WO1993014046A1 - Fertilizer mixture and process for production of the fertilizer mixture - Google Patents

Abstract

The invention comprises a product containing water soluble compounds of nitrogen and phosphorus that are rapidly acting fertilisers, and the content of nutrients is leached slowly from the soil where the water soluble nitrogen is in the form of urea and ammonium salts, and the product contains between 5 and 25 % by weight of nitrogen.

Description

FERTILIZER MIXTURE AND PROCESS FOR PRODUCTION OF THE FERTILIZER MIXTURE.

The present invention relates to a product consisting o soluble and fermentable organic compounds and inorgani nitrogen and phosphorus compounds, suitable for use as carbon source for fermentation and as rapidly actin fertiliser and soil improvement agents _t and a method fo production of such products.

The present invention shows that there exists a synergisti effect between water soluble organic substances and fertili sing stocks, which makes it possible for the latter to remai for a longer time in the soil without being washed out tha is the case when the organic substances are not present This is an important factor in limiting the effluence o fertilising stocks into water systems and groundwater. Reference is further made to the fact that mixtures of wate soluble organic substances and common fertilising stock increase the growth of plants per amount of fertilising stoc more than does the latter alone. The present invention show how such growth enhancing and effluent-limiting mixtures ca also be produced from biological sludge by means of a nove and advantageous mode of treatment.

State-of-the art methods for treatment of biological sludges etc., aim to a): destroy pathogenic microorganisms that ma be present in the sludge; b): remove unpleasant odours an c): convert a dewatered sludge "cake" containing typicall 15-25$ solids to a composted material also containin typically 20-25$ solids.

As this material is biologically active and prone to rottin with the concomitant evolution of malodorous substances, i is in some instances dried and granulated and the resultin product offered for sale as a soil conditioner. A typical composting plant entails a treatment period of at least 14 days, during which time the sludge must be mecha¬ nically turned, aerated and maintained at a temperature of about 30°C. The product so made must then be "matured" for a period of 4-6 weeks. Such plants require a considerable capital investment and a large site area.

The materials made in this way have the*^■severe disadvantage in that their properties and behaviour in many respects resemble the sludge from which they are made. Their dry matter contents of compounds of toxic heavy metals such as cadmium and lead and potentially toxic light metals such as aluminium are identical to that of the sludge from which they are made and they cannot be .pumped.

It has been shown that these materials are potentially useful as fertilisers. However, their contents of plant nutrients, primarily compounds of nitrogen and phosphorus, release too slowly for them to be generally useful other than when applied in very large quantities. This in turn is precluded due to their content of toxic heavy metals. A further factor precluding the use of large quantities of such materials is the risk of surface run-off. This may occur if heavy precipitation follows immediately after their application to a slow-draining, heavy soil such as clay and is a consequence of the large particle size of the materials' nutrient- containing compounds.

Furthermore, the cost of investing in plant and equipment to convert sludge to a granular product is considered prohi¬ bitive by most potential users.

This state of affairs has hitherto resulted in the great majority of installations that produce organic sludge either disposing of the raw sludge cake in approved landfill sites, and/or arranging for use of the sludge cake by farmers as a soil improvement agent i.e. to complement their use of anim manure.

In addition to the few plants producing a granular, drie product, some few have facilities for incinerating sludge This method is effective in reducing the volume of produc leaving the treatment facility, but is very expensive an risks release to the air of heavy metal .-vapours and sulphu oxides.

Environmental authorities are greatly concerned about tw developments: i) the high content of heavy metal (primaril cadmium and lead) in annual crops and agricultural soils, an ii ) the growing content of organic and inorganic pollutant (primarily phosphate, nitrate and nitrite) in groundwater.

Other environmentally related concerns revolve around th presence of (soluble) aluminium compounds in a) drinkin water and crops where they are suspected of being implicate in Alzheimers disease and b) in forest soils where they ar suspected of being implicated in premature tree death Aluminium salts are commonly used as flocculants in wate treatment and it is not uncommon for e.g. the dry matter of sewage sludge to contain as much as 10$ by weight o aluminium derived from the flocculant.

The first of these problems, i), has led to a series of ne regulations in many countries, limiting the amount of sludg based soil improvement agents that can be applied to a give area of agricultural land over a given period. This amoun is far lower than that previously permitted, thereby obligin the producing facility to transport sludge to more numerou and distant locations than was the case before. This in tur increases the operating costs for such facilities an exacerbates the problems of disposal logistics. These rulings will also affect the raw sludge cake and the composted and granulated materials mentioned above, as all the products have similar contents of heavy metals. The second problem mentioned above, ii), has led to restrictions in the operation and location of landfill sites and resulted in use of synthetic fertilisers and animal manure in agriculture being put under closer scrutiny than hitherto. It is expected that these activities wiM be the subject of heavier restrictions in the future. Lack of landfill sites suitable for biological sludges, especially sewage sludge, is already a major problem in many countries and this situation is will probably worsen.

A further consequence of ii) is that sewage treatment plants will be required to reduce the amount of nitrogen compounds released to the environment during their course of opera¬ tions. This is expected to lead to demands for denitrifi- cation of waste water leaving such plants. The most usual denitrification method consists of causing microorganisms to multiply in the said waste water and thereby convert the soluble nitrogen either to cell tissue or nitrogen gas. This requires the presence of a carbon source, i.e. a water soluble, metabolisable, carbonaceous compound, which often must be added to the waste prior to or during denitrifi¬ cation. No state-of-the-art sludge derived product is suitable for use as a carbon source for denitrification.

