MEMBRANE-COAGULATION REACTOR FOR WATER TREATMENT

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MEMBRANE-COAGULATION REACTOR FOR WATER TREATMENT

Supervisors: Professor James O. Leckie (Stanford) and Professor Darren D. Sun (NTU)
Other researchers: Dr. A P Robertson (Stanford)

Duration: 4 years

Overview

Figure 1 is a process diagram for a conventional water treatment plant. The combination of the first 3 steps primarily removes colloids (including some microorganisms) and natural organic matter (NOM). Step 4 (rapid sand filtration) is a polishing step that removes much of the colloidal material remaining after step 3 (sedimentation)

Systems of the type outlined in Figure 1 can provide good quality, potable water and their design and operation are well

understood. In recent years membrane alternatives1,2 have drawn increasing interest because membrane technologies have advanced significantly and membrane systems may:

1. Require considerably less space to treat a given flow
2. Reduce chemical requirements
3. Produce a water that is more easily disinfected and less likely to produce undesirable disinfection by-products

Figure 1 Flow diagram of a conventional potable water treatment plant.

We propose to study a membrane-coagulation reactor (MCR) system (Figure 2). The MCR incorporates flocculation, sedimentation and filtration in 1 reactor instead of 3, suggesting the potential for substantial savings in space and capital costs. The potential water quality benefits arise because the membranes may block a substantial fraction of the small colloids, low molecular weight NOM, and microorganisms that do not sediment and pass through conventional sand filters. Reduction in chemical usage is less certain but may result because of the MCR system’s ability to retain even small flocs.

Figure 2 Flow diagram of membrane-coagulation reactor.

Though the potential advantages of an MCR are apparent there are a number of important issues to address.

Membrane fouling is an obvious concern. Both the magnitude of fouling and its effects on flux and permeate quality are likely to depend, in complex ways on membrane properties and configuration, raw water quality, coagulant dose, additives employed, operating parameters of the rapid mix basin, hydraulic residence time, and hydrodynamic conditions in the reactor3-7.

Conditions in the membrane-coagulation reactor will differ substantially from those in conventional flocculation and sedimentation basins. MCR floc properties—size distribution, morphology, composition, mass and number concentration, etc. may differ substantially from those in conventional systems. Relationships relating flow rates, coagulant and additive dose, etc. that have been elucidated over the years to optimize operation of conventional systems may be inappropriate for MCR systems. Aeration is of particular interest as it is likely to influence hydraulic conditions in the reactor and in so doing inhibit membrane fouling5,7 and affect floc properties and their settling.

OBJECTIVES

One or more bench scale MCRs (Figure 2) will be constructed at NTU. The reactor(s) will be computer controlled and equipped with in-line monitoring capabilities (mass flow rate, pH, temperature, pressure, aeration rate, etc.) The student will employ both synthetic and natural waters in parametric studies designed to elucidate the factors that influence treatment efficiency and operational performance. Operational parameters of interest will include:

1. Coagulant dose, chemical additive type and dose, rapid mixing conditions
2. Hydraulic retention time
3. Membrane type, properties
4. Membrane flux, trans-membrane pressure
5. Aeration
6. Reactor configuration

Water quality parameters to be considered:
1. Colloids—turbidity; mineral, microbial, polymeric
2. Organic carbon—TOC, molecular weight distribution; humic, fulvic, coagulant additives
3. Bacteria and viruses
4. pH, temperature, major ions
5. Floc concentration, size distribution, composition, structure

Standard methods will be employed to assess most of the water quality parameters. A range of microscopies (light, SEM, TEM, AFM), and other techniques (electrokinetics, streaming potential TGA-FTIR) will be employed to characterize flocs and evaluate membranes, both virgin and used.

ORGANIZATION AND TRAINING

Professor Sun has an ongoing, complementary research effort focusing on a membrane bioreactor system very similar to the proposed MCR (Figure 2); the student will be a part of Professor Sun’s research group. Research will also be synchronised with membrane projects being conducted at Stanford as part of the Clean Water Programme and the NSF Center for Advanced Materials for Water Purification. When at Stanford, the student will work closely with the researchers involved in these efforts.

REFERENCES

1. Mallevialle, J. et al., eds. (1994). Water Treatment Membrane Processes. McGraw-Hill,
New York, NY.

2. Hillis, P., ed., (2000). Membrane Technology in Water and Wastewater Treatment. The
Royal Society of Chemistry, Cambridge, UK. 269p.

3. Schafer, A. I. et al., (2001). “Cost factors and chemical pre-treatment effects in the
membrane filtration of waters containing natural organic matter.” Water Research, 35(6), 1509-
1517.

4. Fane, A. G., et al., (2000). “Membrane fouling and its control in environmental applications.”
Water Science and Technology, 41(10-11), 303-308.

5. Vera, L., et al., (2000). “Gas sparged cross-flow microfiltration of biologically treated
wastewater.” Water Science and Technology, 41(10-11), 173-180.

6. Choksuchart, P., et al., (2002). “Ultrafiltration enhanced by coagulation in an immersed
membrane system.” Desalination, 145, 265-272.

7. Hwang, E. J., et al., (2002). “Operational factors of submerged inorganic membrane
bioreactor for organic wastewater treatment: sludge concentration and aeration rate.” Water
Science and Technology, 47(1), 121-126.

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