Note: Descriptions are shown in the official language in which they were submitted.
The inven-tion relates -to a method for eliminating
solids from a liquid containing suspended solids, in
which the liquid is passed through a filter bed of poly-
urethane particles for the purpose of precipitating the
solids onto the polyurethane particles and forming a
liquid low in solids, whereby, after the filter-bed is
at least partly charged with precipitated solids, -the
la-tter are removed from the filter-bed by regeneration.
In the field of liquids and sewage-treatment, filtration
has long been one of -the main methods of eliminating
suspended substances from flowing liquids. In the case
of sewage trea-tment in particular, the presence of sus-
pended matter frequently presents a ma]or problem. Fil-tra-
tion has been used to a considerable ex-tent in order
to reduce and/or eliminate concentrations of suspended
matter from flowing liquids, for example communal sewage,
and for re-using certain -types of sewage. In applica-tions
of this kind, considerable use is made of sand-fil-ters
and dual-media or mixed media operating with rising and
falling flows. Generally speaking, these have been found
cost-effective and efficient. In spi-te of -this, prac-tical
experience has shown that although sand and mixed-media
filters usually eliminate suspended matter efficien-tly,
this applies only to conditions where charging wi-th solids
is limited. As a rule, -the solids concentration in the
flow of liquid entering the filter must be less than
about 100 to 200 mg/L. Above this range, the filter-
bed tends to become plugged, and a large pressure-drop
occurs at the fil-ter-bed.
In recen-t years, filtra-tion processes in the applications
mentioned above have been improved -to some extent by
the use of polyurethane as -the filtration medium. As
a rule, polyurethane foam is cut into small pieces which
are placed in a receptacle to form the filter bed Sewage,
or some other liquid, is then passed -through the bed,
whereupon the suspended matter is precipitated onto -the
polyurethane particles, and remains thereon. The liquid
from which the suspended matter has been precipitated
then flows through the bed and is discharged as a liquid
low in suspended solid contaminants.
Generally speaking, the use of polyurethane foam
as a filtration medium provides numerous operational
advantages over beds using sand or conventional mixed
media as filtration substances. For example, higher
absorption of solids, lower losses due to pressure-drop,
reduced tendency to plugging, and elimination of numerous
soluble, organic con-taminants from the flow of liquid
can be achieved.
In spite of many operational advantages, however,
the regeneration of the polyurethane foam, charged with
suspended solids during the filtration process, remained
a serious problem. Various methods were considered for
regenerating the polyurethane-foam filter-bed charged,
at least in part, with suspended matter during -the filtra-
tion process. Among these are reverse flushing witha flow of liquid low in solids, a method in general use
in conventional sand and mixed-media filtra-tion systems.
In the case of polyurethane-foam filter-beds, however,
reverse flushing presents a particular problem because
of the low density of the polyurethane-foam filtration
medium. Since the true density of polyurethane foam
may be as low as 16 to 32 kg/m3, it is generally desired,
in normal operation, to pass the liquid containiny the
suspended mat-ter through the polyurethane-Eoam bed in
S the downward direction. When a filter bed of this kind
is reverse-flushed by passing a flow of clean liquid
upwardly through -the filter-bed in counterflow, -the flow-
velocity required is usually so high that particles of
polyurethane foam are washed out of the bed or -the bed
is ripped. This causes large cavities in the bed, lead-
ing to short-circuits and other unsatisfac-tory phenomena
during subsequent filtration. On the other hand, if
the polyurethane-foam bed is regenerated in the same way
as in normal filtration processes, i.e. by passing a
flow of liquid low in solids downwardly -through the bed
charged with solids, it is usually difficult to flush
enough suspended matter from the polyurethane particles
to ensure satisfactory renewal of -the bed for the subsequent
filtration stage. Moreover, it has been Eound that reverse
flushing generally fails to restore to the polyurethane
foam its original capacity to absorb solids.
