NO178296B - Products containing water-soluble nitrogen and phosphorus compounds which are fast-acting fertilizers, processes for their preparation, and use thereof - Google Patents

Description

The present invention relates to products containing water-soluble compounds of nitrogen and phosphorus which are fast-acting fertilizers produced from organic sludge, a method for producing this, as well as applications thereof.

The present invention shows that there is a synergistic effect between water-soluble organic substances and fertilizing substances, which results in the latter remaining longer in the soil without being washed out than is the case when the organic substances are not present. This is an important point in limiting the discharge of fertilizer substances into waterways and groundwater. It is further shown that mixtures of water-soluble organic substances and general fertilizers increase the growth of plants per amount of fertilizer more than the latter used alone. The present invention shows how such growth-promoting and emission-limiting mixtures can also be produced from biological sludge by a new and advantageous treatment method.

Known methods for treating biological sludge etc. aim to: a) destroy pathogenic microorganisms that may be present in the sludge, b) remove unpleasant odors, and

c) convert a dewatered sludge "cake" containing typically 15-25% dry matter into composted material which also contains

typically 20 - 25 lbs dry matter.

Since this sludge is biologically active and subject to putrefaction with the formation of foul-smelling compounds, the sludge is in some cases dried and granulated and the resulting product sold as a soil conditioner.

A typical composting plant has a processing time of at least 14 days and during this time the sludge must be turned

mechanically, ventilated and kept at a temperature of approx. 30 0 C. The product must then "ripen" for a period of 4 - 6 weeks.

Such facilities require large capital investments and large areas.

The materials produced in this way are subject to major disadvantages in that the properties and behavior of the materials in many cases correspond to the sludge from which they are produced. The materials' dry matter content of compounds with toxic heavy metals such as e.g. cadmium and lead and potentially toxic light metals such as e.g. aluminium, is identical to the content of these metals in the sludge from which the materials are made. These materials cannot be pumped either.

It has been shown that these materials are potentially applicable as fertilizers. However, the content of plant nutrients, primarily compounds of nitrogen and phosphorus, will be released too slowly for them to be generally applicable if they are not supplied in very large quantities. This in turn is ruled out due to the high content of toxic heavy metals. Another reason that precludes the use of large quantities of these materials is the danger of surface runoff. This can happen if large amounts of rainfall occur immediately after adding these materials to a poorly draining, heavy soil such as, for example. clay, as the nutrients are included in large particles that are easily transported in the runoff water.

Furthermore, the investments in facilities and equipment to convert the sludge into a granular product will be prohibitive for potential users.

So far, these factors have led to the majority of facilities for the production of organic sludge either depositing the raw sludge cake in approved landfill sites and/or using the sludge cake as soil improvers in agriculture, i.e. as an addition to the use of livestock manure.

In addition to the few plants that produce a granulated, dried product, there are a few plants that have installations that incinerate the sludge. This method will effectively reduce the volume of the product coming out of the treatment plant, but it is very expensive and involves the risk of releasing heavy metal vapors and sulfur oxides into the atmosphere.

The environmental protection authorities are very concerned about two developments: i) the high content of heavy metals (especially cadmium and lead) in agricultural products and the cultivation soil, and ii) the increasing content of organic and inorganic pollutants (especially phosphate, nitrate and nitrite) in the groundwater.

Other environmental problems relate to the content of (soluble) aluminum compounds in a) drinking water and cultivated soil since they have been believed to be the cause of Alzheimer's disease and b) in forest soil because they are believed to be the cause of forest death. Aluminum salts are commonly used as flocculants in water treatment and it is not uncommon for the dry matter in the sludge from such facilities to contain as much as 10 mass St of aluminum originating from the flocculant.

The first of these problems i) has led to a series of new regulations in many countries, which limit the amount of sludge-based soil conditioner that can be added to a given agricultural area over a given time. This amount is much less than what was allowed previously and therefore means that the production plant must be responsible for transporting the sludge to more and more remote locations than was the case before. This, in turn, will increase the operating costs of the facilities and make the disposal problems even greater.

These provisions will also apply to the raw sludge cake and the granulated and composted material mentioned earlier, since all these products have the same content of heavy metals. The second problem ii) mentioned above has led to restrictions on the operation and placement of landfills and has also led to increased control over the use of artificial fertilizers and animal manure. It is likely that these activities will also be subject to stricter regulations in the future. Lack of suitable landfills for biological sludge and especially sewage sludge is already a major problem in many countries and this situation is likely to worsen.