The present invention relates to a completely new method of treating sewage sludge and other biological sludge wherein all the major problems connected with treatment of such sludge are solved.

The use of the method according to the invention enables sterilisation (I.e., by removal of microorganims), stabilis¬ ation (preventing growth of microorganisms and thus permit¬ ting long-term storage), volume reduction, air removal (i.e., without the odour of sulphur or decaying materials), major reduction of the metal content including heavy metals, a conversion to usable, salable products having uniq properties. The costs associated with treatment of sludge the method according to the invention are also considerab less than those of known methods which yield inferior e products.

The products according to the invention are characterized that they contain water soluble compounds of nitrogen a phosphorus that are rapidly acting fertilisers, and that t content of nutrients leach slowly from the soil, where t water soluble nitrogen is in the form of urea and ammoni salts, and that the product contains between 5 and 25$. weight of nitrogen.

The products, either liquid or dried, contain respectivel between 15$ and 30$ by weight or more than 50$ by weight of water soluble fermentable carbon source and are produc from organic sludge. The weight ratio of nitrogen to carbo the latter in the form of fermentable subtstances, is fro 1:1 to 1:10, and preferably between 1:2 and 1:5.

The products may be used as a carbon source for, e.g. denitrification of effluent streams from sewage treatmen plants, ethanol production or the growing of proteinaceou microorganisms for use in animal feedstuffs.

The products' content of cadmium is below 5 ppm, the conten of mercury is below 2 pp, and the content of lead is below 2 ppm.

The invention also relates to a method for production of th above products, characterized in that organic sludge from e.g., sewage treatment plants, plants that treat waste wate from the chemical, paper and pulp industries, food-relate industries, animal husbandy, etc., is heated to at leas 180°C and preferably between 210°C and 230°C for at least minutes, and preferably between 3 and 7 minutes. The process is catalyzed by addition to the sludge of inorganic acids, e.g. , sulphuric acid, added in an amount such that the pH of the product is less than or equal to 1.5, and preferably between 0.5 and 1.0.

If desired, the process may be catalyzed by use of metallic salts, which are water soluble compounds of iron (III) and/or aluminium, in an amount of at least 100 -ppm, and preferably at least 200 ppm, based on ingoing sludge.

By the method according to the invention, metallic ions from groups 2b, 3a, 4a, 5a, 6b, 7b and 8 in the periodic table are removed by precipitation.

The precipitating reagent has value as a plant nutrient and consists of ammonia, ammonium carbonate, ammonium carbamate, potassium hydroxide or potassium carbonate or mixtures of these.

The precipitating reagent is added stepwise such that certain flocculants added to the raw sludge liquors in order to precipitate particulate matter therefrom can be recovered from the sterile product and reused. These flocculants consist of iron (III) and/or aluminium salts recovered by precipitation at pH below 4.5, in order to minimise the coprecipitation of undesirable metals such as hydroxides.

The precipitating reagent optionally consists in whole or in part of calcium hydroxide.

The volume of the materials containing organic compounds after treatment is reduced by 1/3 or more of the volume prior to treatment.

The invention will now be described in more detail with the aid of the embodiment examples, with reference to the accompanying drawings. Figure 1 shows a flow sheet of an embodiment of the meth according to the invention.

Figure 2 shows a sketch of a column for determination nitrogen leaching.

In Figure 1 is shown a flow sheet for a_n embodiment of t process in accordance with the invention.

Sludge containing at least 10$ and preferably 15$ or mo particulates is mixed with a quantity of reactor produc typically 20-35$ by volume, such that it is rendered easi pumpable and a quantity of mineral acid, preferably sulphur acid, added such that the pH of the resultant mixture below 1.5, preferably 0.5-0.8, this typically requiring 2 50gm sulphuric acid per kg dry matter.

The content of Fe (III) and Al (III) together is if necessa adjusted such that it is equivalent to at least 200 ppm dry matter. The amount required will vary according to t type of sludge, the figures above being representative f sewage sludge.

The resulting mixture is heated to at least 180°C a preferably between 210 and 245°C for a period of at least and preferably 3-5 minutes, as a result of which it i converted to the reactor product, a low viscosity dispersio containing 2-5$ by weight particulates.

The particulates are allowed to settle and any immiscibl oils float to the surface whereupon both are removed int separate containers. The particulates are concentrated in filter to a filter cake containing as much dry matter a possible, typically at least 40$ by weight.

The pH of the clear, aqueous phase is then adjusted, e.g with ammonia or alkali metal carbonate or hydroxide, first t 4.0 ± 0.4 and then to 7.5 ± 0.5, the precipitated matte being removed after each pH-adjustment. The resulting clear fluid is then subjected to evaporation until it contains a least 20 and preferably at least 30 weight$ dissolved solids.

For sludges low in phosphorus and/or heavy metals, e.g. sludge from the paper and pulp industry and depending upon the application envisaged for the end product, finely divided calcium hydroxide and/or carbonate can be used as preci- pitants.

Dewatered biological sludge containing at least 10$ by weight solid phase is led into a mixing tank A where an acid, preferably sulphuric acid, is added in an amount of at least lOg and typically between 20 and 50g per kg dry matter together with a quantity of product from the reactor (see above), typically 10-40$ by weight of the ingoing sludge. The purpose of the latter is to convert the original sludge into an easily pumpable dispersion. As the addition of acid may cause the evolution of a small amount of gas, tank A is provided with an outlet, such that gas leaving the tank can be led, e.g. , to admixture with boiler feed air or to an ad¬ sorption tower. All parts of the tank and mixer/impellor are constructed of acid resistant materials.

