Process for the treatment of wastewater based on granular / densified biomass

Abstract

A process for the treatment of wastewater based on granular / densified biomass, comprising the steps of: a) feeding a COD-loaded wastewater batch into a first reactor, in which there are granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities; b) in said first reactor, putting said COD-loaded wastewater batch in contact with said granular and / or densified activated sludges, subjecting said COD-loaded wastewater batch to a step of transformation under anaerobic conditions through the action of said PAO and / or GAO bacteria, COD being sequestered by the same; c) precipitating said COD-containing granular and / or densified activated sludges, thereby obtaining a wastewater batch depleted of COD and possibly enriched with phosphates, said phosphates being released by said PAO bacteria; d) discharging said wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor; e) feeding said wastewater batch depleted of COD and possibly enriched with phosphates into a second reactor and subjecting said wastewater batch depleted of COD and possibly enriched with phosphates to a nitritation step possibly combined with a nitratation step under aerobic conditions, thereby obtaining a wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly obtaining a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen; f) discharging said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, from said second reactor; g) feeding said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, to said first reactor and, in said first reactor, putting said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, in contact with said COD-containing granular and / or densified activated sludges, so as to bring about a step of transformation under anoxic conditions through the action of said PAO / GAO bacteria, phosphorus being possibly sequestered by the same; h) precipitating said granular and / or densified activated sludges possibly containing phosphorus, thereby obtaining a purified wastewater batch, said purified wastewater batch being depleted of COD, nitrogen, and possibly phosphorus.

Description

[0001] Title: Process for the treatment of wastewater based on granular / densified biomass

[0002] DESCRIPTION

[0003] Field of application

[0004] In its most general aspect, the present invention relates to an improved process for the treatment of wastewater based on granular / densified biomass.

[0005] Prior art

[0006] The use of granular / densified activated sludges in the context of the treatment of wastewater is known in the art.

[0007] This technology differs from conventional treatment systems, which use activated sludges in the form of flakes.

[0008] The treatment of wastewater through activated sludges in the form of flakes, which is a conventional technology of biological purification, generally involves the use of at least two tanks.

[0009] In one tank, a biological-oxidation step is carried out, during which organic-carbon removal and nitrogen nitrification take place concurrently, nitrification being a process consisting of the successive combination of nitritation and nitratation under strictly aerobic conditions.

[0010] Downstream of said tank, a secondary- sedimentation tank is allocated, said tank having the function of solid-liquid separation of the flow exiting the biological compartment.

[0011] The clarified sewage is discharged from the sedimentation tank and sent to tertiary treatments or returned to the environment, whereas a fraction of the sedimented sludge is commonly recovered and conveyed towardsthe first tank.

[0012] According to some known technologies, this recirculation operation allows to keep the biomass activated inside the treatment system during the biological-oxidation step and to guarantee a certain sludge concentration and age, which is useful to optimize the wastewatertreatment system.

[0013] It is understood that there are also more complex systems, developed from this first approach, which have not been considered herein.

[0014] A line for the recovery of an excess fraction of the sedimented sludge or waste activated sludge is conventionally provided, said line being conveyed to disposal or recovery treatments.

[0015] However, the technology based on activated sludges in the form of flakes has some drawbacks related above all to the flakiness of sludge, to the high energy consumption required for handling recirculation flows and for aeration, and also to the considerable production of waste activated sludge, which must be disposed of, thereby causing additional system complexities and energy and operation costs of the plant.

[0016] The processes based on granular / densified aerobic biomass for wastewater purification are a rapidly expanding technology towards which there is growing interest since it is competitive compared to the conventional treatment systems.

[0017] Unlike conventional treatment systems, the processes based on granular / densified aerobic biomass involve the use of granules and / or thickened formations, defined as microbial-origin aggregates having variable size that sediment with significantly higher speeds compared to the conventional activates sludge.

[0018] Processes of this type are disclosed for example in CN 104891646A and CN2 12687690U, which involve the treatment of wastewater through aerobic granular sludges.Instead, US7060185B2 discloses the treatment of wastewater through a process based on aerobic granular sludges in multiple reactors.

