Flow equalization is the damping of flowrate variations so that a constant or nearly constant flowrate is achieved. Equalization tanks during wastewater treatment can be placed either “in-line” (major flow damping) or “off-line” (minor flow damping). The principal benefits of flows equalization are: (a) biological treatment is enhanced because shock loadings are eliminated or minimized, inhibiting substances are diluted and pH is stabilized; (b) the effluent quality and thickening performance of secondary sedimentation tanks following biological treatment is improved through constant solids loading; (c) effluent-filtration surface-area requirements are reduced, filter performance is improved and more uniform filter-backwash cycles are possible; and (d) in chemical treatment, damping of mass loading improves chemical feed control and process reliability.
The volume requirements are found by determining the biggest difference between the highest volume accumulation in the tank and the lowest volume accumulation in the tank, as calculated from the supplied experimental measurements of inflow in several time intervals (ideally around a full day, or 24h period). In practice the actual tank volume will be larger than the theoretically determined to account for the following factors: (a) Continuous operation of aeration and mixing equipment will not allow complete drawdown, although special structures can be built, (b) Volume must be provided to accommodate the concentrated plant recycle streams that are expected, if such flows are returned to the equalization basin, and (c) Some contingency should be provided for unforeseen changes in the incoming flow.
In Design Mode of calculation the model predicts the minimum required liquid volume for equalization based on the sampled data supplied (in the constant outlet flow mode) or based on the total liquid volume supplied (in the mode where outlet flow is equal to input flow). Then based on the liquid to total volume ratio, it estimates the tank volume, surface area and number of units.
In summary, an equalization tank set in design mode calculates as follows:
Given
● Sample Measurements of Inlet Flow and Concentration
● Operation Mode (constant outlet flow, or constant storage volume)
● Tank Shape (rectangular vs. circular)
● Tank Depth
● Liquid to Total Ratio and,
● Maximum Surface Area
Calculate
● Number of Units Required
● Surface Area for Each Unit
In Rating Mode of calculation, the program considers the supplied value for surface area, number of units and depth (or in the case of constant storage flow, total liquid storage) and based on the supplied measurements decides if there’s enough equalization volume (in case outlet flow is considered to be equal to the time average of the inlet flow measurements). Note that in case where the operating mode is set to ‘outlet flow = inlet flow’ (constant storage volume) the equalization criterion is always satisfied; also, in that case the user cannot set the surface area (otherwise the model becomes over-specified).
In summary, an equalization tank set in rating mode calculates as follows:
Given
● Sample Measurements of Inlet Flow and Concentration
● Operation Mode (constant outlet flow, or constant storage volume)
● Tank Shape (rectangular vs. circular)
● Tank Depth
● Liquid to Total Ratio and,
● Number of Units Required
● Surface Area (unless mode is set to constant storage flow)
Calculate
● Feasibility (unless mode is set to constant storage flow)
For volatile organic compound (VOC) emission calculations, it is assumed that the equalizer behaves like a quiescent tank. The calculations are identical to those of the Clarification Model
1. Metcalf & Eddy, Inc. 3rd Ed.(1991) Wastewater Engineering, McGraw-Hill, Inc.
2. D. W. Sundstrom, H. E. Klei, (1980) Wastewater Treatment, Prentice Hall, Inc.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Equalization: Oper. Conds Tab
● Sampling, see Equalization: Sampling Tab
● Vent/Emissions, see Cont. Stoich. Reaction in a Photobioreactor: Vent/Emissions Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab