Water Quality Determinants

The following determinants can be modelled in InfoWorks ICM:

Code

Name

Description

BOD

Biological Oxygen Demand

Biological Oxygen Demand (BOD) is the potential utilisation of oxygen used by microorganisms in the oxidation of organic matter.

BOD is commonly expressed in terms of the 5-day BOD (BOD5), which is the amount of oxygen consumed by the decay of the material over 5 days.

COD

Chemical Oxygen Demand

Chemical Oxygen Demand (COD) is a measure of the amount of oxygen required for chemical oxidation of pollutants in the system. When modelling COD, there is no conversion of COD to ultimate oxygen demand. COD is taken as the equivalent to BODu.

COD can form part of Dissolved Oxygen processes instead of BOD.

TKN

Total Kjeldahl Nitrogen

Total Kjeldahl Nitrogen (TKN) represents the sum of organic Nitrogen, Ammonia (NH3) and Ammonium (NH4+).

Will form part of Dissolved Oxygen processes.

NH4

Ammoniacal Nitrogen

Ammoniacal Nitrogen (NH4) represents Ammonia and Ammonium. Thus organic Nitrogen = TKN - NH3.

Will form part of Dissolved Oxygen processes.

TPH

Total Phosphorus

Total Phosphorus (TPH) represents the sum of all forms of phosphorus.

When modelling Algae (ALG) and Macrophytes, nutrient limitation factors due to phosphates are calculated.

Attached TPH is used to model adsorbed phosphorus which may limit the supply of phosphate to algae and macrophytes.

PLn

User determinant n

User defined determinant 1 - 4.

DO

Dissolved Oxygen

The Dissolved Oxygen process models the oxygen balance due to the decay of organic material, the nitrification of ammonia and reaeration from the atmosphere.

Dissolved Oxygen requires either BOD or COD to be modelled. TKN, NH4, NO2 and NO3 are also required in order to model the process.

TW and SALT determinants are also used. If TW and/or SALT have not been selected for modelling in the QM Parameters Dialog or time varying profiles are not available, the constant temperature and salinity values set in the Water Quality and Sediment Parameters will be used.

See the Dissolved Oxygen topic for details.

NO3

Nitrate

The Nitrate (NO3) and Nitrite (NO2) determinants form part of the Dissolved Oxygen process.

See the Dissolved Oxygen topic for details.

NO2

Nitrite

The Nitrite (NO2) and Nitrate (NO3) determinants form part of the Dissolved Oxygen process.

See the Dissolved Oxygen topic for details.

PH

pH

pH (PH)is a measure of acidity or alkalinity and is defined as the negative logarithm of the concentration of hydrogen ions:

Where:

H = concentration of hydrogen ions (mol/l)

InfoWorks ICM simulates pH by converting it to a hydrogen ion concentration which is then treated as a conservative substance.

pH can only be modelled when Dissolved Oxygen is also modelled.

In the absence of a pH pollutograph profile, the water quality engine will set pH to 7; any inflow from subcatchments will have pH set to 7.

Concentration of unionised ammoniacal nitrogen is a derived result from pH and total ammoniacal nitrogen (NH4):

Where:

UA = unionised ammoniacal nitrogen (mg/l)

A = total ammoniacal nitrogen (mg/l)

x is a function of pH and temperature (T, °C):

SALT

Salt

The SALT determinant is used to model salinity and is also used in the Dissolved Oxygen process.

In the absence of a salt pollutograph profile, the water quality engine will use the Constant concentration value specified in the Water Quality and Sediment Parameters.

Salt is treated as a conservative substance.

TW

Water temperature

The temperature (TW) determinant is used to model water temperature and is also used in the Dissolved Oxygen process.

In the absence of a TW pollutograph profile, the water quality engine will use the Constant temperature value specified in the Water Quality and Sediment Parameters.

When a variable profile exists, heat transfer between water and the air above in open channels is simulated by the following equation:

Where:

KT = heat transfer coefficient (s-1)

Tbase = equilibrium water temperature (°C)

b = width of flow (m)

A = cross sectional area (m2)

COL

Coliforms

The coliforms (COL) determinant is used to model the variation of E-coli or other bacteria. It does not interact with any other variable.

