Constructed Wetlands

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Constructed wetlands are becoming increasingly popular worldwide for removing nutrients, organics, trace elements, pathogens, or other pollutants from wastewater and/or runoff water. A multi-component reactive transport module CW2D (Constructed Wetlands 2D, Langergraber, 2001, Langergraber and Simunek [2005]) was developed as an extension of the earlier HYDRUS-2D variably-saturated water flow and solute transport program (Šimůnek et al., 1999) and also implemented into current HYDRUS. CW2D was developed to model the biochemical transformation and degradation processes in subsurface-flow constructed wetlands. The mathematical structure of CW2D is based on the IWA Activated Sludge Models (Henze et al., 2000). Monod-type expressions are used to describe the process rates. All process rates and diffusion coefficients are temperature dependent. The biochemical components defined in CW2D include dissolved oxygen, three fractions of organic matter (readily- and slowly-biodegradable, and inert), four nitrogen compounds (ammonium, nitrite, nitrate, and dinitrogen), inorganic phosphorus, and heterotrophic and autotrophic micro-organisms. Organic nitrogen and organic phosphorus are modelled as part of the COD. Heterotrophic bacteria are assumed to be responsible for hydrolysis, mineralization of organic matter (aerobic growth) and denitrification (anoxic growth). Autotrophic bacteria are assumed to be responsible for nitrification, which is modelled as a two-step process. All micro-organisms are assumed to be immobile. Lysis is considered to be the sum of all decay and loss processes.


A new multi-component reactive transport module CWM1 (Langergraber et al., 2009) was additionally included into the version 2.0 of HYDRUS. While in CW2D aerobic and anoxic transformation and degradation processes for organic matter, nitrogen and phosphorus are described, in CWM1 aerobic, anoxic and anaerobic processes for organic matter, nitrogen and sulphur.


Parameters for the constructed wetlands are entered in the Constructed Wetland Model Parameters I and Constructed Wetland Model Parameters II dialog windows.


Comparison of CW2D and CWM1 components.


CW2D

CWM1

[Langergraber and Šimunek, 2005]

[Langergraber et al., 2009]

Components

    1. SO: Dissolved oxygen, O2.
    2. CR: Readily biodegradable soluble COD.
    3. CS: Slowly biodegradable soluble COD.
    4. CI: Inert soluble COD.
    5. XH: Heterotrophic bacteria
    6. XANs: Autotrophic ammonia oxidizing bacteria (Nitrosomonas spp.)
    7. XANb: Autotrophic nitrite oxidizing bacteria (Nitrobacter spp.)
    8. NH4N: Ammonium and ammonia nitrogen.
    9. NO2N: Nitrite nitrogen.
    10. NO3N: Nitrate nitrogen.
    11. N2: Elemental nitrogen.
    12. PO4P: Phosphate phosphorus


Organic nitrogen and organic phosphorus are modeled as part of the COD.


Nitrification is modeled as to-step process.

Bacteria are assumed to be immobile.

Soluble components, S

    1. SO: Dissolved oxygen, O2.
    2. SF: Fermentable, readily biodegradable soluble COD.
    3. SA: Fermentation products as acetate.
    4. SI: Inert soluble COD.
    5. SNH: Ammonium and ammonia nitrogen.
    6. SNO: Nitrate and nitrite nitrogen.
    7. SSO4: Sulphate sulphur.
    8. SH2S: Dihydrogensulphide sulphur.

Particulate components, X

    1. XS: Slowly biodegradable particulate COD.
    2. XI: Inert particulate COD.
    3. XH: Heterotrophic bacteria.
    4. XA: Autotrophic nitrifying bacteria.
    5. XFB: Fermenting bacteria.
    6. XAMB: Acetotrophic methanogenic bacteria.
    7. XASRB: Acetotrophic sulphate reducing bacteria.
    8. XSOB: Sulphide oxidizing bacteria.


Organic nitrogen and organic phosphorus are modeled as part of the COD.


Comparison of CW2D and CWM1 processes.


CW2D

CWM1

[Langergraber and Šimunek, 2005]

[Langergraber et al., 2009]

Heterotrophic bacteria:

    1. Hydrolysis: conversion of CS into CR.
    2. Aerobic growth of XH on CR (mineralization of organic matter).
    3. Anoxic growth of XH on CR (denitrification on NO2N).
    4. Anoxic growth of XH on CR (denitrification on NO3N).
    5. Lysis of XH.


Autotrophic bacteria:

    1. Aerobic growth of XANs on SNH (ammonium oxidation).
    2. Lysis of XANs.
    3. Aerobic growth of XANb on SNH (nitrite oxidation).
    4. Lysis of XANb.


Heterotrophic bacteria:

  1. Hydrolysis: conversion of XS into SF.
  2. Aerobic growth of XH on SF (mineralization of organic matter).
  3. Aerobic growth of XH on SA (mineralization of organic matter).
  4. Anoxic growth of XH on SF (denitrification).
  5. Anoxic growth of XH on SA (denitrification).
  6. Lysis of XH.


Autotrophic bacteria:

  1. Aerobic growth of XA on SNH (nitrification).
  2. Lysis of XA.


Fermenting bacteria:

  1. Growth of XFB (fermentation).
  2. Lysis of XFB.


Acetotrophic methanogenic bacteria:

  1. Growth of XAMB: Anaerobic growth of acetotrophic, methanogenic bacteria XAMB on acetate SA.
  2. Lysis of XAMB.


Acetotrophic sulphate reducing bacteria:

  1. Growth of XASRB: Anaerobic growth of acetotrophic, sulphate reducing bacteria.
  2. Lysis of XASRB.


Sulphide oxidizing bacteria:

  1. Aerobic growth of XSOB on SH2S: The opposite process to process 13, the oxidation of SH2S to SSO4.
  2. Anoxic growth of XSOB on SH2S: Similar to process 15 but under anoxic conditions.
  3. Lysis of XSOB.