**2. Urban water cycle sustainability: new challenges to ensure safe water**

Urban water cycle management involves the fields of water supply, urban drainage, wastewater treatment, reutilization, and sludge handling with a river basin scale approach.

Conventional approaches to urban water management for providing water supply and sanitation services are often costly, inefficient, and not integrated. Hence, there is a need for finding new ways for improving and assess the urban water systems to enable better sustainability of these systems [16] to face new challenges in a climate change context.

In an urban water systems context, life cycle assessment (LCA) can provide a pertinent holistic approach supporting the critical processes identification and the potential improvements of these systems, including the water and wastewater treatment facilities, as well as, its interactions with source or receiving waters. Several researchers used LCA approach for comparing water treatment technologies sustainability [17, 18], as well as the major environmental impact changes resulting from centralized wastewater treatment systems commutation to decentralized ones [19].

This kind of approaches allowed to identify new threats for the urban water cycle sustainability, concerning with the obligation to ensure safe drinking water in order to safeguard public health and urban aquatic ecosystems.

#### **2.1. Occurrence of emerging micropollutants in urban water systems**

Aquatic ecosystem pollution is particularly problematic due to the cumulative effect of pollutants on aquatic organisms during its life cycle. This cumulative effect can occur so slowly that major impacts may remain undetectable until the hatching of irreversible ecosystem changes [20]. The hydrodynamics and the longitudinal dispersion patterns presented by receiving water systems have a decisive role in its ability to self-regenerate [21] and to washout inflow pollutants like nutrients and xenobiotics [22].

During the last decades, the impact of chemical pollution has focused almost exclusively on the conventional priority pollutants, especially those acutely toxic/carcinogenic pesticides displaying persistence in the environment.

At the same time (but receiving much less attention), the anthropogenic activities increased the diversity and load discharge of another groups of bioactive hazardous chemicals into urban water systems (**Figure 1**), namely:

• Contaminants of emerging concern (CECs), such as pharmaceutical compounds (PhCs), diagnostic agents, steroids, phthalates, and disinfectants.


The widespread use of antibiotics as a therapy for bacterial infections in humans and animals (even for promoting it growth) has led to the concentration increase of antibiotic-resistant bacteria (ARB) in surface waters and urban waterways [23–25], used for domestic sewage, hospital wastewater, and livestock feeding operations drainage. As opposed to the conventional persistent priority pollutants, PhCs need not be (necessarily) "persistent" if they are continually introduced to surface waters, even at very low concentrations.

The use of conventional water treatment technologies against these emerging contaminants is limited due to their ineffectiveness and incomplete biodegradation of the waste products as outlined in the applicable EU directives.

The presence of PhCs, PCPs, and EDCs in drinking water indicates that conventional and most commonly used water treatment technologies may not be enough to completely eliminate these compounds from source waters [26], which can be polluted because existing Wastewater Treatment Plants (WWTPs) were usually not designed to remove antibiotics present at trace levels, implying the need for its urgent improvement. Indeed, if urban WWTPs play a vital role in minimizing the discharge of many water pollutants, including antibiotics [27] and pathogenic microorganisms [28] to the aquatic ecosystems, they are also potential breeding grounds and point sources for environmental dissemination of antibiotic resistance [29].

Indeed, the very high bacterial density into biological reactors (e.g., activated sludge) promotes selective elimination and/or changes in the proportions of phenotypes within effluent bacterial populations turning WWTPs into important reservoirs of enteric bacteria which

**Figure 1.** Threats to urban water cycle sustainability due to xenobiotic load increase.

carry potentially transferable resistance genes. For these reasons, higher frequency of multiple resistant coliform bacteria in treated sewage than in raw sewage [30, 31] for most antibiotics, especially for ciprofloxacin and tetracycline, have been found.

• Endocrine disrupting compounds (EDCs), like natural and synthetic estrogenic or andro-

The widespread use of antibiotics as a therapy for bacterial infections in humans and animals (even for promoting it growth) has led to the concentration increase of antibiotic-resistant bacteria (ARB) in surface waters and urban waterways [23–25], used for domestic sewage, hospital wastewater, and livestock feeding operations drainage. As opposed to the conventional persistent priority pollutants, PhCs need not be (necessarily) "persistent" if they are

The use of conventional water treatment technologies against these emerging contaminants is limited due to their ineffectiveness and incomplete biodegradation of the waste products as

The presence of PhCs, PCPs, and EDCs in drinking water indicates that conventional and most commonly used water treatment technologies may not be enough to completely eliminate these compounds from source waters [26], which can be polluted because existing Wastewater Treatment Plants (WWTPs) were usually not designed to remove antibiotics present at trace levels, implying the need for its urgent improvement. Indeed, if urban WWTPs play a vital role in minimizing the discharge of many water pollutants, including antibiotics [27] and pathogenic microorganisms [28] to the aquatic ecosystems, they are also potential breeding grounds and point sources for environmental dissemination of antibiotic resistance [29].

