The influence of temperature in a range of 15–5°C, fluctuations in soluble chemical oxygen demand (sCOD) and suspended solids (SS) of the influent and also long hydraulic retention times (HRT) of 36 or 50 d on the performance of a laboratory septic tank were studied. A laboratory-scale septic tank with a volume of 20 L, started up at 15°C was fed with domestic wastewater from two settlements with sCOD up to 450 and up to 2,000 mg/L, respectively. The efficiency of the septic tank was assessed based on pollutant removal and biogas production. A stepwise decrease in temperature by 5°C in the range of 15–5°C halved the daily emission of biogas. Removal efficiencies for sCOD and SS were in the range of 74–86 and 86–88%, respectively. An increase in sCOD of influent from 450 to 4,000 mg/L resulted in a decreased performance of the septic tank (to ~25% less). The highest biogas emission was observed at 10°C and at retention time (36 d), owing to increased consumption of CO₂ and CH₄ by autotrophic microorganisms over the longer retention time (50 d). At the 15°C, biogas emission was lower than in case of lower temperatures applied. In the range of 15–5°C, operational temperature did not correlate significantly (p > 0.05) with the removal efficiency of sCOD or SS, assuming as a consequence of the long HRT.

In a moving bed biofilm reactor (MBBR) at moderate temperature of 20°C sulfate reduction along with ammonium oxidation were established. In an upflow anaerobic sludge blanket reactor (UASBR), the same process took place at 36°C. Sulfate-reducing ammonium oxidation (SRAO) was performed using reject water as a substrate for micro-organisms and a source of , with being added as an electron acceptor. Bacterial strains belonging to the phylum Planctomycetales were detected from the biofilm of the MBBR; from the UASBR species representing phylum Verrucomicrobia were found. Average volumetric TN removal rates were 0.03 kg-N/m³/d in the MBBR and 0.04 kg-N/m³/d in the UASBR. concentrations exceeding 1000 mg/l had an inhibiting effect on the SRAO process. The stoichiometric ratio of removal was significantly higher than that expected from the extent of reduction. This phenomenon can primarily be attributed to complex interactions between nitrogen and sulfur compounds and organic matter present in the wastewater. Addition of hydrazine and hydroxylamine up to 12.5 mg/l had a positive effect on SRAO process performance, particularly in the UASBR. Hydrazine was naturally present in the reaction medium, indicating occurrence of the anammox process.

Equilibrium processes involving dissolved CO₂ play a vital role in many biological and technological systems. Dissolved CO₂ is an available carbon source for autotrophic organisms that have great natural and technological relevancy. In many places, including northern Estonia, the soil bedrock is limestone (CaCO₃), which has a relatively small ion product and hence low solubility. However, in the equilibrium system CaCO₃ ↔ CO₃^2– ↔ HCO₃^– ↔ H₂CO₃ ↔ CO₂ the solubility of CaCO₃ is elevated, which causes a relatively high alkalinity (pH > 8) of surface and ground water as a result of increased concentration of CO₃^2–, HCO₃^–, and OH^– ions. In this paper the equilibrium processes involving CaCO₃ (limestone) in liquid–gas–solid phase systems are described. The equilibrium distribution of ions and molecules in the system is described on the basis of a structural scheme. A mathematical model is presented for the calculation of pH and concentrations of ions and molecules in the equilibrium system CO₃^2–, HCO₃^–, CO₂, Ca²⁺, H⁺, and OH^– at different concentrations of CO₂ in its liquid phase using an iteration method. The developed model was experimentally validated.