[0019] However, although the technology based on aerobic granular sludges brings about a considerable progress compared to conventional treatment systems, the processes mentioned above in connection with the prior art and, in general, the processes based on granular / densified aerobic biomass have some disadvantageous aspects, including high energy consumption due to the need of extensively using aeration systems, the small but still significant production of waste activated sludge, and a high mineralization of organic carbon into carbon dioxide, resulting in a huge dispersion of climate-changing gases into the atmosphere.

[0020] CN201458907U and CN101628772A propose a system based on anaerobic / anoxic granular sludges developed through two variablevolume SBR reactors, the first of which being designated to the process based on granular sludges and the second one limited to a process that only provides nitrite.

[0021] There is therefore a particularly pressing need to provide a process for the treatment of wastewater that does not have the drawbacks pointed out with reference to the known processes.

[0022] The object of the present invention is precisely to provide said process, which involves low energy consumption, a lower production of sludge to dispose of, and a lower degree of mineralization of organic carbon, in favor of a recovery of the latter in the form of energy or material.

[0023] Summary of the invention

[0024] According to the invention, this object is achieved by process for the treatment of wastewater based on granular / densified biomass comprising the following steps:

[0025] a) feeding a COD-loaded wastewater batch into a first reactor, in whichthere are granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities;

[0026] b) in said first reactor, putting said COD-loaded wastewater batch in contact with said granular and / or densified activated sludges, subjecting said COD-loaded wastewater batch to a step of transformation under anaerobic conditions through the action of said PAO and / or GAO bacteria, COD being sequestered by the same;

[0027] c) precipitating said COD-containing granular and / or densified activated sludges, thereby obtaining a wastewater batch depleted of COD and possibly enriched with phosphates, said phosphates being released by said PAO bacteria;

[0028] d) discharging said wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor;

[0029] e) feeding said wastewater batch depleted of COD and possibly enriched with phosphates into a second reactor and subjecting said wastewater batch depleted of COD and possibly enriched with phosphates to a nitritation step possibly combined with a nitratation step under aerobic conditions, thereby obtaining a wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly obtaining a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen;

[0030] f) discharging said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, from said second reactor;

[0031] g) feeding said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, to said first reactor and, in said first reactor, putting said wastewater batch enriched in nitrites and nitrates and depleted of organicand / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, in contact with said COD-containing granular and / or densified activated sludges, so as to bring about a step of transformation under anoxic conditions through the action of said PAO / GAO bacteria, phosphorus being possibly sequestered by the same;

[0032] h) precipitating said granular and / or densified activated sludges possibly containing phosphorus, thereby obtaining a purified wastewater batch, said purified wastewater batch being depleted of COD, nitrogen, and possibly phosphorus.

[0033] Preferably, the process for the treatment of wastewater based on granular / densified biomass according to the present invention comprises the following additional step:

[0034] i) discharging said purified wastewater batch from said first reactor.

[0035] Preferably, the process for the treatment of wastewater based on granular / densified biomass according to the present invention is carried out repeatedly and according to a plurality of successive cycles, wherein, in said feeding step a) of each successive cycle, a COD-loaded wastewater batch is fed to the first reactor, said COD-loaded wastewater batch being different from the COD-loaded wastewater batch fed to the first reactor during the feeding step a) carried out in the preceding cycle or in the preceding cycles.

[0036] More preferably, when the process for the treatment of wastewater based on granular / densified biomass according to the present invention is carried out repeatedly and according to a plurality of successive cycles, in said feeding step a) of each successive cycle, said granular and / or densified activated sludges correspond to said granular and / or densified activated sludges, possibly containing phosphorus, that have been precipitated during step h) of precipitation carried out in the preceding cycle.In particular, it is specified that, during step b) of transformation under anaerobic conditions, COD is accumulated intracellularly by GAO microorganisms, with concurrent phosphorus release by PAO bacteria. Consistently, during step e) of transformation under aerobic conditions, wastewater transformed through the second reactor, thus containing nitrites and nitrates, possibly containing essentially only nitrates, is also enriched with phosphates released by PAO microorganisms inside the first reactor and during step b) of transformation under anaerobic conditions.

