The Management of Legionella in Air Washers, Wet Scrubbers, Particle and Trivial Gas Scrubbers

Transcription

1 THE WATER MANAGEMENT SOCIETY The Management of Legionella in Air Washers, Wet Scrubbers, Particle and Trivial Gas Scrubbers Guidance produced by Water Management Society Working Party First published July 2010 Revised August 2017 Working Party members: John Alvey David Bebbington Alan Elsworth Giles Green

2 The Management of Legionella in Air Washers, Wet Scrubbers, Particle and Trivial Gas Scrubbers The management of legionella in this type of system is dealt with in a scant manner in L8 and HSG274 Part 3 The control of legionella in other risk systems. In order to help the industry in general, and its members in particular, the Water Management Society (WMSoc) has undertaken to draw together people with expertise in the subject and those with knowledge in the control of legionella, as a working group. The group has been charged with the task of giving detailed guidance in the control of legionella in these systems and the guidance offered here is the considered opinion of the Water Management Society. This paper is part of a series of guidance documents being produced by the WMSoc in the control of legionella in Other Risk systems. Risk Assessment L8 states that certain conditions increase the risk from legionella if: (a) The water temperature in all or some parts of the system may be between 20-45ºC, which is suitable for growth; (b) It is possible for water droplets to be produced and if so, they can be dispersed; (c) The water is stored or recirculated; (d) There are deposits that can support bacterial growth, such as rust, sludge, scale, organic material and biofilms. This type of plant could therefore act as a device that may allow growth and dissemination of legionella bacteria and so must undergo a risk assessment. The assessment determines the level of that risk and what, if any, mitigation actions are necessary. It should also identify the person responsible for these actions and determine the frequency that they are to be carried out. The recommendations made in this guidance document should be taken into account, but each system must have its own specific risk assessment validated by the owner/operator. For each system local factors such as design, use and environment, as well as the previous history of the system, need to be considered. Scrubber systems can be complex in design and can sometimes involve complex chemical considerations in order to control the risk. It is therefore important that the person chosen to risk assess such systems has a suitable degree of training and competence in order to complete the task satisfactorily. Legionella will grow in any water system where: The temperature of the water is between 20 C and 45 C in any part of that system There is sediment, sludge or other nutrients (possibly from the air or gases being cleaned) which will support microbial growth There are biofilms (or slimes) which may support the potential for legionella survival and growth There is scale and/or corrosion (iron is a growth factor for legionella.) Scrubber systems may show all of the above at different times (or continuously) to some extent or other. Even systems that are engineered to the highest standards may produce large amounts of aerosol when operated. These systems can operate continuously and much of the aerosol produced will be released into the atmosphere. If there are conditions that would allow legionella to grow,

3 many people could be affected. Uncontrolled or dirty systems may well result in an increased risk of growth of legionella thus increasing the risk of Legionnaires disease. Contaminated gas inlet Cyclonic mist gas separating movement Scrubber liquid inlet Contaminated liquid reservoir Dirty liquid particle Contaminated scrubber liquid outlet Gas Scrubbers These devices are designed to remove dust and other contaminants from an air-stream or contaminated gas stream and then allow the exhaust to be discharged. The area of this discharge of the exhaust gases is one of the concerns. Frequently, small scrubbers, i.e., those handling less than 5m³/s, will be located adjacent to the process that generates the dust. They will be in a warm environment, as most factories work at about 22ºC and the small scrubber will usually discharge into the workplace. They will discharge saturated air which will contain a small quantity of scrubber water droplets, some of which will be small enough to be inhaled but not exhaled. There will also be very fine particles (too fine to be collected) plus any reaction product from interaction of the dust with water. Even if it is convenient to discharge exhausts into the open air; there will still be a need to manage the risks from these units. Sometimes small, but more usually large, units (about 25m³/s) are located outside. In this location the units will A small portable gas scrubber have to comply with the Environmental Protection Act and should contain a vertical section with an up-flow rate of less than 9m/s and a discharge/ efflux velocity of greater than 15m/s at a height 3m above the apex of the nearest building. This should ensure that the large droplets are not exhausted and ensure good dispersion of the exhaust. There are however several local councils that will not allow the erection of these discharge stacks.