Another consequence of ii) is that facilities for wastewater treatment will be required to reduce the amount of nitrogen compounds emitted during operation. This will probably lead to a requirement for denitrification of the waste water from such plants. The most common denitrification method involves allowing microorganisms to multiply in the waste water and thereby convert the soluble nitrogen either into cell tissue or nitrogen gas. This requires the presence of a carbon source, i.e. a water-soluble, metabolizable carbonaceous compound, which often has to be added to the waste water before or during the denitrification process. No sludge-based products are known that can be used as a carbon source in denitrification.

The present invention relates to a completely new method for the treatment of sewage sludge and other biological sludge where all the main problems associated with the treatment of such sludge have been solved.

By using the method according to the invention, sterilization is achieved, (i.e. by removing microorganisms), stabilization (prevents the growth of microorganisms so that long-term storage is possible), volume reduction, odor removal (i.e. without the smell of sulfur or decaying materials), a large degree of reduction of the metal content, including heavy metals, and conversion to usable, salable products with outstanding properties. The costs associated with the treatment of sludge by the method according to the invention are also considerably less than by known methods which give inferior end products.

The products according to the present invention are characterized by the fact that the water-soluble nitrogen is in the form of urea and ammonium salts, which products are liquid or dried and respectively contain 15 - 30 mass-56 or more than 50 mass-56 of a water-soluble fermentable carbon source, that the mass ratio between nitrogen and carbon, the latter in the form of fermentable substances, is from 1:1 to 1:10 and preferably between 1:2 and 1:5, that the content of nitrogen is between 5 and 25 mass-56, that the content of cadmium is less than 5 ppm, the content of mercury is less than 2 ppm and the content of lead is less than 20 ppm and that the content of fertilizers is slowly washed out when the product is added to the soil.

The products can be used as a carbon source for, for example, denitrification of effluent streams from sewage treatment plants, ethanol production or growth of protein-rich microorganisms for use in animal feed.

The invention also relates to a method for the production of the products above which is characterized by organic sludge containing at least 15% dry matter being heated to at least 180°C and preferably between 210 and 230°C for at least 2 minutes, and preferably between 3 and 7 minutes, after which the insoluble organic components are hydrolyzed to water-soluble compounds with the aid of a catalyst, preferably inorganic acids, undissolved solids are separated, and metal ions from groups 2b, 3a, 4a, 5a, 6b, 7b and 8 in the periodic table are removed by precipitation by adding a precipitation reagent, which precipitation reagent is added stepwise so that flocculants added to the raw sludge liquid are recovered from the product and reused, which flocculants are iron (III) and/or aluminum salts recovered by precipitation at pH less than 4.5.

The process is preferably catalyzed by adding inorganic acids to the sludge, preferably sulfuric acid, which is used in an amount such that the pH in the products is less than or equal to 1.5, and preferably between 0.5 and 1.0, or by adding metal salts to the sludge, which metal salts are water-soluble compounds of iron ( III) and/or aluminum in an amount of at least 100 ppm and preferably 200 ppm based on added sludge.

The precipitation reagent is preferably ammonia, ammonium carbonate, ammonium carbamate, potassium hydroxide or potassium carbonate or a mixture of these, which precipitation reagent has a value as plant nutrient f, or that all or parts of the precipitation reagent is calcium hydroxide and that the ratio between nitrogen and carbon is between 1:1 and 1 :10, preferably between 1:2 and 1:5.

The invention will now be described in more detail by means of exemplary embodiments with reference to the accompanying drawings. Figure 1 shows a flowchart of an embodiment of the method according to the invention. Figure 2 shows a sketch of a column for the determination of nitrogen leaching.

Figure 1 shows a flowchart for an embodiment of the method according to the invention.

Sludge containing at least 10% and preferably 15% or more particles is mixed with an amount of the reactor product in a mixing tank, typically 20-35% by volume, so as to make it easily pumpable and an amount of mineral acids, preferably sulfuric acid, is added so that the pH of the the resulting mixture is below 1.5, preferably 0.5-0.8. This typically amounts to 20 - 50 grams of sulfuric acid per kg. dry matter.

The total content of Fe(III) and Al(III) is adjusted if necessary so that it corresponds to at least 200 ppm of the dry matter. The required quantity will vary according to the type of sludge and the figures above are representative of sewage sludge.

The resulting mixture is heated to at least 180°C and preferably between 210 and 245°C for a period of at least 2 and preferably 3-5 minutes. This treatment causes the mixture to be converted into the reactor product, which is a low-viscosity dispersion containing 2-5 mass-£ particles.

The particles settle and any immiscible oils float to the surface and both of these fractions are removed in separate containers. The particles are concentrated in a filter 9 to a filter cake that contains as much dry matter as possible, typically at least 40 mass-56.

