Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~his invention is concerned with impruve~ents in
and relating to the treatment of organic material~
containing wastes~
~he treatment of solid organic material-containing
waste (that is dustbin refuse, garbage or trash or the
like, hereinafter simply referred to as "solid organic
wastes") presents considerable difficulties with regard to
its ultimate disposal.
mere are, in general, three broadly applicable
methods for the disposal of such organic wastas namely:
(1) ~ipping or sanitary land fill
(2) Incinera-tion; or
(3) Composting.
~he first method, tipping, has been widely practised
in the past and is still widely used but is coming
increasingly into disfa~-our since suitable sites for
tipping are becoming increasingly scarce and the
- environmental disadvantages of this method are becoming
- increasingly apparent-. ~onventional incineration o~
organic wastes requlres relatively complex combustion
equipment and in view of the fact that the water conte~t
of solid organic wastes may be as high as 50% by weight
and, accordingly, auxiliary fuels are required in the
incineration process thereby adding to its cost and
complexity. Further the relatively high ash content of
the material may lead to ash disposal problems. Composting
; of organic wastes offers an apparently more ecological
and environmen-tally satisfactory approach to the treatment
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- of organic wastes since the final product is one which can
be used as a soil conditioning agent or, if there is insufficient
demand for the material for this use, this material is somewhat
less unsightly and objectionable to dump than is the original
untreated waste. Even so, composting has not met with too
great a success in view of the difficulties of persuading the
agri.cultural industry of the usefulness of the end product
(possibly in view of the trace element or inert material, e.g.
glass, content of the compost) and in view of the capital cost
re~uired for establishing suitable composting plant. Thus, of
the three generally available techniques tipping is the most
generally operated in view of its generally low costs and also
due to the fact that tipping practice has been established over
many years.
~ he present invention is based upon the discovery
that the composting process may be so operated as to give a
- product having a relatively low water content ~i.e. less than 20%
by weight) and that this product has a sufficient calorific
value ~e.g. of the order of 3,000 to 5,000 calories per gram on
:. 20 a dry basis) to render it suitable for use as a fuel, that is
the moisture content and calorific value of the product are such
i' that the combustion thereof is not merely self-sustaining but
: can be carried out to produce useful heat energy.
Accordingly, one aspect of the present invention is
concerned with a method for the production of heat which com~
prises composting a solid organic waste in a rotating drum for
an average residence time of from 12 to 48 hours to produce a
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relatively dry product (e.g. having a moisture content
of less than 20,~ by weight) and subsequelltly burning -the
` composted product.
It will be appreciatad that one important step of
the method of thls invention involves composting an organic
waste so as -to produce a relativel~ dry product~ ~he
term "composting" as used herein is intended to refer to
a process of aerobic fermentation of the organic waste
during the course of which carbon dioxide is evolved and
the temperature of the fermenting material is raised `
whereby water vapour is driven off from the fermenting
mass so that it is dried or dewatered. Many processes
and apparatus have been proposed for the fermentation or
compostlng of organic wastes and, in essence, all of these
involve the above indicated steps, namely involve allowing
the organlc waste~to ferment under aerobic conditions.
However, such previously proposed processes have generally
been carried out so as to produce a product having a
- relatively high moisture content (e.g. 40% by weight of
water or mol~e) and to this end steps have been taken -to
add sufficlent water to the organic waste or to the
fermenting mass so as to maintain its moisture level at
~ the desired relatively high level. In accordance with
- the present invention the moisture content and other
process conditions are so controlled that the final product
has a relatively low moisture content. Additionally, the
fermentation process operated in accordance with the
inventlon may ~e so operated as to achieve a lower overall
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fermentation cf the material thereb~ raducin~ wastage
of combustible material. In the past conventional
composting processes have been carried out to give a
product in which the carbon: nitrogen ratio ~i.e. C : N
ratio) is as low as possible, e.g. 10:1 as compared with
about 35:1 for the starting mixture. In the fermentation
according to the invention composting need onl~ be carried
out to produce a C : N ratio of, say, 20 : 1 ~ 25 : 1 In
many cases~ the previously known fermen-tation process or
apparatus can be modified to produce a relatively dry
; product, as is required in the case of the presen-t invention,
by simple modifications, that is by ad~us-ting the water
content of the s-tarting material or the amount of water
added at various stages during the ~rocess or by varying
other process conditions such as the rate of air flow over
rh e
or through th0 fermenting material.
