WO2008084396A9

WO2008084396A9 - Process for biostabilization and humification of biological sludge in planted filtering beds

Info

Publication number: WO2008084396A9
Application number: PCT/IB2008/000069
Authority: WO
Inventors: Massimo Aiello; Paolo Peruzzi; Brunello Ceccanti; Grazia Masciandaro; Rocco Sturchio; Marco Arbi; Eleonora Peruzzi
Original assignee: Cnr-Consiglio Nazionale Delle Ricerche; Acque S.P.A.
Priority date: 2007-01-12
Filing date: 2008-01-14
Publication date: 2009-10-15
Also published as: ITPI20070003A1; EP2144855A2; CN101687675A; WO2008084396A8; WO2008084396A2; WO2008084396A3

Abstract

A process for stabilization of biological sludge provides the step of laying in a containing volume, or basin (1), a draining material (2). In particular, the layer of draining material (2) can be a layer of gravel having granulometry set between 40 and 70 mm and a height (h1) about 25 cm. On the first layer (2) a second draining layer (3) can be laid comprising a material, for example gravel having granulometry less than the first layer (2), set between 4 and 6 mm. The second layer (3) has preferably a height (h2) about 15 cm. Once made the draining layers (2, 3) a planting step is made in the second layer (3) of a plurality of seedlings of an aquatic macrophyte, preferably Phragmites australis (10). The base granular filtering gravel layer has also the function of supporting the root of the seedlings. To ensure a high efficiency of the process a measured density of plants is chosen, corresponding to a relative distance between two seedlings of Phragmites australis set between 40 and 60 cm. once achieved a measured vegetative growth, a first spilling in the containing volume (1) is carried out of an amount of biological sludge (12). The amount of sludge (12) fed in the containing volume (1) corresponds to a predetermined height, set between 2 and 4 cm, of the layer of sludge (12) on the draining layer (3).

Description

TITLE

PROCESS FOR BIOSTABILIZATION AND HUMIFICATION OF BIOLOGICAL SLUDGE IN PLANTED FILTERING BEDS

DESCRIPTION Field of the invention.

The present invention relates to a process for stabilization of industrial or urban sludge, in particular, by means of an "in situ" treatment.

Description of the prior art.

As well known, the disposal of biological sludge produced by purification installations is essential on management of treatment plants for industrial and urban waste water, both economically and operationally.

The sludge represents, in fact, the main waste of each purification plant and a choice on disposing of or recycling them has apparent consequences both on the balance of the operation of the plants and on their environmental impact .

According to its nature, the sludge can be processed in many ways.

In case of sludge coming from the purification of industrial waste water, it can be disposed of in special dumps. Before being disposed of, however, the sludge has to be turned into a loose dehydrated material, by mechanical dehydration through a filterpress, and mixed with chemical additives. The latter have the function of eliminating the electrostatic charge present on the particles of sludge, assisting the separation of the water from the sludge. The disposal of the sludge is a more expensive step than the process of purification, and the more the sludge is hydrated the more the disposal costs arise.

A sludge treatment plant can also have a drying system to remove up to 90% of the water contained in the sludge, so that has a lower disposal cost is achieved. Another solution provides thermodestruction of the sludge. This solution can be used only for organic sludge, which can thermodestroyed, in particular by ovens that produce thermal energy, which however is quite low for this kind of waste, unless removing the high content of water.

If instead the sludge comes from urban waste water treatment, it can be used as fertiliser. In particular, the agronomic features of some organic sludge can be exploited solving at the same time typical agricultural problems, enriching the content of organic substance of the soil.

However, the sludge coming from the urban waste water treatment can contain an amount of heavy metals, more or less high, which can affect the chain food.

Other organic sludge recycling systems provide embedding the sludge in a material for the building industry, mixing them in clays, concrete and other material.

This solution is expensive for transportation costs of high volumes of material and is in any case limited to particular types of biological sludge.

Furthermore, solutions have been proposed that provide the stabilization of the sludge by particular plant species, like those used for phytopurification of waste water. However, such processes have low efficiency mainly owing to problems of clogging draining layers that causes excessive liquid accumulation on the draining bottom and consequent onset of extreme anaerobiosis.

Summary of the invention It is then a feature of the present invention to provide a process for stabilization of biological sludge that allows to obtain a high reduction of the volume and to obtain a stabilized, hygienic and rich of humus material, to prepare a compost for agricultural use. It is another feature of the present invention to provide a process for stabilization of biological sludge that has a very low environmental impact.