4 Particle and Trivial Gas Scrubbers Particle and trivial gas scrubbers are devices that use water to remove by scrubbing (washing) a variety of particles and / or gases from the air. These devices are usually associated with dust and fume control within industrial environments. When used for trivial gas removal, acid or alkali is commonly added to the water to neutralise the content of the gas. Usually the ph value of the liquor is monitored and controlled. Air is extracted via a hood from some industrial process to control the atmosphere within a factory. Typical applications include, but are not limited to, the removal of metal dusts following grinding; the removal of airborne dust from a conveying system; the removal of fine dust after mineral crushing or screening operations and domestic / commercial waste transfer applications. They may involve the removal of trivial amounts of acid gases, acid anhydrides or ammoniacal gases by the use of chemical neutralisation. Paint spray booths are used to remove paint and solvent fumes from the air. These devices are usually self-contained and consist of a liquid (usually water) storage tank and contact area for the air and the liquid to mix, followed by a zone where the droplets of water are removed from the air (i.e., a disengagement zone). The storage tank will usually have some device to ensure that the liquid content is maintained. This may be (and usually is) connected to the town mains water supply. In systems where the water / liquid is recycled back to the tank, the tank may be designed to encourage sedimentation of the particles scrubbed from the air. The sediment will contain the solids generated by the industrial process and particles naturally contained in air such as pollens, insects, bacteria, and mineral matter. Contact of the water or other liquid with the contaminated air is achieved either by pumping the water / liquid into a venturi or by self-induction. The diameter of the droplets generated is inversely proportional to the energy used to generate the droplet. The economical size of droplet that is generated is no smaller than 2 micron but most devices generate droplets in the size range of 5 to 15 microns. For the devices to function effectively the momentum of the water droplets has to be similar to the dust particle to be removed. The dust adheres to the water droplet. These droplets are the right size to allow any legionella that might be present to affect those breathing the aerosol. The exhaust air and the dust now adhering to the water droplets enter into a zone to allow disengagement of the droplets from the air stream. This disengagement can be achieved by several mechanisms. There can be inertial separators in which the air stream is caused to change

5 direction frequently. The high momentum particles carry straight on and impact on the sides within the separator. They then coalesce to form large droplets which flow downward under the influence of gravity. High-speed cyclonic separators, which fling the droplets against the side of a vessel, can also be used. All of these techniques can work well when the machinery is in good condition but work less well when the process does not work as designed (for example after modification) or if there is insufficient air flow or water flow. Venturi Scrubbers Risk Assessment When carrying out the risk assessment the following need to be considered: The location of the equipment with especial consideration of the location of the exhaust. The nature of the dust/gas/solvent to be collected. Both the temperature of the air passing through it and any chemical reaction between the dust and the water will affect the temperature of the water. If the resulting water temperature is consistently below 20ºC or above 60ºC then the risks may be considered trivial. Are any chemicals added to the system to reduce bacterial growth (or additives used for operational reasons which might act as nutrients)? Is there any risk of exposure to aerosol contaminated with legionella during the operation of the unit, or when cleaning or maintaining it? Can contaminated droplets be discharged from the unit? Should there be a constant bleed to prevent debris and scale deposition within the unit? If water temperature is within the growth range for legionella, is a more comprehensive programme of treatment required in addition to a bleed system? Is the scrubber liquor hostile to microbiological growth? If the risk assessor concludes that there is a risk that legionella could grow within the system and could be discharged during normal operation or routine maintenance he should recommend actions to be taken to minimise those risks. He will need to consider the appropriate actions and include them (with priorities) in his written report.

6 Gas Scrubber Operation and Control The responsibility for the control of the system is that of the system owner. He will appoint a responsible person to take managerial responsibility for the operation of the system and the keeping of records. The responsible person will take note of the recommendations made in the risk assessment and devise a programme of work to ensure they are put into place and maintained. It is desirable that all systems have sufficiently good records so that the owners can point to evidence that proves their systems are maintained in an acceptable condition. Any deterioration in condition needs to be noted prior to the point when the system would be deemed high risk. Owners are responsible for keeping these records even though suppliers of chemicals and other service providers may be involved. Possible Control Strategies Good practice requires that every six months these systems should be cleaned and disinfected. All wetted surfaces and make up tanks should be included and descaled as necessary. It also says that any non-chemical water treatment present is to be checked on a weekly basis. This approach is likely to be inadequate in many systems. The responsible person should discuss the options open to him to control the risks with the risk assessor or other competent persons. They will agree what must be put in place and how the measures will be controlled, i.e. what checks will be put in place and how often those checks are to be carried out and how often visual/physical observations need to be made. If water is being recirculated and the conditions for growth are good, then it is perhaps prudent to consider the unit as being analogous to a cooling water system and put similar chemical and physical controls in place as would be required for open recirculating cooling systems. These are well documented in L8 & HSG274 Part 1 and elsewhere. Diagram of a scrubber with a recirculating water system through a sump Such controls would include the addition of biocides to prevent biological growth (including legionella) and a blend of chemicals to prevent scale, corrosion and fouling. Corrosion may be engineered out of these units by having them manufactured using materials that will not corrode. Fouling is likely to be a continual problem due to the nature of the unit (being designed to remove debris from the air). A bleed system would also be necessary to remove debris and prevent over concentration of nutrients. There is some debate about the correct class of chemical to use to reduce the possibility of bacterial growth. One school of thought suggests that this should not be based upon chlorine as it is volatile. However, chlorine release agents are used to prevent legionella growth in cooling water systems. Cooling systems have a similar air scrubbing action to gas scrubbers, and so theoretically it should be acceptable to use chlorine in gas scrubbers. The gas/dust/fume

