Add-on packages are dedicated process units to cover different technologies or areas such as mobile carrier technologies or detailed sewer network modeling. The chapter provides description about these models.
The addons are available for download on our website.
Mobile carrier can be described as free moving, submerged biofilm carrier which can leave the reactor boundaries with the biofilm attached to it.
Mobile carrier (MC) model is the extension of the SumoBioFilm© model by allowing the carrier and the attached biofilm to move between process units. The model uses Xcarrier variable to calculate the amount of carrier and attached biofilm transported via connections. The sub-model allows the simulation of recirculation, back mixing, media removal and addition.
Biofilm carrier migration model describes reactor performance, Water Science & Technology, 75.12, 2017, Joshua P. Boltz, Bruce R. Johnson, Imre Takács, Glen T. Daigger, Eberhard Morgenroth, Doris Brockmann, Róbert Kovács, Jason M. Calhoun, Jean-Marc Choubert and Nicolas Derlon
The sewer network add-on contains a pipe segment process unit, a service cover and compatible gas flow element. The pipe segment is consider as a plug flow reactor and can model force main and gravity sewers with fixed biofilm or sediments.
In gravity sewers, the gas phase is allowed to flow both directions – same as sewage and counter direction as well. The service cover predicts fresh air intrusion and gas transfer.
This add-on includes a Densified sludge separator process unit to simulate the granules selection obtained with a hydrocyclone. A specific model extension is required to simulate separate biological reactions in the flocs and granules: the user can choose from either the Sumo2 or Sumo4N with granule extension models.
The concept is based on the aerobic granular sludge modeling approach described by Baeten et al. (2018) wherein separate modelling state variable components are assigned for floccular and granular organism groups. The diffusion resistance of substrate into the granules is accounted for using higher “apparent” half-saturation coefficients for granular components. The selective retention and accumulation of granules is predicted by the model as a result of the differential effect on the floccular and granular organism groups of biological selector zones and external selectors, for example wasting from the overflow of hydrocyclones.
The granule fraction is calculated as a function of the relative ratio of granular to total organism groups (granules + flocs) and by assigning portions of influent particulate material to either the floccular or granular groups.
Under normal circumstances, the floccular components of ordinary heterotrophs (XOHO), and PAO and GAO carbon storing organisms (XCASTO) will dominate the growth processes because the model applies a “Diffusion Resistance” parameter to the growth of granular organisms. This diffusion resistance accounts for the reduced access to substrate and oxygen in the inner layers of the granules. It is applied using the “apparent Ks” concept described by Baeten et al. (2018). For example, in the equation below the Monod saturation term for some substrate “S” is defined with the half-saturation coefficient “MsatS” increased proportionally to the diffusion resistance “DR”. The default value of DR is 1.2 which implies a 20% increase in the half-saturation coefficient.
In fact the term (Ks*DR) acts as the “apparent Ks” investigated by Baeten et al. Table 1 shows how this might be applied in a simplified model matrix. Figure 1 provides a conceptual framework for how modeling direction of influent substrate to growth of floccular and granular organisms in the mixed liquor can then be used to calculate the densified fraction of the mixed liquor.
Table 1 - Gujer matrix presenting separate state components for floccular and granular PAOs, storage products (PHA) as well as “apparent Ks” applied to VFA sequestration and O2 utilization in granules
Because the higher “apparent Ks” has less impact when substrate concentrations are high, the model will simulate the beneficial effects of an internal selector for sludge densification. It is then easy to model the benefits of an external selector to enhancing densification, such as a hydrocyclone, in which granular state variables are selectively retained.
J. Baeten, M.C.M. van Loosdrecht, E. Volcke (2018) “Modelling aerobic granular sludge reactors through apparent half-saturation coefficients.” 146. 134-145. Water Research.
You can modify an existing Sumo model to work with the densification model. In this example, we will select the A2O plant from the list of Examples in the home screen.
To select the densification biokinetic model, go to the MODELS tab in the Sumo ribbon, select the “Advanced” option, and then the Sumo2_Granules option from the Focus models. This model is based on Sumo2, a 2-step nitrification model, with the modification that certain particulate state variables are divided between floccular and granular components.
To add a hydrocyclone to the flowsheet, select it from the “Separators” in the CONFIGURE tab of the Sumo ribbon. Make sure to select “Densified sludge separator” from the Options in the bottom left pane.
