Note: Descriptions are shown in the official language in which they were submitted.
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TREATING SEWAGE ~1g DIKE SL,IUDGE
This invention relates to treating sewage sludge or similar organic sludge,
such as that obtained from industrial sites such as waste water treatment
plants.
~P 0 283 153 B 1 discloses a method of treating wastewater sludge to
provide a fertiliser for agricultural lands which can be applied directly to
the land. The
method involves mixing the sludge with alkaline material of specified fineness
to raise
the p~I of the mixture to at least 12 for at least a day and to effect
pasteurisation, and then
drying the mixture. Drying is done either a) by aeration and maintaining the
pI~ above
12 for at least seven days until the solids levels reach and maintain a
minimum of 65%
bulk solids, or b) by aeration and heating to a temperature of at least
50°C and so that
the solids levels reach and maintain a minimum of 50% solids. In b) the
temperature
should not be so high as to destroy all non-pathogenic organisms. The
elevation of the
pI~ and the drying are effected so as to reduce odour and undesirable viruses,
bacteria,
parasites and vector (e.g. flies) attraction to the sludge and prevent
significant pathogen
regrowth while not eliminating beneficial non-pathogenic microorganisms. At
least
some of the temperature rise is due to the exothermic reaction with the
alkali. The
product is allowed to air cure for about 10 days after achieving the desired
solids content.
Drying and curing may be effected by windrowing, turn-over or other forced air
methods,
and curing or aeration time is dependent on the aeration procedure and other
factors and
clearly need to be determined, for any particular circumstances,
experimentally to see that
the required end point has been reached. This involves taking samples to
measure the
solids content, testing for
animal viruses - less than one plaque forming unit per 100 ml
salmonella bacteria - less than three colony forming units per 100 ml
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~ parasites - less than one viable egg per 100 ml
~ significant regrowth of the pathogenic microorganisms - there should be
none
and assessing the reduction of odour to a level tolerable in a closed room and
that this
reduction is maintained indefinitely under any climatic conditions, and
whether flies are
less attracted to the product. At the same time, the presence of at least some
beneficial
non-pathogenic microorganisms must be established.
If method b) is used, the minimum recommended time for maintaining the
temperature at least 50°C (but not so high as to destroy all non-
pathogenic
microorganisms) is at least 12 hours and the heat treated alkaline stabilised
dewatered
sludge cake is then air dried (while the pH remains above 12 for at least 3
days) through
intermittent turning of windrows until a minimum of 50% solids content is
achieved.
GB 2 276 876 A, which refers to EP 0 283 153, describes treatment of
sewage sludge having a solids content of at least 15°1° by
weight with an alkaline material
containing free lime, and storing and/or drying and/or composting the
resultant mixture,
adding sufficient lime to the mixture to achieve a pH of at least 10. Higher
pH values,
even higher than 12, are not ruled out, but it is suggested that excellent
reduction in
pathogens can be achieved at pH levels below 12 with reduced evolution of
ammonia,
which renders the treated sludge and its surroundings less unpleasant and
means that the
treated material retains more nitrogen increasing its value as a fertiliser.
The method of GB 2 276 876 involves dewatering the sludge, e.g. in a press,
and measuring it using a weigh hopper into a mixer into which is delivered
alkaline
material in the appropriate amount, the mixed material being delivered to a
skip to
deliver it to a windrowing area where it is turned periodically, normally for
a period up
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to seven days. It is suggested also that provided the amount of calcium oxide
that has
been introduced is sufficient to raise the pIi to a value greater than 12, and
sufficient,
indeed, to ensure that it stays above 12 for at least 2 hours, then it is
possible to store the
mix for not less than two hours, but then apply it directly to the land.
FIowever, GB 2
276 876 notes that such a product is not sterile.
It is apparent that the methods described in these two specifications are
labour intensive and to a large extent dependent upon a subjective judgement
(the
assessment of odour in a closed room, for example) or upon results of
biological tests
which themselves could take days ox weeks before you have a result.
