SWMM - Treatment of Pollutants

Removal of pollutants from influents that enter any node is required. of the drainage system is This is modeled by assigning a set of treatment functions to the node.

Handling expressions have the general form:

or

Where:

• R - fractional removal
• C - output concentration (use the name of the pollutant to represent its concentration)
  For non-storage nodes, this is the way to go. is the mixing concentration of all flow streams entering the node, while for storage nodes it is the pollutant concentration within the stored volume of the node
• P - one or more pollutant names
• R_P - one or more pollutant removals (append R_ to pollutant name)
• V - one or more process variables:
  • FLOW - for flow rate in the node. (in user-defined flow units)
  • DEPTH - for the depth of water above the node. inverted (feet or m)
  • AREA - for the surface area of the node
  • DT - for routing time interval (seconds)
  • HRT - for hydraulic residence time (hours)
• Any of the following mathematical functions (which are case-insensitive) can be used in a handling expression:
  • Abs(x) - for absolute value of x
  • Sgn(x) - what is it? +1 for x ≥ 0 or -1 otherwise
  • Step(x) - what is it? 0 for x ≤ 0 and 1 otherwise
  • Sqrt(x) - for the square root of x
  • Log(x) - for base logarithm e (natural) of x
  • Log10(x) - for base 10 logarithm of x
  • Exp(x) - for e to the power of; power x
  • sin, cos, tan, cot - standard trigonometric functions
  • asin, acos, atan, acot - inverse trigonometric functions
  • sinh, cosh, tanh, coth - hyperbolic trigonometric functions
  • +, -, *, /, ^ - standard operators

Example:

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

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

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 calculate the fractional removal of TSS