NUMEROSAS BARRERAS EN EL ACCESO A LA ATENCIÓN EN

A generic modelling development approach, which included the five main steps of (i) project definition, (ii) data collection, (iii) plant model set-up, (iv) data collection from the model, and (v) simulation and results interpretation, was implemented for developing all of the models discussed in this thesis.

The approach to this model development was adapted from numerous model studies, such as Petersen et al., 2000; Boltz et al., 2013; and Brockmann et al., 2013 and from experience gained while modelling the PFBR. This approach was used in the development of all models in both GPS-X and AQUASIM. The overall goal was to summarize the approach to model development by following a series of generic steps (Figure 4.1).

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Definition

Data Collection From the Model Data Collection Simulation and Results Interpretation Plant Model Set-up Objectives Data Analysis Define Model Scenarios Initial Model Run Specify known Influent Characteristics Requirements Undertake a Sampling and Monitoring Program Sensitivity Analysis Assume Typical Kinetic & Stoichiometric Model Parameters Data Collection Plant Simulation

Calibration Present and

Interpret Results Validation Specify the Physical Dimensions of the Plant Report Model Checks Report

Figure 4.1 – Model development and calibration steps

Initial work in this chapter focused on developing a model that could accurately represent the physical operation of the LS-PFBR (notably the cyclic variation in reactor levels due to the alternate pumping of wastewater between the two connected reactors in the system). The objects considered for modelling the LS-PFBR in GPS-X included trickling filters, rotating biological contactors and submerged biological contactors. However, an initial investigation showed that two linked SBR objects most accurately and efficiently simulated the physical operation of the LS-PFBR. Each SBR object could incorporate typical PFBR stages, comprising fill, anoxic, aeration, settle and draw.

The three SBR objects available in GPS-X were considered namely; (i) the simple SBR object, (ii) the advanced SBR object and (iii) the manual SBR object. All three objects have the same functionality, appearance and choice of biological models but they differ in the manner in which the user specifies the operation of the SBR unit.

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The simple and advanced SBR objects require the specification of the timing and flow rates to define the phases. The manual SBR object requires that the entire operation cycle be defined by the user by having the liquid flows, air flows, and mixing as file inputs.

Suitable layouts for representing the hydraulic characteristics of the LS-PFBR were developed using each of the three SBR objects available in GPS-X. Some results from the initial investigations using the advanced SBR object, the simple SBR object, the RBC and the trickling filter are shown in Appendix C. Table 4.1 shows a number of plant layouts and different process objects trialled in GPSX in order to model the PFBR. It is important to note that this is just a small sample of the layouts trialled to model the PFBR, the rest are shown in Appendix C.

Table 4.1 – GPS-X layouts trialled to model the PFBR

GPS-X object GPS-X layout Comments

Simple SBR The layout involved using 4 SBR objects and 1 modelling toolbox. One SBR object was used for each process of the PFBR i.e.

SBR 1 – Fill SBR 2 – Anoxic SBR 3 – Aerobic SBR 4 – Settle

The hydraulics were modelled successfully using this layout; however, it took the model nearly 60 minutes to simulate a 5 day hydraulic model so it was not practical to use.

Also, between every SBR object, a splitter was required before the model could be built. Hydromantis was informed of this problem and are working on a solution.

Simple SBR The layout involved using 3 SBR objects and a 4 way splitter. One SBR object was used for each of the following processes.

SBR 1 – Fill

SBR 2 – Anoxic, Aerobic SBR 3 – Aerobic, Settle

The 4 way splitter controlled the influent and the recycle flow.

The hydraulics could be modelled using this layout; however, the 4 way splitter did not accurately divide the influent and the recycle flow in every cycle. Hydromantis were informed of this problem and they said they would work on a solution.

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GPS-X object GPS-X layout Comments

Simple SBR The layout involved using 2 SBR objects however this layout required 5 modelling toolboxs modelling the on/off flows and was extremely complicated to set up.

The flows were initially modelled correctly however after one cycle the 5th modelling toolbox which controlled the recycle on/off flow did not read correctly so the hydraulics couldn’t be modelled after the first cycle. Hydromantis were informed of this problem and they said they would work on a solution.

