Page 47 - ro membanes

30 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
inactive state, bacteria do not have ability to adhere to the membrane surface and they behave as most other small-size solid particles in the source water: they enter and travel through the space between the membrane leafs (feed/brine spacers) and exit the RO membranes with the concentrate. Because most bacteria in their inactive unicellular state are smaller than 1 mm in size and because RO membrane spacers are 700e800 mm wide, in this state, bacteria do no accumulate in large quantities within feed/brine spacers and on the membrane surface and do not cause accelerated membrane fouling.
At any given time, some of the aquatic bacteria naturally occurring in surface water bodies are in an active state and others are in an inactive state. The predominant state of aquatic bac- teria (active or inactive) depends on how favorable the ambient environment is for bacterial survival and growth. Most aquatic bacteria will transform from an inactive to an active state under favorable environmental conditions, such as algal bloom events, when high concentra- tions of easily biodegradable organics released from the decaying algal biomass (which serve as food to these bacteria) are readily available in the source water. Since bacteria can attach to the membrane surface and grow colonies there at a very high rate, red tides or other intense algal bloom events are usually the most frequent cause of RO-membrane biofouling, espe- cially in desalination plants with open intakes.
The membrane biofouling process (i.e., the formation of a microbial biofilm layer on the surface of an RO membrane) usually follows five key steps (Fig. 2.3):
1. Formation of a primary organic conditioning film;
2. Attachment of colonizing bacteria;
3. Formation of a biofilm matrix layer;
4. Establishment of a mature secondary biofilm; and
5. Biofilm equilibrium and die-off.
The primary organic conditioning film is a microthin layer on the surface of the membrane
that is rich in nutrients and easily biodegradable organics, which creates suitable conditions for bacteria to convert from an inactive (particulate-like) state into an active state (Phase 1). In this state, bacteria are capable of producing EPS, which are adhesive substances that allow bacteria to attach to the membrane surface and to each other.
During the next step of the biofilm formation process, active bacteria adsorb to 10%e15% of the membrane surface and establish breeding ground from which they spread to the rest of the membrane surface (Phase 2). Bacteria multiply at an exponential rate, and within 5e15 days, they colonize the entire membrane surface and form a biofilm matrix layer that is several micrometers thick (Phase 3). The biopolymer matrix formed on the membrane surface entraps organic molecules, colloidal particles, suspended solids, and cells of other microorganisms (fungi, microalgae, etc.) over time to form a thicker layer with a higher resis- tance to permeate flow (Phase 4).
At the last phase (Phase 5) of membrane colonization, bacteria growth reaches equilibrium established by the availability and quantity of the food source, the rates of production and removal of site products from the bacterial growth process, turbulence of the cross-flow on the surface of the membranes, and space constraints. At this phase the membrane spacers and surface contain large quantity of microbial foulants that result in a significant increase (over 10% of RO feed pressure) and often over 50% of increase in DP.