The only parameter required to model coliforms is decay rate expressed in terms of a T90 value, which may vary. T90 (hours) is specified in the Water Quality and Sediment Parameters and indicates the time taken for a population to reduce to 10% of its original density.

The decay of coliforms is simulated in InfoWorks ICM by a first order decay equation:

Where:

K = decay rate (s-1)

C = concentration of coliforms (user defined units)

K is obtained from T90 (hours) by:

CFn

Coliforms n

Coliforms determinants (CF1 to CF4) are used to model the variation of E-coli and other bacteria. These coliforms can interact with other variables.

ALG

Algae

The Algae (ALG) determinant is used to model the growth and decay of microscopic organisms (plants and certain types of bacteria).

Dissolved algae represents organisms that are in suspension and are liable to movement by currents.

Attached algae represent organisms living on the surface of the bed.

Algae can only be modelled when Dissolved Oxygen is also modelled.

See the Algae topic for details.

SI

Silicate

The silicate (SI) determinant is used to model the variation of silicate as a nutrient available to algae and plants.

Silicate can only be modelled when Algae (ALG) is also modelled.

Silicate is modelled as a dissolved substance. It limits algal growth by a Michaelis-Menten relationship in the same way as phosphates and nitrates. When Macrophytes and/or Attached Algae are also simulated, detrital silicon is tracked in the water column and on the bed.

MAC

Macrophytes

The macrophytes determinant is used to model the growth, nutrient uptake and death of macrophytes - large plants rooted on the river bed.

Macrophytes can only be modelled when Algae (ALG) is also modelled.

To model macrophytes, check the Model macrophytes option on the QM Parameters Dialog.

See the Macrophytes topic for details.

SUL

Hydrogen Sulphide

The hydrogen sulphide determinant is used to model the concentration of dissolved hydrogen sulphide in the system.

Hydrogen sulphide can only be modelled when BOD is also modelled.

To model hydrogen sulphide, check the Model Hydrogen Sulphide option on the QM Parameters Dialog.

See the Hydrogen Sulphide topic for details.

Determinant inflows into the model

Determinant inflows into the model can come from the following sources:

Surface Pollutants	Sediment builds up on the various surfaces of the subcatchment during the simulation and is eroded by surface runoff and carried into the network. This sediment can have pollutants attached to it. It is possible to model two different sediment fractions with different characteristics. You can also define dissolved pollutants that are treated as entering the network through gully pots at each node. You can run the simulation for a build-up period prior to each simulation to allow sediment depth on the surface to reach a steady state. Build-up of surface sediment will continue during the simulation. The Surface Pollutant Model calculates the inflow of sediment and pollutants from the catchment surface and gully pots. You can define the initial masses on the surface prior to the build-up period.
Wastewater Inputs	Wastewater Profiles define inflows from the population of a subcatchment. Additional fields allow you to define concentration, potency and time varying multiplier values for pollutants. The data requirements for wastewater profiles are: Per capita flow Average sediment concentration Average dissolved pollutant concentrations Average potency factors (for attached pollutants) Multiplying factors (to create time varying profiles) See Trade and Waste Water Events for more information.
Trade Waste Inputs	Trade Waste Profiles define a single inflow from, for example, industrial premises. The data requirements for trade waste profiles are: Average sediment concentration Average dissolved pollutant concentrations Average potency factors (for attached pollutants) Multiplying factors (to create time varying profiles) See Trade and Waste Water Events for more information.
Point Pollutant Inflows	Pollutographs are a set of point inputs to the system. The pollutograph is used in conjunction with an Inflow or Level event defining inflows at the same nodes. Pollutographs are more flexible than Waste Water or Trade Waste events, which are based around a 24 hour cycle. Use a Pollutograph to model pollutant inputs that do not follow a pattern, or that have a different pattern. An example would be the weekly flushing of tanks at an industrial premises. A pollutograph contains a section for each potential pollutant type: sediment fraction dissolved pollutant pollutant attached to sediment detrital pollutant Each section contains concentration or potency values for inflow at any number of nodes.

