Wastewater treatment plants may be distinguished by the type of wastewater to be treated. There are numerous processes that can be used to treat wastewater depending on the type and extent of contamination. The treatment steps include physical, chemical and biological treatment processes.
The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of most wastewaters is carried out through separation of solids from liquids, usually by sedimentation. By progressively converting dissolved material into solids, usually a biological floc or biofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.
Oxidation reduces the biochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities. Secondary treatment converts organic compounds into carbon dioxide, water, and biosolids through oxidation and reduction reactions. Chemical oxidation is widely used for disinfection.
Advanced oxidation processes (AOPs) utilizing powerful hydroxyl or sulfate radicals as a major oxidizing agent were first proposed in the 1980s for potable water treatment. Later, AOPs were broadly applied for treatment of different types of wastewaters because the strong oxidants can readily degrade recalcitrant organic pollutants and remove certain inorganic pollutants in wastewater. The objective of this study was to review the fundamentals of and recent advances in the advanced oxidation processes for wastewater treatment. In particular, AOPs for treatment of landfill leachate are discussed in detail.
Based on the classical Fenton treatment scheme, three modified Fenton processes are proposed, including the Fenton-like system, photo-Fenton system, and electro-Fenton system. In the Fenton-like reaction, Fe2+ is replaced by ferric ion (Fe3+), namely, the series of reactions in the Fenton system are initiated from Eq. 13 in the Fenton-like system, rather than from Eq. 12 in the traditional Fenton treatment. In the photo-Fenton reaction, UV irradiation is applied with the traditional Fenton system with a major purpose of enhancing the UV-induced reduction of dissolved Fe3+ to Fe2+. In the electro-Fenton reaction, either or both of the Fenton reagents may be generated through electrochemical methods.
Rapidly expanding population, escalating water consumption, and dwindling water resources have severely aggravated the water shortage problem on a global scale, particularly in arid and water-stressed areas, making water reuse a strategically important approach to meet the current and future water demand [60, 61]. Water reclamation has been long practiced for non-potable urban, industrial, and agricultural scenarios, as well as to augment potable water supplies through indirect or direct reuse . Among various reclaimed water sources, biologically treated secondary effluents (BTSE) produced from wastewater treatment plants (WWTPs) represent a stable non-seasonal source, generally meeting 87 of the 93 numerical primary and secondary drinking water standards without further treatment . Effluent organic matters (EfOM) in BTSE, similar to other BTSE pollutants such as nutrients, play an essential role in tertiary wastewater treatment and water reuse. EfOM is a complex mixture primarily comprised of extracellular polymeric substances (EPS), soluble microbial products (SMPs), and natural organic matter (NOM) derived from drinking water sources [63, 64]. Detailed information regarding the EfOM characteristics is available elsewhere [65, 66]. Despite a few benign effects in limited cases, EfOM has, at least, five adverse impacts on physical, chemical and biological treatment of BTSE , including the following:
Traditional hydroxyl radical-based advanced oxidation processes have been studied for treatment of wastewaters over the past three decades. The major purpose of HR-AOP is to remove recalcitrant organic matters, traceable emerging contaminants, in addition to certain inorganic pollutants. The treatment efficiency relies primarily on the AOP types, physical/chemical properties of target pollutants, and operational conditions. Recently, SR-AOP has also attracted attention for wastewater treatment. Sulfate radicals have a similar strong oxidative capacity and a short lifespan but different reaction patterns from hydroxyl radicals. For example, SR-AOPs are able to readily oxidize ammonia nitrogen in wastewater, which can be rarely removed by HR-AOPs. Particularly, AOPs for treatment of landfill leachate and EfOM in BTSE have been reviewed. Previous studies have demonstrated that AOPs are a technically viable option for leachate treatment and water reuse. In the future research, the development of cost-effective AOPs needs to be investigated.
Manganese deposits are also known to cause problems in drinking water systems, stemming from increased tuberculation in pipes and coating development in concrete tanks . Manganese can deposit on other surfaces such as filter media . In the distribution system, Mn deposits can be formed by either chemical or microbial oxidation, depending on numerous water quality parameters. These particles can impact a dark color to the water and may lead to noticeable amounts of discrete particles in delivered water . This process can occur when soluble Mn concentrations are greater than 0.02 mg/L . A finished water Mn concentration below 0.02 mg/L is a common treatment goal for preventing chronic aesthetic and operational problems associated with manganese.
