ITVR20110230A1 - PROCEDURE FOR PROCESSING DIGESTATO - Google Patents

Description

PROCEDURE FOR PROCESSING DIGESTATE

DESCRIPTION

The present invention relates to a digestate treatment process.

More particularly, the present invention relates to a digestate treatment process deriving from the anaerobic digestion of the organic fraction from separate collection (OFMSW) and having a weight percentage of dry matter equal to about 16% and high concentrations in terms of COD and ammonia (NH3).

In the current state of the art, various treatment processes are known for the purification of the liquid fraction of the digestate from separate collection or OFMSW, in which various biological and physico-chemical treatment systems are usually associated in series.

The typical configuration of a liquid digestate treatment plant involves the coupling of biological treatments with membrane filtration, for the finishing and discharge of purified water or for its industrial reuse.

In particular, biological treatments are divided into suspended biomass systems and adherent biomass systems.

In turn, suspended biomass systems are divided into classic NITRO / DENITRO systems, which include: conventional or SBR (Sequencing batch Reactor) activated sludge systems; systems with combined technologies (physical-biological), among which we can mention the MBR (Membrane Bioreactor) and MBBR (Moving Bed Biofilm Reactors) systems; and innovative systems, such as those known as SHARON / ANAMMOX / CANON.

The aforementioned treatments can operate continuously (conventional activated sludge) or discontinuous (SBR Sequencing batch reactor, MBR Membrane bioreactor, MBBR Moving Bed Biofilm Reactor).

Given the great variability of the chemical-physical characteristics of the digestate, with regard to biological treatments, discontinuous treatments are becoming predominantly established, as they allow a better adaptability to the variability of the digestate, require less areal surfaces and involve less production of sludge. of waste to be disposed of and / or recovered.

In particular, the SBR (Sequencing batch reactor) systems consist of a discontinuous flow biological treatment, consisting of one or more basins for biological oxidation processes, sedimentation, extraction of purified effluent and excess sludge, and take the form of an activated sludge process, in which the different process phases follow each other in a temporal rather than a spatial sequence.

The SBR systems have the advantage of allowing the possibility of varying the duration of the times, according to the real needs of wastewater treatment, and, moreover, they allow to regulate the progress of the process according to the quantitative and qualitative characteristics of the influent wastewater. , so they can be considered as plants that operate in a â € œcontrolled non-stationary stateâ €, thus allowing to vary the duration of each cycle and the duration of each phase of the cycle and to modify the growth rate of microorganisms.

However, SBR systems have some efficiency limits, including an 80-90% reduction in COD and a 50-90% reduction in nitrogen.

In fact, a reduction of more than 60% of nitrogen can lead to cost increases due to the need to add COD (methanol or molasses) as a substrate for the activity of bacteria and / or the growth of energy consumption (air insufflation).

Other disadvantageous aspects of SBR systems are of a biological nature and concern the production of sludge (not very sedimentable, filamentous) and the growth of microorganisms (excessive selection, acclimatization time).

The aforementioned MBR systems, on the other hand, are systems that combine two treatment technologies: biological, for purification treatment, and physical, with membrane filtration to reduce sludge production.

For the digestate, MBR systems usually provide a tank for the aerobic treatment with activated sludge for the biological oxidation of the digestate and the subsequent treatment of the flow by means of hollow fiber ultrafiltration membranes external to the oxidation tank, with type filtration regime tangential and with biomass recirculation.

MBR systems offer some advantages, among which the fact of requiring reduced spaces, of being able to be used in modular sections, of allowing a low production of sludge and the elimination of sedimentation problems as well as the rapid start-up of the sludge treatment.

It should be noted that the purified liquid downstream of the MBR systems, to be reused as industrial water, requires further refinement by reverse osmosis membrane purification. Among the disadvantages, MBR systems denote high costs and frequently show problems of clogging (fouling) of the membranes.

The technology called MBBR (moving bed biofilm reactor) consists of a hybrid system that couples suspended biomass and attached biomass. Its peculiar characteristic is constituted by the provision of plastic elements with a high specific surface, immersed in the reactor, which allow the development of a biofilm on their surface, thus increasing the concentration of bacterial biomass in the aerated tank.

The MBBR systems allow, compared to the biological systems described above, an increase in efficiency, due to the synergistic effect of the two biomasses, the reduction of volumes, the flexibility of expansion with an increase in the filling rate, without any modification or © from a structural point of view nor with respect to existing equipment.

Furthermore, the biomass adhered to the plastic elements gives MBBR systems greater resistance to any load variations, more favorable biomass / liquid separability characteristics, higher sludge ages for equal volumes, and therefore more stabilized and easily dehydratable excess sludge. .