The pH in the clear aqueous phase is adjusted, e.g. with ammonia or alkali metal carbonate or hydroxide, first to pH 4.0 ± 0.4 and then to 7.5 ± 0.5, and the precipitated material is removed after each pH adjustment. The resulting clear liquid is then evaporated to a content of 20 and preferably at least 30 mass pounds of dissolved solids.

Sludge with a low content of phosphorus and/or heavy metals, e.g. sludge from the paper and cellulose industry, and depending on the application of the final product, finely divided calcium hydroxide and/or carbonate can be added as a precipitating agent.

Dewatered biological sludge 1 containing at least 10 mass-# of solid matter leads to a mixing tank 2 where an acid 3, preferably sulfuric acid, is added in an amount of at least 10 grams and typically between 20 and 50 grams per kg. dry matter, together with a quantity of the product 4 from the reactor 5 (see above), typically 10-40% by mass of the incoming sludge. The purpose of this is to convert the original sludge into an easily pumpable dispersion. Since the addition of acid can lead to a small formation of gas, the tank 2 is provided with an outlet, so that gas 6 from the tank can be led, e.g. for mixing with the feed air to the boiler 7 or to an adsorption tower. All parts of the tank and the mixer/impeller are made of acid-resistant materials.

If analyzes of the incoming sludge show that the content of Al^<+>and Fe^<+>is lower than 200 ppm of dry matter, a quantity of aluminum and/or Fe(III) salt should be added, preferably in the form of sulphate, to achieve this concentration.

Mixing tank 2 should have a capacity that corresponds to at least 1 hour of production.

The dispersion from the tank 2 is pumped to the first holding tank 8, the purpose of which is to ensure a steady liquid flow to the reactor 5. All parts of the holding tank 8 and the mixer/impeller are made of acid-resistant materials.

The purpose of the reactor 5 is to heat the supplied sludge flow so that a large amount of the particulate material, which in the case of sewage sludge and sludge from the agricultural, paper and cellulose industry consists of cellulose, is hydrolysed within a short time to water soluble compounds. This requires the use of acid and the previously mentioned aluminum and iron (III) salts as hydrolysis catalysts and opportunities to achieve a high temperature in the reactor 5.

Considerations regarding optimal conditions with regard to maximum reduction of the amount of particulate matter, variable costs for catalysts and energy and fixed costs for paying off the plant indicate that this time should not exceed 15 minutes and should be between 2 and 10 minutes. The optimum temperature range for such sludge has been found to be at least 180 °C and preferably between 210 °C and 245 °C. The amount of acid should be such that the pH of the reaction products is below 1.5 and preferably 0.5 - 0.8.

The first holding tank 8 should have a capacity corresponding to at least 2 hours of sludge production.

Since it has previously been reported that cellulose hydrolysis requires much longer time under similar conditions regarding time, temperature and acidity, it is very unexpected that the operating conditions here are sufficient to reduce most of the sludge's cellulose content to water-soluble compounds.

We have been able to show that this happens due to small amounts of Fe(III) and/or Al salts in the sludge. Although these compounds are present in sufficient quantities in, e.g., sewage sludge, there may be a need to add them in the case of e.g. sludge from the paper industry.

The reactor 5 is designed as a long tube that is coiled up on itself, so that the supplied liquid is heated by heat exchange with the outgoing, treated liquid, while the middle part is further heated to the previously mentioned temperature range by means of an external heat source which e.g. a thermal fluid. The temperature of the outlet liquid should be lower than 100 °C and preferably 60-80 °C. This saves energy and is beneficial for the further treatment.

Part of the liquid from the reactor 5 is led to the settling tank 9 and part is led back to the mixing tank 2 via the pipe 10.

All parts of the reactor 2 must be made of acid-resistant materials.

The purpose of the settling tank 9 is to let particle residues fall out and allow fatty acids, which would otherwise cause foaming later in the process, to float to the surface. The tank 9 is equipped with a skimming device, which may be in the form of a suitably placed drainage pipe, to remove liquid substances 11 (including plastic residues), oil and grease to a storage tank.

The material that settles at the bottom of the tank 9 is pumped to a filter 12, either directly or via a drum filter.

The tank 9 must be made of an acid-resistant material and should have a capacity corresponding to at least 6 and preferably 12 hours of sludge production.

The purpose of the main filter 12 is to remove as much water-soluble material as possible from the particle residues. The liquid that is pressed out of the filter is pumped via line 13 to the first pH control tank 14.