In view of the above-mentioned difficulties with
the use of conventional reverse flushing in the case
of a polyurethane-foam filter-bed, various methods have
been proposed for regenerating the polyurethane foam
in the filter-bed by compression. For compression-regenera-
tion of this kind, use is generally made of mechanical
means in order to "squeeze out" the polyurethane-foam
particles, thus "wringing" out the water con-tained -therein,
-together with the precipi-tated solids. Although a
~8~ 7
regenera-ting process of -this kind is easily carried out
in the laboratory, or on an experimental scale, the mechani-
cal complexity of such a device, and the investmen-t involved
in its application on an industrial scale, have consider-
ably restricted the use of polyurethane-foam filter-beds
in prac-tice.
U.S. Patent 4,162,216 discloses another mechanical
method of regenerating a filter-bed consisting of discon-tinuous
polyurethane particles. In this case, in order to renew
the filter-bed, the precipitated solids are removed from
the polyurethane particles by adding to the filter-con-
tainer at least one bed-volume of a regenerating liquid,
the filter-bed of polyurethane particles being -thoroughly
mixed mechanically in a total volume of liquid located
in the filter-container holding -the same regenerating
liquid and the liquid held back in the filter-bed when
the flow is cut off, thus forming a liquid enriched with
solids which is then withdrawn from -the filter-container.
Although this method generally produces satisfac-tory
results in practice, i-t has the disadvantage that constant
mechanical mixing of -the total amount of liquid is necessary,
and this increases not only the investment but also the
operating costs of the filtration system.
The present invention too provides a method for
regenerating polyurethane particles in a Eilter-bed in
a subs-tantially more cost-effective manner than existing
methods.
According to the invention, an amount of regenerating
liquid equal to between one and five times -the volume
of the bed is introduced into the filter-con-tainer; and
gas is blown through the regenerating liquid in an amount
such that the liquid and the polyurethane par-ticles are
moved through each other. The regenerating liquid and
polyure-thane particles are, if necessary, also moved
mechanically through each other to provide sufficient
movement of the regenerating liquid and polyurethane
particles such that the precipitated solids are released
from the particles and pass into the regenerating liquid.
The regenerating liquid becomes enriched with solids
and is withdrawn from the filter-container.
According to a preferred embodiment of the invention,
the amount of gas blown through the filter-bed is between
0.15 and 1.5 m3/min/m2, preferably be-tween 0.3 and 0.6
m3/min/m2 of cross-sectional area of the filter-bed.
According to a further aspect of the invention, the
mechanical movement of the regenerating liquid is terminated
immediately before the regenerating liquid enriched
with solids is withdrawn from the filter-container.
Furthermore, several regenerating cycles, preferably
two to four, are carried out consecutively. According
to one advantageous configuration of the method according
to the invention at the final regeneration cycle, -the
amount of regenerating liquid enriched with solids drawn
off is such that no more than one bed-volume enriched
with solids remains in the filter-container, and in
that the polyurethane particles are flushed, before
the filtration-process is resumed, with an incoming
liquid containing between one and five bed-volumes,
preferably between one and two bed-volumes, of suspended
solids.
"Bed-volume" is to be understood to mean a volume of
liquid equal to the volume of the filter-bed. In prac-tice,
the regenerating liquid may consist partly of the liquid
held bac]c in the filter-bed when the feed is shut off.
The expression "liquid held bac]c in the Eil-ter-bed"
relates to the liquid between the polyurethane particles
and the liquid retained in the pores of the polyurethane
particles in the fil-ter-bed after the flow of liquid
through the said filter-bed has been shut off.
Preferred embodiments of the invention are shown
in the drawings wherein:
Fig. 1 is a diagrammatical representation of a filtra-
tion-system having a filter-bed with polyurethane
particles, and
Fig. 2 is a diagrammatical representation of another
filtration-system in which the regeneration
of -the polyurethane particles is carried out
in a separate regenerating vessel.
