CA1111158A

CA1111158A - Purification of waste water

Info

Publication number: CA1111158A
Application number: CA278,336A
Authority: CA
Inventors: Dieter Disselbeck; Karl-Heinz Ott
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1976-05-15
Filing date: 1977-05-13
Publication date: 1981-10-20
Also published as: BR7703125A; SE7705632L; CH637905A5; DK212277A; JPS52139257A; ES458813A1; NO771696L; EG12563A; AU509536B2; FR2351060A1; NL7705328A; IT1086194B; FR2351060B1; AU2517277A; GB1583074A; FI771542A; AT357115B; ATA345977A

Abstract

A B S T R A C T
This invention describes a process for purifying waste water including reduction of the chemical and biological oxygen demands needed in connection with any further purification thereof which comprises the steps of admixing the waste water with one or more flocculating agents to effect separation of the majority of the colloidal components and passing the resultant admixture under gravity through a fabric located in a sus-pension device whereby a combined filter is formed comprising a) the fabric and b) a primary filter layer of alluvial solids, thereby to effect pre-clarification of the waste water and the removal of the majority of the particles of solids including flocculated colloidal components from the aqueous phase, and an apparatus wherein the purification of the waste water is processed.

Description

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"Improvements in or relating_to the purification of waste water"
This invention relates to a process and apparatus suitable for use in the purification of waste water.
A conventional process for purifying communal and in-dustrial waste water consists of the steps of mechanical puri-fication (preclarification), biological purification, subsequent clarification and sludge treatment. In the mechanical purifi-cation stage, impurities are removed by means of sieves, rakes, sand traps and/or by sedimentation in preclarifying tanks. In the biological step, colloids and dissolved materials are con-verted into a removable form by microorganisms and may be then separated off by sedimentation in a subsequent clarifying tank.
The sludges with a high water content which are formed in the preclarification and subsequent clarification steps are treated by various methods in order to separate off excess water and stabilize the sludge ingredients, thereby to obtain a residue which can then be removed without endangering the environment.
Owing to the relatively slow process of sedimentation in the preclarifying and subsequent clarifying tanks, the re-moval of the sediment from the preclarification and subsequent clarification steps and the treatment of those sludges with high water contents in thickeners, drying beds, digestion towers, and by mechanical dehydration processes, etc., conventional clarifying plants are characterised by expensive treatment tanks and constructions, generally made from concrete.
Waste water from, for example, clearing and collecting tanks, presents another problem. It generally has the consis-tency of a thin sludge and has a high chemical oxygen demand (COD), for example, of up to 20,000 mg/l or more. It can only be processed by conventional methods at considerab]e expense;
the problem of disposing of it in an enviromentally safe way has hitherto remained unsolved. Thus, according to present practice, such waste water is used in agriculture or stored in sludge beds or the like.
~'or the reasons of water conservation, hygiene and odour this practice is subject to an increasing number of restrictions.
A process is also known for removing small quantities of impurities, such as turbidity or colouring from soiled water, by adding polyelectrolytes, flocculating and filtering agents to the water. The process is carried out under conditions wherein the flocculating agents togethcr with the impurities are precipitated onto the filtering agent in the form of a coating. Suitable filtering agents inclllde diatomaceous earth, perlite, other silicon-containing compounds, charcoal and fibrous material such as asbestos or cellulose. The water .~ .

pre-treated in this manner is pumped by over- or underpressure through standard commercial filter media, to which initially a layer of filtering agent has been applied. The filtering agent becomes coated with the flocculating agents and con-tinuously builds up a porous filter cake whereby a substantial part of the suspended particles is removed from the water.
As indicated above this process may be used for removing turbidity in waste water. In a modified form, i.e. by using only flocculating agents such as aluminium sulphate which cause a high degree of flocculation, it is possible to substantially clarify industr;al waste water containing turbidity in the form of a stable oil emulsion by means of standard commercial filters.
We have now found it possible to purify waste water by a process not requiring the expensive treatment plants of the conventional processes.
Thus according to one aspect of the present invention there is provided a process for purifying waste water including reduction of the chemical and l-iological oxygcn dclnallds needed in connection with any Eurther purification thereof which comprises the steps of admixing the waste water with one or more flocculating agents to effect separation of the majority of the colloidal components and passing the resultant X