[0037] In the same way, during step g) of transformation under anoxic conditions, said PAO / GAO bacteria degrade the previously sequestered COD into carbon dioxide and possibly accumulate phosphorus.

[0038] In other words, during step g), the wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen is subjected to a step of transformation under anoxic conditions through the action of said PAO and / or GAO bacteria, wherein intracellular COD and the possibly present nitrates and nitrites are degraded by denitrification, whereas phosphates are intracellularly accumulated by PAO.

[0039] Specifically, phosphorus can be conveniently sequestered by said PAO / GAO bacteria and therefore stored in said granular and / or densified activated sludge, so as to be possibly be provided during the following cycles of carrying out the present process.

[0040] Through oxidation-reduction, the organic substance is oxidized, whereas nitrite, which is possibly present, and nitrate are reduced to gaseous molecular nitrogen, thereby depleting wastewater of both nitrite and nitrate and of phosphorus, which is possibly present. Upon the subsequent step h) of precipitation, said purified wastewater batch is thus obtained.

[0041] Consistently, during the subsequent step h) of precipitation, a granular and / or densified activated sludge, possibly containing phosphorus, isprecipitated.

[0042] According to the present invention, the expression “densified and / or granular activated sludges” means a water suspension of activated biomass, namely comprising microorganisms, including PAO / GAO, in the form of granules / sedimentable densified formations. Specifically, the microorganisms contained in said activated biomass comprise, but are not exclusively consisting of, PAO / GAO bacteria, in the form of granules / sedimentable densified formations. In particular, the microorganisms contained in said activated biomass additionally and optionally comprise fermentative microorganisms, preferably acidogenic and / or methanogenic microorganisms.

[0043] Specifically, inside said granules / sedimentable densified formations, a segregation of the microorganisms occurs depending on the mass transfer of the substrates, namely of the wastewater and of the material dispersed therein, such as COD, and of the metabolic products from the outer to the innermost regions, thereby causing the coexistence of aerobic, anaerobic and anoxic areas.

[0044] According to the present invention, the expression “PAO / GAO bacteria” means a bacterial population comprising bacteria capable of accumulating phosphorus (PAO, Polyphosphate-Accumulating Organisms) and bacteria capable of accumulating glycogen (GAO, Glycogen-Accumulating Organisms) .

[0045] According to the present invention, the term “COD” means the total mass of organic and / or inorganic material capable of consuming oxygen, also called Chemical Oxygen Demand (COD), which represents the oxygen amount required for the complete chemical oxidation of the organic and inorganic compounds that are in a certain amount of water, namely, in the present case, in a wastewater batch.

[0046] In general, during step e) of transformation under aerobic conditions, the wastewater batch depleted of COD and possibly enriched withphosphates undergoes a biological-oxidation process, which comprises a step of nitritation, possibly followed by, at least partial, nitratation of organic and / or ammonia nitrogen due to autotrophic mechanisms of the microorganisms residing in the second reactor.

[0047] Specifically, step e) of transformation under aerobic conditions may provide a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, that is to say that said wastewater batch exiting said step e) is substantially enriched in nitrates, namely, the presence of nitrates in said batch is prevalent compared to nitrites.

[0048] This means that, after nitritation under aerobic conditions, the wastewater batch depleted of COD predominantly undergoes nitratation under aerobic conditions, resulting in a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen.

[0049] In particular, the nitrification process occurs in a substantially complete manner, wherein, once formed, nitrites are mainly oxidized to nitrates, in other words, an amount equal to at least 95%, preferably equal to at least 99%, of the nitrites formed in the above-mentioned wastewater batch is transformed into nitrates.

[0050] Preferably, according to a totally preferred embodiment, said first reactor is a constant-volume sequencing batch reactor (SBR) .

