Elsevier

Chemosphere

Volume 286, Part 2, January 2022, 131742
Chemosphere

Enhanced wastewater nutrients removal in vertical subsurface flow constructed wetland: Effect of biochar addition and tidal flow operation

https://doi.org/10.1016/j.chemosphere.2021.131742Get rights and content

Highlights

  • Tidal flow operation with effluent re-circulation improved DO load remarkably.

  • The tidal flow with biochar addition improved COD (96.08%) and NH4+-N (88.16%) removals.

  • Improved oxidation and adsorption were the main pathways for nutrient removals.

Abstract

Dissolved oxygen (DO) and carbon stock in substrate medium play a vital role in the nutrient removal mechanism in a constructed wetland (CW). This study compiles the results of dynamics of DO, ammonium N (NH4+-N), nitrate (NO3–N), sulfate (SO4−2), phosphate (PO4−3), chemical oxygen demand (COD), in three setups of vertical-flow constructed wetlands (TFCWs) (SB: substrate + biochar; SBP: substrate + biochar + Colocasia esculenta plantation; SP: substrate + Colocasia esculenta (SP), operated with tidal flow cycles. Experimental analyses illustrated the continuous high DO level (2.743–5.66 mg L−1) in SB and SBP after the I and II cycle of tidal flow (72 h flooding and 24 h dry phase). COD reduction efficiencies increased from 15.75 – 61.86% to 48.55–96.80% after tidal operation among operating TFCWs. N (NH4+-N) and N (NO3–N) removal were found to be 88.16%, and 76.02%; 49.32, and 57.85%; and 40.23%, and 48.94 % in SBP, SP and SB, respectively. The theory of improved nitrification and adsorption through biochar amended substratum was proposed for TFCW systems. PO4−3 and SO4−2 removal improved from 22.63 to 80.50%, and 19.69 to 75.20%, respectively after first tidal operation in all TFCWs. The microbial inhabitation on porous biochar could promote the transformation of available P into microbial biomass and also helped by the plant uptake process while SO4−2 reduction in TFCWs could be mainly due to sulfate-reducing bacterial activity and nitrate reduction process, mainly facilitated by high DO and biochar addition in such setups. The study suggests that effluent re-circulation through tidal operation and biochar supplementation in the substratum could be an effective mechanism for the improvement of the working efficiencies of CWs operated with low energy input systems.

Introduction

Effluent management before the discharge needs specific attention as the excessive release of nitrogen (N) into the water bodies leads to eutrophication and degradation of freshwater resource quality (Zhou et al., 2017; Lai et al., 2020). Water quality maintenance has become a serious matter throughout the globe due to “the exponential increase in human population, industrial growth and technological advancement of societies”. In general, owing to the advantages with low construction cost, minimum operational and maintenance cost, and is an eco-friendly technique, constructed wetlands (CWs) are being used extensively for onsite treatments of domestic wastewaters (Tan et al., 2017; Lai et al., 2020). However, the poor N removal in conventional CWs is still an issue, which is limited by the insufficient oxygen supply in such systems especially during nitrification processes in the substratum (Jia et al., 2010). Saturated conditions in CWs and quick microbial degradation (under significant input load) decrease oxygen accessibility in conventional CWs. To conquer such downsides, tidal flow constructed wetlands have been introduced to increase the availability of oxygen in CWs. Among various sorts of CWs, TFCWs are in consideration, because of their easy operational concept (fill and draw), which are separated into a few successive stages: fill, contact, drain and rest periods (Chang et al., 2014; Saeed et al., 2019). The whole process provides living space for both oxygen consuming and anaerobic organic entities. TFCW infers repeated fill and draws of wastewater load as a result of which media pores are filled by air through atmospheric diffusion which allows the development of aerobic conditions inside the reactor bed. The repeated cycle ensures alternating aerobic and anaerobic conditions which led to nitrification and denitrification simultaneously (Shan et al., 2016; Zhang et al., 2021).

