Secondary Settling Tank Research Articles

NUMERICAL MODELLING AND MEASUREMENT IN A TEST SECONDARY SETTLING TANK

A numerical model and measurements of flow and settling in activated sludge suspension is presented. The numerical model is an attempt to describe the complex and interrelated hydraulic and sedimentation phenomena by describing the turbulent flow field and the transport/dispersion of suspended sludge. Phenomena such as free and hindered settling and the Bingham plastic characteristic of activated sludge suspensions are included in the numerical model. Further characterisation and test tank experiments are described. The characterisation experiments were designed to measure calibration parameters for model description of settling and density differences. In the test tank experiments, flow velocities and suspended sludge concentrations were measured with different tank inlet geometry and hydraulic and sludge loads. The test tank experiments provided results for the calibration of the numerical model and for comparing measured and calculated results. The numerical model could, fairly accurately, predict the measured results and both the measured and calculated results showed a flow field pattern identical to the flow fields in full-scale secondary settling tanks. A specific calibration of the Bingham plastic characteristic was needed and further knowledge of activated sludge suspension rheology will be necessary to improve the numerical model.

Retrofitting and Operation of Small Extended Aeration Plants for Advanced Treatment – Some Experiences in Japan

Some experiences concerning a retrofit design and operation for nutrients removal in small extended aeration plants are presented. In this study a new biological phosphorus removal process as well as a biological nitrogen removal process based on a sequential oxic-anoxic-oxic process is investigated. The denitrification in the first oxic zone has a high removing effect for nitrogen, and the oxidation reduction potential (ORP) value and dissolved oxygen (DO) concentration are useful indexes to maintain the optimum conditions of the first oxic zone for simultaneous denitrification. It is verified that the simultaneous denitrification can alleviate the alkalinity deficit problem against full nitrification by a stoichiometric analysis of alkalinity throughout the entire process. Additionally, a biological phosphorus removal process which uses a sludge blanket zone of secondary settling tank for phosphorus release is proposed.

A model for optimum design of activated sludge plants

An optimization model of the activated sludge wastewater treatment systems was presented. Such a system was relatively complex, containing a series of unit processes, each designed to achieve a specific goal. The design basis of each unit in the system, which consists of primary settling, aeration and secondary settling tanks was given. The model was run at the varying dissolved oxygen concentration. A relationship was developed between sludge volume index and sludge age using data from literature. The relationships between sludge volume index and coefficients of solids flux model, V 0 and n, were also found separately. An optimum design model was developed for the system. In the optimization model, relationships mentioned above were considered. The model was solved by using the Box-Complex Algorithm. The results obtained from the model were discussed.

Neural model for the operational control of activated sludge processes

A modelling and simulation investigation of an activated sludge process shows that the relationships between the recycle ratio (or recycle flow rate), the sludge wastage rate and the influent organic loading at steady state operation are complex due to the inherent nonlinearity and multivariable features of the process. With a limited amount of operational data obtained from the simulation, a multi-layer feedforward neural network was formulated and trained. This exhibited an excellent capability for both interpretation and prediction of these relationships in terms of accuracy. For the further illustration of this neural net-based approach, another neural model was developed based on plant data for the operational prediction of the secondary settling tank performance of a pilot plant.

Modeling of Rectangular Settling Tanks

A mathematical model for predicting the velocity field and suspended solids transport in secondary rectangular settling tanks is presented. The model is applied to investigate the effects of density stratification on the hydrodynamics in the settling tank, such as the behavior of the bottom current and surface return flow and solids concentration distribution. The model can be used to simulate the so‐called density waterfall phenomenon at the front end of the settling tank. With the application of a double exponential relationship to describe the settling process of suspended solids with nonuniform particle sizes, the simulations of the solids concentration distribution in the low‐concentration environment can be significantly improved. By comparing the predictions with field measurements, this model is verified and generally good agreement is obtained.

Flow regulation by automatically controlled overflow weirs

In larger sewage plants, motor-regulated overflow weirs are sometimes used to achieve a uniform flow distribution into the secondary settling tanks, backed up by simultaneous control of the immersion depth of the upstream aerating units. It is demonstrated that, if these overflow weirs are arranged in series on one side of the feed channel, they are unable, with a fixed setting, to ensure equal flow distribution when there is a variable inflow. If flowmeters are mounted in the discharge pipes, with feedback controls to the motor-regulated weirs, it is then possible to achieve equal flow distribution.

