Chemical industry produces several types of wastes. Parts of these wastes have been already treated, but certain process wastewater (PWW), after appropriate treatment, should be recycled and reused according to the principle of circular economy. In most cases, the treatment must be developed individually, respectively, according to the composition of waste. Special treatment/cleaning of chemical equipment must be completed regularly in the fine chemical factories and electronic industry; therefore, large amount of wastewater is generated. Typical example of it is the high content of organic compounds, in most cases with surfactant materials. Before such wastewater can be discharged to the sewage plants, it must be treated in some ways to decrease its organic content under the emission limit. Among the possible treatment concepts, physicochemical approaches got into the focus of interest lately. Approaches like these offer relatively small environmental impact, and the polluting organic substances can be recycled and/or reused.
The high-contaminant saturated washing water is collected in containers in a fine chemical company. The less-polluted flush water goes into pH neutralizer, and then, it is allowed to run into the public sewer. The liquid waste containing higher surfactant material causes serious environmental problem to the companies, because its chemical oxygen demand (COD) value is usually high above the sewer limit, which is 1000 mgO2/L (28/2004. (XII. 25.) Ministry of Environment Regulation). The aim of this study is to develop a method to reduce the COD value of process wastewater under 1000 mgO2/L and to reuse it from the beginning to the end of the process if it is possible.
More methods are available in the topic of reducing the surfactant material concentration of process wastewater and to treat the liquid waste of fine chemical industry (Kowalska et al. 2005; Moreira et al. 2017; Wang et al. 2009). The decrease of COD value of wastewater with polysulfone content was over 85% with ultrafiltration process in case study of Kowalska et al. (2005). Wang et al. (2009) describing the removal efficiency of COD in the treatment of simulated laundry wastewater using electrocoagulation/electroflotation technology. The experimental results showed that the removal efficiency was 62%, when ultrasound was applied to the electrocoagulation cell. Abdelmoez et al. (2013) introduce an integrated method for detergent-contained car wash wastewater consisting of coagulation, flocculation, settling, oxidation and sand filtration. The initial 1430 mgO2/L COD value can be reduced under 200 mgO2/L, which means 86% reduction. Busetti et al. (2015) demonstrates an effective reverse osmosis (RO) and UV combination treatment over 90% removal effectiveness for complex wastewater, which contains benzotriazole and galaxolidone detergents.
Several physicochemical methods are suitable for treating PWW, which primarily remove the organic solvents, surfactant materials and reduce the COD (Koczka and Mizsey 2010). The selection of these methods depends on many factors, such as local conditions, economic parameters, environmental laws, composition of the process wastewater and the pollutant(s) (Toth 2015). The main physicochemical methods are stripping, absorption, adsorption, ion exchange, extraction, wet oxidation, distillation, evaporation and membrane processes. In this study, the last three treatments were examined.
The distillation of organic compounds can reduce significantly the COD of processed wastewater. The distillation can be performed in discontinuous (batch) and continuous mode. There were two factors to consider: the quantity of the material and the need for a stripping column section. A batch distillation is suitable for the separation of small amounts and in the case of feed with frequently changing characteristics. Batch distillation can be realized if total column is rectified or stripped. (Toth 2015).
Nowadays, volatilizing large amount of water with evaporation is a realistic option; therefore, only a small amount of waste needs to be treated, for example incinerated. The increased costs and penalties made this method competitive (Koczka and Mizsey 2010). The evaporation solution for treatment of industrial process wastewaters is really advantageous in the case of PWW, which does not contain volatile compounds (gases, organic solvents) or where the biological purification cannot be executed. The pollutant compounds destroy the biomass, or the high salt content of PWW cannot be broken down by the microbes. Typically, the evaporation can be economical where the PWW is considered as a hazardous waste and it is forbidden to emit it into the public sewer.
The main advantage of PWW treatment with evaporation is the distillate water product, which can be recycled in the technology approaching the ideal case for zero emission. The treatment does not require significant amount of chemicals. In many cases, the evaporator is a compact apparatus, which does not require difficult installation. The energy consumption means the main disadvantage of the evaporation, because of the heat of the water vaporization, which is really high. However, some evaporator apparatuses can reuse the waste heat with the reuse of tired steam and cooling water of gas engine significant savings can be achieved (Bin et al. 2016; Gupta et al. 2012).
The benefits of membrane processes are the high separation efficiency, the flexibility and the energy-efficient operation (Mulder 1996). The application of membrane technology is a realistic option for the treatment of PWWs, because it is suitable for reducing the COD value of PWW, reducing PWW quantity by using hybrid separation technology (Toth et al. 2011), cleaning heavy metals from PWWs (Koczka 2009). Reverse osmosis belongs to the group of pressure-driven membrane processes, where the driving force is the transmembrane pressure between the two sides of the membrane. High-quality product can be resulted by RO as permeate water (Buonomenna 2013; Galambos et al. 2004; Li et al. 2017; Razali et al. 2015). The COD rejection can be calculated by the following equation (Toth 2015):
$${\text{COD}}_{\text{Rejection}} = \left( {1 - \frac{{{\text{COD}}_{{{\text{Distillate}}\,{\text{or}}\,{\text{Permeate}}}} }}{{{\text{COD}}_{\text{Feed}} }}} \right) \times 100 \,[\% ]$$
(1)
Yield (or recovery rate) is calculated according to Eq. (2)
$$Y = \frac{{V_{{{\text{Distillate}}\,{\text{or}}\,{\text{Permeate}}}} }}{{V_{\text{Feed}} }}\,\,[{ - }]$$
(2)
which is the ratio of the volume of the distillate (VD) or permeate (VP) and the volume of the feed solution (VF). Objective function can be defined in order to find the appropriate separation technology: COD rejection and yield have to be maximized and common optimization is necessary. Finally, the economic feature of the selected method is confirmed with cost calculation.