According to Fig.l, liquid containing suspended
solids (suspended matter) enters the -treatment-system
through a line 100 containing a control-valve 115 which
is open during the initial operating phase. This liquid
may come from a variety of sources, Eor instance from
a biological-treatment installation, in which case it
may contain between 40 and 60 ppm of suspended matter,
or from river-water fed to a communal water-conditioning
installation. In other cases, the liquid may be un-treated
water intended for use in boilers or for internal use
in an industrial installation. It may also be a flow
of liquid from an algae-reservoir (oxidizing -trench)
i7
-treated to prevent flakes from needle-algae from reachiny
the main drainage channels.
The liquid passes from line 100 into a filter-
container 102, entering the upper end thereof through
a spray-distributor or some other device 101. Filter-
container 102 may, with advantage, be made of C-steel
or of some other structural material and is partly filled
with a filter-bed with polyurethaneparticles 103. The
said filter-container has an upper inlet-end and a lower
outlet-end. The filter-bed or polyurethane particles
rests, within the container, upon a support 104 which
may be in the form of a grid, screen, or the like or
fine-mesh material. This forms at the lower end of
container 102 a filling chamber 105 in which is collected
the liquid which passes through the filter-bed.
Liquid containing suspended matter is passed, through
the filter-bed located in the filter-con-tainer, to -the
lower outlet-end, in order to deposit solids from the
liquid onto the polyurethane particles, and thus effective-
ly eliminate the solids by means of the filter-bed.
This forms a liquid low in solids which collects in
lower filling chamber 105 and is withdrawn from the
filter-container through a line 106 in which an open
control-valve 116 is arranged. From line 106 the liquid
low in solids reaches the main drainage channel any/or
other treatment or final-consumption locations.
The above-mentioned flow of liquid containing sus-
pended matter is maintained until the filter-bed is
at least partly charged with precipitated solids, where-
upon the flow through the filter-bed is cut off and
s~
the filter-bed is ready for renewal.
In order to improve the performance (collecting
and eliminating solids) of the filter-bed, it may
in many cases be desirable to add a coagulant to the
liquid, for example~ an organic coagulant, sodium silicate
or alum, as it passes to the filter-bed. Such coagulants
are desirable in certain applications in order to lncrease
flocculatlon of suspended matter in the treated liquid
and to reduce clouding of the liquid leaving thefilter-
bed treatmen-t-system.
The polyurethane particles used in the present
filter-bed may consist of any desired and appropriate
foam-material possessing the necessary solids-charging
capacity for the proposed duration of the filtration
phase. The discontinuous polyurethane of the filter-
bed particles may be present in the form of shredded
particles of non-uniform size. They may also be in
the form of cubes such as those obtained, for example,
when apiece of polyurethane foam is cut into small parti-
cles, for example cubes the sides of which are between
6 and 26 mm in length. Suitable polyurethane foam material,
which has been successfully used in practice, include
ester or ether cross-linked polyurethane foam having
a cell-density of between 4 and 40, and preferably between
12 and 24 cells/cm. Open-pore cross~linked or non-
cross-linked polyurethane-foam materials, suitable for
the polyurethane particles of the filter-bed, should
possess a porosity of 24 to 76, preferably 30 to 52
m3/air/min/m2, these values being based upon the flow
velocity of air at 23C and 50% atmospheric humidity.