It is still a feature of the invention to provide a process for stabilization of biological sludge that is cheap and whose production is simple.

These and other features are accomplished with one exemplary phytostabilization process of biological sludge in a containing volume, or basin, whose characteristic is to provide the following steps:

- arranging within said containing volume a bottom layer of a draining material having a measured granulometry, said draining material being appropriate for rooting of at least one vegetable species; - planting a plurality of aquatic macrophyte seedlings in said bottom layer; whose main feature is of providing, furthermore, the following steps:

- spilling on said draining layer a measured amount of biological sludge to treat, surrounding said seedlings and waiting a predetermined time;

- calibrating the process by means of analysis of at least one sample of sludge, said analysis comprising the measurement of at least one among the following parameters:

- pH;

- electric conductivity;

- humidity;

- hydrosoluble carbon; - ammoniacal nitrogen;

- fulvic acids;

- humic acids;

- computing a sludge limit height on the basis of said analysis;

- spilling further biological sludge up to reaching said limit height;

- iteration of the spilling step by spilling further biological sludge when a previously spilled layer of sludge has reached a determined dehydration rate;

- removing the sludge from said volume once a measured height of a stabilized sludge has been reached. In particular, the moment of removing the sludge is chosen on the basis of the course of some parameters, such as the content of hydrosoluble carbon, the content of fulvic acid as well as the height of the side wall of the containing volume. More in detail, the sludge can be removed when the content of hydrosoluble carbon is less than 70% with respect to an initial value, advantageously when it is less than 80% with respect to an initial value, preferably when it is less than 90% with respect to an initial value.

As above described, in addition, or alternatively, to the content of hydrosoluble carbon, the moment of removing the sludge can be decided on the basis of the content of fulvic acids. In particular, the sludge is removed when the content of fulvic acid is less than 50%, advantageously less than 60%, preferably less than 70% with respect to an initial value.

The predetermined time of waiting after spilling the measured amount of biological sludge to treat on the draining layer depends on the rate of dehydration of the sludge and can be set between 1 and 30 days, advantageously between 2 and 10 days.

Preferably, the step of computing the sludge limit height is done on the basis of a detected rate of humidity, i.e. of the content of water sludge, and in particular:

- for a content of water less than 60% the limit height is equal to, or larger than 20 cm; - for a content of water set between 60 and 70% the limit height is set between 10 and 20 cm;

- for a content of water set between 70% and 80% the limit height is set between 5 and 10 cm;

- for a content of water set between 80% and 90% the limit height is equal to, or lower than, 5 cm.

Also the rate of dehydration is determined on the basis of the rate of humidity of the sludge, i.e. of its content of water.

In particular, the height of the spilled sludge must never affect the vitality of the rhizomes and the vegetative capacity of the plants.

Advantageously, before carrying out the removal of the stabilized sludge from the containing volume, can be provided a rest period can be provided for the sludge bed during which further sludge spills are not carried out. The duration of the a rest period of a sludge bed depends in general on the chemical -physical and agronomic features of the sludge.

Advantageously, said draining layer is split into a first and a second draining layer, said second layer comprising a material having granulometry less than said first layer.

In particular, the dehydration of the sludge made by the aquatic macrophyte is monitored measuring periodically the height of the layer of sludge.

Advantageously, the calibration provides a first spilling step of a measured amount of biological sludge in the containing volume and the analysis of samples, drawn periodically.

In particular, a bottom surface on which the first layer is laid can be at an angle in order to assist the outflow of a percolate, produced by the draining action of the bed, towards an outlet duct.

Advantageously, the granulometry of the first layer is set between 30 and 80 mm, preferably between 40 and 70 mm. The second draining layer has advantageously a granulometry set between 2 and 10 mm, preferably between 4 and 6 nun.

In particular, before a first spilling step an irrigation step is provided of said seedlings comprising a first irrigation with fresh water and a following irrigation with water taken from the exit of a waste water purification plant. This allows a quick starting growth and rooting due to the presence of enough amount of nitrogen, phosphorus and microelements in the purified waste water and reduces the stress from transplantation.

Advantageously, a first sludge spilling step is stopped once achieved a layer of sludge having height set between 1 and 6 cm, preferably between 2 and 4 cm. The first spilling step represents a critical step of the process and is used for evaluating the stress of adaptation of the aquatic macrophyte to the sludge and for evaluating the amount of nutrients and of possible potentially toxic substances that are present in it.

