Phosphorus as phosphate and nitrogen as ammonium or nitrate are the main nutrients in wastewaters and agricultural sludges. They runoff easily to waterways and cause eutrophication in water bodies. However, ammonium and phosphate could be precipitated simultaneously and used as recycled nutrients. In this research, dolomite calcined at 650 °C, 750 °C, or 950 °C and commercial MgO were used as precipitants in simultaneous phosphate and ammonium removal from synthetic (NH4)2HPO4 solution and agricultural sludge. Calcination at 750 °C was the preferred option as dolomite was decomposed to MgO and CaCO3 for optimal struvite precipitation. Molar ratios of 1.1–1.6:1–2:2 (Mg:P:N) were employed in the experiments. Very robust ammonium removal was obtained with MgO (57%), dolomite 650 °C and dolomite 750 °C (75%). MgO removed almost all phosphate, while dolomite 650 °C removed 65%, and dolomite 750 °C removed 60% (70% from agricultural sludge). Some part of the phosphate was adsorbed, most likely by CaCO3, during dolomite precipitation. Struvite was the only identified reaction product in all samples after 24 h of precipitation. Calcined dolomite had great potential in ammonium and phosphate precipitation from both synthetic waters and agricultural sludges and the precipitates could be used as recycled fertilizers.

The European Union’s circular economy strategy aims to increase the recycling and re-use of products and waste materials. According to the strategy, the use of industry waste material should be more effective. A chemical precipitation method to simultaneously remove phosphorus and nitrogen from synthetic (NH4)2HPO4 solution and the liquid phase of anaerobic digestate using fly ash as a precipitant was tested. Fly ash is a waste material formed in the power plant process. It mainly contains calcium oxide (CaO) and magnesium oxide (MgO). Saturated precipitant solution was prepared from fly ash, which was added in small proportions to (NH4)2HPO4 solution during the experiment. Fly ash’s effectiveness as a precipitant was compared with that of commercial CaO and MgO salts, and it can be observed that fly ash removed as much ammonium and phosphate as commercial salts. Fly ash sufficiently removed ammonium nitrogen and phosphate from the liquid phase of anaerobic digestate, which led to the formation of ammonium magnesium hydrogen phosphate hydrate, struvite (NH4MgPO4·6H2O), and calcium hydroxide phosphate, monetite, CaPO3(OH). In this study, we have shown for the first time that fly ash can be used to manufacture recycled, slow-release fertilizers from anaerobic digestate.

Humic substances end up in water from the drainage basin causing eutrophication and the spread of algae.These natural organic substances have an influence on the physical, chemical and biological properties of the water system. Adsorption and electrocoagulation (EC) are commonly used purification methods in the water and wastewater treatment. Both methods are used by themselves for removing a wide range of impurities. In this research, the novelty was to study the effect of combined activated carbon (AC) adsorption and electrocoagulation method on the removal of organic substances as total organic carbon (TOC) from the peat solution. With the hybrid method, TOC content was efficiently removed (~95 %) by using first adsorption treatment followed by electrocoagulation with Al-or Fe -electrode. Instead, when using the adsorption and electrocoagulation separately, the removal of TOC was 79-89 %. Based on this study, the hybrid method could be efficiently used for removing organic substances from the water system.

The European Union's (EU) circular economy strategy aims to increase recycling and re-use of products and waste materials. According to strategy, the use of industry waste material and side flows should be more effective. In this research, a chemical precipitation method to simultaneously remove phosphate and nitrogen using calcined paper mill sludge was tested. Paper mill sludge is a waste material formed in the paper-making process. It mainly contains calcium carbonate and cellulose fibers. Objective of this research was to test whether the industrial waste could be used as low cost precipitation chemical for ammonium and phosphate removal from wastewaters or the precipitate could be suitable for fertilizer use. The study results indicated that calcined paper mill sludge removed as much ammonium nitrogen and phosphate as commercial CaO. Furthermore, the results showed that calcined paper mill sludge removed efficiently phosphate from the liquid phase of anaerobic digestate, which leads to formation of hydroxyapatite, Ca5(PO4)3(OH). In this study we have shown, that calcined paper mill sludge can be used to manufacture recycled, slow-release fertilizers. Other possible mechanisms for the removal of phosphate and ammonium were also considered.