Biofouling & Biomonitoring: Mussels help keep Mussels at Bay

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Courtesy of Sweco Nederland BV

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Dykes, shipwrecks, junk and just about anything else submerged in water is soon colonized by aquatic animals, particularly mussels. Unless something is done to stop them, mussels will also establish themselves in cooling water systems, where their presence can cause all sorts of problems like reducing cooling water flow or even blocking system components. Companies that operate cooling water systems therefore have to make them unattractive to mussels. One of the most common ways of doing this is to add chlorine to the cooling water. In 1998, KEMA came up with an innovative way of using chlorine to prevent mussel colonization: Pulse Chlorination®. According to Henk Jenner and Harry Polman, KEMA's experts in this field, the technique provides optimal protection against biofouling while minimizing chlorine use. Amazingly, the system's efficiency is down to the mussels themselves, whose services are employed in the so-called Mussel Monitor®.

Cooling water systems are liable to colonization by aquatic organisms (biofouling). Companies that operate such systems therefore have to take action to prevent mussels, oysters, barnacles and other 'pests' such as bacteria becoming established, since biofouling can cause serious problems. If fouling goes unchecked, cooling water flow can be reduced to an inadequate level, undue load placed on the circulation pumps, or condensers and heat exchangers blocked by shells. As a result, fouling ultimately leads to production losses, equipment damage, unscheduled non-availability and high maintenance costs. To avoid such problems, system operators naturally seek to combat or prevent biofouling. For many years, biofouling has been controlled at plants all around the world by adding chlorine - in the form of sodium hypochlorite - to the cooling water. Other techniques include mechanical cleaning and temporary heating of the cooling water. 'KEMA has a great deal of expertise in the field of cooling water treatment and the prevention of colonization,' acknowledges Jenner.

Pulse Chlorination

Drawing on this expertise, KEMA has developed a new approach to the use of chlorine: Pulse Chlorination. 'Pulse Chlorination is based on what we know about the time it takes for mussels to recover after chlorination,' Jenner explains. 'If you add chlorine in short bursts with pauses in between, the effect on the mussels is much the same as if the water is permanently chlorinated. We have found this out by using our Mussel Monitor, a unique device that enables us to accurately record the mussels' valve activity. By attaching sensors to the mussels, we can keep a record of what the mussels are doing underwater and then analyse our data. Once you know how long it takes the mussels to recover from a given dose, you can chlorinate at intervals that exactly coincide with the point at which the mussels would otherwise become active again. The presence of chlorine either kills the mussels or dislodges them. However, there's no one interval that is right in all cases. The recovery period varies, not only from species to species, but also from site to site, depending on the water chemistry.'

Less chlorine

Harry Polman takes up the story: 'Over the last year, we have installed Pulse Chlorination systems at three plants. First, we talk to the operator and arrange a trial at the relevant site. The trial usually lasts three or four weeks, during which we bring in KEMA's mobile laboratory. The mobile lab is a huge sea container fitted with a range of sophisticated equipment. In the lab, we reproduce the conditions in the plant's system, so that the data we collect can be transferred directly to practical situations. In addition to the Mussel Monitor, the mobile lab has on-line measuring equipment that we use to determine water chemistry parameters. Using the Telediagnostic System, the recorded data is transferred to KEMA on a daily basis by cellular phone. For the plants where Pulse Chlorination is used, we have developed a new chlorine administration system, which cuts the amount of chlorine required by up to half.' Jenner winds up by adding, 'Sodium hypochlorite remains an excellent anti-biofouling agent for cooling water systems. By using Pulse Chlorination, mussel colonization can be controlled using a fraction of the chlorine that the conventional approach requires. As well as saving our clients money, less chlorine means less impact on the aquatic environment. The new system also enables companies to respond to pressure from the authorities, who are anxious that anti-fouling systems should be as efficient as possible and make as little use of chemical additives as possible.'


Government pressure

In addition to paint, salt and pills, Akzo Nobel makes sodium hypochlorite. 'It's used for disinfecting swimming pools and for keeping mussels out of cooling systems,' explains Wim Barentsen, Process Technology Group Manager at Akzo Nobel. Akzo has also been involved in development and introduction of the Pulse Chlorination method. On the face of it, this might seem a little odd, given that the method reduces the amount of sodium hypochlorite Akzo's customers will need. 'True,' says Barentsen, 'but a large company such as ours has a wider responsibility to act in the interests of society at large. And we try to live up to our responsibilities. Besides, new legislation is based on the so-called 'stand still principle': no increase in the discharge of blacklisted substances into the surface waters of a region is allowed, and those responsible for discharges are obliged to use the best available technique (BAT) to control pollution. So the government is pushing everyone in this direction. And, of course, we want to give our customers the best service we can. We want to share our expertise in this field and to offer the customer the best possible option.'

Less chlorine

Harry Polman takes up the story: 'Over the last year, we have installed Pulse Chlorination systems at three plants. First, we talk to the operator and arrange a trial at the relevant site. The trial usually lasts three or four weeks, during which we bring in KEMA's mobile laboratory. The mobile lab is a huge sea container fitted with a range of sophisticated equipment. In the lab, we reproduce the conditions in the plant's system, so that the data we collect can be transferred directly to practical situations. In addition to the Mussel Monitor, the mobile lab has on-line measuring equipment that we use to determine water chemistry parameters. Using the Telediagnostic System, the recorded data is transferred to KEMA on a daily basis by cellular phone. For the plants where Pulse Chlorination is used, we have developed a new chlorine administration system, which cuts the amount of chlorine required by up to half.' Jenner winds up by adding, 'Sodium hypochlorite remains an excellent anti-biofouling agent for cooling water systems. By using Pulse Chlorination, mussel colonization can be controlled using a fraction of the chlorine that the conventional approach requires. As well as saving our clients money, less chlorine means less impact on the aquatic environment. The new system also enables companies to respond to pressure from the authorities, who are anxious that anti-fouling systems should be as efficient as possible and make as little use of chemical additives as possible.'

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