fermentation process for use in accordance with the present
invention is one which is carried out in a rotary drum,
especially one in which the organic waste is fed to one end
of a slowly rotating drum and slowl~ advanced there-through
whilst undergoing fermentation or decomposition, -the final
product being taken out at the other end of the rotating
drum. Air will generally be fed to the drum to assist
aerobic fermentation and this may be warm air to assist
in water vapour removal~
As in the conventional composting of dustbin or like
refuse, the refuse is first treated to remove large,
generall~ irc~mbu3tible objeots, for example by hand sorti~g,
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a~d is then -treated, for example on a magnet1c separator,
to remove metallic objects. Preferably, in acoordance
wlth the present in~ention, the refuse is pulverized
after or~ prefera~ly, before magnetic separation prior
to passing to fermentation. It is also preferred that the
` pulverized refuse be screened or sieved before being
passed to the fermentation drum and before or after
magnetic separation ~for example to remove materia] having
a si3e of great-er than 2 inches or more). ~he rejected
material from this screening operation, which may comprise
larger pieces of combustible material such as paper or
cardboard, need not necessarily be re~ec-tedbut is,
; preferably, first passed to a shredder to reduce its
particle size and then blended in with the composted refuse
at a later stage. ~nen screening is carried out before
magnetic separa~ion lt may be convenient to subject the
rejected material to magnetic separation~
- ~he screened refuse is then fed to the fermentation
: apparatus (hereinafter simply referred to, for the sake of
convenience as a "drum") where fermentation takes place.
The moisture content of the screened refuse will commonly
be of the order of from 25 to 50%, more commonlv from
30 to 40% by weight and whilst this moisture content is
generally s1litable for composting the refuse it is
generally preferred to add additional water to the refuse
since this is believed to moisten the drier portions of
the refuse and to assist in mechanical breakdown of the
refuse in the drum. Thus, it is generally desirable thRt
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the initial material fed to the fermen-tation drum have a
water content of from 25 to 50% by weight, preferably from
30 to 45% by weight. ~he refuse may be moistened by simply
adding water or by adding sewage sludge, the addition of
which has the advantage of;providing addltional organic
matter to the férmentation mass. In accordance with the
present inven~ion it is particularly preferred to moisten
the refuse by the addition of sewage sludge, especially a
partially dewatered sludge ha~ing, for example, a water
content of from 60 to 85% by weight. ~he use of sewage
sludge to moisten the refuse not only increases the organic
matter content of the fermentation mass, but , also, serves
- as a very useful method of dewatering sewage sludge. ~hus,
the disposal of sewage sludge represents a considerable
problem and whilst mechanical methods of dewatering sewage
sludge(e.g. filter presses) ca~ reduce its water content
to, say 75% by weight without too much difficulty, it is
very difficult to reduce the water con-tent below this level
by mechanical means. By introducing the sew~ge sludge as
moisturising medium for the refuse~ the sludge is dewatered
during the course of fermentation due to the heat evolved
during the fermentatio~ so that the final product,
containing sewage sludge solids, has a very much reduced
water content as compared with the starting sewage sludge
material. ~he weight ratio of sewage sludge to organic
waste fed to the fermen-tation drum will, of course, depend
upon the moisture content of the two materials and the
desired water content of their mix ure. I~ practice,
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however, it has ~e~n found that the weight ratio o~
sewage sludge to organic waste or refuse is suitably from
1 : 4 to 1 : 12 by weight, preferably from 1 : 5 to 1 : 10
by weight, for an 80% water content sludge. lhe ratio of
sludge bO refus~e~will general-ly be comparably higher for
drier sludges and lower for wetter sludges.