If air flow through a piece of polyurethane foam 12.7
mm in height and having a cross-sectional area of 127
x 127 mm (a-t righ-t angles to the direction of the flow
of air), at an inlet-pressure of 1 bar, there arises
a pressure-difference of 2.5 x 10 4 bars between -the
upstream and downstream flow of air through the piece
of polyurethane foam. The numerical value of the porosity
is determined as the value of -the above-determined flow
velocity in m3/min/m2 of cross-sectional area of the
filter-bed. The actual density of the polyurethane
par-ticles in the filter-bed should preferably be above
21 kg/m3 in oxder to be of sufficient strength for a
bed. The polyurethane particles in the filter-bed preferably
possess a solids-charging capacity of at least 8 kg
of suspended matter/m3 of polyurethane particles, i
they are to have an adequately high solids-charging
capacity during the fil-tration-phase. For example,
a suitable polyurethane foam material for the present
invention may be present in shredded form having the
following properties: size of particles - between 25
and 51 mm, 12 cells/cm, particle -thickness of -the order
of 0.6 mm, true density (measured without cavities)
of about 28 kg/m3, tensile strength of about 16.2 N/cm2.
During the filtration process, the volumetric flow-
velocity of the liquid containing suspended ma-tter through
the fil-ter-bed is kept to preferably between 0.4 and
4.1 l/min/cm of cross-sectional area of -the filter
bed, in order to ensure adequate contact be-tween the
liquid and the polyure-thane particles in the filter-
bed, wi-thout leading to short-circuits or any other
s~
abnormal flow phenomenaO After -the fil-ter-bed has been
at least partly charged with precipitated solids, the
flow of liquid through the fil-ter-bed is shut off by
closing valves 115 and 116 in lines 100 and 106. There-
after, -the precipitated solids are removed from the
polyurethane particles in the filter-bed, which renews
the said filter-bed for the elimination of solids from
liquids containing suspended solids.
After -the flow of l.iquid through the filter-bed
has been shut off, held-back liquid remains therein.
This held-back liquid may amount to the major part of
a bed-volume and is attributable to the high porosity
of the filter-bed of polyurethane particles. For example,
the porosity of such a filter bed may be of the order
of 80~, so that when the flow of liquid -through the
fil-ter-bed is shut off, the filter-container holds 0.80
of a bed-volume of held-back liquid in the form of liquid
in the gaps between the polyurethane particles and liquid
in the pores of the said particles in the filter-bed.
In manyapplications it may be desirable to remove all
of the liquid from the filter-bed. This may be achieved,
for example, by keeping valve 116 open for a suitable
length of time after valve 115 has been closed, thus
permitting complete removal of all liquid in the filter-
bed through line 106.
For the purpose of regenerating the filter-bed
polyurethane particles, between one and five bed-volumes
of regenerating liquid are added, -through a line 113
having an open valve 11~, to the filter-bed containing
polyurethane particles at least partly charged with
8~
precipita-ted solids. The regenerating liquid entering
-through line 113 may come from an external source of
"clean" liquid. Alternatively it may be a liquid low
in solids, returned from the filter-bed, whlch was collected
during the filtration process. In the lat-ter case,
it is desirable for the said regenerating liquid -to
contain less than 150 mg of suspended solids/l. The
regenerating liquid may also consis-t of incoming liquid
fed into the fil-ter-container -through line 100, in which
case line 113 with valve 114 may be dispensed with.
In the system according to Fig.1, according to the invention
between one and five bed-volumes of regenerating liquid
are fed -through inlet-line 113 and line 122 into filter-
container 102, after which valve 114 is closed. The
change-over of the various valves may be effected by
a timing control.
After the regenerating liquid has been introduced
into filter-conta:iner 102, all valves are closed~ There-
after, valve 118 in gas-supply line 117 is opened and
gas passes through line 117 to distributor 119 arranged
in filling chamber 105 in the said filter-container.
The gas is blown through the regenera-ting liquid, in
an amount such that the liquid and the polyurethane
particles are moved through each other, causing the
precipitated solids to be released from the polyure-thane
particles and to pass into the regenerating liquid,
thus forming a regenerating liquid enriched with solids.
The gas supplied through line 117 is preferably air.
