Aid To Coagulation
Particulate removal begins with coagulation, which involves the addition of a chemical to neutralize the charges on particles and facilitate their agglomeration during the slow mixing provided in the flocculation step. Coagulation also helps prepare the particles for filtration.
Many of the particles that must be removed in water treatment are very small and are referred to as colloids. These particles carry an electric charge on their surface: because of their small size this surface charge has an important influence on their behavior. Because of the particles carry the same charge (usually negative), they tend to repel one another. The purpose of coagulation is to neutralize these charges so that the particles can combine to form large agglomerations if they contact one another. Larger particles settle much faster, and in addition neutralized particles are much more susceptible to filtration. Coagulation is always followed by a period of low-energy mixing called flocculation, which promotes particle-particle contact and the growth of particle aggregates known as flocs. If sedimentation is used; flocculation will occur in a separate chamber, if filtration is used; flocculation may take place in the liquid above the filter, or even in the pores of the filter.
The most common chemical coagulants include aluminum salts (e.g. alum, and polyaluminum chloride), iron salts, and organic polymers.
Coagulation & Ozonation
Particulate removal can be accomplished through the coagulating effects of ozonation. This phenomenon may take the form of a wide range of secondary effects, such as a shift in particle size distribution towards larger sizes; the formation of colloidal particles from dissolved organic matter; the improved removal of TOC (Total Organic Carbon) or turbidity during subsequent settling, flotation, or filtration; a decrease in coagulant dose necessary for achieving desired effluent turbidity or TOC; an increase in floc settling velocities; and extended filter run length due to slower head loss buildup or delayed breakthrough. These are called secondary effects because they do not occur instantly at the moment of ozone reaction, but begin to manifest themselves. What this all means is that the clarity of the water will improve and material will settle out of suspension. This benefit of ozonation may be achieved at a relatively low ozone dose of 0.5 – 1.5 mg/L.
Mechanisms for the Coagulating Effects of Ozone
- Loss of Organic Coating
Ozone strips the organic coating from clay particles returning them to their destabilized state. These particles would be relatively free of the high negative surface charges and thick adsorbed organic layers that serve as a barrier to particle agglomeration. Therefore the particles would flocculate much more easily. It should be noted that this mechanism cannot account for a significant TOC removal since only a minor fraction of the organic content in most natural waters is associated with clay particles.
- Increased Aluminum Complexation
Carboxylic and phenolic functional groups in natural organic matter (NOM) form complexes with aluminum oxide surfaces, forming organic matter-aluminum surface associations. Removal of DOC by activated alumina is increased with increasing numbers of acidic groups up to perhaps an optimum of three. Ozonation can therefore increase the degree of adsorption of NOM to activated alumina and to aluminum hydroxide by increasing the concentration of oxygenated functional groups (especially carboxylic acids).
- Increased Calcium Complexation
An increase in the number of carboxylic acid groups may also lead to an increase in the degree of calcium complexation, resulting in direct precipitation of the organic matter. An increase in the complexation of Calcium may also lead to improved adsorption of organic to alum floc, as well as to other precipitating metal oxide surfaces such as MnO2.
- Organic Polymerization
Ozone induced oxidative coupling may also play a role in the coagulating effects of ozone. Oxidative coupling involves the joining of small soluble molecules via C-C and C-O bonds to give large fulvic acid type molecules and eventually the larger and relatively insoluble humic acid type molecule. Oxidative coupling is thought to be responsible for the presence of nearly all colored substances. These reactions increase the molecular size improving the removability of organic material by coagulation
In addition to the ability of ozone to immediately oxidize some organohalide precursors (TOX), it may also improve the removal of organic precursors in subsequent coagulation. Preozonation can improve the coagulation of THM precursors through both subsequent coagulation and filtration.