~he sewage sludge may be a digested sludge or a
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crude, undigested sludge. ~ince the calorific value of
` the solids contained in undigested sludges is generally
markedly higher than for digested sludges it is often
useful to employ an undiges-ted sludge in admixture With the
solid waste since, other things being equal, the final
composted product will then have a higher calorific value.
After passing through the fermentation drum the
product may be subjected to a secondary screening operation,
for e~amp~e on a further magnetic separator and/or by an
air classifier and is -then stored before being passed to
suitable combustion apparatus. Due to its low water
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content the fermented material will no longer ferment and
can thus be stored without loss of calorific value and
does not evolve unpleasant odoursO
It has been noted that during drying of the composted
material salts originally contained in the mother liquor
(e.g. those contained in the se-.rage sludge) crystallize
out and it is believed -that these contribute usefully to the
combustibles content of the material.
order that the invention may be well understood
reference will now be made to the accomp&nylng drawings
in which: 7
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~igure 1 is a schematic block diagram illustrating
one embodiment of the process of the in~ention;
; ~igure 2 is a lon~itudinal section through a fer~ent~ng
drum for use in accordance with the process of the invention;
,5 ~igure 3 is a perspective view of the final section
of the drum shown in Figure 2; and
- Figure 4 is a schematic sectional vlew illustrating
the feed-of materials to and from a fermenting drum.
In accordance with the process illus-trated in the
drawings, collected refuse is brought into a refuse reception
and storage area 1 generally comprising one or more enclosed
reception hoppers on concrete bases. ~ollec-tion c~cd
delivery to the storage area is, of course, an intermittent
process whereas it is preferred to carry out the fermentation
procèss as a continuous opera-tion, and accordingly, ~he
recep-tion and storage area should be sufficiently large to
store a sufficient quantity of refuse for co~tinuous
operation of the fermentation apparatus.
Refuse to be fermented is then passed from the storage
zone to a primary sorting zone 2, which will generally take
the form of a conveyor belt, a~d in which large or
unfermentable objects are removed from the refuse as are
those which are unpulverisable or may block the pulverisor~
Such objects will commonly be dumped in accordance with
usual practice. If desired waste paper may be added to the
refuse at this stage ~if there is no readily available
market for such waste paper) since such material will,
cleerly, lncrease tOe calor1 ric va lue of the rinal produ_t
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(this is not generally carried ou-t in conventional
composting processes since such materials add little t~
;`r'' the manurial properties of the compost). ~e material fro~
the primary sorting zone is then passed to a pulveriser 3
- 5 in which it is pulverised,that is it is reduced in particle
size to say from 2 to 4 inches. Suitable pulverisers are
- swing hammer pulverisers. Pulverisation prior to introduction
into the fermen-tation drum serves, it is believed, to
- increase the efficiency of the fermentation in the drum by
providing a relatively finely divided material for
fermentation therein~ ~he pul~erised material from pulveriser
~ is then passed to a screening apparatus 4 (e.g. a vibratory
screen) wherefrom oversize reaects (e.g. having a size of
2 inch or more)are passed to a shredder 6 via a magnetic
separator 5. lhe material passing through screen 4 is passed to a
magnetic separator 7 where ferromagnetic material is removed
and may thence be dumped or, possibly, passed to a metal
baler for ultimate sale as scrap. ~he design and construction
of magnetic separators 5 and 7 is well known ln the art and
requires no further elaboration at this point. The material from
separator 7 is then passed to a fermentation drum 8, which
~s divided into three general seetions, a first section 9, a
principal fermentation section 10 and a drying section 11
;~ 25 (see Figures 2 and 3). Commo~l~, a moisturising medium
(hereinafter referred to as "sewage sludge") will be in-
60rporated with the refuse to moisten it and this sewage
sludge is preferably brought in-to contact with the refuse in
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the prim~ry section 9 of drum 8 or immediately before it
is introduced into section 9 of drum 8. ~here is thus
foxmed in or fed to primary section 9 of drum 8 an
intimate mix-ture of sewage sludge and screened refuse
and this primary section is preferably provided with
internally projecting knives or blades 17 which further
serve to disintegrate or pulverise the sludge/refuse
mixture. Section 9 of drum 8 may also be provided with
curtain chains 18, i.e. chains one end of which is attached
to the inner periphery of the drum and the other end of
which is fre~, and festoon ohains 19, to further enhance
the mixing/pulverisation action in the section. In the
second section 10 of drum 8 the sludge/refuse mixture
undergoes i-ts principal fermentation and in order to
achieve efficient aeration of the fermenting mass in this
section the interior of the drum is preferably provided
with lifters so that upon rotation of the drum the
fermenting mass is continuously stirred and/or showered.
