IES83378Y1 - Sludge and slurry destruction plant and process - Google Patents
Sludge and slurry destruction plant and processInfo
- Publication number
- IES83378Y1 IES83378Y1 IE2001/0342A IE20010342A IES83378Y1 IE S83378 Y1 IES83378 Y1 IE S83378Y1 IE 2001/0342 A IE2001/0342 A IE 2001/0342A IE 20010342 A IE20010342 A IE 20010342A IE S83378 Y1 IES83378 Y1 IE S83378Y1
- Authority
- IE
- Ireland
- Prior art keywords
- sludge
- reactor
- liquor
- plant
- waste
- Prior art date
Links
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- 238000000034 method Methods 0.000 title claims description 124
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- 229910052760 oxygen Inorganic materials 0.000 claims description 33
- MYMOFIZGZYHOMD-UHFFFAOYSA-N oxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 claims description 33
- 239000002699 waste material Substances 0.000 claims description 33
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 4
- IOVCWXUNBOPUCH-UHFFFAOYSA-M nitrite anion Chemical compound [O-]N=O IOVCWXUNBOPUCH-UHFFFAOYSA-M 0.000 description 4
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- 210000002268 Wool Anatomy 0.000 description 2
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- 239000010828 animal waste Substances 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- CURLTUGMZLYLDI-UHFFFAOYSA-N carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
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Description
Sludge and Slurry Destruction Plant and Process
Field of the Invention
The present invention relates to a process for the destruction of organic waste
and slurries from a variety of sources which hitherto have been difficult to dispose of.
In particular the invention relates to a biological process for the destruction of organic
slurries that arise from intensive animal rearing units and sludges from the biological
treatment of industrial and domestic waste water.
Background to the Invention
In recent years changes in Environmental Regulations have resulted in stringent
legislation coming into force in relation to effluent from waste water treatment plants.
However, whilst the quality of effluent being released into the environment from waste
water treatment plants may have improved, there is still the problem of disposal of the
residual solids which are produced as a by product of the waste water treatment.
Sewage treatment plants treat the waste water from domestic and industrial sources.
Such treatment results in the formation of residual solids which are termed sludge. The
disposal of the sludge can prove problematic and can be very costly and labour
intensive.
The liquid or solid organic sludges that arise from intensive animal rearing units
and the sludges arising from the treatment of domestic sewage or industrial waste water,
are often quite intractable and the routes for the disposal thereof are often difficult and
expensive.
In this document, the term slurry is defined as the waste product arising from the
treatment of animal waste. Sludge is defined as the waste end product of the biological
treatment of domestic and industrial waste water. Sewage is defined as the Waste water
from domestic or industrial sources or a blend of these. Heneeforth, and for the purpose
of brevity, the term “sludge” in this document shall encompass the term “slurry”.
Such organic waste may be an inevitable consequence of the process itself such
as in intensive animal rearing. The composting process may allow the recycling of some
sludges to land, but the sheer volume of waste from such intensive processes often
makes the disposal an expensive and environmentally sensitive issue.
Other sludges such as those arising from the waste water treatment of industrial
eftluent, food and drink processing plants, paper manufacturing, pharmaceuticals
manufacture or domestic sewage treatment plants will arise as a liquid waste and may
be amenable to further treatment such as anaerobic digestion processes to reduce the
overall volume. The liquid sludge is contacted with anaerobic bacteria in a tank over a
period of about fourteen days. Such secondary sludge processing will produce methane
gas as a by-product and provide a source of power to help offset treatment costs.
Aerobic digestion is also used by contacting the liquid sludge with blown air sometimes
at elevated temperatures and in the presence of thermophilic bacteria, although it does
not produce methane.
Currently known processes for treating sludge are in essence sludge reduction
processes. There is currently no process of total organic sludge destruction available,
other than incineration.
Sludge reduction programmes only produce at best a reduction of forty to sixty
percent of the mass of sludge. The sludge from the digestion processes also still have to
be disposed of.
German Patent No. 19502856 describes a waste water treatment plant which
produces more methane and less sludge than conventional anaerobic digestion
processes. The supply to the waste water treatment plant passes to a primary
sedimentation tank. The plant comprises at least one transfer device and thickening
system for sludge produced. Settled sewage passes to an aerobic activated sludge plant
and an anaerobic reactor digests the sludge. The system uses centrifuges to treat the
thickened reactor sludge prior to the digester and the waste sludge from the digester.
However, some solid waste product is still produced for disposal from the final
centrifuge. The system described uses centrifuges which allow high-g forces to act on
the biomass and lyse a proportion of the cells thus releasing nutrients for the digestion
process and therefore producing more methane.
There are a number of disposal routes commonly used to deal with both raw and
digested waste sludges. In Europe, a common route until recently was simply dumping
the liquid sludge at sea, sufficiently far from land to ensure that the material did not
find its way back to the shore. However the environmental effect of millions of tonnes
of sludge, some of which was contaminated with heavy metals or chemical toxins, was
deemed to be highly inappropriate, and the practice was banned by the European Union
member countries. However this method of sludge disposal is still practised in other
areas but it is unlikely to be allowed to continue in view of the environmental
implications.
The liquid sludge arising from, for example, a digestion process would contain
typically about 5% solids in water. A dewatering process would convert this to a solid
sludge containing 25-35% solids and will give a significant reduction in volume.
Dewatering techniques used would include for example, filtration, perhaps using high
pressure filter presses or belt presses or, alternatively, the use of centrifuges to separate
solid from liquid. However, the waste water stream from the dewatering process is
usually highly contaminated and it is necessary for it to be returned to the inlet of the
treatment works. The solid cake is transportable but still has to be disposed of, thereby
increasing costs.