The filter cake from the filter 12 will typically contain 10-30% of the added sludge's dry matter and will typically contain 40-60 mass-Sé of dry matter. The composition will vary greatly depending on the type of sludge it originates from. Typical dry matter analyzes show that a filter cake after treatment according to the invention of domestic sewage sludge contains 20-40 rnasse-^ ash-forming compounds (mainly silicates and calcium compounds), 40-60 mass-& insoluble organic compounds, probably cellulose and 5-15 mass -^ water-soluble materials from the accompanying aqueous phase. The content of heavy metals in such a filter cake will be proportional to the amount in the accompanying aqueous phase, i.e. typically in the range of 10-30% of the concentration of such metals in the raw sludge.

This material can be used as a soil conditioner.

The purpose of the first pH control tank 14 is to adjust the pH so that a larger proportion of Al and Fe(111) salts, which can be used as flocculants for raw sludge liquids, are precipitated while avoiding an unwanted precipitation of heavy metals. The optimal pH range for this is between 3.5 - 4.5. The higher the pH, the better the yield, but at the same time increasing the risk of precipitation of heavy metals. The choice of the highest pH regulation level therefore depends on the added sludge's content of heavy metals, but will not in any case fall below the mentioned range.

The precipitating agents can be chosen from among the following: alkali metal hydroxide, alkali metal carbonate, ammonia or ammonium carbonate. Ammonia gas is the precipitant of choice as it is inexpensive, easy to use, does not add water to the system and improves the nutritional value of the final product. An excellent alternative to ammonia is leached wood ash, if this is available. Pot ash, i.e. potassium carbonate, should be preferred if the content of cadmium or mercury in the added sludge is high (respectively > 100 ppm or > 10 ppm dry matter) to reduce the risk of heavy metal amines being formed, i.e. complexes of heavy metal compounds and ammonium hydroxide or ammonium salts. Calcium hydroxide can be used as a precipitating agent for sludge with a low phosphate content.

The mixed metal hydroxides that precipitate in the tank 14 are removed from the bottom of the tank 14 and filtered, e.g. in a bag filter 15. The filtrate is pumped to the second pH adjustment tank 16, while the residues are dissolved in mineral acid, selected from sulfuric acid or hydrochloric acid, and used again as a flocculant.

The purpose of the second pH control tank 16 is to adjust the pH so that a greater proportion of the heavy metals are precipitated. The optimal pH range where this occurs is between 6.5 and 8.5, depending on the precipitant chosen, but the pH will in all cases lie within the suggested range.

The precipitating agent can be selected from the following: alkali metal hydroxide, alkali metal carbonate, ammonia and ammonium carbonate. Although ammonia can be used as a precipitant, pot ash, i.e. potassium carbonate, is preferred for products derived from sludges with a high content of heavy metals, as it ensures maximum precipitation of such metals at pH 7 (as carbonates), reduces the risk of formation of soluble cadmium complexes and contribute to the plant nutritional value of the final product. If washed wood ash is available, this is an excellent alternative to pot ash. Calcium hydroxide can also be used as a precipitation reagent for sludge with a low phosphate content.

The precipitated mixed heavy metals are led out of the tank 16 and pumped to a filtering device 17, e.g. a bag filter. The filtrate is pumped to evaporators (not shown), and the residues are deposited, e.g. on a filling. The quantities of dry matter in these residues are, in the case of sewage sludge, approx. 1 mass-^ of the dry matter in the added sludge.

The first and second pH control tanks, 14 and 16 respectively, can be combined as precipitation in a single step in case

a) small plants (< 5000 tonnes/year added dry matter) and/or b) if the content of heavy metals is low (e.g. Cd < 1.5 ppm)

dry matter) and/or c) if Al-containing compounds are used as flocculants in the form of aluminates. In case c) the residues can be washed at pH 10.5 ± 1, e.g. with a solution of sodium hydroxide, for the specific removal of aluminum as aluminate. For sludge containing large amounts of mercury, the aqueous phase can be treated with a precipitation reagent specially selected for this, e.g. Degussa<R>TMT 15.

The purpose of the evaporators (not shown) is to reduce the volume of the product leaving the plant and to provide a stable aqueous phase, the osmotic pressure potential of which is such that microorganisms cannot multiply. While a content of dissolved solids of approx. 20% is satisfactory to meet the last requirement, transport and associated costs will require the aqueous phase to contain more dry matter, typically 30 rnasse-^ or more.

The aqueous phase from several types of biological sludge, including sludge from slaughterhouses, sewage treatment plants and the _food industry, will contain fatty acids. These fatty acids will not be completely removed in the settling tank 9 due to their low solubility in water, and they will be converted into soaps at pH higher than approx. 3, i.e. during the first pH adjustment step. These soaps are very effective surfactants and will cause foaming in an inappropriately designed evaporator, i.e. an evaporator where boiling with bubble formation takes place. We have therefore found it appropriate to use thin-film evaporators or spray dryers to reduce the volume of the liquid.

