ITPC20120008A1 - METHOD AND PLANT FOR THE CHEMICAL-PHYSICAL TREATMENT OF BIOLOGICAL PURIFICATION SLUDGE - Google Patents

Description

METHOD AND PLANT FOR THE CHEMICAL-PHYSICAL TREATMENT OF SLUDGE

BIOLOGICAL PURIFICATION

DESCRIPTION

The present invention relates to a method and plant for the chemical-physical treatment of biological purification sludge. More specifically, the present invention relates to a method and plant for the chemical-physical treatment of biological purification sludge aimed at increasing dehydration, reducing the concentration of heavy metals, polluting organic compounds and eliminating organisms pathogens.

As is known, biological purification sludge can represent a valid contribution to soil amendment and plant nutrition. In fact, in these materials, in addition to the organic substance, considerable quantities of nitrogen, phosphorus, potassium, calcium and sulfur are contained.

However, in the purification sludge, pollutants of various kinds and undesirable substances deriving from the treatment of water and / or wastewater and / or liquid waste of industrial origin can also be contained and, when they are contained in high concentrations, they can inhibit subsequent use. agronomic analysis of the sludge itself or of the products for agricultural use derived from them. Briefly, said pollutants are represented by:

â € ¢ heavy metals;

â € ¢ synthetic organic compounds;

â € ¢ pathogenic bacteria.

Various processes are known for the chemical and / or physical treatment of biological purification sludge aimed at eliminating one or more of the pollutants mentioned above, which, however, are not able to provide totally satisfactory results overall, even in view of a possible reuse of the sludge. treated and purified.

It would therefore be desirable to have a process capable of solving the problem from the purification of the biological sludge in view of a further use of the purified sludge. The aim of the present invention is therefore to provide a method for the chemical-physical treatment of biological sludge from waste water, suitable for reducing or eliminating the presence of said contaminants.

Within this aim, one of the purposes of the present invention is therefore to provide a method for the chemical-physical treatment of biological wastewater sludge suitable for increasing the dry matter.

A further object of the present invention is to provide a method for the chemical-physical treatment of biological sludge from waste water which favors the recovery of useful substances through the reuse of sludge, directly in agriculture or as components / precursors of fertilizing products.

Another object of the present invention is to provide a plant for the chemical-physical treatment of biological sludge from waste water which is suitable for practicing said method.

Not least object of the present invention is to provide a method and a plant for the chemical-physical treatment of biological wastewater sludge which offer guarantees of safety, reliability, ease of maintenance and which allow to be operated at competitive costs.

The aforementioned purposes are achieved by means of a method for the chemical-physical treatment of biological sewage sludge which includes:

- a first step of fluidification of said sludge until a content of dry matter of between 1% and 20% is obtained;

- a second step of abatement of synthetic organic compounds contained in said sludge and partial mineralization of the organic substance;

- a third phase of acidification of the product obtained in said second phase;

- a fourth step of filtering the product obtained in said second step;

- a fifth phase in which the liquid separated from the mud (â € œleachateâ €) in the previous phase is recovered by introducing it into a special tank and reused for the part necessary for the dilution of other mud in the first initial phase;

- a sixth phase in which the liquid separated from the sludge (â € œleachateâ €), after being used for a few dilution cycles, is demetallized by electrochemical and / or chemical means;

- a seventh phase in which the demetallized â € œleachateâ € obtained in the previous phase is treated with a common chemical-physical process for the purification of waste water in order to be discharged into the public sewer system or surface water body.

It has been seen that by operating with the method according to the present invention it is possible to reduce or eliminate the presence of contaminants from the biological sludge of waste water, increasing the dry matter and at the same time favoring the recovery of useful substances through the reuse of sludge, directly in agriculture or as components / precursors of fertilizing products.

Preferably, in said first phase, the sludge as commonly produced by the dewatering plants of the purifiers is diluted until it reaches a dry matter content of between 5% and 10%.

In said second phase, the fluidized sludge is preferably pumped into a chemical reactor where a Fenton reaction is carried out for the abatement of the synthetic organic compounds and the partial mineralization of the organic substance as described in the application for patent PC2011A000014, entitled â € œMethod for the preparation of an agricultural corrective precursor and related productâ €, the description of which is incorporated herein by reference.