[0051] In particular, therefore, the process for the treatment of wastewater based on granular / densified biomass according to the present invention involves the application on granular / densified activated sludges through sequential anaerobic / anoxic conditions for the biological treatment of wastewater with concurrent removal of COD and nutrients and phosphorus recovery based on the cyclic operation of a constant-volume sequencing batch reactor (SBR) combined with an aerobic reactor designated only to the nitritation step, possibly combined with a nitratation process.

[0052] Advantageously, compared to the processes based on aerobic granularactivated sludges known in the art, the process according to the invention allows to bypass the supply of oxygen as an electron acceptor for the oxidation of the organic substance. Such result is obtained by decoupling the destination of the treated wastewater from the destination of the biomass throughout the process: the wastewater is put in contact with the granular / densified biomass in a first anaerobic step, to be then extracted from the first reactor and sent to the second reactor.

[0053] In the second reactor, the wastewater undergoes a process of nitritation possibly combined with a nitratation process under aerobic conditions. Once this step is completed, the wastewater is put in contact with the granular / densified biomass again inside the first reactor.

[0054] The operating principle is based on the possibility of being capable of effectively separating the densified / granular biomass from wastewater due to its sedimentability features and of being capable of replacing the electron acceptor, which is conventionally oxygen, by nitrite / nitrate formed in the intermediate step e) of nitritation combined with a nitratation process.

[0055] Therefore, the process is very suitable to the treatment of wastewater with suitable C / N ratios and allows to limit oxygen consumption to what is strictly necessary for autotrophic nitrogen oxidation, which takes place inside the second reactor and during the above-mentioned step e) of aerobic transformation.

[0056] Preferably, the first reactor comprises a bottom and a top, wherein, during the feeding step a), said granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities reside on the bottom of said first reactor and said COD-loaded wastewater batch is fed at the bottom of said first reactor.

[0057] More preferably, during the discharging step d), said wastewater batch depleted of COD and possibly enriched with phosphates is discharged from the top of said first reactor.In an equally preferred manner, during step g) of transformation under anoxic conditions, said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, is fed at the bottom of said first reactor.

[0058] More preferably, during the optional discharging step i), said purified wastewater batch is discharged from the top of said first reactor.

[0059] Preferably, the feeding step a) is carried out without mechanical mixing. Preferably, during the feeding step a), said COD-loaded wastewater batch has a volume between 10% and 70%, more preferably between 10% and 50% of the operative capacity of said first reactor, even more preferably between 20% and 40% of the operative capacity of said first reactor. According to the present invention, the expression “operative capacity” means the volume of the fluid mass that is inside the first reactor after feeding said COD-loaded wastewater batch during the feeding step a), to which the volume of the mass of granular / densified activated sludges inside said first reactor during the feeding step a) must be added.

[0060] As specified, the process for the treatment of wastewater based on granular / densified biomass according to the present invention may be carried out repeatedly and according to a plurality of consecutive cycles, wherein in each cycle and in said feeding step a), said granular and / or densified activated sludges correspond to said activated sludge, possibly containing phosphorus, that has been precipitated during step h) of precipitation carried out in the preceding cycle.

[0061] Therefore, it is understood that the expression “operative capacity” can also mean a fluid mass that is inside the first reactor after feeding said COD-loaded wastewater batch during the feeding step a), to which the volume of the phosphorus-containing activated sludge that has been precipitated during step h) of precipitation carried out in the preceding cycle in the first reactor must be added.Preferably, during step b) of transformation under anaerobic conditions, the temperature inside the first reactor is between 10-30°C, more preferably between 18-22°C.

[0062] Preferably, during step e) of transformation under aerobic conditions, the temperature inside the second reactor is between 10-30°C, more preferably between 18-22°C.

[0063] Preferably, during step g) of transformation under anoxic conditions, the temperature inside the first reactor is between 10-30°C, more preferably between 18-22°C.

[0064] In general, said COD-loaded wastewater batch may comprise wastewater of civil and / or industrial origin, preferably said wastewater of industrial origin comprising butchering industrial wastewater, dairy-industry wastewater, textile-industry wastewater, zootechnical-origin wastewater, pharmaceutical-industry wastewater, and landfill leachate.