Whilst conventional blade lifters or flights may be used
in this section it is preferred to use lifters in the form
of festoon chains 20 extending along the side walls o~ the
drum and generally parallel to the longitudinal axis
thereof. ~`hese chains achieve a very favourable mixing
action and also achieve a useful heat transfer action
whereby the heat regenerated by the fermenting mass can
be used to drive water therefrom. ~o further achieve
aeration of the fermenting mass in section 10 air (possibly
warm air) should be introduced into the drum and this is
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convenientl~ achieved by drawing air along -the length of
the drum by means of a variable speed induction fan 12.
Where ~n installation for carrying out the procéss of the
invention is located on the same site as the plant in
which the.refuse is burnt, warm air may be drawn from the
vicinity of the combustion plant; for example in the case
where (as described below) the composted xefuse is burnt in
a calcination kiln warm (or indeed hot) air may be drawn
from the vicinity of the hot end of the kiln or may be drawn
from the coolers used to cool the calcined product.
Exhaust air from -the induction fan 12 may be exhausted to
atmosphere via a suitable stack, preferably after washing
in washers.Alternatively~ the exhaust air may be exhausted
through a furnace in which the composted refuse is burnt
e.g. may be introduced at the hot end of a rotary calcining
kil.n. Tn this way unwant.ed gases.evolved dur:ng fermentation
may be burnt.
During the course of fermentation in drum 8, especially
in section 10 -thereof, heat is evolved and this serves to
dewater the fermentation mixture. It desired, in order
to conserve heat within the drum, its walls may be lagged
(with lagging 21) to inhibit heat loss since it is desirable
that any heat loss should serve to accomplish one of the
principal objects of the fermentation reaction, namely the
removal of moisture.
~he final section 11 of drum 8 is the drying zone of
the drum and in this zone the drum is preferabl~ provided
with lifters or flights 22 so that the mass therein is
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subjected to a continual showering ac-tion to be brollght
into contact with air passing through the drum under the
action of ran 12. ~o this end, the lifters in section 11
preferably extend radially inwards to a greater exte~t
than do the lifters in section 10 so that a greater showering
action is obtained and ma~ also be so constructed as to
urge the mass of composted refuse towards the outlet end
of drum 9. Section 11 may, if desired, be constructed as
a separate drum from the drum constituting zones 9 and 10.
~urther, the mixing section 9 of the drum may be
replaced by a separate mixer~ e.g~ a paddle mixer, and in
this case it is generally most convenient to feed the sewage
sludge to the mixer for mixing with the pulverized refuse
from magnetic separator 7u ~he use of a separate mixer in
place of the mixing section 9 of -the drum makes it possible
to simplify the construction of the drum and reduce its sizeO
If desired, in order to monitor the fermentation
operation, thermocouples 23 are placed in each of zones
9, 10 and 11 or at the boundaries thereof so that the
temperature of the material therein can be observed and the
process condi~ions i.e. rate of air flow, rate of rotation
of drum, etc., can be adjusted to obtain the optimum
conditions, e.gO to maintain the temperature at the end of
fermentation zone 10 at between 70 and 80C, preferably
between 73 and 77a. In this connection i~ will, of course,
be appreciated that drum 8 will be rotated at a relatively
low rate, e.g. 0.5 - 4 r.p.m., preferably about 1 r.p.m.