Advanced SBR

This layout included 2 SBR objects, 4 splitters and 5 modelling toolboxes.

The hydraulics and the DO were modelled successfully using this layout. However the model was slow to run due to the 5 modelling toolboxes.

Advanced SBR

This layout included 2 SBR objects, 3 splitters and 3 modelling toolboxes.

The hydraulics and the DO were modelled successfully using this layout.

Rotating biological contactor

The RBC was trialled as an object to model the PFBR. It was proposed to fill the RBC with wastewater and move the biofilm in and out of the tank in order to expose it the atmosphere and mimic the aerobic and anoxic stages of the PFBR.

The option of moving the wastewater in and out of the RBC was also examined.

The submerged fraction of biofilm in an RBC cannot be changed during a simulation.

It was also not possible to move the wastewater in and out of the biofilm as initially thought.

RBC based layouts took about 30 minutes to simulate 1 operating day.

Rotating biological contactor

Layout 1: This layout included

one RBC and involved

changing the KLa value to turn on and off the oxygen throughout a cycle.

This layout did not work and also took about 15 minutes for the plant to start running.

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GPS-X object GPS-X layout Comments

Rotating biological contactor

Layout 2: This layout included 2 RBC objects to mimic the 2 reactors in the PFBR.

It was not possible to transfer wastewater between the two RBCs.

Submerged biological contactor

The SBC was examined as an object to model the PFBR.

The same problems as the RBC arose.

Trickling Filter

The trickling filter was trialled in GPS-X as it provided biofilm biological treatment.

The limitations of the object to model the PFBR concerned the hydraulics. It was not possible to circulate wastewater between two trickling filters.

Table 4.2 shows the general layout of the three SBR objects in GPS-X.

Table 4.2 – Layout of SBR object in GPS-X

Influent: connection point where the

wastewater is filling from.

Overflow: connection point where wastewater can overflow. However, if the model system is set up correctly, the overflow should be zero throughout the cycle.

Decant pump: connection point for decanting. The pump always decants from the upper layer.

Waste Pump: connection point from where

the wastewater is discharged.

The manual SBR object was chosen to model the LS-PFBR as it enabled maximum user flexibility in defining key parameters, such as wastewater flow rates, mixing and air flow rates. The manual SBR object enables the entire operational cycle to be defined by the user, either by directly defining cycle parameters or via file inputs.

The final plant model for the LS-PFBR comprised two manual SBR objects, an influent object, two flow combiners and a flow splitter (Figure 4.2). The two SBR units were used to model the different stages of the treatment cycle, i.e.

 SBR1 – fill, anoxic and aerobic stage  SBR2 – aerobic, settle and draw stage

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This is an exact replication of what happens in the LS-PFBR. The fill and discharge from the system typically takes place in R1 and R2 respectively. The anoxic stage takes place in R1 and the aerobic stage takes place in both R1 and R2.

Figure 4.2 – GPS-X layout of the LS-PFBR model using two manual SBR objects

Table 4.3 shows the physical dimensions of the SBR reactors used in the laboratory and the model.

Table 4.3 – Physical design parameters (measured and modelled) Physical dimensions Size

Surface area of tanks 0.0576 m2 Maximum water level height 0.4 m

The LS-PFBR was operated under two cycle regimes (Study 1 and Study 2). The treatment cycle comprised anoxic, aerobic and settle stages of varying lengths (as described in Section 3.4.2) and summarised in Table 4.4.

qeff frzpump 2toeff Reactor 1 Reactor 2 qinrawinf qinzpump 1 qconzpum p2 Pump 1 Pump 2 Influent Discharge

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Table 4.4 – Operational cycle of LS- PFBR (Study 1 and 2) PFBR Stage Duration (minutes)

Fill (t1) 8

Anoxic (t2) 150

Aerobic (t3) 106

Settle (t4) 60

Draw (t5) 5

ASM1 was chosen to calibrate the LS-PFBR model as the model was mainly concerned with predicting COD and N removal concentrations both in the effluent but also during individual treatment cycles within the PFBR. The model calibration process was initiated with all the ASM1 values set to default.