THERMAL TREATMENT OF INDUSTRIAL WASTEWATER

The paper provides an overview on the major methods of thermal wastewater treatment in the power industry. Here, we present the main advantages and disadvantages of methods based on the concentration of inorganic substances (evaporation or distillation) or the burning of organic compounds (combustion neutralization). The study suggests the possible future directions for the development of thermal wastewater treatment.


Introduction
To date, there is a constant increase in production capacity in various sectors of world industry. This leads to more intensive environmental pollution. Nowadays, anthropogenic impact on the environment, including the hydrosphere, is not compensated by biosphere cleaning processes to the necessary extent. The problem of the rational use of water and its cleaning from various contaminants is becoming more and more urgent under modern conditions [1][2][3].
Wastewater contaminants are characterized by considerable variety. Moreover, the composition and concentration of such contaminants is dependent mainly on the technological process. Therefore, wastewater treatment methods vary.
The study [4] suggested that one of the major industries in Russia, which is characterized by a significant amount of wastewater discharge (about 50% of all discharges), is the power industry. Power plant wastewater contain calcium and magnesium carbonates, ash, soot, corrosive acids, sulfates, ammonia, hydrazine, arsenic, vanadium, oil, petroleum products, and etc. In particular, it is possible to remove these impurities by thermal wastewater treatment. The main arguments in favor of using thermal wastewater treatment are the ease of its implementation and high efficiency.
The aim of this paper is to analyze the current stage of the development of thermal wastewater treatment in the power industry.

RESULTS AND DISCUSSION
Among the major methods of thermal wastewater treatment is the evaporation of inorganic substances. This is one of the simplest methods to implement. Also, along with its simplicity, this method is characterized by high removal efficiency, high productivity and relatively low-cost technology.
The process of evaporation is used for different purposes -desalination, the separation of mixtures, the concentration of solutions, and etc. The method involves the heating of water with   Large size, design complexity, the possibility of mixing a heat transfer agent and a solution.
Large size, high material consumption, the need to maintain a vacuum in the system. Among methods of thermal wastewater treatment from oil and other flammable substances, there is combustion neutralization [6,7]. The method is as follows: polluted water is sprayed into the flow of combustion products at the temperature 900÷1000 0 ɋ. During the reaction, organic substances are burnt, while incombustible impurities precipitate or emit with exhaust gases. Vapor, generated as a result of water evaporation, is removed from a combustion chamber and condenses. It should be noted that this method of treatment is also used for the neutralization of toxic wastewater. Fig. 2 shows a diagram of a plant with heat recovery for wastewater combustion neutralization. The advantages of wastewater combustion neutralization include high efficiency and the simplicity of operation. However, along with these advantages, there are some disadvantages of the method, which should be considered: the need for using additional fuel, large size installations.
Reducing costs for the implementation of evaporative and combustion wastewater treatment and for increasing their efficiency would certainly widen the scope of their application. This would allow them to replace more expensive and complicated methods of wastewater treatment. It seems appropriate to study heat and mass transfer processes, occurring at the implementation of these technologies. In particular, it is possible to conduct research using complex software and hardware systems, specialized data processing algorithms, panoramic methods and numerical models [8][9][10].

Conclusion
This work presented the overview of the major methods of thermal wastewater treatment. Results proved that there are some advantages, disadvantages and difficulties in the implementation of these methods in practice. The information about identified disadvantages may help when choosing the direction of further research to improve the effectiveness of wastewater treatment in industry.
The study was financially supported by the Ministry of Education and Science of the Russian Federation (the scholarship of the President of the Russian Federation for young scientists and graduate students SP-1350.2015.1). The analysis of the effectiveness of the evaporative method of wastewater treatment was funded by the grant of the Russian Foundation for Basic Research (project ʋ 14-08-00057).