Treatment Technologies and Guidelines Set for Water Reuse

Abstract

Water reuse is considered a practice that is currently embraced worldwide owing to the exacerbated water crisis, which is the result of several factors such as the increasing world population, urbanization, industrial sector, global climate change, limited water resources, and agricultural activities. Water reuse is not used intensively only in arid and semi-arid regions, which are characterized by limited water supply but can also be applied in countries that possess sufficient water resources (e.g., Brazil and Canada are implementing policies for water reuse). This chapter discusses the treatment technologies proposed for water reuse and presents some recent guidelines set for water reuse. Treatment technologies typically have three main processes: primary, secondary, and tertiary. There are several set guidelines worldwide for water reuse, however, a universal standard guideline to facilitate the reuse of reclaimed water has not been established. No federal regulations for reusing recycled water have been established in the United States; however, several individual states and territories have established specific regulations to manage reclaimed water for various purposes, including agricultural irrigation, animal watering, and crop production.

1. Introduction

Globally, a big amount of potable water is being seawater, whereas less than 3% can be considered safe for use. This 3% of potable water exists in groundwater which accounts for 20% (requires energy for extraction and pumping) and glacial ice (79%), accordingly, the available amount of potable water that is suitable for direct use is account for less than 1% [1, 2]. In China, water crisis can be observed in two phases, scarcity and deterioration, in which two-thirds of Chinese cities suffer from water deficiency, river pollution, and lake eutrophication [3]. At present, the global economy withdraws approximately 4000 km³ of water per year from natural resources, among which 45% is the discharged wastewater that cannot be handled in the currently available wastewater treatment facilities (only 11% of the total discharge is being treated through different treatment processes). The wastewater may be comprised agricultural runoff (56%), industrial effluents (28%), and household water in an urban area (14%) [4]. At present, half of the world’s natural water bodies are severely contaminated, and by 2030, it will be imperative to reduce the proportion of untreated wastewater by half, according to the SGD agenda 2030 [5].

The scenario of water reuse is considered ancient as human civilizations themselves (e.g., several civilizations, including Egypt, Mesopotamia, and Crete civilizations utilized sewage (domestic wastewater) for agricultural irrigation from the beginning of the Bronze Age (approximately 3200–1100 BC). Afterward, Greek and Roman civilizations adapted water reuse during 1000 BC–330 AD [6].

Almost all continents at present such as Europe, Australia, Africa, Asia, and North America embrace the notion of potable reuse [7]. The amount of water being reused differs from one country to another (e.g., 46% in California, 7% in Japan, 32% in Asia, 75% in Israel, and 44% in Florida). Reclaimed water is being reused for environmental applications in northern Europe (51%). Also, water reuse is account for 44% in southern Europe for agricultural irrigation, 25% in Tunisia, and 25% in Spain for agriculture. In Singapore, approximately 500 Mm³/year of treated wastewater is being reused to fulfill its water demands and by 2060, this amount is expected to be increased to 55% [5, 8].

Although some countries have sufficient water resources (e.g., Canada and Brazil), the arid regions may suffer in the future owing to the limited water supply. This is particularly true for expanding cities, with the situation being exacerbated by decreasing glaciers and depleting water sources due to climate change [9, 10]. The agricultural sector consumes a huge percentage of the global freshwater (70% of the withdrawal and 90% of the consumption), and the water consumption of this sector can grow in the future, as 56% of the globally irrigated crops experiences extremely high water stress [5, 11]. Approximately 12% of the globally irrigated land (36 million ha) irrigates with some urban wastewater, of which only 15% is reclaimed water [6].

Water crises are more evident in megacities, such as London, where water infrastructure and population growth pose severe challenges. Similar issues and incidents have been reported in Mexico City and Tokyo. In these areas, water reuse is implemented at two scales (large and small). At the large scale, reclaimed water is dedicated to drinking water (DW) (e.g., Texas, Orange County, and California), whereas at the small scale, reclaimed water is used for flushing toilets, cleaning streets, and irrigating urban areas [12]. A recently emerging term, “One Water,” should be embraced worldwide; the term is used to promote the ideology that all water has value and should be managed in a sustainable, inclusive, and integrated way. Water reuse may be considered one of the available solution, to ensure sustainable water use and address food insecurity. The “One Water” approach may involve the following provisions: (1) all water types, from raw source water (for DW treatment plants) to water flushed down toilets and drains, should be viewed through the lens of source water protection, and (2) different types of water pollutants must be treated to establish appropriate standards for their intended downstream applications [11].

The main aim of this chapter is to provide an overview of treatment technologies and guidelines set for water reuse as is considered an important factor to overcome the future water crisis.

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].

Figure 1.

Wastewater treatment technologies.

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 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 ammonium-saturated 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 quartz- and 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

3.1 Food and Agriculture Organization (FAO), World Health Organization (WHO), and United States Environmental Protection Agency (USEPA) guidelines

The guidelines for the safe use of reclaimed water in agricultural irrigation, according to FAO, WHO, and USEPA are discussed by Hacıfazlıoğlu et al. [50]; the guidelines comprise several parameters, such as electrical conductivity (EC), sodium absorption ratio (SAR), total dissolved solids (TDS), total nitrogen, total suspended solid (TSS), $N{O}_3-\text{N}$, $P{O}_4-\text{P}$, PAR, $HC{O}_3$, boron, chloride, sodium, free chloride, $pH$, and turbidity. These parameters were classified according to the degree of restrictions: none, slight–moderate, and severe. For example, reused water having EC values <0.7 mS/cm is considered to not have any harmful effects on the ecosystem, whereas EC values >3 mS/cm will cause severe effects. Additionally, there is a high correlation between the SAR values and EC, based on the three classes of restriction on soil permeability (none, slight–moderate, and severe) [50].

