Title

Sewage sludge treatment method

Description

The present invention concerns an improved method for dewatering industrial or sewage sludge, in particular a method for dewatering to give a high solids content sludge concentrate more suitable, where appropriate, for subsequent incineration.

Treatment of industrial or sewage sludge typically comprises the steps of primary and secondary treatments. Primary treatment removes fine solid materials suspended in the sewage. In this treatment the sewage is pumped into settlement tanks and left for several hours to allow the fine particles to settle out of suspension and form a primary sludge. Water and suspended detritus is removed from the top of the tank for secondary treatment. In the secondary treatment process the water is either trickled through a biological filter bed or is put into further tanks and aerated. The water is then pumped into settlement tanks for the detritus to settle out and form an activated sludge. Some of this activated sludge is returned to the treatment process and some is retained. Sludges from the primary and secondary treatment processes can be combined, optionally placed in a digester for anaerobic decomposition to produce combustible gases. This leaves a digested sludge which is then dewatered for disposal. Dewatering is a process whereby the sludge is concentrated to increase its solids content.

Historically this digested sludge has been used in the form of a fertilizer, which can be spread on fields. However, various legislation, such as European Council Directive 86/278/EEC on the protection of the environment places stringent limits and controls on this use. In particular the legislation seeks to address issues associated with the introduction of heavy metal contamination into the soil and, thereby, into the human food chain. The disposal of the digested sludge on the land in this manner requires substantial

time and effort in the provision of comprehensive analytical support. Such disposal also carries with it the risk of crop contamination, collateral land contamination and soil poisoning. Member states are also required to prohibit land spreading where prescribed heavy metal levels in the soil have already been attained or have been exceeded. Accordingly, land spreading of the digested sludge is no longer considered a viable long- term option for sludge disposal.

An increasingly important option for sludge disposal is incineration. However, digested sludge typically comprises only 2-3% solids suspended in water, which currently makes incineration a relatively inefficient option

However, the solids content of the sludge can be increased (thereby improving the efficiency of incineration) through a two-step process involving:

(i) chemical conditioning of the sludge; rendering it suitable for (ii) mechanical dewatering

The chemical conditioning process typically comprises one or more sequential chemical treatments such as:

(a) Coagulation. This involves the addition of divalent or trivalent metal salts and/or polyamines and/or polyquaternary ammonium compounds, often followed by controlled pH adjustment in order to achieve the required coagulation.

(b) Flocculation. This involves the addition of a suitable flocculant polymer to cause suspended particles or agglomerates to form larger, aggregated masses suitable for separation by mechanical means (such mechanical means might include, for example, filtration, centrifugation, sedimentation).

The sludge concentrates generated by such processes may be as low as 15% dry solids but are typically in the range 20-28% dry solids by weight. At such low solids levels, incineration can only be achieved by the use of an additional energy input in order to

drive off water and thereby allow the sludge concentrate to be incinerated. When sufficient water has been driven off, the sludge concentrate is said to be 'autothermic' a state in which the energy required for driving off water exactly matches the energy derived from the incineration process. When the sludge concentrate is below its autothermic point, the incineration process will be a nett consumer of energy. If the sludge concentrate has solids contents of around 33% (depending on the calorific value of the sludge solids), the incineration process can be a no-cost option, or even a nett generator of valuable energy.

An object of the present invention is to produce a sludge concentrate with solids content greater than that achieved by existing techniques, so as to produce a sludge concentrate suitable for incineration without the nett consumption of energy.

Another object of the present invention is to provide an improved sludge dewatering process for the preparation of an incinerable sludge concentrate.

Another object of the present invention is to provide a process for the treatment of sludge so as to reduce the time required for filtration or centrifugation in dewatering.

Another object of the present invention is to reduce or eliminate the nett consumption of energy by a sludge incineration process.

Another object of the present invention is to give users of pre-existing coagulant technology the option to combine pre-existing coagulant technology with the present invention allowing users a transitional method from pre-existing coagulant technology to the present invention.

The present invention in its various aspects is as set out in the accompanying claims.

According to one aspect of the present invention there is provided a method of producing a sludge concentrate, which method comprises the following sequential steps:

a) providing a sludge; b) adding a fibrous additive to the sludge; c) adding a polymeric flocculant to the product of step b);

d) concentrating the product of step c) in a filter press, centrifuge or other mechanical dewatering device. e) removing the product formed in step d) from the mechanical dewatering device.

Step b) and step c) may be carried out at the same time or may be sequential ie step c) following step b).

The fibrous additive may comprise one or more natural fibres or synthetic fibres or a mixture of such fibres. Natural fibres are preferably cellulosics, such as fibres derived from one or more byproducts from the harvesting of grain (straw, cobs, stalks, husks), grain processing (bran), sugar processing (bagasse), fruit and edible vegetable processing (seeds, husks, shells, stones), oilseed plants (shells, husks, lint, fibre, sludge, presscake), timber processing (wood chips, wood shavings, sawdust), pulp, paper processing (fibre waste, sulphite liquor), community lignocellulose waste (newsprint) and textile processing. Such by-products may be rendered into fibres suitable for use in the invention by any appropriate means known in the art. Synthetic fibres, suitable for use in the present invention include, without limitation, fibres derived from textile processing such as polyamide, polypropylene, polyester and other synthetic polymers. Due to the varied and individual nature of digested and industrial sludges, the final selection of fibre will require an optimisation step, carried out by a person skilled in the art. A fibrous additive suitable for use in the invention can be supplied as a solid or as liquid slurry.

