GB2295146A

GB2295146A - Agricultural product from waste

Info

Publication number: GB2295146A
Application number: GB9423079A
Authority: GB
Inventors: Alan Paul Whipps; Andrew Philip Whorton; Joseph George Doidge
Original assignee: SOUTH WEST WATER SERVICES Ltd
Current assignee: SOUTH WEST WATER SERVICES Ltd
Priority date: 1994-11-16
Filing date: 1994-11-16
Publication date: 1996-05-22
Anticipated expiration: 2014-11-16
Also published as: GB2295146B; GB9423079D0

Abstract

A process for preparing a product useful in agriculture from waste materials, comprises the steps of: (a) partially dewatering an aqueous suspension of a fine particulate mineral material; (b) partially dewatering sewage sludge to a solids content of at least 60% by weight; and (c) mixing the partially dewatered sewage sludge formed in step (b) with the partially dewatered fine particulate waste mineral material formed in step (a) to form a granular product. The mineral material may be limestone or dolomite and may be mined with cement kiln dust, lime kiln dust, gypsum, basic slag, lignite or ash from the burning of biological waste material.

Description

Agricultural material containing mineral and biological waste. and its production This invention relates to a product beneficial in agriculture which incorporates finely divided particulate mineral waste material and biological waste from sewage sludge, as well as to a process for producing such a product.

It would be attractive both from an economic viewpoint and from an environmental point of view if sewage sludge could be processed to form a beneficial product which could be sold for a price which was more than sufficient to cover the processing and transport costs. Sewage sludge contains compounds of nitrogen and phosphorus, and may be regarded as being of potential value as a fertiliser and soil conditioning agent in agriculture and horticulture, provided that it is treated in such a way that it no longer offensive or dangerous to human or animal health.

At present some sewage sludge is used in agriculture in the form of partially dewatered cake or liquid digested sludge. This is transported to the site of use, and spread on the land, at the expense of the water treatment company. There is a risk of polluting nearby water courses, when liquid digested sludge is used, particularly on sites with steep slopes. A further disadvantage of this method of disposal of raw sewage sludge is that there is a statutory requirement to incorporate the material into the top layer of the soil rather than using it as a top dressing.

It is possible to dewater the liquid digested sludge to form a cake and to apply this to the land, but filtration of a slurry of digested sludge is difficult because of the high content of very fine particles. The cake does have an odour, but there is no statutory requirement for the cake of digested sludge to be incorporated into the upper layer of the soil.

There is an increasing trend for sludge produced at major sewage treatment works to be subjected to a thermal drying operation. This treatment renders the material free from harmful pathogens, reduces the volume of the material and renders it easier and less costly to transport. The thermally dried sludge may be applied to the land as a top dressing, but there is potential for dust to be generated during the spreading operation. The present invention makes it possible to transport thermally dried sewage sludge to the site of use, and to spread it on the land, in a form in which the dust problem is substantially eliminated.

A waste disposal problem also exists in the stone quarrying industry. In quarries which produce crushed stone for use as an aggregate for concrete or as road stone, the stone as initially removed from the quarry face is generally comminuted by crushing or grinding to produce a product having the required particle size distribution. The stone which is fed to the comminution equipment seldom comprises pure rock material, but is nearly always contaminated with soil or clay materials. Also, as the comminution product which is contaminated with rock dust, earth or clay is considered undesirable for most purposes, the product is washed with water. As a consequence of this washing operation, there is formed a relatively dilute aqueous slurry of dust-sized waste material particles.This slurry is difficult and expensive to dewater, and, even when the material has been successfully dewatered, there has hitherto been no way of profitably exploiting the dewatered material. As, for environmental reasons, it is unacceptable to discharge this slurry into a river, it has been necessary to confine it in a lagoon on land which could be used more profitably for other purposes. A significant proportion of the quarried stone is lost as dust, accounting for up to about 5% of the total weight of the quarried stone.

Co-pending British Patent Application No.

