Treatment of laundry wastewater containing residual surfactants using combined approaches based on ozone, catalyst and cavitation

Highlights

• Study of treatment of laundry wastewaters containing surfactants.

• Combined approaches based on ozone, catalyst and cavitation evaluated.

• Catalytic ozonation is the best treatment approach giving almost 90% reduction.

• Biodegradability of the solution also increases favorably.

Abstract

The present work investigates the treatment of wastewater by degrading detergents and residual surfactants using individual and combined approaches based on ozone, catalyst and cavitation. The intensification of ozonation has been investigated by the addition of solid ZnO and CuO catalysts. The extent of degradation increased with catalytic ozonation as compared to non-catalytic ozonation. In comparison, the combination with cavitation did not yield any beneficial results. The study of the effect of initial pH of the solution and effect of different catalyst loadings in catalytic ozonation with ZnO established that the maximum degradation as 89.3% was obtained at initial pH of 9 and 0.1 g L⁻¹ of catalyst loading. It was also demonstrated that the reaction followed first order kinetics with rate constant as 0.067 min⁻¹. BOD analysis at optimized conditions demonstrated that catalytic ozonation efficiently converts non-biodegradable harmful surfactants in wastewater to easily digestible compounds, which means that this can be a complimentary process to biological oxidation. Overall the work has clearly demonstrated the efficacy of catalytic ozonation approach for treating the laundry wastewater.

Introduction

In the recent years, the study of treatment of domestic wastewater has been getting considerable attention, including developing approaches for the decentralized treatment methods, especially in developing countries [1], [2], [3]. A relatively large portion of domestic wastewater is the effluent from washing machines. Washing machines can typically produce from 50 to 200 L of effluent per wash [4]. A large contributing factor to the pollution due to the washing machine effluent is the detergent, composed mainly of surfactants like linear alkylbenzne sulfonates (LAS or LABS), other long carbon chain backbones like dodecyl sulfates and sulfonates, olefin sulfonates, alkylamides, polyoxyethylene ether sulfates, and buffers like sodium tripolyphosphate [5]. The reported values for concentrations of LAS in domestic effluents range from 3 to 21 mg L⁻¹ [6]. The presence of surfactants in water can cause damage to the ecosystem thereby affecting the environment. Consumption of surfactants above 0.5 mg L⁻¹ can be harmful to health, justifying that the treatment of surfactants from water is essential [7] before any recycle options are explored.

Biological treatment methods have been extensively studied and considered to be the most feasible for the treatment of surfactants [8], [9], [10], [11], [12], [13]. However, concerns over surfactant treatment grew globally with development of recalcitrant and non-biodegradable surfactants with higher activity, toxic metabolites, and partially degraded surfactants being found in wastewaters [14], [15]. Over the last decade, various treatment methods have been developed for remediation of anionic surfactants in wastewater such as coagulation and flocculation [16], [17], wet air oxidation [18], [19], membrane separation [17], [20], ion exchange [21], [22], ozonation [23], photocatalytic degradation [24], [25], Fenton oxidation [26], [27], [28], and other advanced oxidation processes (AOPs) [29], [30], [31], [32], [33], [34], [35], [36]. The possibility of complete mineralization of pollutants with the production of zero sludge makes AOPs an attractive alternative to conventional technologies. Advanced oxidation processes involve the use of ozone, H₂O₂, UV, TiO₂ photocatalysis, Fenton, and persulfate, individually or in combination, for the production and consequent attack of the strongly oxidizing hydroxyl (^•OH) radicals for degradation and mineralization of compounds [37]. Previous studies on ozonation of wastewater containing surfactants have reported COD reduction percentages ranging from 25 to 60% [23], [38], [39]. Ultrasonic irradiation has also been previously reported to degrade the surfactant, sodium dodecylbenzene sulfonate with a COD reduction of 20% [40]. Studies have shown that the use of catalysts during ozonation can further enhance the degradation of recalcitrant chemicals in wastewater [41], [42], [43] though not many studies deal with surfactants. Various metal oxides with different surface characteristics like porosity, surface area, and pH behavior have been reported to enhance the reactivity of ozone in conventional ozonation systems [44].

Within the domain of advanced oxidation processes, an emerging technology, cavitation has been getting considerable attention for degradation of emerging and recalcitrant compounds. Ultrasonic or Hydrodynamic cavitation, in synergy with advanced oxidation processes, have been reported to be effective in the degradation of various organic compounds [41], [45], [46], [47], [48], [49], [50]. Recent studies have reported effective degradation of industrial wastewaters and provided comprehensive evaluations on the synergy between various advanced oxidation processes (O₃, catalytic ozonation, H₂O₂, Peroxone (O₃ + H₂O₂)) and cavitation (acoustic and hydrodynamic) [51], [52], [53], [54]. Cavitation involves the formation and subsequent collapse of vapor-filled cavities using ultrasonic waves or hydrodynamic constrictions, creating local hotspots that drive chemical mineralization [55]. The formation of oxidizing radicals during cavitation synergistically supplements the hydroxyl radical formation and attack in advanced oxidation processes, often leading to higher degradation.

To our knowledge, combined processes based on ozone and cavitation have never been reported for surfactant treatment or laundry wastewater treatment. The present work investigates the use of ozone, ultrasound and hydrodynamic cavitation, individually and in combination for degradation of detergent and residual surfactants from synthetic and actual wastewater. Further, the effect of solid catalyst (ZnO and CuO) has been studied in the case of ozonation. The objective of this work is to demonstrate a high COD and BOD reduction by developing an effective individual or combined process as well as by optimizing the pH and catalyst loading. The novelty is established based on the first application exploring catalytic ozonation and cavitation (different forms) for treatment of surfactant containing wastewaters.