# Storage Treatment Plant

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- Created by Unknown User (reynard.juanir), last modified by Abraham Toribio on Nov 16, 2020

**The Storage/Treatment module** (S/T in sanitary mode) or BMP Treatment Process can simulate the routing of flows and pollutants through a dry or wet weather storage/treatment plant containing a number of units or processes. Each unit may be modelled as having detention or non-detention characteristics.

This option is selected when a Storage-Treatment Plant (or BMP) is to be modelled. BMP is enabled by selecting one of the following options in Job Control.

Sanitary | Pollutant List |

Hydraulics | Pollutant List |

Runoff | Water Quality |

Each unit may be modelled as having detention or non-detention characteristics. The various units may be linked in a variety of configurations. Sludge handling may also be modelled using one or more units. Additionally, capital cost and operation and maintenance cost may be estimated for each unit. If the BMP or S/T Plant button is greyed-out it may be enabled by the **Job Control** setting.

The module takes input from the node it belongs to in the network. Pollutants may be characterised by their magnitude alone or by magnitude and their particle size/specific gravity distribution. Pollutant removal may be done by flexible user-defined equations.

This dialog allows the definition of a BMP or Storage/Treatment plant in terms of a number of interconnected units. Each unit is individually defined, as is the general characterisation of pollutants through the unit.

A Plant is made up of a number of treatment units with one unit entered on each line of the list below. The unit type is selected by clicking on the type field of the list. Each unit has an outflow, a bypass flow and a residual flow. The destination of each of these flows is entered as a downstream unit number, for example a storage treatment plant like this would be represented as:

Unit No. | Type | Outflow | Bypass | Residual | X | Y |
---|---|---|---|---|---|---|

1 | Screen Process | 2 | 3 | 4 | 20 | 100 |

2 | Storage Process | 3 | 3 | 4 | 150 | 100 |

3 | Node Outfall | 400 | 25 | |||

4 | Disposal | 300 | 100 |

## Unit No.

The Unit Number is a integer number that identifies each unit uniquely. Note that any unit to which flows are directed must have a unit number greater than the source unit. It is recommended that unit numbers begin from 1.

## Unit Type

The unit type may be one of Screen Process (Screen Process Unit), Storage Process (Storage Process Unit), Node Outflow or Disposal (Ultimate Disposal).

A **Screen Process** is a non-detention unit; there is no storage associated with it, thus all flow is passed instantaneously. Examples of Screen Processes are microscreens, fine screens, and other forms of screening.

A **Storage Process** has detention properties; these units allow processes such as sedimentation, dissolved air flotation, chlorination, and sludge thickening to be modelled.

This option inserts a Node Outflow unit into a row of the Treatment Unit list. The Node Outflow is not a Storage/Treatment unit as such; it is used to transfer outflows to the node outlet. Node Outflow Units redirect any incoming flows to the outflow of the connected node.

The Disposal unit provides a mechanism for "losing" flow from the system. Disposal units remove any incoming flows from the simulation.

### Screen Process (Screen Unit)

This option inserts a screen process unit into a row of the Treatment Unit list and accesses the Screen Process dialog. A Screen Process is a non-detention unit; there is no storage associated with it. A typical example of a Screen Process is a fine screen filter.

A Screen Process (or Unit) is a non-detention unit; there is no storage associated with it, thus all flow is passed instantaneously. Examples of Screen Processes are microscreens, fine screens, gross pollutant traps and other forms of BMP's. A "critical" particle size or settling velocity is specified and any particles with a larger particle size or settling velocity are removed (along with the associated pollutants). Although the algorithm is simple, the interpretation of "critical" size or velocity is more complex.