The use of two or more thin-film evaporators in series ensures the removal of a sufficient amount of water to give a product containing 30-40% water-soluble solids. It is not advisable to use such evaporators for these higher solids aqueous phases, because the viscosity and the risk of crystallization become too high for pumping. Alternatively, a washing tower or spray drying tower, operated so 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 fed back to the first settling basin in the biological treatment plant, where it will constitute an easily accessible carbon source and thereby speed up the denitrification processes.

The product storage tank (not shown) contains that part of the supplied dry matter, typically 3/5 for sewage sludge, which has been converted to water-soluble compounds and adjusted to pH 7.5 ± 0.5, concentrated to 30-40 mass-£ dry matter. This represents a significant reduction in volume, which is particularly valuable in areas with cold climates, since such products must be stored for up to 6 months before they can be used^

The plant described above is very compact and can be placed in a building with an area of approx. 200 m<2> for a plant with a capacity of 15,000 tonnes/year, 20 £ dry matter, not including areas for energy production. Storage of finished products does not need to take place under a roof.

In the case of sewage sludge, approx. 1/4 to 1/7 of the dry matter will be removed in the settling step and the pH adjustment step respectively and will give the remaining 3/5 plus added chemicals in solution.

The filter cake from the main filter 12 contains partially hydrolysed cellulose, soluble carbon sources and important plant nutrients (mainly phosphorus and nitrogen), which contain less heavy metals than the sludge from which it was produced. The filter cake's low pH means that it cannot be broken down by microorganisms.

However, we have found that the filter cake quickly composts and provides an excellent growing medium for plants if the pH of the filter cake is increased, e.g. by adding finely ground lime, wood ash, ammonia and the like.

The oils from the settling tank 9, mainly saturated and unsaturated fatty acids, which are removed and stored in a tank (not shown) can be used in the manufacture of soap and surfactants and as a component in animal feed. They can be dissolved in a small amount of alkali and used as a carbon source for the denitrification. The hydroxide sludge will contain metals in addition to alkali and alkaline earth metals. If two precipitation steps are used, Fe(III) and Al(III) will make up the majority of the sludge that is precipitated at pH 4. These can preferably be dissolved in sulfuric acid for reuse as flocculants. If a single precipitation step is used, Al(III) can be recovered as aluminate by ashing with alkali at pH > 10. __

The dissolved solids in the liquid from the evaporators (not shown) contain mainly alkali metal and/or ammonium phosphates, alkali metal and/or ammonium sulphates and water-soluble carbonaceous materials, mainly carbohydrates and derived compounds. This liquid can be used as a carbon source for the denitrification or as a unique combined fertilizer and soil conditioner. The product can also be spray-dried into a pelletizable, water-soluble powder containing approx. 90% solids.

The composition of the product from the evaporators will vary according to the sludge used and the choice of operating characteristics within the scope of protection of the invention.

While sewage sludge-based products produced using known technology typically contain 20-25 dry matter and 1-2% nitrogen (compost) or 85-95% dry matter and 4-7 mass-Sé nitrogen (pellets), where the nitrogen is slowly absorbable compounds (proteins), the product produced according to the present invention from the same sludge will contain at least 20 56 and generally at least 30 mass-^ dry matter, of which approx. 1/3 are inorganic materials, part of which is readily available nitrogen in the form of ammonium sulphate. The product that is . produced by spray drying this solution will contain up to 25% nitrogen, depending on the composition of the raw sludge and the choice of pH adjusting agent.

While the content of certain heavy metals in known sewage sludge-based compounds is typically Cd > 10 ppm, Pb > 100 ppm, it is significantly lower in the products manufactured according to the invention and typically Cd < 5 ppm and Pb < 25 ppm.

The products manufactured according to known technology, e.g. composted and/or pelletized sludge, are solids containing fermentable material and which are subjected to fermentation in the presence of sufficient moisture. This will lead to the development of odors and other handling problems.

The product produced according to the invention, on the other hand, is completely stable and incapable of being fermented. These properties are very surprising since the components are highly fermentable, but it has been found that fermentability decreases when the concentration of dry matter exceeds approx. 20%. The reason for this is probably that the osmotic potential of liquids containing more than approx. 20 5é dry matter, is too high for the microorganisms to multiply.

Since it is likely that the inorganic compounds in liquids containing more than 20 mass-56 inorganic compounds and 30-40 mass-56 organic compounds will be able to precipitate, it is preferred to spray-dry liquids containing 40 mass-£ or more dissolved solids , if a more concentrated final product is desired.

