6. Dynamic Modeling
6.41 Reactor Failure Due to OH - Dosing
The first scenario investigated was that of a failure of the NaOH dosing system required for pH control. This can happen due to the NaOH storage tank running empty or a dosing pump malfunction. In the first simulation (Figure 6.21 - OHin[-1] and Figure 6.22) a pH decrease from the normal 7.1 to 6.8 was evaluated. The NaOH dosing was decreased from its normal 0.8 molOH/d (1285 mgNaOH/L) to 0.24 mol/d (385 mgNaOH/L). This caused the system to acidify, resulting in the desired pH decrease to 6.8. After 72 hours, the OH- dosing was reset to the original level (Figure 6.21 – OHin[-1]) and the pH in the reactor recovered back to 7.1.
Figure 6.21, OH- dosing for pH control
Therefore based on the pH inhibitions as included in the model, if the pH is lowered to a level of 6.8 due to a dosing malfunction, the AnMBR can recover completely if this is not continued for longer than 4 days (Figure 6.22). If this period is exceeded, the system fails catastrophically (similar to Figure 6.23).
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Figure 6.22, Maximum Time at Decreased pH = 6.8 for Full Recovery
Since the peak H2(aq) and SCFA concentration was still fairly low for the pH<6.8 scenario, no excessive OH- dosing was not required for full system recovery – NaOH dosing only needed to be recovered to normal and the SCFAe concentration recovered shortly afterwards.
In the next simulation the goal was to decrease the reactor pH to 6.7 (Figure 6.21 – OHin). In this simulation the NaOH dosing was decreased to 0.095 molOH/d (160 mgNaOH/L) to observe the pH decrease (Figure 6.23).
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Figure 6.23, Minimum Time at Decreased pH = 6.7 for Reactor Failure
After approximately 36 hours, the system failed catastrophically. The investigation of the effect to pH fluctuation on the AD-FTRW system could not be continued since the system shows immediate failure for pH values below ~6.68. Similar results were obtained for decreasing the system pH above the optimal (> 7.4), which can happen in a NaOH overdose-situation.
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6.4.2 System Failure Due to Temperature Fluctuation
The next inhibitory effect that was investigated was that of temperature fluctuations (Figure 6.24).
The actual FTRW stream is cooled from 100 oC to 37 oC before it enters the full-scale wastewater treatment plant (Section 2.1). Thus a scenario of temperature increase above the optimal is quite plausible if the cooling system fails. Secondly, a temperature decrease is also plausible if the ambient temperature decreases or if the reactor’s backup internal heating system fails.
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Figure 6.24, Minimum Time at Decreased Temp (305 K) for Reactor Failure
Unlike pH, temperature is a control parameter in the dynamic AD-FTRW model i.e. it can be varied directly (Figure 6.24A). The model predicts very high temperature sensitivity. Even a gradual 5 oC temperature decrease (over 4 days) still leads to catastrophic failure of the biomass (Figure 6.24).
Similar results were obtained for increasing the temperature above 37 oC. The effects of both temperature and pH appeared to be described quite well when compared with experimental observations during the investigation, but this aspect of the model was not formally validated.
6.4.3 System Failure Due to Overloading
The effect of an increase in SCFAe an H2(aq) is interlinked and the one usually happens in response to the other. To simulate the effect on an increase in these two parameters, an organic overload is applied to the AnMBR. An OLR overload is the most common type of inhibition observed in the AnMBR. This is especially prevalent in the start-up phase (Section 4.2) when the OLR is increased on a daily basis. In the next two presented simulations, the smallest OLR increase (for 24 hours) which leads to a catastrophic system failure were investigated (Figures 6.25 & 6.26). For the first simulation, the influent flow rate was increased by 15% (24.9 L/d to 28.635 L/d) for 24 hours and
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then it was reset to the initial flow rate. Correspondingly, the OLR also increased with 15% since OLR = Qi.Sti/Vr and the influent COD (Sti) and the reactor volume (Vr) is constant at 18 500 mgCOD/L and 23 L respectively.
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Figure 6.25, Minimum 24 Hour OLR increase for Reactor Recovery
For an OLR increase of 15% the model predicts a full system recovery (Figure 6.25). A small spike in the SCFAe and H2(aq) is observable, but once the influent flow (and OLR) is reset to normal, the system recovers completely. However, a 17% OLR increase, corresponding to a 17% influent flow rate increase, leads to a catastrophic failure (Figure 6.26). The reason for the failure is that inhibitory compounds (SCFAe and H2(aq)) accumulate in the reactor because it cannot be removed at a high enough rate by the active biomass. This leads to inhibitory conditions that shut down the bio- processes. Experimentally, this value was observed as slightly lower. The laboratory scale AnMBR could handle an OLR increase of ~12%, but a 24 hour increase of 15% would lead to complete system failure.
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Figure 6.26, Minimum 24 Hour OLR increase for Reactor Recovery
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6.4.4 System Failure from Too Low Sludge Age
Next the minimum sludge age for a given OLR was investigated. This simulation was done by allowing the system to reach steady state at a long sludge age (~300 days) for a given OLR. Then the sludge age was decreased to ~100 days and the simulation was allowed to run to steady state (> 3 sludge ages). If the system reached steady state, the sludge age was again decreased to ~95 days and the process was then restarted at a lower sludge age. This procedure was then continued until the minimum sludge age was identified where the system was still able to reach steady state for the chosen OLR. In other words, if the sludge age was decreased any more the biomass in the system would not be able to grow fast enough to remove all of the SCFA. This would lead to a SCFA overload and catastrophic system failure. Figure 6.27 presents the ‘failure sludge age’(RSfail) vs.
OLR and also the corresponding reactor solids concentration (MLSS). As a matter of interest the steady state MLSS as predicted by the steady state model was also plotted for the given OLR and sludge age. A good correlation exists between the steady state and dynamic model predictions and is typically within 10% of one another (Figure 6.27 & Table 6.3).
Figure 6.27, Predicted Minimum Stable Sludge Age & Corresponding MLSS vs. OLR
From Figure 6.27, as the OLR increases, so does RSfail. Furthermore it can be seen that the minimum MLSS of 12 gTSS/L for the membranes to function properly (Section 3.1.1), a minimum sludge age
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of 70 days and a minimum OLR of 13.5 kgCOD/m3/d will be required. To observe how effluent quality and MLSS changes with sludge age at a constant OLR, Figure 6.28 was constructed.
Figure 6.28, Effluent COD and Reactor MLSS vs. Sludge Age at OLR = 15 kgCOD/m3Vr/d
Ideally, the effluent COD (Sbe) should be maintained below 300 mgCOD/L, this not only ensures a high quality effluent, but also avoids SCFA inhibition which could lead to system failure and high OH- dosages to neutralize the unutilized SCFAs accumulating in the reactor. To maintain a Sbe < 300 mgCOD/L the system should be operated at a sludge age > 110 days (for OLR = 15 kgCOD/m3/d). If these criteria are met, sufficient solids (MLSS > 12 gTSS/L) should be generated for membrane scour (Figure 6.27).