**2. Recently proposed wastewater treatment technologies for water reuse purposes**

#### **2.1 Primary treatment (PT)**

Sewage water is pretreated to remove grease, grit, and gross solids, which are considered an obstacle for the subsequent stages of treatment. Afterward, PT is

conducted to settle and remove either inorganic or organic suspended solids using settlers and septic tanks [6]. The PT can be exploited to provide water for the controlled irrigation of forestland and parks, as long as the safety precautions are fulfilled. The PT can reduce 30–40% of the organic load and pathogens (**Figure 1**) [5].

### **2.2 Secondary treatment (ST)**

The ST is conducted to remove or degrade soluble biodegradable organics via biological processes (e.g., aerobic or anaerobic processes using bacteria and protozoa). In this treatment, nutrients such as nitrogen and phosphorous may be also removed. The ST involves activated sludge, aerated lagoons, oxidation ditches, trickling filters, and constructed wetlands (CWs) [6]. The application of ST reduces the organic load and pathogens by ~95% and provides disinfection in some cases. This stage of treatment is convenient and can provide suitable water for the irrigation of trees (e.g., in olive orchards and vineyards), as long as there is no direct contact with the crops (**Figure 1**) [5].

### **2.3 Tertiary treatment (TT)**

The TT primarily involves coagulation, flocculation, sedimentation, filtration, and UV treatment, aiming to purify the outlet discharge before ultimate reuse. Notably, TTs also remove nutrients and residual suspended matter via suitable filtration as well as microorganisms and provide disinfection through the application of UV radiation, ozone, and chlorine. Membrane filtration (micro-, nano-, ultra-, and reverse osmosis (RO)), activated carbon, and filtration/percolation are typically a part of the TT; however, their applications are not widespread in developing countries [6]. The TT is fully capable of reducing the organic load by 99%, and UV disinfection completely removes the pathogens. Therefore, TT may be convenient for all types of edible crops (**Figure 1**) [5]. In Spain, 1.4% (63,000 m3 ) of the TT water produced from Lloret de Mar City (northeastern Mediterranean coast) was used for irrigation, and the rest was discharged into the sea [13]. Integrating different wastewater treatment technologies can also produce treated water suitable for special purposes. In the

following paragraph, we have summarized the recent integrated approaches proposed for wastewater reuse.

The advanced wastewater treatment plant (AWTP) may be used for potable water production based on the wastewater effluent in terms of the feed. For example, AWTP was construed to serve Australian Antarctic Division's Davis Station at the Selfs Point Wastewater Treatment Plant, Hobart, Australia. This AWTP comprises seven barriers: ozone, ceramic microfiltration, biologically activated carbon, RO, UV radiation, calcite dissolution and chlorination, and activated sludge [14]. Moreover, the Groundwater Replenishment Scheme in Australia is operated by the Western Water Corporation; they manage the outlet discharge of the Beenyup Wastewater Treatment Plant by applying three advanced processes, ultrafiltration (UF), RO, and UV disinfection, to recharge the Yarragadee and Leederville aquifers using special recharge bores. The same concept was used in other areas to augment surface water and lake reservoirs [15]. The Old Ford Water Recycling Plant in London collects and treats wastewater from a combined sewer (Northern Outfall Sewer) using a membrane bioreactor, which is then treated by granular activated carbon following which sodium hypochlorite is used to produce reclaimed water suitable for irrigation and toilet flushing [12]. The Aqueous Phase Reforming approach has been recommended for removing the total organic carbon and micro pollutants (e.g., carbamazepine, caffeine, ibuprofen, and diclofenac) from sewage at high percentages (90%) with advantages of H2 and CH4 production (via the valorization of organic matter) [8]. Onsite chlorination has been proposed for vertical flow CWs, together with a small-scale solar-driven system, resulting in the reduction of total coliforms and *Escherichia coli* to ≥5.1 and ≥ 4.6 counts, respectively [16]. A review of the best available technologies and treatment trains in the EU countries, to overcome the deficiency of water supply through urban wastewater reuse, is published in the literature [17].

In the north of Spain, the performance of five WWTP was investigated based on the presence of pathogenic, intestinal protozoa, and nematode eggs. The fifth WWTP does not contain any primary or pretreatment stage; however, wastewater is directly allowed to be treated in an Imhoff tank. In the fifth WWTP, either nematode eggs, Cryptosporidium spp., and Entamoeba spp. were detected, demonstrating high resistance to wastewater treatments. Moreover, the produced sludge contained Cryptosporidium spp., even after aerobic digestion [18].

Mulugeta et al. (2020) suggested that the installation of low-cost bio-sand filters in the decentralized municipal wastewater produces reclaimed water that complied with the WHO and USEPA guidelines [19]. The integrated suspended growth biological process and postozonation (O3) decreased the organic compounds by 92.1% diesel oil, 97.4% chemical oxygen demand, and 97.9% MB dye. This combination also established the efficacy of integrated industrial and domestic wastewater treatments that is ultimately capable of producing reclaimed water appropriate for agricultural irrigation [20]. The RO technology may be replaced with the ozone-biological activated carbon approach, owing to the reduction of capital and operation and maintenance costs, as well as the comprehensive enhancement of the system [21]. The integrated biochar vertical flow and free-water surface CW system is an effective approach for removing pollutants in wastewater, which is inversely correlated to the hydraulic loading rate [22]. A hybrid pretreatment process was proposed before the desalination of the cooling tower water effluents that included vertical subsurface flow, open water, and horizontal subsurface flow CWs [23]. The process of diluted desalination (using UF– RO) was explored as a preferable economical method to conduct the RO of seawater, even when both the UF recovery rate and water flux complied with the least values

