Elsevier

Science of The Total Environment

Volume 639, 15 October 2018, Pages 67-74
Science of The Total Environment

Biochar enhances plant growth and nutrient removal in horizontal subsurface flow constructed wetlands

https://doi.org/10.1016/j.scitotenv.2018.05.146Get rights and content

Highlights

  • Biochar enhanced nutrient uptake by plants roots from HSSF filters.

  • Biochar promoted plant biomass growth and nutrient removal from wastewater.

  • Biochar promoted pH reduction induced by plants in filter media.

Abstract

Biochar has shown great potential as an amendment to improve soil quality and promote plant growth, as well as to adsorb pollutants from water. However, information about the effect of biochar on the wastewater treatment efficiency in horizontal subsurface flow (HSSF) constructed wetlands (CWs) is still scarce. In this study, we assessed the effect of biochar amendment on the purification efficiency of pretreated municipal wastewater in planted (Typha latifolia) experimental horizontal subsurface flow filters filled with lightweight expanded clay aggregates (LECA). The addition of wood-derived biochar (10% v/v) to LECA significantly increased plant biomass production and enhanced the wastewater treatment efficiency of the planted filters. Both the aboveground plant biomass and belowground plant biomass were higher (1.9- and 1.5-fold, respectively) in the filters of the LBP (LECA + biochar + plants) treatments compared to the LP (LECA + plants) filters. The water pH was significantly lower in the planted filters (LBP < LP < LB-LECA + biochar). The efficiencies of TN and TP removal from wastewater were highest in the LBP filters (20.0% and 22.5%, respectively), followed by the LP (13.7% and 16.2%, respectively) and LB (9.5% and 15.6%, respectively) filters. More N and P were incorporated into the plant biomass from wastewater in the presence of biochar in the filter medium. The study results confirm that biochar can be an advantageous supplement for planted HSSF CWs to enhance the treatment efficiency of these systems.

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.

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