Page 49 - ro membanes

32 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
magnitude higher content of bacteria and algae than source waters collected using deep in- takes (i.e., intakes obtaining water at depth higher than 12 m, equivalent to 40 ft). Therefore, source water from shallow/onshore open intakes usually has higher biofouling potential than that collected using deep open water intakes or intake wells.
Not all aquatic microorganisms can generate EPS. Bacteria most commonly encountered in seawaters worldwide are capable of EPS generation. During algal blooms, aquatic organism speciation changes toward an incremental increase of bacteria, which can create EPS. In addi- tion, especially in the initial first week to two-week phase of the bloom, the number of bacteria in the source water increases exponentially.
Brackish groundwater may contain bacteria, which are not found in other saline aquatic environments such as sulfur, iron, and manganese bacteria. Some sulphur-reducing bacteria can generate H2S gas as a side product of their metabolism, which impacts source and prod- uct water quality in terms of odor.
Iron and manganese bacteria can thrive in both brackish water and saline water and can create brown or black discoloration of the water and leave residue of similar color on treat- ment plant structures and equipment. Some marine sulfur bacteria could be quite long in size and could form strings of biological material up to 5 cm long in source water wells and intake storage tanks/pump wet wells.
2.6.3.2 Content of Easily Biodegradable Compounds in the Source Water
Under normal, non-algal bloom conditions, saline source water and most brackish ground waters have relatively low content of biodegradable organics (TOC <0.5 mg/L). However, during algal bloom conditions, the level of biodegradable organics could increase signifi- cantly. If the TOC content of the saline source water reaches 2 mg/L or more, typically the biofouling potential of this water increases to a very high level and use of such water for desa- lination results in accelerated biogrowth on the surface of the RO membranes and in rapid increase of DP through the membrane vessels (as much as 1.0e2.0 bars/week vs. normal DP increase of 1e2.0 bars/3e4 months).
The source of easily biodegradable organics in the source water could not only be a natural event, such as an algal bloom, but also organic conditioning chemicals such as polymers and antiscalants (if they are overdozed) or it may be the type and operation of the pretreatment processes and systems used upstream of the RO facility.
In order to protect the aquatic environment of the area receiving the discharge from the desalination plant, all inorganic source water-conditioning chemicals (acids, bases, and coag- ulants) have to be environmentally safe and all organic conditioning chemicals (e.g., poly- mers and antiscalants) have to be biodegradable. If for example, an organic polymer or antiscalant are overdosed, the excess unreacted chemical will cause or accelerate biofouling because by design these chemicals are created to be easily biodegradable. Bioavailability of source water-conditioning chemicals also increases with their chlorination. Another potential source of biofouling are the biodegradable organic impurities contained in low-quality/low cost source water-conditioning chemicals, such as antiscalants, polymers, or acids (Vrouwenvelder and van der Krooij, 2008).
One reason for accelerated biofouling could be the continuous chlorination of the saline source water, which often is applied to inactivate aquatic microorganisms and reduce biofouling. Since chlorine is a strong oxidant, it can destroy the cells of active aquatic bacteria