A vent-valve 51 is fitted in a ven-t-line 50 at the upper
end of filter-container 102 and serves to reduce any
excess pressure built up therein by the air blown through
the liquid. According to one preferred me-thod of operation,
between 0.15 and 1.5 m3/min/m2 of cross-sec-tional area
of the filter~bed of gas is blown for between 0.5 and
5 mlnutes through the regenerating li.quid containing
the polyurethane particles. Where water contalning
few solids for example 200 mg/l, is to be filtered,
it is desirable, during the regenerating process, to
blow between 0.3 and 0.6 m3/min/m2 of cross-sectional
area of the filter-bed of gas, for between 1 and 2 minutes,
through the regenerating liquid.
It is also possible, if necessary in addition to
the blowing of gas, to move -the regenerating liquid
and the polyurethane particles through each other mechanically
with the aid of a stirring device with blades 159, shown
in dotted lines, connected through a shaft 158 to a
drive~motor 157. This causes the precipitated solids
to be released from the polyurethane particles and to
pass into the regenerating liquid, forming a regenerating
liquid enriched with solids. Gas may also be blown
through the regenerating liquid containing the polyurethane
particles throughout the duration of this mechanical
movement. Alternatively, the gas may be blown through
the regenerating liquid only in the initial stages of
the mechanical movement, as needed and desired in a
particular application.
After the mechanical movement of theregenerating
liquid and polyurethane particles effected by blowing
gas and, if necessary, by additional mechanical movemen-t,
has been carried out for as long as is necessary, this
5i7
movement is terminated ky deactivating the mechanical
stirring device, whereupon valve 121 in line 120 is
opened and the regenerating liquid enriched with solids
is withdrawn from filter-container 102 through line
122and outlet-line 120. In -the practical application
of the method according to the invention, it has been
found advantageous to withdraw the liquid enriched with
solids from the filter-bed in an amount equal to between
4 and 16, preferably between 8 and 12 1/min/dm2 of cross-
sectional area of the filter-bed. The regenerating
liquid enriched with solids, which is withdrawn through
line 120 as a waste-water drain, may then pass to other
treatment-stages, such as dehydration. ~fter the regenerating
liquid has been withdrawn from the fil-ter-container,
15 valves 121 and 118 are closed. In cer-tain applications,
the filter-bed with the polyurethane par-ticles is cleaned
with the aid of the above-mentioned regenerating steps
to such an extent that the normal fil-tration process
maybe resumed; in other applications, however, it may
be desirable to repeat the aforesaid regenerating steps
one after the other as consecutive regenerating cycles.
Not more than two to four regenerating cycles should
normally be carried out. If several regenerating cycles
are carried out, it is of advantage, at the final cycle,
to remove only enough regenerating liquid enriched wi-th
solids such tha-t not more than one bed-volume of regenera-
ting liquid enriched with solids is left in the fil-ter-
container, and to flush the polyurethane particles,
before filtration is resumed, with between one and five,
and preferably between one and two bed-volumes of incoming
liquid containing suspended solids. For this flushing
phase, valve 115 is opened and incoming liquid containing
suspended solids is passed through the filter-bed and,
through line 106 (with valve 116 open), directly bac~
to inlet-line 100. Al-ternatively the flushing liquid
may be passed through waste-water line 120 (with valve
121 open). The flushing phase is preferably carried
out in order to remove residual suspended solids from
the spaces in the polyurethane particles in the filter-
bed, thus fully cleaning the latter for the next filtrationcycle. After the flushing phase, valves 116 and 115
are opened, all other valves are closed, and a new fil-tration
cycle begins with the incoming liquid containing solids
being passed through the bed and the liquid low in solids
being withdrawn through line 106.
In contrast to Fig.l, which illustrates a filter
bed with polyurethane particles which is re-tained in
the filter-container during the regeneration process,
Fig.2 illustrates a system in which the filter-bed poly-
urethane particles are removed from the filter-container
and are passed to a regenerating container in which
they are regenerated before returning to the filter-
container.
