Emerging trends in cooling water systems

Although water technology(chemistry of water) in industrial water treatment has made large advances over the past 60 years, the objectives of successful cooling water systems treatment programme have remained the same: extend equipment life and maintain efficient heat transfer. This has been accomplished by minimizing corrosion of system metallurgy as well as keeping metal surfaces free from deposition. From a historical perspective, even the earliest Cooling Water Systems programmes have addresses these concerns by the incorporation of concerns related to corrosion and scale inhibitors and deposit control agents. Not much of the changes or breakthroughs have taken place in atleast last two decades in terms of speciality chemicals technology .

Emerging trends in cooling water systems programme are mainly in following areas..

• Use of environmentally safe chemicals
• Alkaline treatment
• Health related concerns
• Automatic dosing and monitoring

 - Use of environmentally safe chemicals
Scanning of the recent work on cooling water treatment reveals that the published literature has been mainly dedicated to the search for environmentally safe corrosion and scale inhibitors. The present research deals with types, structure, efficiency, biodegradability and advantages of new compounds "mixtures". green water chemistry and green chemicals(speciality chemicals) are defined and the approach adapted in finding such compounds and the method of evaluation are highlighted. A promising approach is represented by the combination of biodegradable polymers and environmentally safe amounts of phosphorous and chromium. New corrosion and scale inhibitors other than the green ones, and the recent work on ozone have been considered as well. Guidelines for the future work on industrial water treatment chemicals are being redefined.

Cooling towers are used by office buildings and manufacturing plants throughout the world to dissipate waste heat from air conditioning, industrial and power generation processes. Recirculating water transfer thermal energy from the building or industrial process to the atmosphere. Atmosphere air blown through the cooling tower carries away the heat. Environmental problems arise when water escapes from the system in the form of droplets. Such water droplets carry with them various chemicals that are used in the system. Some of these chemicals are environmentally harmful. Hexavalent chromium is the one that is of the most concern and warrants immediate attention.

Hexavalent chromium-based ("chrome") compounds are among the most efficient and cheapest corrosion inhibitors available. The trouble is, hexavalent chromium is a suspected carcinogen, and is highly toxic. Chrome emissions from cooling towers in New Delhi alone could cause as many as 700 cancer cases over a 70 year exposure period.

Various Governments have banned the use of hexavalent chromium water treatment chemicals in open water circulating systems that are potentially capable of emitting respirable hexavalent chrome. This prohibition is expected to reduce the risk of cancer cases due to cooling tower emissions to virtually zero. Additionally, elimination of hexavalent chromium-based treatment chemicals will eliminate the amount of hazardous and toxic wastes discarded through blowdown. Non-chromate chemicals may also have some adverse impacts on the environment. For example, while zinc based chemicals are particularly dangerous to humans, they are highly toxic to marine life. Similarly, phosphate discharges into lakes and ponds may cause excessive algal growth leading to eutrophication problems. But in comparison to the highly toxic chromate inhibitors, the substitute chemicals are relatively innocuous and do not present the same environmental problems that chromates do. Nevertheless, the impact of substitute chemicals on the environment must be carefully analyzed before actually using them.

- Alkaline treatment in cooling water systems
Conventional chromate/zinc and chromate/zinc/orthophosphate programmes have traditionally been controlled at pH limit below 7 (and often much lower) to avoid excessive precipitation of water salts. In modern water treatment "chromate" has been discontinued long back due to "environment considerations." On the other hand now days almost all the good companies use "alkaline cooling water technology" and take advantage of lower corrosivity of high pH waters super saturated with respect to calcium carbonate. Alkaline waters are generally less aggressive towards steel than near neutral pH water because of their higher buffering capacity. The deposit control position of the program utilizes a "polymer technology" that allows the pH to be controlled at levels previously thought to be too high.

