Secondary Settling Research Articles

Technical Note 6 - Applicability aerobic granular sludge technology in the Brazilian context

Aerobic granular sludge (AGS) is currently considered one of the most promising technologies for biological wastewater treatment. This fact is mainly due to the combined removal of organic matter and nutrients in the same reactor, eliminating secondary settlers, making it one of the most compact technology on the market. However, there are several reports of problems, including: (i) instability of the granular biomass; (ii) long start-up periods due to slow granule formation, especially when using sewage with a low concentration of organic matter; (iii) accumulation of nitrite in the final effluent; (iv) high demand for automation; and (v) operational process not yet fully established. Thus, despite considerable advances in sewage treatment with AGS systems and the increase in the number of full-scale sewage treatment plants (STPs) in several countries around the world, an integrated assessment of its application in Brazil is necessary. This technical note aims to present the state-of-the-art of AGS system, as well as to bring up a more in-depth discussion, in the Brazilian context, on the need or not to adopt it, and in what situations it should be implemented to replace other sewage treatment technologies with greater mastery of national engineering and usual lower investment and operating costs.

Deep Learning of Preconditioners for Acceleration of Secondary Settling Tank Simulations

Deep Learning of Preconditioners for Acceleration of Secondary Settling Tank Simulations

ОСОБЛИВОСТІ СПУХАННЯ АКТИВНОГО МУЛУ В МЕМБРАННИХ БІОРЕАКТОРАХ

Membrane bioreactor (MBR) technology, the practical application of which is growing steadily, is a combination of classical technology of biological treatment with activated sludge with a membrane system of micro- or ultrafiltration. MBR are an alternative to the widely used method of separating activated sludge from wastewater in secondary settling tanks used in traditional biological treatment systems. MBR exclude large-scale negative consequences, as in system with traditional aeration tanks and secondary settling tanks in the "swelling of activated sludge". But "swelling of activated sludge" can affect the nitrification process and minimize the efficiency of ammonium nitrogen removal. The purpose of the work is to study the swelling of activated sludge swelling on the existing membrane bioreactor. The research was carried out by hydrochemical analysis of wastewater in MBR and determination of technological properties of activated sludge in the bioreactor. The negative effect of activated sludge swelling in MBR on nitrification activity in the system was established, which caused a decrease in the efficiency of ammonium nitrogen removal. Filamentous bacteria were identified according to the classification of D. Eickelbum, which caused swelling of activated sludge in the studied MBR, Type 021N.

Modeling of wastewater treatment processes with hydrosludge.

The pressure for Water Resource Recovery Facilities (WRRF) operators to efficiently treat wastewater is greater than ever because of the water crisis, produced by the climate change effects and more restrictive regulations. Technicians and researchers need to evaluate WRRF performance to ensure maximum efficiency. For this purpose, numerical techniques, such as CFD, have been widely applied to the wastewater sector to model biological reactors and secondary settling tanks with high spatial and temporal accuracy. However, limitations such as complexity and learning curve prevent extending CFD usage among wastewater modeling experts. This paper presents HydroSludge, a framework that provides a series of tools that simplify the implementation of the processes and workflows in a WRRF. This work leverages HydroSludge to preprocess existing data, aid the meshing process, and perform CFD simulations. Its intuitive interface proves itself as an effective tool to increase the efficiency of wastewater treatment. PRACTITIONER POINTS: This paper introduces a software platform specifically oriented to WRRF, named HydroSludge, which provides easy access to the most widespread and leading CFD simulation software, OpenFOAM. Hydrosludge is intended to be used by WRRF operators, bringing a more wizard-like, automatic, and intuitive usage. Meshing assistance, submersible mixers, biological models, and distributed parallel computing are the most remarkable features included in HydroSludge. With the provided study cases, HydroSludge has proven to be a crucial tool for operators, managers, and researchers in WRRF.

A mathematical optimization model for secondary settler

The study of secondary settler modelling, which aims to establish the main model (one-dimensional-1D model), which is involved in some fundamental processes of the hydrodynamic behaviour of this liquid/solid separation unit and to engender variations of the sludge blanket height as a function of the operating parameters and maintaining of the municipal wastewater treatment plant of Setif. The objective of this research is focused on solid/liquid separation in the secondary settler by attempting a mathematical model that allows us to evaluate the sedimentation velocity as a function of the sludge settleability parameters.