9321789.1 describes a process for preparing a product useful as a soil conditioning agent from an aqueous slurry of dust-sized waste particles from a stone quarry. The aqueous slurry is partially dewatered to form a cake, and the dewatered cake is mixed with a dry, comminuted alkaline earth metal carbonate mineral having a particle size distribution such that substantially all of the particles have a diameter smaller than 75'ism, to form a granular product.

According to one aspect of the present invention, there is provided a process for preparing a product useful in agriculture from waste materials, the process comprising the steps of: (a) partially dewatering an aqueous suspension of a fine particulate mineral material; (b) partially dewatering sewage sludge to a solids content of at least 60% by weight; and (c) mixing the partially dewatered sewage sludge formed in step (a) with the partially dewatered fine particulate waste mineral material formed in step (a) to form a granular product.

In step (a) the particulate mineral material preferably consists predominantly of finely divided waste material comprising particles of a calcareous mineral such as limestone or dolomite as these minerals will contribute a valuable lime content to the product of the present invention. Advantageously the material will be partially dewatered to form a cake having a dry solids content of from about 50% by weight to about 85% by weight. The aqueous suspension of the mineral material will generally be recovered from a lagoon or pond, and is preferably dewatered by means of a filtration device which operates at a pressure differential in excess of 85psig (590kPa). A convenient filtration device for this purpose is a recessed plate filterpress or a membrane plate filter.

Alternatively, a high pressure dewatering device such as a tube pressure filter of the type described in British Patent Specification No. 1240465 may be used.

A device of this type may be operated at pressure differentials of up to about 1,500 to 2,000 psig (10.313.8MPa), and will produce a cake having a dry solids content in the range from about 75 to about 85% by weight. The dry solids content of the cake will preferably comprise at least 35 % by weight of calcium carbonate. The mineral material will advantageously be the fine dust which is washed from crushed aggregate and road stone which is produced at a limestone quarry.

This fine dust typically comprises from about 60% by weight to about 86% by weight of limestone particles, the remainder being predominantly particles of clay.

The particle size distribution of the mineral material is preferably such that at least 90% by weight, and more preferably at least 95% by weight, of the particles are smaller than 75'ism.

Preferably the dewatered mineral is mixed with a dry comminuted mineral material having a particle size distribution such that at least 90% by weight of the particles, and more preferably at least 95% by weight of the particles, are smaller than 75cm. The mineral material is advantageously natural in origin and preferably consists predominantly of an calcareous mineral such as limestone or dolomite. Alternatively the mineral material may comprise cement kiln dust, lime kiln dust, comminuted gypsum, comminuted basic slag, powdered lignite or the ash which results from the burning of biological waste material to generate power. These materials would introduce different valuable chemical elements into the product of the invention. For example, gypsum would introduce sulphur and powdered lignite carbon.The partially dewatered waste mineral material produced in step (a) is preferably mixed with the dry comminuted mineral material in a weight ratio of partially dewatered waste material to dry comminuted mineral material of from 1:1 to 5:1. The cake of the waste mineral material and the dry comminuted mineral material are conveniently mixed in equipment which imparts a tumbling motion to the components to be mixed and is also provided with a high speed impeller which is effective in disintegrating the lumps of waste material cake. The mixing of the dry comminuted mineral material with the partially dewatered cake of the waste material produces a product which has an overall moisture content which is below 20% by weight, and generally within the range of from 5 to 15% by weight.The product may, if desired , be formed into granules before mixing with the partially dewatered sewage sludge which is formed in step (b).

In step (b) the sewage sludge is preferably dewatered to give a dry solids content in excess of 708 by weight. The sewage sludge may be dewatered by a process of filtration or centrifugation, or by air drying, or by a combination of such processes. Most preferably the sewage sludge is subjected to a thermal drying process as this effectively sterilises the sludge. The dry solids content of thermally dried sewage sludge is typically in the range from about 80 to about 95% by weight. The dry solids content of digested sewage sludge typically contains from about 60 to about 90% by weight of organic matter, from about 1 to about 6% by weight of nitrogen, from about 0.5 to about 1.5% by weight of phosphorus and up to about 0.2% by weight of potassium.The bulk density of the partially dewatered sewage sludge is advantageously in the range from about 0.4 to about 0.8. The partially dewatered sewage sludge may, if desired, be formed into granules before being mixed with the partially dewatered mineral material formed in step (a).