Another route for disposal of sludge is the use of landfill. Landfill is used, often
for dewatered sludges that may be contaminated with traces of heavy metals or with
toxins. Landfill regulations are however usually very stringently enforced since the site
must be efficiently sealed to prevent leaching into the water courses and potentially
causing contamination of rivers and other sources of potable water. The landfill option
is therefore becoming ever more expensive as suitable land areas become scarce and
environmental legislation increases costs.
Other methods of disposal of dewatered sludges include sludge drying as
exemplified by the Swiss Combi process which takes a centrifuged cake and passes it
through a specially designed dryer. The dried sludge is pelleted, bagged and sold as a
fertiliser or may also be used as a low grade fuel. An alternative fate, and one that is
being adopted as the method of choice for treatment of sewage sludge in large
communities is the use of incinerators. The sludge cake in this case must be
autothermic, i.e. its moisture content must be sufficiently low to allow combustion to
occur without the addition of extra fuel except at start—up. The dewatering system
selected must be appropriate for this. Using this system, the sludge is destroyed but
leaves an ash which still has to be disposed of. The ash may be used for brick making or
in the case of contaminated industrial sludge, calcined and land filled.
Studies have been carried out on the use of blends of sewage sludge and
domestic waste for incineration in local refuse disposal incinerators or as a fuel in
cement kilns. It is not known if these routes have ever been proven as commercial
propositions. However, blends of sewage sludge and domestic garden and other organic
waste have been used in a commercial composting process.
Both the drying and the incineration process require the building of very large
and expensive plants which adds significantly to the cost of disposing of the sludge.
Additionally, the drying process requires a ready market for the sale of the pelleted dry
material, which may or may not be available.
One outlet for the dried sludge, assuming it has no toxic contaminants such as
heavy metals, is its use as a fertiliser. Sewage sludge in all its forms has always been
used as a fertiliser. At least half the sewage sludge generated is used for agricultural or
land reclamation purposes. Some types of liquid sewage sludge because of their content
of harmful bacteria have to be injected into the soil. Other types, particularly those
which have been subjected to the digestion process which pasteurises the sludge, may
be sprayed on to the land. Dewatered cake may also be used by the farmer or forester.
The stabilisation of sludge with lime has long been used to assist in the sludge
pasteurisation process and assists in its use as a fertiliser product. One development has
been to take the waste ash from the production of cement, called cement kiln dust and
blend this with dewatered sewage sludge cake. The product is a pasteurised and easily
handled product that has the appearance, texture and odour of soil. It is known as the N-
Virom process and has been used in a number of countries to assist in the presentation
of sewage sludge as an acceptable product to agriculture and horticulture. However
beneficial, the product is still the blend of two wastes and doubles the quantity of
products to be sold.
U.S. Patent No. 4079003 describes a process for transforming sludge into
ecologically acceptable solid material. The process involves the use of lime to stabilise
sludge and render it inert. The sludge is converted to a form which is useful as an
agricultural fertiliser.
U.S. Patent No. 5,948,261 describes a process for the treatment of biosolids
resulting from waste water treatment which results in the destruction of organic
contaminants, which results in biosolids which can be disposed of on land or under
ground. Likewise, U.S. Patent No. 4277342 discloses a method for the biological-
chemical detoxification of sewage sludge.
U.S. patent number 4341632 relates to a process for the biological purification
of waste water and in particular to the problem of undesirable "bulking sludge"
formation caused by the proliferation of certain microorganisms. The document
suggests that elements of the bulking sludge can be destroyed by using a centrifuge to
place a high mechanical shear upon the bacteria. Normal bacteria found in the activated
sludge are not affected. The process does not deal with the destruction of sludge.
Japanese patent specification no. 9066298 describes a process whereby after
organic sludge is anaerobically digested, digested sludge is subjected to solid-liquid
separation treatment and the solid part of the digested sludge is solubilized by alkali
treatment before being returned to an anaerobic digestion process.
Japanese patent no. 9052100 describes a method for reducing the volume of
organic sludge by using an aerobic digestion method. The method comprises a first
aerobic digestion process aerobically digesting organic sludge, a solid-liquid separation
process separating digested sludge into a solid and into a liquid, a solubilizing process
heating at least a part of the separated sludge from the solid-liquid separation process or
treating the same with alkali.
U.K. patent number 2 298 195B describes a process in which the production of
sludge is reduced or even eliminated by the deactivation of a proportion of the bacteria
in the biomass. In this way the total number of bacteria in the reaction zone can be
maintained constant while maintaining a high rate of bacterial growth and aerobic
digestion. It is a feature of activated sludge processes that they depend upon the facility
of the bacteria to clump together naturally (flocculation). This ensures good and quick
settlement in the separation phase and little carry over in the final effluent. The patent
relates to a system in which the settled biomass is returned in full to the reactor and a
high shear is applied to the iloc, breaking them up and thereby allowing easier
predation by rotifer and protozoan microorganisms in the digestion process.
In Europe the banning of sea dumping as an acceptable route for sludge disposal
has led to an increase in the building of incinerators to take up the huge volumes to be
destroyed. Additionally the increasingly stringent regulations on the water industry in
Europe to treat domestic and industrial waste water to high standards before disposal
and will continue to lead to an increase in sludge volumes. There is a limit to the
amount of sludge that can be used for agriculture, land reclamation, forestry or
horticulture.
Increasing global populations will make the safe disposal of sludge, whether industrial
or domestic a major problem for all developed and developing countries.