Pollutant removal by screens is a result of two actions: the straining of the screens, and the additional filtration provided by the mat produced by the additional screening. Screens may vary widely in the size of the aperture and the manner in which wastewater flows through them. The model ignores any effect of the mat. Assuming that the specified size corresponds to the screen aperture is probably valid. If the specified size falls between the high and low ends of any size range, the pollutants are removed by linear interpolation. If the pollutant is characterised by a settling velocity distribution (in lieu of a size distribution), then the portion associated with velocities greater than or equal to the "critical" value are removed.

**Bypass Flow Threshold**

This item contains the maximum inflow into this unit, beyond which all flows and pollutants are bypassed. This variable is shared with the Bypass Flow threshold in Storage Processes.

This variable may be set to an abnormally high value for design purposes (i.e. responses to all possible inputs) or set at a realistic value for modelling existing or proposed facilities.

**Residual Flow Fraction**

This item contains the residual flow as a fraction of the inflow. Residual flows occur only during dry periods (ie. no inflow or treated outflow), and serve to drain the unit between storms.

**Pollutant Removal (Screen Removal)**

This button and the associated dialog allows the pollutant removal characteristics to be defined for a particular unit. A list of pollutants appears (as entered in Job Control), and the removal option for each pollutant is entered.

**Pollutants List**

The list of pollutants available is from the 'Use Pollutants' item in Job Control, and represents the list of pollutants for Sanitary analysis. As pollutants are highlighted, the pollutant removal options change to reflect the selected pollutant.

**Critical Particle Size**

This option selects the entered Critical Particle Size as the means of pollutant removal. All particles above this size are removed from the influent. This option allows the modelling of Screen Processes such as microscreens, fine screens, and coarse screens.

A Critical Particle Size can be used to model non-detention units such as screens. An approximation of the removal effectiveness of screens may be obtained by letting the Particle Size equal the aperture size of the screen.

This option is enabled for the highlighted pollutant by selecting the 'Particle Size Distribution' option under 'Storage Treatment', 'More', 'Pollutant Characterisation' dialogs.

**Critical Settling Velocity**

This option uses the entered Critical Settling Velocity in the estimation of pollutant removal.

This option is enabled for the highlighted pollutant by selecting the 'Settling Velocity Distribution' option under 'Storage Treatment', 'More', 'Pollutant Characterisation' dialogs.

**Removal Equation Flag**

This option selects the equation entry dialog for the selected pollutant. This option is enabled for the highlighted pollutant by selecting the 'Concentration Only' option under 'Storage Treatment', 'More', 'Pollutant Characterisation' dialogs.

See Pollutant Removal Equation for more details.** **

**Costs Flag**

This flag controls the calculation of the capital costs and the operation and maintenance costs for this unit. If OFF, cost calculations are not computed for this unit. The flag is shared between the Storage Process type and Screen Process options for the same unit.

**Screen Unit Costs**

Initial capital and operation and maintenance costs are calculated at the end of the simulation. These costs are computed using only the information processed for the simulation period; ie. no attempt is made to derive costs for particular time intervals (eg. annual).

*Capital Cost*

The capital cost for each unit is computed as a function of a specified design flow or volume, or is calculated by the model as a function of the maximum value recorded during the simulation.

The costs are calculated from a list of variables as described below:

**QMAX**- Maximum allowable inflow. This value is found in the 'Bypass Flow Threshold' field under the 'Screen Process' dialog.**QIMAX**- Maximum inflow during simulation. This value is calculated by the program. Note that if no selection is made, the program will ignore the term in the equation.

The Equation used is of the form:

COST = a * X^b

*where*

COST = initial capital cost, dollars

a = coefficient

b = coefficient

X = either QMAX (Maximum Available Inflow) or QIMAX (Maximum inflow encountered during the simulation.

Caution is needed when using these calculated cost results. In a single simulation when the simulated event is a design event the calculated capital cost can only be an estimator of the true capital cost.

*Op**eration and Maintenance Costs*

The operation and maintenance costs are calculated as a function of a specified design flow or volume. The Equation used is of the form:

COST = d * X^f + (h * Dop)

*where*

COST = operation and maintenance cost, dollars

d = coefficient

f = coefficient

h = coefficient

X = either QMAX (Maximum Available Inflow) or QIMAX (Maximum inflow encountered during the simulation.

