Now entering its 6th year, BHRSolutions' water and wastewater mixing, collaborative research programme, WWM, is providing quantified design and selection rules for pipe, channel, stirred tank and sludge mixers, thanks to support from Water plcs, Consultants, Chemicals and Equipment Suppliers.

WWM was set up in 1996 to meet the needs of the industry's Engineers, who had access to little or no information on efficiency or quantified performance of mixing equipment other than that supplied by the manufacturers, which inhibited their ability to select the most appropriate equipment for the application. The information deficit is particularly severe in relation to open channel blending. The chemical industry, the usual source of all pipe blending data, does not use open channels and hence has had nothing to share for these applications.

Water and wastewater treatment processes frequently involve the addition of low flow-rate chemical streams to high flow-rate bulk streams. Examples of chemical additives include coagulants, flocculent, sludge conditioners, disinfectants, acids and bases. The rate at which the additive and bulk streams are blended can have a significant impact on process effectiveness and whole-life costs. Inadequate blending rates can lead to over-dosing, poor control, non-uniform process streams and limited choice of additives. Excessive blending, or use of an inefficient mixer incurs unnecessary power consumption and excessive process operating costs.

A wide variety of equipment and structures is used in the water industry to promote blending, including spargers, dosing lances, weirs, baffles, proprietary static mixers and stirred flash mixers. The mixing performance and efficiency of flash mixers is not understood, resulting in many inadequate installations. The blending of sludge conditioning polyelectrolytes is critical to thickening or de-watering performance. This also is poorly understood.

BHRSolutions measures mixer performance using additive concentration variation (CoV) at a particular pipe or channel cross section. Its dedicated laser induced fluorescence (LIF) pipe test facility enables mixture quality to be measured non-intrusively for fluids of differing rheological properties at a range of scales. During the first two years of WWM, work focused on blending in pipes and channels without dedicated mixers present. During years three and four, pipe static mixers, channel static mixers, weirs and stirred tank flash mixers were investigated. The third phase concentrated on blending of additives into sludge, and the effect of additive viscosity on blending rates. The scope for the fourth phase will include mixing in sludge tanks and in-line, as well as investigating untested mixers to achieve a deeper understanding for key applications.

The main deliverable of WWM is a Design Guide for assessing the performance of existing mixing installations, upgrading existing and designing new mixer installations. The Design Guide enables the user to select the most efficient mixing or dosing arrangement within the process, site and cost constraints. The electronic format Design Guide is updated as research results become available. The benefits of application include: savings in capital cost; chemical and power reduction; improved quality of treated water or effluent; and more reliable process monitoring and control.

Current members of WWM include: Yorkshire Water; Northumbrian Water; North West Water; Statiflo International Ltd; SNF(UK) Ltd; Paterson Candy International. More information about BHRSolutions' services to the Water Industry can be found on its web site at www.bhrsolutions.com. Organisations interested in joining WWM should contact Dr Mick Dawson at BHRSolutions.

PRW 12.10.01

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