Fouling and Scaling

Steam drums and water-wall headers should be inspected for evidence of pitting corrosion, chelant corrosion, cracks, erosion, thinning, sludge fouling, and scale deposition. Also, the waterline should be checked for surging and priming problems. 

A bottleneck in all membrane processes, applied in practice, is fouling and scaling of the membranes. These processes cause a decrease in water flux through the membrane and a decrease in retention. Much attention is paid, especially in case of nanofiltration and hyperfiltration, to prevent fouling of the membrane by an intensive pretreatment and the regular removal of fouling and scaling layers by means of mechanical, physical or chemical treatment. 

In the practical application of electrodialysis there are two main process operation modes. The first one is referred to as the unidirectional electrodialysis and the second as electrodialysis reversal [22]. In a unidirectional operated electrodialysis system the electric field is permanently applied in one direction and the diluate and concentrate cells are also permanently fixed over the period of operation. Unidirectional operated electrodialysis plants are rather sensitive to membrane fouling and scaling and often require a substantial feed-solution pretreatment and stack-cleaning procedures in the form of periodical rinsing of the stack with acid or detergent solutions. The unidirectional operating concept is mainly used today for applications in the... 

In theory, cross-flow is a continuous operation, as the scouring process keeps the membrane surface free of foulants. In practice, however, the scouring action of cross flow is not always enough to prevent all fouling and scaling. Periodically, the membranes will need to be taken off line and cleaned free of material that has accumulated at the surface. 

There is a tendency to want to increase throughput shortly after start up or after a successful membrane cleaning, when membranes are performing their best. However, if changes are made without regard to consideration of the other variables in the system that depend on flow and recovery, that will hasten fouling and scaling as a result. 

Figure 8.1 Shows the projected performance of an RO membrane system with ideal, marginal and inadequate pretreatment.1 After an initial period over which time new membranes stabilize performance, a system with ideal performance will show only a slight decline in performance with time due to compaction and the inevitable fouling and scaling that will occur despite good pretreatment and system hydraulics. Marginal pretreatment exhibits more rapid decline in performance than the system with ideal pretreatment. Initial cleaning may be able to revive most of the performance, but after time, foulants and scale that were not removed become irreversibly attached to the membrane and cannot be cleaned away. The RO system with inadequate pretreatment will show very rapid decline in performance that typically cannot be recovered by cleaning the membranes. An RO system with less than ideal pretreatment faces frequent cleaning intervals and short membrane life. Frequent cleaning and membrane replacement costs money, time, and the environment.

Mechanical pretreatment involves physical techniques to reduce turbidity, suspended solids, SDI, bacteria, hardness, and heavy metals present in RO influent water. Table 8.1 lists some mechanical treatments and what species they will treat. It is important to reduce or eliminate these species from RO influent water to minimize fouling and scaling of the membranes. 

Tables 9.2 and 9.3 list the recommended feed water and concentrate flow rates, respectively, as functions of feed water source quality.1 Higher feed water flow rates result in water and its contaminants being sent to the membrane more rapidly, leading to faster rates of fouling and scaling. As Table 9.2 shows, an RO operating on a well water source can have a feed flow rate as higher as 65 to 75 gpm per pressure vessel, while a surface water source RO should not exceed 58 to 67 gpm per pressure vessel. The well water RO would require 12% fewer pressure vessels than the surface water RO.

Conventional wisdom calls for Beta values less than 1.2 in an RO design to minimize membrane fouling and scaling.9 Table 9.4... 

Membrane age. For general projections, 3 years should be selected which assumes a 3-year membrane life. This input works closely with the flux decline and salt passage increase inputs discussed below. Selecting performance at end-of-life for the membranes will yield the operating parameters necessary after years of fouling and scaling of the membrane. 

Membrane fouling and scaling can both lead to a loss in normalized permeate flow. Additionally, membrane compaction will result in decreased permeate flow as well. 

The flush can be programmed into the PLC and thus occurs automatically when the RO skid shuts down or it can be manually initiated. Note that not all RO systems are equipped with this flushing feature. Without this feature, fouling and scaling of membranes will be exacerbated, particularly if the RO unit cycles on and off on a regular basis. 

The stand-by flush is used intermittently when the RO skid is off line in stand-by mode. It can also be used during extended lay-up of the skid. The objective is to remove particles and salts that have collected on the membrane surface while the membranes are idle. This minimizes the potential for membrane fouling and scaling while the membranes are at rest. 

Figure 14.1 Causes of membrane fouling and scaling for 150 autopsied membranes. Courtesy ofM. Fnzel and The Engineering Society of Western Pennsylvania.

Design of an RO system has a great effect on the potential for fouling or scaling the membranes. As discussed in Chapter 9, feed water flow, concentrate flow, water flux, and recovery all affect the ability of the membranes to foul and scale. Flow rates affect the concentration polarization boundary layer where fouling and scaling occur (see... 

Fouling and Scaling Figure 14.14a shows a feed spacer with foulants adhering to the spacer, while Figures 14.14 b and c show feed spacers virtually completely blocked with foulants and scale, respectively. This... 

Fouling and scaling mechanisms are similar for spiral-wound NF and RO membranes. In general, NF feed water should meet the following characteristics to prevent fouling with suspended solids (refer to Table 7.1 for a more detailed description of spiral-wound RO feed water requirements) ... 

A significant amount of pretreatment is required to minimize fouling and scaling of the membranes in a CEDI system. Table 16.7 lists general feed water quality requirements for CEDI systems.15 21 Due to the stringent feed water quality requirements, most CEDI systems are preceded by RO. Common configurations used to pretreat CEDI feed water include the following 15... 

Despite efforts to comply with the limitations on feed water quality, CEDI systems can still foul and scale with microbes, organics, iron and manganese, and calcium- and silica-based scales. This usually occurs due to upsets in the pretreatment system or a deficiency in the system design that result in excursion in feed water quality to the CEDI system. Organics, metals, hardness, and silica problems are usually found on the membranes and sometimes on the resin (as is the case with organics). Biofouling is typically found on the... 

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