The challenge of biofouling

When surface water is in contact with man-made solid surfaces such as the conduits and pipework in a cooling water system, living organisms will colonise the surfaces. Colonisation happens in a standard pattern whereby organic molecules deposit on the surface first. Within 24 h, bacteria attach, after which their metamorphosis to sessile creatures starts to produce slime (extracellular polymeric substances). These bacteria form a biofilm where the conditions may become optimal for specific types of bacteria, for example, sulphur- reducing bacteria. Once a surface is colonised by bacteria, it becomes suitable for macroinvertebrates such as mussels, barnacles, and hydroids. Figure 1 shows the typical timeline for biofouling settlement.

The type of fouling depends on the geographical location of the plant, the surface water’s specification, and seasonal influences. A biofouling community forms a special ecological entity in an industrial environment that is nearly optimal for all settled organisms, i.e., no predators, a continuous flow of water with nutrients and oxygen, optimal water velocities (0.5–2 m/s) and, above all, a surplus of substrate. The latter is normally the regulating factor.

It is conventional to divide biofouling into two types: microfouling involving bacteria and fungi that produce extracellular polymeric substances; and macrofouling involving organisms such as mussels, oysters, barnacles, and hydroids.

Macrofouling causes gross blockages of heat exchanger tubes and results in heat loss owing to increased wall roughness in the intake pipes. The predominant effects of microfouling are reduced heat transfer efficiency in heat exchangers and microbially induced corrosion on metal surfaces. All these are undesirable in industrial plants.

Biofouling control

The search for the most efficient ways to control biofouling in cooling water systems has been ongoing for a very long time. The first major paper on the subject, which referred to power stations, was in 1927.

Considerable effort has gone into developing new technologies or assessing the applicability of known methodologies for eradicating organisms (larvae and spat) from cooling water systems.

Many of the proposed technologies are often adapted from applications other than the treatment of seawater. The chemicals (biocides) are of two types: oxidising and non-oxidizing compounds.

Oxidising biocides include chlorine and bromine that act by destroying cell membranes or their extracellular enzymes, which leads to cell death. Non oxidizing biocides include chemicals that act by interfering with a necessary life function such as metabolism or reproduction. However, the most common chemical remains sodium hypochlorite

Driven by environmental regulations aiming to reduce the amount of chlorine going into the environment, plant owners are looking for more environmentally friendly and cost-saving alternatives. Ecodosing (previously called Pulse- Chlorination®) from H2O Biofouling Solutions BV is such an option.

In 2000, this dosing method was declared the best available technology under the terms of the EU Integrated Pollution Prevention and Control Directive for macrofouling mitigation using chlorine in once-through cooling water systems.

Local authorities in many other countries also recognise Ecodosing as the best available technology for biofouling control.

Controlling biofouling at a refinery

The cooling water distribution system at the refinery is about 50 years old. Over the years, the system has been developing integrity and reliability challenges pertaining to leaks in the primary distribution system and process heat exchangers. One important underlying cause of the leaks is under deposit corrosion caused by a combination of biofouling and sludge formation. To guarantee the availability and extend the longevity of the cooling water system, the owner of the system developed a cooling water master plan in 2013/14. Part of this master plan was to develop a strategy to control biofouling in the cooling water network to prevent future problems.

The cooling water system network has three parts fed by different cooling water intakes. It was decided to implement a biofouling control procedure in phases, starting at one intake, which supplies a specific cooling water network. This networks area’s brackish cooling water system is a once-through, honeycomb (“Manhattan” style) distribution system fed with river water at a flow rate varying between 10,000 and 30,000 m3/h. The cooling water is distributed through five main headers to the different utilities spread across the area.

The water in the intake harbour area is affected by the runoff from the River Maas and the tide pushing seawater into it. This means that the water at the intake varies between fresh and slightly brackish during the day and throughout the seasons.

Biofouling

The salinity variations in the water mean that the biofouling in this area includes both fresh- and brackish-water species. In addition, surface water quality improvements and the invasion of new fouling species have increased the biofouling problems in the last decade. The main biofouling species are freshwater mussels, brackish water barnacles and hydroids.