Should analysis of the ingoing sludge show that the content of Al³⁺ and Fe³⁺ is below 200 ppm of dry matter, an amount of aluminium and/or iron (III) salt, preferably in the form of sulphate, should be added in order to obtain this concentra¬ tion.

The mixing tank should have a capacity corresponding to at least 1 hr production of sludge.

The dispersion from tank A is pumped into the first holding tank, B, whose purpose is to ensure a regular flow of fluid into the reactor. All parts of the holding tank an mixer/impellor are constructed of acid resistant materials.

The purpose of the reactor is to heat the incoming sludg stream such that a large amount of the particulate matter which in the case of sewage, agricultural and paper and pul industry sludge consists of cellulose, is hydrolysed in short time to water soluble substances. -This entails the us of acid and the aforementioned aluminium and iron (III) salt as hydrolysis catalysts and the possibility of achieving high temperature in the reactor.

Considerations of achieving an optimal balance betwee maximum reduction in the amount of particulate matter variable costs for catalysts and energy and fixed operatin charge to amortise the plant indicate that this time shoul not exceed 15 minutes and should lie between 2 and 1 minutes. The optimal temperature range for such sludges ha been found to be at least 180°C and preferably between 210° and 245°C and the amount of acid such that the pH of th reaction products is below 1,5, preferably 0,5 - 0,8.

The first holding tank should have a capacity correspondin to at least 2 hrs. production of sludge.

As it has been previously reported that cellulose hydrolysi requires far longer times under similar conditions of time temperature and acidity, it is most unexpected that th operating conditions here are sufficient to reduce the bul of the sludge's cellulose content to water soluble compounds

We have been able to show that this is due to the presence o small amounts of iron (III) and/or aluminium salts in th sludge. Whilst these substances are present in sufficien quantity in e.g. sewage sludge, they may need to be added i the case of e.g. a paper industry sludge. The reactor C is designed in the form of a long tube coiled upon itself such that the ingoing fluid is warmed up by heat exchange with the outgoing, treated fluid, whilst the central portion is heated further to the aforementioned temperature range by an external heat source such as a thermal fluid. The outlet fluid temperature should be below 100°C, preferably 60-80°C. This conserves energy and is efficacious for the further treatment.

Part of the fluid leaving the reactor passes into the settling tank, D and part is returned to the mixing tank A via pipe b.

All parts of the reactor must be constructed of acid resistant materials.

The purpose of the settling tank, D, is to allow residual particulates to settle and fatty acids, which might otherwise cause foam formation later in the process, to float to the surface. Tank D is furnished with a skimming device, which can be a suitably located bleed pipe, in order to remove floating substances (including plastic remnants), oils and fats, to a storage tank, E.

Matter settling at the base of tank D is pumped to a filter, F, either directly or by way of a drum filter.

The clear fluid in the center of tank D is pumped into the first pH-regulating tank, G.

Tank D must be constructed of an acid resistant material and should have a capacity equivalent to at least 6 and prefer¬ ably at least 12 hours of sludge production.

The purpose of the main filter F is to remove as much as possible of the water soluble materials from the residual particulates. Fluid pressed out of the filter is pumped vi pipe c to the first pH-regulating tank, G.

The filter cake from F will typically contain 10-30$ of th ingoing sludge's dry matter and will typically contain 40-60 by weight dry matter. Its composition will vary greatl according to the type of sludge from which it is derived Typical dry matter analyses show a filter cake from domesti sewage sludge treated in accordance with the invention, t consist of 20-40$ by weight of ash forming substances (mainl silicates and calcium compounds), 40-60$ by weight o insoluble organics, presumably cellulose, and 5-15$ by weigh of water soluble materials from the entrained aqueous phase The heavy metal content of such filter cakes will b proportionate to the amount of entrained aqueous phase, i.e typically 10-30$ of the concentration of such metals in th raw sludge.

This material can be used as a soil improvement agent.

The purpose of the first pH-regulating tank, G, is to adjus the pH such that a greater part of aluminium and iron (III salts, useful as flocculating agents for raw sludge liquors are precipitated whilst at the same time avoiding majo coprecitation of undersirable heavy metals. The optimum p range where this occurs is between 3.5 and 4.5; the highe the pH, the greater the yield but also the greater the ris for coprecipitation of heavy metals. The choice of first pH regulation level will therefore depend upon the ingoin sludge's content of heavy metals, but will in any event fal within the abovementioned range.

Precipitant can be chosen from the following: an alkali meta hydroxide, an alkali metal carbonate, ammonia * or ammoniu carbonate. Ammonia gas is the precipitant of choice as it i cheap, easy to use, does not add water to the system an enhances the plant nutrient value of the end product. A excellent alternative to ammonia is however leachant fro wood ash where this is available. Potash, i.e. potassiu carbonate should be preferred where ingoing cadmium or mercury levels are high (>100 ppm or >10 ppm dry matter, res pectively), in order to reduce the risk of soluble heav metal amines forming. Calcium hydroxide can be used as precipitant for sludges low in phosphate.

The mixed metal hydroxides precipitating in tank G are removed through a pipe, d, in the base of the tank and filtered e.g. in a bag filter. Filtrate is pumped to the second pH-adjustment tank, H, whilst the residue is dissolved in mineral acid, chosen from sulphuric or hydrochloric acid and reused as flocculant.

