7. Conclusions & Recommendations
7.1 Conclusions
7.1.1 Literature Review
The literature considered (i) the Sasol process, (ii) anaerobic digestion, (iii) anaerobic bio-reactors, (iv) membrane technologies and (v) Anaerobic Membrane Bio-reactors (AnMBRs). From the literature review, the following conclusions were identified:
• In the anaerobic (and aerobic) treatment of SCFA rich waste waters – such as FTRW – biomass settleability poses a major issue. This leads to biomass washout and large required reactor volumes. In a response to this, fixed bed anaerobic technologies were developed. The anaerobic treatment of FTRW has the added benefit that > 90% of the biodegradable organics that enter the system is converted to methane, which can be used for energy generation.
However the fixed bed anaerobic systems produces a effluent with high TSS and SCFA concentrations and required an aerobic polishing step as post treatment. This further increases the operating cost to prepare the treated wastewater for reverse osmosis and recycling into the Sasol process (Section 2.2).
• Anaerobic systems relying on granulation such as the USAB, EGSB and IC reactors show poor process performance in the long term treatment of SCFA streams. This problem is overcome by the 100% solids-liquid separation imposed by the membranes in the AnMBR (Section 2.4.2).
• The capital cost of membranes has traditionally been one of the main factors hampering the full-scale implementation of MBRs. However, from the early 90’s to the present, membrane costs have shown a >95% decrease. The same has also been observed for the operating cost of MBR plants. If these trends continue the competitiveness of membranes over conventional solid-liquid separation systems will continue to increase into the next decade (Section 2.5.8).
• In the early 90’s there was a significant increase in the number of research outputs generated by research facilities investigating the aerobic MBR. In response to these research outputs
University
of Cape
Town
and skills developments, full-scale aerobic MBR plants rapidly increased in number in the past decade. For the AnMBR, a 10 fold increase in publications has been observed over the past 4 years. It is thus anticipated that the AnMBR will similarly find increasing full-scale application over the next decade (Section 2.5.8).
7.1.2 Feasibility Study
After the design and construction of a novel AnMBR system (Section 3.1) for the treatment of FTRW the feasibility study was commenced. This part of the research project comprised an investigation into (i) the response of the anaerobic biomass to the FTRW, (ii) the effect of the dispersed anaerobic biomass on the trans membrane pressure and flux through the membranes and (iii) overall process performance characteristics of the AD-FTRW process. The following conclusions were drawn:
• The AnMBR can treat Fischer-Tropsch Reaction Water at organic loading rates (OLR) of up to 30 kgCOD/m3/d within 5 months from start-up. The system yields an effluent with a total COD < 400 mgCOD/L, an effluent SCFA < 250 mgAc/L and no particulates > 0.45 µm (Section 4.2).
• Since FTRW is chemically created water, it has no natural nutrients and very little alkalinity (~800 mgCaCO3/L). Nutrients have a significant effect on the growth rates of the anaerobic micro-organisms and as a result also on the OLR. From the nutrient optimization done after start-up it was shown that N, P, S and Fe is of primary importance in the AD-FTRW system and should be dosed as macro nutrients (~ 80, 10, 4, 1 mg/Lfeed respectively for 18 000 mgCOD/L FTRW). Yeast extract, Ca and Mg is of secondary importance and can be dosed as micro nutrients (< 1 mg/Lfeed) along with other micro nutrients normally required for anaerobic digestion (Section 4.3).
• Because of the 100% solid-liquid separation imposed, and long sludge ages (> 100 days) required to increase the MLSS into the optimal range for membrane scour (> 12 gTSS/L), the membranes trap all the particulate COD and endogenous biomass inside the AnMBR, where it can be hydrolyzed almost to completion. This induces an abnormally high nutrient recycle
University
of Cape
Town
within the biomass. It was found that the system can operate with a COD:N:P ratio of 2000:10:1 without nutrient deficiency. This is more than three times higher than the typical COD:N:P ratio of 650:10:1 used for anaerobic systems treating acidic substrates. An operational effluent Ammonia (FSA) < 50 mgN/L can be maintained (Section 4.3).
• Provided the system is operated below the critical flux (<4.3 L/m2/h), MLSS concentrations > 30 gTSS/L were maintained without deterioration of membrane fluxes, even though the DSVI (3000 ml/g) indicates that the sludge cannot be settled by traditional methods. The AnMBR system produced 1/10th of the sludge mass traditionally observed in aerobic systems(0.0215 gTSS/gCODremoved vs. 0.22 gTSS/gCODremoved) and has zero oxygen requirements (Section 4.6.1)
• More than 98% of the COD entering the AnMBR is converted to methane (Section 4.6.1).