Object of the Invention
It is therefore an object of the present invention to provide a process and plant
for the substantial destruction of sludge resulting from the treatment of waste water or
from agricultural slurries. It is a further object to provide a process for sludge
destruction without the requirement of chemical additions.
Summary of the Invention
The invention provides a process for the substantial destruction of sludge
comprising diluting the sludge to 1,000 ~5,00()mg/litre COD, reacting the sludge with a
bacterial population at oxygen levels greater than lppm, under conditions of high
intensity mixing.
In the present application high intensity mixing is used to indicate that the
mixing process is sufficiently intense to break up the flocs of bacteria formed within the
l‘63CIOI‘.
In the present application the intensive mixing produces a shearing force within
the reactor system generated suitably by an aeration source and directed at the sludge
and the bacterial population. This ensures that the sludge and bacterial population are
always maintained in a finely divided form and in maximum contact with each other
and the dissolved or micro bubble oxygen.
The process is preferably carried out at ambient temperatures.
Preferably in the process according to the invention the sludge is blended and
macerated to allow the intimate homogenisation and blending of waste organic sludges
in the solid or liquid form with a diluent liquor. Organic material, both dissolved and
suspended is thoroughly mixed and blended and is bacterially treated in the reactor.
There are a number of naturally occurring genera of bacteria such as Sarcina,
Staphylococcus, Bacillus, Nitrosomonas, Vibrio, Pseudomonas or Nitrobacter which
llourish in the strongly aerobic and fertile conditions found in the reactor. These
bacteria are preferentially selected because of the rich supply of nutrient and ready
supply of oxygen all presented at suitable temperatures and acidity to encourage
digestion of the organic waste. The reactor may however be seeded with blends of
micro—organisms if necessary. The ammonia present formed by the degradation of some
of the nitrogenous waste is used by other bacteria and is oxidised to nitrite and nitrate .
Preferably, the incoming sludge is diluted with water or more preferably, raw
sewage. The latter can provide a ready source of micro nutrients such as essential
vitamins and minerals necessary for biological growth and activity. Such nutrients may
be deficient in some sludge.
Preferably the sludge is enriched with nutrients, e.g. nitrates, phosphates, trace
elements, vitamins and micro nutrients. Preferably the enrichment process is carried out
by diluting the incoming sludge with raw sewage.
Preferably the process comprises mixing the sludge and diluent under high
intensity mixing. High intensity mixing is essential to break up any solid sludge and
provide a homogeneous mix. This aids bacterial digestion of the sludge.
Preferably, the level of dissolved oxygen in the destruction reactor is maintained
at levels greater than lppm. This allows sludge destruction to occur. The presence of a
ready supply of oxygen and food in the form of waste allows the bacteria to respire and
multiply resulting in a bacterial population which destroys the sludge.
Preferably the process allows a rapid reduction of Biological Oxygen Demand
(BOD) in the reactor with a dissolved oxygen (DO) level of at least 3ppm at all times.
This allows the rapid uptake of organic material and optimises the conversion of
ammonia to nitrate. The bacterial population is provided with a source of food in the
form of sludge and the population of bacteria rises very rapidly to meet the level of
food available. Sufficient oxygen must be supplied to encourage this growth. Organic
material is destroyed by bacteria oxidation to carbon dioxide and water. The food level
is however constant and soon the bacterial population exceeds the food available. This
point is known as endogenous respiration since some of the bacteria die off increasing
the food level. Maintaining the feed of waste sludge to ensure that endogenous
respiration is just maintained while ensuring a healthy biomass is an essential part of the
sludge destruction process.
Further preferably, in the process according to the invention the sludge is
retained in contact with the bacterial population in the reactor for 1-24 hours.
Preferably, in the process according to the invention the suspended solids in the
reactor mixed liquor is less than 10,000mg/litre and is preferably in the range
()0()-90()()mg/litre.
Preferably the process comprises separating reactor solids from suspending
liquor, suitably in a primary clarifier. This clarifies the resultant liquor. All the
separated solids are returned to the reactor inlet. Other means of separation may be
used such as hydroeyclones, centrifuges or one of a variety of conventional clarifiers.
Further preferably, the process comprises (1) treating the resultant liquor with
denitrifying bacteria; (2) separating denitrifying biomass from supernatant treated
liquor; (3) clarifying supernatant treated liquor by adsorption means; wherein blended
and waste sludge is contacted with waste sludge from the denitrification process.
Preferably the process comprises contacting the resultant clarified liquor with
denitrifying bacteria within the denitrifying Vessel for between 3 and 15 hours.
Further preferably, the process comprises agitating the clarified liquor. Agitation
may be carried out by stirring or other suitable means of agitation.
Preferably the process comprises separating the denitrification vessel solids from
the suspending liquor. Preferably separation is carried out in a secondary clarifier.
Preferably the process comprises returning at least a portion of denitrification
vessel solids to the denitrification Vessel, the balance of solids being returned to the
reactor inlet. The return of a portion of denitrification Vessel solids to the denitrification
vessel ensures a good concentration of viable biomass in the vessel. The balance of
solids is returned to the reactor for destruction.
Further preferably, between 5 and 20% of the solids are returned to the
denitrification Vessel, the balance of solids being returned to the reactor.
Preferably the process comprises treating the supernatant liquor from the
secondary clarifier in an adsorption filter. Current technology is used wherein contact
between the clarified liquor and iron salts allows dissolved phosphate ions to
precipitate.
