Pilot-scale hybrid constructed wetlands for the treatment of cooling tower water prior to its desalination and reuse

Gita Putri,Muhamad Mutaqin,Vinnie De Wilde,Bert Willemsen,Julia Opdam,Huub H.M Rijnaarts,Pim De Voogt,John R Parsons,Thomas V Wagner,Alette A.M Langenhoff

doi:10.1016/j.jenvman.2020.110972

Abstract

Cooling towers are responsible for a large part of the industrial fresh water withdrawal, and the reuse of cooling tower water (CTW) effluents can strongly lower industrial fresh water footprints. CTW requires desalination prior to being reused, but various CTW components, such as total organic carbon (TOC), conditioning chemicals and total suspended solids (TSS) hamper physico-chemical desalination technologies and need to be removed from the CTW. A cost-efficient and robust pre-treatment is thus required, which can be provided by constructed wetlands (CWs). The present study is the first study that determined the CTW pre-treatment efficiency of hybrid-CWs and the impact of winter season and biocides in the CTW on the pre-treatment efficiency. The most efficient CW flow type and dominant removal mechanisms for CW components hampering physico-chemical desalination were determined. Subsurface flow CWs removed PO43−, TSS and TOC as a result of adsorption and filtration. Vertical subsurface flow CWs (VSSF-CW) excelled in the removal of benzotriazole as a result of aerobic biodegradation. Horizontal subsurface flow CWs (HSSF-CW) allowed the denitrification of NO3− due to their anaerobic conditions. Open water CWs (OW-CWs) did not contribute to the removal of components that hamper physico-chemical desalination technologies, but do provide water storage options and habitat. The biological removal processes in the different CW flow types were negatively impacted by the winter season, but were not impacted by concentrations of the biocides glutaraldehyde and DBNPA that are relevant in practice.For optimal pre-treatment, a hybrid-CW, consisting of an initial VSSF-CW followed by an OW-CW and HSSF-CW is recommended. Future research should focus on integrating the hybrid-CW with a desalination technology, e.g. reverse osmosis, electrodialysis or capacitive ionization, to produce water that meets the requirements for use as cooling water and allow the reuse of CTW in the cooling tower itself.

Highlights

Human intervention in water catchments has severely aggravated downstream fresh water scarcity over the last 40 years (Veldkamp et al, 2017) and especially delta areas around the globe experience significant fresh water stress (Yao et al, 2015; Mekonnen and Hoekstra, 2016)
We investigated the potential of different separate constructed wetlands (CWs) removal mechanisms for the removal of cooling tower water (CTW) conditioning chemicals (Wagner et al, 2020a, 2020b; 2020c)
The effluents of the all the compartments of the hybrid-CWs were analysed for electric conductivity (EC), PO43À, NO3À, NO2À, total organic carbon (TOC), total suspended solids (TSS), TDS, turbidity, benzoic acid and benzotriazole because these are critical CTW components that could hamper physico-chemical desalination and re-use of CTW in the cooling tower

Summary

Introduction

Human intervention in water catchments has severely aggravated downstream fresh water scarcity over the last 40 years (Veldkamp et al, 2017) and especially delta areas around the globe experience significant fresh water stress (Yao et al, 2015; Mekonnen and Hoekstra, 2016). After the cooling tower water (CTW) reaches a mineral concentration threshold, the CTW is generally discharged to the surface water without further treatment, and new fresh ‘make-up’ water is taken in. The treatment and reuse of CTW in the cooling tower could prevent water scarcity and unwanted environmental problems and lower the fresh water footprint of cooling towers (Pan et al, 2018). To reuse the CTW, it needs to comply with various water quality requirements, such as limited concentrations of PO43À , total organic carbon (TOC) and total suspended solids (TSS) (Groot et al, 2015; Pan et al, 2018). Con structed wetlands (CWs) could be a suitable nature-based pre-treatment option for CTW (Wagner et al, 2018)

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