Dop = the total operating time during the simulation period, hours

Caution is needed when using these calculated cost results. The calculated operation and maintenance costs can only be a valid estimator of the true costs when a long-term simulation is performed, when operating time is a factor in computing operation and maintenance costs.

### Storage Process (Storage Unit)

This dialog deals with the characteristics of Storage Process (detention) units. Sedimentation, dissolved air flotation, chlorination and sludge thickening are some of the processes that maybe modeled by a detention unit.

**Hydraulic Properties (Storage Unit)**** **

This dialog describes the geometry and hydraulics of the detention unit. Standard level-Puls reservoir routing techniques are used to simulate flow routing. A relationship between depth, surface area, treated outflow, residual flow, and storage volume is required. The relationship can be entered directly, or indirectly with outflow conditions of weirs, orifices, or pumps. Evaporation losses are also accounted for in detention units. The evaporation values used are entered in Job Control.

**Initial Volume (Hydraulic Properties)**

This item contains the total volume of water in the Storage Unit at the start of the simulation, in ft^3 [m^3].

**Depth (Hydraulic Properties)**

This column contains depths for the Detention Unit, in ft [m]. This is the primary variable in the hydraulic depth-storage-outflow relations.

**Surface Area (Hydraulic Properties)**

This column contains the Surface Area corresponding to Depth in the previous column, in ft^2. [m^2]. The only required hydraulic parameters are the depth and surface area. Volumes can be calculated by the model using the trapezoidal rule and surface areas.

**Volume Flag (Hydraulic Properties)**

If this flag is ON, a depth-volume relationship needs to be entered directly. If this flag is OFF, the model will calculate a depth-volume relationship by averaging the surface area between adjacent values of depth, multiplying by the difference in depth, and adding the incremental volume to the accumulated total.

If the above flag is OFF, no data is required for this column, and the model computes volume by trapezoidal integration of surface areas.

**Residual (Hydraulic Properties)**

This flag enables a residual stream to be drawn from the storage unit. Residual flows are drawn off during periods of no inflow or treated outflow, and are handled in the same manner as treated outflow for the purposes of reservoir routing.

When a residual stream occurs from a plug-flow unit the entire unit contents (including the removed pollutant quantities) are mixed (ie. the remaining plugs lose their identity) and drawn off until the unit is empty or inflow occurs. If inflow begins before the unit is empty, the remaining contents are placed in a single plug for further routing. In a completely mixed unit, the pollutant concentrations in the residual flow are identical to the concentrations in the treated outflow. Again, the flow is suspended when inflow occurs.

If this flag is OFF, a residuals stream is never drawn off, and the accumulated pollutants remain in the unit. In this case, the residual power equation or table of Residuals is not used. If the flag is ON, the depth-residual flow relationship may be specified directly or by a power equation.

**Outflow (Hydraulic Properties)**

This column provides options in describing how the discharge of treated outflow is related to depth. The options available include direct entry of a treated outflow for every depth, or a power equation to approximate the relation, or a pump that specifies a constant pumping rate between certain depths. The data in the column is only required if the 'Direct' option above the column is enabled. When `Direct’ is on the data is entered directly in ft^3/s [m^3/s] for the corresponding depth,

**Residual Flow (Hydraulic Properties)**

This item contains the residual flow from the Detention Unit, when entered directly, for the corresponding depth, in ft^3/s [m^3/s]. This data is only required if the 'Direct' option above the column is ON and the 'Residual' check box is ON.

In addition to treated outflow, a residual stream may be drawn from the storage unit during periods of no inflow or treated outflow. This dialog specifies the manner in which residual flows are drawn off. Residual flows are drawn off during periods of no inflow or treated outflow, and thus serve to drain the detention unit between storms. The unit can be drained after a specified number of dry time steps, or on a scheduled basis.

