Treatment of Water and Wastewater: Challenges and Solutions

1. Introduction

Water is widely recognized as a fundamental factor in ecosystems and environments, playing a crucial role in human life and health [1]. However, only a limited portion of water bodies is accessible to humans [2], and many such water sources have become contaminated due to industrialization and urbanization [3]. According to the World Health Organization (WHO), more than 80% of all diseases worldwide can be associated with low water quality [4]. In 2010, the United Nations recognized access to clean water as a human right [5]. Therefore, the removal of contaminants from water sources is urgently required to overcome issues related to the environment and human health.

There are various contaminants in water and wastewater effluents (comprising organic contaminants, heavy metals, emerging contaminants, nutrients, and pathogens), each of which requires a specific treatment process [6]. Several types of these contaminants resist removal and degradation by conventional water and wastewater treatment plants (WWTPs) and methods. For instance, the presence of heavy metals in aquatic environments has a significant ecotoxicology effect on the environment because of the long persistence, bioaccumulation, and biomagnification of such metals in the food chain. Metals resist biodegradation and their quantities continue to increase in freshwater bodies [7]. In addition, Mojiri et al. [8] stated that WWTPs have mostly failed to remove emerging contaminants (such as pharmaceuticals and personal care products (PPCPs), pesticides, and microplastics). Consequently, researchers have employed various physical, chemical, and biological methods to treat water and wastewater, and each of these methods has certain advantages and drawbacks.

However, accessible information about the performance, mechanisms, and process of each method during the removal of different kinds of pollutants remains limited. Thus, this article aims to highlight recent research efforts published in Separations and to discuss some novel approaches to the removal of contaminants from water and wastewater.

2. Treatment Challenges of Water and Wastewater

This section reviews various papers addressing novel techniques to remove diverse types of pollutants. These papers are discussed based on the types of methods employed, including physicochemical and biological methods.

One promising method to treat wastewater is adsorption. Adsorption efficiency can be affected by the chemical and physical properties of the adsorbent and adsorbate [9]. Farch et al. [10] removed methylene blue (MB), a dye contaminant, using walnut shell. Their study indicates that the maximum adsorption capacity (19.9 mg L⁻¹) was achieved at pH (6.9) and 50 mg L⁻¹ of the initial concentration of MB. In another study, Mortada et al. [11] eliminated chemical oxygen demand (COD) from domestic wastewater by using zinc chloride-activated carbon. A maximum adsorption capacity of 45.9 mg g⁻¹ was reached in this study. Adenosine and o-dihydroxybenzene, as PPCPs, were removed by the phenyboronic acid-functionalized Fe₃O₄ magnetic nanoparticles (Fe₃O₄@PBA), and maximum adsorption capacity exceeded 91.9 mg g⁻¹ for adenosine and 80.6 mg g⁻¹ for o-dihydroxybenzene [12]. Further, Mg-modified biochar was applied to recover nitrogen (N) from domestic wastewater to be reused as a soil fertilizer. The study indicated that after sorption of N by biochar, a considerable amount of the carried N (34.7–42.7%) from biochar was uptaken by plants [13]. A maximum sorption capacity of up to 9.5 mg g⁻¹ was reported in the removal of cobalt by crop residues as a natural adsorbent [14].

Membranes (comprising nanofiltration-NF, reverse osmosis-RO, and ultrafiltration-UF) have been extensively employed in the treatment of water and wastewater because of their high effectiveness, wide application, and high selectivity [15]. Jafarinejad and Esfahani [16] investigated the performance of NF in the treatment of petroleum industry wastewater. They reported almost 100% removal of total suspended solids (TSS), >95% removal of total organic carbon, and >99.9% removal of grease and oil content. In another study, 99% of COD, N, and color was removed by an integrated vibratory shear-enhanced process and RO [17]. More than 60% of MB was eliminated by a modified catalytic (molybdenum disulfide-iron oxyhydroxide) membrane (UF) [18].

Advanced oxidation processes (AOPs), which cause the destruction of the compounds and generate oxidation active radicals (such as hydroxyl radicals, ·OH), are noted to be a promising technique in the removal of contaminants (especially organic contaminants) from water and wastewater [19]. TiO₂-photocatalysis has demonstrated high performance in the degradation of organic pollutants without the production of residual sludge [20]. Furthermore, 100% of carbamazepine, as a PPCP, was removed by a boron/sodium fluoride co-doped titanium dioxide (B/NaF-TiO₂) photocatalysis system after a 4 h reaction [21]. In another study, photocatalytic coatings of TiO₂/ZnO/CuPc degraded 80% of ibuprofen, as a PPCP [22]. Moreover, >60% of benzene was degraded by a photocatalysis modified by BiOI/TiO₂ [23]. Ozonation, as a AOP system, is an efficient method of removing organic pollutants [24]. Integrated ozone-based AOPs are more effective than ozone alone in the removal of organic compounds [25]. Yan et al. [26] stated that the ozone/peroxymonosulfate (O₃/PMS)-based technique has a high potential in the treatment of organic wastewater effluents; their investigation found that 85.6% of COD, 81.7% of chromaticity, and 77.6% of dissolved organic carbon (DOC) were removed. Another study found that >49% of alachlor, as an emerging contaminant, was removed by ozone/hydrogen peroxide (O₃/H₂O₂) [27]. Mojiri et al. [28] investigated the performance of integrated electrochemical oxidation and ozonation (EO-O₃) in the treatment of wastewater and found that they removed 77% of tonalide and 84.5% of dimethyl phthalate, as PPCPs, and 58.2% of lead (Pb).

Biological methods offer a green means of eliminating contaminants from wastewater with minimum operation and installation costs [29]. In a pilot-scale study, >98% of ammonium and COD was removed from wastewater with a membrane bioreactor (MBR) modified with zeolite and bentonite [30]. Bioaccumulation of heavy metals with two bacteria species (Actinomycetes and Streptomycetes) was performed. The study showed that these bacteria could remove a significant ration of several heavy metals (such as chromium, nickel, and zinc) [31].

Although these studies found that the methods in question demonstrated high performance in the removal of several contaminants, we are still far from devising a system which can remove all kinds of pollutants with minimum cost and energy requirements.