Industrial sewage treatment plants 

Mechanical and physical methods

Industrial wastewater contains in its composition undissolved parts of various sizes and different types of composition and form of aggregation. Solid impurities with larger dimensions are separated on grates, sieves and belt filters, while suspensions with various degrees of dispersion are removed in flotators.

Larger solid impurities must be removed from raw wastewater, regardless of the adopted treatment technology, due to the safety of equipment operation, while the desirability of removing suspensions depends on the further applied methods of biological wastewater treatment.

Biological methods

Methane fermentation method

It is the basic, biological method of pre-treatment of industrial wastewater to parameters similar to those of municipal wastewater, enabling its further purification using the activated sludge method. As a result of this process, 85% – 95% reduction of organic compounds and 20% – 30% reduction of biogenic compounds is obtained and it is treated as the first stage of biological purification.

The process of methane fermentation carried out by anaerobic bacteria is ideally suited to the decomposition of complex organic compounds, because it proceeds in four basic phases, in which they are gradually reduced to the final products, which are biogas (a mixture of methane and carbon dioxide) and water.

The first stage of methane fermentation is the hydrolysis of complex organic compounds contained in wastewater in dissolved form and in suspensions. Hydrolytic bacteria break down proteins into amino acids, complex carbohydrates into simple sugars, and fats into higher fatty acids. The products of hydrolytic decomposition are used as a substrate for acidogenic bacteria, which in the second stage of biodegradation carry out acidogenesis, resulting in simple organic acids. In the third stage of decomposition, acetateogenesis, these acids are reduced to acetic acid, from which methane, carbon dioxide and water are obtained in the methanogenesis stage. Methane is also obtained, but in a much smaller amount, by reducing carbon dioxide with previously produced hydrogen. The conditions necessary for the proper conduct of the process are the appropriate biological quality of the anaerobic sludge, the process temperature in the range of 36⁰C – 38⁰C and the reaction at the level of 6.8 to 7.6 pH.

Activated sludge method

It is a widely used method of wastewater treatment in municipal wastewater treatment plants. Activated sludge is a complex biocenosis of many groups of microorganisms, from bacteria, through protozoa and worms, to nematodes. They all exist only in the presence of oxygen, so aeration is necessary. Unlike anaerobic sludge used in methane fermentation, aerobic activated sludge is adapted to the decomposition of relatively simple organic compounds that are quickly biodegradable. For this reason, industrial wastewater treatment plants use activated sludge reactors for the final treatment of wastewater in order to obtain the parameters of treated wastewater that allow it to be discharged to the surface water receiver.

A very important issue regarding the technological system in which the oxygen reactor operates is the issue of removing biogenic compounds of nitrogen and phosphorus in it. While the removal of phosphorus compounds from wastewater can be carried out both by biological and chemical methods through precipitation in the form of insoluble phosphates, in the case of biogenic nitrogen compounds, only biological methods are available, consisting in the oxidation of ammonium ion to nitrate ion, which in turn is decomposed under appropriate conditions. to gaseous nitrogen in the denitrification process and thus removed from wastewater. Removal of nitrogen compounds is possible in a system of at least two activated sludge chambers with optimal conditions for this process, i.e. aeration in the nitrification chamber and hypoxic conditions in the denitrification chamber. The effectiveness of the denitrification process also depends on the proper ratio of the concentration of organic compounds defined as COD to the concentration of nitrogen compounds in the wastewater flowing into the denitrification chamber.

Characteristics of industrial wastewater

Industrial wastewater is generated in technological processes involving the processing of various raw materials, during which water is utilized. Particularly high water demand is observed in agro-food processing, and it is in this sector of the economy that the largest quantity of industrial wastewater is produced. These wastewater, containing organic compounds in dissolved and suspended form, originate from processed agricultural raw materials. These include complex carbohydrates, proteins, and fats. The concentration level of these compounds determines the level of wastewater pollution, which can vary significantly. Therefore, the main principle in designing industrial wastewater treatment facilities is the individual selection of the appropriate treatment technology based on the qualitative and quantitative characteristics of the wastewater. Industrial wastewater also exhibits a high variability in composition, even within facilities of the same industry. The concentration of organic pollutants in wastewater, measured by the COD (Chemical Oxygen Demand) indicator, ranges from several thousand mgO2/dm3 in dairy and brewery wastewater to over a hundred thousand mgO2/dm3 in wastewater from biofuel production. Another crucial aspect is the concentration of biogenic nitrogen and phosphorus compounds in industrial wastewater and the technological possibilities for their removal.

It should be emphasized that industrial wastewater containing natural organic compounds, even in very high concentrations, undergoes biological degradation using an appropriate method and technological conditions. Currently, the most effective method for treating such wastewater is anaerobic fermentation, resulting in an 80-90% reduction in organic compounds and a 20-30% reduction in biogenic compounds. This process is considered the primary stage of biological treatment.

Table 1. Degree of pollution of industrial wastewater in relation to the COD concentration in the specified industries.

Industry	Unit	Chemical Oxygen Demand Value
Dairy	mg O2/dm3	3000 – 6000
Sugar		5000 – 15000
Brewery		3000 – 6000
Potatoes		5000 – 15000
Fats		5000 – 8000
Fish processing		5000 – 20000
Juice production		5000 – 15000
Vegetable and fruit processing		7000 – 20000
Poultry and slaughterhouse facilities		5000 – 10000
Distilleries		30000 – 100000
Biofuel production		50000 – 150000
Pharmaceutical industry		10000 – 20000

The presented values of wastewater pollution levels from various industries in the table indicate significant diversity, primarily caused not only by the range of manufactured products but also by water conservation technologies. These technologies typically lead to an increase in the degree of wastewater pollution.

The selection of an optimal industrial wastewater treatment technology

 is a fundamental issue in the investment process. It ensures the effective operation of the entire facility and the achievement of the intended wastewater treatment outcomes. The chosen treatment technology must also be tailored to the investor’s needs regarding the expected degree of wastewater purification. This involves determining whether the wastewater only needs to be partially treated through anaerobic fermentation and further processed in a municipal treatment plant or fully treated to meet specified parameters before being discharged into surface water receivers.

Economic considerations are also crucial both in the investment and operational phases. Achieving effective treatment outcomes is possible through the application of either anaerobic fermentation alone or a two-stage anaerobic-aerobic method. In the first stage, anaerobic fermentation achieves a reduction in organic compounds of approximately 90%, and in the second stage, any remaining organic compounds and biogenic compounds in the pre-fermented wastewater are further removed.

The development of the appropriate technology is dependent on the composition and type of wastewater. This is especially relevant to the presence of suspended solids and fats in the wastewater, as well as compounds that may act as inhibitors to microorganisms, such as disinfectants used for cleaning equipment in the facilities.

At present, two technological solutions are preferred with respect to the operation of the anaerobic section of the treatment plant:

• “At high concentrations of organic suspended solids in raw wastewater, there is the possibility of removing them at the initial stage of mechanical treatment through filtration or flotation. The separated suspended solids can then undergo fermentation. The clarified raw wastewater, devoid of suspended solids, will be treated in an aerobic reactor.
• At high concentrations of dissolved organic compounds and a low fraction of suspended solids, these wastewaters should be directly subjected to anaerobic fermentation and then further treated in an aerobic reactor. This system is much simpler technologically and more cost-effective both in the investment and operational phases.

Comparison of treatment methods in terms of efficiency and the quantity of generated biological sludge

The biological treatment of industrial wastewater is also associated with the generation of microbial biomass, which must be removed from the treatment system in the form of excess sludge. The increase in biomass in terms of pollutant removal efficiency in anaerobic digestion is approximately 10–15 times smaller than with aerobic treatment conditions. Therefore, in two-stage biological treatment plants for industrial wastewater, efforts should be made to minimize the formation of excess aerobic sludge. Aerobic sludge is unstable (meaning a portion of the organic matter in the sludge undergoes further decomposition) and requires additional biological treatment, i.e., it must undergo anaerobic digestion.For this reason, the most economical approach is to perform anaerobic fermentation of raw wastewater and further purification in an aerobic reactor. For example, by treating 1000 m3/day of industrial wastewater with a COD concentration of 5000 mg O2/dm3 only in the aerobic reactor, one can obtain 2500–3000 kg of dry sludge. After dewatering to 20% dry solids, this corresponds to a volume of 12.5–15 m3/day.In contrast, treating the same wastewater with anaerobic-aerobic technology results in only about 430 kg of dry sludge, or approximately 2.15 m3/day after dewatering.

Pre-treatment of industrial sewage using methane fermentation

Facilities generating a large volume of wastewater with a significant pollutant load are required to construct their own wastewater treatment plants. However, for smaller facilities where building a complete wastewater treatment plant may not be economically justified, a highly effective solution is the preliminary treatment of wastewater in an anaerobic reactor. Subsequently, the treated wastewater can be conveyed through the sewer system to the municipal wastewater treatment plant. The reduction of organic compounds in the anaerobic fermentation process ranges from approximately 85% to 95%. Pre-fermented wastewater, containing residual amounts of simple organic compounds, poses no significant challenge for further treatment in an aerobic reactor at the municipal treatment plant. This is especially true in cases where there is an influx of an increased pollutant load.

Full sewage treatment with discharge to the receiver

The conditions for discharging treated wastewater into the receptor are specified in the Regulation of the Minister of the Environment of November 18, 2014, concerning the conditions that must be met when introducing wastewater into water or soil, and regarding substances particularly harmful to the aquatic environment.