AU661703B2 - Improvements to organic waste treatment processes - Google Patents

Description

;d Regulation 3.2(2)

AUSTRALIA

Patents Act 1990 661703

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: IMPROVEMENTS TO ORGANIC WASTE TREATMENT PROCESSES r o r

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Irr~ I r The following statement is a full description of this invention, including the best method of performing it known to 3 t 1 1 |i v IMPROVEMENTS TO ORGANIC WASTE TREATMENT PROCESSES This invention relates to organic waste treatment processes and, in particular, to a process for improving the quality of compost from organic waste by reducing the activity of heavy metals that cause pollution upon interaction with the environment and improved beneficial soil amendment properties.

Of all solid municipal domestic waste produced in Australia, approximately 50 to 55% consists of food and garden waste. Other components are paper plastics glass metal (5 to and other inorganics (10 to The organic fraction is the most environmentally polluting and can be hazardous. The reason for this are 1) the enormous volume occupying up to 75% of landfill space, 2) its putrescible nature, causing it to be a source of pathogenic organisms, 3) the large volume of polluting gases such as ammonia and methane, released during the uncontrolled decomposition of the organic fraction, and 4) the decomposing organic matter is a major contributor to groundwater pollution through dissolution and its function as a carrier for inorganic pollutants, including heavy metals. Furthermore, it creates odour, attracts seagulls and is a breeding ground for rodents and insects.

Main sources of organic waste besides domestic waste are industry (breweries, abattoirs, food), agriculture, the timber sector and sewage treatment 20 plants. Some of this is used on soils as a fertiliser, e.g wood, sewage sludge, reused as a basis for other products, e.g brewery waste, or used as a stockfeed. However, most of the waste is landfilled.

Combined organic waste production in Australia alone is approximately million tonnes annually, therefore, the major challenge in any integrated waste S 25 management strategy is to deal appropriately and effectively with organic waste. The current practice of landfilling organic wasie whether it includes compaction and lining or other more advanced landfilling techniques is rapidly becoming an inappropriate waste treatment practice and unsustainable in the long term. For the abovementioned reasons, and also the fact that landfill consumes large tracts of land, causes devaluation of land and SC 30 is strongly objected to by residents, landfill is fast disappearing in major cities as a single waste management strategy. Particularly in low density cities, transport costs become prohibitive as suitable landfill space is only available well away from waste generation centres.

There are three major alternative treatments for organic waste which reaiise some of the potential of organic waste. Firstly, incineration has attracted 2 considerable interest from city planners for its potential to destroy organic waste, reducing the waste buik while at the same time generating heat. A close examination of the process, however, shows high capital and operating costs; the process does not generally destroy pollutants such as heavy metals, but in fact concentrates them, requiring more advanced further treatment or disposal. Furthermore, the risk posed by toxic organic molecules, such as dioxins, benzopyrene, polycyclic chlorinated hydrocarbons, etc. is of much concern and their destruction is very costly.

The second alternative, anaerobic digestion of organic waste is costly as far as capital and operating costs are concerned. The main reasons for this are that the 1 0 process requires full enclosure to avoid aeration and the process is relatively slow, requiring a large treatment capacity. Further, the residue may not be suitable for land application as it is odorous, has lower nutrient value than compost and may suffer from contamination by a combination of pollutants, including heavy metals and pathogens.

However, the process does recover energy in the form of methane as a result of the 1 5 biomethanation process.

The third alternative, direct composting of organic waste is considerably cheaper than the above processes. Further, the organic waste has considerable potential S. as a resource when stabilised through composting. Compost is high in organic matter (60 to nutrients, such as nitrogen phosphorus (0.5 to potassium 2 0 (0.7 to and trace elements. Composting also has the advantage of reducing the waste volume by 40-50% and furthermore all the advantages associated with the production of a soil conditioner.