Preferably the process comprises contacting diluted and macerated feed sludge
with returned sludges from the primary and secondary clarification steps for a period of
O.5—2 hours. The diluent may be a waste water stream, a final effluent from a treatment
plant or plain water. Other diluents include domestic waste water, industrial waste
water, or blends of water and influent and effluent.
Preferably, the process comprises converting nitrate and nitrite adsorbed on the
incoming sludge and that produced by ammonia oxidation, to nitrogen using
denitrifying bacteria.
Preferably the process comprises maintaining oxygen levels in the denitrifying
step at less than lppm. This enables the denitrifying bacteria to flourish. Denitrification
requires a different bacterial species than those used in the sludge destruction step. The
levels of oxygen within the sludge destruction step work against selection of
denitrifying bacteria.
Preferably the denitrification process is operated in a manner to allow sufficient
carbonaceous material to be available. Preferably the carbon content of the feed to the
denitrifier is kept sufficiently high by controlling the llow rate through the reactor. Too
low a [low will degrade all the carbonaceous material and the denitrification step will
cease to work effectively.
Preferably the flow rate through the reactor is selected prior to commencing the
process and is dependent upon the sludge type and the form of diluent liquor.
Preferably the level of dissolved oxygen within the denitrification tank is
maintained in the range 0.1 to 0.3ppm. Such levels of oxygen encourage the
denitrification organisms to use the oxygen from nitrated or nitrite ions rather than
oxygen in the surrounding water.
Preferably the level of organic carbon is kept at a level sufficient to allow the
bacteria to flourish. The level of carbonaceous material is dependent upon the residual
nitrate levels to be converted to nitrogen and can vary substantially.
Preferably the level of carbonaceous material in the process according to the
invention is calculated empirically during plant set up by controlling flow within the
reactor and therefore residence time in the denitrification vessel.
Preferably the process comprises aerating the liquor prior to allowing the
denitrified liquor to settle. This aeration step removes micro—bubbles of nitrogen by
established principles of mass transfer and assists clarification.
Preferably in the process according to the invention, resulting liquor from the
denitrifying step is allowed to settle. The liquor is allowed to settle in a secondary
clarifier which separates any remaining sludge.
Preferably, the process comprises maintaining the population of the denitrifying
bacteria in the denitrilication step at a viable level by returning a proportion of settled
sludge to the denitrifier. This also provides a further source of carbonaceous material to
assist the denitrification reactions.
In a preferred embodiment, between 5 and 20% of the settled sludge is returned
to the denitrification step.
In general, the discharge of nitrate, nitrite and phosphate from a waste water
treatment plant is legally regulated. These nutrients may be treated biologically or they
may partially rely on adsorption on the sludge usually wasted. As there is no waste
sludge in the process according to the invention, there is no waste sludge available to
carry adsorbed nutrients to alternative disposal processes. Therefore it is essential to
treat all the nutrients in order to ensure removal from the system.
Preferably, the present invention provides a process having an efficient nitrogen
removal regime comprising contacting blended and waste sludge with waste sludge
from the denitrilication process. The balance of the sludge in the secondary clarification
step is returned to a pre-contact step. This allows preliminary denitrification of
adsorbed nitrate from the waste sludge to occur. This should commence the
fermentation process. Nitrogenous material present on the incoming sludge feed and
biomass is converted to ammonia and eventually nitrite and nitrate within the reactor
using microorganisms.
Preferably the process comprises operating the pre-contact step for 0.5 to 1 hour
under anoxic conditions. The pre-contact step improves flocculation of the reactor
biomass.
Conventional biological phosphate removal systems rely on phosphate being
adsorbed on the conventionally wasted sludge. In the present invention as there is no
wasted sludge, such biological phosphate removal systems are not applicable.
Preferably the process comprises removing phosphate from the effluent. The
process provides for removal of dissolved phosphate ions and heavy metals by means of
an adsorption filter. The filter allows the final effluent to contact materials with an
affinity for phosphate and heavy metals.
In one embodiment the process involves treating and reducing the level of
phosphate from reactor effluent using sacrificial iron oxidation at a pH Within the range
from 6.0 to 7.5. This may be carried out by oxidising iron in the form of coarse steel
wool for example, in the Water stream by natural processes. Provided that there is little
oxygen in the water and the pH is not above 7.5, the phosphate ions contained in the
water will react with the ferric iron and precipitate. Alternatively the effluent may be
passed through an iron containing mineral such as latterite allowing the phosphate to be
reacted in the same way.
Further preferably, the process according to the invention comprises removing
heavy metals from the effluent. Such heavy metals may sometimes be found in waste
sludge, (particularly sludges arising from the treatment of industrial waste) and are
liberated from the sludge into solution by the process. Heavy metals may be removed
by passing the effluent through an ion—exchange column. Preferably the phosphate and
metal filters comprise two packed beds of media in series. Preferably, the media
comprises Latterite, for removal of phosphate, and a suitable ion-exchange medium for
removal of heavy metals.
Preferably the process comprises clarifying the final liquor and removing
nutrients. The clarification of the final liquor is carried out by passing the liquor
through a secondary clarifier, preferably of inclined plate design and of appropriate area
for separation of denitrified effluent and the denitrifying sludge. The resulting clarified
liquor is passed through a phosphate filter.
Further preferably the process comprises passing the clarified liquor through a
heavy metals filter. The resulting effluent is within regulatory limits for discharge to
inland or coastal waters. Such limits are set by the appropriate regulatory authorities
and may be subject to change. Typical discharges must be at least: BOD (Biological
oxygen demand) Ztlppm; Suspended Solids 30ppm; Phosphate Zppm; Nitrate Sppm and
.0 ppb (parts per billion) or less for heavy metals.