Variation in the concentration of dissolved carbon dioxide [CO₂]_W causes changes in the solubility of limestone and in the pH of an equilibrium system. An elevation of the pH will shift the equilibrium of the reversible reaction NH⁺₄⇌ NH₃ + H⁺ towards the formation of free ammonia (NH₃). This results in the inhibition of the activity of microorganisms that perform the biological waste- and reject-water treatment. The model of the system H₂O–(CO₂)_W−CaCO₃ was upgraded on the basis of proton transfer principles and taken as the basis for modelling the closed system H₂O–(CO₂)_W–CaCO₃–NH₄Cl. The distribution of ions and molecules in the closed system H₂O–(CO₂)_W–CaCO₃–NH_X is described in terms of a structural scheme. A novel proton transfer model was developed to calculate the pH, concentrations of the formed ions and molecules, and proton transfer parameters of the closed equilibrium system using an iteration method. In the formation of the equilibrium system H₂O–(CO₂)_W−CaCO₃, as a result of the dissolution of CaCO₃, the CO^2-₃ ions are released and these will accept a certain quantity of protons (Δ[H⁺]_CO^2-₃), which originate from two sources: the reversible dissociation of water (Δ[H⁺]_H2O) or H₂CO₃ (Δ[H⁺]_H2O3), which is the product of the reaction between H₂O and (CO₂)_W0. In case the final closed system H₂O–(CO₂)_W–CaCO₃–NH₄Cl includes small initial concentrations of [CO₂]_W0, the main amount of protons (Δ[H⁺]_NH⁺₄) comes from the dissociation of NH⁺₄, or if there are higher concentrations of [CO₂]_W0, the source of protons is H₂CO₃ (Δ[H⁺]_H2CO3). The developed models were experimentally validated.

The aim of the current study was to investigate and model particle size dependence of the dissolution process of Ca(OH)₂, CaCO₃, and oil shale ash. In the samples of the self-regulating equilibrium system H₂O–dissolved CO₂–solid CaCO₃ filtered through a 0.45 μm filter no nanoparticles were detected. The nanoparticles appeared when the pH had been increased to 12.3 and the value of the product of the molar concentrations of the Ca²⁺ and OH^– ions reached the value of the solubility product constant K_sp. When different amounts of Ca(OH)₂ were added to degassed water, the pH increased and nanoparticles started to appear in the solution containing 0.5 g/L of solid Ca(OH)₂ (pH = 12.3). Up to this value of the pH all particles had dissolved. Due to the better solubility of small particles connected to the rise of the value of the solubility product constant K_sp for nanoscale particles, the solubility of Ca(OH)₂, the concentration of Ca²⁺ ions, and the pH increased. The pH of oil shale ash leachate increased with the increasing of the amount of ash, as the quantity of nanoparticles increased in a similar manner as described for the solubility of Ca(OH)₂.

Construction of road embankments in peatlands commonly involves replacement of the peat with a fill-up soil of an adequate load-bearing capacity. This usually requires a lowering of the water level, turning a peatland from a carbon sink to a source of greenhouse gases. Thus, alternatives are sought that are less costly in both economic and ecological terms. Mass-stabilization technology can provide a cheap substitute for Portland cement. Calcareous ashes (waste materials), supplemented with pozzolanic and alkali additives to facilitate and accelerate the setting and hardening processes, are attractive alternatives to soil excavation or replacement techniques. Silica fume and waterglass were used as pozzolanic agents and KOH as a soil-alkalizing agent. X-ray fluorescence (XRF), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analyses and stress–strain tests were performed for the hardened samples. Crystallization of alkali feldspars was observed in all test samples. Comparable hardening of peat soil was achieved for both ashes. It was shown that the ashes of Estonian kukersite (oil shale) from both pulverized firing and a circulating fluidized bed incineration process (produced in energy sector as quantitatively major solid waste in Estonia) can be used as binding agents for peat stabilization, even without the addition of Portland cement. Hardened peat soil samples behaved as a ductile material, and the cellulose fibers naturally present in peat gave the peat–ash composite plasticity, acting mechanically in the same way as the steel or glass fiber in ordinary reinforced concrete. The effect of peat fiber reinforcement was higher in cases of higher load and displacement of the composite, making the material usable in ecological constructions.