Note: Descriptions are shown in the official language in which they were submitted.
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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.,
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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
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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
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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
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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
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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
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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
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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-
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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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