There are a number of available composting systems with varying capital and operational running costs. For composting to be environmentally and cost effective S 25 and practical, it is of paramount importance that the end product, compost, has commercial value in a market of sufficient size to absorb the quantity produced.

Potential markets include local governments, market gardeners, nurseries, viticulture i "agriculture and householders. However, the potential of the market is related to the quality of the compost.

30 Quality of compost, or indeed any usable organic waste, relates in the first instance to the level of contamination by heavy metals. Heavy metals research in the United States and Europe has shown that composting of municipal waste is legislatively limited only because of contamination by heavy metals. Most soils demand protection from such contamination in order to sustain land use. For this purpose, soil protection J 35 legislation has been put in place to restrict the application of heavy metal containing '0 i- ;1 r;i I iiL~IIIII---l i) D 04 0

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2111 organic wastes such as compost and sewage sludge to soils, based on their actual content of heavy metals. The level of heavy metals in compost thus determines the appropriate application volume as a soil conditioner and, as a result, the size of the market.

Heavy metals occur in domestic organic waste because domestic refuse is only separated after collection, allowing metal containing waste such as electronic components, alkaline batteries, paint, cosmetics, rubber, etc., to mix with the organic fraction. The metals are present in the waste, complexed into various chemical components and materials in various metal species. Once mixed, the resulting heavy metal contamination is impossible to reverse without incurring excessive costs and affecting the suitability of the waste to be composted. As during the composting process, a reduction in solids of up to 50% occurs, and metals cannot be decomposed biologically any contaminants present will concentrate in the end product.

One way of avoiding such contamination is to separately collect the putrescible fraction, a practice recently adopted in Germany and the Netherlands.

However, this practice requires educational programmes, householder co-operation and increased waste collection costs. Clearly, a more practical and economic solution is required.

Contamination is defined as either the presence of xeno-biotic substances eg cadmium or lead that have no proven function in life processes or an excess of biotic substances eg copper or zinc which function as essential trace elements. In the case of metals, it is the concentration of metals at a given time at a certain location that is mobile, the so-called flux, that causes contamination. In a practical sense it restricts application so as to prevent uptake by eg vegetables or groundwater contamination. From this, it follows that contamination occurs when a metal adversely affects an ecosystem or 2 5 gains an entry point into the food chain.

The presence of metals itself is not pollution until it interacts with the environment at levels that potentially interfere with life processes present. This means that removal of the interactive flux of metals through reducing the available levels over an extended period of time would remove the potentially adverse effects on the environment.

Thus there is a need to develop a cost-effective method which reduces the mobility of heavy metals in various organic wastes. With this end in view, the inventor discovered that the availability of heavy metals to the soil solution and to plants is reduced when the metals are absorbed into the inorganic soil fraction as opposed to the organic fraction, the reason being that metal release from the organic fraction is mainly

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c~ r 1~1 r i c ill I e,~ 4 governed by rate of decomposition and solubilisation of the organic fraction. Further, the release of heavy metals that are complexed or bound to an inorganic fraction is generally the result of re-establishing an equilibrium between the solid and liquid soil fractions through ion exchange and solubilisation eg hydroxides and carbonates. These processes are strongly pH dependent. The release of metals from the inorganic fraction is easier to control through pH, and is at pH>6 much lower than the release from the organic fraction. The reduced availability of metals can form a safe basis for land application of metal containing organic waste as opposed to total metal concentrations or metal loading.

On the basis of this finding there is provided as the object of this invention an organic material treatment process which enables reduction in the levels of activity and/or mobility of contaminants such as heavy metals thus producing an organic waste, for example compost, that is suitable for soil amendment.

With this object in view, the invention provides a process for the treatment of organic materials containing cont-aminants wherein clay is added to the 15 organic material prior to commencement or in the early stages of the material treatment process to react physically and chemically with said contaminants to complex the contaminants to an inorganic fraction and to thereby reduce the mobility of the contaminants. By reduced "mobility" can be taken to mean reduced extractability and interaction with the environment at levels that interfere with the life processes present; 20 ideally, down to a natural background level of interaction.