3.2 Food and drug administration (FDA) guidelines

The regulations set by the United States FDA as part of the Food Safety Modernization Act (FSMA) for water quality metrics were discussed by Rock et al. [51]. The regulations set by the FSMA are applied to the crops that are subject to raw agricultural commodities and do not receive commercial processing required to decrease the growth of microorganisms (to ensure public health). The FSMA has set standard processes, practices, and procedures that reduce the risk of serious health consequences or biological hazards in fresh raw crops (e.g., edible leafy greens) to decrease foodborne diseases resulting from the consumption of contaminated crops [51]. According to the Produce Safety Rule, farmers establish a microbial water quality profile (MWQP) for irrigation water sources (e.g., untreated surface and ground water) and are requested to perform annual surveys that can be used in the following years. The levels of generic E. coli are considered a basic parameter for the water quality profile in agricultural (pre-harvest) water. First, the MWQP must be performed, with not less than 20 water samples collected as near to the harvest period as possible, over not less than 2 years and not more than 4 and 5 years for surface water and ground water, respectively. The geometric mean (GM) or statistical threshold value (STV) is to be calculated from the collected samples using a minimum of five samples. The MWQP comprises both GM (126 CFU/100 mL) and STV (410 CFU/100 mL). The GM is an average that reflects the central tendency of the water source, whereas the STV represents the variations in water quality. Further details about the FDA FSMA regulations are presented in the literature [51].

3.3 State-level guidelines in the United States

In the United States, there are no established federal regulations for reusing recycled water. However, several individual states and territories (e.g., Arizona, California, Colorado, Florida, Virginia, Delaware, Massachusetts, New Jersey, Hawaii, North Carolina, Idaho, Minnesota, and Washington) have established specific regulations to manage reclaimed water for various purposes, including agricultural irrigation, animal watering, and crop production. Details about these state-level regulations are presented in the literature [51]. Notably, the regulations established by state standards for irrigation purposes to promote the use of recycled water to grow food crops are considered restrictive compared to the FSMA Agricultural Water metrics. This is indicated by the higher permitted concentrations of E. coli, total coliform, or fecal coliform bacteria, which are comparatively lower than those detailed in the FSMA metrics [51].

3.4 Europe (Spain, Italy, Greece, and Cyprus) guidelines

A comparison of guidelines for wastewater reuse in irrigational applications in Europe (Greece, Italy, Spain, and Cyprus) and the United States are discussed by Otter et al. [16]. These guidelines contain uncommon parameters for water quality, prescribing a limit for the number of pathogen indicators, to represent the effectiveness of the wastewater treatment plants in removing the nutrients, organic matter, and pathogens. Accordingly, the disinfection approach (e.g., ultraviolet [UV] radiation, membrane filtration, onsite chlorine generation system, and ozonation) is also a mandatory step to minimize public health risks resulting from potential exposure to reclaimed water [16]. In May 2020, a new regulation was proposed by European Union (EU) regarding the minimum quality requirements (MQR) for reclaimed water (EU, 2020/741) to be reused in the agriculture sector; this regulation will be implemented on June 26, 2023, in all the member states. There is also a growing concern about potential noncompliance situations regarding the MQR [52].

3.5 Egypt

The Egyptian government (Ministry of Housing, Utilities, and Urban Communities) has published the Egyptian Code, with two versions, for treated wastewater (TWW) reuse (code 501, 2005, and 2015). The 501 code in the 2005 version classifies the TWW into three categories, whereas four categories were proposed in (ECP 501, 2015) based on the treatment level. In each category, the Egyptian Code specifies the crops that can be cultivated. In general, the Egyptian Code prohibits the cultivation of raw vegetables, such as cucumber and tomatoes, using TWW [53].

4. Conclusions

The deficiency of water supply is considered a global issue and in the near future (2030; 160% of currently available resources) should be provided to overcome this threat. In the present chapter, we have discussed both treatment technologies and guidelines set for water reuse that may be considered one of the available solutions presented to overcome the future water crisis. The treatment technologies are divided into three main categories including pretreatment, PT, ST, and TT based on the main endeavors suggested for water reuse. The PT can reduce 30–40% of the organic load and pathogens, accordingly, it may be exploited to provide water for the controlled irrigation of forestland and parks, as long as the safety precautions are fulfilled. The ST is capable of reducing the organic load and pathogens by ~95% and provides disinfection in some cases. The ST is suitable water for the irrigation of trees (e.g., olive orchards and vineyards), as long as there is no direct contact with the crops. Lastly, The TT may be convenient for all types of edible crops owing to it being fully capable of reducing the organic load by 99%, and UV disinfection completely removes the pathogens. The standard guideline for the application of water reuse at a global scale is unavailable, whereas, several guidelines for water reuse were discussed in the present chapter including FAO, WHO, USEPA, FDA, State-level guidelines in the United States, Europe guidelines, and Egypt.

Acknowledgments

This work was funded by Science and Technology & Innovation Funding Authority (STIFA) known also as Science and Technology & Development Fund (STDF) (project number: 39369).

Conflict of interest

The authors declare no conflict of interest.