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Drum 8 should be of sufficient length to accommodate the
mass passing therethrough having regard -to the rate at
which the material does pass therethrough and, in general,
the material will pass through the length of the drum in a
period of from 12 to 48 hours. In order to ensure that ~he
material passes through the drum this latter may be
inclined.at a slight angle to the horizontal e.g. 5 ~ ~0
preferably about 7O
As indicated above, the fermentation process in
accordance with the invention is so operated tha-t the
produce emerging from the end of fermen-tation drum 8 is
relatively dry. ~his is achieved by suitably adjusting the
water content of the starting refuse/sludge'mixture (but
of course, not to a level insufficient for satisfactory
fermentation) and arranging for a suitable flow of air
through or past the fermenting material. In addition, in
the drum described above a definite drying zone, Yone 11
is established. In many conventional fermentation drums
for composting organic refuse additional moisturising
20` liquid ~i.e. water or sewage sludge) is introduced into
the fermenting mass as it passes along the length of the
drum. When opera-ting in accordance with the invention
this further additio~ need not take place or ma~r take place
only to a limited degree. In any event it is most desirable
that no water is introduced'into the final section of the
~ drum which is a drying zone, and the lifters or flights
thereof are designed to give increased aeration (and 'hence
drying) as compared with the preceding, principal fermentati~n,
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zone of the drum. By these means the moisture conten~ Gî
the composted material emerging from the and of the drum
may be reduced to the required levels.
In this regard it ma-~ be noted that it is generally
preferred that the water content of the f inal composted
pxoduct be from 20 to 10% by welght, more preferably from
20 to 15~ b-y weight.
~hus if it is attempted to obtain moisture levels
below about 10% by wei~ht it is found that -the temperature
required are such as to severely inhibit fermentation and
that the calorific value of the final product is not use-
fully enhanced.
The dried product from the drum 11 is then conveniently
subjected to a secondary screening operation on a second
magnetic separator 12 ( whence separated material may be
dumped or passed to a metal baler for sale as scrap) and
an air classifier 13 to which material rejected from
screen 4 is added after shredding in shredder 6, the
rejected material from air classifier 12 (such as ashes,
non-ferrous metal, glass and other particulate incombustible
mat-erials) being dumped in a con~entional manner. ~he
product from the secondary screening oparation is then
suitable for use as a fuel and should be stored in the
hopper 14 before being passed to final combustion in
furnace 15. Before combustion the material may be further
ground, for example in an attritor, and may then be burnt
in any suitable heat energy raising furnace. ~his furnace~
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for example, may be a steam raising or water heating
furnace or may be a furnace used in some other industrial
process, ~or exa~ple a calcining operation. ~he product may
be used as the sole fuel of the furnace or may be used
as an auxiliary fuel together wi-th conventional gases,
liquid or solid fuels but, in any even-t, will have a
sufficiently high calorific value to render its combusti.on
capable o~ evolving useful heat.
, ~he following ~able illustrates a t~pical mass
balance for the operation of the process as shown in the
drawings when treating a typical domestic re~useO
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h J O
o ~ h h ~d h
E~ ~ C) C~ h 0 ~
~ m ,0
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h 0 0 ~h ~ ~ ~ h ~ ~ o
. ~- r __ ___ . _
. h ~' ~
2~ 2
. oo~
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o ~ oo o c~l o Lr~ o oo o Lr\ .
