Evaluation of Innovative Filter Materials and Operating Conditions in Vertical Flow Wetlands for Greywater Treatment

Evaluation of Innovative Filter Materials and Operating Conditions in Vertical Flow Wetlands for Greywater Treatment M. E. Karpuzcu*, C. Kahraman*, A. T. Cetinkaya* and F. A. Maviengin* * Department of Environmental Engineering, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey (E-mail: [email protected]; [email protected]; [email protected]; [email protected]) Abstract The reuse of greywater has benefits such as both the protection of water resources and the reduction of the economic burden of water supply and wastewater treatment. Greywater must be treated in accordance with legislative requirements for reuse. Natural treatment systems such as constructed wetlands are increasingly being used for this purpose. This study investigated greywater treatment performances of vertical flow (VF) constructed wetland systems with different filter materials under different operating conditions. Perlite and gravel reactors exhibited similar performance in COD removal when the reactors were operated without a rest phase, while perlite reactor performed slightly better when there was a distinctive flood and rest phase. Reactor operating conditions significantly impacted TKN removal efficiencies. It was hypothesized that operating the VF wetland system with a fully saturated flood phase followed by a rest phase created a hybrid system effect where anoxic and aerobic zones develop in turn conducive to nitrification and subsequent denitrification reactions. The higher removal efficiency observed in the perlite reactor was attributed to the better oxygen transfer capacity of perlite due to its porous structure. The innovative filter material perlite exhibited superior performance compared to gravel and proved to be effective as a promising alternative material in VF wetland reactors. Keywords Circular economy; greywater reuse; nature-based solutions; perlite; sustainability, VF wetlands INTRODUCTION With the rapid population growth and the rise of industrialization, the intensification of water use stands out as the main reason for the concept of global water scarcity. The intensification of water use increases water demand and places greater pressure on water resources. This situation will cause the current water scarcity to turn into a “water crisis” in the coming years. A substantial part of the world still does not have access to clean drinking water. Unless a precaution is taken, this problem will inevitably reach a more critical level in the future (UN Water, 2021). In order to prevent this problem, the concepts of sustainability and circular economy should be integrated into water resources management. Wastewater reuse is an important step towards this integration. Ever since the reuse of wastewater has gained attention, domestic wastewater has been categorized as less concentrated greywater which originates from showers, washing basins, laundry, dishwashers etc., and concentrated black water from toilets (Patil et al., 2022; van Voorthuizen et al., 2008). Although greywater is categorized as wastewater with a low pollution load, it must be treated in accordance with legislative requirements for reuse. The reuse of greywater has benefits such as both the protection of water resources and the reduction of hydraulic load to the wastewater treatment plants, reducing the economic burden of water supply and wastewater treatment. Nature-based solutions such as constructed wetlands stand out as sustainable options for greywater treatment. In this study, the performance of vertical flow wetlands with different filter materials and operating conditions were investigated for greywater treatment. The substitution of conventional gravel and sand with other materials such as zeolite (Du et al., 2020, Muniz Sacco et al. 2024), lava sand (Morandi et al., 2021) and biochar (Brunhoferova et al., 2022; El Barkaoui et al., 2023, Muniz Sacco et al. 2024) have recently been studied to increase the efficiency of pollutant removal in vertical flow (VF) wetlands. Perlite, a naturally occurring volcanic glass, is widely used in industries such as construction, horticulture, and filtration. Perlite has been found to possess excellent adsorption properties, making it a promising material for the removal of pollutants such as heavy metals and dyes from industrial wastewater (Khoshraftar et al., 2023). With its lightweight and porous nature, perlite is a good candidate to be used as a filter material in VF wetlands, however, its treatment performance has not been well documented. This study aims to shed light on the performance of perlite as an innovative filter material in VF wetlands. We hypothesize that the properties of filter materials and the reactor operating conditions have a significant impact on greywater treatment performance of VF wetlands. METHODS Greywater samples were taken from the Istanbul Technical University main dining hall dishwasher effluent. Characterization studies of raw greywater were carried out measuring the following parameters: Chemical Oxygen Demand (COD), Total Kjeldahl Nitrogen (TKN), Ammonia (NH3), Nitrate (NO3), Phosphate (PO4), pH and Dissolved Oxygen (DO). NO3 and PO4 measurements were carried out by Ion Chromatography (IC). All other aforementioned parameters were determined using Standard Methods (APHA, 2017). The VF wetland reactors consisted of 30x40x60 cm plastic containers (Figure 1). In one of the reactors, perlite was used as filter material, while the other was set up using gravel as commonly employed in other studies. Particle sizes for the gravel reactor were as follows: pea gravel 6-8 mm, gravel 8-16 mm and coarse gravel 32-42 mm. The reactors were planted with Anubias barteri. In order to evaluate the effect of operating conditions, the reactors were operated in two different modes. The first operation mode did not have a distinct rest mode and was called “non-rest mode”. The reactors were fed with greywater for 1.5 hours until they get saturated. The feed mode was operated with a hydraulic loading rate (HLR) of 24 mm d−1, a specific pulse volume (SPV) of 150 L m−2, and a specific hydraulic loading rate (SHLR) of 1.67 L m−2 min−1. Following the feed mode, the reactors were drained at the same rate. Once the reactors were completely empty, the second feed mode started immediately. This cycle was repeated four times. Samples were taken at the end of the feed mode (fully saturated state) and towards the end of the drain mode (unsaturated state) to determine removal kinetics. The second operation mode differed from the first one in that it had a distinctive flood and rest mode. Following the 1.5-hour feed mode, the reactors entered a 1-hour flood mode where the valves were kept closed and the greywater stayed inside the reactors under fully saturated state to foster anoxic conditions. After that, the reactors were drained and remained in the rest mode for 3 hours. After the rest period, the same cycle was repeated. Samples were taken at the end of the feed mode, at the end of the flood mode (fully saturated state), and towards the end of the drain mode. Figure 1. Vertical flow wetland reactors (left: perlite reactor; right: gravel reactor) RESULTS Average influent and effluent concentrations in the VF wetland reactors under different operating conditions are shown in Table 1. Both reactors exhibited similar COD removal efficiencies (perlite reactor 72% and the gravel reactor 71%) when they were operated without a rest phase (non-rest mode). Under rest conditions, the perlite reactor achieved a COD removal efficiency of 62%, while the gravel reactor demonstrated a slightly lower removal efficiency of 55%. PO4 concentrations rapidly decreased and remained under 1 mg/L for both reactors under rest conditions mostly due to the adsorption to the filter media. Under non-rest conditions, influent PO4 concentrations fed to the reactor happened to be below 1 mg/L and no further reduction was observed. Table 1. Average influent and effluent concentrations in the VF wetland reactors under different operating conditions Perlite Reactor Gravel Reactor Rest Non-Rest Rest Non-Rest Influent (mg/L) Effluent (mg/L) Influent (mg/L) Effluent (mg/L) Influent (mg/L) Effluent (mg/L) Influent (mg/L) Effluent (mg/L) COD 537 203 814 226 537 239 814 235 TKN 7.03 0.64 5.3 3.91 7.03 1.3 5.30 2.90 NH3 5.11 0.85 1.75 0.38 5.11 nd 1.75 nd NO3 1.95 0.08 1.58 1.53 1.95 nd 1.58 1.55 PO4 3.96 0.77 0.32 0.36 3.96 0.81 0.32 0.32 The operation mode had a significant impact on TKN removal efficiencies. TKN removal efficiencies under non-rest mode remained low, determined as 26% and 45% for perlite and gravel reactors, respectively. When the reactors were operated with a distinctive rest mode, TKN removal efficiencies of 91% and 81% were achieved in perlite and gravel reactors, respectively. During the rest period, NH4-N concentrations decreased accompanied by an increase in NO3-N concentrations. During the flood period when the system remained saturated, NO3-N concentrations decreased and TKN removal was observed. These results suggest that simultaneous nitrification and denitrification occurred in the system when the reactors were operated with distinctive flood and rest modes. It has been shown that the feeding mode has a great influence on the oxygen transfer to the VF wetland systems and operating the system with the filter media partially or fully saturated causes anoxic zones to develop (Bassani et al., 2021). Thus, it can be said that operating the reactors with a full saturation feeding mode followed by a flood phase and the rest phase created a hybrid system effect where aerobic and anoxic zones develop in turn conducive to simultaneous nitrification and denitrification. The higher removal efficiency observed in the perlite reactor was attributed to the better oxygen transfer capacity of perlite due to its porous structure. CONCLUSION In this study, the effect of operating conditions and filter materials in VF wetlands on greywater treatment performance was investigated. Both perlite and gravel reactors exhibited similar performance in COD and PO4 removal when the reactors were operated without a rest phase, while perlite reactor performed slightly better when there was a distinctive flood and rest phase. Reactor operating conditions significantly impacted TKN removal efficiencies. The presence of a distinctive rest and flood phase helped create aerobic and anoxic conditions conducive to nitrification and subsequent denitrification reactions within the same reactor. The innovative filter material perlite exhibited superior performance compared to gravel and proved to be effective as a promising alternative material in VF wetland reactors. Waste perlite from industrial sources could potentially be used, further contributing to the circular economy. 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