Water Quality in SWMM5

Water Quality Modeling Features

In addition to modeling the generation and transport of runoff flows, SWMM can also estimate the production of pollutant loads associated with this runoff. The following processes can be modeled for any number of user-defined water quality constituents:

Water Quality

Water quality related data are supplied to a SWMM model using the following types of objects:

Pollutants

SWMM can simulate the generation, inflow and transport of any number of user-defined pollutants. Required information for each pollutant includes:

Co-pollutants can also be defined in SWMM. For example, pollutant X can have a co-pollutant Y, meaning that the runoff concentration of X will have some fixed fraction of the runoff concentration of Y added to it.

Pollutant buildup and washoff from subcatchment areas are determined by the land uses assigned to those areas. Input loadings of pollutants to the drainage system can also originate from external time series inflows as well as from dry weather inflows.

See Also

Pollutant Editor

Land Uses

Pollutant Buildup

Pollutant Washoff

External Inflows Editor

Water Quality Modeling Features

In addition to modeling the generation and transport of runoff flows, SWMM can also estimate the production of pollutant loads associated with this runoff. The following processes can be modeled for any number of user-defined water quality constituents:

Pollutant Washoff

Pollutant washoff from a given land use category occurs during wet weather periods and can be described in one of the following ways:

Exponential Washoff

The washoff load (W) in units of mass per hour is proportional to the product of runoff raised to some power and to the amount of buildup remaining, i.e.,

where C1 = washoff coefficient, C2 = washoff exponent, q = runoff rate per unit area (inches/hour or mm/hour), and B = pollutant buildup in mass units. The buildup here is the total mass (not per area or per curb length) and both buildup and washoff mass units are the same as used to express the pollutant's concentration (milligrams, micrograms, or counts).

Rating Curve Washoff

The rate of washoff W in mass per second is proportional to the runoff rate raised to some power, i.e.,

where C1 = washoff coefficient, C2 = washoff exponent, and Q = runoff rate in user-defined flow units.

Event Mean Concentration

This is a special case of Rating Curve Washoff where the exponent is 1.0 and the coefficient C1 represents the washoff pollutant concentration in mass per liter. The conversion between user-defined flow units used for runoff and liters is handled internally by SWMM.

(Typical EMC's for selected constituents).

Note that in each case buildup is continuously depleted as washoff proceeds, and washoff ceases when there is no more buildup available. It is also possible to use the Event Mean Concentration option by itself, without having to model any pollutant buildup at all.

BMP Removal Efficiency

Washoff loads for a given pollutant and land use category can be reduced by a fixed percentage by specifying a BMP Removal Efficiency that reflects the effectiveness of any BMP controls associated with the land use.

Removal of pollutants in surface washoff can also occur when runoff is captured by Low Impact Development (LID) controls. The concentration of a pollutant released from an LID unit's underdrain flow can be reduced by a user-specified percentage. These removal percentages are assigned through the LID Control Editorfor each generic LID design.

Pollutant Editor

The Pollutant Editor is invoked whenever a new Pollutant object is created or an existing pollutant is selected for editing. It contains the following fields:

Name

The name assigned to the pollutant.

Units

The concentration units (mg/L, ug/L, or #/L (counts/L)) in which the pollutant concentration is expressed.

Rain Concentration

Concentration of the pollutant in rain water (concentration units).

GW Concentration

Concentration of the pollutant in ground water (concentration units).

Initial Concentration

Concentration of the pollutant throughout the conveyance system at the start of the simulation.

I&I Concentration

Concentration of the pollutant in any Infiltration/Inflow (concentration units).

DWF Concentration

Concentration of the pollutant in any dry weather sanitary flow (concentration units). This value can be overridden for any specific node of the conveyance system by editing the node's Inflows property.

Decay Coefficient

First-order decay coefficient of the pollutant (1/days).

Snow Only

YES if pollutant buildup occurs only when there is snow cover, NO otherwise (default is NO).

Co-Pollutant

Name of another pollutant whose runoff concentration contributes to the runoff concentration of the current pollutant.