The polymeric flocculant in step c) can be any such flocculant or mixture of flocculants suitable for purpose, as would be well understood by a person skilled in the art. Non- limiting examples of suitable flocculating polymers and copolymers can be chosen from one or more of polyacrylamide, poly N,N'dialkylaminoalkylmethacrylate (for example:

poly [dimethylaminoethyl acrylate / acrylamide] copolymer, poly [dimethylaminoethyl methacrylate / acrylamide] copolymer), acrylamide, acrylamide copolymer, polyvinyl alcohol, polychitosan, polyethyleneimine, polyvinylimidazoline, polydiallyl amine. Due to the varied and individual nature of digested and industrial sludges, the final selection of polymeric flocculant will require an optimisation step, carried out by a person skilled in the art.

Step b) may further comprise adding a cationic or amphoteric dewetting agent (or a mixture of both) into the sludge at the same time as or sequentially before or after the addition of fibrous additive. Such agents are capable of modifying the wetting and dewetting characteristics of the fibres in order to optimise capillary and surface water shedding.

A suitable cationic dewetting agent may be an ampiphile comprising a quaternary ammonium or phosphonium compound in which the the N or P atom is bonded to one or more C ₁ -C ₂₂ alkyl groups and the hydrophobic portion of the molecule carries a positive charge. A second type would include pyridine derivatives in which the nitrogen atom assumes a quaternary form as in, for example, an alkyl (C ₆ -C ₂₂ ) pyridinium halide structure. A third example would comprise an alkyl (C ₆ -C ₂₂ ) alkylimidazolinium structure in which the nitrogen atom assumes a quaternary form.

A suitable amphoteric dewetting agent may be selected from alkyl (C ₃ -C ₁₀ ) polyamine carboxylate, 2-alkyl (C ₆ -C ₁₈ )(dimethyl) ammonioacetate, 2-dimethyl(3 -alkyl (C ₆ -C ₁₈ ) carboxamidopropyl) ammonioacetate, 3-dimethyl(3-alkyl(C ₆ -C ₁₈ ) carboxamidopropyl) ammonio-2-hydroxy-l -propane sulphonate, 3-alkyl (C ₆ -C ₁₈ ) -(dimethyl) ammonio-1- propane sulphonate, 3-dimethyl(3-alkyl(C ₆ -C ₁₈ ) carboxamidopropyl) ammonio-2- hydroxy-1 -propane phosphate, sodium 2-[2-hydroxyethyl (2- alkyl(C ₆ -C ₁₈ ) carboxamidoethyl)amino] acetate, disodium 2-carboxylatomethyl[2-(2-hydroxyethyl alkyl(C ₆ -C ₁₈ ) carboxamido)ethyl]amino acetate, 3-alkyl (C ₆ -C ₁₈ ) aminopropanoic acid

In the method of the invention, it is preferable to disperse the fibrous additive in the sludge using an appropriate agitation or incorporation technique which may include (but not be limited to) simple stirring and/or high-shear mixing and/or sonication, serpentine or baffled flow. After dispersion, the flocculating polymer may be added with further mechanical agitation, such agitation being undertaken in a manner currently known in the art in the relevant industry.

In the method of the invention, the relative addition level (F ₁ -) of the fibrous additive is expressed as 'kg of fibre per kg of sludge dry solids'. The sludge may be treated with fibrous additive so as to give an F _r value between 0.2 and 20, more preferably between 0.5 and 5 and most preferably between 0.8 and 1.2.

In the method of the invention, the relative addition level (P ₁ ) of the flocculant polymer is expressed as 'kg of flocculant polymer per tonne (1000kg) of sludge dry solids'. The sludge may be treated with flocculant polymers as to give a P _r value between 1.0 and 20, more preferably between 3.0 and 15 and most preferably between 6.0 and 10.0.

In the method of the invention, the relative addition level (DW _r ) of the dewetting additive is expressed as a percentage of F _r . The sludge or the added fibrous additive may be treated with dewetting additive so as to give a DW _r value between 0.005 and 0.5, more preferably between 0.01 and 0.1 and most preferably between 0.02 and 0.07.

For batch operated mechanical dewatering processes, such as filter pressing, step c) preferably takes no longer than 3 hours, preferably no longer than 1.5 hours, e.g. 1 hour, or less.

The sludge is preferably a sewage sludge. The term sewage sludge in the present invention includes any solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Though the present invention is mainly concerned with the treatment of sludges from domestic sewage, a person skilled in the art would realise that it can also be employed in the treatment of other substantially organic

industrial effluent or by-product sludges, such as those effluent or by-product sewage sludges from food and brewing industries

Known methods of sewage sludge dewatering yield sludge concentrates containing around 20 to 28% by weight solids content. Optionally, further drying can be employed to give a concentrate suitable for incineration, i.e. one with a solids content of around 33% or greater. Such drying processes require significant energy input, such that the sewage sludge disposal including incineration uses more energy than it generates.

