Chemical Oxidation Applications for Industrial Wastewaters 

Industrial pollution control is the most important field of environmental protection. Its importance originates from the impact of wastewaters, and other pollutants, emanating from a variety industrial activities. This impact is mostly through toxic, hazardous components of the wastewaters which demonstrates itself as acute responses e.g. toxicity as well as long-term consequences such as carcinogenicity and bioaccumulation in the environment. Another challenging feature of industrial pollutions is its diversity in terms of both manufacturing processes and the pollutants involved in the waste streams. Micropollutants, a most characteristic outcome of industrial pollution, are the compounds with which the nature cannot deal and mitigate their impact to prevent excessive damage on natural processes of all kinds. Therefore, industrial pollution control and treatment, mostly aims to either totally destroy or convert the micropollutants and other toxic pollutants into such forms that the nature can cope with them properly. Within this context, one of the most effective tools for the treatment and control of toxic pollutants is chemical oxidation.

The primary function of chemical oxidation is to destroy a pollutant e.g. mineralization of an organic matter or to modify its structure so as to be accommodated by conventional treatment methods or natural processes. That is why the role of chemical oxidation in the control of industrial wastewaters has become increasingly important. One of the advantages of chemical oxidation is its almost innumerable modes of operation as far as the oxidants, combination of methods and environmental conditions are concerned. As a result of these chemical oxidation applications to industrial wastewaters has long been a focus of research giving rise to a significant accumulation of knowledge and literature.

Content Table

Textile Industry

The extreme complexity and variety of textile preparation, dyeing and finishing industry wastewater make the overall evaluation and global unification of treatability studies devoted to the effluent originating from this huge sector rather difficult. For instance, operating costs of electrochemical, photochemical, photocatalytic treatment systems as well as processes where oxidant production are mainly based on the use of electrical energy (ozone, electron beam irradiation, sonolysis, etc.) are mainly assessed with respect to electrical energy consumption calculations. On the other hand, in solar irradiation applications the costs of the collector material per unit area may play a decisive role in determination of investment costs. In other applications (wet air oxidation, wet peroxidation, supercritical water oxidation, or catalytic wet air oxidation) capital investments are relatively high; reactor materials and associated equipments (pressure-tight valves, high pressure pumps, heating devices, etc.) are mainly contributing to the overall costs, but energy self-sustainable systems can be operated below a certain calorific value provided by high-strength waste streams. The colour parameter is one of the major concerns of dyehouse effluent due to ecotoxicological risks imposed by its metabolites in the discharged environment and for aesthetic reasons. All the above exemplified chemical oxidation processes are quite effective in the removal of colour from dyehouse effluent. In most treatment cases complete oxidation cannot be achieved under technically/economically feasible treatment conditions. It can be concluded and in particular the more energy (cost)-intensive advanced treatment process should be applied in combination with conventional ones for pre- or post- treatment purposes. Depending on the chemical oxidation process employed, more recalcitrant and/or toxic oxidation intermediates or end products can be formed during treatment of textile industry wastewater. For instance, investigators of electrochemical treatment processes have reported the formation of active chlorine species as well as organically-bound organic halogens (AOX) that increase the toxicity of the treated wastewater to levels being more detrimental than that of the original dyehouse effluent not being subjected to chemical oxidation. Considering the fact that the main purpose of chemical oxidation is to reduce the toxicity and inertness of textile industry wastewater, it is important to verify in preliminary treatability experiments under what chemical reaction conditions potentially toxic oxidation intermediates build up in the reaction solution. Most of the emerging, advanced chemical oxidation processes are still not affordable in large scale for textile industry wastewater treatment; in particular no application exists for treatment of real dyehouse effluent via supercritical oxidation or sonolysis. Studies devoted to these advanced oxidation processes are limited to aqueous dye solutions. More efficient means of cavitational energy production from different sources and the development of more efficient heterogeneous catalysts that promote the application under milder, e.g. sub-critical conditions could be encouraged in future studies.