Coronaviruses and SARS-CoV-2 in sewerage and their removal: Step by step in wastewater treatment plants

The fate of Coronaviruses (CoVs) and in particular SARS-CoV-2 in wastewater treatment plants (WWTPs) has not been completely understood yet, but an adequate knowledge on the removal performances in WWTPs could help to prevent waterborne transmission of the virus that is still under debate. CoVs and SARS-CoV-2 are discharged from faeces into the sewer network and reach WWTPs within a few hours. This review presents the fate of SARS-CoV-2 and other CoVs in the primary, secondary and tertiary treatments of WWTPs as well as in sludge treatments. The viral loads decrease progressively along with the treatments from 20 to 3.0E+06 GU/L (Genomic Units/L) in the influent wastewater to concentrations below 2.50E+05 GU/L after secondary biological treatments and finally to negative concentrations (below detection limit) in disinfected effluents. Reduction of CoVs is due to (i) natural decay under unfavourable conditions (solids, microorganisms, temperature) for relatively long hydraulic retention times and (ii) processes of sedimentation, filtration, predation, adsorption, disinfection. In primary and secondary settling, due to the hydrophobic properties, a partial accumulation of CoVs may occur in the separated sludge. In secondary treatment (i.e. activated sludge) CoVs and SARS-CoV-2 loads can be reduced only by about one logarithm (∼90%). To enhance this removal, tertiary treatment with ultrafiltration (Membrane Bioreactors) and chemical disinfection or UV light is needed. CoVs and SARS-CoV-2 in the sludge (1.2E+04–4.6E+08 GU/L) can be inactivated significantly in the thermophilic digestion (55 °C), while mesophilic temperatures (33–37 °C) are not efficient. Additional studies are required to investigate the infectivity of SARS-CoV-2 in WWTPs, especially in view of increasing interest in wastewater reclamation and reuse.

Cultivation of microalgae for lipid production using municipal wastewater

The shortage of energy and the aggravation of eutrophication are two major problems in China. The use of municipal wastewater to cultivate lipid-producing microalgae for the production of biodiesel not only reduces the production cost of biodiesel and alleviates energy shortage, but also mitigates eutrophication and protects the water environment. This study explored the effects of light, pH, and nutrients on the growth and lipid production of Scenedesmus obliquus, as well as the lipid production and pollutant removal capabilities of algae during microalgae cultivation in municipal wastewater at different treatment stages. The optimal conditions for the growth and lipid production of S. obliquus were determined as light intensity of 14,500 lx, total nitrogen (TN) content of 97 mg L−1, total phosphorus (TP) content of 11 mg L−1, and CO2 content of 9.8%. Under these optimal conditions, the lipid yield from microalgae was 0.367 g L−1, which was superior to other optimisation results in the literature. The total lipid yields of S. obliquus cultivated in the wastewater from the primary and secondary settling tanks were 0.38 g L−1 and 0.33 g L−1, respectively. The TN and TP removal rates were 99.8% and 83.1% from the wastewater of the primary settling tank, and 98.9% and 97.6% from the wastewater of the secondary settling tank, respectively. In addition, fluctuations in the quality of raw wastewater may lead to fluctuations in the final lipid yield, and a regulating tank should be used when necessary. In short, these results indicate the feasibility of cultivating lipid-producing microalgae in municipal wastewater for the production of biodiesel.

Recovery of Water from Secondary Effluent through Pilot Scale Ultrafiltration Membranes: Implementation at Patras' Wastewater Treatment Plant.

Fresh water shortages affect larger areas each year due to the increased human population combined with climate change. Reuse of treated sewage water (mostly for nonpotable uses) can have a significant impact on reducing water scarcity. Ultrafiltration membranes are widely considered as a very good candidate for the remediation of this type of water. The case of Patras’ sewage treatment plant was examined for the treatment of its secondary settling tank effluent using a pilot ultrafiltration unit to produce permeate water suitable for reuse according to Greek legislation. The physicochemical characteristics of the membrane permeate stream showed significant improvements in the quality of the produced water. Turbidity was reduced by 99%, total suspended solids were decreased by more than 94%, while COD was reduced by 37%. E. coli and Enterococcus were detected at high concentrations in the feed stream but were eliminated in the membrane permeate. The results presented herein indicate that the installed equipment is capable of producing improved quality water suitable for reuse even with the strictest limits imposed by Greek legislation.