The organic waste may conveniently be sewage sludge, domestic waste, commercial, industrial or agricultural organic waste and the method is especially suitable where the organic waste treatment process used is microbial decomposition by composting. The contaminants may be heavy metals, that is cadmium, copper, chromium, lead, nickel and zinc.

Preferably, the clay is zeolitic and may, for example, be a bauxite refining residue or "red mud", or any of the clay minerals it consists of, which is a sodium saturated loam derived from Bayer process aluminium refinery plants. Such "red mud" has a high alkalinity (pH 9 to 12) and a high cation exchange capacity (15 to 30 45 meq/100g), properties that make it suitable for reducing the solubility and thus the mobility of complexed heavy metals by metpl hydroxide and carbonate precipitation and adsorption by cation exchange. A further adva'tage is that the adsorption complex of red mud can retain applied fertiliser thereby reducing nutrient leaching. A typical mineralogical composition for a bauxite refining residue is: 1i h' mi Quartz 16.3% Zeolite 10.6% Boehmite 6% Anatase/Rutile 1.6% Haematite 13.9% Alumina Goethite Gibbsite 9% Tricalcium 3.8% Aluminate Goethite 15.4% Muscovite 10.1% Calcite 4.6% Preferably the amount of clay added ranges up to 40 percent by weight fresh organic waste, and more preferably ranges in the range 1 to 30 percent by weight 1 0 fresh organic waste.

Conveniently, gypsum or an acidic salt can be used as an amendment to the clay or red mud to reduce the alkalinity to levels acceptable for soil amendment and to reduce the sodicity of bauxite residue. Such salt can thus be used to adjust the pH of the soil to which treated organic waste, compost, is added. Preferably, such salt is added 1 5 in the range 5 to 10 percent by weight fresh organic waste to obtain a pH level similar to a calcareous soil.

The proposed process presents several advantages: 1. Heavy metals are immobilised by strong adsorption to clay particles. Such immobilisation causes the reduction of the soil 20 solubility and thus availability of heavy metals to plants as compared to organic waste untreated with clay.

2. The product, compost, has greater stability and density.

3. The half-life of the organic fraction of the product is up to 5 times higher due to the formation of very stable clay-organic matter S. 25 complexes.

4. Increased product volume for sale. (Value added for clay and organic waste).

The product, compost, has improved water and nutrient retention, and soil pH buffering capacity over conventiona! soil conditioners.

6. An inorganic fraction remains in the soil after further decomposition of the organic fraction.

7. The large amount of carbon dioxide released during the organic waste treatment process is partly absorbed by the alkalinity in i i i 1 i i 6 clay or red mud through formation of bicarbonate resulting in increased buffer capacity for the end product and reduced carbon dioxide emission during the process.

8. Effective odour control because of pH control (no free carbonic acids or formation of malodorous Maillard products).

9. Clay amendment dilutes the metal concentration and increases the legislative allowable quantity of compost that can be applied to soils, given requirements relating to the amounts of heavy metals that can be added to soils through soil amenders.

1 0 A description of two examples that illustrate the advantage of the invention in reducing heavy metal mobility in compost on a batch and pilot plant scale follows. A comparative example is also included. It will be understood that the Examples are nonlimiting of the scope of the invention.

Example 1. Batch Experiment :o 15 Method An installation was designed and built to accommodate seven composting incubators of 24 litre each. The installation included continuous independent monitoring Sof temperature and effluent gas analysis for oxygen and carbon dioxide. These data were data logged on a Hyundai 286E persmal computer Temperature was controlled to 530C by the computer through varying aeration rates at the solenoid valves between 1 Sand 3 I/min. A schematic of the installation is shown in Figure 1.