In practice, 50% sulfuric acid is added to the mud kept under stirring until a pH of between 3.4 and 4.2 is reached. Then a solution of ferrous sulphate and finally of hydrogen peroxide is introduced. The reaction is normally completed in about 60 minutes.

Preferably, said third phase is carried out in a second chemical reactor into which the sludge is discharged after being subjected to the Fenton reaction. In this way the reactor of the previous phase is empty and can receive more sludge for a new treatment. In said second reactor then added 50% more sulfuric acid up to having pH 2. This pH is controlled and maintained for a defined time, with progressive inputs of acid. At these pH conditions the heavy metals contained in the sludge are found in solution as ions and can be removed by separating the liquid phase which is carried out in the subsequent phase.

In the fourth phase, preferably the sludge is sent to a filter press and filtered at a pressure of between 10 and 15 bar. The dewatered sludge is transferred to storage by a conveyor belt. Through the treatments undergone, the concentrations of heavy metals and organic pollutants were reduced in the solid fraction. It has also been sanitized, as the pathogenic bacteria have been destroyed. An increase in dry matter is also obtained with respect to that of the sludge before treatment up to levels between 50 and 60%. This material constitutes the precursor of agricultural corrective agents referred to in the aforementioned patent application PC2011A000014.

Preferably, in the fifth phase, the liquid separated from the mud (â € œleachateâ €) is recovered by introducing it into a special tank and reused for the part necessary for the dilution of other mud in the first initial phase.

After a few treatment cycles, the concentration of heavy metals in the â € œleachateâ € is such that it must be reduced, preferably through the following phase.

The â € œleachateâ €, after being used for a few treatment cycles, is pumped from the tank into an electrolytic cell. Being an excellent conductor, the application of direct current raises the pH of the liquid. A part of the metals present is precipitated as hydroxides, another part is deposited at the cathode in metallic form. The metal removal process can be made more efficient by adding sodium sulphide to the â € œleachateâ €, at the end of the electrochemical treatment, which increases its precipitation. This precipitated chemical sludge is removed from the electrolytic cell and sent to a bag filter. The liquid that separates from the filter bags and the supernatant present in the cell are pumped into a tank to undergo the seventh stage.

Preferably, in the seventh phase, the electrochemical and / or chemical demetallized â € œleachateâ €, which has a pH close to neutrality and very low concentrations of heavy metals, is treated with a common chemical-physical process for the purification of waste water, not yet suitable for discharge into public sewers or surface water bodies. In practice, keeping the â € œleachateâ € stirred, the pH is brought to complete neutrality with sodium hydroxide or milk of lime, then iron chloride is introduced as a coagulant and a polyelectrolyte to precipitate the chemical sludge. The sludge is pumped into the same bag filter mentioned in the sixth phase, the supernatant is discharged after passing through a sand filter. The efficiency of the purification will determine the choice of discharging into the sewer or into a stream.

In a further aspect, the present invention relates to a plant for the implementation of a method for the chemical-physical treatment of biological purification sludge as previously described, characterized in that it comprises:

- a mixer for diluting said sludges;

- a first reactor for said second step of abatement of synthetic organic compounds contained in said sludges;

- a second reactor for said third phase of acidification of the product obtained in said second phase;

- a filter press for said fourth filtering phase of the product obtained in said third phase - a third tank for collecting said liquid separated from the sludge (â € œleachateâ €);

- an electrochemical cell for said sixth phase of demetallization of said liquid separated from the sludge (â € œleachateâ €);

- a fourth tank for said seventh purification phase;

- a sand filter for the filtration of sludge obtained from â € œleachateâ €.

Further characteristics and advantages of the present invention will become evident from the description of the preferred embodiment, illustrated by way of non-limiting example in the attached figures, in which:

Figure 1 is a perspective view of an embodiment of a plant for the chemical-physical treatment of biological purification sludge according to the present invention; - Figure 2 is a plan view of the plant of Figure 1;

- Figure 3 is a schematic representation of an embodiment of the method for the physico-chemical treatment of biological sewage sludge according to the present invention.

With reference to the attached figures, a detailed description of a possible embodiment of the method and plant 100 for the chemical-physical treatment of biological purification sludge according to the present invention now follows.