In fact, periodical sludge sampling is provided measuring pH, electric conductivity, humidity, hydrosoluble carbon, ammoniacal nitrogen, fulvic acids, humic acids and heavy metals. In particular, the sampling is preferably carried out each 3 months for pH, electric conductivity, humidity, hydrosoluble carbon, ammoniacal nitrogen and each six months for fulvic acids, humic acids and heavy metals. On the basis of the results of the analysis, the feasibility of the process is evaluated and the optimal height is set of a next sludge spill.

In particular, successive sludge spills are stopped to reach layers of sludge having height set between 5 and 20 cm. More in detail, the layers of sludge must not have height too high to avoid an excessive accumulation of liquid on the draining bottom and an onset of extreme anaerobiosis conditions in case of forced operation. Preferably, the vegetable species used belongs to Phτagmxtes australis.

In particular, Phragmites australis is an aquatic plant capable of resisting to a heavy environment like those made by the presence of the sludge, where the roots are mainly exposed to anaerobic conditions. Furthermore, Phragmites australis is an emerging roots aquatic plant and as it has a high capacity of conveying the oxygen from the aerial parts to parts dipped in the sludge.

In particular, said seedlings are planted in said draining layer at a relative distance set between 30 and 70 cm.

Preferably, said seedlings are planted in said draining layer at a relative distance set between 40 and 60 cm.

Advantageously, to achieve a measured height of the layer of dehydrated sludge the seedlings are cut and chopped mixing them with the sludge to obtain a high quality compost.

Normally, the containing volume may comprise sludge draining beds already present in the plant or they can be made ex-novo .

In particular, the containing volume can be made near a waste water treatment plant from which the biological sludge comes. In this case, the percolate at the outlet of the sludge draining beds can be recirculated again into the waste water treatment plant.

In particular, the containing volume can be also made in order to contain the waste water treatment plant. The vegetative growth of the aquatic macrophytes allows this way to reduce the visual impact of the waste water treatment plant .

Brief description of the drawings.

The invention will be made clearer with the following description of an exemplary embodiment thereof, exemplifying but not limitative,, with reference to the attached drawings wherein:

Figures 1 and 2 show diagrammatically a possible exemplary embodiment of a plant that carries out the process for stabilization of biological sludge, according to the invention;

Figure 3 shows the course of the hydrosoluble carbon (WSC) , of the activity of the Dehydrogenase enzyme (DH-asi) and of the metabolic potential (DH- asi/WSC) in samples of sludge drawn regularly;

Figure 4 shows the course of the ammonia and of the nitrates in samples of sludge drawn regularly;

Figure 5 shows the capacity of Phragmites au$tral±s to remove a part of the heavy metals of the sludge and to distribute it into its vegetal tissues, i.e. root- trunk-leaves ;

Figure 6 shows the results of a germination test carried out at the beginning and at the end of the process according to the invention; - Figures 7 and 8 show diagrammatically a possible layout for a plant of stabilization of biological sludge, according to the invention.

Description of preferred exemplary embodiments . With reference to figure 1, a process for stabilization of biological sludge, according to the invention, provides the step of laying in a containing volume, or basin 1, a draining material 2. In particular, the layer of draining material 2 can be a layer of gravel having granulometry set between 40 and 70 mm and a height h₁ that is about 25 cm.

On first layer 2 a second draining layer 3 can be laid comprising a material, for example gravel, having granulometry less than said first layer 2, set between 4 and 6 mm. The second layer 3 has preferably a height h.₂ that is about 15 cm.

Once laid the draining layers 2 and 3, in second layer

3 a plurality of seedlings is planted of an aquatic macrophyte 10, preferably Phragmites australiε. The granular filtering base layer of gravel has then also the function of housing the plants and allowing rooting.

To ensure a high efficiency of the process a measured density of plants is chosen, corresponding to a relative distance between two seedlings of Phragmites aυstralis set between 40 and 60 cm.

The seedlings of Phragmites are then irrigated in a first step with water, for example fresh water and/or water exiting from the plant of treatment. The latter contains nutritive substances capable of assuring the vegetative development of the plants. Furthermore, the use of treated waste water for watering the seedlings 10 of Phragmites reduces the stress from trasplantation.

