Applications of Advanced Oxidation Processes (AOPs) in Drinking Water Treatment

Raw water is groundwater, surface water, or rainwater that has not received any treatment in order to be suitable for drinking. Its quality must be good enough to produce when treated a safe and acceptable drinking water, and it must come from a source that can consistently provide sufficient required quantity.

Polluted or contaminated water sources can contain chemical or microbiological hazards which can lead to sickness and require treatment before consumption. In many cases it is better to protect the quality of the raw water providing sustainable management than to treat it after it has become deteriorated.

This chapter gives an overview of the more traditional drinking water treatment from ground and surface waters. Water is treated to meet the objectives of drinking water quality and standards. Water treatment and water quality are therefore closely connected.

The objectives for water treatment are to prevent acute diseases by exposure to pathogens, to prevent long-term adverse health effects by exposure to chemicals and micropollutants, and finally to create a drinking water that is palatable and is conditioned in such a way that transport from the treatment works to the customer will not lead to quality deterioration.

Traditional treatment technologies as described in this chapter are mainly designed to remove macro parameters such as suspended solids, natural organic matter, dissolved iron and manganese, etc. The technologies have however only limited performance for removal of micropollutants. Advancing analytical technologies and increased and changing use of compounds however show strong evidence of new and emerging threats to drinking water quality. Therefore, more advanced treatment technologies are required.

The water sources of drinking water generally contain natural organic matter (NOM) as a result of the interactions between the hydrologic cycle and the environment. The amount, character, and properties of NOM vary considerably according to the origins of the waters and depend on the biogeochemical cycles of their surrounding environments. NOM can negatively influence water quality in drinking water supply systems, and it can significantly influence the performance of drinking water treatment processes. Hence, NOM removal is an important issue in order to optimize drinking water treatment operation and to reduce the risks of water alteration in the distribution systems. Several treatment processes can be applied for NOM removal depending on water quality, the nature of NOM, and the treatments already existing in the supply system. Among the most effective conventional solutions coagulation/flocculation, filtration, and carbon adsorption are available. An interest has recently increased toward nonconventional solutions based on membrane filtration and advanced oxidation processes (AOPs). An overview on the AOPs will be presented and discussed. Moreover, the AOP with ozone and UV radiation, with two low pressure UV lamps, at 254 and 185 nm wavelength, was experimented on a surface water in order to study the removal of odorous and pesticide, organic compounds (UV absorbance and THMs precursors) and bromate formation. Different batch tests were performed with ozone concentration up to 10 mg L⁻¹, UV dose up to 14,000 J m⁻², and a maximum contact time of 10 min. The main results show that metolachlor can be efficiently removed with ozone alone while for geosmin and MIB a complete removal can be obtained with the advanced oxidation of ozone, with concentration of 1.5–3 mg L⁻¹ and contact time of 2–3 min, with UV radiation (with doses of 5,000–6,000 J m⁻²). As concerns the influence of the organic precursors, all the experimented processes show a medium removal of about 20–40% for UV absorbance and 15–30% for THMFP (trihalomethane formation potential).

NOM usually reaches drinking water supply sources through metabolic reactions and soil leaching. It has been, in general, considered that NOM is still one of the most problematic contaminants present in this kind of influents. Therefore, in the present chapter, most relevant technologies used for removal of NOM and its constituents from water have been examined, emphasizing in the past few years. An overview of the recent research studies dealing the NOM removal by catalytic wet peroxide oxidation and other closely related heterogeneous Fenton-like AOPs is presented. As revealed from recent literature reports, heterogeneous Fenton processes including CWPO are still emerging, promising catalytic technologies for NOM removal from water. A wide variety of catalytic solids reported within the past few years has been examined focusing on their potential in the NOM removal from water. Main findings offered by several types of catalysts like zeolites, Fe-functionalized activated carbons, carbon nanotubes, but mainly pillared and other clay minerals have been critically discussed emphasizing on the NOM removal by CWPO.

Removal of natural organic matter (NOM) in drinking water treatment systems has been a matter of thorough study in recent years. NOM affects organoleptic properties of water and causes membrane fouling; it may act as energy source for microorganisms in distribution systems and leads to the formation of undesired disinfection by-products through its interaction with chlorine. Currently the role played by advanced oxidation processes in the removal of NOM has gained great interest; understanding the composition and behaviour of NOM throughout such a kind of processes may allow to get significant insight in order to improve efficiency. In this chapter the main techniques useful for characterization are described, and their use to investigate the changes undergone by NOM throughout several AOPs has been reviewed.

