Coagulation-flocculation studies of wastewaters

Although coagulation-flocculation processes have been practiced world-wide for almost a century in water treatment, several problems both in the theoretical and in the applied field have not been resolved yet. Especially interpretation of practical results with respect to governing coagulationflocculation mechanisms, the extrapolation of data from laboratory experiments, and the optimal design of a coagulation-flocculation plant have not been fully elucidated.The present studies have been carried out to develop a coagulation-flocculation treatment system for raw sewage of medium strength, as part of an advanced wastewater treatment plant and to elucidate some of the mechanisms involved in coagulation-flocculation of wastewater and effluent.Chapter 2 describes a simple method to measure the power input by an impeller in a vessel. There are many studies on impeller power dissipation in cylindrical vessels, with or without baffles and only a few on power dissipation in the square tanks used in practice. The experiments showed that the dimensionless Power number for square tanks can be estimated quite well using the Power number of a cylindrical, baffled tank provided with 4 radially placed baffles with a distance between two opposite baffles equal to the diameter of the square tank. Differences between the results of coagulation-flocculation experiments with different tanks and stirrers at equal power imput are probably due to a difference in distribution of energy different tipspeeds of the impellers studied.Chapter 3 deals with the strength of ferric hydroxide flocs. Floc size was determined by taking pictures of the flocs under agitation. The experiments showed that the addition of a coagulant aid increases floc strength. Other conclusions from these experiments were that floc break up takes place by erosion of small particles from the exterior surface layers of the flocs and not by fracture of flocs and that once disrupted flocs do not grow again.Chapter 4 gives a report of a modification of an equation for the kinetics of flocculation. The suggestion is made to use the floc volume fraction in flocculation rate expressions instead of the particle concentration.Chapter 5 describes coagulation-flocculation studies of sewage with lime in order to indentify and quantify the organic matter (TOC) fraction removed by magnesium hydroxide precipitation and the fraction removed due to calcium carbonate precipitation. The relation between removal of TOC and Mg(OH) ₂ precipitation showed a behaviour similar to that of an adsorption process.Chapter 6 gives a report on the optimalization of coagulant dose in coagulation-flocculation of sewage. Experiments showed that it is possible to estimate the required coagulant dose from the sewage characteristics . A comparison between a constant coagulant dose (as also applied in previous experiments) and a dose related to the ortho-phosphate concentration of the sewage leads to equal removal with respect to TOC and resulted in a coagulant reduction of 35%. This method of calculating coagulant dose in relation to characteristic sewage parameters is of great economic importance, especially for coagulation-flocculation plants, connected to a combined sewerage system.Chapter 7 deals with some design criteria for a coagulation-flocculation plant for sewage. The study shows that it is not disadvantageous to use only two flocculator chambers with downward tapered velocity of the impellers. The design criteria of the horizontal flow sedimentation tank only could be obtained in pilot plant experiments. The maximum surface loading in the pilot plant was 1.5 m h ^-1, with an average detention time of about 50 minutes in a sedimentation tank with a depth of 1.8 m.In chapter 8 a comparison is made of the effects of different coagulants on the removal of TOC and of some specific compounds from the treated sewage in a small pilot plant and in batch experiments. The experiments showed that the effluent of a coagulation-flocculation plant for raw sewage consists to a large extent of soluble, easily biodegradable organics.Chapter 9 shows that the application of the coagulation-flocculation process on wastewater of a sugar mill is very promising but that the succesful application of such a system also depends on the kind of biological wastewater treatment applied in conjunction with the physical-chemical plant.This also applies for the treatment of raw sewage, as coagulation-flocculation, even if followed by sand filtration and active carbon adsorption removes the easily biodegradable compounds very poorly.Mere coagulaion-flocculation can be applied succesfully to some (industrial) wastewaters containing a high amount of suspended solids, heavy metals or phosphorus.