The incubators were filled with 10 kg mixtures of grass clippings and sawdust and red mud 1, 2, 2.5, 3 and 4kg). The mixtures were then spiked with heavy metals to the following levels in mg/kg dry matter, cadmium 10, chromium and lead 50, nickel 20, copper 100 and zinc 500. Composting was then initiated.

Samples were taken for analysis. The various analyses are listed in Table 1.

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7 Table 1 RELEVANT PROCESS PARAMETERS AND FREQUENCY OF MONITORING OR ANALYSIS.

Parameter Method pH pH probe in 10:1 extract Moisture content drying (2 hrs at 700C) Temperature Pt100 probe Oxygen micro fuel cell Carbon dioxide infra red analyser 1 0 Salinity EC, 10:1 sol:soil extract Heavy metals (Cd, Cr, AAS in DTPA, (plant Cu, Ni, Pb, Zn) available metals) CaCl2 (soil soluble metals) and acid extracts (HNO3,

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4 i C/N ratio COD/Kjeldahl N I E4/E6 .atio UV spectrophotometer 1 5 Chemical Oxygen Demand Chromic acid digest Trace elements (Ca, K, AAS or colorimetric in acid S' Mg) digest Results The red mud reduces the mobility of heavy metals in compost through adsorption onto cation exchange sites, precipitation of metal hydroxides and carbonates, and adsorption of soluble organic acids, containing metals on their functional groups.

Precipitation of metals in the form of hydroxides or carbonates is mainly governed by pH and ionic strength of the soil solution. At the pH levels measured in the red mud clay amended compost some precipitation is likely to have taken place. The same applies for the presence of a high percentage of carbonates in the product compost.

The effect of red mud addition prior to composting, on the extractable concentration or mobility of heavy metals in product compost at 20 days, compared with the situation where no red mud is added, is given in Table 2.

Table 2 EFFECT OF RED MUD ADDITION ON THE MOBILITY OF HEAVY METALS IN COMPOST AFTER THE THERMOPHILIC STAGE AT 20 DAYS (IMMATURE COMPOST) Aaal Reduction Leachable Plant availale Acid digest Chromium >99 >99 9 9" Cadmium >99 67 69 Copper 66 54 69 Nickel 96 73 76 Lead 80 53 66 Zinc 95 72 044* 94 4 9* Li 4 'ii More chromium was extracted than was added.

The dependence of reduction in heavy metal mobility on the weight percentage addition of red mud prior to composting is shown in Figures 2 to 9 in which 15 Figure 2 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the leachable chromium, cadmium, nickel and lead after composting.

Figre 3 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the leachable copper after composting.

Figure 4 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the plant available copper after composting.

Figure 5 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the plant available zinc after composting.

Figure 6 shows the effect of red mud addition to metal spiked organic inatter prior to microbial decomposition on the leachable zinc after composting.

Figure 7 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the plant available chromium, cadmium, nickel and lead after composting.

Figure 8 shows the effect of red mud addition to metal spiked organic matter prior to microbial decomposition on the acid extractable zinc and copper.

Figure 9 shows the effect of red addition to metal spiked organic matter prior to microbial decomposition on the acid extractable chromium, cadmium.

nickel and lead.

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c ,ri t ic* 9 Each of the above Figures also incorporates the statistical error for the analytical results.

The data show that the red mud clay has an overall effect of reducing ,he availability of the metals to the soil solution and for plant uptake with the optimum reduction being achieved at 20-30 weight percent addition of red mud to fresh organic waste. The red mud appeared to contain some chromium, which explains the increased total chromium content extracted with acid.

The recovery of metals from the red mud clay with the acid digest was incomplete for all metals except uhromium of which more was recovered than the 1 0 substrate was initially spiked with. The additionally extracted chromium is naturally occurring in bauxite.