It is necessary, for effective treatment, to mix the alkali with dewatered
sludge, but sludge, nonetheless that comprises a substantial quantity of
water, as it is only
in the presence of water that the desired exothermic chemical reaction takes
place. The
end product - as is clear from the specification discussed - is required to
have less water
content than is appropriate during the heat-development stage. It is an
essential part of
the process, then, that the product be eventually dried, and this is the
reason for treating
it by windrowing. Windrowing, especially aeration by turning over windrows, is
not only
labour intensive and floor space intensive, it is a slow method of drying.
There are further problems and disadvantages with prior art methods such
as those disclosed in EP 0 283 153 and GB 2 276 876. For example, it is
generally
accepted wisdom within the art that the step of pasteurisation should take at
least a day:
the reader is directed to EP 0 283 153 B 1 for a detailed discussion of this
point.
International Publication No. WO 98/29348 discloses a different approach
to the treatment of sewage sludge to those disclosed in EP 0 283 153 and GB 2
276 876.
In WO 98/29348, drying is effected solely by heating caused by exothermic
reaction with
an alkaline admixture. Thus, the approach of WO 98/29348 has the considerable
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advantage that the time and space consuming step of aeration is not required.
This
results in improvements in processing speeds - near continuous or continuous
operation
is possible. Furthermore, there are substantial reductions in the amount of
floor space
required for a processing plant which employs the method of WO 98/29348.
However, it would be highly desirable to provide further reductions in the
processing time required to treat sewage sludge, whilst maintaining or
improving the
quality of the f nal product, since throughput can then be increased.
The present invention overcomes the aforementioned problems and
disadvantages, and overturns the received wisdom in the art regarding the
nature of the
pasteurisation step. Further the present invention does not utilise time and
space aeration
techniques.
According to the invention there is provided a process for treating sewage
sludge or similar organic sludge in which dewatered sludge cake and an
alkaline
admixture are mixed and then dried under pasteurisation conditions utilising
heat from
the exothermic reaction with the alkali; in which drying is effected by
extraction of
moisture evaporated from the mixture by the exothermic heat;
characterised in that the pasteurisation conditions comprise maintaining a
pasteurising temperature for less than 10 hours at a pH of at least 12, the pH
of the
mixture being maintained at 12 or above for less than a day.
Surprisingly, it has been found that such treatment, hitherto regarded as too
short, can effect pasteurisation of the sludge. In particular, E. coli 157 and
Ascaris
species can be completely destroyed, or killed to an extent to which only a
few organisms
or eggs survive. Thus, the processing time required in order to treat the
sludge is
reduced.
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Preferably, the pasteurising temperature is maintained for less than 8 hours,
more preferably less than 6 hours, most preferably for about 4 hours. The time
during
which the mixture is at 12 or above is, necessarily, longer than the time
spent at the
pasteurising temperature. However, it is desirable that the mixture is at 12
or above for
as short a period of time as possible.
The drying may be effected to dry the product to at least 50%, preferably
at least 55%, dry solids content.
The drying may be effected to dry the product to between 50 and 65%,
preferably between 55 and 60% dry solids content. The drying product may pass
two or
more times through a drying hopper. Drying may be assisted by an air flow,
which may
be fan generated. Cases given off during drying may be passed through an
ammonia
scrubber.
It is important that the product is relatively dry, since microorganisms can
survive and regenerate in a wet environment, whereas they cannot do so in an
aggressive
dry environment. The combination of high pH and high dry solids - in excess of
50% -
offers long term stability of the product without pathogen regrowth.
The drying may be effected in a first in first out (FIFO) hopper.
Drying may be effected in a cycling operation until a predetermined
moisture content is detected, then the dried material is discharged.