Leather Tanning Industry

The leather tanning industry is of a considerable pollution load in terms of both organic and toxic parameters. Water pollution control in the industry can be best managed by segregation of waste streams into three main flows: sulphide containing flows, chrome containing flows and other waste flows. Evaluation of other in-plant control methods together with newly developed process modifications has also been made. Control of reduced sulphide species in the segregated waste stream has been carried out by chemical oxidation. Catalytic air oxidation, which has been the most common method, has been evaluated in terms of theoretical and practical aspects. A critical assessment of design and operation parameters has been made. Use of other oxidants was also evaluated. Hydrogen peroxide was suggested for use as a polishing step. Among other oxidation applications reviewed electrooxidation and and peroxymonosulphate oxidation have been evaluated as promising methods. Other chemical oxidation applications to combined raw, mechanically treated and biologically treated effluents were also critically evaluated in terms of enhancement of biodegradability, reduction of toxicity, colour removal and reduction biologically inert COD. Electrochemical oxidation methods applied to pretreated wastewater were evaluated as promising technologies. Ozone was determined to preferentially oxidize biodegradable COD and suggested for use within or after biological treatment. The assessment of advanced oxidation methods indicated their high potential of application to secondary effluent as a polishing step.

Metal Finishing Industry

The metal finishing industry is one of the largest industrial activities in the world using a wide range of chemicals. If not properly managed, the chemicals used in the metal finishing operations may adversely impact public health and the environment. The best management policy encompasses information efficient raw material, energy and water usage; (ii) the substitution by less harmful chemical in the production; and (iii) minimisation, recovery and recycling of wastewater and wastes. Capital, operation and maintenance costs as well as achievable treatment levels are important key factors for wastewater and waste minimisation, recovery and recycling implementations. When these factors are considered, stream segregation seems to be the most feasible approach in the treatment of metal finishing industry wastewaters with different character in the view of maximisation of removal performance and reduction of treatment cost. Within this context, the application of oxidation/reduction techniques to stream segregated flows of the metal finishing industry can be evaluated as follows.        

Oxidation/reduction techniques used in the treatment of cyanide, hexavalent chromium and precious metals bearing wastewaters are well-established and straightforward methods. Among them, alkaline chlorination for the destruction of cyanide, chemical reduction by reduced sulphur compounds for reduction of hexavalent chromium and electrolytic metal recovery for precious metals are the most widely used treatment methods in the metal finishing industry facilities because of moderately low costs. When properly applied, satisfactory removal efficiencies ensuring discharge standards can be satisfied by these methods. Some oxidation/reduction techniques, particularly those used in the treatment of complexed metals, are sensitive to reaction conditions and their misapplications result in unsatisfactory pollutant removal efficiencies. Some methods yield partial oxidation products of pollutants or certain by-products such as cyanate, ammonia, nitrate and sulphate which necessitate an additional treatment step prior to discharge to receiving media. Some processes are expensive to operate and/or energy-intensive methods and hence they are considered economically applicable only for the source-based treatment or for the treatment of small wastewater volumes. Some techniques such as photochemical and photocatalytic oxidations required specific reactor designs. Application of some techniques such as electrocoagulation produces treatment sludge and brings on an additional sludge treatment costs. Therefore, optimisation of operation conditions is a crucial step to minimise sludge production and operating costs as well as to maximise process performance. Besides these limitations and economical constrains, all oxidation/reduction techniques introduced in this chapter of the book are effective treatment methods in the removal of specific waste groups of the metal finishing industry under the defined operation conditions.

