4. Results & Discussion
4.6 AnMBR Steady State Performance Evaluation
University
of Cape
Town
slope of line 2 and 3 on Figure 4.8 gives an indication of how effective the hypochlorite chemical clean is.
Comparing line 1 and 3, it can be seen that the change in flux through the clean membranes changed very little over the 530 days between the two chemical cleans. From the data in Figure 4.8, at a flux of 15 L/m2/h (0.35 m/h), the TMP increased from 15/0.1336 = 112 mmH2O to 15/0.1152 = 130 mmH2O in 530 days. This is 18/530 = 0.034 mmH2O/day permanent fouling. In the activated sludge (AS), this permanent fouling rate at an operating flux of 10-15 L/m2/h was around 0.1 mmH2O/d (Ramphao et al. 2004). Based on this observation it would appear that the permanent fouling rate of activated sludge is at least twice that of the AnMBR; however the operating flux of the AnMBR at 4.3 L/m2/h was significantly below that of the AS system (10 – 15 L/m2/h). If a 50 % decrease in flux is allowable before membrane replacement this data points to a membrane life span in the AD- FTRW environment of at least 7 years (Appendix 4.3). However, it should be emphasized that this prediction is an extrapolation from a small dataset (2 tap water flux tests) and relatively short period of investigation. The investigation of permanent fouling of flat panel membranes in the AD-FTRW environment probably merits as significantly larger dataset and evaluation period to yield a concrete prediction of membrane life span. Note; because lines 1 and 3 lies close to each other, a t-test was done to verify if these lines are in fact statistically deferent, which the case was indeed.
University
of Cape
Town
effluent evaluation criteria was total COD (Ste), particulate COD (Spe), dissolved COD (Sse), Total Settlable Solids (Xte), Short Chain Fatty Acids (SCFAe) and Total Nitrogen (Nte). Daily samples were taken for both the AnMBR and AnPBR with the average values and 95% confidence intervals (CI95%) represented in Table 4.2. The 95% confidence interval is that range of variance around the mean in which 95% of the data points are distributed.
Table 4.2, AnMBR & AnPBR Steady State Comparison
Parameter AnMBR AnPBR Units
Avg CI95% Avg CI95%
Reactor OLR 15.3± 0.15 15.2± 0.28 kgCOD/m3/d
Reactor Volume 23± - 23± - L
Reactor Temperature 37± 1 37± 1 oC
Reactor pH 7.08± 0.05 7.11± 0.05
Reactor MLSS 20.2± 0.37 gTSS/L
Reactor MLVSS 15.4± 0.28 gVSS/L
Sludge Age 61± 3.6 d
Influent COD 18500± - 18500± - mgCOD/L
Influent Alkalinity 875± - 875± - mgCaCO3/L
Influent N 84± - 84± - mgN/L
Effluent Total COD 35.1± 6 1749± 91 mgCOD/L Effluent Particulate COD 0.0± - 768± 80 mgCOD/L
Effluent TSS 0.0± - 512± 266 mgTSS/L
Effluent Alkalinity 2213± 29 3031± 107 mgCaCO3/L
Effluent N 37.6± 3.2 35.7± 30 mgN/L
Effluent SCFA 10.41± 4.8 775± 30 mgAc/L
Effluent Na 1441± 188 1997± 289 mgNa/L
Specific NaOH Consumption 0.067± 0.01 0.11± 0.02 kgNaOH/kgCODremoved
COD Removal Efficiency [%] 99.81 90.55 [kgCODin*100/kgCODout] Biomass Production 7.57 10.60 gTSS/d
N Consumption 0.003 0.0029 kgN/kgCODremoved
For virtually identical influent concentrations and reactor loading and environmental conditions, the AnMBR effluent COD (Ste) is more than an order of magnitude lower than that of the AnPBR. If the particulates (Spe and Xte) are compared, the AnMBR yields virtually zero value, where this is a significant contribution (43 %) to the COD in the AnPBR effluent. Because of the low effluent
University
of Cape
Town
COD, the Removal Efficiency (RE) of the AnMBR is significantly higher (by 9 %) than that of the AnPBR. Biomass production and nitrogen consumption are both 30 % lower in the AnMBR than in the AnPBR.
4.6.2 Alkalinity Requirements
FTRW is an acidic, alkalinity deficient industrial effluent. To provide buffer capacity for anaerobic digestion at neutral pH, a significant amount of alkalinity is required. This alkalinity requirement is the main operating cost of any AD-FTRW treatment system. For this reason, the steady state alkalinity consumption of the AnMBR and AnPBR was evaluated. Figure 4.9 displays the Specific NaOH consumption for the 35 day steady state evaluation period.
0.000 0.050 0.100 0.150 0.200 0.250
0 5 10 15 20 25 30 35 40
Time [days]
Specific NaOH Consumption [kgNaOH/kgCOD]
AnMBR AnPBR
Figure 4.8, AnMBR & AnPBR Specific Alkalinity Consumption vs. Time
From Figure 4.9 it can be noted that the Specific NaOH consumption which reacts with CO2 to from HCO3-
of the AnMBR is consistently lower than that of the AnPBR. The average for the AnMBR over the evaluation period is 0.067 kgNaOH/kgCODremoved (Table 4.2), which is nearly 30% lower than that observed in the AnPBR. The effluent NaOH concentration was also proven to be in the order of 25 % lower in the AnMBR.
University
of Cape
Town
4.6.3 Shock Loading Responses
When a biological water treatment system undergoes a period of stress due to a shock load, the biological activity can decrease, particularly with the acidic FTRW which can depress reactor pH and cause acetoclastic methanogen inhibition. When this happens, the system needs to be operated at reduced OLR until the biomass activity recovers and a low effluent SCFA (< 150 mgAc/L) concentration is again achieved. During this ‘down-time’ the wastewater that cannot be treated needs to be stored or treated by another means, both having significant cost implications. Shock loading responses of the AnMBR and AnPBR were therefore observed and compared.
[A] [B]
Figure 4.9: AnMBR [A] and AnPBR [B] shock loading responses.
Shock loads were imposed on both systems after a period of steady state operation. Note the on-line control system was deactivated for this part of the study. The shock loads were introduced by increasing the feed flow rate to the reactor. In this overloaded state there is not enough active biomass to remove the SCFAs introduced by the FTRW in the feed. Thus an accumulation of SCFAs occurs in the reactor. If the SCFAs go above a maximum value - 800 mgAc/L in the AnMBR and 1600 mgAc/L in the AnPBR – the pH decreases below 6.8 which has an inhibitory effect on the anaerobic biomass resulting in a reduction in activity. If this happens the flow rate to the reactor needs to be decreased or stopped completely, depending on the severity of the overload. This allows
University
of Cape
Town
time for the biomass to utilize the excess SCFAs which in turn increases the pH (and alkalinity). If after 24 hours, if the effluent SCFA is below the required minimum (150 mgAc/L), the OLR can be increased; if higher, it must be decreased. This procedure allows the system to recover from shock loads. Figures 4.10A & B show the shock loading responses of the AnMBR and AnPBR respectively.
In both reactors the OLR overload was ~15 % of the steady state value, in the case of the AnMBR (day 604 to 605), a sharp increase in SCFAs was observed within the first 24 hours and the feed was stopped (Day 606). In the AnPBR (day 372 to day 374) the overload only became apparent after 48 hours and the feed was immediately stopped (Day 374). The SCFA increase in the AnMBR was significantly (60%) smaller than that of the AnPBR system and the recovery time of the AnMBR was 15 days compared to only 10 days for the AnPBR.