Hydraulic behavior and removal efficiency of settling tanks

To have accurate runoff velocity, there is need to improve dye tracer method for estimating surface runoff velocity. This can enhance the calculations of relevant hydrologic parameters that will lead to a better understanding of hydrological processes and soil erosion. In this study, an integrated dye tracer and image processing method (IPV) and dye tracer method (AOV), respectively, were used to estimate runoff velocity under three slope gradients (5°, 10°, and 15°) and three slope positions (up-slope, mid-slope, and down-slope). The results showed more variation in runoff velocity under IPV than AOV. Both IPV and AOV were positively correlated with slope gradient. IPV values were close to AOV ones for slope gradients ≤5°, but were significantly different for slope gradients ≥10°. The mean AOV value was 10.6% higher than that of IPV. Regression analysis showed that compared with AOV, IPV overestimated and underestimated runoff under low and high runoff velocity conditions, respectively. The use of image processing in IPV was advantageous because of its ease of use with fewer artificial errors and its suitability for lateral diffusion of runoff. Irrespectively, additional studies are needed to verify and/or improve further the use of this method in runoff velocity analysis.

A theoretical and experimental study was made of the quiescent settling of the particles of a suspension in order to determine what governs the removal of particles when the suspension passes through a sedimentation basin. Settling column experiments were performed in tubes ranging from one foot to four feet in depth. Glass spheres in water, alum and clay in water, and Pasadena sewage were used as suspensions. While the suspensions settled in the columns, samples were withdrawn at time intervals at several depths. These were analyzed for concentration of suspended particles. It was found that for free, quiescent settling of discrete particles, the removal of particles can be calculated in advance if the frequency distribution of particle settling velocities and the particle concentration are known as functions of position in the suspension at the beginning of settling. For a suspension of flocculent particles it was almost impossible to predict removal without measuring the settling properties of each individual suspension. There is no single universal property of a settling tank with which removal is correlated. It was found that while for some flocculent suspensions removal may be strongly dependent upon detention time, no strong dependence on overflow rate was indicated. Since so many suspensions encountered in settling tanks have flocculent particles, the kinetics of flocculation was studied qualitatively by theory and experiment.

Circulation regions always exist in the settling tanks. These regions reduce the tank’s performance and decrease effective volume of the tank. Recirculation zones would also result in short-circuiting and high flow mixing problems. Inlet position would also have effect on these, too. Using good baffle configuration may increase performance of settling tanks. One method for comparison of different tanks with each others is to use FTC. In this paper, by using FTC the effects of inlet position and baffle configuration on the hydraulic performance of primary settling tanks are studied. The best position of baffle is also determined.

Settling tanks (clarifiers) are key wastewater treatment facilities used for gravity separation of settleable solids contained in the wastewater. Depending on their geometrical shape settling tanks could be rectangular or circular. Clarifier shape most suitable for a given application depends on a number of factors and has to be selected based on a cost‐benefit analysis. Depending on their function, settling tanks are classified in two general categories: primary and secondary. The main purpose of the primary settling tanks is to remove suspended solids from the wastewater treatment plant influent. Secondary clarifiers are located downstream of the plant's biological treatment facilities, such as aeration basins or tricking filters, and are used to separate the biomass generated during the secondary treatment process from the treated plant effluent. Key design criteria used for sizing primary settling tanks are surface overflow rate and hydraulic detention time, while secondary settling tanks are usually designed based on surface and solids loading rates. Recommended values for these design criteria according to various design guideline sources are presented in a table format.

Velocity measurement of water flow within gravel layer with electrolyte tracer method under virtual boundary condition