Pharmaceutical Industry

The occurrence and fate of pharmaceuticals in the environment and particularly in aquatic media have received considerable attention by scientists during the last two decades. Although they appear at relatively low concentrations, they may impose serious negative effects on water quality. Pharmaceutical processes use numerous raw materials and generate significant wastes and emissions of considerable variability and complexity.  However, compared to other industries, production and process profiles are relatively well established and environmental management procedures have been standardised. In designing bulk manufacturing processes special consideration has been given to the availability of the starting materials and their toxicity as well as wastes (mother liquors, filter residues, by-products, cleaning and rinse water) and emissions generated. Due to purity concerns, solvents that are considered as the most toxic and/or hazardous compounds of pharmaceutical manufacturing activities are often not reused but sold for non-pharmaceutical uses, fuel blending, recycled or destroyed via incineration. Specific chemicals produced in pharmaceutical manufacturing processes enter aquatic and terrestrial environments mainly through non-point sources and different routes via discharge of industrial, agricultural and domestic wastewater. A considerable scientific literature exists in the treatability of different pharmaceuticals in aqueous solutions or real pharmaceutical wastewater, however scientific reports have focused on the chemical oxidation of aqueous model pollutants (analgesics, antibiotics, hormones, etc.) being typical for this industry. Consequently, it becomes very difficult to draw general information from the available data about the merits of chemical oxidation processes for the treatment of actual pharmaceutical wastewater. Recent treatability studies have demonstrated that most pharmaceutical formulations can be efficiently treated with emerging processes including AOP and ozonation. From the practical point of view, treatment-at-source may be a more realistic option at drinking water plants where ground and surface waters can be chemically oxidised to achieve destruction of pharmaceuticals together with other micropollutants as well as at pharmaceuticals manufacturing plants where formulation effluents are generated.

Pulp and Paper Industry

Pulp and paper industry is a globally growing industry that consumes a significant amount of natural resources, raw materials and energy. Due to this fact, minimising the environmental impact of this sector is important. The characteristics of the wastewaters generated from various processes of the pulp and paper industry depend upon the types of processes and wood materials, process technology applied, management practices, internal recirculation of the effluent for recovery and the amount of water being used in a particular process. The majority of pollutants released in pulp and paper industry originate from the pulping and bleaching stages. The high organic content of pulping wastewater, coupled with the presence of chlorine, results in the production of many highly toxic chlorinated organic compounds. Of prime concern are chlorinated phenols, guaiacols, catechols, furans, dioxins, aliphatic hydrocarbons. Some members of this family are known to be toxic, mutagenic, persistent, and bio-accumulating and are thought to cause numerous harmful disturbances in biological systems, and pose a human health risk through long-term exposure via drinking water and through bioaccumulation along the food chain. End-of-pipe treatment of wastewaters can be accomplished by integration of traditional biological treatment processes with chemical/advanced oxidation applications. Chemical oxidation technologies have revealed that the applied processes are effective and promising applications for the treatment of pulp and paper industry wastewaters. Ozonation is efficient in removing COD, TOC and colour as well as increasing the biodegradability of the wastewater in many cases. However, it is rather an expensive process. Wet oxidation in its modifications (catalytic wet oxidation) is an effective technology in reducing the COD content of the wastewater and enhancing the biodegradability. AOPs such as Fenton, photo-Fenton, H2O2/UV-C proved to be effective when they are applied as tertiary treatment process. Electrochemical methods proved to be an efficient treatment option for the treatment of pulp and paper industry wastewaters. Combinations of two or more physicochemical processes can be used for the enhancement of removal efficiencies. Currently, the most important task seems to be the assessment of the operating conditions of chemical oxidation processes with regard to specific use of chemical oxidation and its way of coupling with biological treatment.

