The article is the literature review on the importance of trace elements supplementation in the methane fermentation process. The production of biogas, including methane, as well as the efficiency of the process depend on the substrates to be fermented. Substances supplied with the substrate as well as products generated in the decomposition phases can inhibit the process. The factor limiting fermentation is the rate of enzymatic hydrolysis of substrates. Certain compounds, such as alkanes, alkenes, biphenol, aromatic hydrocarbons, alcohols and ketones, are not directly susceptible to hydrolysis. They undergo this process in the presence of extracellular enzymes. The instability of the methane fermentation process described in the literature may be related to the lack of trace elements or micronutrients. Trace elements (Co, Ni, Cu, Mn, Fe, Zn, Se and Mo) are components of enzymes, some bacterial nucleic acids and essential for the synthesis of vitamins. The role of some trace elements, eg. Fe or Mo, has been well understood, while the importance of others still needs to be clarified. Literature data indicate that supplementing trace elements not only prevents process inhibition, but can also improve its performance by providing higher methane production.

Conventional quality parameters such as Chemical Oxygen Demand (COD) or Biochemical Oxygen Demand (BOD) give information about the quantity of organic matter present in wastewater, but do not give a clear indication of the biodegradability of the pollutants flowing in the WWTP. Detailed knowledge can be obtained by dividing the total COD into fractions. Fractionation and balancing of COD can be determined in various ways and with varying accuracy. Good wastewater characteristics are obtained on the basis of COD fractionation in accordance with ATV-A 131 guidelines, especially when the wastewater characteristics are in high compliance with the assumptions of the method. The article proposes a modification of the ATV-A131 method that increases the accuracy of determining the COD fraction. In order to reduce errors in the calculation of COD fractions, the value of fraction X_S was calculated on the basis of the biochemical degradation rate determined in studies (k) for raw wastewater, whereas the S_I fraction was calculated from the difference between SCOD and BOD_Tot of filtered treated wastewater. BOD_Tot of the treated wastewater was calculated taking into account the rate of biochemical degradation determined in the studies (k) for treated wastewater. The shares of individual COD fractions in raw wastewater calculated on the basis of the standard and modified procedure differed by approx. 10% in the case of suspension fractions. Modification of the methodology to determine the COD of the treated wastewater S_S fraction significantly influenced the contents of all fractions in treated wastewater.

The results of investigations on heavy metals and polycyclic aromatic hydrocarbons (PAHs) content in mixed soil fertilized with sewage sludge are presented. The concentration of these micropollutants in various parts of a basket willow Salix viminalis grown on this substratum was also analyzed. Aerobically stabilized, dewatered, and hygienized sewage sludge (a dose of 90 tons of dry matter per hectare) was used to fertilize the soil. S. viminalis was planted on the soil amended with sewage sludge. The samples for analyses were taken after five vegetatives; they were both soil samples and parts of plants (roots, stems, and leaves). The concentration of heavy metals and PAHs was analyzed in the samples. The concentration of heavy metals in soil after five seasons of fertilizing with sewage sludge was at most cases higher than defined in soil at the beginning of the experiment. The concentration of Cr in soil after five years of S. viminalis cultivation was, however, lower than at the outset. The average concentration of Cr in biomass of a basket willow equaled to 9.3 mg/kg_d.m. and it was higher than the average content of this element in not fertilized soil samples (4.6 mg/kg_d.m.). The content of other heavy metals in willow biomass was lower than the soil concentration and was equal to 0.7, 7.9, 1.3, 13.2, and 47.0 mg/kg_d.m. for Cd, Cu, Ni, Pb, and Zn, respectively. The concentration of 15 PAHs total in soil did not exceed 700 μg/kg_d.m.. The total concentration of 15 PAHs was also analyzed in particular parts of S. viminalis. The average total concentration of these micropollutants in roots was equal to 67 μg/kg_d.m.. The total concentration of 15 PAHs in above ground parts of plants was equal to 115 and 55 μg/kg_d.m. for leaves and stems, respectively.

Wastewater treatment plants designed to meet the requirements of discharging wastewater to a receiving water body are often not energy optimised. Energy requirements for conventional activated sludge wastewater treatment plants are estimated to range from 0.30 to 1.2 kWh/m³, with the highest values achieved using the nitrification process. This article describes the energy optimisation process of the wastewater treatment plant in Gubin (Poland) designed for 90,000 PE (population equivalent) using renewable energy sources: solar, biogas, and geothermal. At the analysed wastewater treatment plant electricity consumption for treating 1 m³ of wastewater was 0.679 kWh in 2020. The combined production of electricity and heat from biogas, the production of electricity in a photovoltaic system, and heat recovery in a geothermal process make it possible to obtain a surplus of heat in relation to its demand in the wastewater treatment plant, and to cover the demand for electricity, with the possibility of also selling it to the power grid.

The aim of the paper is to present information from the literature concerning the course of electrochemical wastewater treatment processes in regard to organic micro-pollutant removal. Most often, in order to remove xenobiotics that are difficult to degrade biochemically, advanced oxidation processes and photochemical processes with or without catalysts are used. The efficiency of these processes can be supported by the flow of electric current through the solution being purified in a special system. This paper presents the theoretical foundations of processes such as electrocoagulation, electroflotation, and advanced chemical and photochemical oxidation supported by electric power. Among the processes where the Fenton’s reagent is the oxidant, the electro-Fenton and photo-electro-Fenton processes are also described. This information is supplemented with examples of the use of these processes for removal/degradation of selected organic compounds such as pesticides, dyes, pharmaceuticals, cosmetic ingredients, and other organic xenobiotics from wastewater.

Operating and research experiments indicate that the potential benefits of thermal treatment of sewage sludge before methane fermentation include increasing the biodegradability of substrates, reducing the amount and improving the dewaterability of the fermentate and its hygienization, reducing the emission of odours during stabilization, higher production of biogas, and improving the energy balance of the process. The process of disintegration (liquefaction) can be carried out, for example, through the use of mechanical homogenization, microwaves and ultrasonic waves, chemical agents, thermal methods, and biological processes. The article reviews the literature data on thermal hydrolysis research, from the first source information to the present. The thermal hydrolysis achieved enhanced hydrolysis, biogas potential, and faster sludge degradation during anaerobic digestion without compromising the quality of the end products.