The fertilizing and soil improvement properties of (sewage) sludge are described in the literature ("Sludge spreading on arable land, Nilsson, K., Edner, S., Avfallsaktiebolag, 9.90 and "Operational experiences of sludge application to forest sites in southern Scotland" Arnot, J.M. et al., Seminar at the University of York 5-7/9/89).It is particularly mentioned that the leaching of nitrogenous compounds into the groundwater is much lower per kg of added N when (composted) sludge is used as nitrogen- source instead of artificial fertiliser. This is explained by the fact that nitrogen in such sludge is organically bound and released slowly, so that the soil's nitrogen-fixing capacity is not exceeded, while the opposite is the case with artificial fertilizers such as ammonium salts, alkali and alkaline earth metal nitrates, urea, etc. .

However, sludge and known materials based on sludge are not generally accepted as fertilizers, since the nitrogen content is often too late and unevenly available to be used in this way, for example for crops in cool climates. It has been found that the amount of nitrogen added in the form of known sludge-based products must be 2-3 or more times greater than what is added in the form of ordinary artificial fertilizers to give the same growth improvement in such climates. Since sludge-based products typically contain only 1/5 - 1/10 of the mass of nitrogen in artificial fertilizers, this means that the amount of sludge-based product used is 10-30 times greater than the amount of artificial fertilizers to give the same yield of crops such as grass , wheat, vegetables etc. Such large volumes are difficult and expensive to handle, and they also add unacceptably large amounts of (heavy) metals to the soil.

While the products produced according to the invention contain significantly larger amounts of nitrogen than known sludge-based materials, the nitrogen is present almost entirely as ammonium salts, i.e. inorganically bound. The understanding of the mode of action of organic and inorganic nitrogen will lead to the assumption that the products manufactured according to the invention contain nitrogen which a) is easily available and b) is easily leached into the groundwater.

It is therefore completely unexpected and surprising that while a) can be shown to be similar to what happens when inorganic nitrogen is used, b) is similar to what happens when organic nitrogen is used, and it is shown that the application of the products manufactured according to to the invention leads to an increase in total soil + leached + plant nitrogen which is more than what is added as products according to the invention. Furthermore, it can be shown that the degree of soil improvement is greater when using the products according to the invention than when using known sludge-based products and of course much more marked than when using synthetic fertiliser.

Example 1. Nitrogen leaching

A) Experimental procedure

6 columns constructed as shown in fig. 2 was filled as follows: a-b 1 part peat to 7 parts water-washed molding sand b-c 1 part peat to 10 parts water-washed molding sand c-d water-washed molding sand

The column was saturated with water and the excess water was drained away within 5 days through the outlet.

Each column was filled with a solution with the following composition (i) 150 ml of a product prepared from a biological sludge according to the invention; (ii) 150 ml of a solution of ammonium sulphate and ammonium dihydrogen phosphate with the same N and P content as the product used in (i); (iii) a quantity of pelletized sewage sludge-based product with the same N content as (i); (iv) a quantity of untreated sewage sludge which (i) was based on and with the same N content as (i); (v) a quantity of liquid organic fertilizer (Vadheim Groplex) with the same N and P content as (i); and (vi) distilled water.

The outlet was open for a period of 5 minutes after the addition of the liquids and the liquid drained from the columns during this time was collected, measured (ml-6xl) and examined for N content. The outlet was then closed for a period of 5 days and reopened so that liquid that had drained through the column during this time could be collected, measured (ml-6X2) and examined for N.

An amount of distilled water corresponding to 1.3 times that which had been drained out was added to the column and the procedure repeated. This routine was repeated two more times, after which the column was dismantled and strata a-b, b-c and c-d separated from each other, weighed wet and dry, individually mixed to ensure homogeneity and examined for N. The results are given in Table 1.

B. Interpretation of results

The increase in total nitrogen shown is so suspicious in the case of the products prepared according to the invention that it is believed to be due to the fact that these products have a tendency to accelerate the growth of nitrogen-fixing bacteria. It is further assumed that this in turn is a consequence of the carbon-containing materials of the products manufactured according to the invention being more easily assimilated by the bacteria (fermentable) than those from, for example, columns (iii) to (v).

The extent of nitrogen leaching, which is marginally higher in the case of the product manufactured according to the invention than for the products (iii) to (v), is nevertheless dramatically lower than for the artificial fertilizer system in (ii). It should also be noted that this quantity decreases with time while it remains high in the case (il). The larger amount of available N in column (i) suggests that less N must be added as a product manufactured according to the invention than for the other products to ensure equal plant growth.

Example 2. Soil improvement properties

A. Moisture Absor<p>p. salary and retention. lon in sandy soil The fertility of sandy soils is very dependent on the extent to which they are able to hold a film of moisture and nutrients around the soil particles that are accessible to the plant roots. Since the packing material in columns (i) to (vi) contains sandy soil, the wet and dry weights of strata a-b, b-c and c-d were measured for each column before and after the routine described under A. experimental method, and these results are shown in Table 2 .The weight loss was also measured as a function of drying time at 5CPC.