Once achieved a measured vegetative growth, a first spilling step is carried out in the containing volume 1 of an amount of biological sludge 12, for example through a duct 15 (figure 2) . The amount of sludge 12 fed into the containing volume 1 corresponds to a predetermined height, set between 2 and 4 cm, of the layer of sludge 12 on the draining layer 3.

In particular, the rooted seedlings 10 of Phragmites australis participate actively to the dehydration of the sludge through a process for evapo-perspiration and set the favourable conditions to a development of a plurality of processes that have as a result a progressive mineralization of the organic fraction owing to bacteria adhering to the thick root structure, i.e. the rhizosphere, and the hygienization of the organic material. There is, in particular, a creation of an anaerobic zone that is distant from the root and an aerobic zone near it where the atmospherical oxygen transported by the hollow stem of the vegetable species spreads. The microbial biomass present in the vicinity of the root activates a process of mineralization of the organic nitrogen in the form of proteins, aminoacids, etc. that turns into ammoniacal nitrogen, i.e. ammonium. This form of nitrogen is oxidized into nitrate by other bacterial populations "nitrobacter" that consume the oxygen transported by the aquatic plant in the phyto- treatment layer. The plant assimilates part of these nutrients, along with other elements, such as micronutrients, heavy metals, phosphorus, calcium, magnesium, potassium, etc., for its metabolism and growth, "purifying" the mass of sludge, and also turns the sludge into a material for use in the agricultural field.

In addition to the mineralization process, in the phyto-treatment mass also a process of humification is activated. The humification process of the organic substance is followed by the evolution of the humic fraction, which represents the steadier component of the organic substance, since it consists of polyfenolic and polycarboxylic aromatic polymers that are degradable in a difficult way. This fraction gives a added value to the sludge, because it produces a quantity of effects on the agronomic soils and on the crops: it keeps humidity, holds the microbial populations involved in the cycle of the nutritive elements, releases metal-organic chelates that stimulate the physiology of the plants and block residues of pesticides inglobating them in the polymer network of the humic structures .

In an exemplary embodiment of the invention, containing volume 1 can be built near an active sludge treatment plant from which biological sludge 12 derives.

In the case shown in figures 7 and 8, the containing volume 1 encircles the waste water treatment plant. In particular, the sludge 12 coming from the sedimentation tanks 60 is conveyed by means of ducts 61 to a storage reservoir 50. From here, the biological sludge is fed in a controlled way by means of ducts 55 to the containing volume 1 arranged as above described. This reduces the visual impact of the active sludge treatment plant and avoids a transportation on trucks of biological sludge to be treated towards treatment sites, such as dumping or drying installations.

The process above described is advantageous both concerning economical and environmental reasons, since it allows keeping the cycles on the sludge directly at the purification plant, without the need of removing with tankers the semiliquid sludge and transporting it far from the plant for a mechanical dehydration.

Hereafter, an example is given that is to be intended as not limitating the extent and the features of the present invention.

Example 1 In a purification plant, on which measurements have been carried out, an amount of sludge was treated produced by 3.000 equivalent inhabitants (e.i.) and producing about 1.200 mVyear (2% solid) of biological sludge. For the phytostabilization process, six sludge drying beds have been used sized 10 x 2,5 x 0,7 m for a volume of 17,5 m³. Of these beds, only 4 have been used for the experiments, whereas the other two have been left free as backup for possible emergency of the plant. The bottom layer of the phyto-drying bed consists of a draining material, 25 cm of gravel having diameter 40-70 mm and 15 cm of gravel 0.5 cm, where an apertured duct is arranged for collecting percolated liquid-

In May 2004, in an intermediate layer between larger gravel and smaller gravel, seedlings of Phragmites australis have been located in a number of about 140 for each basin. To ensure the development and the growth of the plants, at first fresh water and then water rich in nutritive ^' elements coming from purification has been supplied. After about 4 months the seedlings had grown enough to allow a first sludge spill.

The sludge has been added as layers in the basins, for an amount of about 5 m³ every 15 days, i.e. about 40 m³ of sludge/month for 4 beds. Every week the height of the sludge in the sludge bed was measured for determining the reduction of volume, whereas monthly measurements of pH, electric conductivity and humidity were effected. An analysis concerning the processes of mineralization and stabilization of the organic substance of the sludge was carried out on samples drawn each 3 months, December, May,

August and October corresponding to the different seasonal periods. For carrying out this sampling, the sludge spill has been discontinued for about three weeks, and for each basin 6 sub-samples of sludge have been drawn in different points and near the gravel layer. The sub-samples have been homogenized and preserved at 4⁰C for biological analysis, and instead dried in the air for chemical analysis. After about one year and a half from planting the step has been carried out of cutting the plants.