Advantageously, the process according to the invention allows the waste sludge
from a number of sources to be treated biologically in a manner to provide complete
destruction of the organic material contained within the sludge. A highly efficient and
cost effective process is provided.
Further preferably, and to allow for situations where the effluent from the sludge
destruction plant may be returned to an existing water treatment plant, the process may
be operated to provide a discharge effluent having a higher COD (Chemical Oxygen
Demand) and BOD (Biological Oxygen Demand) and Suspended Solids than those
exemplified above.
The invention provides a process wherein the treatment and destruction of the
waste organic sludge generated from the biological treatment of organic waste liquors
includes the integral treatment and removal to regulatory levels, of nutrient products
such as nitrates, phosphates and heavy metals.
The invention provides a process whereby the organic component of waste
sludge is totally destroyed using a combination of mechanical and biotechnological
processes. Preferably, all of the sludge is destroyed thereby dispensing with the need for
disposal routes.
Use of the process according to the present invention allows for complete
destruction of sludge. Therefore there is no requirement for pre-treatment or post-
treatment storage.
The invention provides a process whereby the final effluent has a BOD of l()—
mg/litre.
The invention provides a process whereby the final effluent has a COD
(chemical oxygen demand) in the range 4()—500mg/litre. The COD provides a quick and
convenient method of determining the organic content of a waste water by using a
chemical oxidation technique rather than the biological methods required for BOD
determination. It is possible with a reasonable degree of accuracy for those skilled in the
art to correlate the BOD figures with those obtained from the COD result. Once such a
correlation is achieved, it is possible to control plant operating parameters using COD
measurements. Therefore remote process management becomes achievable.
The invention provides a process wherein the level of suspended solids in the
final effluent is in the range 10-200mg/litre.
The level of nitrates in the final effluent is in the range 2—lOmg/litre, the level of
phosphates in the final effluent is in the range l—l()mg/litre, and the level of heavy
metals is in the range 0.1-O.5mg/litre.
The invention provides a process wherein the levels of total unoxidised nitrogen
(TUN) as N is in the range 2.0-10mg/lt and the level of ammonia as N is less than
lmg/litre.
The process according to the invention does not produce an offensive odour.
Further preferably, the by-product of the process described does not have an odour.
This feature makes the process more environmentally acceptable, particularly in urban
areas.
The invention also provides a plant for the biological destruction of organic
sludge using microorganisms comprising; (a) a reactor adapted to receive liquor and
retain it in contact with a bacterial population, (b) an aeration means adapted to provide
oxygen levels above lppm within the reactor in order to maintain bacterial population
growth, and (c) a means for intensive mixing of the reactor contents.
Preferably the reactor is adapted to be seeded with a bacterial population capable
of digesting sludge. The bacterial species feed on the dissolved, colloidal and suspended
organic matter.
Preferably the aeration means is suitably adapted to maintain oxygen levels
within the reactor at a level of greater than lppm.
Preferably the aeration means has an oxygen transfer efficiency of at least 40%
and allows vigorous agitation to produce high shear forces.
Preferably the aeration means comprises a jet aerator. The aerator allows for
vigorous aeration and maintains an even homogeneous blend of solids suspension with
an intimate mixing of biomass and oxygen to allow rapid respiration and bacterial
growth.
Preferably the plant comprises a high intensity mixing system, more preferably a
high shear macerator/mixer. The high shear mixer provides high tangential fluid forces
that act upon clumps of sludge breaking them up into their constituent parts. The high
shear maeerator uses knives to cut and mince large masses of foreign bodies which
could block the plant. The combined high shear macerator/mixer allows for intimate
mixing of the liquor and sludge, and the breaking up of flocs of bacteria.
Further preferably the plant comprises a blender means adapted to receive liquid
macerated sludge or liquefied and macerated solid dewatered cake and combine it with
a liquid stream which acts as diluent.
Preferably the blender means is provided with a high shear macerator. The raw
sludge is macerated prior to blending with diluent. The high shear ensures the breaking
up of any clumps of sludge and assists in the degradation of cellular material so as to
form diluent liquor in a form suitable for biological fermentation.
Preferably the plant comprises a pre—contact tank. the pre-Contact tank is
suitably adapted for further diluting the blended sludge and mixing it with sludge
returned from a primary clarifier.
Preferably the plant comprises a denitrifying system having a primary clarifier, a
denitrification tank and a secondary elarifier.
Further preferably, the primary clarifier is adapted to receive liquor from the
reactor and to allow sludge to settle.
Preferably the plant comprises a denitrification tank adapted to receive liquor
and retain it in contact with denitrifying organisms for a period of 3 to 15 hours.
In a preferred embodiment of the invention the denitrifying tank contains an
agitator.
Preferably the plant comprises a secondary clarifier, adapted to return at least a
portion of the sludge to the reactor and the balance to the denitrifying tank.
Further preferably, the secondary clarifier is adapted to return all the settled
sludge to the reactor.
Further preferably the plant comprises an adsorption filter adapted to receive
clarified liquor and remove phosphate. The adsorption filter comprises an adsorption
media which converts a proportion of the dissolved phosphate to an insoluble form.
Preferably, the adsorption filter is a packed bed of material which has an affinity for
phosphate ions. This may, for example, be naturally occurring material such as Laterite
or other iron containing materials or further preferably alternative artificial phosphate
absorbers.
In a further embodiment of the invention the adsorption filter is adapted to
remove heavy metal ions.
