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carcinogenic risks mentioned, VOCs can produce teratogenic and mutagenic effects; therefore, their emission must be controlled (Yeh et al., 2011; Zhang et al., 2019). Industrial wastewaters are important sources of VOCs. Significant amounts of benzene, toluene, xylenes, and other compounds are released during wastewater treatment and can harm the health of staff working at treatment plants and people living in nearby communities (Zhang et al., 2019).
Studies of VOC emissions focus mainly on municipal wastewater treatment plants. However, the treatment of industrial wastewaters has attracted attention. In the research conducted by Zhang et al. (2019), a full‐scale plant for coke‐oven wastewater treatment was evaluated, estimating the VOC emissions in each treatment unit and discussing factors that influence these emissions.
Another category of organic compounds that cause concern is POPs. They are defined as carbon compounds that remain immutable for a long time in the environment. In living organisms, they can accumulate in adipose tissues and are toxic to both animals and humans: they can cause different forms of cancer, reproductive problems, and changes in the nervous system (Stockholm Convention, 2001; Trojanowicz, 2020). Table 1.3 presents a list of POPs according to the Stockholm Convention.
The organic content of wastewaters can be estimated by the BOD, COD, total organic carbon (TOC), or total oxygen demand (TOD). The BOD test measures biodegradable organic carbon, and the COD test measures total organic carbon with the exception of certain aromatics, such as benzene, that are not completely oxidized in the reaction. Because it is an oxidation‐reduction reaction, other non‐organic reduced substances, such as sulfides, sulfites, and ferrous iron, will also be oxidized and reported as COD, except NH3‐N. The TOC test measures all carbon as CO2, and hence inorganic carbon (CO2, HCO3‐, etc.) present in wastewater must be removed before the analysis or corrected for in the calculation. The TOD test measures organic carbon and unoxidized nitrogen and sulfur (Eckenfelder et al., 2008). TOC measures are used to monitor organic compounds in complex industrial wastewaters because they are more reliable than conventional BOD or COD measures, although the latter parameters are still widely implement (Dubber and Gray, 2010; Barak et al., 2020).
Table 1.3 Listing of persistent organic compounds according to the Stockholm Convention.
Source: Adapted from Trojanowicz, 2020.
| Category of compounds | Examples of compounds |
|---|---|
| Compounds for elimination(Should be eliminated from the production, use, and trade, although their import and export can occur under specific and restrictive conditions) | Aldrina‐ and b‐hexachlorocyclohexaneChlordaneChlordeconeDieldrinEndrinHeptachlorHexabromobiphenylHexabromodiphenyl ether and Heptabromodiphenyl etherHexachlorobenzeneLindaneMirexPentachlorobenzenePolychlorinated biphenyls (PCB)Tetrabromodiphenyl ether and pentabromodiphenyl etherToxaphene |
| Compounds whose production should be restricted(Subject to restrictions concerning their production and use) | DDT (1,1,1‐trichloro‐2,2‐bis(4‐chlorophenyl) ethanePerfluorooctanesulfonic acid (PFOS), its salts, and perfluorooctane sulfonyl fluoride |
| Compounds that are unintentionally produced(Should reduce and eliminate their release from unintentionally produced POPs) | Hexachlorobenzene (HCB)Pentachlorobenzene (PeCB)Polychlorinated biphenyls (PCB)Polychlorinated dibenzo p‐dioxins and dibenzofurans |
Several processes are used in industry to treat organic compounds. Biological treatment is one of the most‐used processes since it is considered a reliable and economical technology in comparison to chemical (coagulation‐flocculation, oxidation) and thermal (incineration) treatment (Barak et al., 2020; Chávez et al., 2019). Barak et al. (2020) studied full‐scale membrane bioreactors (MBRs) for the treatment of wastewater from one of the major factories in an industrial park and observed good results in relation to the microbial diversity of the bioreactor and the removal of TOC.
However, some industrial wastewaters contain sufficient toxic and refractory compounds to affect the COD and removal of other parameters, not allowing them to reach the expected discharge standards. Thus, AOPs can be an alternative to achieve a lesser impact of the final effluent on the receiving ecosystem (Chávez et al., 2019; Li et al., 2019). AOPs have been extensively studied for treating industrial wastewaters such as those from the textile industry, not only for the removal of COD and TOC but also for the effective removal of colors and other toxic byproducts (Bilińska et al., 2020). They are usually implemented independently or can be combined with other processes (Bilińska et al., 2020; Buthiyappan and Raman, 2019; Chávez et al., 2019).
Catalytic ozonation has also occupied space for the treatment of industrial effluents. Bilińska et al. (2020) used catalytic ozonation to remove organic matter and toxicity in raw effluent from the textile industry, evaluating three types of catalysts. As a result, they observed that, with the best‐performing catalyst (activated carbon), 35% of TOC and 40% of COD were removed, and toxicity decreased by 30%. Chávez et al. (2019) evaluated the feasibility of a photocatalytic ozonation treatment preceded by aerobic biodegradation in a sequencing batch reactor (SBR). The system was assembled on a bench scale to treat wastewaters from petrochemical and cosmetic products with a high organic load (TOC > 3 g/L, COD > 12 g/L, BOD5 > 2 g/L), other toxic compounds, and metals. Due to the quality of the effluent, it was diluted with urban wastewater (1:5) for treatment in the SBR. The implemented approach achieved a final effluent suitable for disposal according to environmental regulations (COD < 125 mg/L, BOD5 < 25 mg/L).
Zeolites are also widely used as an adsorbent to remove organic contaminants in the treatment of industrial wastewaters. Zeolites are porous materials with a three‐dimensional structure used as an adsorbent base during treatment (Hashemi et al., 2018). Using a surfactant‐modified zeolite, Hashemi et al. (2018) reported that the TOC content of olefin plant wastewater was reduced up to 89%.
Contaminants of Emerging Concern (CECs)
CECs are any substances originating from human activity or natural occurrence not generally monitored in the environment (Nawaz and Sengupta, 2019). They mainly result from the discharge of wastewaters (industrial or domestic), and conventional treatment processes are not capable of degrading these compounds. They can also be present in surface and drinking water. Even in very low concentrations (μg/L or ng/L), they can be bioaccumulative and become potentially dangerous to the ecosystem and human health (Prada‐Vásquez et al., 2020).
CECs include pharmaceutical and personal care products (PPCPs), pesticides, POPs, EDCs, flame retardants (FRs), artificial sweeteners (ASWs), various industrial chemicals, etc. (Salimi et al., 2017). Two criteria for the long‐term classification of a compound as a CEC are persistence in the environment and/or potential ecotoxicological and harmful effects to humans (Nawaz and Sengupta, 2019). Countless compounds fit in this classification, and over time, new substances and their effects are discovered, so the list of CEC has been constantly updated. Salimi et al. (2017) listed a series of CECs, some of which are shown in Table 1.4.
Table 1.4 Classification of some CECs such as PPCPs, EDCs, FRs, pesticides, and ASWs.
Source: Adapted from Salimi et al., 2017.
| Classes | Used | Examples |
|---|---|---|
| PPCPs | ||
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Analgesics
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