The purpose of the second pH-regulating tank, H, is to adjust the pH such that a greater part of heavy metal components are precipitated. The optimum pH range where this occurs is between 6.5 and 8.5, dependent upon the choice of preci¬ pitant, but the pH will in any event fall within the abovementioned range.

Precipitant can be chosen from the following: an alkali metal hydroxide, an alkali metal carbonate, ammonia or ammonium carbonate. Whilst ammonia can also be used as precipitant here, potash, i.e. potassium carbonate is the precipitant of choice for products arising from sludge high in heavy metals, as it ensures maximum precipitation of such metals at pH 7 (as carbonates), reduces the risk of soluble cadmium complex formation and contributes to the plant nutrient value of the end product. Leachant from wood ash, where this is avail¬ able, is an excellent alternative to potash. Calcium hydroxide can also be used as precipitation reagent for sludges low in phosphate.

The mixed heavy metal rich precipitate leaves the tank through pipe e and is pumped into a filtering device e.g. a bag filter. The filtrate is pumped to the evaporators, and the residue disposed of e.g. in an approved dump sit This residue's dry matter amounts typically, in the case of sewage sludge, to about 1$ by weight of ingoing sludge d matter.

The first and second pH-regulating tanks, G and H respec tively, can be combined in a single stage precipitation i the case of a) small plants (< 5000 tons/yr. ingoing dr matter) and/or b) where the heavy metal content is low (e. Cd < 1.5 ppm dry matter) and/or c) where aluminium containin substances are used as flocculants in the form of aluminates In the case of c), the residue can be washed at pH 10.5 ± e.g. with a solution of sodium hydroxide so as specificall to remove aluminium as aluminate. For sludges containing hig levels of mercury, the aqueous phase can be treated with precipitation reagent specially chosen for this purpose, e.g Degussa ^RTMT 15.

The purpose of the evaporators, I_j_ and I2 respectively, i to reduce the volume of product leaving the plant and t yield a stable aqueous phase whose osmotic pressure potentia is such that microorganisms cannot multiply. Whereas dissolved solids content of about 20$ is sufficient to mee the latter requirement, the costs related to transport an associated areas dictate that the aqueous phase shoul contain rather more solids, typically 30$ by weight or more.

The aqueous phase from many types of biological sludge including that from abattoirs, sewage treatment plants an food industry plants, will contain fatty acids. These fatt acids will not be completely removed in the settling tank, D due to their slight solubility in water and will be converte to soaps at pH greater than about 3, i.e. during the first p adjustment stage. These soaps are highly effective surfac tants and will cause foaming in an incorrectly designe evaporator, e.g. an evaporator where boiling takes place wit bubble formation. We have therefore found it prudent to us thin-film evaporators or spray driers in order to reduce th volume of liquid.

The use of two (1^ and l ) or more thin film evaporators i series ensures the removal of sufficient water to yield product containing 30-40$ water soluble solids. It is no advisable to use such evaporators for these aqueous phases beyond this dry matter content as the viscosity and risk o crystallisation become too high for easy pumping. Alter¬ natively, a washing tower or spray drying tower operated such that the product contains 30-40$ solids can replace the thin film evaporators.

It should be noted that the steam from the evaporators will contain small amounts of organic compounds, primarily furfural. This should preferably be condensed and returned to the first settling basin in the biological treatment plant where it will provide an easily accessible carbon source and thereby assist denitrification processes.

The product storage tank J contains that portion of ingoing dry matter, for sewage sludge typically 3/5, that has been converted into water soluble compounds at pH 7.5 ± 0.5, concentrated to 30-40 weight$ dry matter. This represents a considerable reduction in volume, which in turn is parti¬ cularly valuable in cold climates, as such products need to be stored for a period of up to 6 months before they can be used.

The plant described above is highly compact and can be accommodated in a roofed building area, exclusive of offsites for energy production, of ca. 200 m² for a plant with capacity 15,000 tons/yr., 20$ dry matter. Finished product storage does not require a roofed building. In the case of sewage sludge, roughly 1/4 to l/7th of t dry matter is removed in the settling and pH adjustment ste respectively, leaving the remaining ca. 3/5 plus add chemicals in solution.

The filter cake from the main filter, F, contains partial hydrolysed cellulose, soluble carbon sources and essenti plant nutrients (mainly phosphorus and nitrogen), whi contains considerably less heavy metals than the sludge fr which it is derived. The cake's low pH renders it incapabl of being broken down by microorganisms.

However, we have found that the cake is rapidly composted a yields an excellent growth medium for plants if its pH i raised, e.g. by adding finely ground lime, wood ash, ammoni etc.

The oils, primarily saturated and unsaturated fatty acids that are removed from tank D and stored in tank E, can b used in the manufacture of soaps and surfactants and as component in animal fodder. They can also be dissolved in small quantity of alkali and used as carbon source fo denitrification purposes.

The hydroxide sludge will contain metals other than alkal and alkaline earth metals. Where two stage precipitation i employed, Fe (III) and Al (III) will constitute by far th major portion of the sludge precipitated at pH 4. These ca be dissolved preferably in sulphuric acid for reuse a flocculent. Where a single stage precipitation is employed Al (III) can be recovered as aluminate by washing with alkal at pH >10.