#### *Treatment Technologies and Guidelines Set for Water Reuse DOI: http://dx.doi.org/10.5772/intechopen.109928*

of thresholds [24]. The integrated biological trickling filters and CW may be suitable for treating wastewaters derived from food industry and have several advantages: (a) tolerance to loading shocks, (b) simplicity of operation, (c) durability, (d) low capital and operation costs, and (e) high pollutant removal efficiencies [25]. The combination of osmotic membrane bioreactor and RO could be used to treat wastewater before its subsequent reuse [26]. Five scenarios, namely, RO, evaporation, crystallization, de-supersaturation, and precipitation, have been evaluated for treating petrochemical unit effluents, in which the energy consumption and chlorine-derived compounds used in the pretreatment are considered the most influential factors [27]. The integration of ozonation (oxidation) and biofiltration (adsorption and biological degradation) is also considered an effective approach to overcome several obstacles during the conventional water reuse treatment process, owing to its effectiveness in removing trace organic pollutants, byproduct precursors, biodegradable organic matter, and concerning substances [28]. Integrating membrane distillation and RO can purify RO-concentrated wastewater for potable water reuse with high recovery percentages; however, subsequent treatments, such as advanced oxidation (posttreatment), may be required to treat nitrosodimethylamine [29]. A UV-based advanced oxidation can be used to treat RO concentrates characterized by high content of dissolved organic matter; this can ultimately improve the sustainability of water reuse systems [30]. Notably, the use of RO was suggested to reclaim wastewater that is characterized by high salinity [31]. In general, the membrane distillation process has been proposed for potable water reuse following the coagulation and filtration pretreatment processes to avoid severe membrane fouling [32]. In a previous study [33], the feasibility of combining a vertical flow CW and membrane system for the treatment and reuse of decentralized gray and black water was highlighted. Additionally, a breakthrough dynamic-osmotic membrane bioreactor/nanofiltration (NF) hybrid system was proposed for the treatment and reuse of real municipal wastewater, in which membrane fouling was minimized while maintaining high water quality [34]. An adsorption technology was proposed as a promising TT to treat anodized industrial wastewater for reuse purposes [35]. The intensification of supercritical water oxidation through ion exchange with zeolite was proposed to treat landfill leachates for reuse purposes. The intensified process was conducted without using auxiliary substances or oxidants, thereby promoting this method as more eco-friendly and less expensive; however, further improvements are required regarding arsenic concentrations and ammoniumsaturated clinoptilolite [36, 37]. A photocatalytic membrane reactor was proposed to treat produced water; the reactor efficiently decomposes and mineralizes organic pollutants, inactivates viruses, detoxifies heavy metals, and recovers valuable minerals [38]. An integrated biological and advanced oxidation process followed by microfiltration and ultrafiltration, suggested to treat laundry wastewater, is a promising and highly efficient combination process for water reuse [39]. Integrated adsorption, RO, and TiO2/Fe3O4 photocatalytic oxidation were proposed for the reuse and recycling of aquatic center sewage comprising shower wastewater, whirlpool tub discharge, pool overflow, and sauna wastewater. This method yielded satisfactory results; however, the process could not remove organic matter (3.3 mg/L) [40]. Jain et al. (2021) indicated that the removal of silica (reactive and colloidal) using diatom biofilms developed from water generated from cooling towers of thermal power plants may save annually 1485 MLD of fresh water, in addition to the generation of value-added products (biogenic silica) [41]. In a previous study, the water reuse systems were optimized using an advanced control system for RO, in which more than Euro 10,000,000 could be saved per year in large RO facilities (>30.000 m3 /day) [42]. Additionally, a study combined

the mechanisms of sedimentation and flocculation for the in-situ treatment of quartzand chlorite-containing water at the Sijiaying Iron Ore Mine; approximately 65.10% grade and 86.50% recovery rate were achieved for iron concentrate under optimum conditions [43]. The denitrifying woodchip bioreactor approach was proposed for treating nitrate-rich water-containing aquaculture effluents. The tannins-lignin and total ammonia nitrogen concentrations were simultaneously increased with Cu and Zn concentrations. It was not recommended to immediately reuse the outflows following start-ups or restart after a dry period [44]. Integration of decolorization and desalination using *Escherichia fergusonii* was recently proposed for remediation of textile effluents for water reuse; the water was subsequently treated with rice husk-activated charcoal treatment to ensure the disinfection and detoxification. This method was not energy-intensive, in addition to being simple, economic, and a two-step process as a complete solution for textile effluent treatment [45]. The process of ultrafiltration was proposed for municipal wastewater reuse as a TT; in this process, the treated water was used only for nonpotable applications, according to the national and international guidelines [46]. The pond-in-pond wastewater treatment system, in which anaerobic and aerobic ponds are combined into a single pond, was also proposed for water reuse. This system is considered highly efficient, with low costs and low maintenance [47]. Xing et al. (2021) revealed that the integrated chlor(am)ine-UV oxidation and UF may be considered a promising alternative for efficient wastewater recycle and reuse [48]. Additionally, molecularly imprinted polymers (MIPs) can be used for removing various contaminants of emerging concern (CECs); however, the MIPs cannot be applied at a large scale and are limited to the lab/bench scale. However, MIPs can remove a wide range of CECs, such as diclofenac, atrazine (pesticide), ketoprofen and ibuprofen (nonsteroidal anti-inflammatory drugs [NSAIDs]), ciprofloxacin and sulfamethazine (antibiotics), triclosan and parabens (personal care products [PCPs]), and bisphenol A (plasticizer) [49].

### **3. Recent guidelines set for water reuse**