The dramatically higher moisture content of the samples from column (i) and the lower gradient of the drying curve for the strata, strongly indicate that the products manufactured according to the invention have a superior ability to improve the property of the sandy soil to retain the moisture (and thus also the nutrients). thereby improving its fertility.

B. Moisture absorption and porosity in clay soils

Soils containing clay retain moisture and nutrients better than sandy soils, while the large amount of fine particles (<10 u) which are densely packed - prevents the easy transport of air, moisture and nutrients. Such soils tend to crack when drying, a factor that can expose plant roots and lead to poor growth.

In order to determine whether the products manufactured according to the invention acted as soil improvers for such soils, the procedures described under experimental method and moisture absorption and retention in sandy soil were carried out with the columns whose strata a-b comprised 4 parts porcelain clay, 3 parts sand and 1 part peat fiber. To reflect the fact that nutrients and soil conditioners must penetrate into the clay if they are to be effective, the interval between each addition of water was increased to 7 days and the upper 1/3 of strata a-b was removed before analysis. The results are shown in table 3.

As shown in Table 3, the column containing the products manufactured according to the invention exhibits superior moisture (and nutrient) penetration and retention properties that these products are capable of improving clay soils. It shows that only sample a-b from the column treated with products manufactured according to the invention formed a crumb-like structure upon drying. Together with the lower drying rate, this indicates that the sample is both more porous and retains moisture better than the others.

C. Ionebvtting properties

The soil's ion exchange properties are important since certain ions, such as soluble Al^<+>, are toxic to plants and fish, while others, such as Cd2+, are also toxic to higher animal species. Soils with a high ion exchange capacity will bind these ions and limit their leaching into the groundwater. It is known that low pH in the soil, which is for example the result of using ammonium salts or urea fertiliser, increases leaching (Lindmark, J.-E., Våxtpressen no. 1, 2/90).

Since sandy soil is particularly susceptible to such leaching, the ion exchange properties of the strata in columns (i) to (vi) were investigated by adding a quantity of cadmium sulphate and aluminum sulphate so that the content of each Cd<2+> and Al^<+> ions in the nutrients used at the start of the procedure experimental method, was at least 500 ppm of the solution [water in column (vi)]. The content of each of these two ions in the leached solution from the column was measured and the results are shown in table 4.

As shown in Table 4, the column containing products prepared according to the invention showed that the mixture in this column had a superior ion exchange capacity. This is very surprising since known theories would assume that the use of ammonium sulphate will increase leaching due to the fact that it would lower the pH of the soil [cf. column (i)] and as a solubility improver for cadmium compounds. We hypothesize that the presence of a readily available carbon source increases the growth of microorganisms that bind metal ions and buffer pH.

Example 3. Growth properties

Having shown that the product produced according to the present invention has unique soil improvement properties that are not exhibited by either synthetic fertilizers, known sludge-based products or commercial organic fertilizers, the growth-enhancing properties of these three product groups were investigated.

A. Experimental method

5 seed trays [(i) - (v)] were filled with a mixture of 6 parts sand, 1 part porcelain clay and 1 part peat fibres. On the surface of each dish, 2 g of grass seeds (festuca spp.) were sown and covered with 2-3 mm of soil. The dishes were watered and kept at 23°C, each of which was given 12 hours of artificial daylight, 12 hours of darkness and was examined for germination (about 7 days).

When the seeds germinated, the dishes (i)-(iv) were watered with 250 ml of a solution containing b$ > N, 1% P and 4£ K prepared from (i) product prepared according to the invention, (ii) a mixture of ammonium sulfate, ammonium dihydrogen phosphate and potassium sulfate, (iii) commercial organic fertilizer, (iv) a slurry of 150 g of sludge from which product (i) was prepared in 150 ml. To (v) 250 ml of distilled water was added as a control. Potassium sulfate or hydrogen phosphate or ammonium sulfate or ammonium dihydrogen phosphate was added to samples (i), (ii) and (iv) to obtain the aforementioned contents of N, P and

K.

500 ml of distilled water was sprayed evenly on each dish every 5 days over a period of 30 days and the drained liquid was collected from each dish every 5 days.

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

B. Conclusions

The results from the bowls containing the products manufactured according to the invention are remarkable and cannot be explained from the current understanding of the mode of action of fertilizers. It must therefore be concluded that the components of the product produced according to the invention work synergistically with each other in a way that has not been described in the literature so far.