The dissolved solids in the liquid from the evaporators, 1 and ∑2 respectively, contain primarily alkali metal and/o ammonium phosphates, alkali metal and/or ammonium sulphate and water soluble carbonaceous materials, primarily carbo hydrates and their derivatives. This liquid can be used as a carbon source for denitrification or as a unique combined fertiliser and soil improvement agent. The product can also be spray dried to a pelletisable water soluble powder containing ca. 90 $ dry matter.

The composition of the product from the,- evaporators 1*^ and 12 will vary according to the sludge employed and the choice of processing characteristics within the scope of the invention.

Whilst sewage sludge based products made by state-of-the-art technologies typically contain 20-25$ dry ^' matter and 1-2$ nitrogen (compost) or 85-95$ dry matter and 4-7 weight$ nitrogen (pellets), the nitrogen being slowly available organic compounds (proteins), the product made in accordance with the invention from the same sludge will contain at least 20$ and generally at least 30$ by weight dry matter, of which about 1/3 is inorganic materials, part of which is rapidly available nitrogen in the form of ammonium sulphate. The product made by spray drying this solution will contain up to 25$ nitrogen, depending on the composition of the raw sludge and the selection of pH adjustment level.

Whilst the content of some heavy metals in state-of-the-art sewage sludge based compounds is typically Cd > 10 ppm, Pb > 100 ppm, it is considerably lower for products made in accordance with the invention and typically Cd < 5 ppm and Pb < 25 ppm.

Products made from the state-of-the-art technology, e.g. composted and/or pelletised sludge, are solids containing fermentable matter and are subject to fermentation in the presence of sufficient moisture. This will lead to the development of odours and other handling difficulties. The product made in accordance with the invention is, on t other hand, completely stable and incapable of bei fermented. These attributes are very surprising as t components are highly fermentable, but it has been found th this fermentability ceases once the concentration of d matter exceeds about 20$. This is presumably due to t fact that the osmotic potential of the liquors containi more than about 20$ dry matter is too high to perm microorganisms to multiply.

As the inorganic compounds in liquors containing more th 20$ by weight of inorganic and 30-40$ by weight of organi compounds are liable to precipitate, it is preferable t spray dry liquors containing 40$ by weight or more dissolved solids if a more concentrated end product i desired.

The fertilising and soil improvement characteristics o (sewage) sludges are described in the literature ( "Slam spredning pa akermark", Nilsson, K. , Edner S. , Avfallsaktie bolaget, 9.90, and "Operational experiences of sludg application to forest sites in Southern Scotland" Arnot J.M., et al , Seminar at the University of York 5-7 Sept 1989). Particular mention is made of the fact that leachin of nitrogen-containing compounds to groundwater is far lowe per kg N added when (composted) sludges are used as nitroge source than when synthetic fertilisers are employed. This i understood to be due to the fact that the nitrogen in suc sludges is organically bound and only slowly released suc that the soil's nitrogen binding capacity is not exceeded whereas the opposite is purported to be the case fo synthetic fertilisers such as ammonium salts, alkali an alkaline earth metal nitrates, urea, etc.

However, sludges and state-of-the-art materials based upo sludge are not generally appreciated as fertilisers, as thei nitrogen content is often too slowly and irregularly available to be useful for this purpose e.g. with annual crops in cooler climates. It has been found that the amount of nitrogen added in the form of state-of-the-art sludge based products must be 2-3 or more times greater than that added in the form of the common synthetic fertilisers to yield the same growth enhancement in such climates. As sludge based products typically contain -by weight only 1/5- l/10th of the amount of nitrogen in a synthetic fertiliser, this implies that the amount by weight of sludge based product used must be 10-30 times greater than the amount of synthetic fertiliser to achieve equal yields of annual crops such as grass, wheat, vegetables, etc. Such large volumes are cumbersome and costly to handle, and they also add unacceptably large amounts of (heavy) metals to the soil.

Whereas the products made according to the invention contain considerably greater proportions of nitrogen than state-of- the-art sludge based materials, the nitrogen is present almost entirely as ammonium salts, i.e. inorganically bound. The present understanding of the modus operandi for organic and inorganic nitrogen would presuppose that products made according to the invention thus contain nitrogen that is a) readily available and b) easily leached out to groundwater.

It is therefore totally unexpected and most surprising that whilst a) can be demonstrated to be similar to that which occurs when using inorganic nitrogen, b) is similar to that which occurs when using organic nitrogen; indeed it is shown below that the use of products made according to the invention leads to an increase in total soil + leached + plant nitrogen over and above that added as products in accordance with, the invention. Furthermore it can be shown that the extent of soil improvement is greater when using products of the invention than when using state-of-the-art sludge based products and, of course, far more marked than when using synthetic fertilisers. Example 1. Nitrogen leaching

A) Experimental method

Six columns constructed as shown in fig. 2 were filled follows:

a-b 1 part peat to 7 parts water washed builders' sand b-c 1 part peat to 10 parts water washed builders' sand c-d water washed builders' sand.

The column was saturated with water and the excess wat allowed to drain away over a period of 5 days through t outlet.

Each column was filled with a solution of the followi composition: (i) 150 ml of a product made from a biologic sludge according to the invention; (ii) 150 ml of a soluti of ammonium sulphate and ammonium dihydrogen phosphate havi the same N and P content as the product used in (i); (iii) quantity of pelletised sewage sludge based product equal in content to (i); (iv) a quantity of the untreated sewa sludge upon which (i) was based and equivalent in N conte to (i); (v) a quantity of liquid organic fertiliser (Vadhe Groplex) equivalent in N and P content to (i); and (v distilled water.

