Following or during coagulation another process known as flocculation occurs. In flocculation the primary means of attraction is by weak van der Wall’s forces. These forces are encouraged through increased retention time and decreased mixing energy. The process yields particles that are visible to the naked eye, unlike coagulation which yields microscopic microflock’s.
In flocculation rapid mixing is not promoted, instead high retention time and low energy agitation is desirable. The low energy agitation promotes greater microfloc/particle contact and in turn allows the formation of larger suspended particles known as pinfloc’s. These pinfloc’s continue to grow as retention time increases. The resulting macrofloc’s are visible to the naked eye, and can be removed from the medium by mechanical or gravitational means. Flocculant reagents can be added to increase strength, weight and decrease settling time. They can be anionic, cationic or non-ionic. Care should be taken when using a coagulant of the opposite charge. This will avoid the reagents neutralising charges and becoming ineffective.
The effectiveness of flocculation is greatly dependant on the mixing energy and velocity of the medium. The weak inter-molecular van de Waal’s forces holding the flocs together can be sheered or broken easily when subjected to a high energy environment. In general the greater the velocity and higher the mixing energy, the smaller the floc’s.
The flocculation process is also affected by pH, salts, alkalinity, turbidity, temperature and degree of mixing.
Retention time for flocculation varies greatly but typically is longer than 15 minutes. Through extensive jar testing an optimal time can be found.
To find the theoretical retention time, use the equation for retention time (also known as detention time)
Retention time = basin volume / volume per minute flow
Obtain the rise rate from,
Rise rate = volume per minute flow / net up-flow area of basinGreen, Don, and Robert Perry. Perry’s Chemical Engineers’ Handbook. McGraw-Hill Companies, 2007. G.L.Hayett. “Engineering Handbook: Technical Information.” New York: Industrial Press Inc., 2004. http://www.uacg.bg/filebank/att_1846.pdf http://www.who.int/water_sanitation_health/hygiene/emergencies/fs2_13.pdf