A method-of-lines formulation for a model of reactive settling in tanks with varying cross-sectional area

Abstract Reactive settling denotes the process of sedimentation of small solid particles dispersed in a viscous fluid with simultaneous reactions between the components that constitute the solid and liquid phases. This process is of particular importance for the simulation and control of secondary settling tanks (SSTs) in water resource recovery facilities (WRRFs), formerly known as wastewater treatment plants. A spatially 1D model of reactive settling in an SST is formulated by combining a mechanistic model of sedimentation with compression with a model of biokinetic reactions. In addition, the cross-sectional area of the tank is allowed to vary as a function of height. The final model is a system of strongly degenerate parabolic, nonlinear partial differential equations that include discontinuous coefficients to describe the feed, underflow and overflow mechanisms, as well as singular source terms that model the feed mechanism. A finite difference scheme for the final model is developed by first deriving a method-of-lines formulation (discrete in space, continuous in time) and then passing to a fully discrete scheme by a time discretization. The advantage of this formulation is its compatibility with common practice in development of software for WRRFs. The main mathematical result is an invariant-region property, which implies that physically relevant numerical solutions are produced. Simulations of denitrification in SSTs in WRRFs illustrate the model and its discretization.

Full-scale vacuum degassing of activated sludge – A case study over 2-years of operation

The effect of vacuum degassing of activated sludge on biological wastewater treatment over a total period of 4 years was examined. In half of the period studied, the treatment system was classic, where wastewater, after a biological reactor with nitrification-denitrification and biological dephosphatation, was directed to secondary settlers. In the second period, the treatment system was modified so that the sludge from the biological reactor was directed to the vacuum degasser, followed by the secondary settling tanks (vacuum degassing activated sludge system; VDAS). As a result of using the VDAS, a significant improvement in sludge settling ability was recorded, which allowed the plant operators to increase the amount of biomass in the biological reactor. Differences in the quantity and quality of the wastewater influent were found during both periods. However, the quality of treated wastewater and the efficiencies of nutrient removal and removal rates were usually better in the system with vacuum degassing. An improvement was achieved not only for the elimination of the different forms of nitrogen, which by definition were to be removed as ammonia and gaseous nitrogen, but also for organic compounds expressed as COD and phosphorus. As a result of the lower sludge loading, sludge production expressed in g SS per g COD removed was significantly lower (20 %) with the VDAS.

Secondary wastewater treatment processes optimization

Introduction. The level of water pollution in Ukraine continues to grow, despite the strengthening of requirements for the treated water, so the existing approaches to the water treatment processes control need to be revised and improved.
 Materials and methods. Mathematical Programming methods for formalization and solving the optimization problem are used. Computer simulation research using the program developed by the authors are applied to verify the compliance of the obtained results with the normative values and data of normal operation.
 Results and discussion. The optimization of processes control was performed on the example of a typical process of secondary wastewater treatment, which is used for wastewater treatment in Kyiv and nearby settlements. The secondary treatment unit consists of an aeration tank, into which air is forcibly supplied and evenly distributed, and a secondary settling tank with recycling. The problem of optimization of biological wastewater treatment control process is formalized: minimization of pollutant concentration in treated water is chosen as the main aim of optimization, air flow to aeration tank is chosen as control parameter, quadratic deviation of current concentration values from normative ones is chosen for target function. To solve the optimization task, a software module in JavaScript was developed using a client-server architecture that works in real time and allows to obtain such values of oxygen consumption in the aeration tank, which provide a minimum deviation of the concentration of pollutants from the normative values. The simulations showed that the calculated control values provide a reduction in the concentration of pollutants to the normative values within 6-10 hours, which corresponds to the data of normal operation, and the difference between the calculated and actual data does not exceed 5%.
 Conclusions. The obtained results allow to find a set of technological influences to ensure optimal control according to the selected criterion, and are also the basis for calculating the control system. The results of calculations can be used for short-term - up to 8 hours - forecasting of water quality indicators.