** Every `x' Hours (Residual Draw-off). **This option draws residual flows off starting at a certain number of time step intervals, but the flow is delayed if inflow and/or treated outflow is in progress. This option corresponds with the situation in which the unit is drained on a regular basis. The value entered represents The number of time steps after which residuals will be regularly drawn off. Time step size is specified in Job Control.

** After `x' Hours of No Flow (Residual Draw-off). **This option draws off residual flows only after the entered number of time steps of no inflow or treated outflow. This option applies to the case in which the unit contents are drained after each runoff event.

**Bypass Flow Threshold**

This item contains the maximum inflow into this Storage Unit beyond which all flows and pollutants are bypassed. This value can be set to an abnormally high value for design purposes (to allow a response to all possible inputs) or set to a realistic value for modelling existing or proposed facilities.

**Pollutant Routing Method**

Pollutants are routed through a detention unit by one of two modes: Complete Mixing or Plug Flow:

**Complete Mixing**

Selecting this option causes the program to route pollutants for this unit assuming complete mixing. Complete mixing is most applicable to small tanks where the primary purpose is to thoroughly mix the contents (rapid-mix chlorination flocculators and mixing tanks).

In this method, the concentration of pollutant in the unit is assumed to be equal to the effluent concentration. Pollutants are removed at a rate proportional to the concentration present in the unit (ie. a first-order decay reaction is assumed).

Removed pollutant quantities are not allowed to accumulate under this option. Strictly, pollutants cannot settle under such conditions. Therefore, the residual stream is effectively another route for treated outflow. All pollutant removal is assumed to occur bynon-physical means (eg. biological decomposition). Several processes such as flocculation and rapid-mix chlorination are essentially completely mixed detention units.

**Plug Flow**

Selecting this option causes the program to route pollutants for this unit assuming perfect plug flow. Perfect plug flow is recommended for long, rectangular tanks where settling is the most important removal mechanism and is required when any pollutant is characterised by a particle size/specific gravity or settling velocity distribution.

Removed pollutants are accumulated in plug-flow units, without decay, until removed by the residual flow.

*Travel Length (Plug Flow)*

This item contains the travel length for plugs in this unit. This value is used in the computation of a turbulence factor used in particle settling calculations.

*Detention Surface Roughness (Plug Flow)*

This item is the Manning's roughness coefficient for the surfaces of this Detention Unit. This value is used in the computation of a turbulence factor used in particle settling calculations.

*Sludge Generation (Plug Flow)*

This data is used to indicate the build up of sludge in a plug-flow detention unit. The method requires a pollutant used to calculate the sludge volume, he concentration of that pollutant in the sludge, and the acceptable level of sludge. The model assumes that the sludge volume has no effect on the available storage volume and that no compression occurs. This information is used to warn the user of possible maintenance/performance problems.

*Predominant Pollutant in Sludge (Plug Flow)*

This flag is used to enable computations on sludge depth. When this flag is ON, a warning message will be output if the accumulated sludge exceeds the depth entered below. The selected pollutant is used to calculate the sludge volume. The adjacent button selects the pollutant used to calculate the sludge volume.

*Pollutant Concentration. in Sludge (Plug Flow)*

This item represents the concentration of the predominant pollutant in the sludge. The sludge volume is increased by dividing the amount of pollutant removed each time step by this concentration. The concentration is given in units consistent with the pollutant, as defined in the Pollutants Global Database.

*Max Sludge Depth (Plug Flow)*

This item contains the maximum depth that sludge in this detention unit may reach before a sludge depth warning message is output. The message will warn of possible maintenance/performance problems.

See also Plug Flow Theory

**Pollutant Removal (Storage Unit)**

This dialog allows the pollutant removal characterisation to be defined in concrete terms for this unit. A list of pollutants appears, and the removal option for that pollutant is entered. The only option available is that for which the pollutant has been characterised in this Storage/Treatment plant in 'Pollutant Characterisation', under 'More' in the 'User Built Treatment Plant' dialog.

