By using a flow sheet of surface water clarification, it is possible to show how the principles of coagulation and flocculation comes in play and applied towards wastewater treatment plant designs. Theoretically, with lower amount of suspended solids that are present in the feed stream, the more critical it is for proper mixing to take place to achieve good results. With high requirement in terms of achieving a good final effluent clarity, wastewater treatment plants should be designed in order to incorporate both flash mixing and flocculation mixing.
Basically, there are several ways to achieve flash mixing and the most common approaches are by using in-line hydraulic mixing or either high speed mixing carried out in a small basin. Coagulant is usually added before the flash mix but depending also on the feed stream, some designs can have the coagulant added in even before flash mixing take place. Proper mixing can be accomplished and assisted either through use of hydraulic pumps in open channels or somehow also using venturi flumes. Normally use of flash mixers is compulsory so that chemicals are homogenously dispersed throughout the wastewater prior to flocculation.
For flocculation, the mixing usually occurs in gently stirred compartments. Common designs based on the flocculation principle are the horizontal reel and another one is the turbine mixer. For example, a small and medium-sized municipality will incorporate a direct filtration put in place before flash mixing and flocculation process so that this will help towards better treatment of wastewater. Conventional-designs will have a reel-type flocculation units combined together with rectangular sedimentation basins working together in parallel. In certain cases, some treatment plants will employ use of variable speed motors to provide better process control while options using hydraulic flocculation can be considered as well but it will have limited use due to decreasing effectiveness at reduced flow rate.
To achieve better clarification on the final discharge, choice of hydraulic or mechanical mixing can be considered to improve and assist removal of heavy particles and suspended solids in flocculator and also subsequent process steps. Depending on required effluent discharge quality, sometimes use of sand filters maybe feasible but however, problems related to clogging and constant need for backflushing will discourage its application and its use will be limited to small-scaled treatment plants with low volume of wastewater generation.