The treatment times and methods described below and relating to the various phases depend on the characteristics of the mud to be treated. It will therefore be essential to analyze the sludge, determining the percentage of dry matter, organic carbon, the content of heavy metals, synthetic organic substances and the microbiological load, so as to be able to calibrate the treatment and the quantities of reactants, in order to maximize its efficiency. PHASE 1

Each phase of the treatment is designed to work with a fluid material. The biological purification sludge, as it is produced by the purifiers, at the exit of the dehydration lines, has an average dry content of between 15 and 30%. They are shovelable and therefore lend themselves to being handled with wheel loaders or clamshell buckets, they can be pumped with great difficulty and high energy consumption. In this physical state they do not lend themselves to easy mixing with chemical reagents. It is therefore necessary to thin them to subject them to the treatment.

At the beginning of the cycle, a defined dose of sludge is placed inside a hopper 1 equipped with a bottom screw that mixes it, while a pipeline adds water. A quantity of water is introduced until a dryness of between 6 and 8% is obtained. With this density, the sludge can be pumped with single screw or lobe pumps 10 and can subsequently be kept in mixing with propellers in order to obtain a perfect interaction with the liquid chemical reactants that will be introduced.

The initial volume of the sludge, for dilution with water, therefore increases from 3 to 4 times, according to the percentage of starting dry substance.

The mixture is then introduced into a tank through the bottom auger of the hopper, opening a bayonet gate.

STEP 2

The load is complete when the volume of fluid sludge introduced is approximately half the geometric capacity of the reactor, consisting of a cylindrical fiberglass tank 2, placed horizontally.

This precaution is necessary in order to guarantee a sufficient air draft in the reactor to contain the foams that are generated in this phase. The introduced mass is kept in slow agitation by means of a propeller, moved by an external motor.

Inside the reactor, in the upper part free from the liquid, there are nozzles that spray water, in small drops for the abatement of the foams, which are formed during the oxidative reaction.

The oxidative reaction of the sludge with hydrogen peroxide and iron ions takes place inside the reactor, in accordance with the aforementioned patent application PC2011A000014 entitled â € œMethod for the preparation of a precursor of agricultural corrective agents and related productâ €.

In fact, in the purification sludge, in addition to chemical stabilization, ie the denaturation / hydrolysis of proteins and the percentage reduction of organic carbon, subject of the aforementioned patent application, other useful reactions occur jointly.

It is known that the use of H2O2 (in this case contained in the tank 14) and iron ion (in this case contained in the tank 13) in particular in waste water and liquid waste, known by the name of process â € œFentonâ €, is sometimes applied with advantage for the elimination or reduction of various organic and inorganic pollutants such as: aromatic solvents, formic, tartaric, lactic, propionic acids, amines, ketones, phenols, pesticides, cyanides, hypochlorites, etc.

It is applied with good results in acid solutions (pH included in the range 3-5), and is also controlled by the Fe2 +: H2O2 ratio and by the Fe2 + content.

The chemical-physical treatment phases find wide application in those cases in which the wastewater to be treated is characterized by a content of polluting substances mainly of an inorganic, or organic, difficult or non-biodegradable type.

Many metals have special properties of transferring oxygen, improving the effectiveness of hydrogen peroxide.

Among these, the most common is iron, which, when used in a certain way, promotes the formation of highly reactive hydroxyl radicals (OH).

The Fenton process is used for the treatment of a variety of industrial waste, containing the most varied types of toxic compounds (phenol, formaldehyde, pesticides, plastic additives). Through laboratory determinations it has been ascertained that the process can be applied to sludge with the following additional effects:

â € ¢ Denaturation / hydrolysis of proteins

â € ¢ Percentage reduction of organic carbon content

â € ¢ Breakdown of polluting organic molecules;

â € ¢ Reduction of toxicity;

â € ¢ Improved filterability;

â € ¢ Reduction of BOD / COD;

To optimize the results of the oxidative reaction on the sewage sludge in the fluid state, the following process conditions were tested in the laboratory:

â € ¢ pH ideal for the development of the reaction in sludge in a range between 3 and 5;

â € ¢ concentration of the Fe catalyst in the mixture (for example as FeSO4);

â € ¢ H2O2 concentration

â € ¢ reaction time

From the experimentation conducted it was understood that:

â € ¢ pH: for the optimal treatment of a biological sludge in a fluid state it is necessary to keep the reaction within a narrow pH range.