The outlet remained open for a period of 5 minutes aft addition of the liquids and the liquid draining out of t columns during this time collected, measured (ml-6_xι ) a tested for N content. The outlet was then closed for period of 5 days and then reopened to allow liquid that h drained through the column in this time to be collecte measured (ml-6_X ) and tested for N. 20

A quantity of distilled water equivalent to 1.3 times that which had drained out was added to the column and the procedure repeated. This routine was repeated twice further, after which the column was dismantled and the strata a-b, b-c and c-d separated from one another, weighed wet and dry, individually mixed to assure homogeneity and tested for N. The results are given in Table 1.

TABLE 1. mg Nitrogen

1. 2. 3. 4. 5. 6. 7. Added Leached in a-b in b-c in c-d loss*¹ 6./1, xlOO

Synth. fertiliser 1890 365 390 260 180 1060 56.1

Organic fertiliser 2090 25 1090 365 160 475 22.7

Sludge product 1850 60 750 585 540 (25) (1.3)

Untreated sludge 1780 30 1360 135 80 205 11.5

Pelletized sludge 1820 20 1480 120 40 180 9.9

Water

(control) 90^c 10 30 20 10 30 33.3

^a N as ammonium + nitrate + nitritt ^b 1. - (3.+4.+5. ) ^c N present in the wash water

B. Interpretation of results

The increase in total nitrogen shown so conspicuously in the case of products made according to the invention is assumed to relate to a propensity that these products have for encouraging the growth of nitrogen fixing bacteria. It is further assumed that this in turn is a consequence of t carbonaceous materials of the products made according to t invention being more readily assimilable by bacteri ("fermentable") than those from e.g. columns (iii) to ( inclusive.

The amount of nitrogen leaching, whilst marginally higher i the case of products made in accordance,* with the inventio than for products (iii) to (v) inclusive, is nonetheles dramatically lower than for the synthetic fertiliser syste in (ii). It is also noticeable that this amount decrease with time whilst it remains high in the case of (ii). Th greater amount of N available in column (i) implies that les N need be added as product made according to the inventio than for the other products to ensure equal plant growth.

Example 2. Soil improvement characteristics

A. Moisture absorption and retention in sandy soils

The fertility of sandy soils is highly dependent upon th extent to which they are able to retain a film of moistur and nutrients around the soil particles which is availabl for plant roots. Inasmuch as the packing material in column (i) to (vi) contains sandy soil, the wet and dry weights o strata a-b, b-c and c-d were measured for each column befor and after the routine described in Example 1 (A) (Expermenta method) and the results are shown in Table 2. Weight loss wa also measured as a function of drying time at 50°C. TABLE 2. g Water

1. 2. 3. 4. 5. 6. 7. 8.

Added Leached in a-b in b-c in c-d 2.-1. °./2. Spec^a §ynth. fertiliser 491 414 45 22 10 77 0.19 0.59

Organic fertiliser 519 346 92 48 23 173 0.50 1.32

Sludge product 656 372 102 88 84 284 0.76 2.17

Untreated sludge 505 321 121 45 18 184 0.57 0.71

Pelletized sludge 517 336 132 33 16 181 0.54 1.38

Water (control ) 471 319 90 27 14 131 0.41 1

^a Specific water absorption, 6. product/6, control Average of 3 runs.

The dramatically higher moisture content of the samples from column (i ) and the lower gradient of the drying curve for the strata there Implies very strongly that products made in accordance with the invention are superior in their ability to improve the propensity of sandy soils to retain moisture (and thus also nutrients ) and thereby improve their fertility.

B . Moisture absorption and porosity in clay soils

Soils rich in clay retain moisture and nutrients rather better than sandy soils , but their large amounts of fine particles ( <10u ) pack tightly and hinder the easy transport of air, moisture and nutrients. Such soils also tend t crack upon drying, a factor which may expose plant roots an lead to poor growth.

In order to determine whether products made according to th invention acted as improvers for such soils, the procedures in Example 1 (A) (Experimental method) and Example 2 (A) (Moisture absorption and porosity in clay soils) were repeated with columns whose strata a-b consisted of 4 parts china clay, 3 parts sand and 1 part peat fibre. In order to reflect the fact that nutrients and soil improvement agents need to permeate the clay if they are to be effective, the interval between each addition of water was increased to 7 days and the top 1/3 of strata a-b was removed prior to analysis. The results are shown in Table 3.

TABLE 3. g Water

1. 2. 3. 4. 5. 6. 7. 8.

Added Leached in a-b in b-c in c-d 3. +4. ⁶./2. Spec^a

+5 Siynth. fertiliser 585 293 62 47 15 124 0.42 0.73 Organic fertiliser 656 309 76 61 32 169 0.54 1 Sludge product 771 308 93 107 88 288 0.94 1.70 Untreated sludge 614 291 77 45 18 140 0.48 0.83 Pelletized sludge 517 236 64 33 16 113 0.48 0.67 Water

(control) 604 316 81 59 29 169 1.53 1

^a Specific water absorption, 6. product/6, control As can be seen from table 3 the column containing products made according to tne invention demonstrates the superior moisture (and nutrient) penetration and retention properties that these products are able to impart clay soils. It shows that only sample a-b from the column treated with products made according to the invention formed a crumb-like structure upon drying. Taken together with the slower drying out rate, this indicates that this sample is both more porous and better at retaining moisture than the others.

