Chemistry: molecular biology and microbiology – Measuring or testing process involving enzymes or... – Involving viable micro-organism
Reexamination Certificate
2002-07-23
2004-03-02
Weber, Jon P. (Department: 1651)
Chemistry: molecular biology and microbiology
Measuring or testing process involving enzymes or...
Involving viable micro-organism
C435S004000, C435S968000
Reexamination Certificate
active
06699684
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to membrane separation and, more particularly, to methods for monitoring and/or controlling biofouling in membrane separation systems.
BACKGROUND OF THE INVENTION
Membrane separation, which uses a selective membrane, is a fairly recent addition to the industrial separation technology for processing of liquid streams, such as water purification. In membrane separation, constituents of the influent typically pass through the membrane as a result of a driving force(s) in one effluent stream, thus leaving behind some portion of the original constituents in a second stream. Membrane separations commonly used for water purification or other liquid processing include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), electrodialysis, electrodeionization, pervaporation, membrane extraction, membrane distillation, membrane stripping, membrane aeration, and other processes. The driving force of the separation depends on the type of the membrane separation. Pressure-driven membrane filtration, also known as membrane filtration, includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis, and uses pressure as the driving force, whereas the electrical driving force is used in electrodialysis and electrodeionization. Historically, membrane separation processes or systems were not considered cost effective for water treatment due to the adverse impacts that membrane scaling, membrane fouling, membrane degradation and the like had on the efficiency of removing solutes from aqueous water streams. However, advancements in technology have now made membrane separation a more commercially viable technology for treating aqueous feed streams suitable for use in industrial processes.
Furthermore, membrane separation processes have also been made more practical for industrial use, particularly for raw and wastewater purification. This has been achieved through the use of improved diagnostic tools or techniques for evaluating and monitoring membrane separation performance. The performance of membrane separation, such as efficiency (e.g. flux, membrane permeability, permeate recovery, energy efficiency, time between membrane cleanings or time to conduct a cleaning cycle) and effectiveness (e.g. rejection or selectivity) are typically reduced by membrane fouling.
Membrane separation processes are prone to fouling by microbes, i.e. biofouling. The growth of microorganisms during membrane separation is a constant concern particularly in aqueous streams which provide optimum conditions for microbial growth. Biofouling is particularly detrimental to membrane separation systems because once it is started, the growth rate accelerates and biofouling can facilitate other types of fouling as well. For example, the exopolymeric substances (“EPS”) or slime layer of the biomass can trap and hold scales and other particulates that might otherwise pass out of the membrane separation system during operation. Furthermore, a thick EPS layer can also decrease turbulent flow within the membrane. This can lead to an increase in the concentration polarization layer which is a known contributor to membrane scaling phenomena.
The immediate and most obvious effect of biofouling is a decrease in membrane permeate output and/or a rise in the pressure drop along the length of a membrane element on the feed and concentrate side of the membrane, referred to herein as “differential pressure.” Under a constant pressure, this results in a loss in the production of permeate. Pressure can be increased in order to maintain a constant flux, but this increases energy consumption and further accelerates fouling. In addition, continued operation under these conditions (i.e., a loss of permeate flux, an increase in pressure differential and an increase in the pressure driving force) would necessarily require an increased number of cleanings over the life time of the membrane, thereby decreasing the membrane life and potentially increasing water costs if a significant amount of down time is required due to the cleanings. Less obvious effects include reduced solute rejection, contamination of permeate and deterioration of membrane modules, such as biodegradation on membrane glue lines. The review article written by H. F. Ridgway & H. Flemming entitled “Membrane Biofouling” and appearing in
Water Treatment Membrane Processes
, McGraw Hill, pp. 6.1 to 6.62, 1996, is incorporated herein by reference.
In general, biofouling is controlled through the use of biocides and other biocontrol agents, i.e., chemicals that can inhibit microbial growth by destroying the cell wall or cellular constituents of microorganisms. Mechanical means and radiation means are additional possibilities. Intermittent use of biocides is typically encouraged since biocides can be both expensive and toxic. Thus, to prevent waste, constant monitoring and testing of the water system and of the membrane process parameters are required to determine the proper quantity of biocide for controlling microbial growth.
However, known monitoring techniques may not provide an adequate level of sensitivity, specificity and/or accuracy with respect to monitoring the effects of biofouling on membrane separation. Typical monitoring techniques include pressure and flow measurements and grab sampling to determine microbial population. With respect to pressure measurements, monitoring is generally conducted by evaluating changes in the differential pressure along the length of the membrane. With respect to flow measurement, the flow meters generally employed in such systems are subject to calibration inaccuracies, thus requiring frequent calibration. However, the changes in pressure and flow are not necessarily specific to biofouling, as they can be influenced by any suitable increase in scalants, foulants and/or like constituents that can build-up and remain in the system during membrane separation. As previously discussed, the microbial growth layer can enhance other types of fouling since it can trap or hold scales and other particulates that might otherwise pass out of the system during membrane separation.
For grab samples, water samples are typically taken from the feed stream and/or from the exit stream. Samples from the permeate stream can also be taken to determine if there is any contamination in the permeate. A typical technique involves withdrawing a sample, diluting the sample, and applying the sample to the surface of a nutrient agar medium. After incubation for 24 to 28 hours, the sample is checked for the presence of microorganisms and, where appropriate, the organisms are counted by manual or video means. A variation on this method includes withdrawing a sample and culturing it for a predetermined time, and then observing the culture medium by nephelometry or turbidimetry. In other words, the presence of microorganisms is revealed by the opacity of the culture medium.
A significant problem associated with grab sampling is the time lag between withdrawing the sample and completing the analysis to determine the level of microbiological activity in the sample. In this regard, the time lag can be exacerbated when the samples have to be transported off-site for analysis which can further delay obtaining the results.
Another problem associated with grab sampling is that it can underestimate the overall microbiological activity in the industrial water system because grab sampling is only sufficient to provide an indication of the planktonic microbiological activity, not the sessile activity. Planktonic microbiological populations are alive and exist suspended within the water of a water system. As used herein, the term “sessile” refers to populations of microorganisms that are alive, but immobile. It is possible to get an industry-acceptable measurement of planktonic populations by grab sampling since planktonic microorganisms are suspended within the water sample that is removed and tested for microorganism concentrations. In contrast, sessile populations are strongly a
Chattoraj Mita
Ho Bosco P.
Wu May W.
Zeiher E. H. Kelle
Breininger Thomas M.
Brumm Margaret M.
Nalco Company
Weber Jon P.
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