Treatment of Wastewater from the Foundry Industry Using Fenton Process

The increase in automation and intensification of production processes and the reduction of the amount of water used in these processes leads to a significant increase of pollutants in industrial wastewater. Bearing in mind the progressive degradation of the natural environment, there is a need to limit the negative impact of industry on various elements of the environment, including on surface waters through the use of effective methods of wastewater treatment. This situation applies to various industries, including the foundry industry.

Important environmental aspects of the non-ferrous metal foundry industry include the emission of organic pollutants from pyrolysis and thermal decomposition of binders, protective coatings, etc. (including phenol, formaldehyde, amines, hydrogen cyanide, polycyclic aromatic hydrocarbons – PAHs, benzene, and Volatile Organic Compounds – VOC), as well as the generation of wastewater, e.g. from wet dedusting, and – in the case of pressure casting – transport of hydraulic fluids or coolant to the water circuit in an industrial plant. Significant amounts of rinse water are also generated, used baths containing high levels of resins, as well as wastewater from technological equipment containing worked hydraulic fluids, coolants, release agents and lubricants for casting moulds [1, 2]. The composition of many preparations used in production processes is subject to patent protection of the manufacturer, which is why it is often difficult to determine the exact qualitative composition of the wastewater flowing into the treatment plant when it contains preparations mentioned above.

Industrial wastewater treatment is based on biological, physical, chemical and physicochemical methods. The use of individual methods or their configuration depends on the qualitative and quantitative characteristics of the supplied wastewater and the technical and economic capabilities of the unit dealing with their treatment. Due to the diverse composition of wastewater and their high variability in time, it is often necessary to use several methods. Conventional methods, which are simple to implement and economically efficient, such as straining, sedimentation, flotation, precipitation or coagulation, are not as efficient and allow only for the transfer of pollutants from the liquid phase (wastewater) to another, e.g. solid (deposits). They are often characterized by low efficiency in reducing the concentration of dissolved organic pollutants, in the case of which the use of biological methods is not possible due to the coexistence of a large amount of non-biodegradable or toxic fractions. The solution to the above-mentioned disadvantages may be the inclusion of alternative methods in the wastewater treatment plant line, such as advanced oxidation processes (AOPs). These methods, as a result of the generation of OH• highly reactive hydroxyl radicals, which react with almost all organic substances, allow for their degradation, also in the case of toxic and biodegradable-resistant pollutants. The hydroxyl radical undergoes chemical reactions with almost all organic pollutants, as well as with many inorganic substances, without producing harmful by-products [3–10].

One of the most common and effective methods for producing hydroxyl radicals is a mixture of hydrogen peroxide (H₂O₂) and iron salts (II), i.e. Fenton’s reagent [11]. An additional advantage of the Fenton process is the simultaneous removal of pollutants contained in the treated solution by means of coagulation, caused by the presence of Fe³⁺ and precipitation (OH^–+ Fe³⁺) [4, 7, 12].

The classic Fenton process is the subject of many studies on the possibility of using it to eliminate individual organic substances, as well as to treat real wastewater from various industries. For example, in [13] research was conducted on the treatment of effluent collected from the municipal waste landfill in Kozodrza near Rzeszów that was subjected to reverse osmosis. This effluent had a pH of 7.5 and COD at the level of approx. 7500 mg/L. The wastewater pH was not corrected in these tests. The highest level of COD reduction was obtained using the Fe²⁺/H₂O₂ of 0.07 (approx. 85%). In turn, research on the use of Fenton's reagent for the treatment of concentrated wastewater from the production of maleic anhydride presented in [14] showed approx. 91% reduction of wastewater COD with the following parameters: (pH = 3.0, Fe²⁺/H₂O₂ = 0.33, process time 120 min, 5% CaO solution to pH 7.0). Fenton’s reaction has also found its application in the treatment of wastewater from the textile industry [15]. Studies have documented the effect of color removal exceeding 97%, as well as a significant reduction in COD (approx. 90%) for wastewater containing five types of dyes (suspension, acid, basic, direct and reactive) that are most commonly used in the textile industry. The selection of appropriate parameters of the Fenton reaction also allows for a significant reduction in the concentration of surfactants in the solution [16]. For example, for reaction parameters of 60 mg H₂O₂/L, 90 mg FeSO₄/L, pH 3 and 50 min of process time, over 95% reduction in the concentration of two anionic surfactants – alkylbenzene sulfonate (ABS) and linear alkylbenzene sulfonate (LAS) – was achieved.