Apart from that, substrates used in CWs is one of the factors on which removal efficiency in recent past, the tailoring of substratum has been investigated by various previous researchers to improve the wastewater treatment efficiency of CWs (Yang et al., 2018). Previous literature suggests that phosphorous removal is also restrained in the conventional wetlands because the substrate used such as gravels, sand, rock and pebbles could only achieve limited removal efficacy (Brix et al., 2001; Jia et al., 2010; Yang et al., 2018). Therefore, selection of the substrate becomes an important factor for the removal of pollutants, as the variety of microorganisms growing in the substrate layer, which is responsible for the removal of a pollutant not just relies upon the activity conditions such as tidal, aerobic, anaerobic, etc. but also their growth depends on the characteristics of utilized substrates (Hussain et al., 2019). A couple of studies employed natural and waste materials as fundamental media in TFCWs which reported promising performance of TFCWs (Liu et al., 2014; Tan et al., 2019; Zhao et al., 2011).

To overcome such substrate problems in CWs, biochar as an amendment in the substrate has been widely utilized due to its large surface area, strong cation exchange capacity and high porosity enhances the removal of pollutants and reduces the emission of greenhouse gas (GHG) (Dalahmeh et al., 2012; Zhou et al., 2019). Previous literature indicates that dosing of carbon in CWs not only improves the microbial colonization and their functioning but also enhances the overall removal efficiencies by providing extra absorption surface for charged chemical substances from wastewater (De Rozari et al., 2016; Kizito et al., 2017; Yadav et al., 2018). The role of different operating conditions in CWs in the removal of major wastewater parameters is summarized in Table 1. Gagnon et al. (2010) found that the addition of external carbon sources in the CW system could improve nitrogen removal efficiency by 95%. Further, adding to that an investigation made by Saeed et al. (2019) in TFCWs, as comparative evaluation of different substrate material, organic (coal, biochar, and coco-peat), waste (slag), and constructional (concrete block, gravel) in which they revealed promising results for nitrogen and organic matter with the organic substrate (71–85% and 84–96%) in comparison to the constructional substrate (49–69% and 74–95%), respectively in TFCWs. A similar type of study by Li et al. (2019) investigated four systems with and without biochar-based (TFCW and with re-circulation) in which they reported the best performance in TFCW with biochar with efficacy of 98.30% for the NH4+-N removal. A study by Feng et al. (2020), revealed the improvement in nitrogen removal efficiency in a constructed wetland while treating the anaerobically-digested swine wastewater in a batch setup. Their study found that the addition of biochar improved the total nitrogen removal by 30.92–40.12% compared to setups without biochar. Also, this study indicates that N2O emission fluxes could be reduced after biochar application in such treatment setups. In another study by Jia et al. (2020) reported high-rate NO3–N and heavy metal reduction in groundwater treated with a constructed wetland amended with iron-carbon-based (Fe–C) substrate in the bottom medium. They found about 87% nitrate reduction in such systems with low nitrous oxide (N2O) emission rates during experimentations. Also, heavy metals total Cr and total Pb reduced by 19–43% in Fe–C spiked CWs. Zhong et al. (2021) revealed that biochar amendment in CWs could be a sustainable approach to enhance the nitrogen removal efficiency of such systems. In their study, they found that biochar addition effectively improved the removal of total N, and NO3–N in wastewater by 59.88–93.03%, and 83.14–100%, respectively, possibly due to improved microclimatic conditions for microorganisms in the substrate and synergetic impact by biochar on heterotrophic and autotrophic denitrifiers in CWs. Bolton et al. (2019) reported the enhanced removal of phosphorous in sewage water treated in a CW amended with biochar. They narrated that adsorption could be the possible mechanism of P-removal from wastewater during the treatment process. Analysis of used biochar revealed that P in biochar increased by 77% over the experimental period. The sink of P on biochar could be a source of P for the rooted plant in CWs. Some studies indicate that biochar in combination with zeolites showed better results of wastewater contaminates removal. For example, Guo et al. (2021) in their study found that the addition of biochar-zeolite in CW significantly increases the arsenic removal by 53.56% in arsenic-containing biogas slurry of a pig farm. Apart from that zeolite-biochar amended CWs showed high removal of NH4+-N, TP and COD, showing scaling impact of adsorptive properties of biochar on contaminates removal rates. A similar study by Abedi and Moriji (2019), investigated the effect of biochar-zeolite on removal efficiency of CWs and their results showed about 99.9% removal of COD, NH4–N, phenol, Pb, and Mn from wastewater. The N2O emission rate was also reduced in CWs amended with biochar. Apart from improvement in oxygen supply in a CW, amendment of biochar could be a sustainable approach to enhance the working capacity of such systems to treat domestic wastewater. Ji et al. (2020) in a study found that biochar amended CW with improved aeration could significantly improve the working capacity of a CW and also helps in the reduction of GHGs from CWs during the treatment process. Biochar enhanced the microbial community biodiversity and richness in the substratum.