The reduction in metal mobility over the relatively short period of days strongly suggests that the metals are bound through an inorganic reaction, since organic complexation of metal ions to the organic fraction takes piace mainly during the 15 humification stage of composting (20-60 days). Thus addition of clay such as red mud during the early stages of composting generates effective reduction in heavy metal mobility.

The stabilihy of inorganic heavy metal complexes is, apart from their individual properties, primarily dependent on the ionic strength and pH of the medium.

20 The stability is reduced at lower ionic strength and pH. Maintaining a stable and slightly alkaline pH is essential for the red mud amended compost to raidin the metals. This can be achieved by a high buffer capacity.

The buffer capacity of red mud amended compost, expressed in CaCOs, is shown in Table 3.

The slightly alkaline pH of 7.4 is satisfactory for the metal complex stability as well as use of the compost for soil conditioning. The pH did not differ from that of the plain compost. However, the buffer capacity of the red mud amended claycompost is significantly higher than the plain compost. The high buffer capacity ensures the stability of the pH and thus the inorganic metal complexes. The red mud contributes 3 0 to the calcium content. The high calcium content and the ability of the red mud to adsorb soluble organic acids reduced the chemical oxygen demand (COD) in a water extract to The lower nitrogen and trace element content is due to a lower organic matter content.

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Table 3 QUAUTY PARAMETERS AND ELEMENTAL CONTENT OF COMPOST AND CLAY-COMPOST pH C-CO 2 CaCO 3 C/N N% Ca% K% Mg% COD lost ratio mpost 7.4 489 0.01 22 1.5 0.26 0.35 0.11 5420 cor 0 0 I I

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claycompost 7.4 370 3.16 22 0.8 1.10 0.21 0.07 572 Figure 10 shows the relation between oxygen respiration and temperature. For a short period the temperature control failed to maintain the temperature at 55 0 C and it raised to 60 0 C. The oxygen consumption, or carbon dioxide release, was at its highest during the thermophilic stage of composting. The fluctuating concentrations are caused by air flow rate adjustments for temperature control.

Example 2 Pilot Plant Experiment.

Method A pilot plant with a capacity of 5 m 3 consisting of a rotating drum with agitators, and special gearing was constructed. A council compactor truck filled with 150 bins of domestic waste from a randomly picked area in Perth, Western Australia, delivered to the plant. The refuse was sorted for the recyclables metals, plastics and glass. The drum was filled with the remainder for composting. The drum was rotated continuously at 6 rpm for 48 hours to pulverise and homogenise the refuse. After 48 hrs the drum was rotated at hourly intervals to satisfy oxygen demand and achieve thermophilic conditions throughout the mass.

Temperature and pH moisture content was monitored daily. Twice a 25 week a water extract was analysed for chemical oxygen demand to monitor maturity.

Runs were carried out with and without the addition of red mud prior to composting.

Information gained from the batch experiments was used to determine the optimum red mud addition which is approximately 20% of fresh organic waste weight.

The various types of composts were analysed for total (acid digest), soil 3 0 solution soluble (CaCI 2 and plant available (DTPA at pH 7.3) heavy metals.

Results.

The composition of the domestic, waste received at the plant is given in Table 4. The composted fraction is high and though it contained some inorganics it produced good quality compost.

(C I i I- i i- The metal content of both types of domestic compost was within recently adopted European guidelines for food producing soils as reported by Bertoldi de M et al, 1990, MSW Compost Standards in the European Community, Biocycle. 31. pp 60-62.

Table 4.

COMPOSITION OF DOMESTIC SOLID WASTE SORTED FOR COMPOSTING TRIALS.

Fraction Victoria Park East-Perth Wembley glass 3.3 6.5 metal 4.3 9.3 4.3 plastic 12.7 17.4 10.5 composted% 79.7 66.7 80.8 Total 100 100 100 Sample size (kg) 3000 2580 2350 The extractable metals in the compost to which red mud had been added prior to composting, were analysed at 5 and 60 days and lraturity. The results for a 1 5 mature compost with red mud additions prior to composting are shown in Table Comparative data for a mature compost without red mud addition are also included. The time dependence of the extractability of metals in the case of red mud additions prior to composting is shown in Table 6 for the second rur of clay compost production.

