Mixing is fundamental to both water and wastewater treatment processes. However, mixing design criteria are frequently poorly defined and mixer specification, selection and sizing carried out in a haphazard fashion.

The Water and Wastewater Mixing (WWM) research programme, (an offshoot of our fluid mixing processes research programme, FMP), is systematically quantifying mixer performance and efficiency. Easy to use Design Guides are enabling improved design, better process performance and control as well as operational cost savings through reduced power and chemical consumption.

Benefits of WWM
The main WWM deliverable is a Design Guide for assessing the performance of existing mixing installations, upgrading existing mixing installations and designing new mixing installations, leading to:

• Capital cost savings
• Chemical cost reduction
• Improved equipment design and selection
• More reliable process monitoring and control
• Increased throughput for existing plant
• Improved quality of treated water or effluent
• More efficient use of power

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 results arising from each phase of research work become available.

The Problem
The main mixing operations in the water industry are classified as:

• Dosing and mixing chemical additives
• Blending water, wastewater or sludge streams from different sources
• Suspending solids.

Examples of chemical additives include coagulants, polymeric flocculants, 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.

Currently, engineers have little or no quantified performance and efficiency information enabling them to select or size mixers other than the advice of equipment suppliers themselves.

The Research Programme
The work is supported by Water PLCs, consultants, chemical and equipment suppliers.

WWM is providing quantified design and selection rules for pipe, channel, stirred tank and sludge mixers.

Blending performance is measured using additive concentration variation (CoV) at a particular pipe or channel cross-section.

A dedicated Laser Induced Fluorescence (LIF) pipe test facility has been commissioned to enable mixture quality to be measured in a non-intrusive manner for fluids of differing rheological properties at a range of scales.

For further information about the consortium, or about single-client research or consultancy, please contact us.

Phase 6 Work Programme
Autumn 2007 onwards
Phase 6 concentrates technically on the wastewater and sludge applications:

• Jet Mixer for P-Removal Dosing: On-site Demonstration
• Design Guide for Sludge Tank & Advanced Anaerobic Digester Mixing Systems
• Sludge System Pumping Losses Prediction Tool
• Further Development of the BHR Sludge Rheology Database
• Energy Saving in Flash Mixing & Flocculation

Members are also entitled to the design guides and reports from the first 5 phases:

Phase 1: 1996 - 1998
During the first two years of the project work focused on blending in pipes and channels without dedicated mixers present. Deliverables included:

• Design Guide for pipe and channel mixing
• Correlations to predict mixture quality in both pipes and channels
• Comparison of pipe and channel mixing

Phase 2 1998 - 2000
Phase 2 work included the effect of operating variables and dosing on mixture quality (CoV) and headloss for:

• Static mixers in pipes
• Static mixers in channels
• Weirs
• Stirred tank flash mixers (CSTRS)

Phase 3: 2000 - 2002
During the third project phase inline dosing of additives into sludge and the effect of additive viscosity on blending rates were investigated.

Phase 4: 2002 - 2004
The fourth project phase concentrated on the wastewater and sludge applications of:

• sludge conditioning
• sludge mixing and blending
• dosing for P removal
• anoxic zone mixing.

The results and deliverables from Phase 4 enabled participants to improve mixer specification and selection, reduce chemical and powers costs, reduce sludge volumes and improve process performance.

The work also assisted Water Companies in achieving compliance with their treatment requirements and legislation.

Phase 5: 2005 - 2007

Phase 5 concentrated on:

• extending the range of sludges covered by validated sludge rheology data to cover the new sludges being generated by modern processes
• incorporating the data into a new database for WWM members
• using both physical and computer modelling to improve sludge tank design
• using physical and computer modelling to improve anoxic zone design and mixer design for phosphorus removal.