This energy rich biogas can be used for digester heating, electricity generation or even recycled for fuel production. It is estimated that if the biogas produced from the anaerobic digestion of the entire Secunda-FTRW stream (29 ML/d at 18 gCOD/L) is converted to electricity, it will exceed the wastewater treatment plant’s electricity requirements by approximately 23 MW (571 MWh/d) (at 33% thermal efficiency), enough to power 17000 average South African house holds (33 kWh/household/d). The carbon footprint of the AS- AnMBR plant will also be 48% less than that of the current waste water treatment system (Appendix 1.1).Alternatively, the biogas can be blended into the natural gas line before auto- thermal reforming which will then be converted to synthesis gas and polymerized via the FTRW process. It is estimated that if the 128 ton/d of methane is converted to diesel, 52 000 L/d can be produced extra, resulting in a further capital gain of ~65 R million per year (Chapter 1).
• The Specific NaOH Utilization of the AnMBR is affected by three parameters, (i) the feed flow rate through the reactor, (ii) the reactor pH and the (iii) effluent SCFA concentration. It was found that the NaOH requirement of the AnMBR was on average 0.067 kgNaOH/kgCOD at a reactor alkalinity and pH of 2200 mgCaCO3/L and 7.05 respectively (Section 3.5 & Section 4.6.1).
University
of Cape
Town
• In the design of the AnMBR, the membrane surface area was significantly oversized, since there was no way of predicting how the membrane performance would be affected by the anaerobic biomass. For the first 650 days of the study membranes were operated at 1 to 10 % of their capacity. The recirculation of biogas for reactor mixing and membrane scour was successful. No noticeable deterioration of membrane performance was observed over the test period. However, inorganics including rust or precipitates can cause a reversible fouling that has a detrimental effect on membrane performance, but once removed, membranes recover completely (Section 4.5.1).
• In the last 50 days of the experimental phase, an effluent recycle was incorporated into the AnMBR design to simulate the effects of an increased flux over the membranes. For fluxes below 4.3 L/m2/h no correlation could be found between TMP and flux. However, when the flux increased over 4.3 L/m2/h, a sharp increase in the TMP was observed. Under the conditions imposed, the flat panel membranes appear to have a critical flux of ~4.3 L/m2/h.
At fluxes of higher than this critical value, biological cake layer formation cannot be controlled by biogas scour, even at increased scour gas flow rates up to 150% (Section 4.5.2).
The normal scouring rate was 750 L/m3/h as recommended by Kubota®.
• The first irreversible fouling test was done before commissioning the AnMBR, the second was done 530 days later. The change in flux through the clean membranes changed very little over the 530 days between the two chemical cleans. If a 50 % decrease in flux is allowable before membrane replacement this data points to a membrane life span on the AD-FTRW environment of at least 7 years. However, it should be emphasized that this prediction is an extrapolation from a small dataset and relatively short period of investigation. It would appear that the permanent fouling rate of activated sludge is at least twice that of the AnMBR (Ramphao et al., 2004); 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) (Section 4.5.3).
University
of Cape
Town
7.1.3 Performance Evaluation
In response to the positive outcomes of the feasibility study, a lab-scale down-flow Anaerobic Packed Bed Reactor (AnPBR) was also constructed and operated to benchmark the performance of the AnMBR. The AnPBR was a scaled-down version of a pilot plant system currently under development by Sasol - its volume was exactly the same as the AnMBR (23 L) and its packing density and down flow velocity the same as the pilot plant (Section 3.2). Both systems are fed the same artificial FTRW and nutrient mix. The two systems were operated at a steady state organic loading rate (OLR) = 15 kgCOD/m3/d for a period of 35 days and their performance compared. The flowing conclusions were drawn:
• The AnMBR effluent (35 mgCOD/L) is free of particulates and TSS compared with the AnPBR where 57 % of the effluent COD (1750 mgCOD/L) is in particulate form.
Furthermore the total COD of the AnMBR is only 2% of the total effluent COD of the AnPBR at an OLR of 15 kgCOD/m3/d. This difference would result in a significantly reduced operating and capital cost for the downstream processing system (Section 4.6.1).
• The membranes act as a positive barrier retaining biomass in the AnMBR. Because of this and the long sludge age, the dead biomass in the reactor gets hydrolyzed and is reintroduced as substrate to be utilized by the anaerobic biomass. This re-utilization of biomass results in a
~30% lower sludge production - and sludge incineration cost - if compared to the AnPBR (0.022 gTSS/gCODremoved vs. 0.031 gTSS/gCODremoved at OLR = 15 kgCOD/m3Vr/d) (Section 4.6.1).
• The main operating cost in anaerobic systems treating acidic waste water is the alkalinity (NaOH) dosing cost. Because of the high effluent SCFA and slightly higher reactor pH of the AnPBR to maintain a SCFA/Alkalinity < 0.3, the alkalinity consumption of the AnMBR system was ~40 % higher than that of the AnMBR under the same operating conditions (0.067 gNaOH/gCODremoved vs. 0.11 gNaOH/gCODremoved) (Section 4.6.2).
• The AnPBR can handle far greater (3 times) shock loads than the AnMBR and also show a 30% shorter recovery period before complete recovery. From an operational point of view it