|Ph.D Student||Kotzer Eli|
|Subject||Early Biofouling Detection of RO Membranes|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Robert Armon|
|Full Thesis text - in Hebrew|
Desalination market is under continuous development and in the last years a massive advance occurred in effluent desalination area. One of the most severe problems faced during the operation of effluent desalination by reverse osmosis is rapid biofouling of these membranes. The main cause for this rapid biofouling phenomena is based on bacterial adhesion onto membranes surface area and further regrowth as a result of nutrients presence in the feed water. To avoid this biofilm development, a field apparatus that can predict biofoulin potential of the feed water is very much required. The information obtained from such a device will facilitate to adopt the correct pretreatment of the feed water, a treatment which can prevent bacterial regrowth on RO membranes. One major demand for such a reliable field apparatus is to obtain fast and non destructive result and highly predictive of membranes biofouling.
The present study aims were to develop a biosensor and an experimental method for biofouling prediction for effluent desalination; to investigate the correlation between the developed biosensor due to bacterial activity and the rate of biofilm development on the RO membrane surface. Achieving those goals can be a solid scientific base for a potential development of an on-line apparatus which will enable early detection of RO membranes biofouling in effluent desalination plants or other types of desalinated water.
In the present study a novel biosensor was constructed comprising a low rate organic biodegradable porous polymer matrix embedded with a chromogenic dye material that under oxidation-reduction conditions reacts by color formation. The color formation is related to bacterial activity on the biosensor surface closely simulating the bacterial activity inside the RO module. The constructed biosensor was applied to field studies in order to verify its potential and also to be used as "devise" for chemical optimization required when supplementing the feed water with disinfectants.
The basic assumption of this research for the development of the biosensor, was to use field conditions such as: nutrient and bacterial loads found at an operational RO facility (Dan Region Effluent Treatment Plant) and real RO membranes.
The developed biosensor reaction can be described as first-order reaction, a result of bacterial metabolic activity and nutrients dependence present in real feed water.
The constructed biosensor consists of four basic components:
The biosensor: the biosensor was made of a low rate biodegradable polymer matrix mechanically confined into a multi-well plastic mold. The polymer was previously doped with chromogenic dye material that responds to redox biological reactions. To improve the mechanical bond between the dye material and the polymer porous matrix and in order to minimize dye leaching, kaolinite and ethanol were used.
The flow chamber: was a box that contains the biosensor enabling a thin laminar and uniform flow of the circulated feed water on the biosensor surface.
The scanning system: consisted of a digital camera/scanner connected to computer in order to convert the obtained developed analog colored picture of the biosensor surface to a digital file to be mathematical analyzed using a software.
The software: the designated software was used for mathematical analysis of the developed color through biofilm formation on the biosensor to convert numerical results into a biofilm index.
Validation of the biosensor was performed by comparison of the developed color vs. biofilm biovolume developed on RO membranes throughout the different runs, with various water types. The experimental biofilm regrowth took place on tiny RO membranes device as bypass to feed effluent water from Dan Region Effluent Treatment Plant. The results indicated a fine correlation between the biosensor (after 24 hours) and the fouled RO membrane biovolume (after 15 days).
Finally, optimization experiments were performed in order to find out the chemical consumption in order to reduce the disinfectant dose required for feed water preparation (to reduce bacterial load, the primary factor in biofouling). These optimization experiments proved that the present device and method enable in a short time to deduce the minimal concentration of chloramines capable to prevent biofilm development on the RO membranes on one side and to prevent irreversible membrane oxidation and destruction by excess disinfectant (free chlorine). Under previous common procedures this optimization test would take longer periods of time at high expenses.
The same biosensor was also tested with sea water revealing very good results similar to effluent quality waters. Interestingly, due to oligotrophic characteristic of sea water, the reaction obtained by the biochemical device was slower (requiring longer periods of time) but with the same outcome, emphasizing the usefulness of the method for sea water desalination plants (that is the major desalination market in Israel and worldwide). The chemical and biological characteristics of the device and method were thoroughly tested under various conditions and the results are presented.
The device and the method described in this abstract are under patent application, therefore described only briefly.