Improvements and guidance on best designs and operation of cleaning systems (to minimise airborne contamination) - AFM 131
Sponsor: DEFRA
Partners: CCFRA, Fenner Fluid Power, Wallgate Ltd, Walter-Broadley Machines Ltd, Hazlewood Foods plc, Hygrade Foods Ltd, Northern Foods plc, Unigate plc, ASDA Stores Ltd, Waitrose Ltd, M&W PearceLtd, Silsoe Research Institute.
Total Project Cost: £220,809
Start date/End date: 02/10/2000 – 30/09/2002
Project Co-ordinator: Dr. D. Burfoot, Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4HS. Tel: 01525 860000 Fax: 01525 860156 Email:
Abstract
Cleaning operations can contaminate foods because they produce aerosols that may contain microogranisms. Based on the results from this project, guidelines were produced that consider the relative risks posed by various cleaning operations. They present data on the number of droplets made airborne by each cleaning operation, show examples of the distances travelled by the airborne droplets and present a method of estimating risk. Finally, practical recommendations on reducing risk are provided. The guidelines can be obtained from address above.
The research carried out included measurements of droplet sizes and concentrations using laser-based systems, measurements of organism concentrations using airborne samplers and settle plates, and the use of computational fluid dynamics models to predict the detailed movements of droplets. The investigations considered in detail hand washing, boot cleaning, floor scrubbing and the use of hoses, but many of the conclusions are relevant to other cleaning operations.
Cleaning operations produce a wide range of droplet sizes. All operations produce very small droplets (2.5 µm and below). Droplets of this size disperse easily in air and they can move large distances and stay airborne for long periods. Droplets above 20 µm diameter tend to settle quickly and do not disperse well in food factories. Droplets up to several millimetres diameter can be produced by equipment such as hoses. These large droplets can splash directly onto a food or disperse by landing on clothing or the floor and can then be transferred around the factory.
The studies showed that a hose (low pressure, 100 psi, trigger-type) produces far more droplets than the other cleaning devices tested. Droplet concentrations near to hosing operations were up to 12 million droplets per cubic metre. This value is around 20 times greater than those near to any other cleaning operation. For this reason, low-or-high-pressure hoses should not be used during production periods.
Much of the airborne contamination from cleaning operations, other than hosing, settles close to the operation. Much of the contamination produced by the boot scrubber, hand washer and floor scrubber settled within 2, 1, and less than 1 metre, respectively, of the operation. Contamination from hosing spreads much further. The distance depends on how the hose is used; even at distances of 5 metres the contamination from the hose can still be very significant.
All the cleaning operations produced many small droplets that could contain organisms. Unlike the large droplets that settle quickly and produce high deposits near to the cleaning operation, these small droplets tend not to produce high deposits of contamination but they produce low levels over large areas. These droplets disperse very easily, they can travel throughout a food production area, almost irrespective of its size, and they can remain airborne for up to 2 hours in a high-care area. This flight time depends on the air flow in the factory but, in most factories with typical air change rate and efficiency of filtration, such droplets are removed within 30 minutes. At least 99% of these small droplets are removed by the filtration system or they leave through doorways and other openings. Measurements in factories found that the number of airborne droplets containing an organism varied from 1 in 200, directly next to staff during hand washing, to 1 in 30000 in a well designed factory during a period of no activity.
A procedure for estimating the risk of foods being contaminated by airborne droplets from cleaning operations is presented in the guidelines. This is a simplified approach, compared to the use of advanced computational techniques, but it does allow a realistic estimation of risk that compares well with measurements obtained with settle plates. The cleaning operation can be implemented if the estimated risk is acceptable. Settle plates can then be used to measure the actual incidence of contamination.
Approaches for reducing the risk from airborne contamination are also described in the guidelines including changes in factory operation and the operation and design features of cleaning equipment.