The dried material may be tested for moisture (or dry solids content) by a
sensor the output of which determines whether the product is recycled or
delivered to
another location, e.g. a stockpile. The dried mixture may be tested by an
infra-red
moisture measuring transducer.
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At the beginning of the process, the dewatered sludge may be measured for
water content and the measure used to control the alkaline adrriixture - the
dewatered
sludge may pass beneath an infra-red moisture measuring transducer.
The dewatered sludge cake may be fed to a storage bin from where it is
delivered to a mixing arrangement with the alkaline admixture. The storage bin
may
have a level sensor controlling the supply of sludge cake to the storage bin
and/or
delivery of sludge cake to the mixing arrangement. The delivery of sludge cake
to the
mixing arrangement may be measured and the measurement used to control the
supply
of alkaline admixture to the mixing arrangement. Delivery of sludge cake to
the mixing
arrangement may be effected by a weigh belt.
The moisture content of the mixed dewatered sludge cake and alkaline
admixture may be measured and the measurement may be used to control the
addition of
said admixture to said sludge cake.
The moisture content of the mixed dewatered sludge cake and alkaline
admixture may be measured before drying and the measurement used to control
the
drying process.
Dewatered sludge cake may be fed directly from a dewatering arrangement
such for example comprising a belt press arrangement.
The belt press arrangement may comprise primary and secondary high
pressure belt press arrangements.
The output from the infra-red moisture measuring transducer may be used
to control sludge cake throughout via adjustment of the operation of the belt
process
arrangement.
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The process may be controlled by a microprocessor.
Feedback loops may be used to control operational parameters (such as feed
rates, alkaline admixture mixing ratios and polyelectrolyte dosing). The
feedback loops
may comprise proportional integral differential loops.
The steps of mixing, filing, drying and discharging the product may be
performed within 24 hours. The step of filling comprises filling a
pasteurisation unit,
such as a FIFO hopper. The mixing and f fling may be performed in less than ~
hours,
drying in less than 5 hours and discharging in less than 3 hours.
Processes in accordance with the invention will now be described with
reference to the accompanying drawings, in which :-
Figure 1 is a plan view of a sludge cake-alkali mixing apparatus; and
Figure 2 is a schematic diagram of apparatus for introducing sludge
cake t~ the mixing apparatus;
The invention comprises a process for treating sewage sludge and similar
organic sludge in which dewatered sludge cake and an alkaline admixture are
mixed and
then dried under pasteurisation conditions utilising heat from the exothermic
reaction
with the alkali, in which drying is effected by extraction of moisture
evaporated from the
mixture by the exothermic heat. Apparatus for performing the process axe
depicted in
Figures 1 and 2.
Pasteurisation and drying, by aeration, is effected in a first in first out
(FIFO) hopper 10 such as a 100% live capacity Samson hopper fitted with full
width
discharger 10a.
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The drying is effected in a cycling operation until a predetermined moisture
content is detected, then the dried material is discharged via outloading belt
conveyor 12,
and take away belt conveyor 26.
The drying is effected to dry the product to between 50 and 65%, preferably
between 55 and 60%, dry solids content. The drying product passes two or more
times
through the drying hopper 10, carried by an inclined belt conveyor 14,
reversing belt
conveyor 16, screw feeder discharger 18 and recycling belt conveyor 20. Drying
is
assisted by an air flow which may be fan generated by an extraction fan (not
shown).
The air flow is distributed across the cross section and length of the hopper
10.
Gases given off during drying are passed through an ammonia scrubber 24.
Thus discharges to atmosphere are treated, and do not present a health risk
either to the
process operators or to the general public. Residual material from the ammonia
scrubber
24 may be mixed with the final dried product to eliminate a waste stream and
to enhance
the nutrient value of the final product.
The product is held for a maximum of 10 hours at pasteurisation
temperature, which is monitored by temperature probes (not shown). Typically,
the
pasteurisation temperature is in the range 50-55 °C, although the use
of higher
temperatures still is not ruled out.
