Rapid Mixer
Rapid Mixer
In order for coagulation to properly occur, two stages are necessary: 1.) rapid mixing to disperse coagulant chemicals by violent agitation into the water being treated, and 2.) flocculation to agglomerate small particles into well-defined floc by gentle agitation for a much longer time (1) In order for floc to form most efficiently, rapid intimate mixing of the raw water and the coagulant must occur (1). The water treatment plant that we are investigating as part of this project does not contain a rapid mixer. The addition of a rapid mixer could possibly save the plant money because less chemicals would be needed.
For proper design of rapid mixing units, one must answer the following questions: 1.) What mixing device should be used and why? (2) 2.) How intense should the mixing be? (2) 3.) How long should the mixing be carried out? (2) 4.) How and where should the coagulant be added? (Dosage with dilute or concentrated coagulant feed? Point of coagulant addition near impeller or near water surface? A little before the in-line mixer or at its mouth?) (2) 5.) Should coaguant and flocculant aid be introduced at the same point in the process trainand have the same mixing conditions, or should they be introduced separately? (2)
According to the Great Lakes-Upper Mississippi River Board, the detention period should not be more than thirty seconds with mixing equipment capable of imparting a minimum velocity gradient (G) of at least 750 fps/ft (3).
G can be calculated from the equation (4) where G= root-mean-square velocity gradient μ = dynamic viscosity P= power input V= volume Rapid mix design is dependent on temperature because water viscosity changes with temperature (4).
Equipment basins should be equipped with devices capable of providing adequate mixing for all treatment flow rates (3). Static mixing maybe considered where the flow is relatively constant and will be high enough to maintain the necessary turbulence for complete chemical reactions (3). The coagulation and flocculation basins should be as close together as possible (3).
There are several designs that can be chosen from when designing a rapid mixer. These include mechanical devices ina dedicated basin, in-line blenders, hydraulic methods, air mixing, and induction mixing (4). Some of these systems can be coupled together based on the coagulation needs of the plant.
Mechanical mixers use paddle or paddle-type mechanical mixers work in a dedicated basin are most widely used in water treatment plants (4). This system works by blending incoming water with water that entered the basin previously throughout the entire basin, hence the name backmix reactor (4). Mechanical mixers are usually made with a vertical shaft drivenby a speed reducer and electric motor(4). Propeller-type mixers can be arranged so that flow is directed in any direction (4). The coagulant chemical is usually directed to the eye of the propeller on the suction side (4). One drawback to this system is that mechanical mixers are not normally provided with variable-speed drives, so if an energy input adjustment is needed, the propellers or paddles must be changed, or the shaft speed must be adjusted (4). Mechanical mixers can be placed in series, with each mixing basic injecting a different chemical into the water, based on the order that the chemicals need to be added (4). The detention time for each basin can be set, allowing for the needed reaction times for each given chemical. Detention times for mechanical mixers are usually 30 seconds to 10 seconds with a G value of 600 to 1,000 /s (4).
In-line blenders are another option when designing rapid mixers. Recent experience has shown that in-line blenders often provide more efficient rapid mixing (4). In-line blenders operate with very short detention times, less than 1 s, and at high G values (4). This nearly instantaneous dispersion of chemicals is an advantage to in-line blenders (4). However, this short detention time can also be a disadvantage for waters that require more reaction time and more than one chemical to be added (4). To evade this problem, an in-line blender can be placed in series with a mechanical rapid mix basin proceeding it, giving more detention time (4). In-line jet mixers can also be advantageous because either source water without added chemicals or partially destabilized source water can be used in the chemical injection system (4). A valved installed in the pump discharge line can control pumping rate and vary energy input for various plant flows and types of coagulating chemicals (4). In another variation on this design, multiple jets can be used that inject perpendicular to the flow in the pipe (4). This gives even a shorter retention time and a G value of about 1,000 /s (4). Mechanical in-line blenders consist of a propeller in the pipe and an electric drive system and provide rapid mixing of chemicals with water flowing in a pressure pipe(4). One advantage to mechanical in-line blenders is that they can be specified to provide any required any G value (4). Lastly, static in-line blenders use energy of the flowing liquid to produce mixing by using a a static configuration of intersecting bars, corrugated sheets, and plates (4). These mixers attampt to create flowpaths that result in consistent and precictable mixing performance (4).
Air mixing is another alternative that can be especially useful when aeration of the water is necessary anyways (4). These systems can be incorporated into existing water treatment systems in some cases (4). Energy applied by air injection may be varied by adjusting airflow, which is an advantage of this system (4). Air mixing is not widely used and inspection must be carried out to install it (4). A disadvantage to this system is that scum and flotable material can accumulate and become a problem; some coagulants and algae may increase scums (4).
Hydraulic mixing is a nonbackmix method that can sometimes be highly efficient (4). This type of mixing is achieved by using V-notch weirs, Parshall flumes, orifices, throttled valves, swirl chambers, and simple turbulence caused by velocity in a pipe (4). Since this system is stationary and the water mixes by flowing around various static shapes, the effectiveness of this system depends on how constant the flow is. The number of plant mixing modules in operation to maintain more or less constant flows can be varied when flows are not constant (4). Head loss across a throttled valve should not be more than 4 ft (4). If the head loss exceeds this amount, coagulants should be added to the flow downstream of the valve in the zone of decaying energy because excessive confined energy can shear polymers (4). Hydraulic mixing using a weir with a downstram baffle develops G values as a function of flow (4). If the volume (V in G equation) where turbulence dissipates is assumed to be constant, G may vary significantly, but if turbulence volume is assumed to be proportional to the flow Q, there is a lesser variation in G (4). In a weir mixer, chemicals are distrubuted at several points across the length of the weir (4). Weir mixers are usually only used at plants with less than 40 mgd (4).
Induction mixing serves as an all-in-one chemical feed, mixing, and control system (4). This mixing system has typically served for introducing gaseous disinfectants into wastewater, but the concept can be considered for metal coagulants in drinking water (4). Induction mixers use a vacuum via a propeller system to pull coagulant chemicals into the mixing system and inject them into the water stream (4). The chemical reactions that are required for charge neutralization occur within seconds, and the induction system disperses the coagulant into the raw water stream very rapidly (4). Induction mixing takes place in either an in-line piping system or submerged in a channel (4). An advantage of this system is that power costa can be saved since the chemical feed, mixing, and control system are combined into one unit (4).
(1) Spellman, Frank R. Water and Wasteater Treatment Plant Operations Second Edition. CRC Press. 2009.
(2) Srivastava, Ravindra Mohan. Mixing, Al(III) Chemistry, And The Rapid Mixing Process In Water Treatment. Doctoral Thesis University of Illinois at Urbana-Champaign. 1993.
(3) Recommended Standards for Water Works. 2007 Edition. Great Lakes Upper Mississippi Board (GLUMRB). http://10statesstandards.com/waterstandards.html
(4) Baruth, Edward E., Technical Editor. American Water Works Association American Society of Civil Engineers. Water Treatment Plant Design Fourth Edition. 2005.