Co-Fraction

Fraction of the co-pollutant's runoff concentration that contributes to the runoff concentration of the current pollutant.

An example of a co-pollutant relationship would be where the runoff concentration of a particular heavy metal is some fixed fraction of the runoff concentration of suspended solids. In this case suspended solids would be declared as the co-pollutant for the heavy metal.

Pollutant Buildup

Pollutant buildup that accumulates within a land use category is described (or "normalized") by either a mass per unit of subcatchment area or per unit of curb length. Mass is expressed in pounds for US units and kilograms for metric units. The amount of buildup is a function of the number of preceding dry weather days and can be computed using one of the following functions:

Power Function

Pollutant buildup (B) accumulates proportional to time (t) raised to some power, until a maximum limit is achieved,

where C1 = maximum buildup possible (mass per unit of area or curb length), C2 = buildup rate constant, and C3 = time exponent.

Exponential Function

Buildup follows an exponential growth curve that approaches a maximum limit asymptotically,

where C1 = maximum buildup possible (mass per unit of area or curb length) and C2 = buildup rate constant (1/days).

Saturation Function

Buildup begins at a linear rate that continuously declines with time until a saturation value is reached,

where C1 = maximum buildup possible (mass per unit area or curb length) and C2 = half-saturation constant (days to reach half of the maximum buildup).

External Time Series

This option allows one to use a Time Series to describe the rate of buildup per day as a function of time. The values placed in the time series would have units of mass per unit area (or curb length) per day. One can also provide a maximum possible buildup (mass per unit area or curb length) with this option and a scaling factor that multiplies the time series values.

Pollutant Washoff

Pollutant washoff from a given land use category occurs during wet weather periods and can be described in one of the following ways:

Exponential Washoff

The washoff load (W) in units of mass per hour is proportional to the product of runoff raised to some power and to the amount of buildup remaining, i.e.,

where C1 = washoff coefficient, C2 = washoff exponent, q = runoff rate per unit area (inches/hour or mm/hour), and B = pollutant buildup in mass units. The buildup here is the total mass (not per area or per curb length) and both buildup and washoff mass units are the same as used to express the pollutant's concentration (milligrams, micrograms, or counts).

Rating Curve Washoff

The rate of washoff W in mass per second is proportional to the runoff rate raised to some power, i.e.,

where C1 = washoff coefficient, C2 = washoff exponent, and Q = runoff rate in user-defined flow units.

Event Mean Concentration

This is a special case of Rating Curve Washoff where the exponent is 1.0 and the coefficient C1 represents the washoff pollutant concentration in mass per liter. The conversion between user-defined flow units used for runoff and liters is handled internally by SWMM.

(Typical EMC's for selected constituents).

Note that in each case buildup is continuously depleted as washoff proceeds, and washoff ceases when there is no more buildup available. It is also possible to use the Event Mean Concentration option by itself, without having to model any pollutant buildup at all.

BMP Removal Efficiency

Washoff loads for a given pollutant and land use category can be reduced by a fixed percentage by specifying a BMP Removal Efficiency that reflects the effectiveness of any BMP controls associated with the land use.

Removal of pollutants in surface washoff can also occur when runoff is captured by Low Impact Development (LID) controls. The concentration of a pollutant released from an LID unit's underdrain flow can be reduced by a user-specified percentage. These removal percentages are assigned through the LID Control Editorfor each generic LID design.

Initial Buildup Editor

The Initial Buildup editor is invoked from the Property Editor when editing the Initial Buildup property of a Subcatchment. It specifies the amount of pollutant buildup existing over the subcatchment at the start of the simulation. The editor consists of a data entry grid with two columns. The first column lists the name of each pollutant in the project and the second column contains edit boxes for entering the initial buildup values. If no buildup value is supplied for a pollutant, it is assumed to be 0. The units for buildup are either pounds per acre when US units are in use or kilograms per hectare when SI metric units are in use.

If a non-zero value is supplied for the initial buildup of a pollutant, it will override any initial buildup computed from the Antecedent Dry Days parameter specified on the Dates page of the Simulation Options dialog.