Surprisingly, it has been found that such a short heat treatment is sufficient
to effect pasteurisation of the sludge. It is possible to hold the product at
pasteurisation
temperature for less than 8 hours, even less than 6 hours. In fact,
maintenance of the
pasteurisation temperature for about 4 hours has been found to be effective.
It is not yet
known if shorter heat treatment might be effective. It is anticipated that a
treatment time
of 2 hours, for example, would prove to be insufficiently long, but this has
not yet been
confirmed. For this reason, a numerical lower time limit is not provided
herein (in any
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event, such a lower time limit would likely depend on the specific application
and nature
of the sludge). However, the skilled person would have no difficulty in
ascertaining, if
he/she so wished, such a lower limit.
The present invention can effect complete or near complete destruction of
harmful pathogens, in particular E.coli 157 and Ascaris species. Ascaris
species are
regarded as being particularly difficult to destroy-see, for example, EP 0 283
153.
The pH of the mixture is maintained for less than a day, preferably less than
12 hours. This can be ensured by providing an alkaline admixture which
comprises
(Ca~) and one or more further addivitives, such as fly ash. The amount of lime
used is
enough - but just enough - to obtain the required rise in temperature. The fly
ash (which
is at a pH of ca. 9.5) provides granularity and bulk in the final product.
The dried material is tested for moisture (or dry solids content) by a sensor
(not shown), the output of which determines whether the product is recycled or
delivered
to another location, e.g. a stockpile, via the reversing belt conveyor 16 and
take away belt
conveyor 26. The dried material may be tested by an infra-red moisture
measuring
transducer. The process achieves total exhaustion of free calcium oxide with
the water
in the drying product. Additionally, when the product is ready to be
discharged, a
reduction in temperature to around 25 °C is achieved.
Figure 2 depicts apparatus used at earlier stages of the process. At the
beginning of the process, the dewatered sludge is measured for water content
and the
measurement used to control the alkaline admixture. This is achieved by
passing the
dewatered sludge beneath an infra-red moisture measuring transducer 30, and
subsequently controlling certain operations in a manner described more fully
below.
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The dewatered sludge cake is fed - via a belt conveyor 32 - to a storage bin
34 from where it is delivered to a mixing arrangement 36 with the alkaline
admixture.
The storage bin 34 has an ultrasonic level sensor 38 controlling the supply of
sludge cake
to the storage bin 34 and/or delivery of sludge cake to the mixing arrangement
36. The
delivery of sludge cake to the mixing arrangement 36 is measured and the
measurement
used to control the supply of alkaline admixture to the mixing arrangement 36.
Delivery
of sludge cake (exiting the storage bin 34 via a sliding frame discharges 34a
and screw
feeder 34b) to the mixing arrangement is effected by a weigh belt feeder 40,
which
permits the measurement of sludge cake delivery by monitoring the weight of
sludge
cake fed.
The mixing arrangement 36 provides complete and intimate contact between
the sludge cake and the alkaline admixtures without breaking the structure of
the sludge
cake (which would result in a "plasticised", paste-like material). Alkaline
admixture is
supplied from two silos 4~, 44 via screw conveyors 46, 48. Further silos may
be
employed. Additionally, a lime silo 52 is provided. The purpose of the lime
silo 52 is
to provide small, controlled additions of Ca~ or a material having a high free
Ca0
content (for example, lime kiln dust) in the event that the other alkaline
admixtures do
not achieve the required temperature rise and hold criteria. The mixed
dewatered sludge
cake and alkaline admixture is transferred via a swivel belt conveyor 49 to
the inclined
belt conveyor 14 which supplies the hopper 10. The moisture content of the
mixed
sludge cake and alkaline admixture is measured before drying and the
measurement used
to control the drying process. The measurement is made by an infra-red
moisture
measuring transducer 50, the output of which may also be used to adjust the
feed rates
of the alkaline admixture. This is used to achieve the desired mixed product
moisture
setting, typically 50%.