The hydrocyclone is added to the WAS stream together with a “Proportional side flow divider” to enable easy switching “On” and “Off” of the cyclone operations.
In the OUTPUTS tab, select the Cyclone element. Create a table and drag and drop the “Contributors to granular TSS” into the table for each of the Feed, Overflow and Underflow. You can drag and drop the input and output ports of the cyclone to create the different columns.
In the INPUTS tab of the Sumo ribbon, select the proportional side flow divider immediately upstream of the cyclone and check the “Scenario” box.
In the SIMULATE tab, this parameter will now appear as an option in the bottom left pane. Create and name two scenarios to represent Cyclone “On” and “Off” operations.
The model includes fractions to represent the amount of particulate inorganic (XINORG), particulate unbiodegradable (XU), particulate biodegradable (XB) and heterotrophs (XOHO) that are granular. The default values are 20%, 10%, 0% and 1%, respectively. The fractions are applied in the following manner: if the amount of influent XINORG is 75 mg/L then 15 mg/L (20%) will be assigned to XINORG,granule and the remaining 60 mg/L (80%) to regular “floccular” XINORG.
By including these in the scenarios we can use them to calibrate the amount unbiodegradable material that contributes to the densified fraction. For example, changing the fraction of influent unbiodegradable organics (XU) that meets the granule definition from 10% to 5% reduces the Densified fraction from 7.5% to 6% in the Baseline scenario (Cyclone Off).
The diffusion resistance (DR) parameters are defined in the MODELS tab of the Sumo ribbon in the bottom left pane after selecting “Show all” under the heading “Granule diffusion resistance”. We can calibrate the impact of these parameters by selecting the “Scenario” boxes as shown below.
After changing the DR for susbtrates SB and SVFA from 1.2 to 2, simulating dynamically for 100 days shows the Baseline densified fraction (Fgranule) decreasing from 7.5% to 6.5%.
Note that the steady state solver does not always accurately find the “true” long term stable state condition, particularly when the previous solution state was “far” from the current one. It may be advisable to use the “Reset” button in some cases to reinitialize the model to a state closer to a “normal” condition or run long term dynamic simulations until time series charts show stabilizing variables of interest.
On switching to the “Cyclone” scenario, and including the changes to the diffusion resistance parameters from 1.2 to 2, we see the dynamic changes in Densified fraction “Fgranules” decreasing from 28.8% to 21.2% over 100 days of simulation.
The model calculates a difference in ORP for fermentative organisms in the floc and the granules. The ORP for the flocs and granules can be viewed in a table by typing “Oxidation-reduction” into the dialog box in the bottom left pane of the OUTPUTS tab of the Sumo ribbon. As long as the “Variables” option is selected from the drop-down menu then the “Oxidation-reduction potential” for the flocs and granules should be displayed. As can be seen below, the ORP in the granules is lower than in the flocs.
The Cyclone operating parameters can be accessed in the INPUTS tab of the Sumo ribbon. The cyclone model is defined with only four parameters. The first specifies the hydraulic or volumetric split between the overflow and the underflow and has a default value of 10%. The second parameter represents the mass fraction of floccular particulate material captured in the underflow and has a default value of 20%. The last two parameters represent a multiplier applied to this mass capture for granular material and inorganic granular material. So for a default capture of 20% of flocs in the underflow, a “Ratio of granule to flow captured in underflow” of 3 will result in a mass capture of 66% of granules in the underflow.
The plant SRT calculation can be calculated as usual in the TOOLS ribbon, selecting the “Sludge Retention Time” feature and selecting the proper units with drag and drop method.
The calculation of granule SRT can be done on the basis of one of the granular biomass, as XAOB, granule for example. The “Ratio of variables” calculation has to be selected in the top left panel, and “custom” numerator and denominator variables should be set in the bottom left panel as “M_X_AOB_granule” and “F_X_AOB_granule” respectively. In the bottom right panel, the appropriate process units should be drag ant drop at numerator and denominator.
In the following simulation with the cyclone activated, the process is intensified by increasing the WAS pumped flow from 300 m3/d to 900 m3/d. The SRT calculations results in 6.3d SRT for the plant and 8.6d SRT for granules.