The amount of growth (leaf + root) as a function of consumed nitrogen, i.e. added - (leached + in the soil + in the plant + in the roots) suggests that significantly smaller amounts of nitrogen must be added to the soil in the form of the products according to the invention than for for example synthetic fertilizers to achieve an even growth.

The relatively large amount of nitrogen remaining in the soil at the end of the growth trials where the products according to the invention were used suggests that a supply of fertilizer in the form of the products according to the invention will provide sufficient nitrogen for an entire growing season and that a possible surplus remains in the soil and is assimilated in the following year. This mechanism will eliminate the need for a so-called top dressing, i.e. adding fertilizer in the middle of the season. This shows promising savings in both labor and fertilizer costs for, for example, farmers and forest owners.

The low and diminishing leaching of nitrogen over time is an important contribution in the attempts to reduce the nitrogen level in groundwater that runs off from agricultural areas.

Emissions from, for example, sewage treatment plants will often contain nitrogen in quantities that can lead to the rapid growth of algae and other microorganisms in the recipient and pipes that carry the waste liquid. Apart from being a disadvantage due to the necessity of regular cleaning of such waterways and conduits, this growth can also be harmful from the point of view of maintaining a thin aquatic environment for example in lakes and ponds and also in sea water. This has led to such waste liquids being treated by allowing microorganisms to grow in them under controlled conditions so that nitrates and nitrites are both fixed and reduced to nitrogen, thereby reducing the amount of nitrate released into the environment.

This process requires an easily accessible carbon source so that the previously mentioned microorganisms can multiply quickly. Such carbon sources must generally be added and compounds such as methanol and molasses are often used. This results in significantly increased treatment costs.

The method according to the invention, especially when used for the treatment of, for example, paper and pulp industry sludge, gives products which act as readily available carbon sources for such denitrification processes.

Disposal of cellulose-rich sludge that has a low phosphate and metal1 content, for example from the paper and pulp industry, is often expensive and difficult. Treated according to the procedures, it will give liquid reactor products which have a rich content of C₁ sugars. The pH in these reactor products can be fully or partially adjusted with calcium hydroxide so that their nitrogen content is optimized for subsequent fermentation to, for example, ethanol or a protein-rich animal feed.

Claims (5)

1. Products containing water-soluble compounds of nitrogen and phosphorus which are fast-acting fertilizers produced from organic sludge, characterized in that the water-soluble nitrogen is in the form of urea and ammonium salts, which products are liquid or dried and respectively contain 15 - 30% by mass or more than 50 mass-^ of a water-soluble fermentable carbon source, that the mass ratio between nitrogen and carbon, the latter in the form of fermentable substances, is from 1:1 to 1:10 and preferably between 1:2 and 1:5, that the nitrogen content is between 5 and 25 mass-56, that the content of cadmium is less than 5 ppm, the content of mercury is less than 2 ppm and the content of lead is less than 20 ppm and that the content of fertilizers is slowly washed out when the product is added to the soil. 2. Process for the production of products according to claim 1, characterized in that organic sludge (1) containing at least 15% dry matter is heated (5) to at least 180°C and preferably between 210 and 230°C for at least 2 minutes, and preferably between 3 and 7 minutes, after which the insoluble organic components are hydrolysed to water-soluble compounds using a catalyst, preferably inorganic acids, undissolved solids are separated, and metal ions from groups 2b, 3a, 4a, 5a, 6b, 7b and 8 of the periodic table are removed by precipitation (16) by adding a precipitation reagent, which precipitation reagent is added stepwise so that flocculants added to the raw sludge liquid are recovered from the product and reused, which flocculants are iron (III) and/or aluminum salts recovered by precipitation at pH less than 4, 5. 3. Method according to claim 2, characterized in that the method is catalysed by inorganic acids (3) being added to the sludge, preferably sulfuric acid which is used in an amount such that the pH in the products is less than or equal to 1.5, and preferably between 0.5 and 1, 0, or by metal salts being added to the sludge, which metal salts are water-soluble compounds of iron (III) and/or aluminum in an amount of at least 100 ppm, and preferably at least 200 ppm, based on added sludge. 4. Method according to claim 3, characterized in that the precipitation reagent is ammonia, ammonium carbonate, ammonium carbarnate, potassium hydroxide or potassium carbonate or mixtures thereof, which precipitation reagent has a value as a plant nutrient, or that all or parts of the precipitation reagent is calcium hydroxide and that the ratio between nitrogen and carbon is between 1:1 and 1:10, preferably between 1:2 to 1:5 5. Application of the products according to claim 1, as a carbon source for denitrification of effluent streams from sewage treatment plants, ethanol production or growth of protein-rich microorganisms for use in animal feed.