Treatment of cooling tower blowdown water by using adsorption-electrocatalytic oxidation: Technical performance, toxicity assessment and economic evaluation

Highlights

• An eco-friendly process was adopted to treat cooling tower blowdown water.

• PANI/TiO₂ exhibits excellent performance in the removal of organics and P.

• Electrocatalytic oxidation is an effective method for desorbed eluate.

• TELI method was applied to assess the toxicity of effluent and oxidized eluate.

• A reasonable operation cost of the treatment loop was estimated.

Abstract

An eco-friendly process was adopted to treat cooling tower blowdown water (CTBD) and the toxicity of correspondingly produced water/eluate was evaluated using the transcriptional effect level index (TELI) based on toxicogenomics. The objective of the work is to provide a feasible treatment loop including adsorption to remove organics and phosphorus from CTBD, electrocatalytic oxidation to improve the biodegradability of the eluate after desorption. Results showed that PANI/TiO₂ was a promising adsorbent in the removal of organics and phosphorus from CTBD and exhibited a satisfied regeneration ability beyond 30 times of reuse. During the electrocatalytic oxidation process the biodegradability of desorption eluate was gradually increasing and BOD₅/COD of the oxidized eluate reached 0.4 after 4.8 h of treatment, indicating that the treated wastewater could be returned to the biological treatment loop for further processing. The analysis of the quantitative toxicogenomics assay revealed that the toxicity of CTBD was mainly caused by oxidizing biocides of trichloroisocyanuric acid (TCCA), leading to a significant membrane stress response of bacteria. And the toxicity level of CTBD decreased after adsorption treatment while the desorption eluate experienced increase and then decrease during the electrocatalytic oxidation, meaning that certain oxidation duration was needed to keep the eluate safe for biological treatment. According to economic analysis, the operation cost of treatment loop was estimated at around 0.6 dollars/m³, ensuring high reuse water quality and safe eluate for further biological treatment.

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.