Even in the case of MBBR systems, the purified wastewater still requires further membrane refinement treatments. Therefore, even the MBBR system, like the other biological systems described, cannot be used alone if the aim is to produce water suitable for industrial reuse or discharge onto a surface water body.

The SHARON / ANAMMOX system (SHARON: Single reactor High rate Ammonia Removal Over Nitrite; ANAMMOX: ANaerobic AMMonia Oxidation) is an innovative biological system, useful for the treatment of ammonia-rich wastewater, which develops in the following two phases: the SHARON phase , which consists in the partial oxidation NH4 <+> into NO2 in an SBR system, in which 50% of the ammonium must be converted to nitrite and in which the oxidative stage NO2 - NO3 is inhibited with high temperatures (development of Nitrosomonas and inhibition of Nitrobacter); and the ANAMMOX phase, which consists in the isolation of specialized bacteria that oxidize ammonium under anoxic conditions, using nitrite as an electron acceptor, and in which NH4 <+> and NO2 are converted into N2, thus resulting in an anaerobic oxidation process obligata supported by specialized bacteria and at low cellular growth rates, which carry out the aforementioned strictly autotrophic anaerobic process.

The CANON process, which consists of the coupling of SHARON and ANAMMOX, allows reduced management costs, low energy consumption, reduced production of sludge and reduced occupation of surfaces, but at the same time requires long start-up times (6 months) , exploits unstable microorganisms, sensitive to variations from stationary and slow-growing, so it is difficult to adapt to the OFMSW digestate, as well as requiring further refinement treatments for the product waste. The most common physical treatments in the area of digestate purification are membrane filtration treatments, which are usually applied as a finish downstream of biological treatments.

Membrane treatments, depending on the purification efficiency, are divided into microfiltration, ultrafiltration (membranes used in MBR systems), nanofiltration and reverse osmosis (more widespread for the final refinement of the wastewater).

In particular, reverse osmosis provides the guarantee of abatement of the organic substance and residual suspended solids present in the wastewater, of the abatement of nutrients (phosphorus and more) and of all other inorganic pollutants.

Usually in the application to the digestate the membranes operate with tangential flow to limit fouling phenomena and the formation of saline precipitates.

Membrane filtration treatments allow a strong containment of the treatment surfaces, low consumption of chemical products for washing the membranes and a high cost / benefit ratio, however, in the face of high energy consumption and the need to provide in any case a further concentrate management system, which usually represents about 30% of the treated volumes.

Other known types of treatment, such as evaporation / concentration, are still not very widespread and in any case provide for further final refinement treatments, such as ammonia stripping, or adsorption treatments on exchange resins.

The aim of the present invention is to solve the drawbacks of the known art, by providing a digestate treatment process which allows to limit the digestate purification to a single chemical-physical treatment.

Within this aim, an object of the invention is to provide a digestate treatment process which allows to obtain, at the end of its execution, a compound that can be used as a fertilizer and a liquid that can be reused as industrial water.

Another object of the present invention is to provide a digestate treatment process which can be carried out without difficulty using a traditional composting and anaerobic digestion plant.

Not least object of the present invention is to provide a digestate treatment process which can be carried out at extremely low costs.

This aim, as well as these and other objects which will become clearer hereinafter, are achieved by the digestate treatment process according to the invention, as defined in claim 1.

Further characteristics and advantages of the invention will become clearer from the following description of a preferred but not exclusive embodiment of the digestate treatment process according to the invention.

The digestate treatment process, according to the invention, is based, in principle, on carrying out the purification of a digestate, coming from the anaerobic digestion of the organic fraction from separate collection (OFMSW), by means of a single treatment, which essentially consists , in carrying out a concentration of the digestate at acidic pH, with the aim of retaining ammonia (NH3), in the form of ammonium ion (NH4 <+>), inside the digestate itself to be subjected to aerobic composting , for the purpose of its recovery as a composted soil conditioner, and at the same time obtaining, as will be better explained below, a liquid distillate that can be used to satisfy industrial water requirements. It should be noted that this type of digestate to be treated normally has a weight percentage of dry matter equal to about 16% and high concentrations in terms of COD (Chemical Oxygen Demand, up to 50,000 mgO2 / l) and ammonia (NH3). , up to 4000 mg / l).

Going into more detail, the process according to the invention provides a first step which consists in acidifying the digestate, preferably by adding sulfuric acid and, more preferably, substantially 90% pure sulfuric acid.

The quantity by weight of 90% pure sulfuric acid added to the digestate may, for example, be substantially comprised between 0.5 and 0.6% of the total weight of the digestate.

