Lin et al., 2002). Sulfides can be removed in CWs by biodegradation (Higgins et al., 2006). The potential role of dead algae or algal debris as potential microbial carbon source in CWs combined with algal pond has not been well investigated yet.
In this study, we compared the water treatment efficiencies of CWs with/without algal ponds. In this study, we investigated the optimal conditions of CWs combined with algal pond systems with different sewage-filling ways and analyzed the changes of DO concentration and microbial carbon source. Adding an algal pond before CWs may be advantageous to sewage treatment.
Section snippets
Construction of laboratory-scale units
Three laboratory-scale vertical subsurface flow CWs were built with the inner size of 60 cm (length) × 45 cm (width) × 35 cm (height). The CWs were filled with quartz sand as filter medium and planted with Canna (Fig. 1). In each CW, two PVC (5 and 25 cm high) tubes were arranged for water sampling. CWs were filled with two layers of substrates with different diameter ranges. The ranges of substrate diameter in the upper layer and bottom layer were 3–5 mm and 10–15 mm, respectively. The two layers were 15
Nitrogen and phosphorus
During the 15-week experimental period, the mean temperature ranged from 2.2 to 9.5 °C. In Stage 1, pH in System A increased from 7 to 10.15 and pH in System B increased from 7 to 10.26 (Fig. 2). The pH was significantly influenced by sewage-filling way and temperature (Table 2). In Stage 2, the mean pH in System A and System B respectively decreased to 6.6 and 6.8, and pH was significantly influenced by sewage-filling way (P < 0.05).
DO in effluent was high during 15-week monitoring. In Stage 1,
Discussion
This study clearly indicated that CWs combined with algal pond had a high wastewater treatment capacity. CWs are efficient in removing biodegradable organic matters as well as nitrogen and phosphorous in tropical regions (Kantawanichkul et al., 2009). The present study confirmed that the new CWs combined with algal pond had the improved performance in removing nitrogen, phosphorous and organic matters at the low temperature (0–10 °C).
Conclusions
The CWs combined with algal pond showed the enhanced contaminant removal from wastewater at the low temperature. Owing to oxygen enrichment and DO increase by algae via photosynthesis, the better performance in contaminant removal was observed. The dead algae and algal debris might be utilized as microbial carbon source to improve the removal efficiency of nitrogen in CWs. Without adding extra carbon source, we could save both cost and energy. The designed wastewater treatment device was mainly
Acknowledgement
This study was supported by the National Natural Science Foundation of China (Grant Nos. 51279207, 51309503, and 51409267).
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