Land Uses

Land Uses are categories of development activities or land surface characteristics assigned to subcatchments. Examples of land use activities are residential, commercial, industrial, and undeveloped. Land surface characteristics might include rooftops, lawns, paved roads, undisturbed soils, etc. Land uses are used solely to account for spatial variation in pollutant buildup and washoff rates within subcatchments.

The SWMM user has many options for defining land uses and assigning them to subcatchment areas. One approach is to assign a mix of land uses for each subcatchment, which results in all land uses within the subcatchment having the same pervious and impervious characteristics. Another approach is to create subcatchments that have a single land use classification along with a distinct set of pervious and impervious characteristics that reflects the classification.

The following processes can be defined for each land use category:

See Also

Land Use Editor

Subcatchments

Land Use Editor

The Land Use Editor dialog is used to define a category of land use for the study area and to define its pollutant buildup and washoff characteristics. The dialog contains three tabbed pages of land use properties:

Land Use Editor - General Page

The General page of the Land Use Editor dialog describes the following properties of a particular land use category:

Land Use Name

The name assigned to the land use.

Description

An optional comment or description of the land use. (Click the ellipsis button or press Enter to edit).

Street Sweeping Interval

Days between street sweeping within the land use (0 for no sweeping).

Street Sweeping Availability

Fraction of the buildup of all pollutants that is available for removal by sweeping.

Last Swept

Number of days since last swept at the start of the simulation.

If Street Sweeping does not apply to the land use, then the last three properties can be left blank.

Land Use Editor - Buildup Page

The Buildup page of the Land Use Editor dialog describes the properties associated with pollutant buildup over the land during dry weather periods. These consist of:

Pollutant

Select the pollutant whose buildup properties are being edited.

Function

The type of buildup function to use for the pollutant. The choices are NONE for no buildup, POW for power function buildup, EXP for exponential function buildup, SAT for saturation function buildup, and EXT for buildup supplied by an external time series. See the Pollutant Buildup topic for explanations of these different functions. Select NONE if no buildup occurs.

Max. Buildup

The maximum buildup that can occur, expressed as lbs (or kg) of the pollutant per unit of the normalizer variable (see below). This is the same as the C1 coefficient used in the buildup formulas discussed under Pollutant Buildup.

The following two properties apply to the POW, EXP, and SAT buildup functions:

Rate Constant

The time constant that governs the rate of pollutant buildup. This is the C2 coefficient in the Power and Exponential buildup formulas discussed under Pollutant Buildup. For Power buildup its units are mass / days raised to a power, while for Exponential buildup its units are 1/days.

Power/Sat. Constant

The exponent C3 used in the Power buildup formula, or the half-saturation constant C2 used in the Saturation buildup formula discussed under Pollutant Buildup. For the latter case, its units are days.

The following two properties apply to the External Time Series option:

Scaling Factor

A multiplier used to adjust the buildup rates listed in the time series.

Time Series

The name of the Time Series that contains buildup rates (as mass per normalizer per day).

Normalizer

The variable to which buildup is normalized on a per unit basis. The choices are either land area (in acres or hectares) or curb length. Any units of measure can be used for curb length, as long as they remain the same for all subcatchments in the project.

When there are multiple pollutants, the user must select each pollutant separately from the Pollutant dropdown list and specify its pertinent buildup properties.

Land Use Editor - Washoff Page

The Washoff page of the Land Use Editor dialog describes the properties associated with pollutant washoff over the land use during wet weather events. These consist of:

Pollutant

The name of the pollutant whose washoff properties are being edited.

Function

The choice of washoff function to use for the pollutant. The choices are:

The formula for each of these functions is discussed under the Pollutant Washoff topic.

Coefficient

This is the value of C1 in the exponential and rating curve formulas, or the event-mean concentration.

Exponent

The exponent used in the exponential and rating curve washoff formulas.

Cleaning Efficiency

The street cleaning removal efficiency (percent) for the pollutant. It represents the fraction of the amount that is available for removal on the land use as a whole (set on the General page of the editor) which is actually removed.