Advantageously, the acidification of the digestate can be carried out by providing the dosage of the sulfuric acid inside a digestate stream, which is suitably homogenized by mechanical mixing, so as to obtain the salification of the ammonia present in the digestate below. form of ammonium sulfate.

Conveniently, the acidification phase is carried out with a continuous adjustment of the sulfuric acid dosage as a function of the pH value reached by the digestate and measured directly by means of a pH meter. The pH value is adjusted according to the concentration of ammonia and volatile fatty acids measured in the digestate to be treated, in order to obtain the minimum transfer of the aforementioned chemical compounds in the liquid distillate, subsequently obtained.

Subsequently, the acidified digestate is concentrated, preferably up to 35% of dry matter, by means of an evaporator, preferably with a triple effect.

Conveniently, it is possible to use a triple effect evaporator of the type on the market in the food industry, and, more particularly, a triple effect evaporator suitable for the concentration of flows characterized by a high concentration of dry matter and suitable to operate in an acidic environment (with pH even lower than 2), therefore built with corrosion resistant steel (type AISI 316).

The principle of operation of the aforementioned evaporator provides for three evaporative modules, otherwise called â € œeffectsâ €, placed in series with each other and kept under vacuum, which, fed at the inlet with a solution to be treated, are able to produce a liquid distillate, resulting from the condensation of vapors obtained by heating the solution to be treated, and a concentrated solution.

In order to minimize the energy required to produce the liquid distillate, the evaporator works at different temperatures and pressures in the three effects. In particular, the three effects in series operate at decreasing temperatures and pressures, so as to be able to exploit, in the effects subsequent to the first, the latent heat of condensation of the steam produced in the previous effect. Purely by way of example, the heat produced by an external heat source is transferred to a solution to be treated through the first effect obtaining its boiling; the evaporate thus obtained is condensed and its latent heat of condensation is exploited in the second effect, in which there is a greater degree of vacuum, so as to favor the evaporation of the solution to be treated at a lower temperature, compared to first effect. Similarly, the latent heat of condensation of the vapor developed in the second effect is transferred to the solution to be treated in the third effect.

In practice, the acidified digestate is heated in the evaporator, so that a part of it evaporates in the form of vapors, which are subsequently condensed, in order to obtain a liquid, while its remaining part remains as concentrated digestate.

It should be pointed out that liquid distillate indicates in the present description the liquid deriving from the condensation of the vapors obtained in the evaporator.

A third step of the process according to the invention consists in correcting the pH of the liquid distillate separated from the evaporator, in order to subsequently use this condensed distillate to satisfy the water requirements of an integrated anaerobic digestion and composting plant, while the digestate concentrated in when exiting the evaporator, it can advantageously be subjected, in mixture with other organic and lignocellulosic substances, to a controlled biological process of oxidative degradation by bacteria and fungi by aerobic composting that leads to the complete recovery of the concentrated digestate in the form of composted soil conditioner mixed, defined pursuant to Legislative Decree

75/2010.

In particular, the correction of the pH of the liquid distillate involves bringing the liquid distillate back to neutrality conditions by dosing a strong basic reagent, such as, for example, NaOH.

The treatment method according to the invention will be explained in more detail hereinafter with reference to a possible example of embodiment.

It is assumed that the digestate has an alkalinity value equal to 5000 mg CaCO3 / l, which, as can be inferred from literature data, is an upper limit value not to be exceeded for the correct trend of anaerobic digestion.

90% pure sulfuric acid (H2SO4) was chosen as pH corrector (specific weight about 1.8 Kg / l).

The reference reaction during the digestate acidification phase is the following:

CaCO3 H2SO4 = CaSO4 H2CO3 In this case, 0.05 moles of CaCO3 are neutralized with 0.05 moles of H2SO4, therefore:

0.05 mol x 98 g / mol H2SO4 = 4.90 g of H2SO4 Using commercial 90% sulfuric acid (m.p. approx. 1.8 kg / l), add 5.44 g of commercial sulfuric acid for each kg of digestate.

Once the alkalinity present in the digestate has been reduced, to subsequently bring the digestate to a pH of 2.5, a concentration of H <+> equal to 3 · 10 <-3> moles / l is required, therefore:

3 10 <-3> moles x 98 g / mol H2SO4 = 0.294 g of H2SO4, corresponding to 0.33 g of commercial H2SO4 at 90% for each kg of digestate.

Overall, for the preliminary acidification to the concentration treatment, about 6 Kg of commercial H2SO4 / ton of digestate are required.

Chemical evaluations show that already at pH 6 there is a ratio:

NH3 / NH4 <+> = 5.8 · 10 <-10> / 1 · 10 <-6> = 5.8 · 10 <-4>

This means that only 5.8 · 10 <-2>% of the total ammonium present in the digestate to be concentrated is in the state of free ammonia and is therefore able to evaporate and subsequently condense in the distillate.