In the majority of the previous studies on utilities of biochar in CWs, researchers used biochar made from hardwood or plant residues (see Table 1) which seems an inappropriate approach in terms of fresh resource utilization and energy budget. Studies found that animal manure could be a valuable feedstock for quality biochar preparation, which could serve two purposes: safe disposal of cattle dung and low-cost substitute of wood for biochar preparation under energy- and resource-saving approaches (Sharma and Suthar, 2020; Yao et al., 2021). Recent studies indicate the successful application of manure/dung-based biochar in different areas of environmental management such as soil quality improvements, wastewater treatments and removal of industrial chemicals from aqueous mediums (Cao and Harris, 2010; Yao et al., 2021; Lee et al., 2021). Apart from amorphous carbonized structures, cow dung biochar also contains several nutrients like Ca, Na, K, other elements, etc. which could be a valuable source of nutrients for microbial communities and plant biomass in CWs (Chand et al., 2021a, Chand et al., 2021b). Preparing biochar from cattle dung could serve several purposes: low energy required for pyrolysis process; effective use of cattle dung waste; enriched biochar with macronutrients would serve as a source of nutrients for microbes and plant stand in CWs; cost-effective process for low-cost CW designs.

To be best of the author's knowledge, animal dung based biochar is hardly used and tried by any previous worker in TFCWs. Although, a recent work by Chand et al., 2021a, Chand et al., 2021b revealed the efficacy of cow dung-based biochar in vertical flow sub-surface CWs (VSSFCW) in treating domestic wastewater. Their results suggested the high fixed carbon in dung-based biochar and VSSFCW amended with cow dung biochar showed 92.6, 81.7, 81.2, 85.4, 69.5, and 97% removal of COD, NO3–N, NH4+-N, SO4−2, PO4−3, total coliforms, respectively from domestic wastewater. Therefore; the aim of this present study accomplished in the mesocosm vertical flow constructed (VFCW) planted with Colocasia esculenta and filter bed packed with biochar with tidal flow operation was conducted to explore the wastewater treatment having three successive cycles of the cycle of 72 h hydraulic retention time (HRT) each under ambient conditions combination (Chand et al., 2021a,b) The objectives of this study were: 1) to investigate the role of Colocasia-based TFCWs for the wastewater treatment, 2) to see the role of biochar and its relevance in pollutant removal in TFCWs, and 3) to examine the combined effect of the plant stand, biochar and tidal flow operation in pollutant removal. The obtained results are to have a better understanding of biochar dose in wetlands and the impact of tidal flow operation wastewater treatment.

Three independent identical down-flow mesoscales (VFCWs) reactor columns were designed, which were operated as SB, SBP, and SP. All the setups were kept in the outdoors of Doon University, Dehradun, India from May to August with a monsoon climate. The VFCW setups were established in a plastic container of 30 L capacity (ø 33.5 cm and 37 cm height) with the same configuration and four different substrate layers having a depth of 40 cm. The influent wastewater pumped to the reactor column passing through all the layers were collected at the bottom. The reactor bed was layered with gravels (ø = 20–21 mm) to a depth of 5 cm, above it, a coarse gravel (ø = 8–12 mm) layer of 10 cm, a layer of fine gravels (ø = 4–6 mm) of 10 cm, a topmost layer (rhizome layer) of a homogenized mixture of biochar (0.25 ~ 1 mm particle size) and river bottom sand of 15 cm The schematic diagram of VFCWs is described in Fig. 1.

