Laserfiche WebLink
1 <br />H <br />1 <br />1 <br />1 <br />J <br />1 <br />1 <br />1 <br />I� <br />1 <br />1 <br />1 <br />1 <br />1 <br />r <br />Appendix 11-B: Background Information on <br />Chemical Treatment <br />Coagulation and flocculation have been used for over a century to treat water. It is used <br />less frequently for the treatment of wastewater. The use of coagulation and flocculation <br />for treating stormwater is a very recent application. Experience with the treatment of <br />water and wastewater has resulted in a basic understanding of the process, in particular <br />factors that affect performance. This experience can provide insights as to how to most <br />effectively design and operate similar systems in the treatment of stormwater. <br />Fine particles suspended in water give it a milky appearance, measured as turbidity. <br />Their small size, often much less than 1 pm in diameter, give them a very large surface <br />area relative to their volume. These fine particles typically cant' a negative surface <br />charge. Largely because of these two factors, small size and negative charge, these <br />particles tend to stay in suspension for extended periods of time. Thus, removal is not <br />practical by gravity settling. These are called stable suspensions. Polymers, as well as <br />inorganic chemicals such as alum, speed the process of clarification. The added chem- <br />ical destabilizes the suspension and causes the smaller particles to agglomerate. The <br />process consists of three steps: coagulation, flocculation, and settling or clarification. <br />Each step is explained below as well as the factors that affect the efficiency of the pro- <br />cess. <br />Coagulation: Coagulation is the first step. It is the process by which negative charges on <br />the fine particles that prevent their agglomeration are disrupted. Chemical addition is <br />one method of destabilizing the suspension, and polymers are one class of chemicals <br />that are generally effective. Chemicals that are used for this purpose are called coagu- <br />lants. Coagulation is complete when the suspension is destabilized by the neutralization <br />of the negative charges. Coagulants perform best when they are thoroughly and evenly <br />dispersed under relatively intense mixing. This rapid mixing involves adding the coagu- <br />lant in a manner that promotes rapid dispersion, followed by a short time period for <br />destabilization of the particle suspension. The particles are still very small and are not <br />readily separated by clarification until flocculation occurs. <br />Flocculation: Flocculation is the process by which fine particles that have been destabil- <br />ized bind together to form larger particles that settle rapidly. Flocculation begins naturally <br />following coagulation, but is enhanced by gentle mixing of the destabilized suspension. <br />Gentle mixing helps to bring particles in contact with one another such that they bind and <br />continually grow to form "flocs." As the size of the flocs increases they become heavier <br />and tend to settle more rapidly. <br />Clarification: The final step is the settling of the particles. Particle density, size and <br />shape are important during settling. Dense, compact flocs settle more readily than less <br />dense, fluffy flocs. Because of this, flocculation to form dense, compact flocs is <br />2014 Stormwater Management Manual for Westem Washington <br />n <br />Volume 11 - Appendix B - Page 419 <br />