---- - -- ~
Lt\
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c~ d
~ _ h ~ ~ ~ U2 ~ ~ u~ Q
H
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~ ~ æ ~ ~ (U 2 'D ~ oo Lr~ ~ ~o
. ~ o~
. ~P, o, ~ o 0 a)~ o oo
O O ~ ~ ~ 0 OQ Lr~ Lr~ 00
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Fir~ 4 of the drawings diagrammaticall~ illustrates
a preferred arr~lgemen~ ~or feeding materials to and lrom
fermentation drum 8. ~hus pulverised and screened refuse 25
is fed via belt conveyor 23 under a magnetic separator 7
to a chute 24 whence it passes to mi~ing secti~n 9 of
drum 8. ~ne interior of drum 8 will be provided with
knives, chains and li~ters as shown in Figure 2 but these
are omit-t-ed (for clarity) from ~igure 4. Drum 8 is mounted
upon suitable bearings (not shown) and is further provided
with means (not shown) for rotating it, e.g. at a rate of
from 1 to 4 r.p.m. Dewatered sewage sludge 27 is fed via
belt conveyor 26 ~nd is brought into contact with
pulverised refuse 25 at the end of conveyor 23 and is fed
with the refuse to drum 8. ~he input end of drum 8 is
surrounded by a casing 28 carrying said duct 29 in which is
mo1mted variable speed fan 12. Casing 28, in which chute
24 forms the bottom wall surrounds the end of drum 8 so
that fan 12 may draw air through drum 8. In order to
minimise the ingress of air into casing 28 a ~lexible
(e.g. rubber) air seal flap 30 is provided adjacent the
upper links of conveyor 23. Conveyor 23 may be set to the
piston shown at 23' and the orifice in casing 28 may be
retracted to the piston shown at 23' and the orifice in
casing 28 may be provided with a suitable sealing slide
for closing the orifice if it is desired to operate
the apparatus with intermittent feed of refuse and/or
sludge.
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The output end of the drum is surrounded, again in
a generally airtight manner, by casing ~'l which co~nects
wi~h an air inlet duct 32 and has a product discharge
oriflce 32 wherein product energy from drum 11 is
transferred to magnetic se~ OE ator 13 and belt corveyor 33~
The air introduced into duct 22 may be hot air -taken
from a ho-t area of the plan-t as described above and the
air discharge via duct 29 may be discharged, for example,
to a stack or a kiln as described above.
As indicated above the composted material may serve
as a fuel in a calcining operation. ~hus one preferred
embodiment of the invention is concerned with a process
wherein the composted material is burnt as an auxiliary
fuel in the calcination of a calcinable inorganic ma-terial.
~he term "calcinable inorganic material" as used
herein is intended to refer to any inorganic material
which may be calcined in a kil~ to produce a useful calcined
product. Examples of such calcinable inorganio materials
include chalk and limestone (calcium carbonate) which may
be calclned to give lime;bauxitic clays which may be
calcined to give alumina; dolomite which may be calcined
to give magnesia; and mixtures of silicaceous materials
(especially clays) and chalk or limestone which may be
calcined to give cement such as Portland Cement.
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It has already been proposed to use pulverised
d ~tbin or like refuse as an auxiliary fuel in the
production of cement by calcination of a clay/chalk or
limestone mixture which process serves not only as a
method for the removal or dlsposal of dustbin refuse but
also gives rise to savings~in respect of the conventional
fuels used in the calcination process. However, in view
of its r~elatively high water con~ent, which ma~ vary from
~0 to 50~ by weight, pulverised domestic refuse is often
a difficult matsrial to hand]e in that it may give rise to
clogging problems on the ha~dling machinery and it also
has the disadvantage that it is a rather unpleasant
material to handle. ~urther such material will generally
have a variable ash con-tent and accordingly will give rise
to problems of process control in that the composition of
the total cement-firing materlal fed to the process may
vary to such an extent that an lnferior product may be
produced. In contradistinction the relatively dry
composted product burnt in accordance with the present
invention is much more readily handled. ~he problem of
~uality control of the fuel may be calcined, in accordance
with the invention, by air classification to reduce its
ash content, thereby reducing the variable factor.