C. Ion exchange characteristics

The ion exchange characteristics of soil are important inasmuch as certain ions, e.g. soluble AI³⁺, are toxic for plants and fish whilst others, e.g. Cd²⁺, are toxic also for higher animal species. Soil with a high ion exchange capacity will bind these ions and limit the extent to which they are washed out to groundwater. It is known that low soil pH, e.g. resulting from the use of ammonium salts or urea fertilisers, increases leaching (Lindmark, J.E., Vaxtpressen No.l, 2/90).

As sandy soils are particularly at risk for such leaching, the ion exchange characteristics of strata in columns (i) to (vi) inclusive were tested by adding a quantity of cadmium sulphate and aluminium .sulphate such that the content of each of Cd²⁺ and Al³⁺ ions in the nutrient used at the start of procedure 3.3.1.A. (Experimental method) was at least 500 ppm of the solution [water in column (vi)]. The content of each of these two ions in the leachant from the column was measured and the results shown in Table 4. TABLE 4. Added 500 ppm Cd²⁺ and 500 ppm Al^{3+ a}

1. 2. 3.

Water Al³⁺ Al³⁺ added leached Spec*

Synth, fertiliser 4x123 g 385 ppm 1.20 Org. fertiliser 4x123 g 210 ppm 0.66 Sludge product 4x164 g 90 ppm 0.28 Water (control ) 4x125 g 320 ppm 1

^a0.14 g CdS0₄ + 0.47 g A1₂(S0₄)₃ in 149.4 g water phase ^Specific metal leaching (control) = 1

As can be seen from Table 4, the column containing produc made according to the invention demonstrates the superior i exchange capacity of the mixture in this column. This most surprising as current theory would predict that the u of ammonium sulphate would enhance leaching due to i propensity for lowering soil pH [cf. column (i)] and as solubility enhancer for cadmium compounds. We believe th the presence of easily accessible carbon source increases t growth of microorganisms that both bind metal ions and buff pH.

Example 3. Growth enhancement properties.

After demonstration that products made according to t invention possess unique soil improvement characteristi that are not exhibited either by synthetic fertiliser state-of-the-art sludge based products or commercial organ fertilisers, the growth enhancement properties of these thr groups of products were investigated.

A. Experimental method

Five sowing trays [(i) - (v)] were prepared as in fig. jj a filled with a mixture of 6 parts sand, 1 part china clay a 1 part peat fibre. On the surface of each tray, 2 gms of grass seed (festuca spp. ) was sown and covered with 2-3 mm of soil. The trays were watered and maintained at 23°C, and each was alternately given 12 hrs. artificial daylight, 12 hrs. darkness, and watched for the appearance of seedling leaves (ca. 7 days).

Once the seedling leave had sprouted, tnays (i) - (iv) were watered with 250 ml of a solution containing 6$ N, 1$ P and 4$ K derived from (i) product made according to the invention, (ii) a mixture of ammonium sulphate, ammonium dihydrogen phosphate and potassium sulphate, (iii) commercial organic fertiliser, (iv) a slurry of 150gm sludge from which product (i) was derived in 150 ml water. to (v) was added 250 ml distilled water as control. Potassium sulphate or hydrogen phosphate or ammonium sulphate or ammonium dihydrogen phosphate was added to samples (i), (ii) and (iv) in order to attain the abovementioned N, P and K contents.

500 ml of distilled water was sprayed uniformly onto each tray every 5th day over a period of 30 days and the run-off was collected from each tray every 5th day.

After 30 days the grass in each tray was cut down to soil level, weighed and analysed for N. 1/2 of the soil area was removed from each tray together with the grass roots and the soil particles washed away to leave the root mass which was then weighed and analysed for N and P. The remaining 1/2 of soil in each tray, including the grass roots, was weighed wet and dry and analysed for N and P. The results are shown in Table 5.

The above procedure was repeated using carrot seed (Nantes Early, 1 seed per 10 cm, 10 cm between rows) instead of grass seed. The trial was carried out over a period of 7 weeks and the periods between watering increased to 7 days. The results are shown in Table 6. TABLE 5. Grass

Leaf Leaf Root Root Root Soil Soil N-cont Weight N-cont P-cont. Weigjit Weight Weight ($) (g) ($) ($) (g) dry (g) wet (g)

Nitrogen.

TABLE 6. Carrot

Leaf Leaf Root Root Root Soil Soil N-cont Weight N-cont P-cont. Weight Weight Weight

Nitrogen.

() increase ^aAdded = (in soil + in plants)

B. Conclusions

The results from the trays containing products made in accordance with the invention are remarkable and cannot be explained on the basis of existing understanding of the modus operandi for fertilisers. It must therefore be concluded that the components in products made according to the invention act synergistically with one another in a manner hitherto not described in the literature. The amount of growth (leaf + root) as function of nitrog consumed, i.e. added - (washed out + in soil + in plant + roots) suggests that considerably less nitrogen need be add to soil as products of the invention than as, e.g., synthet fertiliser in order to achieve an equal growth.

The relatively large amount of nitrogen remaining in the soi at the end of the growth trials where products of t invention were used suggests that an application of fertili ser in the form of products of the invention will provi sufficient nitrogen for an entire growth season and a excess will remain in the soil to be assimilated in t following year(s). This mechanism will obviate the need fo a so-called "top dressing", i.e., addition of fertiliser i mid-season. This promises considerable savings both i labour and in fertiliser costs for, e.g., farmers and woodlo owners.