In a still further embodiment of the invention the filter comprises an additional
section with alternative adsorbers, for example ion exchange media, which have an
affinity for heavy metals. The filter is designed to be easily removable for adsorber
removal or renewal.
In an alternative embodiment the plant comprises a reactor comprising (a) two
compartments of about equal volume, each provided with an aeration means and (b) a
sump. The sump may collect inorganic matter.
Preferably said aeration means comprises a jet aerator.
Preferably, the plant for carrying out the process according to the invention is
sized to fit into a standard transport container (freight container) attachable to a means
of transport, preferably a container lorry. The plant may be suitable for treatment of
sludge at intensive animal breeding farms, industrial factories or domestic and industrial
waste treatment plants.
In one embodiment the plant is constructed in concrete or steel as a permanent
fixture. The choice of material used in the construction is dependent on the volumes of
sludge to be treated.
In an alternative embodiment the plant is a prefabricated steel structure to be
placed on a flat concrete foundation.
In a still further embodiment the plant is constructed as a package designed to fit
into a standard transport container attachable to a means of transport.
One such arrangement comprises the packaging of the plant to fit a standard
transport container attachable to a means of transport. The term standard container
herein means standard sizes of transport container such as 20 foot and 40 foot
containers. Thus the plant is suitable for the treatment of sludge waste arising from
populations of between 1,000 and 20,000 people or the equivalent volume of industrial
sludge or animal waste. Larger volumes of sludge could be treated by multiple such
container units or by a prefabricated package plant or a concrete or steel structure
according to the invention.
Preferably the plant for carrying out the process according to the invention is a
self contained modular package for drop—in situ installation.
Further preferably, the plant is adapted for multi point connection for quick and
easy commissioning. The plant is suitably adapted for connection to services such as
water and electricity.
Preferably the process and plant according to the invention has the capacity to
treat organic waste sludges from a variety of sources such as domestic and industrial
waste water, including domestic sewage treatment systems, mixed domestic and
industrial sewage treatment systems, chemical and petrochemical, pharmaceutical,
brewery, paper, sugar, oil and gas, food processing and manufacture and agricultural
slurries from the intensive breeding of animals and intensive growing of vegetables.
Preferably the sludge destruction plant allows for a self contained sludge
destruction plant to be placed on the site where that sludge is produced and become a
part of the existing waste treatment process. The treatment process allows for a
substantial reduction in space, typically half the space required for sludge storage and
one third the space required where sludge reduction processes such as dewatering or
anaerobic digestion are carried out.
The sludge destruction plant may conveniently be sited close to the sludge
generating treatment facility. The present invention allows for the sludge destruction
plant and process to become a part of an existing biological treatment facility. This
eliminates the cost associated with conventional sludge disposal routes, including
handling and transport costs.
Alternatively, the sludge destruction plant and process may be a stand alone
facility and be suitable for the treatment of a number of different organic waste sludges
in solid or liquid form and from a number of different biological waste treatment
facilities.
Brief Description of the Drawings
The invention will be further described by way of example only, and with
reference to the accompanying drawings in which:
Fig 1 is a schematic representation of the plant for carrying out the sludge destruction
process according to the invention.
Fig. 2 is a schematic representation of an alternative embodiment of the plant for
carrying out the sludge destruction process according to the invention.
Detailed Description of the Invention
In the process according to the invention, the waste sludge to be treated is pre-
treated according to its origin. Solid sludges such as those coming from domestic or
industrial waste treatment facilities and which have been mechanically dewatered with
centrifuges or belt or filter presses are reslurried in a blending tank (a) which contains a
high shear mixing head . Such mixers are commercially available and are selected
according to efficiency. The sludge is reslurried in plain Water, final effluent from a
water treatment facility or screened and degritted intluent from a water treatment
facility. The selection of the diluent is for convenience and cost.
The mixing and blending of the Waste sludge is an important step in the process.
The mixer must provide sufficient shear to break up any solid sludge and provide a
homogeneous mix. The shear effect will also assist in the breaking of cellular material
and assist materially in its digestion within the reactor system.
It has been demonstrated however that solid sludge such as that produced from
waste water treatment processes after dewatering with centrifuge, belt press or filter
press may be fed in bulk to the reactor. The high shear aerator system proved effective
in reslurrying the sludge in the reactor so that destruction could occur. While this
method is not necessarily the preferred route, a satisfactory rate of sludge destruction
was measured as measured by the suspended solids levels in the reactor.
EXAMPLE
A waste activated sludge containing 18% solids was fed in bulk to the reactor
system of Volume 12M3. The diluent was a domestic sewage stream flowing
continuously 2M3/hr.
The solids were fed twice per day with a six hour gap and were started at a low
rate up to a maximum of 500kg/day over a five day period. The method of feeding can
be regarded as shock loading. “Hie results shown in the table below indicate the
flexibility of the process.
Day Total solid fed (kg. at 18% ds) Reactor solids (mg/lt) Effluentso1ids(mg/lt)
() 0 4340 25
l 28 4588
4472
4715
5680
6230 42
8738 152
6845 38
Incoming liquid sludges such as those from agricultural, industrial or sewage
sources will also be subject to the high shear blending process, to ensure a fully
homogeneous mix and a degree of shear sufficient to produce cell breakdown where
possible.
