In this week, we are going through whole processes of water treatment, from intake to a final stabilizing process, with basic mechanistic ways with previously discussed chemical parameters, such as pKa, logKow, logKaw, and others. Water treatment plant is comprised of various processes; intake of source water, including surface water and groundwater, pre-treatment, such as coagulation and sedimentation, main processes, including sand filtration, activated carbon adsorption, and membrane filtration, and post-treatment processes, including disinfection and stabilizing step. Within intaked source waters, there are many different contaminants, including particle, colloid, toxic ion, organics, salts for desalination, micro organisms, such as virus, bacteria, and protozoa, and etc.
Coagulation followed by sedimentation processes are mainly for removal of both particles and colloids. Some of micro-organisms and pathogenes, such as bacteria and prozoa may be categorized into particles that can be relatively easily removed through settlement because of their sizes, normally bigger than micron. Colloids with size ranges lower than 0.1 micron are problematic as those are suspended thus difficult to remove by sedimentation. Resistance of colloids to sedimentation are explained by relatively smaller size and also repulsive force between the surface of colloids having the same electrostatic charges; for example, negatively charged colloid surface exerted the repulsive force to the other colloid having the same charge. To prompt colloids to settle down to be removed, chemical coagulants that can help bridging between the two same charged surfaces, can be used with either ferric or aluminum. Aggregation of previously separated colloids enables colloids to settle down by forming a big aggregate (named floc), which is called destabilization as opposed to stabilization (i.e., stable colloids from repulsion) (handout page 28). This is why we studied the solubility product concept (i.e., Ksp). To better understand the solubility product concept of iron or aluminum based coagulants, you can practice the example described in handout page 17, also refer to handout page 28. Please draw the corresponding pC-pH lines and a combined curve after setting up the four equations of pC against pH which are derived from the Ksp expressions. The upper region of the combined curve represents precipitated solid and the lower region dissolved species, thus, for effective coagulation, certain amounts of coagulants have to be inserted depending on pH at the concentrations similar to values of the total solubility line (handout page 28). Too much or less dosages of coagulants may lead to low coagulation efficiency because of big solid/precipitation size or too diluted dissolved species.
Here, it is desirable for us to discuss about the colloid surface to better understand the repulsion between the colloids and roles of coagulants (Fe, Al). Colloids may include clays which are comprised mostly of silicon and other atoms, including K, Ca, Mg, Na, etc. Silion oxide, i.e., silica, is ionized positve or negative depending upon pH; posive at lower pH and negative at higher pH. With being charged, water molecules surround the surface of silica because of water polarity and subsequently hydrogen bonding. The bonded waters at the surface do not flow thus the layer with the waters are called as the fixed layer. In the region somewhat away from the surface beyond the first water molecule layer, there is another layer with divalent ions (with the negatively charged surface, Ca, for example). Around this second layer or beyond the line, there is a conceptual line being defined as the shear plan at which zeta potential exist, as opposed to the surface potential (handout pages 25~26, and 27). The zeta potential is a potential at the boundary between the fixed and movable (diffused) layer, thus, we can measure this by introducing electric field so that we have movement of the collid then calculate the zeta potentil by using a relationship between the velocity of colloid and zeta potential with unit of voltage. Zeta potential exhibits less negative charged than a surface potential which is resulted by screening of surface potential influence with opposite charged ions (e.g., calcium).