View Technical Program - Levant Desalination Association

2. Primary Scale Deposition
Parallel to point no. 1, and particularly in the last stages:
Deposition of supersaturated of sparingly soluble salts/compounds in view of:
• Inadequate acid or antiscalent dosage.
• Exceeded system design parameters: e.g. too high recovery and/or too slow
brine flow.
3. Biofouling
a. Primary Bacterial Adhesion
Only a small fraction of bacteria cells (biocolloids) in the feed water adsorbs
on the membrane due to electrostatic repulsion between the negative charge
on the thin film composite NF membrane and that on the bacteria cells at
near neutral pH values. Extent of adsorption is determined by hydrodynamic
factors, water composition, suspended matter, and membrane surface
characteristics.
b. Propagation of Biofilm
During an incubation period of about 14 days at ambient temperature of 20
– 30 °C, the adsorbed bacteria progressively covered the membrane surface
by their life products, the extracellular polymer substance (EPS), in a layer of
several microns thick. The slime gel matrix of the film favors further organic
and biofouling through entrapping the organic molecules, colloidal particles,
suspended matter and bacteria cells which we considered as (secondary
adhesion). This is an example of interaction between the different forms of
fouling.
c. Fouling Film Denaturation
Up to this stage, the adequate chemicals cleaning could remove the fouling
film and restore the initial system performance. However, with time the
accumulating fouling film underwent progressive change of nature. It became
increasingly denser and sticky, more adherent to membrane surface and of
lower permeability. Such modification may be attributed to physic- - chemical
and/or biological factors.
4. Secondary Scale Deposition
Accumulation of the previous fouling forms lead to the formation of secondary
membrane which results in: higher ΔP (feed – permeate) and higher ΔP (feed
– brine). The consequently impaired hydrodynamics conditions cause severe
concentration polarization with localized scale deposition; the figure shows the
film SEM.
Again this is an example of interaction between the different forms of NF
membrane fouling which further complicate the structure and composition of the
film and lead to higher decline of performance, and more difficult film removal by
cleaning.
Water Reuse and Desalination: Experience and Opportunity
TECHNICAL PROGRAM 41