In order to further accelerate the removal of
moisture from the composting material wherein the composted
product, is to be employed in a calcining operation, the
composting apparatus may conveniently be sited in a high
temperature zone o~ the calcini~g plant, for example in
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the vicinity of the coolers where ambiert temperatures
may be of the order of 35 to 65C. By siting -the
composting plant in this area, heat losses from the
apparatus are minimised and any air passing through -the
drum is at a relatively high temperature whereby the
removal of water vapour is facilitated. Al-ternatively,
the air supplied to the fermentation drum may-be drawn from
the vici~nity of the hot end of the kiln or may be drawn
from the exhaus-t gases of grate-type clinker coolers with
the attendant advantage that any entrained fine cli~ker
may be entrained with the composted product and thence
recycled to -the kiln.
The fuel product produced in accordance with the
invention is in many ways comparable with low grade solid
fuels such as low grade coal or lignite~ ~hus, i-t may have
a calorific value of from 3000 to5,000 cal/gm. ~he
composted product from the fermenter will be a relatively
high ash fuel (e.g. containing from 20 to 30% by weight of
~ ash(on a dry basis3 especlally in view of the fact that it
is prepared from a starting material already containing
incombustible materials such as ashes ? small metallic
particles or small pieces of glass. ~his ash content may
be reduced by an air classification process. In certain
operations (for example the production of cement) the ash
itself may form a valuable contribution to the final end
product of the operation.
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IIo~e~er, removal of at least a part o~ the ash
correspondingly increases the calorific value of the
product and the costs of so doing are o~ten acceptable
and lt may be desirable to gi~e a generally constant ash
content to the final product.
~he calorific value of the composted material
produced in accordance with the present application may
be enha~ced by incorporating hydrocarbonaceous oils, such
as waste hydrocarbonaceous oils, crude oils or partially
refined oils, e.g. in an amount of up to 15~ by weight.
~his may be achieved simply by mixing the composted
product with the hydrocarbonaceous oils but it is preferably
achieved by mixing the hydrocarbon oil with the material to
be treated, that is the mi~ture or organic waste material
- 15 and water, partially dewatered sewage sludge or raw sewage
sludge. In this way the hydroc2rbonaceous oil becomes
absorbed b~- the fibrous constituents of the waste organic
material and when these are broken down during the composting
process ths fragments thereof containlng absorbed oil
'become well dispersed within the composted product giving a
readily handleable product. By incorporating hydro-
carbonaceous oils in the composted product of the invention
in this manner not only is the calorific value of the
- product enhanced but, in addition, there is provided a
convenient method for the utilisation of hydrocarbonaceous
oils t e.g. waste hydrocarbonaceous oils, -the disposal of
which has given rise to problems in the past.
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~ `urther, ~he composted product m~y ba mixed with
solid fuels ~o ~ive a product having a higher calorific
value. '~hus, the product may be mixed with washed coal
smalls (e.g. having a particle size of one inch or less)
) to produce a product having a reduced per therm as
compared with ~he coal itself. 'nhis is particularly
applicable to coals, such as wet coals~ or low volatile
coals, which have previously often been considered difficult
to use. 'nhe ratio of compost-ed product to solid fuel may
vary widely.
22
In order tha~ tne i~vention may be well understood the
following Examples are given by way of illustration only.
Example 1
~wo samples (each of 400 kg) were taken from the
pulverizer of a refuse treatment plant. ~hese samples
had the ~ollowing basic characteristics sho~n in lable 1
.~ .
~ _ ~
. Sc~mple A Sample ~
~ ~'
Moisture content
(% by weight) . 24.8 27.6
as recei~ed
. ...... _ . . . ~ .
Calorific value
(on a dry basis) 2~00 cal/~m 3100 cal,/gm
_ _ _ __ __ _. _ . .
Ash content .
(on a dry basis 31.4 . 3204
% by weight)
. _ ,_ _ _ _ _ , . .
.
The composition of each of the samples ~as
investigated to give the results shown in Table 2.
23
~ABLE 2
. . .
_ ~
_ Sample A Sample
_ _ _
Component
% by wt. % by vol~ % by wt. % by vol.
