xpswmm/xpstorm Resource Center xps

The Sanitary pollutant global data dialog contains pollutant-specific information that the Sanitary layer uses when displaying and routing pollutants.

This page contains the following topics:

General Information

This dialog contains pollutant-specific information that the Sanitary layer uses when displaying and routing pollutants. Pollutant names and basic descriptions are shared between Runoff and Sanitary modes. The list of pollutants to be used in analysis is, however, defined independently for the Runoff and Sanitary layers. The 'Pollutant List' item in Job Control controls the list of pollutants for analysis.

Water quality routing through conduits in both the Sanitary and Runoff layers is accomplished by assuming complete mixing within the conduit in the manner of a continuously stirred tank reactor. With this procedure, the concentration of the pollutant in the conduit is assumed to be equal to the outflow concentration. The calculations are based on a mass balance, under the assumption that pollutants are removed at a rate proportional to the concentration in the conduit. An integrated form of the solution to the governing equation is used to avoid inaccuracies due to relatively small conduit volumes.


Options for concentration units are reasonably broad and broken into three categories.

mg/l (J3 - NDIM = 0)

This option defines pollutant units of mg/l. If this item is selected the unit label is ignored. All parameters for which the quantity is measured as a mass should be suited to this option.

Most pollutants are measurable in this unit. Although parameters such as metals, phosphorus or trace organics are often given as micrograms per litre, SWMM output is to 3 decimal places and is expected to be compatible with typical values of these parameters.

'Other'/l (J3 - NDIM = 1)

Used when the pollutant has some per litre units other than mg/l. eg a bacteria count such as MPN/l. This unit accounts for pollutants defined by a count per unit volume.

Other (J3 - NDIM = 2)

This option covers parameters with specialised concentration-type units such as pH, conductivity (umho), turbidity (JTU), color (PCU), temperature (C), etc. For these parameters, interpretation of concentration results is straightforward, but "total mass" or "buildup" is mostly conceptual. Since loads are transmitted in terms of concentration times flow rate, regardless of the concentration units used, proper continuity of parameters is easily maintained.

Unit Label (J3 - PUNIT)

Unit used as heading for tabular output for units other than mg/l.

Daily Decay Rate (J3 - DECAY)

Decay rate of pollutant in units specified per day.


If this flag is ON, Scour/Deposition is modelled for this pollutant during conduit routing. Scour/deposition relates to the deposition of material during dry-weather flow and subsequent scour during wet-weather flow. This phenomenon is assumed to form a significant contribution of solids to combined sewer overflows, and is also evident in the “first flush” (high solids concentrations at the beginning of storm events) found in many sewer systems.

Particle Size

Each entry in the column below represents a particle size division, in mm. Only the metric unit is available.

Percentage Greater

This data represents the percentage of the particle size distribution that is above the particle size in the adjacent column. 

Specific Gravity

The average specific gravity for the given particle size range. Decreasing specific gravities increases the amount suspended. Typical values of specific gravities of particulate matter in sewers range from 1.1 for volatile matter to 2.7 for sand and grit.

Max Size for Dry Weather Flow

The maximum particle size contained in dry-weather flow input, in mm. Only the metric unit is available. This will apply to direct inflows into manholes or inflows generated by the dry-weather flow module.

The Sanitary layer utilises a fixed particle size and specific gravity distribution for each pollutant characterised this way. This distribution is applied to source pollutant loads entering a conduit. The Sanitary layer maintains a time history of the maximum particle size in suspension and the minimum particle size in the bed for each conduit. Mass-weighted values of the particle size distribution of particles in motion are routed downstream for entry into subsequent conduits.

Several assumptions are made in the development of the algorithm. Solids in the sewer system are assumed to behave like ideal non-cohesive sediment. No distinction is made between particle size and specific gravity distributions resulting from different pollutant sources, eg. dry-weather flow and storm water. Only one distribution is used for each pollutant. Shields’ criterion is used to determine the dividing particle size between motion and no motion. Once in motion, no distinction is made between bed and suspended load. Particles in motion (“suspension”) are routed downstream in each conduit by complete mixing, in the same manner as other pollutants.

When a critical diameter is determined for scour, all particles having a diameter smaller than the critical diameter are eroded. Armouring or erosion of the bed layers is not simulated. Scour-deposition is considered only in conduits, it is not simulated in non-conduits, or Storage nodes. The effect of deposited sediment on the bed geometry is not considered. When the hydraulic radius (an important parameter) is calculated to determine the critical diameter for motion, the bed is assumed to have the geometry of the conduit. This may lead to some underestimation of deposited material, mainly at low flows.

For each conduit, the critical diameter is determined as a function of velocity, roughness and specific gravity. At the same time, the maximum diameter of the suspended fraction and the minimum diameter of the settled fraction is maintained. If the critical diameter is less than the maximum of the suspended material, more is settled; similarly, if the critical diameter is greater than the minimum of the settled material, more is suspended.

Continuity of pollutant mass is maintained during scour and deposition. In addition, larger particles can settle upstream in flat conduits and be unavailable for downstream settling. No layering within the sediment is possible; a uniform distribution with sediment depth is assumed.

Dry Weather Flow Generation

These parameters control the method used to generate sub-area dry weather loads.

Use Volume Rate

If this option is selected dry weather loads from sub-areas are based on a load per unit volume.

Use Per Capita Rate

If this option is selected dry weather loads from sub-areas are based on a load per capital rate.