The aim of this work was to evaluate the removal of suspended solids (SS) and 7-day biochemical oxygen demand (BOD7) in a three chamber septic tank depending on theoretical wastewater retention time and the degree of septic tank cleanliness. It was found out that the performance of the septic tank depended on the degree of its cleanliness: when the septic tank was clean and retention time was three days, SS and BOS7 removal efficiency was 77±10% and 67±14% respectively, whereas two months later, after septic tank desludging, SS removal efficiency decreased to 53±22% and BOD7 to 32±31%. The performance of the septic tank also depended on theoretical wastewater retention time: when some amount of solids was accumulated at the bottom of the septic tank and wastewater retention time was one day, SS and BOS7 removal efficiency was 45±40% and 33±16% respectively; when retention time was three days, SS removal efficiency increased to 53±22% but BOD7removal efficiency remained similar to one day retention time, i.e. 32±31%. Santrauka Bandymų metu buvo įvertintas organinių ir skendinčiųjų medžiagų šalinimas trijų kamerų septike esant vienos ir trijų parų teorinėms nuotekų išbuvimo septike trukmėms bei skirtingam septiko švarumo laipsniui – iš karto po septiko išvalymo, kai jis dar yra švarus, ir praėjus tam tikram laiko tarpui, kai ant septiko dugno yra susikaupęs tam tikras nuosėdų kiekis. Tyrimais nustatyta, kad septiko veikimas priklauso nuo septiko švarumo laipsnio: esant trijų parų nuotekų išbuvimo septike trukmei, vidutinis skendinčiųjų medžiagų (SM) šalinimo efektyvumas buvo 77±10 %, o septynių parų biocheminio deguonies suvartojimo (BDS7) – net 67±14 %, kai septikas buvo švarus, tačiau praėjus dviem mėnesiams nuo septiko išvalymo jo veikimas pablogėjo ir vidutinis SM šalinimo efektyvumas po septiko išvalymo buvo 53±22 %, o BDS7 – 32±31 %. Taip pat nustatyta, kad šalinant teršalus didelę reikšmę turi nuotekų išbuvimo septike trukmė: kai septikas buvo švarus, o nuotekų išbuvimo trukmė trys paros, vidutinis SM šalinimo efektyvumas – 77±10 %, o BDS7 – 67±14 %; sumažinus nuotekų išbuvimo trukmę iki vienos paros, SM šalinimo efektyvumas sumažėjo iki 31±38 %, o BDS7 šalinimo efektyvumas buvo neigiamas.

Application of tracer techniques to continuous-flow toxicity testing

Modeling the effect of wind in rectangular settling tanks for water supply

Evaluating the performance of a settling tank is an important issue for wastewater system managers. The relevance of a CFD approach for determining the settling efficiency of a tank has already been demonstrated from experimental data obtained from scale models of basins. The CFD modelling strategy is based on the resolution of Navier-Stokes equations to calculate the flow (Eulerian approach), and then on a Newton equation to calculate particle trajectories (Lagrangian approach). In this study, experimental data on settling efficiency are collected in a 1 scale cylindrical settling tank constructed in the laboratory. Eighteen experiments were carried out to collect data (settling efficiency) for a range of flow rates (between 5 and 30 l/s) and three materials representative of the sediments encountered in sewage networks. These data were then compared with the results obtained by CFD modelling to assess the relevance of the numerical approach for a full-scale structure.

Abstract. Nowadays, a settling tank's removal efficiency is one of the most crucial matters for all water or wastewater treatment plants (WTPs or WWTPs). The unit can affect WWTP performance and improve the provided effluent quality. In this paper, the geometrical aspects of a settling tank were numerically analyzed via tracer curves, the finite-volume method, and ANSYS CFX software in which the baffle depth and diameter of a settling tank were assessed. Firstly, a previous study was similarly remodeled to verify simulation results. The impact of tank depth variation was numerically assessed where the outcomes showed that a deeper tank could raise discharge time or the hydraulic retention time (HRT). Thus, extensive discharge time may result in less polluted effluent, degrading more solids. However, the tank should not be too deeply based on costs. Moreover, the differential effect of baffle height was analyzed and indicated that lower height is more useful for boosting the HRT. An investigation of tank diameter changes also revealed that wider diameters bring about a broader HRT.

A biological-chemical treatment plant for cheese factory wastewater was upgraded to handle increased loads and achieve higher removal efficiencies with respect to organic matter and phosphorus. An existing aeration basin was converted to an aerated equalization tank and two existing trickling filters were replaced by two moving bed biofilm reactors (MBBRs). In the MBBRs biomass is attached to small plastic elements that move freely along with the water in the reactors. A new final settling tank was added after an existing intermediate settling tank and existing flocculation chambers. At loads that were highly variable and often above the design values, the upgraded plant achieved average removal efficiencies of 98% with respect to both total COD and total P. An average effluent concentration of 0.36 mg total P/l was obtained with an average coagulant dose of 2.0 mol Al3+/mol P.