BMP Efficiency

Removal efficiency (percent) associated with any Best Management Practice that might have been implemented. The washoff load computed at each time step is simply reduced by this amount.

Land Use Assignment Editor

The Land Use Assignment editor is invoked from the Property Editor when editing the Land Uses property of a Subcatchment. Its purpose is to assign land uses to the subcatchment for water quality simulations. The percent of land area in the subcatchment covered by each land use is entered next to its respective land use category. If the land use is not present its field can be left blank. The percentages entered do not necessarily have to add up to 100.

Water Quality Routing

Water quality routing within conduit links assumes that the conduit behaves as a continuously stirred tank reactor (CSTR). Although a plug flow reactor assumption might be more realistic, the differences will be small if the travel time through the conduit is on the same order as the routing time step. The concentration of a constituent exiting the conduit at the end of a time step is found by integrating the conservation of mass equation, using average values for quantities that might change over the time step such as flow rate and conduit volume.

Water quality modeling within storage unit nodes follows the same approach used for conduits. For other types of nodes that have no volume, the quality of water exiting the node is simply the mixture concentration of all water entering the node.

The pollutant concentration in both a conduit and a storage node will be reduced by a first-order decay reaction if the pollutant’s first-order decay coefficient is not zero.

Water Quality Characteristics of Urban Runoff
Constituent	Event Mean Concentrations
TSS (mg/L)	180 - 548
BOD (mg/L)	12 - 19
COD (mg/L)	82 - 178
Total P (mg/L)	0.42 - 0.88
Soluble P (mg/L)	0.15 - 0.28
TKN (mg/L)	1.90 - 4.18
NO2/NO3-N (mg/L)	0.86 - 2.2
Total Cu (ug/L)	43 - 118
Total Pb (ug/L)	182 - 443
Total Zn (ug/L)	202 - 633

Source: U.S. Environmental Protection Agency. (1983). Results of the Nationwide Urban Runoff Program (NURP), Vol. 1, NTIS PB 84-185552), Water Planning Division, Washington, DC.

Treatment

Removal of pollutants from the flow streams entering any drainage system node is modeled by assigning a set of treatment functions to the node. A treatment function can be any well-formed mathematical expression involving:

• FLOW for flow rate into node (in user-defined flow units)
• DEPTH for water depth above node invert (ft or m)
• AREA for node surface area (ft2 or m2)

• DT for routing time step (sec)

• HRT for hydraulic residence time (hours)

• abs(x) for absolute value of x

• sgn(x) which is +1 for x >= 0 or -1 otherwise

• step(x) which is 0 for x <= 0 and 1 otherwise

• sqrt(x) for the square root of x

• log(x) for logarithm base e of x

• log10(x) for logarithm base 10 of x

• exp(x) for e raised to the x power

• the standard trig functions (sin, cos, tan, and cot)

• the inverse trig functions (asin, acos, atan, and acot)

• the hyperbolic trig functions (sinh, cosh, tanh, and coth)

along with the standard operators +, -, *, /, ^ (for exponentiation ) and any level of nested parentheses.

The result of the treatment function can be either a concentration (denoted by the letter C) or a fractional removal (denoted by R). For example, a first-order decay expression for BOD exiting from a storage node might be expressed as:

C = BOD * exp(-0.05*HRT)

or the removal of some trace pollutant that is proportional to the removal of total suspended solids (TSS) could be expressed as:

R = 0.75 * R_TSS

Care must be taken to avoid circular references when specifying treatment functions. For example, the above expression would not be computable if it were used to compute fractional removal of TSS.

Treatment Editor

The Treatment Editor is invoked whenever the Treatment property of a node is selected from the Property Editor. It displays a list of the project's pollutants with an edit box next to each as shown below.

Enter a valid treatment expression in the box next to each pollutant which receives treatment. Click the OK button to accept your edits or click Cancel to ignore them.

Any of the following math functions (which are case insensitive) can be used in a treatment expression:

along with the standard operators +, -, *, /, ^ (for exponentiation ) and any level of nested parentheses.

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