Biocide Technical Guide
Biofilm consists of microorganisms contained in a matrix of hydrated extracellular polymeric substances. In most biofilms, the microorganisms account for less than 10 percent of the dry mass, with the balance consisting of inorganic ions and extracellular material. Figure 1 is an SEM image of a biofouled reverse osmosis (RO) membrane showing numerous bacilli imbedded in an organic matrix1.
Compared to cellulose acetate, the main disadvantage of polyamide membranes is their intolerance to chlorine and other oxidants. This disadvantage often leads to biofouling of membrane surfaces and feed passages, which can significantly reduce system performance.
Figure 1: SEM Image
There are five strategies for controlling biofouling in polyamide reverse osmosis (RO) systems:
• Use of non-oxidizing biocides
• Chloramine addition
• Ultraviolet light sterilization
The primary non-oxidizing biocides used today are dibromonitrilopropionic acid (DBNPA) and isothiazolin. In the past, plants have also used formaldehyde and gluteraldehyde. However, their toxicity and potential incompatibility with polyamide membranes has severely restricted their use.
Table 1 lists the kill rate by DBNPA for various bacteria species2. As the table shows, the kill rate by DBNPA is often rapid, and for many systems, periodic 30-minute treatments are sufficient to control biofouling.
Operators of municipal wastewater systems sometimes inject low dosages of choramine (monochloramine) into the feed to control biofouling. However, Zhao4 showed that chloroamine degrades polyamide RO membranes over time, albeit at a lower rate than of free chlorine. Figure 2, from Zhao, shows the TDS passage-chloramine loading response for a typical polyamide element. The use of chloramine to control biofouling in RO systems involves a trade-off, longer element life versus reduced permeate quality.
ULTRAVIOLET LIGHT STERILIZATION
Operators of seawater and brackish water RO systems
have used ultraviolet pre-treatment systems for many years with mixed results in 2005. Harif5 studied the effect of UV pretreatment on biofouling of brackish water RO membranes, which yielded the following results:
• Feed water bacteria count was reduced following UV treatment
• Less extra-cellular polymeric substances (EPS) were found on the membrane surfaces following UV treatment
• Normalized flux decline was reduced by 35 percent by UV treatment
In 2011, Hassan studied the effects of UV treatment on a NF-SWRO pilot system, which provided results similar to those of Harif. One concludes from these and other studies that UV pre-treatment can reduce the severity of biofouling, but it cannot prevent it from occurring. From our experience, results are also site specific, requiring pilot studies to evaluate fully.