â € ¢ Fe concentration: the reaction rate of the process is conditioned by the rate of formation of the OH radicals (that is, the concentration of the iron catalyst). The hydroxyl radical is one of the most energetic oxidizing chemical species, second only to elemental fluorine, with a redox potential of 2.06. In the absence of iron there is no formation of the hydroxyl radical and when, for example, H2O2 is added to the sludge, there is no significant reduction in organic contaminants. When the concentration of the iron level undergoes an increase, the reduction of organic contaminants is accelerated until it reaches a point where a further addition of iron becomes absolutely irrelevant.

â € ¢ H2O2 concentration: the expected effects from the reaction on sludge occur only in a precise range of H2O2 concentration in the mixture.

â € ¢ Reaction time: depends on the type of sludge treated and, obviously, on the concentration of the various reagents. Under non-optimized conditions, the reaction times can vary from a few minutes to several hours. Often, by observing the color variations, it is possible to evaluate the progress of the reaction. Generally, after treatment, the dark-colored wastewater clears up when the reaction is completed.

Following the experiments, the treatment conditions were established with regard to the oxidative reaction that develops in Phase 2 against biological wastewater purification sludge.

They are summarized in the following table.

Process parameters Process conditions

Dry matter in% of the sludge to be subjected to

oxidative process <5% to 10%>

PH range to be reached with H2SO4 injection

(fig. 2 - f) From 3.4 to 4.2

Percentage by weight of iron (eg FeSO4) to be introduced

in the reactor (fig. 2 - 2), depending on the weight of the From 0.25% to 0.6%

dry matter of the sludge introduced

Percentage by weight of H2O2 at 130 Volumes from

introduce into the reactor, depending on the weight of the From 10% to 25%

dry matter of the sludge introduced (fig. 2 - e)

Reaction time From 30 minutes to 90 minutes

At the end of the treatment of Phase 2, the mud is odorless, the percentages of any polluting synthetic organic compounds have been reduced, The Fenton reaction has denatured the proteins, part of the organic carbon has been demolished and part of the heavy metals contained in the mud they have passed into solution due to the low pH.

The sludge thus treated is transferred to a second reactor 3 for the acid treatment in the subsequent Phase 3. The first reactor 2 is then empty and ready for a new treatment cycle.

STEP 3

Reactor 3 where Phase 3 is performed is structurally identical to the previous one. An air tie equal to approximately half of the geometric volume of the tank is kept inside and the mud is kept in slow agitation with a propeller driven by an external motor. From tank 15 50% sulfuric acid is then added up to an optimal pH between 1.5 and 2.2, so that the heavy metals pass from the mud to the solution. The pH is controlled and maintained for a defined time with progressive inputs of acid, by means of a pump automatically controlled by a pH meter. The maintenance of a precise pH range is an essential condition of the process as it has been ascertained in the laboratory that, outside this range, the chemical conditions do not exist for most of the heavy metals to pass to the ionic form and therefore in solution.

Process parameters Process conditions pH range to be reached with H2SO4 injection

at 50% From 1.5 to 2.2

Reaction time From 60 minutes to 90 minutes

STEP 4

At the end of Phase 3 the chemical treatment of the mud is complete. The proteins are denatured, large percentages of metals have passed into solution from the organic matter, the pathogenic bacteria have been eliminated and the filterability of the mass is increased.

In Phase 4 the solid fraction is separated from the process liquid by means of an industrial filtration system. This system can be either a filter press 4 or a belt press, but made with constructive measures that allow the apparatus to resist the acidity of the treated mud.

From laboratory tests it has been learned that filtration is optimized when a pressure between 10 and 15 bar is applied to the sludge weighing on the filter sheets.

As regards filtration with filter press, the chamber between the sheets, into which the sludge is pumped, must not exceed 40 mm.

Process parameters Process conditions

Filtration pressure 10 - 15 kg / cm2

Conditioning of the aluminum polychloride mud

From the experimental tests, under ideal treatment conditions, a percentage of dry matter on the filtrate was measured equal to 57% with a filtration pressure of 14 kg / cm2.

The filtered material therefore has a much higher dryness than the starting sludge and easily lends itself to subsequent processing.