For detention units, the initial concentrations of pollutants at the start of simulation is also required.

** Initial Concentration (Storage Unit Removal. **This item should contain the concentration of the highlighted pollutant in this unit at the start of the simulation. The concentration is given in units consistent with the pollutant, as defined in the Pollutants Global Database.

** Pollutants List. **The list of pollutants available is from the 'Use Pollutants' item in Job Control, and represents the list of pollutants for Sanitary analysis. As pollutants are highlighted, the pollutant removal options change to reflect the selected pollutant.

** Critical Particle Size. **This option selects the entered Critical Particle Size as the means of pollutant removal. All particles above this size are removed from the influent. This option allows the modelling of Screen Processes such as microscreens, fine screens, and coarse screens. A Critical Particle Size can be used to model non-detention units such as screens. An approximation of the removal effectiveness of screens may be obtained by letting the Particle Size equal the aperture size of the screen. This option is enabled for the highlighted pollutant by selecting the 'Particle Size Distribution' option under 'Storage Treatment', 'More', 'Pollutant Characterisation' dialogs.

* Critical Settling Velocity. *This option uses the entered Critical Settling Velocity in the estimation of pollutant removal. This option is enabled for the highlighted pollutant by selecting the 'Settling Velocity Distribution' option under 'Storage Treatment', 'More', 'Pollutant Characterisation' dialogs.

**Costs (Storage Unit)**

Initial capital and operation and maintenance costs are calculated at the end of the simulation. These costs are computed using only the information processed for the simulation period; ie. no attempt is made to derive costs for particular time intervals (eg. annual).

**Capital Cost (Storage Unit)**

**Capital Cost (Storage Unit)**

The capital cost for each unit is computed as a function of a specified design flow or volume or is calculated by the model as a function of the maximum value recorded during the simulation.

The Equation used is of the form:

COST = a * X^b

where

COST = initial capital cost, dollars

a = coefficient

b = coefficient

X = either QMAX (Maximum Available Inflow) or QIMAX (Maximum inflow encountered during the simulation) or VMAX(Maximum allowable storage) or VSMAX (Maximum storage encountered during simulation)

Caution is needed when using these calculated cost results. In a single event simulation the calculated capital cost can only be an estimator of the true capital cost when the simulated event is a design event.

**Operational and Maintenance Cost (Storage Unit)**

**Operational and Maintenance Cost (Storage Unit)**

The operation and maintenance costs are calculated as a function of a specified design flow or volume or is calculated by the model as a function of the maximum value recorded during the simulation.

The Equation used is of the form:

COST = d * X^f + (h * Dop)

where

COST = operation and maintenance cost, dollars

d = coefficient

f = coefficient

h = coefficient

X = either QMAX (Maximum Available Inflow) or QIMAX (Maximum inflow encountered during the simulation) or VMAX (Maximum allowable storage) or VSMAX (Maximum storage encountered during simulation)

Dop = the total operating time during the simulation period, hours

Caution is needed when using these calculated cost results. The calculated operation and maintenance costs can only be a valid estimator of the true costs when a long-term simulation is performed, when operating time is a factor in computing operation and maintenance costs.

**Cost Variables (Storage Unit)**

**Cost Variables (Storage Unit)**

This dialog allows you to select from a list of variables:

**QMAX Maximum allowable inflow**- this value is found in the 'Bypass Flow Threshold' field under the 'Storage Process' dialog.**QIMAX Maximum inflow during simulation**- this value is calculated by the program.**VMAX Maximum allowable storage**- this value is taken as the maximum storage (for this unit) as calculated from the Depth - Surface Area table in the 'Hydraulic Properties' dialog under the 'Storage Process' dialog.**VSMAX Maximum storage encountered during simulation**- this value is calculated by the program.