Technology for biological treatment of urban wastewater and sludge treatment with deep removal of nitrogen and phosphorus

This article is dedicated to increasing of removing of nitrogen and phosphorus in DENIFO technology, and the removing of phosphorus by reagents. Removal of nitrogen and phosphorus from municipal wastewater prevents eutrophication of open water, which has become a global problem. A list of conditions that provide maximum efficiency of the processes of wastewater treatment and sludge treatment is completed. Functions of primary settlers, air tanks (based on technology DENIFO), and secondary settlers is reviewed. Pre-denitrification of the returned active sludge with the partial flow of waste water improves anaerobiosis in the anaerobic zone of biological block, and promotes intensive dephosphorization. The work was made under normal conditions of wastewater treatment plant. Unfavorable periods (snowmelt, torrential rains, floods) are not taken into consideration. Mathematical equations are made for the average flow of wastewater during dry weather. Combined dehydratation of the sludge and sediments (the long stay in the tank) leads to rapid displacement phosphorus phosphate to drain water, and a corresponded increase the content of phosphorus in the treated water. Also recommendations for changing the operating mode of the block of sludge treatment given. The important results included in article was taken on the really functioned wastewater treatment plants of Saint Petersburg. This work is aimed at a comprehensive assessment, development and improvement of technology of removing nitrogen and phosphorus from municipal wastewater, including from secondary pollution coming from the return flow from the sludge treatment units.

Technological Innovation to Reduce the Negative Environmental Impact of Industrial Wastewater

Rational use of natural resources and protection of water bodies is an actual problem of the modern world. Due to the rapidly developing industry, wastewater which contain high concentrations of suspended substances and organic pollutants is discharged to water bodies in a large volume.So design improved treatment facilities that ensure high quality of treated water is an important task.Biomembrane technology is a promising direction in water treatment, providing high efficiency of treatment at minimal cost. Compared to the traditional treatment scheme, it allows you to avoid the use of secondary settling tanks by separating the active sludge from the treated wastewater on the membranes. Due to the very small size of the membrane pores, almost complete removal of suspended substances and microorganisms is achieved. A bioreactor with an external arrangement of the membrane module provides high specific capacity.The small-bubble aeration system in the aeration tank ensures the maintenance of dissolved oxygen at a concentration of 2-3 mg / l for the correct course of the nitrification and oxidation of organic substances. The combination of biological treatment and membrane separation ensures high operational reliability and durability.This technology is used in various industries, but it is most widely used in the food and oil refining industries due to the high content of organic pollutants, which are mostly removed by this installation.The use of biomembrane technology is economically advisable for designing treatment facilities for industrial wastewater treatment from organic pollutants.

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.

Study on the Efficiency of On-Site Sludge Reduction Using Ti/SnO2-Sb and Ti/RuO2-IrO2 Electrodes Based on a Cell Lysis-Cryptic Growth System

The present study investigated the parameters and the mechanism of action of electrochemical cell lysis to reduce the return sludge from secondary settlers based on the theory of cell lysis-cryptic growth. The factors influencing the functioning of two electrodes (Ti/SnO2-Sb and Ti/RuO2-IrO2) were investigated to determine the optimal cell lysis parameters for each electrode, and the effects of the two electrodes on cell lysis were compared under these conditions. Finally, the Ti/SnO2-Sb electrode was selected for the subsequent experiments. The electrolysis reaction was performed using the following parameters: the initial sludge concentration was 7000 mg/L, the working voltage was 18 V, the plate spacing was 1 cm, the initial pH was 6.8 to 7.0, and the electrolysis duration was 90 min. The degree of disintegration of the sludge and the cell lysis rate reached 25.35% and 20.15%, respectively. In summary, electrochemical cell lysis has a good prospect for sludge reduction.

Promotion of full-scale constructed wetlands in the wine sector: Comparison of greenhouse gas emissions with activated sludge systems

The aim of this study was to quantify and compare greenhouse gas (GHG) (i.e. carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4)) emissions from two full-scale winery wastewater and sludge treatment systems (i.e. constructed wetlands (CWs) and activated sludge system) located in Galicia (Spain). GHG fluxes were measured using the static chamber method in combination with an on-site Fourier transform infrared spectroscopy (FTIR) gas analyser in the CWs system. These on-site innovative techniques proved to be very accurate and reliable. In the activated sludge treatment systems, the floating chamber method in combination with the FTIR gas analyser was used. Measurements were carried out during the vintage season, when winery wastewater has the highest flow and loads, and the rest of the year. Emission rates of CO2, N2O and CH4 in the CWs units (i.e. vertical flow, horizontal subsurface flow and sludge treatment wetlands) ranged from 1.35E+02 to 7.54E+04, 1.70E-01 to 3.09E+01 and − 3.05E+01 to 1.79E+03 mg m−2 day−1, respectively. In the case of the activated sludge units (i.e. reactor, secondary settler and sludge storage tank) emission rates of CO2, N2O and CH4 ranged from 1.56E+04 to 1.43E+05, 1.13E+01 to 4.75E+01 and 2.52E+01 to 1.01E+03 mg m−2 day−1, respectively. Seasonally, daily and instantaneous variability in emissions as well as spatial variability was found. Comparing CWs with the activated sludge system, surface emission rates were lower in the CWs system in both seasons considered. Results highlighted that CWs are suitable technologies that can help to reduce GHG emissions associated with winery wastewater treatment.

Improving wastewater treatment plant performance by applying CFD models for design and operation: selected case studies.

Hydrodynamic simulation (CFD: computational fluid dynamics) is one of the major tools for planning the reconstruction and operation of the structures in wastewater treatment plants, and its routine use is commonplace because of the cost savings and efficiency gains that can be achieved. This paper provides examples of how CFD can contribute to substantial improvements in the overall efficiency of wastewater treatment plants.The case studies presented in the paper include rarely investigated issues, such as the operation of aerated grit chambers, performance of primary settling tanks, mixing performance in oxidation ditches and return sludge control. The results show that: (1) air intake rate can be strongly decreased in most of the grit chambers, (2) optimization of the inlet geometry design of primary settling tanks is crucial, especially at high loads caused by storm events, (3) mixer performance design based on current design guidelines is often of an unnecessarily high capacity, (4) sludge recirculation rate should be optimized by CFD investigations based on secondary settling tank performance.

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.

Dynamic Modeling of a Batch Sludge Settling Column by Partial Differential Non Linear Equations with a Moving Interface

Abstract The further goal of the model presented in this paper is to propose a digital tool to predict the behaviour of a main equipment of the water resource recovery facilities, the secondary settling tank. Most of the time, models of this liquid/solid two-phase system are based on a dynamic solid phase mass balance and a settling velocity constitutive expression stemming from the Kynch’assumption. The model presented here is based on both dynamic mass and dynamic momentum balance equations and on an explicit representation of the sludge blanket depth. Therefore, it is able to be used in a wider range of operating conditions than existing models in the literature. It includes effective solid stress appearing when the solid particles concentration is above a given threshold and a moving interface above which there is no solid particles: the sludge blanket. A study of the main physical phenomena into a batch sludge sedimentation column (hydrodynamics of two-phase suspensions) is usually carried out before a faithful representation of continuous sedimentation within a continuous settling tank is deduced. The batch settler is divided into three zones (clarification, thickening and compression). A one dimensional implicit hybrid partial differential non linear equations (Hybrid NL-PDE’s) state space representation is proposed based on the different dynamics in each of the three zones as well as the constitutive equations and the boundary conditions. Some simulations are given and compared with sludge blanket height measurements in a batch settling column.

Profitable wastewater treatment by directly recovering organics for volatile fatty acids production

A strategy of directly recovering organics for the production of volatile fatty acids (VFAs) was introduced to make wastewater treatment profitable. A sludge adsorption section was proposed to add before the conventional wastewater process for adsorbing the organics in wastewater by the excess activated sludge which was produced during biological wastewater treatment and returned from the secondary settling tank. Then, the excess activated sludge combining wastewater organics, called organics-enriched sludge, was anaerobically fermented for the production of high value-added VFAs. Results indicated about 70 % of wastewater organics could be source-recovered within 30 min. Most of the organics in wastewater could avoid being lost via carbon dioxide or synthesized into bacterial organisms, therefore, the total amount and overall bioavailability of the organics recovered from wastewater were greatly improved. Pilot-scale tests indicated VFAs production from wastewater organics was enhanced by 62.2 %. Mass balance analysis indicated 46.8 % of wastewater organics could be finally transferred into VFAs, only 12 % in conventional processes. Economic evaluations indicated source-recovering wastewater organics for VFAs production was a promising alternative to make wastewater treatment plants (WWTPs) profitable. Finally, an idealized organics transformation route for wastewater treatment was proposed based on this concept to make wastewater treatment profitable.