Coagulation pressure, calculation

An important case is the interaction between two identical parallel surfaces of two infinitely extended solids. It is, for instance, important to understand the coagulation of sols. We can use the resulting symmetry of the electric potential to simplify the calculation. For identical solids the surface potential -ipo on both surfaces is equal. In between, the potential decreases (Fig. 6.9). In the middle the gradient must be zero because of the symmetry, i.e. d f(f x/ 2)/df = 0. Therefore, the disjoining pressure in the center is given only by the osmotic pressure. Towards the two surfaces, the osmotic pressure increases. This increase is, however, compensated by a decrease in the Maxwell stress term. Since in equilibrium the pressure must be the same everywhere, we have ... 

Example 6.2 By considering the criterion for effective mixing, the volume of a rapid-mix tank used to rapidly mix an alum coagulant in a water treatment plant was found to be 6.28 m with a power dissipation of 3.24 hp. Assume air is being provided at a rate of 1.12 m per m of water treated and that the detention time of the tank is 2.2 min. Calculate the pressure at which air is forced into the diffuser. Assume barometric pressure as 101,300 nW, the depth from the surface to the diffuser as 2 m, and the temperature of water as 25°C. 

Example 17.11 A flow of 25,000 m /d of treated water is to be disinfected using chlorine in pressurized steel cylinders. The raw water comes from a reservoir where the water from the watershed has a very low alkalinity. With this low raw-water alkalinity, coupled with the use of alum in the coagulation process, the alkalinity of the treated water when it finally arrives at the chlorination tank is practically zero. Calculate the amount of alkalinity required to neutralize the acid produced during the addition of the chlorine gas. 

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