Controlling Problem Particles in Water Treatment

The aim of this work is to develop a new test to monitor the formation and breakage of particles in water treatment processes. Before water reaches a customer's tap, it must pass through a train of processes to remove the impurities from the water before it is safe to drink. The bulk of the contaminants are removed from the water in the process of coagulation and flocculation. Here, a destabilising chemical, such as an iron or aluminium salt is added to the water which enables dissolved, colloidal and particulate material to aggregate into large, fragile particles called flocs. In most water treatment plants in the world, these particles are removed by a sedimentation or flotation process. Any fine residual particles remaining are then usually removed by a sand filter. Failure to remove these particles results in high turbidity in the filtered water, which can reduce the efficacy of disinfection and the particles can be a vehicle for more toxic compounds entering drinking water. For this reason, turbidity in drinking water is strictly controlled. In the UK, a target value for turbidity in drinking water is 1 nephelometric turbidity unit (NTU). Recent revision documents from the European Commission indicate that this will soon move to a mandatory value of 1 NTU, with a target value of 0.5 NTU.The properties of the floc particles that form has a strong influence on how well they are removed by the clarification and filtration processes. If the flocs are too small or break apart too easily, water quality is compromised. The composition and structure of the floc is a function of a number of factors, including the source water matrix, the coagulant and flocculation chemicals used and the mixing regime deployed. Measuring the character and the strength of the flocs can therefore provide some very useful information on how well they may be removed in water treatment systems. To date, floc strength tests have concentrated on characterisation of large scale floc breakage products. This can significantly underestimate the concentration of small particles in a heterogeneous system containing a wide range of particle sizes. However, it is the small particles (normally centred around 1 um for most filtration conditions used in practice) that can cause operational difficulties because they are least well removed in a filter. The intellectual contribution of this work is in developing an operationally applicable and relevant floc strength test that integrates particle size and particle removal in depth filtration. For the first time we will investigate floc strength in terms of the formation and concentration of small particles before and after floc breakage by measuring floc strength in terms of the small particles around 1 um. The project will therefore deliver a methodology to determine floc strength through understanding the formation of problem particles. In addition we will establish how the system water quality and coagulation conditions influence floc strength and the formation of particles that cause turbidity in drinking water. Finally we will develop coagulation control strategies to limit the formation of FBP to enable longer filter run times and more effective filtration which could be applied on full-scale water treatment systems.

Whilst the proposed work will be of great interest to those in other research groups concerned with particle characterisation, the direct beneficiaries of this research will be those working in potable water supply in the UK and across the world. Given that nearly all water treatment plants in the developed world utilise coagulation and floc formation, the outputs from this research will be very relevant to water utilities, contractors and water quality regulators, such as the UK Drinking Water Inspectorate. The development and implementation of a suitable floc strength test will help deliver higher quality drinking water quality by minimising turbidity in the treated water. This will be of direct benefit to the general public who consume this water. The Centre for Water Science at Cranfield University has very close working relationships with the stakeholders involved in potable water production which has given the Centre a strong track record in being able to inform and influence the water community. The results of this research will be disseminated via a knowledge transfer day workshop of the Cranfield led EPSRC Network on Potable Water and Supply (EPSRC seed funding) where opportunities and challenges associated with implementing the research will be discussed with water supply companies. The developed tools would then be available to be used immediately by those working in water production to increase filter run times by developing control strategies in treatment processes to minimise the formation of particles that cause turbidity in drinking water. An increase in filter run times has advantages in terms of reducing water loss and treatment costs associated with filter cleaning where up to 5% of the total water produced by a WTP may be used. Water regulators would benefit from the research by having a tool that could be used to help demonstrate best practice for control of particles in drinking water. One of the objectives of this research is to produce a roadmap to identify future opportunities to indirect beneficiaries where the particle characterisation and floc strength tools developed could be used in applications where control of particle size is critical. By developing the research roadmap, a platform will be developed which will identify exploitation routes of the floc strength test and particle characterisation tools. Firstly, this will enable the principal investigator (PI) and post doctoral research assistant (PDRA) to further develop their academic careers from follow on research proposals in directly linked research areas such as in understanding how upstream floc formation influences filter capture mechanisms. Secondly, this approach will also enable the researchers to identify more indirect applications of the particle characterisation tools such as in microbiology for control of pathogenic microorganisms in drinking water (such as enteric bacteria and parasitic cysts like Crytosporidium), wastewater treatment, paper manufacture and paint production. Here, the timescales of the benefits will be realised over a much longer timescale in 3-5 years time due to the time taken to break into different research fields. In addition to being able to develop their academic profiles from the research, the PI and PDRA staff working on this project will develop key research and professional skills that can be applied in employment sectors. One of the key challenges of academic research is to effectively communicate complex science to less specialised audiences. The main focus of the professional development to be undertaken as part of this project will be in developing the communication skills of the PI and PDRA. The PDRA will also enrol on a bespoke training programme run by the School of Applied Sciences that develops skills in written and oral communication, client relationships and commercialisation of research.