The low and decreasing leaching of nitrogen with time is a important contribution in the effort to reduce nitroge levels in groundwater draining from agricultural areas.

Effluent from e.g. sewage treatment plants often contain nitrogen in quantities that can lead to a rapid growth o algae and other microorganisms in waterways and conduits fe by such effluent. Apart from being a nuisance by dint o requiring regular cleaning of such waterways and conduits this fouling can also be hazardous from the point of view o maintaining a healthy aquatic environment, e.g., in lakes an ponds and even in seawater. This state of affairs ha resulted in such effluents being treated by causing micro organisms to grow in them under controlled conditions, tha both fix and reduce nitrate and nitrites to nitrogen, thereb reducing the amount of nitrate subsequently released to th environment. This process requires a readily accessible carbon source in order for the aforementioned microorganisms to multiply rapidly. Such carbon source must generally be added, and substances such as methanol and molasses are often used. This adds considerably to treatment costs.

The method of the invention described here, particularly when employed for treating e.g. paper and pulp industry sludges, produces products that function as readily accessible carbon sources for such denitrification purposes.

The disposal of cellulose-rich sludges that are low in phosphates and metals, e.g. from the paper and pulp industry, is often costly and difficult. Treated in accordance with the method, they will yield liquid reactor products that are rich in C^-sugars. The pH of these reactor products can in whole or in part be adjusted with calcium hydroxide, such that their nitrogen content is optimised for their subsequent fermentation to e.g. ethanol or a proteinaceous animal feed.

Claims

C l a i m s

1.

Product containing water soluble compounds of nitrogen a phosphorus that are rapidly acting fertilisers, c h a r a c t e r i z e d i n that the content of nutrien leaches slow from the soil where the wat;er soluble nitrog is in the form of urea and ammonium salts, and that t product contains between 5 and 25 $ by weight of nitrogen.

2.

Product according to .Claim 1, c h a r a c t e r i z e d i that the product is in liquid or dried state and contain respectively, between 15 and 30 $ by weight or more than 50 by weight of a water soluble, fermentable carbon source.

3.

Product according to Claims 1 and 2, c h a r a c t e r i z e d i n that the weight ratio nitrogen to carbon, the latter in the form of fermentab substances, is from 1:1 to 1:10, preferably between 1:2 a

1:5.

4.

Product according to Claims 1 to 3, c h a r c e t e r i z e d i n that it is made from organ sludges.

5.

Product according to Claims 2 to 4, c h a r a c t e r i z e d in that it is suitable for use a carbon source for, e.g., denitrification of efflue streams from sewage treatment plants, ethanol production the growing of proteinaceous microorganisms for use in anim feedstuffs.

6.

Product according to Claims 1 to 5, c h a r a c t e r i z e d i n that the content of cadmium is below 5 ppm, the content of mercury is below 2 ppm and the content of lead is below 20 ppm.

7.

A method for production of products according to Claims 1 to 6, c h a r a c t e r i z e d i n that organic sludges from, e.g., sewage treatment plants, plants that treat waste water from the chemical, paper and pulp industries, food-related industries, animal husbandry, etc., is heated to at least 180°C and preferably .between 210°C and 230°C for at least 2 minutes and preferably between 3 and 7 minutes.

8.

A method according to Claim 7, c h a r a c t e r i z e d i n that the method is catalyzed by addition of inorganic acids to the sludge.

9.

A method according to Claims 7 to 8, c h a r a c t e r! z e d i n that the said acid is sulfuric acid used in an amount such that the pH of the product according to Claim 1 is less than or equal to 1.5, and preferably between 0.5 and 1.0.

10.

A method according to Claims 7 to 9, c h a r a c t e r i z e d i n that the method is catalyzed by use of metallic salts.

11.

A method according to Claim 10, c h a r a c t e r i z e d i n that said metallic salts are water soluble compounds of iron (III) and/or aluminium, used in an amount of at least 100 ppm, and preferably at least 200 ppm, based on ingoi sludge.

12.

A method according to Claims 7 to 11, c h a r a c t e r i z e d i n that metallic ions from grou 2b, 3a, 4a, 5a, 6b, 7b and 8 in the periodic table ar removed by precipitation.

13.

A method according to Claim 12, c h a r a c t e r i z e i n that the precipitating reagent has value as a plan nutrient.

14.

A method according to Claims 12 and 13, c h a r a c t e r i z e d i n that the precipitating reagent is ammonia, ammonium carbonate, ammonium carbamate, potassium hydroxide or potassium carbonate or mixtures of these.

15.

A method according to Claims 12 to 14, c h a r a c t e r i z e d i n that the precipitating reagent Is added stepwise such that certain flocculants added to the raw sludge liquors in order to precipitate particulate matter therefrom can be recovered from the sterile product of claim 1 and reused.

16.

A method according to Claim 15, c h a r a c t e r i z e d i n that the flocculants are iron (III) and/or aluminium salts recovered by precipitation at pH below 4.5 in order to minimise the coprecipitation of undesirable metals such as hydroxides.

17.

A method according to Claim 12, c h a r a c t e r i z e d i n that the precipitating reagent in whole or in part consists of calcium hydroxide.

18.

A method according to Claims 7 to 17, c h a r a c t e r i z e d i n that the volume of materials containing organic compounds after treatment is reduced by 1/3 of the volume prior to treatment.