[0065] In general, therefore, the process for the treatment of wastewater based on granular / densified biomass according to the present invention allows to obtain the following advantages:

[0066] lower energy consumption compared to processes based on conventional activated sludges and aerobic granular activated sludges through the complete replacement of oxygen by nitrite, nitrate or other oxidized compounds as electron acceptors;

[0067] - reduction of the production of waste activated sludge to be handled and disposed of compared to the processes based on conventional activated sludges and aerobic granular activated sludges;

[0068] - reduction of the fraction of organic carbon that is mineralized during the oxidation process, with a possible conversion of a fraction into methane that can be valorized through fermentative processes; and - possible effective biological removal of phosphorus and possiblerecovery thereof from the waste activated sludge.

[0069] According to a preferred embodiment, the present invention comprises a step of conversion of a fraction of organic carbon into volatile fatty acids and / or methane through fermentative processes, wherein said step of conversion of a fraction of organic carbon into volatile fatty acids and / or methane takes place during step b) of transformation under anaerobic conditions and through the action of fermentative microorganisms, preferably acidogenic and / or methanogenic microorganisms, which are comprised in said granular and / or densified activated sludges, thereby causing an enrichment of said wastewater batch with a solubilized gaseous mixture rich in volatile fatty acids and / or methane.

[0070] Compared to the process carried out in the absence of a step of conversion of a fraction of organic carbon into volatile fatty acids and / or methane, the embodiment at issue involves a higher C / N ratio, in particular with a higher carbon amount being available for the process.

[0071] The enabling factor from the point of view of the process is the proper definition of the duration of the cycle times, such as to promote on one hand substrate hydrolysis and fermentation, and on the other hand the rapid consumption of nitrite / nitrate so that it does not penetrate into the innermost parts of the granule.

[0072] In addition, maintaining strictly anoxic or anaerobic conditions allows the granular / densified biomass to be enriched with fermentative microorganisms, which are capable of transforming the fermentable organic substance first into volatile fatty acids and then in biogas (mixture of CH4 and CO2), similarly to what happens in an anaerobic-digestion process.

[0073] Preferably, when said step of conversion of a fraction of organic carbon into volatile fatty acids and / or into methane is carried out and after said step h) of precipitation, the wastewater batch thereby purified and - at the same time - enriched with a solubilized gaseous mixture rich involatile fatty acids and / or methane is subjected to a separation operation in which said solubilized gaseous mixture is removed.

[0074] More preferably, said operation of separation of the solubilized gaseous mixture rich in volatile fatty acids and / or methane may be carried out inside said first reactor or in a further unit designed to carry out said separation operation, wherein said further unit is arranged downstream of the first reactor and in fluid communication therewith.

[0075] Even more preferably, said unit designed to carry out said separation operation is a stripping or degasification unit, in particular a unit of stripping through carrier gas.

[0076] According to an alternative embodiment, the present invention may comprise a preliminary step of feeding said first reactor with a carbon fraction already pre-fermented to volatile acids and a subsequent step of conversion of said volatile acids into methane by fermentative processes through the action of fermentative methanogenic microorganisms comprised in said granular and / or densified activated sludges, thereby causing an enrichment of said wastewater batch with a solubilized gaseous mixture rich in methane.

[0077] In a further preferred manner, in accordance with any variant of the above-mentioned present invention, an operation of taking out and separation of the waste activated sludge formed during the above-mentioned step b) of transformation under anaerobic conditions may be carried out.

[0078] Advantageously, since organic carbon is stored in intracellular spaces of the GAO microorganisms in the form of biopolymers, such as for example polyhydroxyalkanoates, a further fraction of COD can be recovered, thereby avoiding its mineralization.

