Biocide dosing system

Some months after installing a biocide dosing system on the cooling water unit, one of the dosing tanks overpressurized and ruptured. The explosion propelled the dosing liquids about 20 ft. (6m) in the air. The unusually warm temperature of the tank was a clue that some form of exothermic chemical reaction occurred. Samples analyzed of the liquids and... 

These systems have been promoted as complete water management packages, which do not require expensive biocidal dosing and address concerns of health and safety (especially the potential proliferation of Legionella sp.) in the built environment. Some models incorporate facilities for proportional... 

Apart from the primary components of a cooling water system, all ancillary equipment such as level controllers, bleed systems, dosing and control systems, low-level alarms, pH controllers, and on-site biocide generation systems should be regularly inspected, cleaned, calibrated, and maintained in good working order. 

In reality, often the only practical benefit of using timer-controlled biocide dosing equipment is the increased assurance it provides, in that the biocide(s) will actually be dosed to the cooling system at the planned time and for the planned duration. If this is of vital concern to the operator or mandated by regulations, then so be it. 

Cooling water systems are dosed with corrosion inhibitors, polymers to prevent solid deposition, and biocides to prevent the growth of microorganisms. 

It is also important to consider the dose of biocide required to give the desired longevity in-use and compare this to the cost of the preservative. In many metalworking fluids, the preservative system can be the single most expensive component in the formulation. Getting the balance between cost and efficacy is key. 

NOTE In certain cases a continuous feed of biocides may be more suitable than the more usual program of shock dosing. A low-level continuous feed provides the cooling tower with a continuous concentration of biocides, which may be particularly useful, say, for cooling systems that suffer from regular process leaks. 

During an on-line clean, the quality of the clean will normally be enhanced by the use of an initial chlorine dosing procedure at a pH of 7.2 to 7.6. This practice is to be recommended, irrespective of whether the current cooling system maintenance biocide program employs chlorine. With severely fouled cooling systems, it may be necessary to provide many hundreds of ppm of HOC1 in order to satisfy the oxidation demand. This in turn may require the use of a temporary, supplementary corrosion inhibitor, and almost certainly the use of some antifoam. 

Fig. 9.2. Typical water treatment program dosing and control arrangements for industrial cooling system (schematic). Nonoxidizing biocides and biodispersant are typically periodically hand-dosed.

Periodic dosing of biocide. This is still most usually achieved by use of electronic timers, although some monitoring systems, similar to inhibitor monitoring, are available. 

However, given that that many biocides are hazardous to handle, a dosing pump may be a useful measure. Thus a suitable compromise may perhaps be to pump biocide to the cooling system, when tests or observation indicate a need, and to employ a manually initiated limit timer to actuate the pump. Typically, limit timers have an adjustable 0- to 90-minute timer cycle, with a 13 to 15 A output to a chemical metering pump. 

Isothiazoline is a widely used biocide, but it is not effective against anaerobic bacteria. It is deactivated by hydrogen sulfide leaks. Also, do not dose to closed systems (because of the presence of 0.2% copper and 2 to 3% magnesium salts, which are used as product stabilizers and can increase the risk of galvanic corrosion). Additionally, the high pH of closed-loop systems will hydrolyze isothiazoline. Automated feed systems may be useful for dosing isothiazolines due to the severe skin irritant nature of this material. 

Because the chlorine can react with many organic substances found in water systems, there is a "chlorine demand" that will affect the biocidal activity of a particular dose. It is usually necessary to add excess chlorine in order to leave a free residual of chlorine in the system to ensure efficient biocidal action. Some comments on chlorine treatment programmes are contained in Table 14.11 [Betz Laboratories Inc. 1976]. 

The most common cause of deposits in polymer/product preparation, filtration and dosing equipment is bacteria. Waterborne bacteria can thrive in make-up systems, causing slimy or pseudoplastic deposits to form. Once this has been identified, the systems need to be thoroughly cleaned, pipe work replaced and a treatment with biocide implemented. Care needs to be taken with the choice of biocide, it must be fuUy water soluble and compatible with the polymer/product otherwise reduced performance will result. 

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