~ ~ _ __
Dust and ashes 18.2 6.4 17-5 6.0
Paper and Cardboard35-8 69.9 36.4 71.1
Plastics 4.1 6~2 3~7 5.8
Metal 8.2 6.0 8~4 6.0
Glass 10.0 3.2 10.5 303
Organic matter 18.4 5~4 19.0 5~6
(vegetables etc.)
Textiles 2~4 1.5 2.0 1.2
llr ~ ss i ri . ~l'~ . 2. 1 . 4 2 . 5 1 0
.
The two samples of pulverized refuse were
thoroughly mixed together and fo*r 25 kg samples (sample A)
of the mixture were taken. F~rther fo~r 25 kg samples
(sample B) taken of the mixture from which metal and
glass had bee~ removed by hand mixing.
i2o lwo samples of digested sewage sludge and two samples
of raw undigested sewage sludge were analysed to give the
results shown in ~able 3.
-- 2
. . .
TABI,E~
__ _ _ __ _ ___ I
Digested sludge Undi~ested sludge
A _ _ _ _ . _ _ _~ ~ _ _
Sampl.e A Sample B Sample A Sample B
. ~ __~ _
Moisture content 68.0 72.5 76.8 81.4
~% by weight)
Ash 2~.4 27.6 25.6 26.2
(% by wei~h-t on a . .
dry basis) .
Calorific value . 4850 495 6700 7100
(on a dry basis, .
cal/gm) .
.. _ _ , - . . _. ~
. :
.
~ he two samples of the digested sludge were thoroughly
mixed together as were the two samples of the undigested
sludge~ Then 5 kg and 2.5 kg sa~ples of each were
thoroughly mixed with refuse samples A æ d ~ respectively
to give the following mixtures. :
.
TABIE 4
__
Sludge
Mixture Refuse _ _ _
Digested Undigested
5 ` _ _ _ __ _ _ _
1 A (25 kg) 5 kg .
2 A (25 kg)2.5 kg _
3 B (25 kg) 5 ~g
. 4 ~ (25 kg)2~5 kg _
A ~25 kg) _ 5 ~g
6 A (25 kg) _ 2.5 kg
. 7 B (25 kg) _ 5 kg
. B (25 kg~ _ _ _ 2.5 kg
~ach of the mixtures was then composted in a rotary
drum for a peri.od of 4& hours and the water content, ash
content and calorific value of each o the composted
products were determ~ned to give the res~lts shown in
~able 5~
.
?6
:.. . . ... . .
. .
~ABLE 5.
. . ___
Compost Water content Ash content of Calorific value
for of compost compost of compost
mixture (% by weight) (% by weight on (cal/gm, on a
a dry bafiis) dr~ basis)
_ ___ _ _ ,
1 ~ 8~4 ~ ~~i1 nO . 4050
2 ` ~ g-2 ~;0~5 3800
~ 14~6 11.0 4600
4 .11~4 1104 4750
. 5 10. 2 28~ 5 4300
6 10 ~ 4 28 o O 4-300
7 12~5 9~0 495
8 14~8 9u 5 5100
_ _ _ _ _
Example 2
1100 ~ons of domestic refuse were having an average
water content of ~0% by weight were screened to remo~e
3 tons of rejects have a size greater than 3 inches
(average waste content 15% by weight). ~he remaining
800 tons of refuse were fed to a "dano" type rotary
composting plant over R period of 16 hours together with
110 tons of a dewatered digested sewage.sludge having a
water content of 80% b~ weight. ~ke residence time of`
the material in the fermentation drum was 48 hours and
there were obtained 600 tons of a composted product having
a moisture content of 17. 5% by weight, an ash content of
24~7% by weight and calorific value (on a dry basis) of
3610 cals/gm.
.. . 27
~hi9 product ~Jas ar app~rently dry product ~ld could
be burnt as c~n auxiliary fuel in a ro-tary cement kiln.
.
. 28
