Non-oxidizing biocides

Non-oxidizing biocides are used on membranes to prevent microbial fouling. By definition, these products will not oxidize polyamide composite membranes and can be used directly on the membranes. There two most common, non-oxidizing biocides used with RO membranes sodium bisulfite and 2,2,-dibromo-3-nitrilo-proprionamide or DBNPA. 

Sodium bisulfite can be used as a biocide on a shock feed basis. Typically, 500 to 1,000 ppm as sodium bisulfite should be fed for 30 minutes. The frequency of use should be dictated by the temperature of the water and the concentration of nutrients for microbes (warmer water and higher concentrations call for more frequent application of the bisulfite). 

For potable water applications, only off-line treatment with DBNPA is recommended. This is to ensure that the single produce active concentration (SPAC) of 90 ppb of DBNPA in the permeate is not exceeded.  

Sodium bisulfite and other reducing agents can decompose the active ingredient in DBNPA formulations. Hence, it is recommended to suspend use of reducing agents during addition of DBNPA to avoid decomposition of the biocide.  

Although DBNPA is non-oxidizing, it does give an ORP response of about 400 millivolts at 0.5 to 3.0 ppm. Chlorine gives a reading of about 700 millivolts at 1 ppm concentration. It is recommended that the ORP set points be by-passed during addition of DBNPA. 

In spite of high effectiveness of oxidizing biocides in cooling water systems, it is sometimes difficult to control problem organisms just with oxidizers alone. High reactivity and low persistence of most of oxidizers can leave some microorganisms unharmed, especially those that proliferate in biofilms. Therefore, the need exists in other classes of biocides that are more persistent and aimed at organisms less effected by oxidizers. 

Most of non-oxidizing biocides are liquids, and they can be classified rather vaguely by mode of action, by speed of kill, by active chemical group, etc. We will address here only significant practical aspects of most commonly used non-oxidizers. An information on other non-oxidizers can be found in the recently published reviews by Puckorius and Associates (Puckorius, 1998a, Puckorius, 1998b). 

Biocide type Target organisms Incompatibilities Strength Weakness Mechanism 

Quats Bacteria High suspended solids, Broad spectrum. Not effective vs. fungi. Cationic surfactant. 

DBNPA [11, 17.5.] Bacteria Unstable at pH 7.5 High efficacy Not persistent at higher pH, Inhibits respiration and enzyme activity 

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Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

Isothiazolone has also been used as a non-oxidizing biocide for RO applications. However, the residence time required is much longer than for DBNPA. For example, a dosage of 50 to 100 ppm requires a 4-hour contact period. Thus, isothiazolone is not recommended for shock feed or continuous feed, but is recommended for cleaning events. Isothiazolone is more effective than DBNPA in high-organic waters. 

Non-Oxidizing biocide Which one(s) is being used and how is the dosage controlled On-line or off-line usage ... 

Neutral-pH cleaners are usually used to address microbes. Most of these cleaners use a non-oxidizing biocide such as DBNPA (dibro-monitriloproprionamide) or isothiasolin, and are pre-packaged by vendors. 

For biocide treatment directly on the membranes, DBNPA is a good non-oxidizing biocide (see Chapter 8.2.5.2). For clean membranes, a dosage of about 100 ppm for 30 minutes 2 to 3 times per week is recommended. For heavier fouling, 100 ppm for 60 minutes 2 to 3 times per week should be fed. Alternatively, DBNPA can be fed continuously at about 2-3 ppm. Note that once biofouling gets out of control, it will be very difficult for DBNPA to work, because it is a non-oxidizer and cannot penetrate biofilm. DBNPA works best as a preventative treatment. 

Non-oxidizing biocides include a variety of organic compounds such as glutaraldehyde, quarternary ammonium salts (QUATS), isothiazolones etc. These compounds are less critical in terms of corrosion because they do not accelerate the cathodic partial reaction. Many of them are active over a wide range of pH and temperatures. THPS, a water soluble organic compound, has been found particularly effective for preventing microbial corrosion due to SRB in oilfields [33]. Biocide... 

Figure 7 Biocidal efficacy of non-oxidizing biocide. Arrows correspond to addition of biocide.

Effect of combination of non-oxidizing biocides. Experimental study shows that the combination of nonoxidizing biocides was more efficacious in biofilm control than biocides when used alone (Figure 9). Simultaneous use of isothiazolones (4 ppm) and quaternary ammonium compound (12 ppm) demonstrated better biofilm control than either isothiazolones or quat alone. This biocidal combination controlled biofilm (based on HTR data) for about 27, 30, and 32 hours after three consecutive treatments. The biofilm DO response was much more significant than any separately tested biocide and was similar to the test when isothiazolone was used together with oxidizing biocide. These laboratory results confirmed practical (field) evidence that combinations of biocides affect biofibns more effectively than biocides used alone. 

General information. Glutaraldehyde is a 5-carbon dialdehyde that has seen extensive use in industrial water treatment applications. In addition to its use as a biocide in oil and gas operations, it is also used in cooling water, paper making, and preservative applications, medical instrument sterilization, and as a non-biocidal crosslinker for leather. X-ray films, and enzyme immobilization. It is often used in oilfield applications in combination with other non-oxidizing biocides such as QAC s and formaldehyde and is compatible with the oxidizing biocides typically used in cooling water applications. 

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