By using the method of the present invention it is possible to produce sludge concentrates with 33% or more by weight of solids. Such compositions are generally sufficiently concentrated that they will readily incinerate in a conventional incinerator. This is a major advantage as no energy-consuming drying step is required, which leads to significant energy savings and transforms the overall sludge incineration process from a nett energy consumer, as experienced by the prior art process, to a nett energy generator. In particular embodiments, it is possible to obtain a sludge concentrate comprising at least 50% by weight of solids.

Further, use of the method of the present invention can enable sludge concentrates to be prepared faster than prior art methods, as they require less filtering or centrifugation time. For example, the method may be used to produce conventional sludge concentrates up to 3 times faster than with current methods

Sludges can be produced which are at or above their autothermic point, i.e. they have a solids content such that they provide a fuel source capable of costless incineration or of giving a nett energy contribution in an incineration process. Such a nett contribution enables power to be generated from incineration or less energy efficient wastes to be burnt alongside a sludge concentrate produced according to the method of the invention. A greater product throughput is also achievable using the dewatering method of the invention.

According to a further aspect of the invention there is provided a treatment process comprising a method of sludge treatment as described herein, wherein the sludge concentrate produced is burnt in an incinerator without requiring a step to dry the sludge (such step adding to the nett energy consumption in the current state of the art).

The invention further provides a process for generating energy by incineration of a fuel comprising a sewage sludge concentrate obtained as described herein.

According to another aspect of the invention there is provided a fuel comprising a sewage sludge concentrate as described herein.

According to another aspect of the invention there is provided the use of fibrous additive for increasing the solids content of a sewage sludge concentrate.

The invention will now be illustrated by means of the following examples:

Example 1 - Fibrous additive

A sample of a digested sludge from a UK sludge processing plant, was taken prior to the addition of any metallic coagulants. The sludge was analysed for solids content and found to contain 2.91% by weight dry solids.

A wood fibre with mean particle size of 650um was added to give an F ₁ - value of 1.0 and the resultant mixture was stirred thoroughly.

A copolymer of acrylamide and dimethylaminoethyl acrylate, having relative molecular mass in the region of 15,000,000 to 20,000,000, was added to the above mixture to give a P ₁ - of 8.97 and mixed for approximately 20 seconds until flocculation occurred. A strong separation was seen between clean water and the flocculated solids.

The treated sludge was then pressed using a laboratory filter press at a pressure of 6 bar for 15 minutes. The resultant filter cake was analysed and found to comprise 52.51% by weight dry solids.

When the same sludge was treated using a conventional ferric chloride coagulation followed by flocculation using the same flocculant polymer at P _r value of 8.97 and

subsequent pressing, the resultant filter cake was found to comprise 24.18% by weight dry solids.

Example 2 - Fibrous additive plus dewetting additive

A sample of a digested sludge from a UK sludge processing plant, was taken prior to the addition of any metallic coagulants. The sludge was analysed for solids content and found to contain 2.62% by weight dry solids.

A wood fibre with mean particle size of 650um was weighed to give a calculated F _r value of 1.0 and was mixed with a solution of cetyl trimethyl ammonium bromide to give a calculated DW _r of 0.02. This mixture was added to the sludge and stirred thoroughly.

A copolymer of acrylamide and dimethylaminoethyl acrylate, having relative molecular mass in the region of 15,000,000 to 20,000,000, was added to the above mixture to give a P _r of 7.86 and mixed for approximately 20 seconds until flocculation occurred. A strong separation was seen between clean water and the flocculated solids.

The treated sludge was then pressed using a laboratory filter press at a pressure of 6 bar for 15 minutes. The resultant filter cake was analysed and found to comprise 56.13% by weight dry solids.

When the same sludge was treated using fibrous additive alone, at F _r value of 1.0, followed by flocculation using the same flocculant polymer at P _r value of 7.86 and subsequent pressing, the resultant filter cake was found to comprise 47.64% by weight dry solids.

When the same sludge was treated using a conventional ferric chloride coagulation followed by flocculation using the same flocculant polymer at P _r of 7.86 and subsequent pressing, the resultant filter cake was found to comprise 19.88% by weight dry solids.

Description of the Drawing

Figurel/1 is a schematic that represents the steps of the method of the sewage treatment in accordance with the embodiment of the present invention.

To a sewage sludge 1 in figure 1/1 there is added a fibrous additive 3. If the user requires the addition of a coagulant 2, then that is added 4 at the same time as or sequentially before or after the fibrous additive 3. A dewetting additive 5 may be added at the same time as or sequentially before or after the fibrous additive 3 and/or the coagulant 2 if de- watering enhancement 6 is required. A flocculant polymer 7 is added, followed by mechanical de- watering (filter press,centrifuge or other mechanical dewatering device) 8. If the product 9 has useable calorific content then it can be used as fuel for incineration 10, if not it can be disposed of via an alternate method 11.