Film-mulched rock outcrops are a proven way to effectively prevent preferential flow at the rock–soil interface in rocky karst areas, but the impact on rainwater redistribution and maize growth in farmland areas has never been studied. This paper used the dye tracer method at a sprinkling intensity of 1 mm min−1 to study the rainwater redistribution in soils for three different treatments: away from rock (AR), close to rock (CR), and close to film-mulched rock (CFMR). The growth situation of maize (Zea mays L.) according to the different treatments was also studied. It was shown in the study that the rainwater gathered by rock outcrops was mainly confined only within a narrow flow path at the rock–soil interface in the CR treatment, with a preferential flow fraction of 35.78~55.54% and dyeing depth of 15.37~20.00 cm across the three dye application amounts (850 mL, 1700 mL, and 3400 mL) in contrast to the uniform distribution of the rainwater in the soil of the AR treatment, with a preferential flow fraction of 12.31~37.15% and dyeing depth of 6.93~14.00 cm. Interestingly, in the CFMR treatment, there was no preferential flow at the rock–soil interface benefiting from the film’s blocking action, and the redistribution of rainwater was mainly based on the matrix flow, with a preferential flow fraction of 24.93~39.62% and dyeing depth of 10.27~18.00 cm, indicating that the film-mulched rock outcrops improved the rock’s output capability for gathering rainwater into the surrounding soil. In addition, film-mulched rock outcrops can promote the growth of maize, as indicated by the results for plant height, stem diameter, maximum leaf area, SPAD value, and floral development of maize, which increased in order from AR, CR, to CFMR. Our study suggests that film-mulched rock outcrops have important implications for the efficient use of rainwater and the growth of crops in rocky karst areas, as well as in regions with similar geological characteristics.

The possibility of enhancing the process performance of the UASB-septic tank for treating strong sewage in Palestine by means of inoculating the reactor with well adapted anaerobic sludge and/or adding a packing media to the upper part of the reactor, creating an anaerobic hybrid (AH)-septic tank, was investigated. To achieve these objectives, two community onsite UASB-septic tank and AH-septic tank were operated in parallel at 2 days HRT for around 8 months overlapping the cold and hot periods of the year in Palestine. The achieved removal efficiencies of CODtot in the UASB-septic tank and AH-septic tank during the first months of operation, coinciding with the cold period and the subsequent hot period, were respectively 50 (+/- 15)% and 48 (+/- 15)% and 66 (+/- 8)% and 55 (+/- 8)%. This shows that the UASB-septic tank performed significantly better (p < 0.05) than the AH-septic tank after rather long periods of operation. The difference in the CODtot removal efficiency was mainly due to the better CODss removal efficiencies in the UASB-septic tank. The removal efficiencies over the last 50 days of operation for CODtot, CODsus, CODcol and CODdis were 70, 72, 77 and 55% and 53, 54, 78 and 45% for the UASB-septic tank and AH-septic tank, respectively. Comparing the here achieved COD removal efficiencies with previously reported efficiencies of UASB-septic tanks operated in Palestine shows that the reactor performance in terms of COD removal and conversion, during the first 8 months of operation, has improved substantially by being started with well adapted anaerobic sludge, simulating and predicting long-term performance. Adding packing media did not lead to an improvement.

This study explores the challenges of sloshing in rectangular tanks, focusing on the use of an internal perforated plate to mitigate its adverse effects. A novel numerical flow model is developed to examine sloshing dynamics involving these plates. The Navier–Stokes equations are solved using the projection method on a Cartesian grid, while a newly adapted immersed-boundary generalized harmonic polynomial cell method is employed to solve the Poisson equation within this framework. The solver's accuracy is validated through comparisons of free surface profiles, pressure, and velocity data with experimental results, demonstrating reliable performance in simulating sloshing phenomena. The research emphasizes optimizing the placement of perforated plates within the tank to maximize energy dissipation and minimize structural forces. The findings demonstrate that strategically positioning perforated plates effectively mitigates violent resonant sloshing caused by horizontal excitation, highlighting the critical influence of plate location on tank performance. These insights are instrumental for enhancing tank stability in engineering applications such as liquid cargo transportation, offshore storage facilities, and fuel tank design for aerospace and automotive industries, where precise control of fluid dynamics is essential for safety and operational efficiency.

The Standards for Water Works issued by the Korean government prescribed the insertion of inclined plates in a clarifier to enhance the settling of the suspended solids. In this study, in order to verify the role of the inclined plates, two identical laboratory-scale rectangular clarifiers were constructed and eight inclined plates were inserted into one of the clarifiers and inflow from same source was treated in both the clarifiers. Dye tests revealed that only the front three of the seven slots received the inflow at 0.57 m3/m2·h, which was the highest SOR (surface overflow rate). Three different SORs, with 12 different SS (suspended solid) concentrations at each overflow rate, were fed to both clarifiers. However, the clarifier with the inclined plates failed to show an improved removal rate for the SS. In order to enable the Boycott effect within the slot, it is suggested that each slot created by the inclined plates receives equalized inflow. Moreover, collision of the inflow with the settled sludge at the bottom of the clarifier has to be avoided. These provisions, which can maximize the Boycott effect, should be added to the Standards for Water Works endorsed by Korean government.