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Membrane Technologies for Point-of-Use and Point-of-Entry Applications 627
adsorption with membranes and caused different degree of organic fouling. With an accelerating
deterioration of water quality, especially for a micropolluted water source, organic
matters and/or microorganisms have become the main foulant in membrane systems.
The last type of fouling, biological fouling or biofouling, results from bacteria that pass the
pretreatment, attach to the membrane or spacer components, and form a foulant layer.
Biofouling arises from microbial biofilms which are ubiquitous in water systems of any
nonsterile unit (72). Undesired effects of biofouling include energy loss in cooling towers and
heat exchangers, decreased water quality and possible contamination of drinking water with
pathogenic bacteria, plugging of filter systems, corrosion or biodeterioration of equipment or
pipeline, and degradation of product quality in paper manufacturing (72). The presence of
organic and inorganic compounds serving as sources of energy for bacteria in drinking water
results in the multiplication of microorganisms in drinking water distribution systems
(regrowth) (73). Bacteria growing attached onto surfaces exposed to drinking water remain
in the distribution system and may form the main part of the amount of metabolically active
biomass present, in particular when drinking water contains a disinfectant residual (74).
Release of bacteria from the walls of the pipes and reservoirs and from sediments may be
responsible for the increase in numbers of bacteria, including coliforms, in drinking water
during distribution (75–77). Several factors affect the formation of biofouling such as nutrient
concentration in water system. In nonsterile water systems, microorganisms attach to any
kind of surface and form biofilms which reach the plateau growth phase within a short time.
Biomass accumulates on the reactor surfaces, and the nutrient-depleted effluent water allows
microbial growth, including the development of biofilms, only to a lesser extent. The lower
the nutrient concentration, the less the biofilm formed (72).
Several techniques are used to prevent the fouling problems. Graham et al. (70) suggested
that periodic cleaning of a membrane to remove foulant accumulations is essential to
maintain quality and quantity of product water and to maximize membrane life. Scaling is
prevented by operating at a relatively low recovery of 85% and hydrochloric acid (HC1)
dosing to the feed (67). To prevent colloidal fouling, a Modified Fouling Index (MFI) lower
than 1 s/L 2 is proposed (67). Hong and Elimelech (71) indicated that the removal of divalent
cations in the cleaning process could reverse fouling. In their work, EDTA was used as the
strong chelating agent. It was demonstrated that this chemical is an effective agent in
removing the natural organic matter (NOM)-calcium fouling layer and restoring permeate
flux. However, for complex fouling, which often includes multiform foulants or interaction
with membranes, little applied information and few cleaning techniques are available regarding
cleaning solutions and procedures to remove these complex foulants from a RO system. In
preventing organic fouling, control of organic specie with the value of dissolved organic
carbon (DOC) lower than 1 mg C/L is proposed (67).
To determine the scaling problem, calculation of the scaling potential is provided in ASTM
standards, which are (1) ASTM D3739-94 (2003): Standard Practice for Calculation and
Adjustment of the Langelier Saturation Index for Reverse Osmosis, (2) ASTM D4582-91
(2001): Standard Practice for Calculation and Adjustment of the Stiff and Davis Stability
Index for Reverse Osmosis, and (3) ASTM D4692-01: Standard Practice for Calculation and