The granulation of the dewatered mineral material and/or of the dewatered sludge and/or of the mixed material formed in step (c) may be achieved by pelletising, shredding, pressure forming or extrusion followed by chopping. Furthermore, one or more of these operations may be performed on the mixed material formed in step (c) as an additional step to improve the granular nature of the product.

In step (c) the dewatered sewage sludge is preferably mixed with the mineral material in equipment which imparts a tumbling motion to the components to be mixed and is also provided with a high speed impeller which is effective in disintegrating any lumps which may be present. The product of a mixing step of this type is a granular material which is readily spreadable on the land, and which does not disintegrate easily on spreading to form dust. Alternatively, both the dewatered mineral material and the dewatered sewage sludge may be provided in an already granulated form, and the two granular materials may be lightly blended together. The dewatered mineral material is preferably mixed with the dewatered sewage sludge in a weight ratio of dewatered mineral material to dewatered sewage sludge of from 8:1 to 0.2:1.Other particulate materials may be incorporated into the mixture formed in step (c). For example, a potash-containing material may be incorporated to increase the overall potassium content and thus enhance the value of the product as a fertiliser.

A second aspect of the present invention provides a granular product useful in agriculture formed by mixing together a partially dewatered mineral waste material and a partially dewatered sewage sludge.

The present invention is illustrated by the following non-limiting Example.

EXAMPLE Waste silt from a lagoon adjacent to a lime stone quarry was dewatered by means of a membrane plate filter operating at a pressure of 90psig (621kPa) to give a cake having a dry solids content of 76k by weight. The silt was of variable composition but, on average, contained approximately 60-80t by weight of limestone, the remainder being predominantly clay particles. The particle size distribution of the silt was such that from 90 to 100k by weight consisted of particles having a diameter smaller than 63cm.

The cake was mixed with pulverised limestone of the type used as a filler in asphalt. The pulverised limestone had a particle size distribution such that 98k by weight consisted of particles having a diameter smaller than 63cm. The components were mixed in the proportions of 1 part by weight of pulverised limestone to 4 parts by weight of the cake formed by dewatering the silt. These proportions corresponded approximately to 3 parts by weight of dry silt solids to 1 part by weight of pulverised limestone.

The mixing was performed in an Eirich (trade mark) mixer which comprises a rotating pan and a high speed rotating agitator which is effective in disintegrating any lumps of the cake which are present. Mixing was continued for a period of 6 minutes. There was then added to the mixer thermally dried digested sewage sludge having a dry solids content of 95.6% by weight and a nitrogen content of 6% by weight in the proportions of 1 part by weight of the digested sewage sludge to 3 parts by weight of the mineral mixture.

The mixed product consisted of granules which were sufficiently dry not to cohere and which had sizes in the range from 0.5 to 4.Omm.

As a control, a second mixture was prepared, in accordance with British Patent Application No.

9321789.1, using further samples of the same waste silt and of the same pulverised limestone as described above and mixing them in the same proportions and in the same manner as described above, except that no thermally dried digested sewage sludge was added. The product consisted of granules which had a dry solids content of 86k by weight and which had sizes in the range from 0.5 to 4.Omm. The neutralising value, or percentage by weight of CaO, of the granules was found to be 33% by weight.

The granules were tested for their soil conditioning and fertilising properties by applying them to plots marked out in an area of arable land.