VFCW reactor columns were operated under tidal flow conditions with effluent re-circulation, having flood and drain system rhythmically which were operated in 3 d cycle, after each cycle the reactor was giving a dry phase of 24 h for the enrichment of the oxygen in the constructed wetlands through atmospheric diffusion into the porous bed and to ensure re-entering of sufficient air to replenished the substratum with oxygen after effluent draining (Zhi and Ji, 2014). Three cycles were performed for each batch study of 9 d to get a significant amount of data to see the average removal performance of VFCWs at different hydraulic periods with fill and draw system (72, 144 and 216 h) during each 9 d cycle. In each setup, the wastewater characteristics were measured at 72, 144 and 216 h during each 9 d cycle. After each cycle, the drain of the first cycle was fed as an influent load in VFCWs. All setups were operated with tidal flow conditions to provide an abundance of oxygen through atmospheric diffusion (natural diffusion) for an alternate cycle of aerobic and anaerobic conditions in the CWs for rapid removal of pollutants (Liu et al., 2016; Li et al., 2019). The wastewater was filled in each experimental setup in morning hours (7:00–8:00 a.m.) and water was filled continuously for 30–45 min until all wastewater creates a flood condition (up to 7 cm headspace) in setup without overflow. The retention time was counted after filling of complete wastewater in VFCWs.

For each cycle, influent and effluent water samples (150 mL) were collected at 8:00 AM to analyse the transformation of nitrogen and other organic and inorganic wastewater pollutants in VFCWs after each 72 h HRT in the studied systems. To get more reliable results the samples from the outlet were collected in three batches with 15 min gap and then taken for the analyses. The characteristics of domestic wastewater (COD, NH4+-N, NO3–N, PO4−3, and TN) were estimated using standard methods as described in (APHA, 2005), apart from that, dissolved oxygen (DO) was determined using DO meter (HQ40d, Hach, USA) and pH values of the samples were determined by pH Meter. All standards and solutions were prepared in AR grade chemicals with 99% purity mostly provided by Merck (Germany).

Section snippets

DO levels in TFCWs

The profile of DO levels in different TFCWs reactors is shown in Fig. 2 The average DO concentration of the wastewater at 0 h was 6.16 mg L−1 and after loading the wastewater in SB, SBP, and SP setup; the DO values of each step up during the 72 h HRT of each cycle were observed as; the first cycle DO values of SB set up were 4.23, 2.30 and 1.88 mg L−1 (24, 48, and 72 h) respectively and after the dry phase of 24 h DO values in Cycle II; were 4.17, 2.22, and 1.01 mg L−1 (96, 120, and 144 h),

Conclusions

This study reveals the significance of the incorporation of biochar with conventional media, and the abundance of oxygen in the CWs due to tidal flow operation mode. The pollutants transformation processes are mainly influenced by operational strategy (TFCW), type biochar, microbial degradation, and the presence of plant stand. The quality of water in the SPB system was best, with maximum efficacy for COD, NH4 +-N, NO3–N, PO4, SO4−2 (96.80, 88.16, 57.85, 75.26 and 80.50%, respectively). This

Credit author statement

Naveen Chand: Data curation, Formal analysis, Methodology, Surindra Suthar: Conceptualization, Supervision, Validation, Writing – original draft, review and editing. Kapil Kumar: Conceptualization, Supervision, Review and Editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The first author is highly thankful to NIT Delhi for providing SRF for research work and also acknowledge the facilities provided to conduct the work smoothly. SS is grateful to Dr Teresa Cutright, editor for her kind editorial handling and four anonymous reviewers for fruitful suggestions to improve the manuscript.

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