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Table HEAVY METAL CONTENT OF MUNICIPAL RED MUD COMPOST IN MG/KG DRY MATTER.

Mg±a1 Mature plain compost (no red mud ddin) Mature~ (with 20% -by weight fresh organic waste r ed mu d European auidejlne

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a. 4 44 Cadmium Chromium 1 0 Copper Nickel Lead Zinc Soil Soluble 0 0.5 1 .5 0.3 0.12 3.5 Plant Available 0.77 0.2 6.9 1 .2 20.4 113.6 1.4 0 57.9 12.2 105.4 255 Soil Soluble 0 0 1 .3 0.1 0.3 2 Plant Availl 0 0 3.2 1 4.2 58 Total 0.1 8.5 45 14 32 205 150 300 750 1000

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Table 6 HEAVY METALS CONTENT IN MUNICIPAL COMPOST WITH RED MUD ADDITION PRIOR TO COMPOSTING AND THEIR AVAILABILITY TO PLANTS AND GROUND WATER LEACHING AS A FUNCTION OF COMPOSTING TIME.

Ma1I leachable plant availbl MLQ imen~ mQnature 0nI-e Cadmium 0 0 0 0.7 .23 0 0.13 Copper 0.8 4.5 1.3 16.3 31.6 3.2 44.7 Chromium 0 0 0 0 0 0 Nickel 0 0 0.1 1.5 2.8 1 14.2 Wead 0 0.3 0.3 7.5 9.5 4.2 32.5 Zinc 1.5 2.8 2 89.4 96 58 205 It appears that a concentration effect occurs during the first part of the process as a result of loss of dry matter. During the maturation phase the metals appear to be bound in a

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i r, :4t:: i rt r r r rr r e 13 It appears that a corcentration effect occurs during the first part of the process as a result of loss of dry matter. During the maturation phase the metals appear to be bound in a less mobile form as shown by a reduced extractability.

The total metal content is very low compared with the overseas data shown in Table In Table 7 the mobile fraction of heavy metals is expressed as a percentage of the total content for each compost. Run I relates to a compost in which red mud was added prior to composting and the Run II relates to a compost prepared without red mud addition prior to composting.

Table 7 1 0 THE LEACHABLE AND PLANT AVAILABLE HEAVY METALS AS A PERCENTAGE OF THE TOTAL CONCENTRATION IN THE COMPOST.

Run I Run II MatureMature Compost Compost (no red mud addition) 15 (with red mud addition) Soil Soluble Plant Available Soil Soluble Plant Available cadmium 0 0 0 copper 2.9 7.1 2.6 11.9 chromium 0 0 33.3 13.3 nickel 0.7 7 2.5 9.8 lead 0.9 12.9 0.1 19.4 zinc 0.9 28.3 1.4 44.5 The table s..ows that the mobility of metals with the exception of leachable copper and lead is significantly lower in the red mud amended compost than in the plain municipal compost, which may thus be suitable, for plant growing.

Now follows a comparative example in which red mud is added after composting.

Comparative Example.

Samples of Auckland compost to which red mud was added were prepared. Mixtures containing 0 to 15% red mud were extracted by mechanical shaking batchwise in a 10:1 liquid solid ratio with 0.005 M DTPA and 0.01 M CaCI 2 to determine the plant available and soil soluble Cd, Cu and Ni.

The results are expressed as mg/kg heavy metals extracted from compost-red mud mixtures.