The requirement for cooling water at the refinery has also changed with time: the closure of some facilities means less demand for cooling water, which has resulted in an oversized cooling water system with some stagnant zones. Decades of using this surface water for cooling purposes without any biofouling control has created ideal conditions for biofouling settlement and growth, see Figure 2.

Applying battery limit intake filters only prevents the larger shells from ending up in the heat exchangers. Most biofouling larvae pass through the filters and grow downstream of them to cause a gradual build-up of biofouling in the narrow passages, thereby increasing the risks to heat exchanger efficiency and integrity.

Hydroid growth has an additional side effect; hydroids form web-like structures that act as nets for sediment particles and increase the build-up of sediment in the pipes. This is a particular problem in the second petroleum harbour because the water has large fluctuations in sediment levels through ship movement.

Furthermore, microfouling, which can cause microbiologically induced corrosion, is an integrity threat to all parts of the system where the conditions are conducive to its growth

Taking action

To control biofouling growth and remove the historical build-up of biofouling in the refinery’s cooling water network, the Ecodosing method was evaluated. This method was chosen because of the very successful control of biofouling achieved using a minimum amount of chlorine at other industrial locations in the Netherlands, including in the Botlek and Moerdijk area.

The Ecodosing system takes advantage of the natural life cycles of biofouling organisms, including mussels and barnacles, and their response to biocides under local conditions. It is based on the principle that these organisms have a recovery period after exposure to a chlorination period before they open fully and restart filtration for oxygen and food uptake.

The Ecodosing system takes advantage of this recovery time by alternating short periods of chlorination with periods without chlorine.

During continuous chlorination, the organisms close and switch from aerobic to anaerobic metabolism and can live on their own reserves for up to 10 weeks.

With pulsed chlorination, the organisms must switch their metabolism continuously from aerobic to anaerobic, which leads to physiological exhaustion. This results in more rapid antifouling compared with continuous chlorination *[Ref 1].

Local conditions are used to determine the most effective dosing programme to control biofouling settlement and growth. Consequently, the required dosing interval and the optimal chlorine concentration were determined during a site test at the refinery.

Before system implementation, the water residence times at all the different utilities in the cooling water network were studied. As some users on-site receive water from different headers, residence times can vary significantly. For effective biofouling control, it is important that the pulses of chlorine arrive according to the defined interval of each utility.

Additional studies aimed to verify the effectiveness of the chlorine dosing at all the utilities. In addition, Biovision® biofouling monitors (Figure 3) were installed a year before implementation to build up a picture of the fouling.

Hypochlorite injection skids are installed to dose sodium hypochlorite at the intakes. Hypochlorite is dosed in the discharge lines of the cooling water pumps using retractable injection quills.

The Ecodosing regime was implemented in the cooling water network at the refinery in 2018. Its efficiency was determined from the cleaning rate of the biofouling monitors. Adult mussels were installed to measure mortality speed and rate. Chlorine measurements were carried out at all the different utilities to ensure that effective pulses of chlorine were reaching all of them.

Results

After six weeks of Ecodosing, the biofouling monitors were free from living biofouling species and the mortality rate of adult species was above 75%. In the following weeks, an increase in the mortality percentage (postponed mortality) of the adult species was expected. During the dosing period, there was no negative impact on the cooling water system as a result of the release of biofouling debris.

At the start of the Ecodosing regime, the fouling coupon (Figure 4(a)) was covered with hydroids, barnacles and a few mussels. Figure 4(b) clearly shows that the Ecodosing regime killed the hydroids, i.e., less sedimentation on the coupon, and the barnacles: the white marks are their remains. The hydroid and shell debris was relatively small (<1 cm), so did not block the heat exchangers. Adjusting the start of the Ecodosing regime according to the size of the biofouling species prevents potential blockages in the heat exchangers

Conclusion

A trial has shown that pulsed hypochlorite dosing using the Ecodosing system will prevent biofouling growth in the cooling water network of the refinery without operational issues. However, careful monitoring of system performance is required to prevent overdosing (permit issues) and underdosing (effectiveness).

The Biovision biofouling monitors provide insights into biofouling settlement and growth rates. This information has helped the refinery to reduce the required Ecodosing period to a maximum of eight weeks per year. During the rest of the year, a daily, 15-min dose of chlorine is applied to control microfouling in the brackish cooling water headers and the heat exchangers.