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admixture under gravity through a fabric located in a suspension device whereby a combined filter is formed comprising a) the fabric and b) a primary filter layer of alluvial solids, there-by to effect preclarification of the waste water and the removal of the majority of the particles of solids including flocculated colloidal components from the aqueous phase.
The present invention now makes it possible to set up prefabricated purifying plants which do not require extensive underground structural work, thus being able to reduce the installation and running costs for plant purifying waste water.
~is also results in flexibility with regard to the choice of site and possibilities of expansion. Due to its compact construction the entire processing plant can be erected in the minimum space, and thus can be cheaply roofed, e.g. with flexible textile building materials. The invention also allows for a reduction in the biological (BOD) and chemical (COD) oxygen demands needed for further purification of waste water, compared to the usual processes where solids are separated by preclarification.
Waste water which may be treated according to the in-vention includes that containing both solid substances and colloidal substances and also genuinely dissolved material, and wherein the impurities are at least partly of an organic ~il , . ~ - . .

nature. The waste water generally has an average biological oxygen demand (BOD) of 250-300 mg/l. However, more contamin-ated waste water having a BOD of up to 12,000 mg/l or more and possibly having an increased solids content, such as waste water from clearing and collecting tanks, may also be treated by the process according to the invention.
The BOD can be regarded as a measurement of the con-tent of organic substances in the waste water which are biologically degradable by microorganisms with a certain consumption of oxygen from the air. The COD constitutes a measùrement for determining the amount of organic impurities, and in this case, the consumption of oxidising agents, e.g.
potassium dichromate, is used as a basis for the measurement.
Owing to the generally increased chemical reactivity of oxidising agents compared with microorganisms, the COD is generally higher than the BOD.
In the process according to the invention it is not necessary to add filtering agents, since the solids contained in the waste water are not removed by a separate prec]arifica-tion s~ep but serve to build up the active filter layer on the support fabric.
If enviromental considerations allow the separated aqueous phase having a COD value which has been substantially ,~

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reduced by the process according to the invention to be fed di-rectly into the main drainage channel, there is no need for further purification of the separated aqueous phase. The solidâ
obtained may, however, be further mechanically dehydrated in a second step and the filtrate from both steps may be biologi-cally treated. This is particularly suitable for sludge obtained from clearing and collecting tanks.
In the process according to the invention the known processes of sedimentation, clearing and sludge thickening and, moreover, partly biological degradation of conventional methods of water treatmcnt a.re replaced by a filtration process using gravity. Thus, the process according to the invention, replaces the preclarificati.on, cleari.ng, thickening and sludge dehydra-tion steps and also at least a part of the biological degradation, this latter essentially by removing the COlloidâ by a si.mple purification process using the separated solids as an alluvial filter layer. Since the substances in suspension are also almost entirely held back by the filter layer, any following biological process is correspondi.ngly less burdened.
It is surprisi.ng that it :Ls possible by the present invention to substantial].y reduce the biological oxygen demand needed for further purification of the waste water running .

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off as the separated aqueous phase, compared with the biological oxygen demand needed for purifying municipal waste water freed from solids in conventional processes. In general 30% of the biological oxygen demand of conventional processes is used by removable substancesJ about 20% by colloidal substances and about 50% by genuinely dissolv0d substances. Howe~er, using the method according to the invention, a separated aqueo-us phase is obtained, the purification of which has a much re-duced biological oxygen demand, being at least ~5%, generally 50 to 70%, possibly even up to 80% or even 90% smaller than the biological oxygen demand of the conventional processes.
This surprising effect appears to be due to the fact that not only are the removable and the majority of the colloidal substances separated off by the filtering means used according to the invention, but presumably also a large part of the genuinely dissolved substances are removed thereby, chiefly as a result of absorption or of adsorption processes.
If desired, the process according to the invention may be extended by subsequently treating the biologically treated filtrate in a physicochemical process step, for example, with the addition of con~entional precipitating agents such as aluminium, iron and silicic acid compounds and either X .'~

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recycling the sludge thus obtained to the initial filtering stage or feeding the sludge to a separate apparatus.
According to another aspect of the present invention there is provided an apparatus for waste water treatment by a process including reduction of the chemical and biological demand needed in connection with any further purification thereof which comprises the steps of admixing the waste water with at least one flocculating agent to effect separation of the majority of the colloidal components and passing the resultant admixture under gravity through a fabric located in a suspension device whereby at first A) a combined filter is formed comprising a) the fabric and b) a primary filter layer of alluvial solids, thereby B) to effect preclarification of the waste water and the removal of the majority of the particles of solids including flocculated colloid-al components from the aqueous phase which apparatus comprises a combined bag-shaped gravity filter located in a suspension device provided with an outlet for the resulting dehydrated sludge, the said combined gravity filter comprising an alluvial filter layer built up from the solids separated during the filter-ing process, and a fabric capable of supporting the ]ayer.
According to one embodiment of the apparatus accordingto the invention the sludge outlet from the gravity filter is connected to a device ~or subsequent mechanical dehydration there-of. If desired, the filtrates obtained from the gravity filter and the mechanical sludge dehydration device are both fed to a biological treatment device.
The process and apparatus are hereinafter particularly described in more detail with reference to the accompanying drawing which shows a flow chart of an embodiment of the C~ g ~.