Sediment fractions

Two separate sediment fractions can be modelled. These sediments are referred to as Sediment Fraction 1 (SF1) and Sediment Fraction 2 (SF2). Sediment fractions advect and can erode or deposit to/from the bed.

Each sediment fraction is defined by two parameters:

D50 - the average sediment particle size (default value 0.04mm)
Specific Gravity - the density of the sediment fraction (default value 1.7)

These parameters are specified in the Surface Pollutant Editor. If these are blank, you can set your own defaults in the Water Quality and Sediment Parameters. If you do not specify any values at all, the default values above are used.

The two sediment fractions make up the Active Layer of sediment in pipes. The Active Layer is the layer that can be eroded and deposited during a simulation. See Sediment.

Conservative determinants

Determinants are modelled as conservative unless decay parameters have been specified for the pollutant in the Decaying pollutants editor in the Water Quality and Sediment Parameters, or the determinant is being used as part of Dissolved Oxygen processes.

The determinant is simply modelled as a transported substance which does not grow or decay over time and does not interact with any other variable.

Decaying determinants

Determinants may be modelled as non-conservative (with the exception of Salt, pH and Water temperature).

To model a determinant as a decaying substance, enter decay parameters in the Decaying pollutants editor in the Water Quality and Sediment Parameters.

Note that decay operates before user defined processes (see below).

The decay types available are:

None
Linear: v = v - a.dt
Exponential concentration: v = v - a (1 - e-bv) dt
Exponential time: v = ve-c.dt
Exponential both: v = v - a (1 - e-b.v) dt; v = ve-c.dt

Where:

v is the determinant value, mg/l or cfu/100ml ^*
a is the constant decay rate or maximum decay rate depending on decay type, (mg/l)/day or or (cfu/100ml)/day ^*
b is the exponential decay, 1/(mg/l) or 1/(cfu/100ml) ^*

^* depending on the selected determinant

c is the exponential decay constant, (1/day)
dt is the simulation engine timestep, s

User defined processes

Three user defined processes are supported.

To model user defined processes, enter process parameters in the User defined processes editor in the Water Quality and Sediment Parameters.

Note that user defined processes operate after decay (see above).

The process types available are:

Growth: Rate = s1 x P x p2^(s2-p3) x s3/(s3-p4)
Growth-product: Rate = P x p2 x s1 x s2
Equilibrium: Rate = P x (p1 - s1)

Where:

P is a Scaling factor type parameter that is either a constant or a variable, which is chosen from a dropdown list in the User defined processes editor.
If a constant, P will be p1 for Growth and Growth-product or p2 for Equilibrium
If a variable:
p1 to p7 are coefficients 1 to 7
s1 to s6 are concentrations of determinants 1 to 6

Each defined process can be assigned to one or more determinants. Open the User processes editor from the User defined processes editor to create a set of determinants and factors for a process.

Dissolved and attached determinants

Dissolved determinants are dissolved in water in the network. Dissolved determinants do not erode or deposit to/from the bed. The quantity of the dissolved determinant is given as concentration (mass per unit volume).

Attached determinants are determinants linked to a sediment fraction. BOD, COD, TKN, TPH, PLn, CFn and ALG can be modelled as attached. Attached determinants advect and erode/deposit to/from the bed along with the sediment fraction.

The quantity attached to the sediment fraction is given by a potency factor. The mass of determinant is calculated by multiplying the sediment mass by the potency factor. Determinants can be attached to more than one sediment fraction with different potency factors.

In a pollutograph, time varying profiles for dissolved determinates are defined in the tabs with a 'D' appended to the first two letters of the determinant code while attached determinants are defined in the tabs with an 'A' appended to the first two letters of the code. Where multiple dissolved or attached determinates can be defined, the initial letter of the determinant code followed by a number and then an appended D or A, e.g. P1D and C1A, are displayed on the applicable pollutograph tabs. These profiles override any fixed potency factor that may be defined in the Profile Properties Dialog for a dissolved determinant profile.