The dewatered sludge cake may be fed directly from a dewatering
arrangement, which comprises a belt press arrangement (not shown). The belt
press
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arrangement involves both primary and secondary belt pressing operations. Such
an
arrangement results in a number of advantages, such as an increase in dry
solids content
from a norm of ca. 25% to a norm of ca. 33 to 38%. Furthermore, substantial
reductions
in sludge cake tonnage feed, alkaline admixture, mixed product and overall
plant size are
found to occur.
The process is controlled by an industrial process controller (PLC) with PC
based data acquisition. The PLC provides automatic control of material
handling,
mixing, and the pasteurisation/aeration process. The control and automation
system
incorporates proportional integral differential (PID) feedback loop controls
to optimise
process economy (e.g. admixture consumption and polyelectrolyte useage during
dewatering) within the constraints of the required operating parameters. 'The
use of other
feedback systems is within the scope of the invention. Inputs from the
moisture sensors
and tachometers fitted to the feed screws (which provide indications of speeds
and thus
feed rates) are used in the control loops, although the master control is
provided by the
weight indication given by the weigh belt feeder 40. Variables such as
polyelectrolyte
dosing, and belt process speed and throughput are also under microprocessor
control.
The output from infra-red moisture measuring transducer 30 may be used to
control
sludge cake throughput via adjustment of the operation of belt press
arrangement, in
particular to optimise dry solids output with respect to polyelectrolyte
additions.
The system software provides both visual and hard copy on line, reporting
on all of the process steps described above. Daily production reports, heat
pulse
temperature graphs, and sludge, mixed and final product dry solids reports can
be made
available. Additionally, a "help line" service from the operating site to Dead
Office is
provided via a modem link, with a remote management reporting facility.
It is possible to provide essentially continuous treatment by careful control
of treatment times and operating conditions. With a single pasteurisation unit
(such as
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the FIF~ hopper 10) it is desirable to complete the entire process of mixing
and filling,
temperature rise and hold, recycling (comprising aeration and drying) and
discharge
within a 24 hour cycle. The major difficulty with the method of CVO 98/29348
is that the
pasteurisation temperature (typically 52 to 60 ° C) should be
maintained for at least 12
hours, with another two hours or so additionally being required to attain this
temperature.
The present invention provides a significant improvement in this regard, since
the
pasteurisation temperature is maintained for a substantially reduced period of
time.
The key process considerations required to achieve the 24 hour cycle aree-
(a) lVtixing and filling operations must be rapid. This involves such
considerations as increased dewatering machine capacities and
operating regimes, sludge buffer storage capacity increase and
resultant mixing machine throughput requirement.
(b) The recycle aeration and drying phase must be also rapid and
efficient as perhaps only one complete recycle can be achieved.
Special considerations during this phase are product discharge and
recycle rates and ventilation requirements.
(c) Finished product discharge to stockpile must also be at a high rate.
In order also to achieve (a), (b) and (c) above, total variable speed control
of the dewatering machines, mixer drivers and the mass discharger is required.
Such is
provided using the PLC system described above.
If a twin pasteuriser arrangement is employed, a continuous operational
cycle can be employed whilst enjoying 100% back-up capacity. In such an
arrangement,
an extra hopper might be provided to the set-up shown in Figure l, swivel belt
49
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alternating between the two hoppers. Alternatively, both pasteurisers can be
used in
tandem.
'The advantages of the continuous treatment include reduced plant
requirements, a reduced plant footprint, increased processing throughput and
reduced
operating labour and power costs per tonne of sludge processed, since most of
the plant
operations are carried out when the site is unmanned.
Modifications to the process described above will be apparent to those
skilled in the art. For example, more admixture silos or an extra hopper for
pasteurisation/drying might be provided.