C 4 t s -i 14 Table 2 Heavy Metals Extracted from Compost-Red Mixtures Red Mud Addition Plant Available Soil Soluble Q: Cu Ni Cu NI 0 2.2 8.e 8.9 1.7 1.8 1.9 8.9 8.1 1.6 1.8 1.7 8.9 8.7 1.5 1.9 1.5 8.9 8.7 1.5 1.9 1.3 9.7 8.8 1.7 1.9 50 1.0 10.3 8.8 1.7 Red mud addition after composting had little impact on the soil solution solubility of Cu and Ni, though the plant availability of cadmium was lowered. Little effect on the plant availability of copper and nickel was noted.

The results for the comparative example indicate generally much lower efficacy in S. 15 reducing the mobility of the heavy metals Cd, Cu and Ni in compost than the results for Examples 1 and 2. It is believed that the reason for this is that, during composting the heavy metals are so strongly adsorbed or complexed onto the organic fraction that they are not readily amendable to adsorption onto red mud added following composting. If the red mud is added prior to composting, however, heavy metal mobility is, in general, more effectively reduced because the heavy metals present are thus adsorbed or complexed onto the inorganic fractions (clay or red mud) and thus not available for adsorption onto the organic fractions. Thus the further decomposition of the organic fractions does not effect a flux of heavy metal release into the environment. Since the metals are complexed to the clay fraction which does not decompose.

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Claims (12)

1. A composting process for the treatment of a first material containing compostible organic materials including the step of adding a clay to said first material involved in said composting process.

2. The process as claimed in claim 1 wherein said clay is added to said first material prior to commencement, or in the early stages of said composting process.

3. The process as claimed in claim 1 wherein said clay is added to the first material at, or near, the conclusion of said composting process.

4. A process as claimed in any one of claims 1 to 3 wherein said compostible organic materials contain contaminants being metals selected from the group consisting of cadmium, copper, chromium, nickel, lead and zinc which are adsorbable by said clay. A process as claimed in any one of claims 1 to 4 wherein said organic material is sewage sludge. t6. A process as claimed in any one of claims 1 to 5 wherein said clay is zeolitic or contains zeolite.

7. A process as claimed in any one of claims 1 to 6 wherein said clay "is a bauxite processing residue.

8. A process as claimed in any one of claims 1 to 7 wherein gypsum or an acidic salt is used as an amendment to said clay.

9. A process as claimed in any one of claims 1 to 8 wherein said clay is added in an amount ranging up to 40 percent by weight fresh rganic waste. A lf A71- A process as claimed in claim 9 wherein said clay is added in an amount of between 1 and 30 percent by weight fresh organic waste.

11. A composted organic material produced by the process as claimed in any one of claims 1 to

12. A soil conditioner comprising the composted organic material claimed in claim 11.

13. A process for the production of a soil conditioner from an organic material wherein a clay is added to said organic material to produce a treated organic material with enhanced soil conditioning properties in terms of reduced contaminant mobility due to physical and chemical bonding of contaminants present in said organic material with said clay, pH buffer capacity, water and nutrient retention and soil conditioning capacity relative to the untreated organic material.

14. A process as claimed in claim 13 wherein said clay is zeolitic or contains zeolite.

15. A process as claimed in claim 13 wherein said clay is a bauxite processing residue. i 16. A soil conditioner produced by the process as claimed in any one of claims 13 to DATED this 21th day of February, 1995. 0 THE UNIVERSITY CO PTY LTD 0 WATERMARK PATENT TRADEMARK ATTORNEYS 4TH FLOOR, "DURACK CENTRE" 263 ADELAIDE TERRACE i PERTH W.A. 6000 AUSTRALIAi iAI -j ~C~A organic fraction, the reason being that metal release from the organic fraction is mainly ii ABSTRACT Disclosed is a process for the treatment of organic waste containing heavy metal ions wherein a clay, such as a red mud obtained from bauxite process residue, is added to the organic waste prior to commencement or in the early stages of the treatment process to thereby react in a physico-chemical manner with heavy metal ions thus reducing their mobility and availability to the environment. 00 0 00 00 @00 0 00 0 *000 0 0 A1