Agitation in Water/ Waste Water Treatment
Agitation in water treatment is required for three unit processes.
- Dosing Tanks,
- Coagulation tank or flash mixer,
- Flocculation tank.
The process of agitation can be achieved by
- Mechanical agitator,
- Jet mixer,
- Static Mixer or in-line mixer,
- Air purging with diffused air system.
Mechanical agitator comprising of motor, gear box, shaft and impeller, creates a thrust at the bottom of the tank to lift water through sideways. Impeller height from tank bottom is generally one third of tank diameter. Impeller diameter app. 1/3rd to 1/5th of tank diameter in speed range 100 – 200 rpm range. About 1/8th in rpm 1400.
Impeller geometry – For dosing tank and flash mixers, it is generally marine propeller, or hydrofoil or aerofoil, all showing similar BHP, thrust and flow characteristics.
Impeller tip linear speed arrived at by pi X Dia X RPM / 60 Mtr/Sec should be better selection criteria than RPM itself. For lime dosing, flash mixers, etc. tip speed may be in range 2-3 Mtr/ Sec. In case of polymers, bio cultures, enzymes, etc, which can break at higher speed, 1-1.5 Mtr/ Sec is ideal. For flocculators, it is 0.6- 0.9 Mtr/Sec.
For agitators in dosing tanks using chemicals to be easily dissolved, 3-5 minutes of agitator run is good enough. For tanks using settable particles, continuous stirring is desired. In case of flash mixers, liquid should be drawn across the impeller 4-5 times towards its path to settling or flocculation. Impeller dia and speed can be designed to achieve that to attain reasonably proper coagulation.
Baffle walls are suggestable for circular tanks to prevent useless vortex, and in case of HDPE tanks make Sintes/ Patton, etc, off-centre mounting of agitator app 1/5th of tank dia is good.
Jet mixing employing a pump (with flooded suction) and recycling with delivery point as jet so as to keeping settable particles suspended and reasonable motion throughout the liquid. Jets can be produced by using nozzles (available with hardware shops) with app. half the pipe dia producing a good speed and thrust. It should be immersed in water, at centre pointing downwards and at 1/3rd (of tank dia) height from tank bottom. If to be placed at tank edge, expert guidance is solicited. Water jet goes forward with a decreasing speed and ingressing water from surrounding, disrupts the sediments and through the tank periphery, goes upward to keep the mixture homogeneous.
Their affectivity with good conscience, is better than mechanical mixers at far lower cost. Very much under-rated by design people.
Static mixer is inline mixer, motionless, powerless. Employing some plates, baffles, helical elements in the pipe, a pump thrusts the liquid for necessary action. It is a good substitute for flash mixers in water treatment applications. Dosing of necessary chemicals is done prior to entering or at static mixer. The molecular chains are disturbed and ready to form new ones being pushed through the geometric constrictions. Not much effective for flow speed below 3 Mtr/Sec across the pipe. Pressure drop can be estimated by the baffle geometry and designed to perform the objective solicited.
Schematic Static Mixer.
Air purging has little use in dosing tanks or flash mixers. It is because, bottom sediments cannot be lifted up and sufficient thrust is not produced to keep a homogeneous mixture/ suspension. Equalization tanks do find their user to keep it equalized and little aerated. Flocculation process is enhanced by coarse bubble air purging. Air volume 2 Cu Mtr/hr FAD per sq mtr area is good for flocculation tank. As water goes up at speed 1-2 Mtr/Sec with air, water also comes down slowly 0.2-0.5 Mtr/Sec, and causes soft collision among particles to get sticked. Diffusers placed intelligently will result in good floc formation.
Flocculation in perikinetic zone occurs due to Brownian motion of solid particles in the liquid. Here particles gets attached to each other 10 to 100 times in diameter (10 power3-4 times volume and weight) from range 10 power -6 to 10 power -3 range. That thickening is also not good enough for lightly packed flocs to form settlable particles. Coagulants help but we do not have any weapon to enhance this process.
After adding coagulants, loading agents and polymers as sticky fibres, orthokinetic floc formation takes place. Here the added agents gets sticked to each other and with particles in the liquid to form particles in dia ranging from 1/100th mm to 1/10th mm. Mechanical aid helps in coalition between them. Here we want flow of liquid across each other very slowly. Ideal flow speed is 0.3 Mtr /sce to 0.7 Mtr/sec. A more vigorous flow will cause disruption of flocs. It is found by experiments that 6-8 % of the tank (cross sectional) area should be covered with pitched gate type impeller to get better performance. The loosely packed floc formed is usually of sp gr 1.01-1.05 settles down and gets thickened at the bottom.
About the author, he is a mechanical engineering graduate passed out in 1994. Presently runs a Kolkata based company named Agitators & Mixing Solutions. He is thorough with the international literatures and developments in this and related fields.