Natural organic matter (NOM) is a heterogeneous complex of organic materials and is ubiquitous in natural aquatic systems. The amount of NOM in the environment is continuously increasing because of global warming and/or changes in precipitation patterns and has negative impact on drinking water as it produces an undesirable colour and as a vector for the introduction of contaminants. For these reasons, several technologies have been proposed to address the impact of NOM in aqueous systems. Among these, advanced oxidation processes (AOPs) refer to oxidation processes that result in the formation of highly reactive radical species. This chapter presents an overview of recent research studies dealing with photon-activated AOPs for the removal of NOM and emerging contaminants in water.

Polluted surface water and groundwater represent a significant human health risk as it is a vehicle for a number of diseases derived from the exposition to untreated drinking water. Historically, chlorination became a great advance on reducing the impact of many pathogens associated with polluted drinking water in developed countries, with the consequent benefits to societies growing and welfare. Nevertheless, other treatments have been investigated during the last decades with the aim of increasing their capability for treating water and overcoming the limitations of chlorination and other conventional technologies including UVC radiation and ozone. Fenton and photo-Fenton process have been demonstrated to be a good option as alternative water disinfection process during the last years. The aim of this chapter is to briefly describe the fundamentals of this process with special focus on particular aspects related to pathogens inactivation in water. Moreover, the most recent scientific contributions on the application of Fenton and photo-Fenton for water disinfection are discussed.

The production of drinking water with good quality including the chemical, microbiological, and aesthetic characteristics remains one of the main contemporary challenges for drinking water industry. As the most predominant and problematic earthy-musty taste and odor (T&O) compounds are recalcitrant to conventional water treatment, advanced oxidation processes (AOPs) have been recently studied and employed in drinking water treatment for taste and odor control. In the light of recent developments, the present chapter reviews the effectiveness of various AOPs for T&O compounds removal from aqueous media. More specifically, an overview of the recent research studies dealing with AOPs for the removal of geosmin, 2-methylisoborneol, 2,4,6-trichloroanisole, 2-isopropyl-3-methoxypyrazine, and 2-isobutyl-3-methoxypyrazine from water reservoirs and drinking water, is presented. The fundamentals and experimental setup involved in relative technologies and the effectiveness of each process are further discussed. Special attention was also given to the degradation products and mechanisms that have been proposed for all the compounds in interest. Future research directions regarding the application of AOPs for T&O control and recommendations for further development are also highlighted.

The treatment of several industrial effluents, such as textile, pharmaceutical, and phenol-containing wastewaters, often face limitations toward conventional treatment processes. Solutions for such problematic include the use of several advanced oxidation processes (AOPs) and particularly the Fenton one. However, most of the research has been focused in the homogeneous process, while recent trends point for the use of heterogeneous systems, with the catalyst immobilized in a solid support. In addition, process optimization and catalyst screening are commonly carried out in batch reactors, which often are not the best solution for continuous industrial units. In this chapter, a review is made regarding the application of heterogeneous Fenton-like advanced oxidation processes in continuous systems (fixed-bed, fluidized-bed, and continuous stirred-tank reactors). The application of this catalytic process for pollutant/wastewater treatment is summarized, giving emphasis to the effect of the key operational parameters (e.g., pH, feed dose of H₂O₂, catalyst load, and feed flow rate) affecting the oxidative performance of such systems. Moreover, the main physicochemical properties of heterogeneous catalysts (e.g., source of support and particle size) and preparation methods (e.g., type of precursor and metal ion) affecting the catalytic efficiency of Fenton’s oxidation, and the stability of the catalyst itself, are also discussed. Finally, some operational issues of concern regarding solid catalysts operating in continuous-flow reactors are addressed.