The ammonia present in the digestate is therefore found ionized in the concentrate in the form of ammonium sulphate in the dissociated form, a salt whose fertilizing properties are known and which is thus recovered in the soil conditioner produced by the subsequent aerobic fermentation of the concentrate leaving the € ™ triple effect evaporator.

The concentration of free acetic acid in the digestate to be concentrated is regulated by the following equation:

CH3COOH = CH3COO <-> + H <+>

Ka = 1.8 10 <-5>

CH3COO- / CH3COOH = 1.8 10 <-5> / H +

According to the stated equation, at pH = 6 there is a CH3COOH / CH3COO <-> ratio equal to 1 · 10 <-6> / 1.8 · 10 <-5> = 0.0556, which means that the 5.56% of the total acetic acid is in the state of free acetic acid.

It should therefore be noted that the acidification phase of the digestate is carried out according to the concentrations of ammonia and volatile fatty acids present in the digestate to be concentrated, as an equilibrium solution in order to produce a distillate with such qualitative characteristics that it can be reused for industrial water requirements and at the same time recovering the greatest possible quantity of ammonia in the form of ammonium sulphate in the concentrate and consequently in the soil conditioner of related production.

With regard to the flow of liquid distillate, the treatment is completed with the adjustment of the pH of the condensate downstream of the evaporator, in order to bring the water back to neutral conditions (pH = 7), by dosing a strong basic reagent, for example NaOH, monitored with a special pH meter.

The distillate thus produced has no dry matter residues and is characterized by low concentrations of COD, deriving only from the fraction of volatile fatty acids present in the treated digestate and possibly dragged into the distillate by evaporation under acid conditions. It has been found that the presence of residual COD is in any case limited by the low temperature at which evaporation is carried out and by the fact that, being this evaporator a system that operates at low pressure, the air extracted by the pump which reproduces the low pressure conditions inside the evaporator convey a part of the volatile fatty acids present in the digestate stream, which evaporate rapidly at the temperatures and pressure typical of such evaporation plants.

The flow of distillate produced can be reused to cover the water requirements of the composting and anaerobic digestion plant, inside which the process according to the invention is carried out.

In practice it has been found that the invention is able to fully accomplish the intended aim and objects.

In particular, it should be noted that the digestate treatment process according to the invention allows, by means of a single chemical-physical treatment, to retain the ammonia, present in the digestate, in the form of ammonium sulphate, in a concentrated matrix of organic nature, to be sent for recovery for the production of composted soil improver enriched with the ammonium sulphate compound, in order to increase its fertilizing characteristics, and at the same time to produce a distillate, which is subsequently condensed, with characteristics that can be reused as industrial water for satisfy the industrial water needs of the plant used.

All the characteristics of the invention, indicated above as advantageous, convenient or the like, can also be missing or be replaced by equivalents. The individual characteristics set forth with reference to general teachings or particular embodiments can all be present in other embodiments or substitute features in these embodiments.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept.

Furthermore, all the details can be replaced by other technically equivalent elements.

Claims (9)

CLAIMS 1. Digestate treatment process characterized by the fact that it comprises the phases which consist of: - acidify a digestate from anaerobic digestion of the organic fraction from separate collection; - concentrate the acidified digestate by means of an evaporator; - correcting the pH of the liquid distillate separated from said evaporator. 2. Process according to claim 1, characterized in that said acidification step is carried out by adding sulfuric acid to said digestate. 3. Process according to one or more of the preceding claims, characterized in that said acidification step involves the metering of the sulfuric acid inside a digestate stream and the homogenization of said digestate stream by mechanical mixing, in order to salify the ammonia present in said digestate in the form of ammonium sulphate. 4. Process according to one or more of the preceding claims, characterized in that said sulfuric acid is about 90% pure and is added to said digestate in a quantity by weight of 90% pure sulfuric acid substantially comprised between 0.5% and 0.6% of the total weight of said digestate. 5. Process according to one or more of the preceding claims, characterized in that said acidification step is carried out with continuous adjustment of the pH. 6. Process according to one or more of the preceding claims, characterized in that said concentration step is carried out by means of a triple effect evaporator. 7. Process according to one or more of the preceding claims, characterized in that said digestate is concentrated to 35% of dry matter. 8. Process according to one or more of the preceding claims, characterized in that said concentrated digestate is subsequently subjected to aerobic composting, for its recovery in the form of composted soil conditioner. 9. Process according to one or more of the preceding claims, characterized in that the step of correcting the pH of the liquid distillate separated from said evaporator involves bringing the liquid distillate back to neutral conditions by dosing a strong basic reagent.