It is encouraging to note that , 85% of the cooling towers operating worldwide had already changed to non-chromate systems or alkaline treatment.Most industrial cooling water systems now run at pH level above 7 in order to take advantage of the alkalinity in the water as an aid in corrosion control. Several different combination programmes are available, offering corrosion and scale control in various ways depending on system needs. The entire programme should include bio-fouling control as an integral part of the operation.

Never let the cooling water system run without treatment since alkaline system are generally well into the scaling range and will precipitate calcium carbonate quickly if scale inhibitor are not present. Recovery from such disasters may be difficult and expensive.

VARIOUS RULES FOR GOOD OPERATIONS OF ALKALINE COOLING WATER OPERATION ARE

@ Be sure that operating personals are well trained and are able to understand the importance of good control of cooling water operations.

@ Install good feeding equipment, with backup parts if needed and insist that it should
be well maintained.
@ Plan, through regular checking of water parameters, for unexpected problems and develop a response plan. (Quick response to upset conditions can mean the difference between a momentary problem and long term loss of good performance or loss of costly equipment.)
@ Start up the system properly. Be sure the system is clean and follow the supplier’s
recommendations for start up procedures and be sure that maximum dosages, particularly with biocides are not exceeded.

- Health related concerns
In 1976, there was an outbreak of 221 pneumonia cases associated with an American Legion Convention in Philadelphia, Pennsylvania. It was determined that the bacterium legionella was the causative agent of the outbreak. Since that time Legionellosis or Legionnaires Disease as it is commonly referred to, has been a controversial subject. Outbreaks of legionellosis continue to occur. Since cooling tower systems have been implicated as sources of outbreaks, it is important that water treatment professionals have an understanding of what is known and what is not known about Legionnaires Disease. Legionella is a fresh water bacterium. It has been isolated from such sources as wet soil, lakes, rivers, and streams, cooling towers and piping systems. The word most often used to describe legionella is "ubiquitous", meaning omnipresent or being everywhere. It is true that this organism is all around us and can be isolated from most fresh water sources.

Legionella is an aerobic bacterium. This means it requires the presence of oxygen to survive and grow. In laboratory tests, the bacterium has been shown to withstand a pH as low as 2.0. Temperatures up to 55 Deg C (131 Deg F) can be tolerated and growth can occur at temperatures up to 45 Deg C (113 Deg F). In general, the cells are 0.5-1.0 micrometers in width and 1.0-3.0 micrometers in length. The legionella bacterium grows at a slower rate than most other fresh water bacteria.

Legionella depends upon the presence of appropriate nutrients and minerals for growth. Two of the most important are an amino acid called L-Cysteine and iron. Laboratory experiments have shown that some species of legionella can multiply intracellularly within certain free-living protozoa. In addition to protozoa, algae and non-legionella bacterial have been shown to play important roles in the proliferation of legionella. Algae provide growth stimulation of legionella through substances produced by the algae. Non-legionella bacteria can be used as sources for the amino acid L-Cysteine.

There are two forms of Legionnaires Disease; Pneumonia and Pontiac Fever. Of the two forms the pneumonia is much more serious. The pneumonia form of legionellosis is transmitted by the breathing in the water vapors containing legionella. This allows the bacterium to infect the lung tissue. The incubation period can be anywhere from 2 to 10 days long. The attack rate is 1-4% of those exposed. There is a 15- 20% mortality rate for persons actually coming down with the pneumonia. The symptoms, which come on gradually, can include fever (39 Deg C, 102 Deg F), headache, cough and upset stomach. This form of legionellosis is generally thought to be seasonal with the greatest number of outbreaks occurring in the summer and fall. Persons most at risk to the pneumonia form of legionellosis include adults, the elderly and those persons who have low immune tolerances.

Pontiac Fever is a mild, self-limiting form of legionellosis. It has an incubation period of only 36 hours. Pontiac Fever has an extremely high attack rate (95%), yet there has never been a fatality attributed to this form of the disease. Pontiac Fever is most likely to occur in working age adults.

