Treatment of cooling tower blowdown water by using adsorption-electrocatalytic oxidation: Technical performance, toxicity assessment and economic evaluation
Graphical abstract
Introduction
Annually, huge amounts of freshwater globally are used for cooling tower systems [1], [2]. For China, the industrial circulating cooling water consumption is up to 100 billion cubic meters, accounting for 60–80% of the total industrial water consumption [3]. A great deal of chemicals, such as scale inhibitors, corrosion inhibitors, biocides etc., are added into the cooling water to maintain the system chemically and biologically stable [4]. Due to evaporation, salts are condensed and need to be discharged regularly [5], [6]. In this regard large volumes (10–20% of the consumed water) of the cooling tower blowdown water (CTBD) rich in chemicals (dozens to hundreds mg/L), phosphorus (several mg/L) and other organics (dozens mg/L) need to be discharged and handled, which otherwise would bring severely negative impact on the accepting water bodies or wastewater treatment plants (WWTPs).
In the CTBD water, the contaminants are mainly synthetic chemicals and a mixture of solvable microbiological products (SMP) and natural organic matter (NOM), contributing to COD, nutrients (e.g. phosphorus) and leading to toxicity. Regarding corrosion and scale inhibitors, the most widely used agents in most countries (e.g. China) are still phosphorus series [7], [8], such as nitrilotrimethylene triphosphonic acid (ATMP, C3H12NO9P3), hexanediamine tetramethylene phosphonic acid (HDTMP, C10H28N2O12P4), etc. which bring organics and nutrients to the water body when incorrectly discharged and would lead to severe contamination and eutrophication [9], [10]. Besides, anti-bacteria reagents such as trichloroisocyanuric acid (TCCA, C3N3O3Cl3) are commonly used in cooling water as oxidizing biocide. Some WWTPs accepting discharged CTBD containing biocides often complained collapse of the microbiological system for wastewater treatment and thus severely dropped treatment efficiency. Some studies showed that the decomposed products of biocides (e.g. TCCA) are readily to react with organics in water and form toxic disinfection by-products (DBPs) which would increase the risks of cytotoxicity, neurotoxicity and genotoxicity [11], [12], [13] due to the use of such contaminated water. In general, although the aforementioned chemicals exhibit good corrosion, scale and bactericidal inhibition efficiency, they are usually environmentally unfriendly and lead to toxicity for organisms and the ecosystem. Therefore, effective wastewater treatment technology is urgently needed to remove COD and nutrients and reduce the toxicity.
At present, the common technologies for CTBD treatment include flocculation, membrane filtration, etc., but the problems of high investment cost, poor stability and secondary pollution have to be considered [14], [15]. From both economy and effectiveness perspectives, adsorption is considered as a promising technology for simultaneous removal of organics and phosphorus [16], [17], [18]. The thus treated wastewater could be either discharged or reused. Among candidate adsorbents polyaniline-modified TiO2 (PANI/TiO2) is a novel composite adsorbent designed by Wang et al., which showed outstanding performance in the removal of dyes, phosphorus and effluent organic matters from treated wastewater [19], [20]. In addition, the adsorbent exhibited excellent regeneration ability [21]. Considering its high efficiency and regeneration ability, PANI/TiO2 was selected in the present work for the treatment of CTBD.
CTBD treated by adsorption would produce desorption eluate which is rich in contaminants [19]. COD of the eluate could usually exceed 10,000 mg/L accompanied by high ion concentration (to the level of g/L). Till present, treatment of eluate has not got enough attention in previous studies while the efficacy of the adsorption itself was always the focus. Among advanced treatment technologies handling such heavily contaminated water electrocatalytic oxidation is one of the most suitable processes for its high efficiency in removing recalcitrant pollutants and the property of easy to use [22], [23], [24], [25], [26]. Usually, Ti/PbO2 is considered as a promising anode material and has been successfully used in electrocatalytic oxidation process due to its strong oxidation ability, low cost and easy for preparation [27], [28], [29]. From economic perspective the refractory eluate does not need to be fully mineralized, but to achieve certain level of biodegradability and then discharged to biological WWTPs [30], [31]. The integration of adsorption, electrocatalytic oxidation could thus form a thorough treatment loop [19].