7 which is removed may result in increasing chlorine demand, however, so reducing its effect and requiring continuous dosage and control of chlorine reserves. Legionella effective non-oxidising biocides might also be considered in these applications. It should always be borne in mind that in scrubbers the water content could be less than 20% of the gas flow per second. i.e. 5m³/s air flow in a scrubber with only 1 m³ of water. Under these conditions, even biocides with low vapour pressure may be stripped from the scrubber. Another view is that continuously dosed oxidising biocides (such as chlorine) are acceptable in many types of systems. The choice of treatment will also depend on the contaminants that are being removed by the unit. Many contaminants may react adversely with any of the treatment chemicals being applied (including chlorine, scale and corrosion inhibitors or non-oxidising biocides) nullifying their effect or even result in hazardous emissions. The choice of control will be risk-assessment led and the complexity serves to emphasise the importance of having a specific risk assessment in place. This complexity also emphasises the need to ensure that the risk assessor is sufficiently competent to carry out the assessment. If the responsible person does not have the expertise available to him within his workforce, to apply and control the treatment himself, he will employ a competent service provider to do this for him. The service provider will carry out frequent checks of the treatment programme and prepare written reports of the conditions found at each visit. Copies of these reports will be made available to the responsible person and he will keep these records in the system logbook for a full five years. The site may carry out testing as indicated by the risk assessment and the results of these tests also need to be recorded and maintained in the logbook. One test that might be recommended by the risk assessment is routine monitoring for legionella bacteria at an initial frequency which might be reduced according to the advice of the risk assessor. Note that scrubber liquors might be more hazardous than cooling system water, so extra care in taking and transportation of samples may be required. If the discharge from the unit does not go to atmosphere but discharge is into the confined workplace, then addition of chemicals and biocides as described would not be appropriate. The workforce would be breathing the treatment chemicals and may be affected by them. Under these circumstances, it would probably be more prudent to put into place a more frequent, e.g., weekly, cleaning and disinfection regime to ensure that all debris is removed and any bacteria that have grown over the previous week are eliminated. Again, the frequency chosen will be risk-assessment led. For small air scrubbers, which may not have bleed systems, the assessor may recommend that the units are emptied and refilled frequently, e.g., every day (or every shift, depending on conditions), and cleaned and disinfected frequently, e.g., weekly. Indeed for small units, it may well be more cost effective, and give better control, to remove the water and clean the unit regularly, rather than attempting to treat it. Once again, a specific risk assessment is needed to determine what is required. Substitution of the system itself for a lower risk system might be possible. If the function of the scrubber is the removal of inert dust, then a dry system such as cyclones could be considered. Beware that some dusts can cause fires or might be explosive, so wet systems might be preferred or even required.

8 References and Further Resources WMSoc: Guide to Legionella Risk Assessment BS8580: 2010 Water quality Risk assessments for Legionella control Code of practice THE WATER MANAGEMENT SOCIETY Disclaimer: the Water Management Society has published this document as part of a series of guidance papers designed to give support in the control of legionella. Guidance in this document does not replace any legislative requirements and should be used in conjunction with any manufacturers recommendations. The Water Management Society accepts no responsibility for misuse or misapplication of the guidance. For further copies of this document, and more information about Water Management Society publications, training courses and future events visit or telephone: The Water Management Society 6 Sir Robert Peel Mill, Hoye Walk, Fazeley, Tamworth, Staffs B78 3QD Telephone: Fax: admin@wmsoc.org.uk W /17 Water Management Society The use of the WMSoc logo or any related imagery is not permitted for personal or commercial use.