Pesticide Industry

The pesticide industry is an important part of the world economy. In pesticide industry, the manufactured pesticides are complicated organic molecules and their production requires the operation of sophisticated chemical processes and due to this sophisticated process, pesticide industry wastewaters contain a variety of pollutants. It is not possible setting one generic wastewater discharge characteristics for all pesticide manufacturing processes because of the differences in wastewater quality generated. Chemical oxidation processes such as ozonation, ozone-based AOPs, H2O2/UV, Fenton, and Fenton-based AOPs etc. are used as a pretreatment stage to enhance biodegradability and reduce toxicity, prior to biological treatment in pesticide industry and these technologies are found to be effective. Biodegradability of pesticide industry wastewaters can be improved, and their phytotoxicity can be reduced after the application of AOPs. An important aspect in the chemical oxidation applications to pesticide industry wastewater is to determine to extent of oxidation in a way that while the toxicity is minimised to keep an adequate level of organic matter to support biological treatment. Beyond the technical feasibility, this point is also important in terms of economical point of view.

Gold and Silver Mining

Many real gold mines processing large pieces are no longer available all over the world. In consequence of this exhaustion, gold has to be mostly beneficiated from their low-grade ores using the extracting techniques. One of the main obstacles in extracting gold or silver from their low-grade ores is the separation from the gangue. In order to separate gold or silver with high recovery efficiencies, cyanidation has been still the most common applied leaching technique, all over the world, since 1890 owing to its inherent advantages on recovery process. Additionally, flotation process applied for the recovery of base metals such as copper, lead, and zinc, is accomplished using low concentrations of cyanide. Thus, these operations realised with either low or high cyanide concentration are addressed potential toxicity issues in regard to the ecologic system surrounding the mine sites as well as human life. This potential hazard to environment has also been exactly pointed out the catastrophic failures of some tailings impoundments resulting in their mismanagement operations or their faulty designs. During the last decade, this serious environmental concern has been triggered efforts on the replacement of cyanide with less hazardous lixiviant in gold and silver mining facilities. Widespread research has been undertaken for the identification and development of less toxic lixiviants. Results obtained from these studies performed at laboratory or full scale demonstrated that leaching with thiourea, thiosulphate, thiocyanate, and halides can replace with cyanidation but there are some limitations hindering their widespread adaptation in gold mining operations. For instance, in spite of their effectiveness, thiourea and thiosulphate are unstable and exhibit low recycle efficiencies. Additionally, their detoxification costs are relatively expensive. Halides are expensive and difficult to handle and control. Recovery with thiocyanate is still under research. The patented coal-oil agglomeration process seems to be replaced with mercury amalgamation as it is only suited to process free gold particles. In view of these findings, cyanide will still continue to be used as the lixiviant in gold and silver mining sites unless a plausible, benign and environmentally friendly non-cyanide alternative is found out and proved with all aspects including occupational safety, environmental safety, availability, selectivity, recyclability, detoxifiability, and feasibility at a large scale. Within this framework, rigorous adherence to the best management practices for cyanidation process in the gold and silver mining sites is crucial.

Chemicals Industry

Chemical oxidation applications are used for the control of chemical industry wastewaters. Organic chemicals manufacturing industry produces wastewaters having high organic matter content which is for the most part biodegradable and can be handled by biological treatment, however, the main pollution control problem lies in the micropollutant content of wastewater. The control of micropollutants used to rely on polishing treatment methods such as adsorption that are employed in the end-of-pipe treatment plant and mostly following biological treatment. The new approach is the control of micropollutants at the source by the use of pretreatment technologies as an integrated part of manufacturing processes. This new approach has been reflected in the evaluation of wastewater control strategies of the industry. The emphasis has been placed upon the control of micropollutants at the source by the application of chemical oxidation methods. Wet oxidation application in its various modification as well as newly developed and quite effective reductive dechlorination applications have been assessed as far as targeted pollutants and operation methodologies are concerned. Among the other chemical oxidation methods the two promising applications; Fenton’s oxidation and electrochemical oxidation, particularly by the use of newly developed quite effective electrode materials, have been critically evaluated.