Historically, the United States Environmental Protection Agency (USEPA) has not established an enforceable health-based standard for the allowable Mn concentration in drinking water. Rather, the USEPA publishes a non-enforceable secondary maximum contaminant level (SMCL) of 0.05 mg/L , with the goal of limiting aesthetic problems. Regular monitoring of Mn is not required under the SMCL. The USEPA conducted a health-based assessment of manganese and established a lifetime health advisory level of 0.3 mg/L for Mn . US states may establish regulations that are stricter than the federal standards. The State of California has a notification level, a health-based advisory level for chemicals in drinking water that do not have a maximum contaminant level, of 0.5 mg/L for Mn . When concentrations exceed the notification level, certain requirements and recommendations apply to all public water systems. More recently, the EPA has placed Mn on the Fourth Draft Contaminant Candidate List (CCL4) to examine the potential impact of enforceable regulation. The EPA could promulgate an enforceable maximum contaminant limit (MCL) for Mn, which would require some utilities to change their Mn management approach. The World Health Organization (WHO) has not set health-based guideline value for allowable Mn concentration, largely because of no health concerns at Mn levels that are likely to cause acceptability (aesthetic) problems, and thus result in treatment or use of another source . Health Canada is currently considering a maximum acceptable concentration (MAC) for Mn concentrations in drinking water.
Particulate Mn can be removed by a range of appropriate particle separation processes. Truly dissolved Mn is almost entirely in the reduced Mn(II) (or Mn2+) form and can be directly removed by physical/chemical processes for dissolved cation removal, or the Mn(II) is oxidized to insoluble Mn(III) and/or Mn (IV) forms for removal in particulate form. An important process combines sorption of dissolved Mn(II), surface catalyzed oxidation to Mn(IV), and ultimate removal in particulate form. Another important removal process is based on uptake of dissolved Mn(II) by media support biofilm, microbially mediated oxidation to Mn(IV), and also ultimate removal in particulate form via backwashing. The following sections provide more detail and review of recent publications addressing the various processes for Mn removal. At the end of the paper, comments on the impacts of co-occurring constituents and the integration of Mn removal within a treatment plant are provided.
Mn particles in source waters or Mn particles formed via direct oxidation may be removed by conventional water treatment processes, such as clarification and media filtration. In either process, Mn particles must be effectively destabilized in order to allow for the particle aggregation or attachment needed for effective process performance [21, 28, 29]. One less common treatment scenario that sometimes illustrates this need is the use of intermediate ozonation following coagulation, flocculation, and clarification but prior to media filtration. Source water dissolved Mn(II) may be oxidized by the intermediate ozonation, creating stable (negatively charged) colloidal manganese that may not be effectively removed by media filtration; destabilization by addition of a pre-filter low dose of a cationic coagulant is necessary and effective.
Biological treatment via media supported biofilm (often called biofiltration) is another viable removal option for Mn; an advantage of this method is that typically there is no, or very little, chemical addition required. There are three known mechanisms by which microorganisms can remove dissolved Mn from water. The first is direct intracellular oxidation of Mn as part of the metabolic pathway of manganese-oxidizing organisms (MOO). Manganese-oxidizing organisms use Mn(II) as an electron donor and oxygen as an electron acceptor . Bacteria genera that perform Mn oxidation often include Leptothrix, Crenothrix, Hyphomicrobium, Siderocapsa, Siderocystic, Metallogenium , and Bacillus . Some specific species include Leptothrix discophora , and Pseudomonas manganoxidans. The second Mn removal mechanism by MOO is extracellular adsorption. Mn(II) can adsorb to negatively charged extracellular polymer substances (EPS) . Biogenic oxides such as γ-Mn3O4 that are generated from bacterial catalytic reactions can also adsorb Mn(II) . The third mechanism is catalysis of Mn(II) oxidation by biopolymers generated by microorganisms . 2b1af7f3a8