Treatment of Domestic Wastewater Using Anaerobic Membrane Bioreactor

Principal Investigator:

Shihwu Sung, sung@iastate.edu (other projects)

Co-Principal Investigator:

• Samir Khanal, samirk@iastate.edu (other projects)

Other Authors: Jaeho Ho, graduate student

Project Status: In Progress

Start Date: 08/01/2004
End Date: 07/30/2006


Research Objective: Application of anaerobic processes for the treatment of low strength wastewater has drawn considerable attention recently, and is becoming increasing popular in developing countries. However, maintaining a long solids retention time (SRT) is one of the challenges of anaerobic treatment processes. Anaerobic membrane bioreactors (AMBR) could essentially retain all of the biomass in the reactor without any fear of sludge wash-out irrespective of short hydraulic retention time (HRT). AMBR also produces superior effluent quality in term of suspended solids, chemical oxygen demand and pathogen count and there is a possibility of reuse and recycling of the treated effluent.

However, two important issues that aggravated the successful application of AMBR are capital and operational costs. Membrane technology is highly energy intensive process especially at higher solid levels. Not to mention, membrane itself is very expensive. These problems can be offset by using low cost materials such as non-woven fabric filter. The relatively low cost of this material allows frequent replacement after severe pore clogging. Based on this premise, this study was conducted to examine the applicability of non-woven fabric filter in retaining the anaerobic biomass.

Tubular non-woven fabric filter and poly-tetrafluoroethylene (PTFE) composite membrane were used to separate anaerobic sludge. The non-woven fabric filter used was made of polypropylene and had a filtration surface area of 0.015m2 with 25 and 12 µm pore sizes and PTFE membrane laminated on the non-woven fabric filter with 10, 5 and 1µm pore sizes were used. The anaerobic sludge was obtained from a local anaerobic digester. The solid concentration of the anaerobic sludge used in this research ranged from 5 to 30g/L. The microscopic structures of the non-woven fabric filter were investigated using scanning electron microscope (SEM). The sludge and permeate particle size were analyzed using Micro-Flow Imaging (MFI) technology. The flux of the membranes was restored by back-flushing with permeate and chemical pretreatment as well as mechanical cleaning.

The anaerobic membrane separation unit was operated continuously during the experiment at a cross flow velocity (CFV) of 0.1-0.2m/sec and a transmembrane pressure (TMP) of 0.5-5psi. The intrinsic membrane permeability was about 250 L/m2/hr/psi. The non-woven material has a random, entangled assembly of fibers with a relatively larger pore size. The rough nature of non-woven filter matrix made it more susceptible to particle deposition on the surface. The capturing of the particles within the non-woven fabric matrix in fact improved the effluent quality through partial clogging. Permeate suspended solid (SS) concentration was also related to the pore size. Initial suspended solids (SS) concentration in permeate was 150, 40 and 20 mg/L for 25 µm and 12 µm non-woven filter, and 9.7 µm PTFE composite membrane, respectively. However, after one day of operation, the secondary membrane forwarded to give almost the same permeate SS.

The performance of PTFE laminated composite membrane was compared with non-woven fabric filter. Figure 2 shows the flux decline of PTFE membrane and non-woven filter for a short term test. One PTFE laminated membrane was pre-wetted using methanol to overcome the hydrophobic barrier, whereas the other one was not pre-wetted. The hydrophobic characteristic caused a lower flux in the beginning, but it increased gradually and eventually overcome after 5 hours operation. PTFE laminated membrane showed less tendency of biomass deposition on the surface compared to the non-woven filter.

The biomass deposition on the PTFE composite membrane and non-woven fabric filter was 4.6g/m2 and 11g/m2, respectively. The flux decline profile of both membranes showed similar trend. However, after mechanical cleaning PTFE had a higher flux than the non-woven filter due to a decreased bio-fouling layer. The surface of the non-woven filter before and after mechanical cleaning was examined through SEM. Even after mechanical cleaning, the entangled fabric was still clogged.

Preliminary testing showed that there was not significant difference in flux between 5,000 and 10,000 mg/L of MLSS levels. However, a long-term operation might be different due to the gel layer thickness and density. TMP. The PTFE composite membrane had a higher flux than the non-woven filter at the same TMP. High TMP leads to high flux as well as high resistance. Therefore, low TMP can extend the frequency of membrane cleaning, but cannot produce a higher flux. Although the non-woven fabric filter is a cost effective material, surface modification to reduce bio-fouling becomes essential to enhance the filtration performance.

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