In the process according to the invention, the blended sludge is prepared as a
slurry at 2-5% solids, is screened and passed to a pre-contact tank (b) where it is further
diluted to bring the COD to a range of 1000-7000mg/litre. It is intimately mixed with
the returned sludge from the primary clarifier (e). The residence time within the pre-
contact tank is in the range 0.25—l.0 hours. The liquor overflows to the reactor (c)
whose volume is determined by the nature of the waste sludge being treated. The
volume typically would be in a range to provide 6-24hrs retention. Once in the reactor
(c) the liquor comes into contact with the bacterial species which feed on the dissolved,
colloidal and suspended organic matter therein. The aerator (d) within the reactor (c)
introduces air into the reactor and also functions as an excellent mixing system which
allows the bacterial species to have a ready supply of oxygen, thus allowing rapid
respiration and bacterial growth. Such growth rates are dependent upon nutrient levels,
oxygen availability temperature and pH. The latter three criteria are controlled by plant
operation and it is therefore the availability of nutrient that will control the growth
patterns of the biomass. Thus where there is a nutrient rich stream feeding a low
biomass population in highly aerated conditions, an exponential growth of bacteria will
occur. As the bacterial population exceeds the level of nutrient able to sustain it, it
ceases to grow and allows the level of predatory higher organisms such as protozoa and
rotifers to increase. The bacteria are therefore exposed to two constraints, first a lack of
food to sustain the population and second a level of predation that is increasing. The
bacteria therefore enter a zone of endogenous respiration. In this phase bacteria begin to
die, the cells break up thus providing some additional nutrient, which in turn will
encourage further growth.
The aeration system provides for intensive aeration. There is always sufficient
oxygen to encourage bacterial growth when sufficient nutrient is available. Thus food
availability is a limiting step and at the point of endogenous respiration with bacteria
dying, the population is vulnerable. If a proportion of the bacterial population are
deactivated, for example by the breaking up of the naturally formed clumps or flocs by
shear forces, then the population pressures on the remaining bacteria are removed and
the growth phase will resume. Such shear forces may be provided by the aeration
system, the flocs are broken up into their constituent bacteria, weakening them and
rendering them vulnerable to predation. The continuous removal of bacterial population
and encouragement to predation by higher organisms, quickly establishes a balance of
growth and digestion within the reactor.
The rate of growth of the biological system can be described by a simple
equation:
dX/dt : uX - keX - kdX
where X is the concentration of microorganisms
U is the specific growth rate. This is a measure of the rate of cell division.
ke in the above is the specific endogenous respiration rate. This is a measure of
utilisation of stored cell materials ending in cell lysis (cell death with membrane
breakdown and release of nutrients.)
kd is the rate of predation. This is a measure of the feeding rate of the other
organisms that rely upon the bacterial population as food (e.g. protozoa or rotifiers).
When the system is in the growth phase, that is when the food supply is
plentiful, specific growths are as high as 5 per day. By contrast, in the endogenous
phase, when the bacteria are using up their own resources and dying, the specific
growth rate is below 0.1 per day.
Negative growth rates are conventionally achieved by holding the system at a
low food to micro-organism ration (F/M), either by a slow rate of feed or maintaining
the population of biomass for a long period (tens of days). An example of this is the
extended aeration process. In such a system the rate of decrease of biomass is slow
because it is governed by the endogenous respiration of the population. The death of the
micro-organisms is therefore by "natural" causes. The transfer of oxygen is a critical
factor in process efficiency. In the process described herein a jet aerator is used as it
offers efficiencies greater than 40% in the transfer of oxygen to the waste water. The jet
aerator provides a very efficient mixing action and ensures that the biomass and oxygen
are at all times in intimate contact. The ready availability of oxygen and the fluid
motion across the bacterial cell surface allows substrate (food) uptake to increase. The
substrate in the water now becomes limiting so that additional energy is now required
for the digestion of bacteria by other bacteria. The factor kg in the above equation
therefore increases.
By maintaining the system in the endogenous respiration phase, conditions are
created to allow the favourable growth of certain protozoa and rotifiers, both higher
forms of life than bacteria whose source of food is the bacterial population. Bacterial
cells naturally floc together to form clumps which act as a defence to predation. In the
process described herein the high shear mixing system confers strong tangential fluid
forces to the clumps, breaking them down into smaller units and also releasing loose
bacterial cells. These cells are captured and digested by the protozoa and rotifiers. This
results in an increased size of population of these predatory organisms. Thus instead of
the cells dying naturally they are consumed by the predators. The factor kd is now
increased.
In the process described herein, the combination of shearing and high oxygen
transfer with the consequent increased substrate uptake means that the biomass in the
reactor is in a continuously highly active state, with a rapid uptake of substrate, high
turnover of bacteria and high predation rates. In such conditions which mimic the
situation of the explosive growth phase in conventional systems a high substrate feed
rate can be achieved. In the present process waste organic sludge is the substrate and it
is rapidly oxidised by the bacterial species.
In the process according to the invention, the influent liquor contains a high
proportion of organic nutrient wholly or partly coming from the blended sludge. The
bacterial species within the reactor will digest the organic material and convert it to
carbon dioxide and water. Some of the nitrogenous material will be converted to
ammonia and then oxidised to nitrates. A proportion of the nitrates present will also
come from the incoming sludges, the proportions being dependent on the source. In
common with many biological oxidation systems, a small proportion of the nitrogenous
material will prove resistant to biological conversion and will pass through the plant as
unoxidised nitrogen. Levels of nitrate will therefore be high and it is essential to treat
this and reduce as much as possible to nitrogen. In addition, the liquor will contain
phosphate released from the sludge treatment and any adsorbed phosphate from the
incoming sludge. In some industrial sludges there may be adsorbed heavy metals which
will remain in solution and pass unchanged through the plant.