**Costs Flag**

This flag controls the calculation of the capital costs and the operation and maintenance costs for this unit. If OFF, cost calculations are not computed for this unit. The flag is shared between the Storage Process type and Screen Process options for the same unit.

## Outflow Unit

The number of the unit accepting the treated outflow is entered here.

## Bypass Unit

The number of the unit accepting the bypass flow is entered here. Each unit must be assigned a maximum inflow, beyond which all flows and pollutants are bypassed.

## Residual Unit

The unit number accepting the residual flow is entered here. Residual flow is drawn from the unit during periods of no inflow or no treated outflow. Residual flows serve to drain the detention unit between storms.

## Drawing Coordinates

Used to construct the plant drawing via the **Draw button**. These two numbers (the X and Y coordinates) define the location of the units making up the plant. The origin of the drawing space is at the upper left, with positive X to the right, and positive Y down.

### Draw Button

This button causes the BMP (or treatment plant) specified in the table above to be drawn on the screen. The Plant is drawn below from the Unit Type and X and Y coordinates of each unit as entered in the table above the drawing.

The origin of the drawing space is at the upper left, with positive X to the right, and positive Y down. The icon of each unit is dependent on the type. The objects linking each unit represent the flows and are drawn as lines (the colour key for each line is under the respective column heading.)

A Plant is made up of a number of treatment units with one unit entered on each line of the list below. The unit type is selected by clicking on the type field of the list. Each unit has an outflow, a bypass flow and a residual flow. The destination of each of these flows is entered as a downstream unit number, for example a **storage treatment plant** would be represented as:

Unit No. | Type | Outflow | Bypass | Residual | X | Y |
---|---|---|---|---|---|---|

1 | Screen Process | 2 | 3 | 4 | 20 | 50 |

2 | Storage Process | 3 | 3 | 4 | 150 | 75 |

3 | Node Outflow | 400 | 25 | |||

4 | Disposal | 300 | 200 |

* Note*: X < = 550 and Y < = 300.

## More Storage/Treatment

This button and the subsequent dialog provides for the entry of additional required data for a Storage/Treatment plant. The data includes the removal characterisation of pollutants, output control, and the waste-stream temperature of the influent.

**Waste Stream Temperature**

This item allows you to select from the global list of Waste Stream Temperatures. Water temperature has a direct effect on the settling velocity of a particle, reflected through the viscosity of the wastewater which is a function of temperature. This data is only required if any pollutant is characterised by a particle size distribution.

**Storage Treatment Print Control **

This button and the associated dialog contains information controlling the output from the Storage/Treatment model.

*P**rint End of Simulation Summar**y*

This group contains a selection on the type of output summaries produced. Options exist to produce monthly, annual, or end-of simulation summaries as described below:

*Annual Summary*

An annual summary and a summary at the end of the simulation is printed for this plant in the output file.

*Annual + Monthly Summary*

A monthly summary, an annual summary, and a summary at the end of the simulation is printed for this plant in the output file.

*None*

A summary at the end of simulation is only printed for this plant in the output file.

*Define Print Period*

This flag defines specific sets of periods for detailed printout. If this flag is OFF, a single period covering the whole of the simulation is assumed. If the flag is ON, the periods are detailed in the list below. Detailed printout is generated at a frequency defined in the 'Detailed Print Control' group within the periods set out below.

*Start Print Period*

The starting date for a detailed print period. The date format is set by the DATE_FORMAT variable in the SWMM.CFG configuration file.

*Stop Print Period*

The ending date for a detailed print period. The date format is set by the DATE_FORMAT variable in the SWMM.CFG configuration file.

*Detailed Print Control *

Detailed printout of results is provided at every time step that is a multiple of this value, ie. if this value is 2, a detailed report will be output at every other time step during the specified periods.

*Detailed Print Periods*

This group defines the periods in which to print out detailed information from Storage/Treatment.

