The growing amounts of sewage sludge (SS) and water pollution caused by heavy metals are major environmental concerns. This study addresses both issues by investigating the potential of biochar derived from SS gasification at an experimental plant as an effective adsorbent for the removal of selected heavy metals, cadmium, chromium, copper, and lead, from synthetic wastewater. A Box–Behnken design was used to determine the influence of the biochar mass, initial heavy metal concentration, pH, and time on the heavy metal removal. For the statistical analysis, 104 experiments were performed. The pristine SS biochar demonstrated an adsorption capacity reaching up to 46.64 mg/g for Cd, 43.89 mg/g for Cr, 42.42 mg/g for Cu, and 72.66 mg/g for Pb from single-component synthetic solutions in acidic-to-neutral conditions, with an over 99% removal efficiency for all four heavy metals under optimal conditions. The removal of all the tested metals followed pseudo-second-order kinetics, with Cd fitting the Langmuir model and Pb, Cr, and Cu fitting the Freundlich model. This paper also provides suggestions for further research focused on the multiple uses of biochar as an adsorbent and later as a substitute material in the construction industry, aiming to achieve an integrated approach and maximizing the overall sustainability of wastewater treatment and waste management by utilizing waste as a resource.

Water conditioning is a method of removing altering minerals, chemicals and contaminants from a water source and it is carried out on facilities equipped with the corresponding electro-mechanical equipment. Although efficient, conventional processes typically use several complex devices connected to a single functional unit, which are often expensive to maintain and occupy large areas. Therefore, the aim of this paper is to present the electrocoagulation (EC) method as an alternative to conventional water conditioning processes. The examples of previous studies of the EC process application is presented in this paper. The focus of the paper is to investigate the influence of the certain operational parameters such as pH, temperature, electrode material, etc., on the efficiency of pollutant removal such as Escherichia coli and elevated concentrations of iron, arsenic, manganese, ammonia and others. Further, an economic analysis is made, which, from an economic point of view, shows when it is feasible to use the EC in the conditioning process. Furthermore, a case study of electrocoagulation process for Total Nitrogen (TN) removal is presented. According to results, 69.7 % of TN was removed with aluminum electrodes after 240 minutes. For this case, total operating costs were 7.60 €/m3.

Wastewater containing oil is becoming a growing problem worldwide due to increasing quantities and existing pollution. The pollutants contained in these effluents, when released into the environment, affect surface and groundwater pollution, endanger human life and health, and pollute the atmosphere. Their sustainable treatment should be cost-effective and meet all requirements to prevent the pollutants from being transferred to the environment or to humans. This study gives a brief overview of some conventional and modern technologies that have been proven in practice for the treatment of oily wastewater. Due to the high concentrations of chemical oxygen demand (COD) and total hydrocarbons (mineral oils) in oily wastewater its treatment is complex, and to achieve optimum treatment conditions and efficiency a combination of different technologies is required. This paper focuses on hybrid electrochemical process combining the electro-Fenton process (EF) using stainless steel (SS), and electrocoagulation (EC) with iron (Fe) and aluminum (Al) electrodes. The influence of the two different types of pretreatment, i.e., pretreatment of the raw wastewater on the overall efficiency of oily wastewater treatment using a hybrid treatment process, which is a combination of AOP and EC, is investigated. Two type of pretreatment were tested, with primary sedimentation and pretreatment of the mixture of raw wastewater and previously generated electrochemical sludge with primary sedimentation. During the applied treatment processes, the concentration of COD, mineral oils, and other elements in the raw and treated wastewater (As, Ca, Cd, Cr, Cu, Ni, Pb, Sn, Zn) and in the generated sludge (K, Ca, Fe, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, Pb, Th) were determined. By combining the primary sedimentation of the raw wastewater with the EF/EC process, a mineral oil removal efficiency of 72% (1.1 mg/L) and COD of 89% (170 mg/L) was achieved. Using primary sedimentation of a mixture of raw wastewater with previously generated sludge as pretreatment and followed by EF/EC treatment, a higher efficiency for mineral oils of 94% (7.6 mg/L) and COD of 98% (43 mg/L) was achieved.

This study investigated the removal of selenium (Se) and strontium (Sr) from water using a three-stage electrochemical reactor with integrated pH control. A total of 102 experiments were performed following a Box–Behnken design that varied the electrode material, applied current, number of electrodes, operating time, and initial pH to evaluate their effects on the Se and Sr removal efficiencies. The complete removal of Se was achieved under multiple conditions, even without pH control, while effective Sr removal required a high current and initial pH adjustment. The top performance for both elements was achieved with a 25 A current, four Fe electrodes, 15 min of operational time per phase (cycle), and a middle range of pH values, which resulted in 97.92% and 99.96% removals of Sr and Se, respectively. This research highlighted the novel approach of using electrochemical pH control to achieve high removal efficiencies of Se and Sr from water in a short operating time, which surpassed the efficiencies reported in previous studies.

The increasing production of oily wastewater as a by-product of industry has become a major environmental problem. Therefore, this article investigates the removal of mineral oil from oily wastewater by a circulating flow sono-electrocoagulation. The Box–Behnken design was used to study the effects of characteristic electrocoagulation and ultrasonic parameters on mineral oil removal efficiency. A total of 34 different experimental setups were performed at a laboratory scale. A reduced cubic regression model with derived coefficients was developed to describe the mineral oil removal rate. The experimental results show that circulating flow sono-electrocoagulation with iron electrodes can effectively reduce mineral oil by 93.3% under the optimum conditions of 10.948 cycles, a current density of 107.12 A m⁻² and a flow rate of 0.468 L s⁻¹. The experimental observations agreed well with the modeled values, and the model was verified experimentally. Under the optimal conditions, the average operating cost was 0.77 EUR/m³.

Chemical kinetics can be a useful tool for determining the optimal operating time of electrochemical processes. The main objective of the study was to determine the mineral oil removal rate by sono-electrochemical treatment. In this study, zero-, first-, and second-order kinetic models were used to determine the reaction rate of mineral oil removal with the sono-electrochemical process. The reaction rate experiments were conducted under the following optimal conditions: 8 min of treatment time, a current density of 53.1 A/m2, and a flow rate of 0.23 L/s. It was found that the changes in mineral oil concentrations follow second-order kinetics with a coefficient of determination of 0.9732. The mineral oil removal efficiency was 94.4%. This study concludes that sono-electrochemical process could be a promising technology for the removal of mineral oil from wastewater, and that the mineral oil removal rate can be determined by chemical kinetics. The results obtained may be useful for the optimization of the sono-EC process and reactor design.