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.