*No detailed printout*

This option defines no detailed printout of simulation results. The set of periods for detailed printout are ignored.

*D**etailed Printout at Intervals*

This option will provide a detailed printout of results at a regular number of time step intervals within the set of print period ranges in this dialog.

**Pollutant Characterisation**

This button enters the Pollutant Characterisation dialog which defines how each pollutant selected in Job Control 'Use Pollutants' is represented for the purposes of removal from storage/treatment units.

Pollutants in Storage Treatment may be characterized by a particle size distribution, a settling velocity distribution or by their concentration only.

Pollutants are characterised by their magnitude (mass flow and concentration) and, if required, by particle size/specific gravity or settling velocity distributions.

Describing pollutants by their particle size distribution is especially appropriate where small or large particles dominate or where several Storage/Treatment units are operated in series. For example, if the influent is primarily sand and grit, then a sedimentation unit would be very effective; if clay and silt predominate, sedimentation may be of little use. Also, if several units are operated in series, the first units will remove a certain range of particle sizes thus affecting the performance of downstream units. Therefore, the need for describing pollutants in more detail is obvious for modelling purposes.

The pollutant removal mechanism depends on the characterisation. If pollutants are characterised only by their magnitude then the model improves the quality of the waste stream by user-defined removal equations. If a pollutant is characterised by a particle size distribution or settling velocity distribution, then it is removed from the waste stream by particle settling or obstruction. Many storage/treatment processes use these physical methods to treat wastewater; sedimentation and screening are amongst the obvious examples.

The method each pollutant is to be characterised by is chosen by highlighting the appropriate pollutant in the list on the left, then selecting the corresponding pollutant characterisation method radio button. Settling the velocity distribution and concentration only should redirect to settling velocity distribution section and concentration only section respectively.

**Particle Size Distribution**

This option characterizes the pollutant by a particle size distribution.

Use the radio buttons to indicate whether XPSWMM is to use the size distribution from the upstream network or the one entered into the dialogue

Describing pollutants by their particle size distribution is especially appropriate where small or large particles dominate or where several Storage/Treatment units are operated in series. For example, if the influent is primarily sand and grit, then a sedimentation unit would be very effective; if clay and silt predominate, sedimentation may be of little use. Also, if several units are operated in series, the first units will remove a certain range of particle sizes thus affecting the performance of downstream units.

XPSWMM uses the unhindered settling by discrete particles as the removal mechanism. This procedure is most applicable to detention basins modeled as plug-flow reactors. The terminal or settling velocity is approximated by the following equation:

Vs = SQRT (4 / 3 * g * d / Cd * (Sp - 1))

Where:

Vs = terminal velocity of particle, ft/sec

g = gravitational constant, 32 ft/sec^2

Cd = drag coefficient

Sp = specific gravity of particle

d = diameter of particle, ft

and

Cd = 24 / Nr, if Nr < 0.5, or

Cd = 24 / Nr + 3 / SQRT(Nr) + 0.34, if 0.5 <= Nr <= 10^4

Cd = 0.4, if Nr > 10^4

where

Nr = Reynold's number, dimensionless

and

Nr = Vs * d / v

where

v = kinematic viscosity, ft^2/sec

and

v ~= 8.46E-4 / (T + 10)

where

T = water temperature, degrees fahrenheit

An iterative algorithm is used to solve this system of equations.

*Obtain from network *

When this option is selected, the particle size distribution is taken from the network upstream. This distribution varies with time and reflects the changes to the particle size distribution from upstream elements in the network.

*Pa**rticle Size Distribution *

When this option is enabled, the particle size distribution in the table below is taken as the grading for input at this plant. This distribution remains constant over time.

*Particle Size*

Each entry in the column below represents a particle size division, micrometers.

*% Greater*

The percentage of the particle size distribution that is above this particle size.

*Specific Gravity*

The average specific gravity of particles in the size range.

Note that first data row must contain particle size = 0 and % greater = 100. In the last data row the % greater must = 0.

