TPU Researchers Determined How The Type And Concentration Of Impurities Affect Mechanical Purification Of Wastewater

Researchers from Tomsk Polytechnic University and the S.S. Kutateladze Institute of Thermophysics of the Siberian Branch of the Russian Academy of Sciences conducted experimental studies and determined the sedimentation and stratification characteristics of impurities in industrial wastewater from metallurgical and pulp and paper enterprises. This data formed the basis of a mathematical model for predicting wastewater treatment in a primary settling tank.

The research was supported by the Russian Ministry of Science and Higher Education (grant No. 075-15-2025-007). The results of the research have been published in the journal Powder Technology (Q1, IF: 5,5).

The metallurgical and pulp and paper industries are leaders in wastewater discharge, and their multi-stage water treatment systems critically depend on water quality. Primary water purification from impurities in settling tanks is a key stage. Before industrial wastewater treatment, the water is allowed to settle: heavy particles sink, and light liquids float. However, real-world effluents contain many impurities simultaneously, which can collide and agglomerate. This complicates calculations: formulas that work for a single particle can provide significant deviations when describing multi-component wastewater.

TPU researchers conducted comprehensive studies of industrial wastewater components typical for metallurgical and pulp and paper enterprises – dispersed wood, iron sulfate, kerosene, sodium chloride, and sodium carbonate. In the first stage, each impurity was studied individually: characteristics of acidity, sedimentation of solid particles, and stratification of liquid components. In the second stage, multicomponent mixtures were created according to the priority of impurities in real effluents.

The experimental and numerical studies conducted have shown that the sedimentation rates of solid impurity particles and stratification of liquid components are not fully described by the classical Stokes and Hadamard-Rybczynski models. Deviations from these models occur due to the confined nature of deposition of solid particles and the "sticking" of liquid impurity droplets. Considering this data, we proposed a mathematical model of an industrial settling tank. It shows how the concentration of impurities spreads along the reservoir height and how long it will take for the water to clear. For example, an increase in impurity concentration from 0.5 to 10 wt.% is accompanied by a 1.9-2.2 fold increase in settling time, which indicates a significant hindered settling effect,— notes Mikhail Mikulich, one of the authors of the research, research engineer at TPU Heat and Mass Transfer Laboratory.

The experiment results showed that when the mass concentration of impurities increased from 0.5% to 5%, the sedimentation rate of wood particles decreased by 73-74%, and that of iron sulfate particles by up to 85-89%.

The rate of kerosene and water component separation decreased by more than 150 times with the same increase in concentration.

Temperature also affects the wastewater sedimentation process. For example, according to the researchers, increasing the suspension temperature just by 20 ^°C accelerated the sedimentation of solid particles by 12-23% due to a decrease in water viscosity. When heated to 60^°C, the rate of kerosene flotation and separation more than doubled.

The TPU researchers separately compared the developed model with calculations using Stokes' formula (without considering agglomeration and hindering). The results showed that the classical model overestimated the sedimentation rate by 25-30%. The TPU model allowed for a reduction in deviation to 15% — this difference is critical when transferring to the scale of a real industrial settling tank.

The parametric studies allowed us to establish the degree of influence of the geometric parameters of a settling tank. The calculations showed that the best sedimentation characteristics are achieved at a tank depth of 3.3 m and a radius of 10 m. Additionally, it has been established that the main mass of the sediment layer forms within the time interval from 0 to 2 hours,— adds Mikhail Mikulich.

In the next stage, knowing the concentrations and composition of impurities remaining in wastewater after settling, the researchers will proceed to study droplet evaporation, a thermal purification method that allows to remove residual contaminants.