The filtered material is then sent via a discharge belt 11 to a tank 12 for accumulating the treated sludge.

The organic substance has been mineralized and stabilized by the treatment with hydrogen peroxide plus iron ions and the mud, thus treated, is the precursor of agricultural corrective agents in accordance with the patent application PC2011A000014, entitled â € œMethod for the preparation of a precursor of agricultural corrective and related productâ €.

This precursor is also used for the production of agricultural corrective agents of which: â € ¢ patent n ° 0001364037 of 20 July 2009 entitled â € œplant for the production of agricultural corrective called defecation plasterâ €;

â € ¢ patent application PC2010A000024, filed on 10/12/2010 with the title â € œMethod and plant for the preparation of agricultural corrective agents â € œ.

STEP 5

The liquid separated from the sludge during filtration (â € œleachateâ €) is collected in a third tank 5.

It contains the heavy metals extracted from the mud treatment, but the pH has been kept at a value of 2 and therefore the metals are in solution and it is therefore possible to reuse it for the dilution of the initial mud of the first phase.

Once in contact with the fresh sludge of the next cycle, the residual sulfuric acid reacts and is consumed, the pH rises and a metering pump is activated to reach the process conditions described in phase 2. The reuse of the liquid deriving from filtration it can only be done for a few cycles.

Their number depends on the concentration of heavy metals in the starting sludge.

From the laboratory tests it is understood that 3 - 4 cycles of reuse are possible when treating a sludge with medium concentrations of metals (eg Zn <2500 ppm)

Due to the extreme dehydration of the material in the filtration phase, the difference in humidity between the starting sludge and the treated material increases the circulating liquid and a quantity, equal to this increase, will be purified and discharged at each cycle, after having separation of heavy metals, as described in the following step 6.

STEP 6

In this phase the â € œleachateâ € is treated, to separate the heavy metals extracted from the sludge through electrochemical extraction and chemical precipitation carried out with reactants.

The â € œleachateâ €, from the storage tank 5, is pumped into an electrolytic cell 6, made with a truncated pyramidal bottom to facilitate the evacuation of electrode sludge and chemical sludge from precipitation. The low pH allows the rapid separation of metals by applying direct current to the cell.

In the cell are placed from 5 to 10 cubic meters of â € œleachateâ € to be processed in direct contact with two flat electrodes of about 1 m2.

The cathode is made of a common sheet of carbon steel with a thickness of 5 mm, while the anode is made of graphite.

Direct current is introduced from a rectifier connected to the network, according to the conditions shown in the following table.

Process parameters Process conditions

Volume of â € œleachateâ € in the cold room From 5 to 10 cubic meters

Current intensity applied to the cell From 0.1 to 0.5 Ampere / dmq

Applied potential difference 2 to 20 Volts

Electrode materials Graphite anode - Iron cathode

Electrochemical extraction time 1 to 2 hours

PH control with reagents (CaO, NaOH) for the

chemical precipitation <7 to 7.5>

As the minutes pass, the pH rises and two phenomena occur:

â € ¢ part of the metals present precipitate on the bottom of the cell in the form of hydroxides;

â € ¢ another part of the metal ions discharges in contact with the cathode and, in metallic form, remains attached to it or precipitates too.

If the pH does not reach neutrality with the electrochemical reaction, alkaline solutions can be introduced which favor the further precipitation of metals or even sodium sulphide with the same effect.

The bottom chemical sludge is pumped to a battery of filter bags 19 which retain the sludge by thickening it and drain the liquid which is pumped to a tank 7.

For the full bags, disposal in landfills or an eventual industrial recovery of metals is envisaged, while the separated liquid is treated with a chemical-physical purification system, in order to comply with the legal limits envisaged for discharge into the sewer or into the surface water body. . The treatment is described in the following step.

STEP 7

The last phase of treatment before the discharge of the â € œleachateâ €, is the chemical-physical purification. In the previous phase 6, the concentration of heavy metals in the liquid was reduced by electrochemistry and through chemical precipitation.

In order to reach the limits established by the Law for discharging into surface water bodies or sewers, it is necessary to refine the abatement through this phase 7.

From the electrochemical cell, the demetallized â € œleachateâ € is pumped into a cistern 7 and stirred through a propeller.