Effect of Activated Sludge Processes on Secondary Settling Tank Efficiencies

Some calculations and experimental work on the turbulence in aeration tanks using different aeration systems and its effect on floc cohesion are presented. Vigorous aeration causes floc dispersion and it can be quantified in standard test conditions using velocity gradient calculations and comparing these with the numbers of particles in the supernatant liquor after 30 minutes settlement of the sludge. Tests on final settlement tanks indicated that the density current is of prime importance and that the harmful effects of this can be overcome by installing deep inlets and arranging to have the outlet weir inset and as near to the inlet as possible. Mixing calculations also indicated that circular tanks are more efficient than rectangular and that if the latter were used transverse flow is better than longitudinal flow. A test on a full scale plant indicated that flocculation occurred in the density current zone of the final tank so that the number of particles in the supernatant decreased to an extent similar to that produced by mild agitation.

Control of Activated Sludge Circulation and of Secondary Settling Tank Hydraulics

The directives issued by the German Waste Water Association (ABWASSERTECHNISCHE VEREINIGUNG (ATV)) and by the British WATER RESEARCH CENTRE (WRC) in respect of the rating and control of activated sludge circuits and of the loading of secondary settling tanks of biological waste water treatment plants were studied for their applicability to papermill effluents. To this end, several years' testing was carried out on an activated sludge pilot plant and on various industrial plants. These tests revealed that secondary settling tanks of papermill effluent treatment plants may safely be rated in accordance with ATV up to a sludge volume index (SVI) of 300 ml/g and a sludge return rate of 3. If SVI values are low, the WRC process permits excessive surface loadings and is thus only practicable for high SVI levels. If sedimentation properties of the activated sludge are poor, load capacity limits are indicated more correctly by the WRC method. A simplified settling velocity method derived from the WRC process gives reliable information on the load capacity of secondary settling tanks and may readily be used for sludge circulation control.

Activated Sludge Process Control by Behaviour of Secondary Settling Tanks

The mathematical models of activated sludge kinetics and solids flux have been investigated; the constants have been derived experimentally. By developing the equations of the two models, an operational chart for activated sludge process control has been plotted.

Disposal of Alum Sludge to the Existing Activated Sludge Wastewater Treatment Plant

Abstract For finding the practicability of discharging water treatment plant sludge into the municipal sewer, the effects of synthetic aluminum hydroxide floc on several major units in an activated sludge wastewater treatment process were studied. It was found that the crucial effects were the increase in sludge production, either in the primary settling tank or in the secondary settling tank, and the retardation of the digestibility of the wasted sludge. The treatment efficiency, in terms of the quality of the treated wastewater, however, was improved. No toxic effects were found in the study of the functions of organic matter removal and the nitrification of activated sludge with the addition of aluminium hydroxide up to 300 mg Al/1 to the mixed liquor. The dewatering property of the wastewater sludge was improved with the addition of this chemical floc. Two processes are suggested. One is to discharge alum. sludge into the sewer at a suitable rate and the other is to discharge the water treatment wastes into a thickener in the wastewater treatment plant. The concentrated underflow can be mixed with the regular digested sludge for dewatering while the supernatant can be discharged either to the primary settling tank or to the aeration tank.

Settling of Activated Sludge in Horizontal Tanks

The settling properties of activated sludge in the aerator effluent from 20 activated-sludge plants were examined in settling tubes. A study of flow conditions and efficiencies of the secondary sedimentation tanks at the plants, all of the horizontal type, was conducted. It was found that density currents caused low volumetric efficiency, but at actual loading conditions all settleable material was removed. The apparent depth of the bottom density current was found in all cases to be about 2 ft. Unsettleable floc in the settled water occurred noticeably when the concentration of sludge in the aeration tanks was low. The investigations indicate that considerable improvements in the design of secondary settling tanks are possible. The design must be made with respect to the highest concentration of activated sludge liquor that will be used in the process.

The Biology of Physa anatina Lea, a Snail Living in a Sewage Treatment Plant

Relatively little specific information is available regarding the biology of any species of Physa, consequently, when Physa made its appearance in extreme abundance in the sewage treatment plant of Urbana-Champaign, Illinois, there seemed to be exceptional opportunity for studying certain aspects of the life cycle. The following statements give a general idea of the plan of the treatment works. Raw sewage enters the Imhoff tanks in which considerable bacterial action takes place. From here it is pumped into the sprinkling filters where jets of the sewage are forced into the air for aeration. As the aerated sewage soaks through the deep beds of crushed stone, organic matter is removed. The sewage is then sent through conduits to the secondary settling tank, from which a part of the final effluent is diverted into an experimental lagoon, the remainder being released into the Saline Drainage Ditch, a tributary of the Big Vermilion River. In the present investigation, collections from different regions of the plant have been studied in an effort to determine possible restrictions or limitations of breeding areas within the system. These field studies have been supplemented by laboratory observations on the snails to determine, in so far as possible, the rate of reproduction and rate of growth.