Other Industries and Municipal Landfill Leachate

Alcohol distilleries

Alcohol distilleries, a common process, worldwide, produce concentrated wastewaters containing only organic matter as a pollutant. This waste stream needs to be pretreated to remove hardly biodegradable polyphenol content and enhancement of biodegradability, prior to aerobic or anaerobic biological treatment. Chemical oxidation processes have been exclusively employed for the pretreatment. The promising technologies, ozonation which removes polyphenols selectively, advanced oxidation processes and electrooxidation have been evaluated regarding to their effectiveness and application bases. Electrocoagulation process with its very high organic matter removal performance seemed to serve not only as a pretreatment but functioning as a part of end-of-pipe treatment.

Olive Oil Industry

The olive oil industry presents a crucial sector for many Mediterranean Countries that account for almost 95% of the olive oil production worldwide. Olive mill effluent constitutes a serious water pollution problem, due to the unique features associated with this type of agro-wastewater, namely seasonal and localized production and its polyphenolic, difficult-to-degrade, high organic carbon content. Olive mill wastewater is considered as eco-toxicologically dangerous affecting all biota in water and soil; its dark, brownish colour inhibits the absorption of sunlight utilised by photosynthetically growing organisms and also forms a film layer on the water surface thus preventing oxygen transport from air to water. Different physical, chemical and biological processes have so far been proposed for olive mill effluent treatment. However, none of them have proven to be successful and therefore advanced treatment combinations are currently being investigated. Among them, Advanced Oxidation Processes (AOPs) have received considerable attention in the past few years including ozonation, wet air oxidation, electrolysis, Fenton, Photo-Fenton as well as TiO2-mediated heterogeneous photocatalytic treatment were intensively investigated for complete toxicity and phenolics removal accompanied with partial oxidation of organic matter. Moreover, attempts have been made to integrate AOPs with biological processes to enhance the overall treatment efficiency and decrease operating costs associated with AOPs.

Municipal landfill leachate

Municipal landfill leachate is a liquid waste percolating through or emerging from solid waste. It can be collected using several systems and contains soluble, suspended, or miscible constituents removed from solid wastes. Biological processes including aerobic and anaerobic systems are the most common treatment methods used to treat landfill leachate with high organic matter contents. The combinations of biological processes with physical and chemical treatment methods are also widely applied to landfill leachates. Among them physico-chemical treatment techniques, oxidation methods such as Fenton, photo-Fenton and other advanced oxidation processes are capable of enhancing the abatement of recalcitrant organic molecules or converting them into more readily biodegradable forms in old and biologically treated landfill leachates. Recently, electrochemical techniques proved to be very efficient in reducing organic matter and colour and oxidising ammonia from raw and pretreated landfill leachate.

Resources

The issues in this article are addressed in the book, Chemical Oxidation Applications for Industrial Wastewaters by Olcay Tunay, Isik Kabdasli, Idil Arslan-Alaton and Tugba Olmez-Hanci

The book aims at presenting and discussing the applications of chemical oxidation processes for the treatment of industrial wastewaters. All important industrial activities have been covered. For all industry categories the presentation followed two main sections. In the first section general information about the manufacturing processes, wastewater sources and characterization as well as conventional treatment systems has been given. In the second section applicable chemical oxidation processes have been presented with regard to their place of use, their application basis and cost. A short review of the processes and concluding remarks has been provided at the end of each chapter. Over one thousand relevant and up-to-date literature has been cited.

References

CHEMICAL OXIDATION APPLICATIONS FOR INDUSTRIAL WASTEWATERS (IWA Publishing 2010)

Prof. Dr. Olcay Tünay, Prof. Dr. Işık Kabdaşlı, Prof. Dr. Idil Arslan-Alaton, Assoc. Prof. Dr. Tuğba Ölmez-Hancı

İstanbul Technical University, Civil Engineering Faculty, Environmental Engineering Department, Ayazağa Campus, 34469, İstanbul, Turkey.

(kabdasli@itu.edu.trtunayol@itu.edu.trarslanid@itu.edu.trtolmez@itu.edu.tr)

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