The pH is now close to neutrality and is corrected to a pH suitable for the coagulating reagent to be used. In fact, the various commercial coagulants are active in specific pH ranges. Using ferric chloride, which has a rather wide range of action (4 <pH <10), it will not be necessary to correct, while deciding to use zinc chloride, which is active in a very narrow pH range (7 , 8 <pH <8.5), it will be necessary to perform a precision dosage.

The alkaline agent for neutralization is conveniently contained in a tank 17. Keeping the â € œleachateâ € in slow stirring, the coagulant is introduced from the tank 16, waiting 15 minutes, then the polyelectrolyte from the tank 18 is added to improve the sedimentation characteristics of the sludge.

Before starting the purification it is necessary to check the dosage of the coagulant and the polyelectrolyte with a special laboratory test (Jar Test), which defines ideal conditions and quantity of reagents to be introduced, in order to obtain the best possible precipitation and the most purification. efficient.

From the tank 7 where the reaction took place, the liquid is finally pumped into a gravity settler 8. The liquid supernatant separates and flows into a sand filter 9 for the separation of any foams or floating materials still present and finally discharged, for example into a drainage well 20.

The efficiency of the purification will determine the choice of discharging into the sewer or into a stream. The chemical precipitation sludges concentrate in the bottom cone of the sedimented sediment, which are removed by pumping them into the bag filters 19 previously described.

The final purification of the â € œleachateâ € takes place according to the conditions shown in the following table.

Process parameters Process conditions

Volume of â € œleachateâ € in the cold room From 5 to 10 cubic meters

PH control with reactants (FeCl3; ZnCl2; etc.) for the variable pH according to the preparation of the coagulating coagulation to be introduced.

Moderate agitation Reactive introduction sequence: Introduction of coagulant and coagulant polyelectrolyte

wait for 2 minutes polyelectrolyte

Coagulation reaction time 15 minutes

Check conditions using All parameters

Jar Test

EXPERIMENTAL TESTS

The following tests were carried out with the aim of developing a chemical treatment effective in breaking down the heavy metal content of the sewage sludge and in increasing the capacity of the sludge to lose water, thus increasing the amount of dry matter.

The characteristics of the mud that has been subjected to the treatments are shown in the following table:

s.s. 105 ° C Zn Ni Cd Pb Cu Cr

% mg / Kg s.s mg / Kg s.s mg / Kg s.s mg / Kg s.s mg / Kg s.s mg / Kg s.s 21.7 1175.61 76.26 3.69 94.01 682.97 149.69

The sludge thus characterized was diluted with water up to a dry matter value of 7%.

The mixture obtained was subjected to two types of treatments that attacked the mud with H2SO4 and H2O2.

The first treatment involved acidification of the solution for sulfuric acid at 50% up to pH = 4, while in the second, pH 2 was reached.

Both treatments involve increasing additions of hydrogen peroxide at 40% m / v, in the percentages of 2%, 5% and 10% of the initial mass of treated sludge.

In summary, the tests were carried out according to the theses set out in the following table:

<pH 4> pH 4 pH 4 <pH 2 2%> pH 2 pH 2 pH 4

2% H2O25% HpH 2

2O210% H2O2 H2O25% H2O210% H2O2 For each thesis the metals remaining in the mud were analyzed, following dehydration carried out on a vacuum filter, followed by pressing at a pressure of 14 kg / cm2, after placing the sample between two draining sheets. .

The effectiveness of the treatment was also evaluated as a function of time.

In fact, the analyzes were performed on sample aliquots taken after 1 hour of treatment, after 3 hours of treatment, after 12 and 24 hours of treatment.

The results, summarized in the grid below, confirm the initial hypotheses, namely that the acid environment favors the solubilization of metals and that hydrogen peroxide promotes this process.

The table values are to be considered as percentages of demetalization (for example â € œ-15â € indicates that the sludge has lost 15% of the metal concentration it had before treatment).