Poor quality in drinking water is primary cause of pathogen transmission and responsible of varied infectious diseases. Methods of water treatment for human consumption must pay special attention on microbiological safe disinfection. Indeed, from the past few years laws all around the world have included new, more stringent water quality parameters. Chlorination and other mainly used conventional disinfection processes usually do not achieve full inactivation of all microorganisms present in real water supplies, whereas the presence of even low concentrations of organic matter can lead to form harmful disinfection by-products. Protozoan parasites Giardia sp. and Cryptosporidium sp. are some of the microorganisms that cannot be completely inactivated via chlorination under the same contact times typical of bacteria or virus elimination. It has increased toxicological and microbiological risks as well as operational costs. Disinfection by the advanced oxidation process more intensively studied in the past few years has been reviewed including Fenton and photo-Fenton processes and photocatalytic and electro-catalytic variants; this vibrant topic still remains partially uncovered in the available scientific background, which has motivated many recent researches and publications. This chapter is then devoted to briefly review the most recent reports studying the disinfecting potential displayed by mentioned AOPs with respect to widely and currently used conventional techniques. Revision of the inactivation of water-borne pathogens including E. coli, total coliforms, parasites as Giardia and Cryptosporidium, and virus such as coliphages has focused on advantages and disadvantages in application of every particular AOP, their disinfecting mechanisms, and the main parameters affecting the disinfection response.

Cryptosporidium, a protozoan parasite, was found responsible for numerous water- and foodborne outbreaks. The high risk introduced by the presence of Cryptosporidium in water is attributed to its low infectious dose and its resistance to environmental stress and conventional disinfection processes. Since Cryptosporidium oocysts are highly resistant to chlorine, the most applied water disinfectant, alternative disinfectants were proposed and applied to reduce the health risks of Cryptosporidium in water, among them advanced oxidation processes (AOPs), based on highly reactive oxidants, mainly hydroxyl radicals. AOPs proved to be efficient in reducing the concentration of micropollutants. The data presented here proved also that AOPs are effective in the inactivation of Cryptosporidium and other waterborne pathogens. Therefore AOPs can be applied as a barrier for reducing the health risks of waterborne Cryptosporidium.

Catalysts (homogeneous or heterogeneous) can be utilized to improve the performance of conventional advanced oxidation processes (AOPs). In general, catalyst activity, selectivity, stability, simplicity of preparation, preparation time, cost, nontoxicity, availability, recycling capability, environmental suitability, etc. can be the important parameters in the catalyst selection. High costs, cumbersome preparations, and environmental unsuitability can usually hinder the industrial applicability of a catalyst. In this chapter, catalytic AOPs (Fenton-based processes, catalytic ozonation, heterogeneous photocatalysis, catalytic wet air oxidation, and catalytic supercritical water oxidation), related catalytic materials, and cost-effective catalytic materials used in these processes are discussed.

Since the inception of drinking water treatment systems, ensuring the production of microbiologically safe drinking water has been a primary objective. While chemical oxidants are often successfully employed to mitigate microbial risks, the chemical reactions that occur between oxidants and the dissolved or particulate constituents present in source waters, e.g., natural organic matter (NOM), can produce byproducts associated with unintended health consequences. These disinfection byproducts (DBPs) are potentially carcinogenic, mutagenic, genotoxic, and/or teratogenic. Since the discovery of DBPs in the early 1970s, considerable effort has been afforded to develop regulations or guidelines striving to simultaneously control microbial pathogens and DBPs. As advanced oxidation processes (AOPs) gain traction as an integral part of advanced treatment trains in water, wastewater, and water reuse scenarios, their impact on DBPs, in terms of both formation and destruction, is an increasingly important consideration and is the focus of this chapter.

This chapter begins with a brief overview of major drinking water disinfection processes, followed by an introduction to common classes of disinfection byproducts (DBPs) and their precursors, and concludes with discussion of the influence of AOPs on DBP formation, formation potential, and removal.

Advanced Oxidative Processes (AOP) have been successfully employed as efficient water treatment methods. The utilization of AOP on drinking and wastewater represents currently an alternative to costly, hazardous, and slow processes. In order to further establish the ground basis for AOP in water safety and security, reliable and consistent methods of analysis are required. As an alternative to basic statistical methods, which may not successfully describe and forecast the application of a given treatment methodology of water, the use of chemometrics has increased significantly over the past decades. Chemometric analyses are an intersection between analytical chemistry and applied statistical models in order to predict and extract information from a given condition. This chapter introduces the concepts of chemometrics in environmental engineering issues and the utilization of experimental design to efficiently analyze experimental data in environmental samples. Two case studies are presented to demonstrate the importance of chemometrics in water analyses: (1) considering a Taguchi L ₁₆ experimental design, and an optimization study using Response Surface Methodology, to evaluate photo-Fenton and ozone AOP-based treatment on an effluent with high concentration of organic matter; (2) using a Taguchi L ₉ array to evaluate the combination of photocatalytic degradation and AOP of an industrial effluent. The results showed in this chapter demonstrate how a given statistical method can be successfully employed within the intersection of environmental analyses and water issues.