Example 2

12 drying tanks have been prepared with an overall volume of 864 .m³. In the process about 4.500 m³ sludge of have been used yearly with respect to the total produced sludge of 6652 m³ (30000 e.i.).

The step of planting seedlings in the beds has been carried out at the end of July 2005 in an area of 0,5 x 0,5 m for each plant. After a period of about one month for adaptation and growth of the seedlings, to which water has been supplied exiting from the plant to decrease the stress from transplantation and to assist a better radication, in the month of September 2005 the first layer of sludge has been spilled and at the beginning of October 2005 the first sampling has been carried out and the plant is still operating. In the first year of operation the volume of sludge has been reduced to 99%, with a rate of 2.42 cubic metres for each square metre.

Example 3 The same as in example 2, with the difference that five drying tanks have been prepared with an overall volume of 225 m³. In the process about 1462 κι³ of sludge have been used yearly with respect to the total sludge produced of 5616 m³ (10000 e.i.). In the first year of operation the volume of spilled sludge has been reduced to 98%, with a rate of 3.16 cubic metres for each square metre.

Example 4 Like in example 2, but a drying basin has been provided having an overall volume of 64 m³ for the phytostabilization process. In the process yearly about

250 m³ of sludge have been used with respect to the total product sludge of 454 m³.

The planting step of the bed has been carried out at the end of September 2005 (plant spacing 0,5 x 0,5 m) , whereas the sludge spill started in March 2006.

Example 5 Eight drying tanks for the phytostabilization process have been provided, with an overall volume of 256 m³. In the process about 998 m³ of sludge have been used yearly with respect to the total product of 635 m³.

Example 6 Five drying tanks have been provided for the phytostabilization process, with an overall volume of 130 m³. In the process about 877 m³ of sludge are planned to be used with respect to the total product of 1098 m³.

The following are the experimental results of some tests carried out on the samples of sludge spilled regularly from the containing volume. Results experimental

The chemical results obtained during the experimentation in the case described in example 1 are given in table 1.

In particular, it is known that electric conductivity (E. C.) increases with time probably owing to the progressive accumulation of the sludge on the bottom of the basins, whereas the pH tends to decrease.

The carbon and the total organic nitrogen decrease significantly (p<0.0l), showing an efficiency of the processes of mineralization of the organic substance owing to the microorganisms of the sludge and of the radical system, i.e. of the rhizosphere of the plants. In fact, the hydrosoluble carbon (WSC) , which represents a product of the mineralization of the carbon but also a substrate that can be used by the microbial activity, shows a high increase at the beginning of the experimentation followed by a following decrease, by processes of degradation of the labile organic substance (figure 3) .

Dehydrogenase (DH-asi) , commonly used as indirect evidence of the total microbial activity, being an intracellular enzyme (Masciandaro et al. 2000), is positively correlated with the hydrosoluble carbon, evidencing the crucial action of the microorganisms in the supporting the metabolic processes.

In fact, the potential metabolic index calculated on the ratio between dehydrogenase and hydrosoluble carbon

(DH-asi/WSC) defines the course of the mineralization process that involves the labile carbon forms. This index decreases with time indicating the progressive degradation of the substrates available owing to the microbial activity.

In figure 4 the course is shown of the potential metabolic index, corresponding to an increase of the ratio between nitrate (NO₃ ^") and ammonia (NH₃) . The increase of this index indicates that the mineralization of the organic substance proceeds in aerobic conditions ensured by the oxygen supplied by the plant. The high concentration of ammonia achieved at the end of the experimentation do has not compromised the development of the Phragmites australis that can tolerate high concentration thereof iuaintainng a high production of dry biomass with respect to each other plant species (Hill et al., 1997).

Contrarily to carbon and to nitrogen, total phosphorus tends to increase with time, probably owing to the immobilization processes of the microorganisms that use phosphate as source of phosphorus (tab. 1) . The humification process of the organic substance has been followed by monitoring the course of the humic and fulvic fractions of the carbon that make up the humic substance (figure 5) . In particular, the humic carbon represents the steadier component of the humic substance, since it consists of polyfenolic and polycarboxylic compounds that can be degraded in a difficult way; instead, the fulvic carbon, being made up mainly of aliphatic components, is considered a less stable humic substance. In fact, a positive correlation has been found between the fulvic carbon and the hydrosoluble carbon. A decrease of the fulvic carbon corresponds exactly to an increase of the humic carbon with time (p<0,01), since the humification of the organic substance starts from a more labile component of the humic substance, i.e. the fulvic component. The decrease of the ratio of humic carbon that is observed at the end of the experimentation is due to the achievement of a balance that corresponds to the stabilization of the sludge. The results obtained agree with the studies published by other authors.

Notwithstanding the continuous sludge spill and the pile of stabilized sludge, the content of heavy metals remains mainly fixed, or tends to decrease during the experiment, except from copper, zinc and of chromium that increase with time. This course is an evidence that the plats have an essential role in the absorption of metals, ensuring the maintainance of a low amount of heavy metals in the sludge. Tn fact, their concentration are below some limits of law for use of the sludge in agriculture.

At the end of the phytostabilization process the effect has been evaluated of the stabilized sludge on Lepidium sativum, a vegetable species that is used for testing the use of the sludge for agricultural purpose. Figure 6 shows an increase of the index of germination at the end of the experimentation confirming the good results of the phytostabili2ation process on a reduction and/or elimination of possible phytotoxic substances.

The phytostabilization process ensured the dehydration and the maturation of the sludge delivered in the tanks owing to evapo-traspiration action of the plants, to hydraulic conductivity of the sludge bed and to a metabolic action of the microorganisms of the sludge and of the rhizosphere.

During the period of the experimentation the purification system has produced about 1.000 m³ of sludge of which about 600 m³ have been disposed of in a dump. The 400 m³ have been put in the phytostabilization sludge beds of that have achieved a final volume of 50 m³, i.e. a reduction of volume of 80%.

Such reduction of volume has granted savings to the purification plant (transportation and disposal of the sludge) of about 30-35%.

In table 4 are given some chemical results obtained during the experimentation in the case described in example 2. In particular, the following data have been obtained: content of total nitrogen (TN) , content of total organic carbon (TOC) , content of total phosphorus (TP) , hydrosoluble carbon (WSC) , dehydrogenase (DHase) , amount of fulvic acid and humic acid.

Always relatively to example 2, in table 5 the following data have been obtained relative to the content of some heavy metals in the sludge.

The chemical results obtained during the experimentation in the case described in example 3 are given in table 6 below:

In table 7 the following data have been obtained relative to the content of some heavy metals in the sludge.

By analyzing the data given in the tables above in the tables relatively to example 2, tables 4 and 5, and in example 3, tables 6 and 7, a significant decrease of the content of organic substance (TOC and TN) has been observed both in total form and in the more labile forms, determined as ammoniac nitrogen and hydrosoluble carbon (WSC) . This shows the efficiency of the mineralization process owing to the microorganisms of the sludge and of the radical system, i.e. of the rhizosphere of the plants.

In fact, the hydrosoluble carbon represents the product of the degradation of the substrates organic complex, but at the same time is the substrate more quickly available for metabolic activity. The dehydrogenase (DHase) , commonly used as direct evidence of the total microbial activity, being an intracellular enzyme, is correlated positively with the hydrosoluble carbon confirming the crucial action of the microorganisms in supporting the metabolic processes.

Furthermore, a high decrease is apparent of the dehydrogenase enzymatic activity contemporaneous to the decrease of the hydrosoluble carbon. The presence of a high content of labile organic substance as available substrate in the drying tanks, stimulates the synthesis of enzymes: when the substrate decreases, also the enzymatic activity decreases.

Contrarily to carbon and to nitrogen, phosphorus tends to increase with time (TP) mainly owing to the processes of immobilization of the phosphates in the microbial tissues. A certain amount of phosphorus is in any case desirable in the production of fertilizers for applications in the agriculture. Concerning the data relative to fulvic acid and to humic acid, both for example 2 (table 4) and for example 3

(table 6) , a decrease is observed of the ratio of humic carbon, i.e. of the sum of the fulvic acid and of the humic acid. This decrease indicates the achievement of a balance that corresponds to the stabilization of the sludge.

Concerning the analysis of the content of heavy metals in the sludge (tables 5 and 7) it is observed that it remains mainly fixed, notwithstanding the continuous sludge spill and the increasing pile of stabilized sludge. This course indicates the essential task of the plants in the absorption of metals. This ensures a maintenance of an amount of heavy metals in the sludge below certain limits of law concerning the use of the sludge of purification in agriculture.

The foregoing description of a specific embodiment will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such an embodiment without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiment. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

1. Phytostabilization process of biological sludge comprising the following steps:

- providing a bottom layer of a draining material having a measured granulometry in a containing volume, or basin,, said draining material being adapted to allow rooting of at least one vegetable species;

- planting a plurality of aquatic macrophyte seedlings in said bottom layer; characterised in that of providing, furthermore, the steps of: spilling a measured amount of biological sludge to treat on said draining layer surrounding said seedlings and waiting a predetermined time; - calibrating the process by means of analysis of at least one sample of sludge, said analysis comprising the measurement of at least one among the following parameters:

- pH; - electric conductivity;

- humidity;

- hydrosoluble carbon; ammoniacal nitrogen; fulvic acids; - humic acids; computing a sludge limit height on the basis of said analysis;

- spilling further biological sludge up to reaching said limit height; - iteration of the spilling step by spilling further biological sludge when a previously spilled layer of sludge has reached a determined dehydration rate; removing the sludge from said volume once a measured height of a stabilized sludge has been reached.

2. Phytostabilization process of biological sludge, according to claim 1, wherein a second layer of draining material is laid above said first layer, said second layer comprising a material having granulometry less than said first layer.

3. Phytostabilization process of biological sludge, according to claim 1, wherein said layer has a bottom at an angle such that it assists an outflow towards an outlet duct of percolate product drained from said first and second layer.

4. Phytostabilization process of biological sludge, according to claim 2, wherein the granulometry of said first layer is set between 30 and 80 mm, preferably between 40 and 70 mm.

5. Phytostabilization process of biological sludge, according to claim 2, wherein the granulometry of said second draining layer is set between 2 and 10 mm, preferably between 4 and 6 mm.

6. Phytostabilization process of biological sludge, according to claim 1, wherein at the end of said planting step an irrigation step with water is provided of said plurality of seedlings up to reaching a determined vegetative development.

7. Phytostabilization process of biological sludge, according to claim I₇ wherein said first sludge spilling step during said calibration step is stopped once achieved a layer of sludge having height set between 1 and 6 era, preferably between 2 and 4 cm.

8. Phytostabilization process of biological sludge, according to claim 1, wherein said sludge spilling step is stopped once achieved layers of sludge having height set between 5 and 20 cm.

9. Phytostabilization process of biological sludge, according to claim 1, wherein said aquatic macrophyte is Phragmites australis.

10. Phytostabilization process of biological sludge, according to claim 1, wherein said planting step is carried out by arranging said plurality of seedlings at a relative distance set between 30 and 70 cm, preferably between 40 and 60 cm.

11. Phytostabilization process of biological sludge, according to claim 1, where in order to achieve a measured height of the layer of dehydrated sludge said seedlings are cut and chopped and eventually mixed with the sludge to obtain a compost,

12. Phytostabilization process of biological sludge, according to claim 1, wherein said containing volume is made near a waste water treatment plant from which said a percolate is recirculated again, into said plant with said waste water.

13. Phytostabilization process of biological sludge, according to claim 12, wherein said containing volume is already present in the plants of purification or can be made ex novo also in order to contain said waste water treatment plant.

14. Phytostabilization process of biological sludge, according to claim 1, wherein said computing step of said sludge limit height is done on the basis of a detected rate of humidity, i.e.: for a content of water less than 60% the limit height is equal to, or larger than 20 cm; for a content of water set between 60% and 70% the limit height is set between 10 and 20 cm,- for a content of water set between 70% and 80% the limit height is set between 5 and 10;

- for a content of water set between 80 and 90% the limit height is equal to, or less than 5 cm.

15. Phytostabilization process of biological sludge, according to claim 1, wherein said removal of said sludge is done at at least one of the following conditions:

- the content of hydrosoluble carbon is less than 70% with respect to an initial value, advantageously when it is less than 80% with respect to an initial value, preferably when it is less than 90% with respect to an initial value;

- the content of fulvic acid is less than 50%, advantageously less than 60%, preferably less than 70% with respect to an initial value.