Rapid Mix Units

 

 

Introduction

Once coagulant chemicals have been added to raw water, it is essential to provide rapid agitation to distribute the coagulant evenly throughout the water. This is particularly true when alum or ferric salts are being used. The water must be briefly and violently agitated to encourage the greatest number of collisions between suspended particles.

The more common types of facilities used for rapid mixing (or flash mixing) are:

 

 

 

Static Mixers

In the field of water treatment, mixing and contacting are important unit operations having a fundamental influence on the performance of individual process stages or even on the results of the complete process itself. The ever increasing demands on water quality call for continuous improvement of the cleansing processes. This has led to a marked increase in the general use of in-line static mixers for mixing and contacting operations in this sector.

 

 

Function

A static mixer consists basically of a sequence of stationary guide plates which result in the systematic, radial mixing of media flowing through the pipe. The flow path follows a geometrical pattern, precluding any random mixing. The mixing operation is therefore completed within a very short flow distance. The formation of fine gas bubbles in a water/gas mixture promotes intensive contact between the two phases. The result is high mass transfer, for instance a high oxygen transfer rate or an excellent ozone utilization factor.

In contrast to stirred tanks or empty pipe systems, static mixers ensure that the complete fluid stream is subjected to compulsory or enforced mixing or contacting.

The energy required for mixing or for mass transfer is taken from the main stream itself, which is manifested by an insignificantly higher pressure drop than in an empty pipe system. This value depends on the design of the mixer and on the relative operating conditions. It is generally in the range 0.02 - 3 m w. g. In comparison to dynamic agitator systems, the energy requirement of static mixers is smaller by at least an order of magnitude. In addition, the energy is evenly dissipated throughout the entire mixer volume.

 

 

Design

Static mixers for mixing and contacting operations are installed in-line in a process stream. The associated pipe or conduit can be round, square or rectangular in shape. The mixers are available in different designs and in a variety of materials, enabling optimum adaptation to meet individual process requirements. In the treatment of water, free of suspended solids, e. g. potable water, it is preferable to use the SMV mixer for both mixing and gas/liquid mass transfer procedures. However, in mixing operations where the media involved contain solid particles, particularly fibres, plugging of the mixer can occur.  In such circumstances, the use of the specially developed SMF mixer is recommended. The SMF mixing elements consist of inclined vanes which cross without touching each other and are fully open to the flow stream from all sides. This type of mixer is therefore not susceptible to plugging and is being used with success in the treatment of wastewater and sludge

 

 

 

 

Mechanical Mixers

Mechanical mixers are widely used for rapid mixing because of their good control features.  They are usually placed in a small chamber or tank and include the propeller, impeller, or turbine types.  The detention time in these chambers is designed to be very short.

Mechanical mixers can also be mounted directly into a pipeline;  they are then referred to as in-line-mixers.  The type of unit provides good instantaneous mixing with little short circuiting, costs much less than a conventional rapid-mixing installation , and still allows for adjustment to provide the correct amount of mixing energy.  All in-line mixers must be located close to the flocculation chamber so that flocculation and settling will not occur within the pipeline.

 

 

 

Pumps and Conduits

Coagulant chemicals can also be added to the suction side of a low-lift pump to use the turbulence in the pump as a mixing mechanism.  The amount of mixing is determined by the speed of the pump, so the turbulence required for proper flash mixing may or may not be provided.  If the turbulence is adequate, it can be used without investing in special equipment and without increasing the system head.

Disadvantages include the fact that there is little or no opportunity to adjust the operation to suit treatment needs and that coagulant may cause pump corrosion.

 

 

Baffled Chambers

The baffled chanber provides turbulence to the water flowing over and under the baffles.  The primary problem is that turbulence is determined by the rate of the flow and normally cannot be controlled.

 

 

Pipe Grids

Grid systems of perforated pipes are used to disperse the coagulant into the water being treated.  These systems provide uniform distribution of coagulant but are susceptible to flow changes.