IES84102Y1 - Method of treating waste material - Google Patents

Description

PATENTS ACT, 1992 S2004/0087 METHOD OF TREATING WASTE MATERIAL ENVIRONMENTAL TECHNOLOGY MANUFACTURING LTD. $0$flIggz METHOD OF TREATING WASTE MATERIAL This invention relates to a method of treating waste material.

In particular, it relates to the treatment of waste by a method that is environmentally safe, involves low energy input and does not generate significant waste streams.

Waste material is typically treated by composting.

Composting occurs either aerobically or anaerobically when organic materials are mixed and piled together. In general aerobic composting is the most efficient form of decomposition and produces finished compost in the shortest time.

In composting, the end product is a humus type material which is high in organic content and nutrients available for assimilation by plants. Composting requires a carbon to nitrogen ratio of approximately :1 and to achieve this, compostable organic materials of different C:N ratios are mixed together. Composting also requires tight controls on moisture, temperature and oxygen levels to achieve optimum composting conditions. As a general rule, if the C:N ratio is greater than 20:1, microbes will immobilise nitrogen into their biomass. If C:N ratio is less than 20:1, nitrogen can be lost to the atmosphere as ammonia gas, causing odour.

It is an object of the invention to overcome or minimise the disadvantages of the prior art.

According to the invention there is provided a method of treating waste material, which method comprises: (a) forming a waste sludge; (b) stabilising the sludge formed in step (a); and optionally (C) composting the stabilised sludge.

Preferably, the sludge is dewatered prior to the stabilisation step (b). The dewatered sludge preferably has a solids content of from S to 50%, more preferably from 10 to 30% by weight. The (dewatered) sludge is preferably stabilised by mixing with soil, preferably in a ratio of sludge (wet basis) to soil of from 1:5 to 5:1, especially about 1:1. The sludge/soil mixture is preferably aerated and is at least partially dried during aeration. Aeration may be achieved by mechanical means, such as, e.g. by trammelling and/or using probes.

In contrast to the conventional composting method, the method of the invention does not require stringent control of moisture, temperature or the C:N ratio. In the method of the invention, the waste sludge is preferably aerobically stabilised in a soil matrix prior to any composting. The soil matrix facilitates some microbial degradation of organic materials and an elimination of odours which would be generated under anaerobic conditions. The stabilisation facilitates the aerobic bioconversion of organics within the waste sludge, which eliminates odour and facilitates the conversion of the sludge into a material comparable to soil.

In the method of the invention, the inert constituents and properties of soil are utilised for improving the texture and particle size of the sludge /soil crumb after mixing. This is a physical process in which moisture from the sludge is absorbed by the soil and a homogenous sludge soil crumb is attained which facilitates contact of the soil micro—flora with the sludge while also producing air spaces between the crumbs which allow more than 5% oxygen and aerobic conditions to prevail in the mixture. Due to the absorption of moisture and the production of the crumb texture, the percentage solids content of the mixture is significantly increased in excess of the solids content of the waste activated sludge. This has advantages in respect of low moisture content, texture and void space production and so generates favourable conditions in the optional composting step of the treatment process.

The method of the invention allows the micro-flora of the soil to utilise the inherent carbon to nitrogen ratio of the waste sludge to initiate stabilisation and remediation of organic compounds in the waste sludge.

Available and readily biodegradable organic materials are converted quickly to more stable materials which results in microbial activity diminishing as the process advances producing a stabilised inert odour free product conditioned for further treatment by composting.

Essentially, the method of the invention comprises a controlled and accelerated decomposition process involving the natural breakdown processes that occur when organic residue comes in contact with soil. This process allows not only the stabilisation of the waste activated sludge but the additional benefit of remediation of the waste sludge into a material suitable for transhipment and further treatment by composting, if appropriate.

The most important factors for adequate stabilisation and remediation in the inventive method are the chemical makeup of the waste activated sludge, for example, quality and quantity of carbon and minerals and pH; the physical size and shape of the waste activated sludge "crumb"; the porosity of the pile; and the population of organisms involved in the process, e.g. macrofauna and mesofauna; micro—organisms including bacteria, actinomycetes and fungi.

In the method of the invention, homogenous top soil with a pH of about 6.0 to 8.0 is typically mixed in a trammelling machine in a ratio of about 1:5 to 5:1, especially 1:1 with dewatered waste activated sludge.

