Coupling effect of ozone/ultrasound with coagulation for improving NOM and turbidity removal from surface water

Highlights

• Addition of the ultrasound/ozone (US/O₃) to the coagulation process improved the NOM and turbidity removal.

• The US/O₃-coagulation could efficiently transform the high molar mass NOM into smaller and more biodegradable compounds.

• The US/O₃-coagulation process did not increase the bromate concentration, and its level was below the WHO acceptable limit.

• No diagnosis of total coliform or Escherichia coli was observed in the samples treated by US/O₃-coagulation.

Abstract

This study investigates the effects of ultrasound–ozonation (US/O₃) pretreatment combined with coagulation on the natural organic matter (NOM) and turbidity removal from surface waters.

The influence of US/O₃ alone and US/O₃-coagulation on the removal of NOM and turbidity was measured by testing total organic carbon (TOC) fractionation and UV absorbance at 254 (UV₂₅₄) and using a turbidimeter. Aluminum sulfate (alum), ferric chloride (FC), and poly aluminum chloride (PACl) were used as coagulants. The disinfection efficiency of the process was studied using total coliforms and bromate concentration was analyzed to examine the formation of disinfection byproducts (DBPs).

The US/O₃ process led to 84%, 33%, and 46% removal of the NOM, turbidity and TOC, respectively. However, a combination of US/O₃ with coagulation increased the removal efficiency to 95% and 50% and 75%, respectively. The optimum working conditions of the US/O₃-coagulation process was US frequency of 80 kHz, power intensity of 200 W/cm², ozone dosage of 3 mg/L, reaction time of 8.06 min, PACl concentration of 0.81 mg/L, and 0.1 mg/L of coagulant aid. Chemical fractionation showed that the US/O₃-coagulation could transform the NOM macromolecules into smaller and lighter compounds. No diagnosis of total coliform or E. coli was detected in samples treated by the US/O₃-coagulation process. Moreover, treatment with this process did not increase the bromate concentration.

According to the results, pretreatment with US/O₃ significantly improves the coagulation process for NOM, TOC, coliform, and turbidity removal. Therefore, the US/O₃-coagulation process has the potential to be considered as an efficient water treatment method.

Introduction

Over the years, the demand of safe and high-quality drinking water has considerably increased due to the population growth and development of different industries. In this regard, dam reservoirs are used to provide water storage for domestic, drinking and irrigation purposes and life of millions of people throughout the world depends on these reservoirs. It has been well demonstrated that the quality of the waters behind these reservoirs is different from that river flowing into them. Because of the long retention time (months or even years), these waters are less turbid, while have high levels of natural organic matter (NOM) [1]. In the past years, conventional water treatment technologies, such as coagulation, sedimentation and filtration, have widely been used for the treatment of these water supplies. However, due to several shortcomings of such conventional methods, they fail to remove the turbidity and a significant amount of NOM to desirable levels [2,3].

The coagulation and flocculation processes are extremely important components of the conventional water treatment process. Since the formation of flocs is mainly dependent on the concentration of suspended sediment [4,5], therefore to achieve high turbidity removal, especially in low turbid waters, polymeric coagulant aids should be applied to increase the performance of coagulation/flocculation process. This can result in the production of high amounts of sludge [4], which needs additional treatment and proper disposal and consequently could increase the operating costs of water treatment plants [5], owing to the high price of polymeric coagulant aids. Moreover, the release of monomeric polymers may also pose potential health risks to consumers [6].

The high amounts of NOM in dam waters, in part, might be due to the excessive growth of algae and other aquatic plants. NOM is a complex mixture of various organic compounds present in natural [3] waters and contributes as the main factor to water color and odor. Of note, NOM has been considered as one of the most important precursors for disinfection byproducts (DPBs), including trihalomethanes (THM), which are associated with adverse health effects. Furthermore, the presence of high amounts of NOM has been regarded as the major contributor to fouling in water systems [3,4,7]. There are numerous studies indicating that NOM can enhance microbial growth in biofilms of the water distribution systems [7,8], and also could lead to the formation of organometallic complexes by combining with available heavy metal ions, which may result in some degree of toxicity [8].

The conventional processes, such as coagulation, under the optimum condition have an average efficiency to remove NOM from water. To improve the performance of conventional treatment methods, higher coagulant dosage is required [5]. On the other hand, the coagulation process mainly removes the hydrophobic fraction of NOM, and other types of treatments are necessary to remove the residual part of the NOM from water resources [9]. Accordingly, the application of an individual conventional method requires the use of more chemicals or materials, which therefore are not efficient and cost-effective [5], and more importantly may lead to both aesthetic and health issues [4]. Moreover, since the optimum conditions for the removal of NOM do not always align with the removal of other forms of contaminants, thus the individual conventional coagulation process cannot efficiently remove NOM from water resources [5,10]. Therefore, additional pretreatment and chemical/chemical-free techniques should be combined with the coagulation/flocculation process to improve the effectiveness of the process in removing NOM [4,105].

The ozonation has been demonstrated as a powerful water treatment process which can efficiently degrade organic compounds [11]. However, the increase in the dose of ozone should be proportional to the amount of organic matter which increases the cost of the treatment process. Additionally, the ozonation of water can result in the production of bromate and organic brominated DBPs [8,12]. Thus, the ozonation process should be further improved to meet the requirement of practical application. There are pieces of evidence indicating that the combined use of ozone with other treatment methods, in addition to increasing the quality of the water provided, could be more cost-effective by reducing both the applied doses of ozone and contact time [8,11,13]. In this regard, previous findings revealed that ozonation combined with coagulation can greatly enhance the coagulation performance, significantly remove NOM and five halo-acetic acid precursors [11], and decrease the formation of trihalomethanes (THM) [13]. Since treatments with ozone would increase the concentration of functional oxygenated groups like carboxylic acid, thus the efficiency of coagulation and flocculation process would be increased in ozonated waters. Also, ozone can polymerize meta-stable organics, which leads to particle aggregation through bridging reactions [13].

Moreover, ultrasound (US), a well-known chemical free water treatment process, has the potential for the application in the removal of NOM [4,10] as well as for disinfection purposes [14]. The US irradiation system is quite economical because of the ease of installation and its low maintenance costs. The degradation of organic compounds by this process in mainly occurs through the generation of short-lived oxidants, such as hydroxyl radicals, thus no formation of DBPs is expected [4,15]. The US has been shown to considerably increase the removal efficiency when combined with ozone, leading to the enhanced and accelerated oxidation of organic matters [16]. It seems that the production of higher amounts of ozone hydroxyl radicals under the ozonation-irradiation process contributes to the significant removal of the NOM [13]. In addition, US irradiation has also been reported to pose a positive effect on the performance of coagulation process [10]. In order to remove NOM, several methods such as air stripping PACl integrated system and H₂O₂ have been employed in the previous studies [17,18]. To the best of our knowledge, there is no study which has investigated the effect of ultrasound-ozonation (US/O₃) pre-treatment on the performance of coagulation-flocculation process in water treatment. Accordingly, in the present study, we aimed to study the effects of (US/O₃) combined with coagulation process on the removal of NOM and turbidity from dam water sources. We also performed TOC fractionation to trace NOM degradation. Furthermore, the concentration of bromate was examined for the formation of disinfection byproducts (DBPs) and total coliforms were used to evaluate the disinfection efficiency of the studied method [3].