An investigation into the strength and thickness of biofouling deposits to optimise chemical, water and energy use in industrial process cleaning

Biofouling is both a human health hazard and detrimental to process efficiency. Biofilm growth is inevitable on exposed surfaces, so an informed approach to cleaning and timely management are essential. Chemicals can readily kill cells, but the biofilm structure must be removed to prevent re-growth and maintain sterility. Chemical agents also pose health and environmental risks, but the typical alternative is to pump unsustainable volumes of cleaning solution through pipelines for mechanical cleaning. The aim of this research was to apply green cleaning principles to biofouling removal in industry, reducing the amount of chemicals, water and energy used in cleaning. Biofilms of Escherichia coli and Burkholderia cepacia were grown on polyethylene, glass and stainless steel 304, in single and mixed species cultures. Fluid dynamic gauging (FDG) utilises hydrodynamics to measure both the thickness and attached strength of the biofilms and therefore the optimum water usage for removal can be estimated, and is both relatively simple and inexpensive to operate. As well as using a static culture method, a drip flow reactor was built to develop biofilms under flow conditions. The use of FDG offers an original way of monitoring both the attachment strength and thickness of mixed species biofilms, and drip flow is an alternative to traditional biofilm growth methods for analysis of removal behaviours, with particular relevance to food production environments. The adhesive and cohesive strengths of both single and mixed species biofilms increased up to 14 days’ growth, and as previous studies suggest that this will be sustained over longer periods under flow conditions, cleaning prior to peak strength would be prudent – at later stages the risk of pathogens developing and contaminating the process would likely become too great, particularly if the biofilm is experiencing significant detachment which increasingly occurs with age. The development of greater, sustained thickness over time can also pose problems with heat transfer and enhanced pressure drop. Protein, a key component of the extracellular matrix, showed a strong correlation with the adhesive strength of mixed species biofilms. Biofilms grown on polyethylene attached more strongly in the early stages of growth than those on glass or steel, which may be due to the greater hydrophobicity of the surface. Chemicals can be used most effectively to weaken the outer layers, and sodium hypochlorite was also shown to be useful for weakening surface adhesion – the required shear stress for 95% removal was reduced by approximately 60% for 5 and 10-day old biofilms. There are more risks associated with chlorine-based disinfectants than the alternative, peracetic acid, although finding a suitable low concentration would be simple using this method. There is no simple solution, complicated further by the unpredictability of the species present in industrial biofouling. The best way of minimising the risk of spoiling and contamination would be to clean surfaces with regularity, in the region of every 5 days rather than after a more prolonged period, which would also serve to minimise the resources used by preventing biofilms from becoming too strongly attached or too thick. A chemical input would need to be determined by testing for the optimum concentration necessary for a suitable effect, thus eliminating excess use, and thereby reducing water and energy use in the process. Taking a multispecies sample from a process flow could offer a more realistic approximation of industrial biofilms. Surface coatings to prevent adhesion are the focus of much research, and could be an alternative to reactive methods.