Indeed, the very high bacterial density into biological reactors (e.g., activated sludge) promotes selective elimination and/or changes in the proportions of phenotypes within effluent bacterial populations turning WWTPs into important reservoirs of enteric bacteria which

• Personal care products (PCPs), such as fragrances, sun-screen agents, and cosmetics.

continually introduced to surface waters, even at very low concentrations.

**Figure 1.** Threats to urban water cycle sustainability due to xenobiotic load increase.

genic chemicals.

128 Application of Titanium Dioxide

outlined in the applicable EU directives.

Human health risk characterization related to the pharmaceutical water ingestion exposure can be performed by the assessment of risk quotients (RQs). This risk index can be estimated dividing the maximum concentration of a pharmaceutical (MPC) found in the water matrix by the respective Drinking Water Equivalent Level (DWEL), which can be obtained as an exposure criteria based on other related parameters, such as acceptable daily intake; body weight, hazard quotient, and drinking water daily ingestion; gastrointestinal absorption rate; and frequency of exposure. So, a RQ value higher than 1 leads to a risk concern related to inadvertent exposure through drinking water, and measures must be considered in order to prevent public health.

A recent monitoring program performed along Lisbon's drinking water supply system [32] showed that appreciable risks to the consumer's health arising from exposure to trace levels of pharmaceuticals in drinking water were yet extremely unlikely, because all risk quotient (RQ) values were less than 0.001. Therefore, a high environmental risk was detected for *Erythromycin* (RQ = 1.55), the urgency of the study and development of new low-cost technologies for an effective removal of the most prevalent antibiotics in WTP raw waters.

### **2.2. Advanced oxidation processes: the role of photocatalysis as a** *low-cost* **alternative technology**

Nanotechnology offers significant opportunities to revolutionize approaches toward drinking water treatment by enhancing the multifunctionality and versatility of treatment systems, while reducing reliance on stoichiometric chemical addition, shrinking large facilities with relatively long hydraulic contact times, and minimizing energy intensive processes [33]. So, it can provide low-cost, safe, and efficient water treatment systems with minimal energy requirements contributing for a more sustainable urban water cycle.

Nanomaterials properties have been explored for applications in water and wastewater treatment, due to its advantages related to the high specific surface area, fast dissolution, high reactivity, and strong sorption. Micropollutants' removal ability of new materials, such as carbon nanotubes, nanofibers, nanoscale metal oxide, nano-zeolites, and magnetic nanoparticles, is being tested and assessed when used in selected treatment unit processes, like adsorption, photocatalysis, membrane filtration, and disinfection.

Different advanced water treatment techniques for antibiotic removal have been studied, especially focus on membrane filtration, activated carbon adsorption, and advanced oxidation processes (AOPs). AOPs are recommended when water pollutants (such pharmaceuticals) have a high chemical stability and/or low degradability, allowing a more useful and cost-efficient combination with biological processes, namely in wastewater treatment [34].

The efficacy of AOPs depends on the generation of very reactive and nonselective free radicals such as hydroxyl radicals (•OH), superoxide radical (O2−), hydroperoxyl radical (HO<sup>2</sup> •), and alkoxy radical (RO•)—involving chemical (e.g., O<sup>3</sup> , O<sup>3</sup> /H<sup>2</sup> O2 ), photochemical (UV/O<sup>3</sup> , UV/ H2 O2 ), or photocatalytic (TiO<sup>2</sup> /UV, ZnO/UV) oxidation processes. In recent years, semiconductor photocatalytic process has shown a great potential as a low-cost, environmental friendly, and sustainable treatment technology to align with the "zero" waste scheme in the water/ wastewater industry. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water [35] and some hazardous inorganic micropollutants (e.g., arsenic, heavy metals, uranium).

Recent research works were mainly focused on AOPs assisted by solar radiation (a clean and renewable energy source), such as heterogeneous photocatalysis, in order to develop more sustainable and *low-cost* processes. The photocatalytic reactors can be divided into two main groups: with suspended nanoparticles (e.g., TiO<sup>2</sup> , ZnO) in the reaction mixture (water and wastewater) and with immobilized nanoparticles on a carrier material (e.g., glass, quartz, stainless steel, zeolites).

When the catalyst is in suspension, the active surface is greater. However, its particles have to be removed from the treated water after the detoxification, and the manipulation of powdered semiconductors are difficult. To ensure complete rejection of TiO<sup>2</sup> nanoparticles, an extensive and relatively costly installation technology is necessary, including pumps. Very promising techniques for solving problems concerning separation of the photocatalyst as well as products and by-products of photo-degradation from the reaction mixture are the use of photocatalytic membrane reactors (PMRs) and the introduction of a magnetic into the nanocomposite [36]. However, the energy costs evolved in membrane processes can compromise the economic sustainability of the water treatment utilities, namely in medium and small water supply systems.

A solution for avoiding the contamination with the photocatalytic nanoparticles is their immobilization on the surface of specified materials by use of suitable coating techniques, as a wet chemical process. Quartz has been found to be the best support for titanium dioxide, because it is the most neutral and stable one at high temperatures. As a consequence, it has been chosen as the ideal support for new experiments with TiO<sup>2</sup> in the photodegradation of organic micropollutants in water [37].