The land had already received treatment with a fertiliser containing 17 by weight of nitrogen, 17% by weight of potash (K20) and 17% by weight of phosphorus pentoxide (P205) at a rate of 250 kg/ha and with a fertiliser containing 34.5% by weight of nitrogen at a rate of 187.5 kg/ha. Each batch of granules was applied at the rate given in the Table below to four randomly distributed plots each of size 4m x 1Sm. As a further control experiment, four plots of the same size, randomly distributed within the same overall pattern, received no additional treatment.

The area including the test plots was then sown with spring barley of the variety Chariot on 26th March, 1993 and the crop was harvested the following August. The average yield of barley for each group of four plots in tonnes of grain at a standardised dry matter content of 85 by weight per hectare was determined. The average nitrogen content of the grain produced in each group of four plots was also determined.The results are set forth in the Table below: Table Treatment Rate of application Yield (85% DM) Grain Nitrogen (t/ha) (t/ha) (% wt) 4 parts silt cake, 1 part 17.98 6.59 2.14 limestone, 1.67 parts digested sludge 4 parts silt cake, 1 part 13.65 5.28 1.50 limestone No treatment - 4.99 - Notes: (i) "85% DM" means that the yield is expressed on the basis of a standardised grain dry matter content of 85% by weight.

(ii) The rates of application are based on a total organic nitrogen rate of 250 kg/ha and lime at the rate of 4.5 t of CaO/ha.

It will be observed that the inclusion of thermally dried sewage sludge in the soil conditioner/fertiliser granules increased the yield of barley by approximately 25% and the nitrogen content of the grains by approximately 43% as compared with treatment with the granules comprising only silt cake and limestone. These results are especially surprising in view of the fact that the land had already received treatment with 42.5kg/ha of each of phosphate and potash and 107kg/ha of nitrogen. It was also observed that, during the growing season, the barley on the plots which had been treated with the granules containing the digested sewage sludge always looked more vigorous and was about 10-15cm taller than the barley growing on the other plots.

Claims

1. A process for preparing a product useful in agriculture from waste materials, the process comprising the steps of: (a) partially dewatering an aqueous suspension of a fine particulate mineral material; (b) partially dewatering sewage sludge to a solids content of at least 60% by weight; and (c) mixing the partially dewatered sewage sludge formed in step (a) with the partially dewatered fine particulate waste mineral material formed in step (a) to form a granular product.

2. A process according to claim 1, wherein the particulate mineral material consists predominantly of finely divided waste material comprising particles of a calcareous mineral.

3. A process according to claim , wherein the calcareous mineral is limestone or dolomite.

4. A process according to claim 1, 2 or 3, wherein the aqueous suspension of the fine particulate mineral material is partially dewatered to form a cake having a dry solids content of from about 50% by weight to about 85t by weight, preferably from about 75% by weight to about 85% by weight.

5. A process according to claim 4, wherein the dry solids content of the cake comprises at least 35% by weight of calcium carbonate.

6. A process according to any preceding claim, wherein the aqueous suspension of fine particulate mineral material is partially dewatered by means of a filtration device which operates at a pressure differential in excess of 85psig (590kPa).

7. A process according to claim 6, wherein the filtration device is a recessed plate filterpress or a membrane plate filter, or a high pressure dewatering device which may be operated at pressure differentials of up to about 1,500 to 2,000 psig (10.3-13.8MPa).

8. A process according to any preceding claim, wherein the particulate mineral material is a fine dust which comprises from about 60% by weight to about 86% by weight of limestone particles, the remainder being predominantly particles of clay.

9. A process according to any preceding claim, wherein the particle size distribution of the mineral material is such that at least 90% by weight, and preferably at least 95% by weight, of the particles are smaller than 75pm.

10. A process according to any preceding claim, wherein the partially dewatered suspension of mineral material is mixed with a dry comminuted mineral material having a particle size distribution such that at least 90% by weight of the particles, and more preferably at least 95% by weight of the particles, are smaller than 75pom.

11. A process according to claim 10, wherein the dry comminuted mineral material is predominantly an calcareous mineral; or comprises cement kiln dust, lime kiln dust, comminuted gypsum, comminuted basic slag, powdered lignite or the ash which results from the burning of biological waste material to generate power.

