Method of and apparatus for continuous flow flocculation and clarification

#The invention relates generally to a method of and apparatus for continuously removing dispersed particles which can be flocculated from a liquid containing said particles, and more particularly, to a method of and apparatus for continuously removing solid colloidal particles, ranging in size from tenths of a micron to tens of microns, from wastewater containing said solid colloidal particles to yield clarified water.

#By way of background as to this specification, attention is called to the Doctoral thesis of the inventor Delichatsios, the work leading to the thesis being done at the Massachusetts Institute of Technology under the supervision of the inventor Probstein. The thesis was deposited in the M.I.T. library system on Mar. 3, 1975 and is of record in said application Ser. No. 446,666. Attention is also called to a journal article entitled "Scaling laws for coagulation and sedimentation," Journal of Water Pollution Control, Vol. 47, No. 5, May 1975, pp. 941-949.

#One method that has been used for removing particles from wastewaters is by flocculating the particles into larger size particles in a flocculation tank in advance of a settling tank wherein clarification is accomplished. Flocculation is usually induced by mixing in the flocculation tank a suitable amount of coagulant (e.g., metal salts) and/or coagulant aid (e.g., organic polymers) by means of a stirrer paddle. Because the flow in the flocculator is both nonuniform and nonhomogeneous, regions of different flow conditions exist, so that deposition and breakup occur simultaneously. This has an adverse effect on the flocculation efficiency and increases the needed flocculator volume.

#The detailed structure and method of operation of the invention is described more clearly with reference to the attached drawings wherein:

#FIG. 1 shows schematically and in block diagram form apparatus in accordance with the present invention and includes a pipe wherein flocculation occurs;

#FIG. 2 is a side section view of a rapid mixing device for the injection of coagulant and/or coagulant aid into the system of FIG. 1; and

#Turning now to FIG. 1, there is shown at 101 apparatus for continuously removing particles that can be flocculated from a liquid containing such particles. The apparatus 101 comprises a relatively long, composite pipe that is labeled 102 and that is composed of three circular pipes of equal diameter D. To simplify this explanation, the three pipes are numbered 8, 9 and 10. Wastewater containing colloidal solid particles ranging in size from tenths of a micron to tens of microns is continuously introduced at the input end labeled 1 of the pipe 102. It is assumed that any larger size particles have been previously removed, for example, by passing the wastewater through a conventional screen and grit removal chamber (not shown in FIG. 1). The required coagulant and/or coagulant aid is continuously injected from a metering device 2 into pipe 102 at station 3 through suitable piping and near the input end 1. Complete mixing of the coagulant and/or coagulant aid, which acts to effect or aid coagulation with said flow is achieved in the pipe 9, at the end of which recirculated dispersed floc is continuously added at a station 4 through a suitable pipe 5. Effective flocculation starts at the station 4, and the small solid particles contained in the wastewater are flocculated in the pipe 10 with the big flocs of the recirculated stream. The flow rate of the recirculated dispersed floc is controlled so that flocculation to the required degree is accomplished before the mixture is discharged through output end 11 of the pipe 102 into the conventional settling tank 13. The clarified effluent is continuously discharged through suitable piping 12. The settled floc (sludge) is continuously withdrawn from the settling tank 13 at discharge station 14. The quantity of dispersed floc required for recirculation is withdrawn through the input end 7 of the pipe 5 by a pump 6, and is conducted through the pipe 5 to the injection station 4. The diameter of the pipe 5 is taken such that the stable floc size corresponding to the flow conditions in this pipe is less than the stable floc size characteristic of the flow conditions in the pipe 10.

#In FIG. 2 there is shown a detail of a device 103 for continuously injecting the coagulant and/or coagulant aid at the station 3 in FIG. 1, to ensure rapid mixing with the flow in the pipe 9. The flow rate of said coagulant and/or coagulant aid is so controlled by the appropriate metering device 2 that it is reduced by an amount equal to the quantity of coagulant and/or coagulant aid concurrently recirculated through the pipe 5 with the recirculated dispersed floc. A suitable entrance design is also provided for the continuous injection of said floc from the pipe 5 at station 4 into the pipe 10.

#In FIG. 3, a typical relationship between particle sizes dispersed in a liquid medium, such as a wastewater containing colloidal solid particles, and the rate of turbulent energy dissipation per unit mass is given with respect to flocculation, breakup and settling of said particles. FIG. 3 also shows the size of the microscale of the turbulent eddies in the flow, for comparison with the size of the said particles. For turbulent pipe flow, the effective rate of energy dissipation per unit mass, .epsilon., is given approximately by: