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.