12. A process according to claim 10 or 11, wherein the partially dewatered waste mineral material produced in step (a) is mixed with the dry comminuted mineral material in a weight ratio of partially dewatered waste material to dry comminuted mineral material of from 1:1 to 5:1.

13. A process according to claim 10, 11 or 12, wherein the mixing of the dry comminuted mineral material with the partially dewatered cake of the waste material produces a product which has an overall moisture content which is below 20% by weight, and is preferably within the range of from 5 to 15% by weight.

14. A process according to any preceding claim, wherein the product of step (a) is formed into granules before mixing with the partially dewatered sewage sludge which is formed in step (b).

15. A process according to any preceding claim, wherein in step (b) the sewage sludge is dewatered to give a dry solids content in excess of 70% by weight.

16. A process according to any preceding claim, wherein the sewage sludge is dewatered by a process of filtration or centrifugation, or by air drying, or by a combination of such processes.

17. A process according to claim 16, wherein the sewage sludge is subjected to a thermal drying process which effectively sterilises the sludge.

18. A process according to claim 17, wherein the dry solids content of the thermally dried sewage sludge is in the range from about 80 to about 95% by weight.

19. A process according to any preceding claim, wherein the bulk density of the partially dewatered sewage sludge is advantageously in the range from about 0.4 to about 0.8.

20. A process according to any preceding claim, wherein the partially dewatered sewage sludge is formed into granules before being mixed with the partially dewatered mineral material formed in step (a).

21. A process according to any preceding claim, wherein in step (c) the dewatered sewage sludge is mixed with the mineral material in equipment which imparts a tumbling motion to the components to be mixed and is also provided with a high speed impeller which is effective in disintegrating any lumps which may be present.

22. A process according to any one of claims 1 to 20, wherein both the dewatered mineral material and the dewatered sewage sludge are provided in an already granulated form, and the two granular materials may be lightly blended together.

23. A process according to any preceding claim, wherein the dewatered mineral material is mixed with the dewatered sewage sludge in a weight ratio of dewatered mineral material to dewatered sewage sludge of from 8:1 to 0.2:1.

24. A process according to any preceding claim, wherein one or more than one other particulate material is incorporated into the mixture formed in step (c).

25. A process according to claim 24, wherein the other particulate material is a potash-containing material.

26. A process according to claim 1, substantially as described in the foregoing Example.

27. A granular product useful in agriculture formed by mixing together a partially dewatered mineral waste material and a partially dewatered sewage sludge.

28. A granular product according to claim 27, whenever produced by a process according to any one of claims 1 to 26.

Amendments to the claims have been filed as follows CLAIMS

1. A process for preparing a product useful in agriculture from waste materials, the process comprising the steps of: (a) partially dewatering an aqueous suspension of a fine particulate mineral material; (b) partially dewatering sewage sludge to a solids content of at least 60% by weight; and (c) mixing the partially dewatered sewage sludge formed in step (b) with the partially dewatered fine particulate waste mineral material formed in step (a) to form a granular product.

4. A process according to claim 1, 2 or 3, wherein the aqueous suspension of the fine particulate mineral material is partially dewatered to form a cake having a dry solids content of from about 50% by weight to about 85% by weight, preferably from about 75% by weight to about 85% by weight.

8. A process according to any preceding claim, wherein the particulate mineral material is a fine dust which comprises from about 60% by weight to about 86t by weight of limestone particles, the remainder being predominantly particles of clay.

9. A process according to any preceding claim, wherein the particle size distribution of the mineral material is such that at least 90% by weight, and preferably at least 95% by weight1 of the particles are smaller than 75pm.

10. A process according to any preceding claim, wherein the partially dewatered suspension of mineral material is mixed with a dry comminuted mineral material having a particle size distribution such that at least 90t by weight of the particles, and more preferably at least 95% by weight of the particles, are smaller than 75pm.