The liquor from the reactor, after the appropriate residence time, overflows into
the primary clarifier (e). This may be of the inclined plate design and allows the sludge
from within the reactor to settle. The settled sludge from the primary claritier (e) is
returned to the reactor (c). The supernatant liquor, containing any unoxidised BOD plus
the nitrate, phosphate and any heavy metals in solution overflows into the
denitrification tank (f). This is ofa volume sufficient to allow efficient denitrification to
occur. Typically the liquor will reside in contact with the denitrifying bacteria for
between 3 and l5 hours.
The conversion of nitrate to nitrogen within the denitrification tank (f) requires
organic carbon to be present. It is convenient therefore to provide the carbon from the
reactor by ensuring that there is sufficient fermentable carbonaceous material present in
the influent to the denitrifier. If this is not possible, external sources of carbon such as
the addition of methanol may be required.
The liquor from the denitrification tank (f) overflows to the secondary clarifier
(g) which may also be of the inclined plate design. This separates the denitrifying
biomass from the supernatant treated liquor still containing phosphate and perhaps
heavy metals. A proportion (between 5 and 15%) of the settled biomass is returned to
the denitrifying tank. The balance is returned to the pre—contact tank This assists in
the preliminary removal of adsorbed nitrate from the incoming sludge. It may be
necessary to aerate the liquor between the denitrification tank (f) and the secondary
clarifier(g). This aeration step will remove micro—bubbles of nitrogen by established
principles of mass transfer and assist clarification.
The clarified and treated liquor overflows from the secondary clarifier (g) into a
phosphate removal filter (h). This is a sacrificial filter packed with a material such as
steel wool which at the appropriate pH will oxidise and simultaneously remove
phosphate ions by precipitation as iron phosphate. Alternatively, other phosphate
adsorption materials such as laterite may be used allowing the effluent to contact the
adsorbent either in the upflow or downflow modes and remove the bulk of dissolved
phosphates. The adsorption material must be renewed or reactivated from time to time.
The phosphate filter may, if required, be combined with a heavy—meta1 ion filter.
This may be a filter through an appropriate ion exchange medium, or an adsorption
system combined with the phosphate removal filter (h).
In the process according to the invention, the final effluent is of a very high
standard, containing low levels of suspended solids and of oxidisable carbonaceous
material. The organic content of the influent sludge is completely destroyed and nitrate
and phosphate levels are reduced to those specified by the regulatory authorities for
discharge to inland or coastal waters. Any heavy metal ions that may have been
adsorbed or contained within the destroyed sludge are easily and conveniently removed
from the effluent liquor. The process is such that simple remote monitoring will allow
for fully automatic operation of the plant once the required operational parameters have
been set.
In some cases it is not necessary to remove all the oxidisable carbon to have a
high quality effluent, it being necessary, simply to destroy the bulk of the incoming
sludge. For example, if the effluent is to be returned to the inlet of a treatment works,
then the residence time within the reactor may be reduced to provide an effluent of
lower quality.
In an alternative embodiment the plant for carrying out the process according to
the invention may be adapted so as to concentrate the inorganic non-degradable
materials that are often found in waste sludges. Some of the inorganic material will be
colloidal or very fine and will therefore be carried over in the waste effluent. However
larger particles will build up as inorganic waste which will have to be removed
periodically.
In an alternative embodiment, the plant for carrying out the process described
herein comprises a reactor having two compartments of about equal volume. The
alternative embodiment showing compartments C1 and C2 is illustrated in Figure 2.
The first section Cl acts as the inlet reactor and is fitted with a jet aerator (d). After an
appropriate residence time in the reactor compartment C1, the liquor is passed into the
second compartment C2 which is also provided with a jet aerator (d). Liquor from C2 is
passed into the primary clarifier (e) as described above. However the returned biomass
from the clarifier is returned to Cl and C2 in different proportions. For example
between 15 and 30% is returned to C1 and between 85 and 70% is returned to C2. The
proportions will vary depending on the feedstock and the rate of destruction. Over time
the proportions of dense inorganic material in C2 will increase and collect in a sump (j)
which is fitted to the bottom of the reactor. Accordingly the inorganic material may be
removed and disposed of at appropriate times.
The words "comprises/comprising" and the words "having/including" when used
herein with reference to the present invention are used to specify the presence of stated
features, integers, steps or components but does not preclude the presence or addition of
one or more other features, integers, steps, components or groups thereof.
Claims (5)
1. A process for the substantial destruction of sludge comprising diluting the sludge to 1,000 -5,000 mg/litre COD. reacting the sludge with a bacterial population at oxygen levels greater than lppm, under conditions of high intensity mixing.
2. A process according to claim 1 comprising (1) treating the resultant liquor with denitrifying bacteria; (2) separating denitrifying biomass from supernatant treated liquor; (3) clarifying supernatant treated liquor by adsorption means; wherein blended and waste sludge is contacted with waste sludge from the denitrification process.
3. A plant for the biological destruction oforganic sludge using microorganisms comprising; (a) a reactor adapted to receive liquor and retain it in Contact with a bacterial population, (b) an aeration means adapted to provide oxygen levels above lppm within the reactor sufficient to maintain bacterial population growth, and (C) a means for intensive mixing of the reactor contents.
4. A plant as claimed in claim 3 further comprising a sump for the collection of inorganic matter.
5. A process for the biological destruction of sludge using microorganisms. substantially as hereinbefore described and/or with reference to the accompanying drawings. seer,-I312 Tomkins & Co.
Publications (1)
Publication Number | Publication Date |
---|---|
IES83378Y1 true IES83378Y1 (en) | 2004-04-07 |
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