The aim of the research undertaken as part of this work was to assess the possibility of using the classic Fenton reaction in the treatment of real wastewater from the foundry industry, after their initial pretreatment in the coagulation process.

The selection of process conditions of the Fenton process was carried out using a sample of coagulated wastewater, without the addition of used baths. Firstly, for doses of H₂O₂ of 1 or 5 g/L of hydrogen dioxide, 120 min of process time and constant Fe²⁺/H₂O₂ mass ratio of 0.3, the reaction pH was selected by testing the pH range from 2.0 to 4.0. After stopping the Fenton reaction and sedimentation of the resulting sediments, COD analysis was performed in the resulting decantates, because this parameter was taken as a measure of process efficiency. The results of these analyses are shown in Figure 1.

Figure 1.

The highest efficiency of COD reduction for each of the applied hydrogen peroxide doses was obtained at the initial acidification of wastewater to pH 2.5. However, the effectiveness of the process clearly depended on the H₂O₂ dose. After H₂O₂ 1 g/L dose, COD value decreased by approx. 11% and after a dose of 5 g/L by approx. 51%.

Subsequently, the dose of hydrogen dioxide was selected. The tests were carried out using a wastewater pH selected on the basis of the results from stage I, i.e. pH of 2.5. The dose of H₂O₂ was selected using the most effective dose from Stage I, i.e. 5 g/L and 4 lower doses (4.00, 4.25, 4.50 and 4.75 g/L) and 3 higher (5.25, 5.50, 5.75 g/L). The process time was 120 min and the mass ratio Fe²⁺/H₂O₂ was 0.3. The test results are shown in Figure 2.

Figure 2.

The highest increase rate in process efficiency was recorded in the H₂O₂ dose range from 4.00 to 5.00 g/L. The dose of H₂O₂ 5.00 g/L allowed to obtain approx. 51% reduction in COD, which was already determined in stage I. The use of higher doses of H₂O₂ resulted in a further increase in the effectiveness of COD reduction, but with a lower intensity. The application of H₂O₂ dose of 5.75 g/L allowed for the reduction of COD by approx. 53%. Comparing this result with the one obtained for the dose of 5.00 g/L, it can be concluded that increasing the dose above this value is not justified. Therefore, the best dose was H₂O₂ 5.00 g/L.

The selection of Fe²⁺/H₂O₂ mass ratio was determined using the reaction pH selected in Stage I, i.e. pH 2.5, and the dose of H₂O₂ selected in Stage II, i.e. 5.00 g of H₂O₂/L. For the process parameters listed above, Fe²⁺/H₂O₂ mass values of 0.1, 0.2, 0.3, 0.4 and 0.5 were used. The process time was 120 min. The results are shown in Figure 3.

Figure 3.

The highest COD reduction efficiency was obtained for the mass ratio Fe²⁺/H₂O₂ of 0.3, which was already assumed in stages I and II. A change in the value of this parameter is therefore not required.

Fenton reaction time was determined using the reaction pH selected in stage I, i.e. pH 2.5, the dose of H₂O₂ selected in stage II, i.e. 5.00 g H₂O₂/L and the mass ratio Fe²⁺/H₂O₂ selected in stage III, i.e. 0.3. The process efficiency was tested for these parameters using different reaction times of 45, 60, 120 and 180 min respectively. The results are shown in Fig. 4.

Also in this case, the process time of 120 min, already tested in stages I-III, turned out to be sufficient for the successful course of the Fenton reaction (effectiveness of COD reduction by approx. 51%). Shortening the process time resulted in a decrease in its effectiveness. In turn, its increase, e.g. to 180 min, only slightly improved the efficiency of the process (reduction of COD by approx. 52%).

Figure 4.

The determined effective parameters of the Fenton process for the tested wastewater without the addition of used baths from the foundry industry were: wastewater pH 2.5, dose of hydrogen peroxide 5 g/L, mass ratio of Fe²⁺/H₂O₂ of 0.3 and process time 120 min.

It should be added that as a result of the Fenton reaction, a clear increase in the concentration of chloride and sulfate ions in treated wastewater was noted. The comparison was made for a sample of wastewater before and after the treatment process with the selected process parameters (Fig. 5). This phenomenon was the result of introducing these ions into the wastewater together with the reagents included in the Fenton reagent, i.e. sulfuric (VI) acid, which was to acidify the reaction environment, and iron (II) chloride, which was the source of Fe²⁺ ions.

Figure 5.