This sludge preferably has a solids content in the region of about 5—50%, more preferably about 10-30% by weight. Trammelling is continued for about 5 to 40 minutes, typically 10 minutes, until such time as a homogenous mixture of sludge and soil is attained with a crumb—like texture. The trammelled material may be placed in windrows, which facilitates microbial activity and some drying. Windrows of this mixed material are preferably constructed to a maximum height of 2 m, preferably in covered and bunded treatment areas which protect the windrows from rain while allowing wind to aid in the aeration and drying thereby aiding stabilisation of the mixture.

In order to ensure aerobic conditions and adequate aeration for bioremediation and stabilisation by microbial action within the windrows, means such as metal probes are inserted into the soils to a depth of approximately 1.5 m. Air is forced through the probes and into the soil using, for example, an air compressor. Aeration of the soils is carried out typically three times a fortnight. In addition, aeration by mechanical mixing (mechanical excavator and trammelling) may also be carried out to disturb the mixture and reduce the potential for anaerobic pockets of soil. This method also provides an opportunity for a visual assessment of the soils to be made in respect of water retention or pockets of unmixed waste activated sludge. No significant runoff is generated during the treatment process.

The invention is illustrated in the following Example.

EXAMPLE Homogenous topsoil with a pH of from 6.5 to 7.5 was mixed in a trammelling machine with dewatered waste activated sludge having a solids content of about 20% by weight. The soil had the following composition: (a) Clay in the size range of 0.002 mm (b) Silt in the size range of 0 002—0.02 mm (c) Fine sand in the size range of 0.02-0.20 mm (d) Coarse sand in the size range of 0.20—2.00 mm (e) Gravel in the size range of 2.0+mm Approximate percentage of the total volume occupied by the various components of normal soil were: Mineral Material 51% Pore space 40% Organic Material (Humus ) 8% Organisms 1% ( bacteria and fungi ) The soil was tumbled with from 10 kg to 150 kg of the biological sludge (dry basis) per 1000 kg of soil. A proprietary mix of ammonium chloride and sodium phosphate were added at approximately 90 g per tonne of soil. Trammelling was carried out for approximately minutes. The resulting mixture had a crumb—like texture. The mixed materials were placed in windrows of approximately 2.0 m in height and allowed to stand for a period of 30 days. To facilitate aeration, metal probes were inserted into the mixed materials to a depth of approximately 1.5 m. Air was forced through the probes and into the mixture using an air compressor. The material was aerated 6 times in the -day period.

The method of the invention has a number of advantages over conventional waste treatment methods.

.Soil is utilised as a microbial seed to facilitate bioremediation and stabilisation of the sludge constituents under aerobic conditions.

. The soil is utilised as a conditioner versus organic materials in traditional composting.

. Trammelling of the waste sludge with soil alters the sludge texture and yields a material of crumb constituency, which facilitates closer Contact between the soil microbes and the sludge.

. Due to utilisation of the soil, which has lower moisture content than the waste sludge the moisture from the sludge migrates to the soil in the crumb binding the material together, lowering the sludge moisture Content and facilitating better handling of the stabilised material. This process also results in a significant reduction in leachate from the windrows when compared with traditional composting which can generate significant quantities of leachate.

.Trammelling and production of the crumb constituency of the mixed material produces void spaces within the mixture, which favour aerobic conditions reducing odour generation significantly.

.The addition of soil ensures that anaerobic conditions within the windrows are not prevalent due to the unique crumb structure which allows air to pass through the material.

. The cation exchange capacity of the soil facilitates the adsorption and binding of sludge constituents which preyents leaching and immobilises the Q . materials for further treatment by composting post transhipment.

Claims (5)

CLAIMS:

1. A method of treating waste material, which method comprises: (a) forming a waste sludge; (b) stabilising the sludge formed in step (a); and optionally (c) composting the stabilised sludge.

2. A method according to claim 1, wherein the sludge is dewatered prior to the stabilisation step (b) to a solids content of from 5 to 50%, preferably 10 to 30% by weight. sp

3. A method according to claim 1 or 2, wherein the stabilisation is carried out by mixing the sludge with soil, preferably in a ratio of sludge (wet basis) to soil of from 1:5 to 5:1, especially 1:1.

4. A method according to claim 3, wherein the sludge/soil mixture is aerated, preferably by mechanical means. W

5. A method according to claim 1, substantially as hereinbefore described, with particular reference to the accompanying Example. F. R. KELLY & CO., AGENTS FOR THE APPLICANTS