Channelling chemical resources

Static mixers are a water industry mainstay for many chemical dosing applications, particularly in downstream processes. It would be difficult to find a UK WTW where static mixers do not feature as a key part of the chlorination/dechlorination disinfection process, or are used for other dosing duties where relatively small quantities of treatment chemical need to be accurately dosed and mixed.

A well-designed static mixer will blend treatment chemical into the main flow, mixing to a condition where monitoring equipment will have difficulty measuring variations in concentration of the chemical.

This level of mixing can be calculated and defined statistically with the Coefficient of Variation (CoV). It is not good

enough to simply design a plant with static mixers, as with any other piece of process plant, they must be correctly specified to suit the application. Crucial features which need consideration are:

• level of mixing – most water treatment applications consider CoV = 0.05 to be fully mixed. Note as CoV defines a variation from the mean, a low CoV value designates good mixing, a high CoV poor mixing,

• how quickly the fully mixed condition is achieved – most well designed mixers will achieve CoV= 0.05 within one hydraulic diameter of the mixer discharge,

• how much energy the

  static mixer needs to achieve full mixing (pressure drop/headloss).

Extensive trials by BHR Group, on behalf of the Water & Wastewater Mixing Consortium, indicated static mixers could offer large cost savings on inlet works applications.

Historically, static mixers have been perceived as flanged pipework components fitted with internal mixing elements. The trend over the

last few years, however, has been to develop mixers in alternate shapes and geometries often to meet complex installation requirements.

Most inlet works are not pipework based but open concrete channels. The requirement for the static mixers to be square/rectangular in section, quite large and to fit in open channels has been overcome by manufacturers.

Statiflo’s Channel Mixer is now beginning to influence inlet works design, because it is now practical to have all inlet works chemical dosing processes carried by static mixers. The benefits include:

• no moving parts,

• negligible headloss, typically around 20-50mm/WG,

• uniform shear throughout the system,

• uniform distribution of dosed chemical. As the mixing elements fill the channel, all water must pass through the device and be mixed,

• very rapid mixing

  minimising unwanted

  hydrolysis reactions,

• control system sees truly mixed and representative condition quickly, enabling immediate process control,

• available in any shape, size or material,

• low cost design – simple mixing elements are installed directly in the channel, there are no flanges or pressure retaining components,

• no motors or any external power consumption – so no electrical wiring and no MCC.

A WTW in Cheshire extracts 150-170M/ld of river water. In the inlet works, following pH control, the water undergoes coagulation with an alum product. Alum solution had for many years been surface dosed into the open channel. Due to the minimal mixing generated by flow in an open channel, this gave poor and variable quality coagulation. The alum dose rate was set by downstream turbidity readings resulting in an average alum dose of 85mg/l.

This was considered high by the operator and as part of a drive to reduce costs and improve water quality, the operator approached Statiflo. A 3.8m-wide Channel Mixer was installed, a simple operation involving lowering the mixer into the channel and bolting it to the walls.

Following re-commissioning, a few hours later, an significant improvement in water quality was apparent. The Channel Mixer instantly blended alum uniformly throughout the water stream resulting in rapid and effective coagulation. This has optimised the system so alum no longer has to be overdosed to compensate for the lack of effective mixing. The alum dose was reduced to 70mg/l while flocs were stronger and turbidity improved.

This 15mg/l reduction in alum dose rate equates to a cost saving of £85/d, or more than £30,000 per year. The Channel Mixer cost around £18,000, installed, and with a service headloss of only 20-60mm/WG was easily retrofitted into the existing channel without affecting the hydraulic profile of the plant.

The success of this installation rapidly lead to a contract for a second unit operating on a second alum dosing application. A payback period of less than five months for both units was achieved.

Additional benefits continue to appear. Improved coagulation not only saves on chemical costs, but also improves the efficiency of downstream clarifiers. Fewer fines in the clarifier overflow mean less solids loading on downstream gravity filters, hence longer cycle times between backwashes. During the same shutdown, the client installed eight conventional static mixers for a variety of pH control and chlorination duties.

Modern static mixers’

combination of chemical

savings, energy savings,

zero maintenance and improved water quality are having an effect on the design of new inlet works. In some instances flash mixers are not sepcified resulting in inlet works with all chemical

dosing and mixing treat-

ment stages carried out by Channel Mixers operating under gravity flow.

A recently commissioned WTW in northern England is an excellent example. Water arrives at the inlet works from five sources, all of differing quality. These streams enter an inlet chamber where a 7 x 5m static mixer, possibly the largest in the world, blends the streams to a single flow of more than 600Ml/d. Because the flow is quickly divided into four equal trains, the ability of this blend mixer to create an exit condition with a uniform velocity profile proved vital in ensuring equal distribution of water down each channel. Of equal importance is the virtually negligible headloss of the mixer, 9mm/WG, a crucial factor in a gravity flow plant.

After division into four streams, each of about 155M/ld, the water is further treated. Firstly by pH adjustment using either lime or sulphuric acid dependent upon the pH of the incoming water. This process is carried out by four 1.5 x 5m Channel Mixers. These mixers are designed to achieve a CoV of < 0.03 within one hydraulic diameter of additive dosing, thus ensuring a stable pH condition prior to the next treatment process.

The design of the chemical injection system in such deep channels required particular attention, particularly considering the elevated density of concentrated sulphuric acid and the usual design requirements for lime slurry systems.

Following pH control, the water undergoes alum dosing in four 1.5 x 5m Channel Mixers. These also achieve a CoV of < 0.03 within one hydraulic diameter of additive dosing, again ensuring complete and rapid mixing of alum thus maximising the flocculation process.

After passing through a relaxation/flocculation zone, the water is further treated with polyelectrolyte to consolidate the flocs prior to rapid gravity filtration. Again four 1.5 x 5m Channel Mixers mix the dosed polyelectrolyte to a CoV of < 0.03 ensuring large strongly flocculated particles ideal for downstream filtration. More than 600Ml/d of inlet water is fully treated, presenting the downstream disinfection system with excellent quality water with an energy consumption of only 271mm/WG of headloss. The ability to design the inlet works with such a low gravity flow headloss eliminated the need for the pumping station which figured in the original plans - with a projected cost in excess of £1M