In such a loop it is necessary to assess the toxicity of the liquid formed in each step, to understand its possible impact on human healthy and the bacteria in accepting water bodies or WWTPs. Among a number of toxicity test methods, transcriptional effect level index (TELI) is a novel, feasible and cost-effective quantitative toxicogenomics-based toxicity assessment method [32], [33]. It could quantify toxicity and simultaneously analyze toxic mechanisms within 2–4 h. It is important to note that TELI is a quantitative evaluation criteria for toxicity level, which exhibited a dose-response relationship and allowed for linking the transcriptional level effects to conventional toxicity endpoints [32]. At present, this method has been successfully applied in some fields for toxicity analysis [34], [35], [36]. Toxicity assessment is important for risk evaluation but has usually been neglected by studies focusing mainly on the treatment efficiency as a sole consideration.
In this work, the combination of treatment processes including adsorption of CTBD and electrocatalytic oxidation of thus formed eluate, were presented to deal with such kind of wastewater. The performance of PANI/TiO2 on the removal of organics and phosphorus was evaluated, the effectiveness of electrocatalytic oxidation on the treatment of desorption eluate regarding COD decrement and biodegradability improvement was proved. TELI method was applied to assess the toxicity of the wastewater quantitatively and understand the toxic mechanisms regarding the activation/deactivation of certain genes after certain treatment processes. In addition, we also evaluated the economic cost of the whole treatment process. The purpose of the present work is to provide an efficient, economical and eco-friendly treatment process for CTBD.
Section snippets
Illustration of the adsorption-desorption and electrocatalytic oxidation treatment loop
The flowchart (Fig. 1) shows the specific processes of the overall CTBD-related treatment: firstly, PANI-TiO2 was used to remove organic pollutants and phosphorus from CTBD. The high-quality water with low COD and P contents was obtained by filtration to separate the solid adsorbents from it. Secondly, the used PANI-TiO2 was desorbed and activated by using acidic-alkaline. Finally, the desorbed and concentrated eluate was electrocatalyticaly oxidized to achieve COD removal and biodegradability
Performance of PANI/TiO2 for CTBD treatment
Fig. S2 shows the performance of PANI/TiO2 in removing COD and TP from CTBD at different dosages and reaction times. With the increment of adsorbent dosage, the removal of both COD and TP increased quickly till to the dosage of 1.5 g/L, and above which the improvement became minor. Under the dosage of 1.5 g/L, the removal of COD and TP reached 58 and 90% within 60 min, respectively. The results indicated that a dosage of 1.5 g/L adsorbent could be reasonable for the treatment of CTBD and more
Conclusions
In this work, a combined process of adsorption-electrocatalysis was used to treat cooling tower blowdown water (CTBD) and delivered an eco-friendly treatment loop. Meanwhile, the water toxicity during the whole treatment process was evaluated by TELI method which was a new detection technology at genes level. The specific conclusions are as follows:
- (1)
PANI/TiO2 is an effective adsorbent for the treatment of CTBD, which could remove the COD and TP by around 55 and 90%, respectively. And after 30
CRediT authorship contribution statement
Xiaoliang Li: Conceptualization, Methodology, Investigation, Writing - original draft. Linjie Wu: Validation, Investigation, Methodology, Data curation, Visualization. Sijia Lu: Validation, Formal analysis, Investigation. Heyun Yang: Resources, Supervision, Data curation. Wenzhou Xie: Resources, Supervision. Huiyan Zhao: Resources, Supervision. Yaozhong Zhang: Methodology, Investigation. Xin Cao: Formal analysis, Investigation. Gang Tang: Formal analysis, Writing - review & editing. Hesheng Li:
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.
Acknowledgments
The present work was supported by National Natural Science Foundation of China (No. 51878555, 51738012), Shaanxi International Cooperation Foundation (No. 2017KW-041) from Shaanxi Provincial Science and Technology Department, Natural Science Basic Research Plan in Shaanxi Province of China (No. 2019JQ-735), which are highly appreciated.
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