process according to the ;nvention. Waste water is fed into a flocculating device 2, possibly after being collected in a tank 1. In the flocculating device 2 flocculating agents are added in order to flocculate solids in the waste water. From the flocculating device 2, the flocculated waste water passes to a combined gravity filter 3 comprising a support fabric and an alluvial filter layer built up from the separated solids, in this stage the removable and also substantially all of the colloidal ingredients of the water are separated off, and thus the separated aqueous phase is practically free from solids due to this particular type of dehydration. Without using any further devices, it is possible to concentrate the solids to more than 15, preferably 20-25% and possibly up to 30% of dry substance in this stage.
If a sludge with a higher solids content is desired, the sludge contained in the alluvial filter may be subjected to a second process step, comprising subsequent mechanical dehydration in device 4, e.g. in a vertically operating drainage press or a rotary vacuum filter, therehy being dehydrated until the desired solids content of e.g. more than 35% is obtained. The sludge de-hydrated in this way is passed through outlet 9a to a container in which it -may be removed.
If desired, the separated aqueous phase obtained from the dehydration 2n steps is subjected to biolo~,ical and/or physicochemical treatment, biological treatment being preferred. As described above the physicochemical treatment may consist, for example, of precipitation with precipitating agents, e.g.
aluminium suphate. For this biological and/or physicochemical treatment~ the separated aqueous phase is fed into a treatment apparatus 6, if desired via outlets 11 and 12 and a filtrate collecting container 5. The biological treat-ment preferably takes place in a plastic trickling filter, i.e. a trickling filter prepared from synthetic resins, for example, of a type described in British Patent 1.395.763. In this biological treatment step, the dissolved organic ingredients in the separated aqueous phase are biologically degraded to give the desired BOD values, converted into a removable form and fed by -10-`

gravity via 13 back to the first step. If desired, the output from the biological treatment step is subsequently treated in a later physicochemical step 7 in order to remove the solids precipitated in the biological treat-ment. Thus for ,xar,ple, phosphates can be precipitated by metal ions and any colloidal substances causing turbidity can also be removed in this stage.
The sludge obtained from biological and/or physicochemical treatment may be dehydrated, for example, by sedimentation or filtration in a gravity filter of the kind mentioned above. The sludge may also be recycled to the gravity filter apparatus 3. The separated aqueous phase from the biological treat-ment step can be fed into main drainage channel 8 without any further processing. Reference numerals ~ and 10 indicate the outlets for preliminary dehydrated sludge and precipitated sludge respectively.
In the process according to the invention large quantities of waste water can be prccessed in a short time, for example, for was~e water with a high solids content, it is possible to ohtain throughput values of up to 1000 l/hour per m of filter area. In the case of waste water with a low solids content, the throughput values may be even higher. Moreover, sub-sequent treatment of the filter, such as rinsing, etc., to remove any residual substances deposited, is not requirod.
The fabric used in the filtcr is not roquired to perform ally filtering action, but merely acts as a supporting and strengthening member and absorbs the forces produced as the liquid is poured in. Generally, it is bag-shaped with a capacity of at least 60 litres, preferably from 1 to 5 cubic metres. In principle it may be constructed from any material which will perform this function, including for example wire mesh. However, it is advantageous to use sack-like fabrics manufactured from man-made filaments or fibres. Particularly suitable materials include those made from polyester fibresJ such as polyethylene tereph~halate, polybutylene terephthalate, poly-1,4-dlmethylolcyclohexane terephthalate or analogous products based on isophthalic acid; polyamides such as poly(hexamethylene adipate), poly-~, r . ~

