Biochar enhances plant growth and nutrient removal in horizontal subsurface flow constructed wetlands
Graphical abstract
Introduction
Constructed treatment wetlands (CWs) are ecotechnological systems designed and built to treat various types of wastewater by imitating natural processes occurring in wetlands, marshes and bogs. CW technology is an economically and environmentally sustainable alternative to conventional treatment plants and is easy to use and operate (Vymazal, 2011). One of the common designs of this technology is the application of hybrid systems: vertical subsurface flow (VSSF) filters followed by horizontal subsurface flow (HSSF) filters. Quite often, these systems are planted either with Phragmites australis or Typha latifolia, whereas VSSF filters can also contain several other species of emergent macrophytes, such as Iris pseudacorus, Schoenoplectus spp. and Carex spp. (Kadlec and Wallace, 2009; Vymazal, 2013a). The pollutant removal in CWs is dependent on several biotic and abiotic components, such as bacteria, fungi, algae, macrophytes, the filtration capacity, the filter material type, precipitation, sedimentation and adsorption (Gupta et al., 2016; Liira et al., 2009; Vymazal, 2011). The main characteristics affecting pollutant removal efficiency in the planted filters are the hydraulic residence time, temperature, oxygen concentration, filter media and vegetation type and microbial activity (Truu et al., 2009). In CWs, organic matter is removed mostly via the activity of aerobic bacteria attached to the filter media and plant roots, and the main removal processes of nitrogen are mostly ammonification, volatilization, plant uptake and microbial nitrification and denitrification (Bachand and Horne, 2000; Shi et al., 2018). Nitrification, i.e., oxidation of ammonium to nitrate, occurs in the aerobic VSSF filters, and denitrification of nitrate, i.e., reduction of nitrate to N2O and N2, proceeds in water-saturated HSSF filters (Vymazal, 2013b), where organic carbon is used as an energy source. In hybrid filter systems where VSSF filters are followed by HSSF filters, the high efficiency of the organic carbon removal in VSSF can be a limiting factor for denitrification in HSSF filters. If VSSF filters have removed most of the organic compounds from water, then the nitrate removal by denitrification in the HSSF part will be suppressed (Shi et al., 2018). Addition of an external carbon source to the wastewater-saturated wetland systems has been shown to substantially increase (up to 95%) the total nitrogen (TN) removal in these systems (Gagnon et al., 2010). If hybrid systems are used in combination, where HSSF filters are followed by VSSF filters or French-type VSSF filters are used for wastewater treatment, then the organic carbon is usually not a limiting factor for denitrification (Yadav et al., 2018). Phosphorus removal is highly dependent on the filter material properties and mostly occurs via precipitation, sorption and sedimentation processes as well as plant uptake in CWs (Vohla et al., 2011).
Biochar is the carbon-rich product obtained when biomass, such as wood, is heated at high temperature in a closed chamber with little or no available oxygen (Zhou et al., 2017). In agriculture, biochar is highly valuable as an amendment for improving the quality of soils (Chan et al., 2007; Glaser et al., 2002; Lehmann et al., 2003; Lehmann et al., 2011; Xu et al., 2012). Studies have shown that biochar will boost soil fertility and improve soil quality by raising the pH, increasing the moisture holding capacity, attracting beneficial fungi and microbes and retaining nutrients in soil (Lehmann, 2007; Lehmann et al., 2006). Many other studies have also shown that biochar has the ability to sequester carbon from the atmosphere-biosphere pool and transfer it to the soil (Chan et al., 2007; Winsley, 2007; Xu et al., 2012). Due to its large surface area, negative surface charge, and charge density (Liang et al., 2006), biochar also has great sorption ability, including the ability to adsorb nutrients such as phosphate ions (Lehmann, 2007).
The effect of soil amendment with biochar on soil fertility and improvement of the quality of polluted soils (Muter et al., 2014; Song et al., 2016) has been studied intensively. However, limited attention has been paid to this material as a beneficial supplement to the widely used filter materials in wastewater treating CWs only during the last few years (Gupta et al., 2016; Mohan et al., 2014; Zhou et al., 2017). Most of these studies have been carried out to investigate the effect of biochar on nitrogen and phosphorus removal from wastewater in VSSF CWs (Gupta et al., 2016; Mohan et al., 2014; Zhou et al., 2017), but the planted HSSF systems have been almost neglected so far (Gupta et al., 2016).
Therefore, the main aim of this study was to assess the effect of biochar amendment on the plant growth and the purification efficiency of the municipal wastewater using horizontal subsurface flow filters planted with broadleaf cattail (Typha latifolia).
Section snippets
The experimental set-up
The experiment was conducted in a mesocosm-scale constructed wetland system located in Tartu County, Estonia during one vegetation period from the 13th of June 2016 to the 12th of October 2016. The municipal wastewater was pumped from the inflow of the Nõo activated sludge treatment plant into a septic tank (2 m3) and then from the interim well (IW) to the VSSF pretreatment filters with a total area of 6 m2. Pretreated wastewater flowed by gravity to HSSF filters. Six rectangular HSSF CW
Characteristics of the biochar
The SEM image of the initial biochar shows that the biochar was composed of irregular forms with very coarse and heterogeneous surfaces and shallow channels originating from tracheid and vessel cells of the original wood of various diameters (Fig. 2A). The average pore size calculated from the SEM image ranged up to 10 μm. The EDX analysis of the initial biochar showed that this material was composed of C (86.7%), O (12.7%), Ca (0.3%), K (0.2%) and Mg (0.1%) (Fig. 2B). In the SEM images of the
Discussion
In the current study, we used biochar as an amendment to the main filter material (LECA) in order to enhance the plant growth and nutrient removal efficiency of the HSSF filters. Studies have found that the addition of biochar to vertical flow systems (CWs and down-flow filter beds) can be an appropriate strategy to enhance the removal of organic pollutants, TN (Kizito et al., 2017; Zhou et al., 2018), and TP (Kizito et al., 2017) as well as to reduce the N2O emissions from these systems (Zhou
Conclusions
Our results show that the application of plants and biochar together with the traditional filter material can significantly enhance the treatment efficiency of HSSF filters. The current study clearly indicated that amendment of filter substrates with biochar could significantly increase the efficiency of cattail roots to take up nutrients, as was reflected in higher aboveground and belowground plant biomass. In future studies, biochar amendment could be accompanied by the addition of a more
Acknowledgements
This study was supported by the Ministry of Education and Science of Estonia grant no IUT13016, EU through the European Regional Development Fund (Centre of Excellence ECOLCHANGE) and Estonian Research Council grant no PUT1125. We thank Saale Truu and Laura Kasak for the assistance in computer graphics and Mae Uri and Holar Sepp for the laboratory analyses.
References (48)
- et al.
Nitrogen removal from sewage and septage in constructed wetland mesocosms using sand media amended with biochar
Ecol. Eng.
(2018) - et al.
Effects of nitrogen removal microbes and partial nitrification-denitrification in the integrated vertical-flow constructed wetland
Ecol. Eng.
(2016) - et al.
Alternative filter material removes phosphorus and mitigates greenhouse gas emission in horizontal subsurface flow filters for wastewater treatment
Ecol. Eng.
(2015) - et al.
Treatment of anaerobic digested effluent in biochar-packed vertical flow constructed wetland columns: role of media and tidal operation
Sci. Total Environ.
(2017) - et al.
Biochar effects on soil biota - a review
Soil Biol. Biochem.
(2011) - et al.
Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent - a critical review
Bioresour. Technol.
(2014) - et al.
The effects of woodchip- and straw-derived biochars on the persistence of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) in soils
Ecotoxicol. Environ. Saf.
(2014) - et al.
Bacterial carbon utilization in vertical subsurface flow constructed wetlands
Water Res.
(2008) - et al.
Microbial biomass, activity and community composition in constructed wetlands
Sci. Total Environ.
(2009) - et al.
Filter materials for phosphorus removal from wastewater in treatment wetlands-a review
Ecol. Eng.
(2011)