This form of legionellosis has a rather sudden onset but recovery generally occurs in 48 hours. The symptoms are a low-grade fever and headache.

There are currently 23 recognized species of legionella. Of these, only 11 have been isolated from infected humans. One of these species, Legionella pneumophila has been implicated as the causative agent for approximately 85% of all known cases of legionellosis. The same species of the bacterium can cause either the pneumonia or Pontiac Fever. Still, it is not known what factors influence the bacterium to cause one form of the disease instead of the other. Using a technique called isoenzyme typing, each serogroup can be further subdivided. The use of this technique has resulted in the discovery of 62 genetic variations of L. pneumophila serogroup.

The identification of a legionella species in a given water source does not prove any association with disease. This is due to the genetic diversity of the organism.

Close surveillance of outbreaks is the primary indirect control measure for legionellosis at this time.

 
So how are responsible water treatment professional supposed to handle questions regarding Legionnaires Disease? The following common sense recommendations are not designed to eliminate legionella, but rather to minimize the potential of the bacteria to get out of control and cause on outbreak of disease.

Keep the cooling water system operating within the established parameters. The accumulation of suspended matter and organic matter can contribute to the proliferation of legionella. Undertake an effective biocide program that includes two, alternating biocides that function in differing manners. (i.e. alternate a quaternary ammonium based biocide with a carbamate biocide.) The control of other microflora within a system can and does have a direct impact upon the growth of legionella.

Monitor levels of other bacteria within the system. Since legionella grows at a rate slower
than most other bacteria, it is probable that the system would experience other problems prior to legionella becoming a problem.

Inspect the site for air intake sites in the vicinity of the cooling tower. By minimizing the
exposure of persons to water vapors from the cooling system, you can minimize the potential for an outbreak of disease.
By following the above recommendations, best possible protection against Legionnaires Disease can be achieved.

- Automatic dosing and monitoring
In most cases, too much emphasis is placed on the control and feed of cooling water treatment chemicals and not nearly, enough emphasis is placed on continuous monitoring of the system. While most end users of cooling water specialty chemicals are aware of the monitoring capabilities of the major specialty suppliers, few are aware that having there on lab. For monitoring helps them in getting better results. There are a number of good reasons for continuous performance monitoring of cooling systems:

Changes in make-up water chemistry.
Changes in temperature due to seasonal variations.
Changes in system heat load due to production level changes.
Changes in biological control requirements due to seasonal variations.
Treatment Chemical quality control problems.
System feed and control problems.
System contamination from atmospheric or process leaks.

Most systems control and monitor pH and conductivity levels on a continuous basis. While these two parameters are of primary importance, two other parameters warrant attention:

1. Chlorine level if oxidizing biocides are utilized.
2. General corrosion rate and pitting tendency.

Chlorine Monitoring and Control
The importance of monitoring and control of chlorine levels in cooling water systems, which utilize oxidizing biocides such as chlorine, chlorine/bromine, or sodium hypochlorite, is very important. If a true chlorine residual is not maintained, biological deposition and/or under deposit corrosion will occur. If chlorine residuals are too high, corrosion will occur.

Corrosion Monitoring

Continuous monitoring of cooling systems for general corrosion and pitting tendency is critical to assuring performance of a cooling system treatment program. Corrosion coupons can provide you data on the average corrosion rate over a given period. Instantaneous readings of corrosion probes provide a snapshot of system corrosion performance at any given time. The cost of these instruments is reasonable; (approx. Rs. 6,000-8000) and the data provided is very valuable

Results of process or atmospheric contamination can be detected immediately.
The effects of the interaction of other additives on corrosion such as non-oxidizing biocides can be evaluated. In some cases, we have found that some non-oxidizing biocides also have a tendency to significantly increase the pitting tendency of some waters.
Corrosion can be monitored on off shifts and weekends when close attention may lack due to the absence of supervisory personnel.