GB2158429A

GB2158429A - Flocculation and clarification of liquids

Info

Publication number: GB2158429A
Application number: GB08411843A
Authority: GB
Inventors: Ross Gregory
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1984-05-09
Filing date: 1984-05-09
Publication date: 1985-11-13
Also published as: GB8411843D0

Abstract

A method of clarifying a liquid comprises adding to the liquid a coagulant and a plurality of pieces of chemically inert solid material having a large surface area. The pieces are preferably each a body of plastics material having a substantial voidage therein with an extensive area of access to said voidage from the outer surface.

Description

SPECIFICATION Flocculation and clarification of liquids This invention relates to chemical coagulation and flocculation for the clarification of liquids.

There is a great range of methods for clarifying liquids, for removing solids in suspension. A common method is to dose either or both coagulating and flocculating agents as a preliminary to the physical removal of the suspended solids, for example, by sedimentation, flotation or filtration.

"Coagulation" is the process involving the addition of a coagulant, usually an aluminium or iron salt, under conditions which will lead to the formation of a "floc" consisting of insoluble hydrolysis products of the coagulant together with the impurities. The mechanism of this process is complex: chemical precipitation is involved in the removal of humic substances, for example, from water, while removal of particulates involves co-precipitation and chemical destabilisation. The definition includes only that mechanism of floc growth which is brought about by what is termed perikinetic transport whereby particle collisions are effected by diffusion through micro-dimensions. The factors involved in the stages of coagulation influence the resultant floc quality and ease of its removal from the suspension.

"Flocculation" is the process of floc growth whereby the products of coagulaton are agglomerated under conditions in which particle diffusion is brought about by either or both hydraulic and mechanical mixing, otherwise known as "orthokinetic flocculation". The resultant size and shape of floc created by this process influences the efficiency of the subsequent separation process. Flocculation is thus mostly influenced by the engineering of the process and can be considerably time-dependent.

Flocculation, besides being time-dependent, is also greatly influenced by the volumetric concentration of the particles and the velocity gradient of the liquid at the particle surface.

Various methods are used to improve the rate of flocculation and floc qualities, such as size, shape, density and structural strength, and hence ease of floc removal. These generally concern the addition of polymers to act as flocculants or the addition of weighting or ballasting agents. Weighting agents work by increasing the volumetric concentration of the particles and so enhance flocculation and by adding weight to the flocs which increase their density and structural strength. This enhances the ease of floc removal from the suspension.

We have now found another way of enhancing flocculation, which is simple and economic to operate. In accordance with a feature of the present invention, we have found that flocculation can be enhanced by adding to the suspension before, during or after the addition of a coagulant, pieces of solids material which are chemically inert but which provide a large surface area.

A wide range of materials can be used but we prefer those described in our copending U.K. patent application no. 8330781 (to which reference should be made) which including matting, usually of bonded synthetic fibres, or rigid or compressible porous pieces of metal, plastic or natural substances, in the form for example of foamed, sintered, felted or woven structures.

When pieces of material are used which are porous, then the coagulation and flocculation also leads to floc accumulating within them, further enhancing the flocculation at and within the pieces of material.

The pieces of material used will preferably always be larger than the floc which will be formed, and normally they will be of a small but conveniently handlable size. For example, for some purposes pieces of matting or foam of about 5cm or less in size are very suitable.

Foam bodies of roughly this size are used (for a different purpose) in the "Captor" activated sludge process and these and other similar bodies are described, for example, in U.K.

specification no. 20061 81 A to which reference should be made.

The method of the invention can be practised in a variety of ways, both on a batch and on a continuous basis. For example, in a batch procedure, pieces of material may be stirred into the coagulant-containing suspension and, after stirring, allowed to settle. We have found that the supernatant so formed is of improved quality over that formed when no material pieces are added. In one particular procedure, we used pieces of material 13cm2 x Icm and in an amount of from 1 to 5% by volume. However, the optimum size and amount of pieces to be used will vary and can be determined, in any particular case, by routine trial and experiment.

The pieces of material can be removed from the water being treated in any convenient manner, e.g. by scree ing, sedimentation, flotation, or filtration. The removed pieces can sometimes be re-used without cleaning, or on other occasions will preferably be cleaned to remove entrapped floc before re-use.

1. A method of clarifying a liquid compris irig adding to the liquid a coagulant and a plurality of pieces of chemically inert solid material having a large surface area.

2. A method as claimed in claim 1, wherein the additions are performed substantially simultaneously.

3. A method as claimed in claim 1, wherein the addition of the plurality of pieces is performed after and/or before the addition

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Flocculation and clarification of liquids This invention relates to chemical coagulation and flocculation for the clarification of liquids. There is a great range of methods for clarifying liquids, for removing solids in suspension. A common method is to dose either or both coagulating and flocculating agents as a preliminary to the physical removal of the suspended solids, for example, by sedimentation, flotation or filtration. "Coagulation" is the process involving the addition of a coagulant, usually an aluminium or iron salt, under conditions which will lead to the formation of a "floc" consisting of insoluble hydrolysis products of the coagulant together with the impurities. The mechanism of this process is complex: chemical precipitation is involved in the removal of humic substances, for example, from water, while removal of particulates involves co-precipitation and chemical destabilisation. The definition includes only that mechanism of floc growth which is brought about by what is termed perikinetic transport whereby particle collisions are effected by diffusion through micro-dimensions. The factors involved in the stages of coagulation influence the resultant floc quality and ease of its removal from the suspension. "Flocculation" is the process of floc growth whereby the products of coagulaton are agglomerated under conditions in which particle diffusion is brought about by either or both hydraulic and mechanical mixing, otherwise known as "orthokinetic flocculation". The resultant size and shape of floc created by this process influences the efficiency of the subsequent separation process. Flocculation is thus mostly influenced by the engineering of the process and can be considerably time-dependent. Flocculation, besides being time-dependent, is also greatly influenced by the volumetric concentration of the particles and the velocity gradient of the liquid at the particle surface. Various methods are used to improve the rate of flocculation and floc qualities, such as size, shape, density and structural strength, and hence ease of floc removal. These generally concern the addition of polymers to act as flocculants or the addition of weighting or ballasting agents. Weighting agents work by increasing the volumetric concentration of the particles and so enhance flocculation and by adding weight to the flocs which increase their density and structural strength. This enhances the ease of floc removal from the suspension. We have now found another way of enhancing flocculation, which is simple and economic to operate. In accordance with a feature of the present invention, we have found that flocculation can be enhanced by adding to the suspension before, during or after the addition of a coagulant, pieces of solids material which are chemically inert but which provide a large surface area. A wide range of materials can be used but we prefer those described in our copending U.K. patent application no. 8330781 (to which reference should be made) which including matting, usually of bonded synthetic fibres, or rigid or compressible porous pieces of metal, plastic or natural substances, in the form for example of foamed, sintered, felted or woven structures. When pieces of material are used which are porous, then the coagulation and flocculation also leads to floc accumulating within them, further enhancing the flocculation at and within the pieces of material. The pieces of material used will preferably always be larger than the floc which will be formed, and normally they will be of a small but conveniently handlable size. For example, for some purposes pieces of matting or foam of about 5cm or less in size are very suitable. Foam bodies of roughly this size are used (for a different purpose) in the "Captor" activated sludge process and these and other similar bodies are described, for example, in U.K. specification no. 20061 81 A to which reference should be made. The method of the invention can be practised in a variety of ways, both on a batch and on a continuous basis. For example, in a batch procedure, pieces of material may be stirred into the coagulant-containing suspension and, after stirring, allowed to settle. We have found that the supernatant so formed is of improved quality over that formed when no material pieces are added. In one particular procedure, we used pieces of material 13cm2 x Icm and in an amount of from 1 to 5% by volume. However, the optimum size and amount of pieces to be used will vary and can be determined, in any particular case, by routine trial and experiment. The pieces of material can be removed from the water being treated in any convenient manner, e.g. by scree ing, sedimentation, flotation, or filtration. The removed pieces can sometimes be re-used without cleaning, or on other occasions will preferably be cleaned to remove entrapped floc before re-use. CLAIMS

3. A method as claimed in claim 1, wherein the addition of the plurality of pieces is performed after and/or before the addition of coagulant.

4. A method as claimed in any one of the preceding claims, wherein the pieces are each a body of plastics material having a substantial voltage therein with an extensive area of access to said voidage from the outer surface.

5. A method as claimed in claim 4, wherein each body is a pad of reticulated foamed polyether.

6. A method as claimed in claim 4, wherein each body is an open woven structure of a plastics filament.

7. A method as claimed in any one of claims 1 to 3, wherein the pieces are each a small sheet of matting.

8. A method as claimed in claim 7, wherein the matting is made from bonded synthetic fibres.

9. A method as claimed in any one of the preceding claims, wherein each of the plurality of pieces of material is 5 cm or less in size.

10. A method as claimed in claim 5, wherein each piece of material is dimensioned in the relgion of 13 cm2 x 1 cm.

11. A method as claimed in any one of the preceding claims, wherein the plurality of pieces of material are added in an amount of between 1 and 5%, based on their overall dimensions, by volume of the liquid.

12. A method of clarifying a liquid substantially as described herein.

13. A method of producing enhanced flocculation in a liquid comprising adding a plurality of pieces of chemically inert solid material having a large surface area.