hexamethylene sebacate, polyundecanoaTnide, polycaprolactam, and also poly-p-phenylene terephthalamide. Bags made from fibres of polyacrylonitrile, copolymers of acrylonitrile and at least one other monomer, the acrylonitrile content of which is at least 85%, poly~inyl chloride or polyolefins such as polyethylene or polypropylene are also suitable.
The fabric should conveniently not have too close a setting in the warp and weft directions, since if the setting is too close the throughput may be impaired. In the case of polyethylene terephthalate fibre with a titre of dtex. 2200, a thread setting of 7.8 to 9.2, preferably 8.2 to 8.8 fibres per cm in the warp and weft directions is preferred. In the case of other yarn titres, the setting should be varied accordingly. If polyamide fabrics are ùsed, the settings can be determined without any great difficulty taking into account the expansion characteristics of the polyamide. This corres-pondingly applies for fabrics made of other materials.
Suitable flocculating agents include, for example: aluminium chloride, iron chloride, calcium hydroxide, polyacrylates or polyacrylamides.
These flocculating agents cause substantially no contamination of the waste water. Conveniently quantities of from 1 to ~50 g, or in the case of com-munal waste water preferably up to l50 g, in particular from 2 to 50 g an~l, in the case of waste water from clearing and collocting tanks, prefcrably 100 to 200 g of flocculating agent per cubic metre of water are sufficient for adequate flocculation. Generally quantities of flocculating agents towards the higher end of these ranges are used when the content of impuri-ties in the waste water is high.
Example Waste water, having a biological oxygen demand (BOD) of 3000 mg/
liter and a solids content of 0~85%, was flocculated with 115 mg/liter of a commercial organic polyelectroly~e based on polyacrylamide. A filter element in form of a funnel-shaped gravity filter, consisting of poly-ethylene terephthalate fibres with a titre of dtex 2200 and a thread setting of 8,5 fibres/cm in the warp and weft directions, with a content of 1,5 m was fed with the flocculated waste water in a rate of 3,5 m3/h. The filter element was completely filled after 5 m3 had been introduced. At the end of the filling operation the sludge had a solids content of approximately

2,5% which increased to about 10% during 5 hours of continuous dewatering.
~he resulting filtrate was practically free from solids, only less than 20 g/m3 were present. The BOD of the filtrate was 200 tc 250 mg/liter which is a reduction of the biological oxygen demand of more than 90%.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for purifying waste water including reduction of the chemical and biological oxygen demands needed in connection with any further purification thereof which comprises the steps of admixing the waste water with at least one flocculating agent to effect separation of the majority of the colloidal components and passing the resultant admixture under gravity through a fabric located in a suspension device whereby at first A) a com-bined filter is formed comprising a) the fabric and b) a primary filter layer of alluvial solids, thereby B) to effect preclarifi-cation of the waste water and the removal of the majority of the particles of solids including flocculated colloidal components from the aqueous phase.

2. A process as claimed in claim 1 wherein the solids removed from the aqueous phase are subsequently mechanically dehydrated in a second step to give a further separated aqueous phase.

3. A process as claimed in claim 1 wherein the aqueous phase, obtained or admixed with another aqueous phase which is produced by an additional mechanical dehydration, is subjected to a biological treatment with the aid of a trickling filter con-sisting of synthetic resins and wherein the major portion of the aqueous phase contained in the sludge thus formed by this biological treatment is separated from the solids content.

4. A process as claimed in claim 1 or 2 or 3 wherein the fabric a) is formed into a bag-shape having a capacity of at least 60 litres.

5. A process as claimed in claim 1 or 2 or 3 wherein the aqueous phase obtained or an aqueous phase which is produced by an additional mechanical dehydration or an aqueous phase which is produced by subjecting the sludge obtained in a biological treatment or a combination of such aqueous phases is subjected to a physico-chemical treatment with a flocculating agent.

6. A process as claimed in claim 1 or 2 or 3 wherein the flocculating agent is added in a quantity of 1 to 450 g of floc-culating agent per cubic metre of waste water.

7. Apparatus for waste water treatment by a process includ-ing reduction of the chemical and biological demand needed in con-nection with any further purification thereof which comprises the steps of admixing the waste water with at least one flocculat-ing agent to effect separation of the majority of the colloidal components and passing the resultant admixture under gravity through a fabric located in a suspension device whereby at first A) a combined filter is formed comprising a) the fabric and b) a primary filter layer of alluvial solids, thereby B) to effect preclarification of the waste water and the removal of the majori-ty of the particles of solids including flocculated colloidal components from the aqueous phase which apparatus comprises a combined bag-shaped gravity filter located in a suspension device provided with an outlet for the resulting dehydrated sludge, the said combined gravity filter comprising an alluvial filter layer built up from the solids separated during the filtering process, and a fabric capable of supporting the layer.

8. Apparatus as claimed in claim 7 wherein the sludge outlet from the combined gravity filter is connected to a device for subsequent mechanical dehydration.

9. Apparatus as claimed in claim 7 wherein the fabric is formed into a bag shape having a capacity of at least 60 litres and consists of polyester or polyamide fabric.

10. Apparatus as claimed in claim 7 or 8 or 9 wherein the outlets for the aqueous phase from the combined gravity filter and the mechanical sludge dehydration device are both connected to a biological treatment device in the form of a trickling filter consisting of synthetic resins.

11. Apparatus as claimed in claim 7 or 8 or 9 which also contains other gravity filters for separating the sludge obtained from biological and physico-chemical purification.