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 4 1 h 5.79 2.66 -15.35 1.45 1.55 6.46 pH 4 3 h 6.22 1.24 -13.87 3.71 1.56 3.17 pH 4 12 h 1.81 -0.57 -10.91 -0.50 0.01 1.69 pH 4 24 h - 3.03 -6.47 -17.49 -5.45 -6.76 -3.75

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 4 2% H2O2 + 1 h -5.05 -9.01 -9.66 2.29 -0.47 -0.53 pH 4 2% H2O2 + 3 h -2.02 -3.32 -7.01 -1.52 -3.50 -5.84 pH 4 2% H2O2 + 12 h -2.00 -0.10 -6.45 -3.79 -4.27 -0.26 pH 4 2% H2O2 + 24 h -4.38 -4.31 -15.27 -6.30 -4.21 -4.54

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 4 5% H2O2 + 1 h -7.86 -6.79 -7.07 1.15 -2.55 0.52 pH 4 5% H2O2 + 3 h -8.12 -8.21 -5.44 -2.42 0.65 -3.37 pH 4 5% H2O2 + 12 h -7.11 -8.19 -3 , 92 -0.88 -1.00 -4.67 pH 4 5% H2O2 + 24 h -11.49 -7.08 -8.05 -7.00 -6.45 -2.56

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 4 10% H2O2 + 1 h -46.70 -19.06 -6.76 -9.59 -8, 68 -6.42 pH 4 10% H2O2 + 3 h -43.18 -17.66 -6.94 -8.98 -7.82 -1.13 pH 4 10% H2O2 + 12 h -44.91 -22 , 64 -5.57 -6.36 -7.27 -8.02 pH 4 10% H2O2 + 24 h -43.58 -16.94 -6.88 -7.32 -7.46 -3.28

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 2 1 h -8.10 -13.77 -25.44 -1.03 -9.87 -14 , 25 pH 2 3 h -13.72 -17.14 -24.32 -6.14 -9.33 -14.07 pH 2 12 h -16.53 -19.83 -22.93 -1.94 -5.86 -8.88 pH 2 24 h -17.78 -22.99 -30.97 -19.98 -23.02 -19.40

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 2 2% H2O2 + 1 h -36.97 -23.61 -18.79 0.44 -0.44 -6.57 pH 2 2% H2O2 + 3 h -41.03 -26.26 -21.70 3.25 -1.41 -8.45 pH 2 2% H2O2 + 12 h -44.81 -25.94 - 21.78 0.31 -4.64 -9.34 pH 2 2% H2O2 + 24 h -48.37 -27.90 -25.32 -2.09 -8.24 -12.32

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 2 5% H2O2 + 1 h -62.04 -28.49 -12.39 0.20 -28.46 -9.19 pH 2 5% H2O2 + 3 h -57.77 -29.32 -8.98 3.13 -23.46 -10.67 pH 2 5% H2O2 + 12 h -64.04 -31.42 - 18.39 1.68 -19.97 -10.38 pH 2 5% H2O2 + 24 h -64.36 -30.50 -21.66 -2.13 -17.69 -9.94

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) pH 2 10% H2O2 + 1h -68.55 -29.67 0.01 -3.71 -52.58 - 12.82 pH 2 10% H2O2 + 3 h -65.64 -29.41 -6.37 -5.12 -52.72 -11.33 pH 2 10% H2O2 + 12 h -67.61 -33.11 -11.61 -3.47 -54.18 -17.10 pH 2 10% H2O2 + 24h -64.60 -32.28 -12.22 -5.53 -52.25 -15.98

The pH 2 treatment proved to be particularly effective for demetalization, and the most important reduction was obtained with the maximum dose of hydrogen peroxide.

The acidic and oxidizing conditions of the solution allowed the removal of 65.64% of Zinc, 29.41% of Nickel, 6.37% of Cadmium, 5.12% of Lead, 52.72% of Copper and 11.33% of Chromium, within 3 hours of treatment.

The analysis of the results from a point of view of real practical and economic feasibility led to a preference for the treatment at pH 2 5% of hydrogen peroxide, less aggressive but still efficient.

It was therefore decided to investigate the effect of these experimental conditions on metals and on the dry matter content reached by the treated sludge following pressing. The test was carried out using the same mud and following the illustrated method, but only on the treatment at pH 2 5% of hydrogen peroxide, with treatment times of 1 hour and 3 hours. A maximum time of 3 hours was considered sufficient to obtain suitable results.

The table below shows the dry matter values reached by the treated sludge (pH 2 5% H2O2) after pressing and the percentages of demetallization.

