Spreading the word on best treatment process practice

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 at BHR Group is systematically quantifying mixer process performance and energy efficiency. In addition, the WWM Research Programme is promoting the adoption by water companies of sensible dosing and mixing design standards that constitute good practice.

The deliverables from the WWM Research Programme are software tools, design guides, seminars and consultancy, which are enabling improved design, better process performance and control as well as operational cost savings through reduced power and chemical consumption.

Major mixing operations in the water industry include:

• Dosing and mixing chemical additives
• Blending water, wastewater or sludge streams from different sources
• Suspending solids
• Contacting wastewater and biomass
• Gas-Liquid mass transfer operations

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, 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. Inadequate blending rates can lead to overdosing, poor control, non-uniform process streams and a limited choice of additives.

Excessive blending rates or use of inefficient mixers incurs unnecessary power consumption and process operating costs. A wide variety of equipment and structures are used in the water industry for dosing and mixing chemical additives. These include dosing lances, spargers, weirs, flumes, valves, orifice plates, baffles, proprietary static mixers and stirred flash mixers.

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

Rapid blending

Anaerobic digestion is key to the sustainable recycling of biosolids as it provides volatile solids and pathogen reduction as well as biogas production.

The desire for sludge treated to an enhanced standard to facilitate recycling to land has led to increasing numbers of hydrolysis plants upstream of digesters. The term Advanced Anaerobic Digestion has been coined to describe digestion with higher feed solids (5-12% DSC, frequently pre-hydrolysed), short retention times, loadings of >3kg VS/m3/day and integration with CHP.

Advanced Anaerobic Digestion can only realise its full potential with improved mixing system design to ensure rapid blending of thick, gassing feeds. All too often digester mixing systems have failed on modern digesters due to poor mixer selection, sizing and sludge rheology understanding.

This has resulted in increased foaming due to poor feed distribution, reduced loading rates and reduced gas production and solids build-up.

BHR Group says it is ideally placed to provide mixer modelling and consultancy for Advanced Anaerobic Digestion because it has:

• A “unique” WWM Sludge Tank & Digester Mixing Design Guide
• One of the world’s largest Sludge Rheology Databases (SRDB)
• The best available validated CFD modelling of digesters and sludge tanks
• Chemical industry performance modelling of impeller and jet mixed tanks
• Development of jet, submersible and gas mixing design correlations.

Chemical precipitation is increasingly being used for phosphorus removal on wastewater treatment plant. Metal salts can be dosed upstream of the primary tanks and upstream or downstream of the secondary tanks.

Chemical dosing

The quantity of metal salts required for P removal is minimised with effective mixing and flocculation. As with potable water coagulation, rapid mixing is required to enable stoichiometric quantities of dosed chemicals to be used.

Long mixing times result in the need to apply higher chemical doses, often nearly double the stoichiometric quantities, treat and dispose of larger quantities of sludge and potentially encounter excessive residuals in the effluent.

Most wastewater treatment works have not been designed with the need for chemical dosing, mixing and flocculation in-mind, consequently retrofitting mixers can be problematic.

Ragging can also be a serious difficulty when selecting mixers for P removal applications.

The WWM Research Programme has investigated a range of options for rapid mixing of chemical dosed into wastewater streams including spargers, flumes, liquid jets, gas jets and stirred flash mixers.

Performance measurements have been carried out at both pilot and full scales. Sizing and selection guidelines for the devices that mix sufficiently rapidly, have low headloss and avoid blockage have been recommended.

www.bhrgroup.com