In the system according to Fig.2, the filter-bed
with polyurethane particles 203 is loca-ted in an elongated,
vertical filter-container 202 of constant cross-sectional
area. Initially all valves in the system are closed,
with the exception of valve 215 in an inlet-line 200
and valve 247 in an outlet-line 248. A liquid containing
30 solids enters fil-ter-container 202 through a line 200
:L4
Éi7
and a spray distributor 201, and thus flows through
the filter-bed wi-th polyurethane particles from top
-tc bottom. This forms a liquid low in solids which
flows, through a support 204, into filling chamber 205
in -the filter-container, whence it is withdrawn through
line 248. This procedure is repeated until -the filter-
bed is at least partly charged with precipi-ta-ted solids,
whereupon valves 215 and 247 are closed.
In order to regenerate the filter-bed polyurethane
particles, a pump 141 is activated and -the said polyurethane
particles pass through line 240, from filter-container
202, into a regenerating container 250, where they are
placed upon a support 251. After all of the polyurethane
particles have passed from filter-container 202 into
regènerating container 250, pump 241 is deactivated.
Valve 214 in line 213 is then opened and between one
and five bed-volumes of regenerating liquid are introduced
through lines 213 and 222 into the said regenerating
container, whereupon valve 214 is closed. Valve 21~
in a gas-supply line 217 is now opened and an amount
of gas, e.g. air, equal to between 0.15 and 1.5 m3/min/m2
of cross-sectional area, is passed through line 217
to a distributor 219 located underneath suppor-t 251.
In this way, enough gas is blown through the regenerating
liquid with polyurethane particles to cause -the said
liquid and particles to move through each other. The
precipitated solids are thus released from -the polyurethane
particles and pass to the regenerating liquid, forming
a regenerating liquid enriched wi-th solids. A ven-t
valve 150 is fit-ted in a vent-line 151 at -the upper
end of regenerating container 250, and is used to reduce
any excess pressure built up in the regenerating container
by -the air blown through. After the gas has been blown
through for a certain length of time, e.g. between 2
and 3 minutes, valve 221 in a waste-water line 220 is
opened and the regenerating liquid enriched with solids
is withdrawn from regenerating container 250. Additional
regenerating cycles may be necessary. In this case,
regenerating liquid is passed through line 213 and 222
into the regenerating container and gas from line 217
is blown through -the regenerating liquid and polyurethane
particles, and through the regenerating liquid enriched
with solids. At the final regenerating cycle, only
enough regenerating liquid enriched with solids is withdrawn
as will leave not more than one bed-volume of regenerating
liquid enriched with solids in the container. At this
stage of the cycle all valves are closed. Valves 245
and 216 are then opened simultaneously and the polyurethane
particles in the regenerating container are flushed
with a liquid containing between one and five bed-volumes
of suspended solids and entering -through lines 200 and
246. After the required volume of flushing liquid has
passed through line 246 into regenerating container
250, valve 245 in line 246 is closed. The flushing
liquid then passes from regenerating con-tainer 250,
through lines 222 and 206, back -to inlet-line 200. Valve
243 is then opened and the now fully regenerated polyurethane
particles pass, by gravity, through line 242, into filter-
container 202. As soon as all of the polyurethane particles
30 are back in filter-container 202, valve 243 is closed
16
and valves 215 and 247 are opened again. The filtration
process now begins again.
In this method of operation, it is again possible,
according to the invention, to move the regenerating
liquid and polyurethaneparticles mechanically through
each other, in addition to blowing air through them,
with the aid of a mechanical stirring device, shown
in dotted lines, consisting of blades 259, a shaft 258
and a drive-motor 257. This releases the precipitated
solids from the polyurethane particles and causes them
to pass into the regenerating liquid, thus forming a
regenerating liquid enriched with solids.
Although preferred embodiments of the invention
have been described herein in detail, it will be understood
by those skilled in the art that variations may be made
thereto without departing from the spirit of the invention
and the scope of the appended claims.