Zn (%) Ni (%) Cd (%) Pb (%) Cu (%) Cr (%) <s.s. 105 ° C> (%) pH 2 5% H2O2

+ 1 hour -80.99 -62.84 -15.50 -4.90 -35.31 -10.55 57.32 pH 2 5% H2O2

+ 3 hours -77.19 -63.11 -19.75 -4.96 -21.35 -9.52 57.84

The percentage of dry matter shown in the table is the average value between 10 replicates carried out in the same test, as regards the percentages of demetallization they represent the average value calculated between 5 replicates.

The treatment chosen allows to obtain a product with a much lower metal content, compared to the same untreated sludge, as regards Zn, Ni and Cu, while Cd, Pb and Cr respond to the treatment with a less evident decrease.

On the other hand, it is important to underline the effect of the treatment on the dry matter content.

The untreated mud, after pressing, reaches a dry percentage of 26.80% (average of 10 replicates), the treatment allows to eliminate a greater quantity of water from the mud bringing it to a dryness of more than 57%.

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Claims (10)

CLAIMS 1. A method for the chemical-physical treatment of biological sewage sludge characterized by the fact of comprising: - a first step of fluidification of said sludge until a content of dry matter of between 1% and 20% is obtained; - a second step of abatement of synthetic organic compounds contained in said sludge and partial mineralization of the organic substance; - a third phase of acidification of the product obtained in said second phase; - a fourth step of filtering the product obtained in said third step; - a fifth phase in which the liquid separated from the mud (â € œleachateâ €) in the previous phase is recovered by introducing it into a special tank and reused for the part necessary for the dilution of other mud in the first initial phase; - a sixth phase in which the liquid separated from the sludge (â € œleachateâ €), after being used for some dilution cycles, is demetallized by electrochemical and / or chemical means; - a seventh phase in which the demetallized â € œleachateâ € obtained in the previous phase is treated with a common chemical-physical process for the purification of waste water. 2. Method according to claim 1, characterized in that in said first step, said sludges are diluted until reaching a dry matter content of between 5% and 10%. 3. Method according to claim 1 or 2, characterized by the fact that in said second phase the fluidized sludge is pumped into a first chemical reactor and sulfuric acid is added to it until a pH between 3.4 and 4,2, then a solution of ferrous sulphate and finally of hydrogen peroxide. 4. Method according to one or more of the preceding claims, characterized in that in said third phase the sludge obtained from the previous phase is introduced into a second chemical reactor to which sulfuric acid is added until it reaches pH 2, controlling and maintaining this pH for a definite time. 5. Method according to one or more of the preceding claims, characterized in that the sludge obtained from said fourth phase is sent to a filter press and filtered at a pressure between 10 and 15 bar, subsequently transferring the dehydrated sludge thus obtained into a storage via a conveyor belt. 6. Method according to one or more of the preceding claims, characterized in that the liquid separated from the sludge (â € œleachateâ €) is recovered by introducing it into a third tank and reused for the part necessary for the dilution of other sludge in the first initial phase. 7. Method according to one or more of the preceding claims, characterized in that, in said sixth phase, the â € œleachateâ €, after having been used for some treatment cycles, is pumped from said third tank into an electrolytic cell and subjected to electrochemical treatment of demetallization. 8. Method according to claim 7, characterized in that at the end of said electrochemical treatment, called â € œleachateâ € is treated with sodium sulphide. 9. Method according to one or more of the preceding claims, characterized in that, in said seventh phase, the pH of the â € œleachateâ € is brought to complete neutrality with sodium hydroxide or milk of lime, thus introducing iron chloride as a coagulant and a polyelectrolyte for precipitate the chemical sludge. 10. Plant (100) for the implementation of a method for the chemical-physical treatment of biological sewage sludge according to one or more of the preceding claims characterized in that it comprises: - a mixer (1) for diluting said sludges; - a first reactor (2) for said second step of abatement of synthetic organic compounds contained in said sludges; - a second reactor (3) for said third phase of acidification of the product obtained in said second phase; - a filter press (4) for said fourth step of filtering the product obtained in said third step - a third tank (5) for the collection of said liquid separated from the mud (â € œleachateâ €); - an electrochemical cell (7) for said sixth phase of demetallization of said liquid separated from the sludge (â € œleachateâ €); - a fourth tank (8) for said seventh purification phase; - a sand filter (9) for the filtration of the sludge obtained from the â € œleachateâ €.