[0079] In one aspect, the present invention relates to plant designed to carry out the above-described process, comprising:- a first reactor, in which there are granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities, said first reactor being possibly equipped with first-reactor stirring means and being designed to carry out the transformation, under anaerobic conditions, of COD-loaded wastewater into wastewater depleted of COD and possibly enriched with phosphates and the transformation, under anoxic conditions, of wastewater enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly enriched in nitrates and depleted of organic and / or ammonia nitrogen, into purified wastewater;

[0080] - a second reactor, in fluid communication with said first reactor, said second reactor being possibly equipped with second-reactor stirring means and being designed to carry out the transformation, under aerobic conditions, of wastewater depleted of COD and possibly enriched with phosphates into wastewater enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly enriched in nitrates and depleted of organic and / or ammonia nitrogen.

[0081] According to a totally preferred embodiment, the first reactor is a constant-volume sequencing batch reactor (SBR) and, more in particular, the second reactor is an aerobic reactor of nitritation possibly combined with a nitratation process.

[0082] When present, said first-reactor stirring means may be and said second-reactor stirring means are designed to be in a deactivated state or in an activated state.

[0083] Preferably, said first reactor comprises a bottom, a top, a first inlet opening, and a first outlet opening, wherein said first inlet opening is positioned at said bottom and said first outlet opening is positioned at said top, said granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities residing on the bottom of said first reactor, when said possible stirring means are present and in a deactivated stateSpecifically, said first inlet opening is designed to feed a COD-loaded wastewater batch into said first reactor and said first outlet opening is designed to discharge a wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor.

[0084] In an equally preferred manner, said first reactor comprises a second inlet opening and a second outlet opening, wherein said second inlet opening is positioned at said bottom and said second outlet opening is positioned at said top.

[0085] Specifically, said second inlet opening is designed to feed a wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, into said first reactor, and said second outlet opening is designed to discharge a purified wastewater batch from said first reactor.

[0086] More preferably, the plant according to the present invention comprises a first manifold, which is in fluid communication with said first outlet opening of the first reactor and with said second reactor, said first manifold being designed to send a wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor to said second reactor.

[0087] In an equally preferred manner, the plant according to the present invention comprises a second manifold, which is in fluid communication with said second reactor and with said second inlet opening of said first reactor, said second manifold being designed to send a wastewater batch enriched in nitrites and / or nitrates and depleted of organic and / or ammonia nitrogen, or possibly a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, from said second reactor to said first reactor.

[0088] According to a preferred embodiment, said plant comprises a control system, said control system being configured to control the actuation ofthe possible said first-reactor mixing means and of the possible said second-reactor mixing means, and also to control the movement of fluids to and from said first reactor and to and from said second reactor.

[0089] Preferably, said first reactor may comprise at least one monitoring probe, said at least one monitoring probe being adapted to monitor the chemicalphysical parameters of the process of treatment, such as pH and the oxidation-reduction potential (ORP), wherein said at least one monitoring probe is designed to communicate with said control system.

[0090] The regulation of flows entering and exiting said first reactor is adjusted through designated completely conventional valves.

[0091] It is understood that the same obtained advantages described above in connection with the process of the invention can be obtained through the use of the above-mentioned plant.

[0092] Preferably, in accordance with a preferred embodiment of the plant according to the present invention, the granular and / or densified sludges contained in said first reactor also comprise fermentative microorganisms, more preferably acidogenic and / or methanogenic fermentative microorganisms.

[0093] More preferably, said first reactor comprises means designed to discharge and collect a gaseous mixture rich in volatile fatty acids and / or methane and / or said first reactor comprises means designed to carry out an operation of separation of the solubilized gaseous mixture rich in volatile fatty acids and / or methane which was obtained by carrying out a step of conversion of a fraction of organic carbon into volatile fatty acids and / or methane, and / or said first reactor comprises a further unit designed to carry out said operation of separation of the solubilized gaseous mixture rich in volatile fatty acids and / or methane thereby obtained, wherein said further unit is arranged downstream of the first reactor and in fluid communication therewith.

[0094] Even more preferably, said further unit is a stripping or degasificationunit.

[0095] Consistently and in an equally preferred manner, said first reactor comprises a third outlet opening, which is designed to discharge said gaseous mixture rich in volatile fatty acids and / or methane from said first reactor, and possibly a collecting reservoir, as well as a manifold, wherein said manifold is in fluid communication with said first reactor through the third outlet opening.

