Intensification of activated sludge sedimentation processes by means of vacuuming

Introduction. The mixed liquor of nitrogen removal wastewater treatment plants is characterized by a high concentration of nitrates and dissolved oxygen at the inlet to the secondary settling tank. In the sludge layer of secondary sedimentation tanks, conditions of decreased oxygen content and uncontrolled denitrification processes take place. This leads to the floating up and removal of sludge with the effluent and secondary pollution of treated water. The purpose of this article is to study the parameters of activated sludge sedimentation in municipal wastewater treatment plants and their intensification by means of vacuuming. Materials and methods. The studies were carried out under laboratory conditions. Activated sludge vacuuming and sedimentation processes were simulated. Diagrams of the “sludge-water” phase reduction (Kinsh curves) were drawn. Mathematical and graphic processing of the results was carried out. Results. Biological treatment of municipal wastewater (aerotank — secondary settling tank) and methods of its intensification by influencing the activated sludge were considered in this article. Trends of activated sludge (at different concentrations of mixed liquor suspended solids) sedimentation were experimentally obtained for municipal wastewater treatment plants. The process of sludge vacuuming was researched, the process efficiency was determined as a function of the treatment time. Conclusions. Vacuuming allows removing gases from the fluid, which accelerates the process of sludge separation from the treated water and prevents it from floating to the surface. The treated sample is characterized by better sedimentation characteristics, density, coarseness and integrity of flakes. The optimal duration of mixed liquor vacuuming before sedimentation is 0.5 minutes; this accelerates the processes of subsequent sedimentation and reduces the removal of sludge with treated water. The results of laboratory tests can be applied to the design of the mixed liquor vacuuming unit before the secondary sedimentation tanks and its sludge separation.

Specific cost modelling for secondary treatment of wastewater

The article is devoted to the study of model cost indicators of wastewater treatment facilities as one of the factors for ensuring sustainable development. It is noted that the greatest contribution in terms of cost formation is provided by the stage of biological wastewater treatment. As part of the study, a techno-economic simulation was conducted to compare two technological treatment schemes: conventional activated sludge scheme in aeration reactor and secondary settling tank, and treatment by means of membrane bioreactors. In the course of technological modeling, the concentrations of pollutants after treatment were obtained and compared for each of the schemes, as well as the values of possible harm to water bodies were calculated. Within the framework of cost modeling, capital and operating costs were obtained for the two variants, and key differences in their formation were analyzed. It is noted that the classical cleaning scheme requires the construction of a larger volume of structures, while for schemes with a membrane bioreactor, a significant cost item is the purchase of the actual membrane modules, but as the system performance increases, the share of these costs decreases.

Experience from the Implementation and Operation of the Biological Membrane Reactor (MBR) at the Modernized Wastewater Treatment Plant in Wydminy

Biological membrane reactors or membrane bioreactors (MBRs) based on pressure separation techniques are placed among the latest and most modern methods of wastewater treatment. Currently, this method is becoming more and more popular and is being implemented in smaller and larger wastewater treatment plants (WWTPs). However, technologists, operators, and managers of small WWTPs often do not understand the MBR technology installed in their sites and need extensive professional and technological information. The aim of this study was to analyze the modernized WWTP in a small town of Wydminy, located in northeastern Poland in the Great Masurian Lakes region, where the traditional secondary settling tank was replaced by an MBR. The effectiveness of wastewater treatment before modernization and after installation of the membrane module was compared. On the basis of the conducted research, it was noted that the operation of the plant after modernization is more cost-intensive. There were additional electricity costs due to ensuring adequate pressure on the membrane. Nevertheless, the obtained results of the removal of contaminants place the plant in Wydminy in the group of the most effective Polish sewage treatment plants, as compared to the results obtained in other facilities. The MBR operation also places high demands on the exploiters, prompting them to observe even the smallest changes. The conducted research is a type of a case study, which could give the readers an understanding of the necessity of traditional WWTP modernization with MBR.

Quantifying the sources of uncertainty when calculating the limiting flux in secondary settling tanks using iCFD

Solids-flux theory (SFT) and state-point analysis (SPA) are used for the design, operation and control of secondary settling tanks (SSTs). The objectives of this study were to assess uncertainties, propagating from flow and solids loading boundary conditions as well as compression settling behaviour to the calculation of the limiting flux (JL) and the limiting solids concentration (XL). The interpreted computational fluid dynamics (iCFD) simulation model was used to predict one-dimensional local concentrations and limiting solids fluxes as a function of loading and design boundary conditions. A two-level fractional factorial design of experiments was used to infer the relative significance of factors unaccounted for in conventional SPA. To move away from using semi-arbitrary safety factors, a systematic approach was proposed to calculate the maximum SST capacity by employing a factor of 23% and a regression meta-model to correct values of JL and XL, respectively – critical for abating hydraulic effects under wet-weather flow conditions.

A Numerical Study on Influent Flow Rate Variations in a Secondary Settling Tank

The secondary settling tank is an essential unit for the biochemical treatment of domestic sewage, and its operational effect influences the quality of effluent. Under the influence of the confluence of rainwater and sewage, wastewater use habits, etc., the inflow of the secondary sedimentation tank changes over time. In this paper, OpenFOAM, an open-source computational fluid dynamics package, was used to study the dynamic behaviors of solid–liquid two-phase flow in the tank under influent flow rate variations. A coupled method including a mixture model, drift equation and a dynamic boundary method is proposed. Numerical investigations were carried out for a 2D axisymmetric sedimentation tank using 12 cases. With increasing influent flow rate, sludge accumulates continuously in the bottom left side of the tank, sludge hopper, and inlet; the sludge blanket thickness near the right end of the tank increases continuously; and the sludge concentration in the tank approximately linearly increases with time, with a low slope. The developed framework is generic and is, therefore, expected to be applicable for modelling sludge sedimentation processes.