Reverse Osmosis (1D)

This reverse osmosis (RO) unit model
  • is 1-dimensional

  • supports a single liquid phase only

  • supports steady-state only

  • supports both solution-diffusion (SD) and Spiegler-Kedem-Katchalsky (SKK) models [1]

  • supports flat-sheet and spiral-wound module designs

  • assumes isothermal conditions

Degrees of Freedom

Aside from the inlet feed state variables (i.e. temperature, pressure, component flowrates), the RO model has at least 4 degrees of freedom that should be fixed for the unit to be fully specified.

Typically, the following variables are fixed, in addition to state variables at the inlet:
  • membrane water permeability, A

  • membrane salt permeability, B

  • permeate outlet pressure

  • membrane area or length or width

Model Structure

This RO model consists of 1 MembraneChannel1DBlock for the feed-side ([t, x]), a StateBlock indexed by time and space for the permeate-side (permeate_side[t, x]), and a StateBlock for the final permeate at the outlet (mixed_permeate).










Normalized ContinuousSet [0 … 1] discretized by number of finite elements






[‘H2O’, ‘NaCl’]*

*Solute depends on the imported property model; example shown here is for the NaCl property model.


Refer to the Variables section in the 0DRO model.


Refer to the Equations section in the 0DRO model.

Class Documentation

class watertap.unit_models.reverse_osmosis_1D.ReverseOsmosis1D(*args, **kwds)
  • rule (function) – A rule function or None. Default rule calls build().

  • concrete (bool) – If True, make this a toplevel model. Default - False.

  • ctype (class) –

    Pyomo ctype of the block. Default - pyomo.environ.Block

    Config args


    Indicates whether this model will be dynamic or not. default - False. Membrane units do not yet support dynamic behavior.


    Indicates whether holdup terms should be constructed or not. default - False. Membrane units do not have defined volume, thus this must be False.


    Property parameter object used to define property calculations, default - useDefault. Valid values: { useDefault - use default package from parent model or flowsheet, PhysicalParameterObject - a PhysicalParameterBlock object.}


    A ConfigDict with arguments to be passed to a property block(s) and used when constructing these. default - None. Valid values: { see property package for documentation.}


    Indicates what type of mass balance should be constructed, default - useDefault. Valid values: { MaterialBalanceType.useDefault - refer to property package for default balance type MaterialBalanceType.none - exclude material balances, MaterialBalanceType.componentPhase - use phase component balances, MaterialBalanceType.componentTotal - use total component balances, MaterialBalanceType.elementTotal - use total element balances, - use total material balance.}


    Indicates what type of energy balance should be constructed. default - useDefault. Valid values: { EnergyBalanceType.useDefault - refer to property package for default balance type EnergyBalanceType.none - exclude energy balances, EnergyBalanceType.enthalpyTotal - single enthalpy balance for material, EnergyBalanceType.enthalpyPhase - enthalpy balances for each phase, EnergyBalanceType.energyTotal - single energy balance for material, EnergyBalanceType.energyPhase - energy balances for each phase.}


    Indicates what type of momentum balance should be constructed, default - MomentumBalanceType.pressureTotal. Valid values: { MomentumBalanceType.none - exclude momentum balances, MomentumBalanceType.pressureTotal - single pressure balance for material, MomentumBalanceType.pressurePhase - pressure balances for each phase, MomentumBalanceType.momentumTotal - single momentum balance for material, MomentumBalanceType.momentumPhase - momentum balances for each phase.}


    Options to account for concentration polarization.

    default - ConcentrationPolarizationType.calculated

    Configuration Options



    Simplifying assumption to ignore concentration polarization


    Specify an estimated value for the concentration polarization modulus


    Allow model to perform calculation of membrane-interface concentration


    Options to account for mass transfer coefficient.

    default - MassTransferCoefficient.calculated

    Configuration Options



    Mass transfer coefficient not used in calculations


    Specify an estimated value for the mass transfer coefficient in the feed channel


    Allow model to perform calculation of mass transfer coefficient


    Options to account for mass transfer model.

    default - TransportModel.SD

    TransportModel.SD”, “Solution-diffusion model for describing water and salt transport for most membrane types” “TransportModel.SKK”, “Speigler-Kedem-Katchalsky model for describing water and salt transport”


    Options to account for geometry differences between flat sheet and spiral wound membranes.

    default - ModuleType.flat_sheet

    ModuleType.flat_sheet”, “Module type option for flat-sheet membrane modules” “ModuleType.spiral_wound”, “Module type option for spiral-wound membrane modules, this option accounts for how membranes in spiral-wound modules are folded which reduces the channel width by half”


    Indicates whether terms for pressure change should be constructed, default - False. Valid values: { True - include pressure change terms, False - exclude pressure change terms.}

    pressure_change_type Indicates what type of pressure change calculation will be made. To use any of the pressure_change_type options to account for pressure drop, the configuration keyword has_pressure_change must also be set to True. Also, if a value is specified for pressure change, it should be negative to represent pressure drop.

    default - PressureChangeType.fixed_per_stage

    Configuration Options



    Specify an estimated value for pressure drop across the membrane feed channel


    Specify an estimated value for pressure drop per unit length across the membrane feed channel


    Allow model to perform calculation of pressure drop across the membrane feed channel


    Options to account for friction factor correlations.

    default - FrictionFactor.default_by_module_type

    Configuration Options



    Friction factor correlation that is specific to the supported membrane modules type


    Argument defining whether area variable should be spatially variant or not. default - DistributedVars.uniform. Valid values: { DistributedVars.uniform - area does not vary across spatial domain, DistributedVars.variant - area can vary over the domain and is indexed by time and space.}


    Discretization method to use for DAE transformation. See Pyomo documentation for supported transformations.


    Discretization scheme to use when transforming domain. See Pyomo documentation for supported schemes.


    Number of finite elements to use when discretizing length domain (default=10)


    Number of collocation points to use per finite element when discretizing length domain (default=5)


    Level of reporting results. default - False. Valid values: { False - include minimal reporting of results, True - report additional properties of interest that aren’t constructed by the unit model by default. Also, report averaged expression values

  • initialize (dict) – ProcessBlockData config for individual elements. Keys are BlockData indexes and values are dictionaries with config arguments as keys.

  • idx_map (function) – Function to take the index of a BlockData element and return the index in the initialize dict from which to read arguments. This can be provided to override the default behavior of matching the BlockData index exactly to the index in initialize.


(ReverseOsmosis1D) New instance

class watertap.unit_models.reverse_osmosis_1D.ReverseOsmosis1DData(component)[source]

Standard 1D Reverse Osmosis Unit Model Class.


Spiegler, K. S., & Kedem, O. (1966). Thermodynamics of hyperfiltration (reverse osmosis): criteria for efficient membranes. Desalination, 1(4), 311-326.