** Settling Velocity Distribution**

This option characterises the pollutant by a magnitude and a settling velocity distribution.

The settling velocity distribution may be used in lieu of a particle size distribution to affect the settling of pollutants in detention units. Refer to the discussion under 'Particle Size Distribution' under the 'Pollutant Characterisation' dialog for guidelines for selecting terminal velocities of particles. Note that the settling velocity distribution is constant over time. The distribution applies to pollutants entering the Storage/Treatment plant and is modified as it passes through the plant. The distribution is not transferred downstream in the network.

*Settling Velocity*

The terminal or settling velocity for a portion of the pollutant. This distribution may be used as an alternative to a particle size distribution. Settling velocities are applied to plant influent and are constant over time.

*Fraction*

The fraction of the pollutant within the settling velocity range to the left. This fraction applies to plant influent and is constant over time. The fractions will be modified as the influent passes through the plant, but is not transferred to the network downstream.

** Concentration only **

This option characterises the highlighted pollutant by concentration, allowing simulated removal through equations.

Removal of a pollutant may be simulated as a function of:

1. Detention time (detention units only)

2. Time step size

3. Its influent concentration

4. Inflow rate

5. The removal fractions of pollutants

6. The influent concentrations of other pollutants

The equation can be any arbitrary expression in terms of variables which cover the above data descriptions. There are some restrictions on choices of variables depending on whether the unit is a detention or non-detention type.

**Particle Size Distribution****Settling Velocity Distribution****Concentration only****Waste Stream Temperature****Storage Treatment Print Control****Pollutant Characterisation**

** **Pollutant Removal Equation

This dialog contains the Removal Equation and Maximum Removal Fraction. To enter an equation, the 'Removal Equation Variables' button must be ON in Job Control, and equation variables defined for references in the equation. If this option is greyed-out check that the 'Removal Equation Variables' check box is enabled.

The 'Variables' button lists the possible variables that may be inserted into the equation. These variable names are defined in the 'Removal Equation Variables' dialog under Job Control.

In the case of detention units, detention time is the most important indicator of pollutant removal ability and, as such, removal equations should be written as a function of detention time (along with other possible parameters).

*Maximum Removal Fraction*

The Maximum Removal Fraction provides an upper bound on the removal fraction generated by the equation. This is usually used in the case where an equation generates Removal Fractions that may exceed a reasonable value or 1.0.

*Removal Equation Variables*

This button provides a list of variables that may be inserted into the removal equation.

Each variable may be selected by placing the cursor under the appropriate position in the equation string, selecting 'Variables', highlighting the appropriate variable and selecting 'OK'. The variables for each pollutant are found in the 'Job Control' dialog under the 'Removal Equation Variables' item.

The removal equation is very flexible and may take almost any form. For example, an equation for removing suspended solids from a sedimentation tank:

Rss = Rmax * (1 - e^(-k*td))

**where**

Rss = suspended solids removal fraction 0 <= Rss <= Rmax

Rmax = maximum removal fraction

td = detention time, sec, and

K = first order decay coefficient, 1/sec

Another example is taken by fitting a power function to the suspended solids removal curve for a 35-micron microstrainer, yielding the following equation:

Rss = 0.0963 * SS^0.286

**where**

Rss = suspended solids removal fraction 0 <= Rss <= Rmax

SS = influent suspended solids concentration, mg/l

Curves representing the reduction in volatile solids in raw sludge by a digester as a function of percent volatile solids and detention time may be approximated by:

Rvs = 1.31 * 10^-4 * (td/86400)^0.33 * Pvs^1.67

**where**

Rvs = volatile solids reduction, 0 <= Rvs <= 1.0

td = detention time, sec

Pvs = percent volatile solids in raw sludge

Pvs = 100 * VS/SS

**where**

VS = influent volatile solids concentration, mg/l, and

SS = influent suspended solids (raw sludge) concentration, mg/l