[0096] Advantageously, according to the embodiment just described, it is possible to convey and collect said gaseous mixture rich in volatile fatty acids and / or methane to temporarily store it, while waiting further treatments such as carrying out a further separation step to separate volatile fatty acids and / or methane from the other gases that are in the gaseous mixture.

[0097] The present invention will be further described with reference to a drawing and to an example provided by way of non-limiting illustration.

[0098] Brief description of the drawings

[0099] Figure 1 is a schematic representation of a plant for carrying out the process according to the present invention.

[0100] Figure 2 shows a schematic representation through a flow chart of the noteworthy steps of the process of the present invention.

[0101] Figure 3 is a schematic representation of a plant for carrying out the process according to a particular embodiment of the invention, wherein the granular and / or densified biomass contained in the first reactor also contains acidogenic and methanogenic fermentative microorganisms. Figure 4 is a graph showing the pH variation as a function of time inside the first reactor during step a) of feeding and b) of transformation under anaerobic conditions (left) and during step f) of feeding and g) of transformation under anoxic conditions (right) .Figure 5 is a graph showing the variation of the oxidation-reduction potential (ORP) as a function of time inside the first reactor during step a) of feeding and b) of transformation under anaerobic conditions (left) and during step f) of feeding and g) of transformation under anoxic conditions (right).

[0102] Detailed description

[0103] With reference to Figure 1, a plant 1 designed to carry out the process according to the present invention is depicted.

[0104] Said plant is configured according to a preferred embodiment of the invention with particular reference to the presence of mixing means in the first reactor R and in the second reactor R’, said means being conventionally completely optional.

[0105] The plant 1 comprises a first reactor R, in which granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities reside.

[0106] The first reactor is optionally equipped with first-reactor stirring means, not shown since they are completely conventional, and it is designed to carry out the transformation, under anaerobic conditions, of COD-loaded wastewater into wastewater depleted of COD and possibly enriched with phosphates and the transformation, under anoxic conditions, of wastewater enriched in nitrites and nitrates and possibly depleted of organic and / or ammonia nitrogen into purified wastewater.

[0107] The plant 1 also comprises a second reactor R’, in fluid communication with the first reactor R.

[0108] The second reactor R’ is optionally equipped with second-reactor stirring means, not shown since they are completely conventional, and it is designed to carry out the transformation, under aerobic conditions, of wastewater depleted of COD and possibly enriched with phosphates into wastewater enriched in nitrites and nitrates and depleted of organicand / or ammonia nitrogen.

[0109] The wastewater flow that can be fed to the first reactor R during the feeding step a) is indicated in Figure 1 with the Roman numeral i).

[0110] The flow i) is influent wastewater loaded with organic pollutants (measured as COD), nitrogenous pollutants (mainly ammonium NH4+and organic nitrogen), and phosphorus (mainly phosphate PO43-and organic phosphorus).

[0111] The wastewater flow exiting the first reactor R and fed to the second reactor R’ during step e) is indicated in Figure 1 with the Roman numeral ii).

[0112] The flow ii) is wastewater low in COD, rich in organic and / or ammonia nitrogen and possibly enriched with phosphates.

[0113] The wastewater flow exiting the second reactor R’ and fed to the first reactor R during step g) is indicated in Figure 1 with the Roman numeral iii).

[0114] The flow iii) is wastewater low in COD, rich in nitrites, nitrates and phosphates.

[0115] The wastewater flow exiting the first reactor R and that is discharged therefrom during step i), namely during step a) of the successive execution cycle, is indicated in Figure 1 with the Roman numeral iv). The flow iv) is purified effluent wastewater, low in COD, nitrogenous compounds and phosphates.

[0116] The first reactor R is a constant-volume sequencing batch reactor (SBR) and, more in particular, the second reactor R’ is an aerobic reactor of nitritation / nitratation.

[0117] ft is restated that also the possibility of making the first reactor R as a constant-volume SBR reactor is completely optional.The first reactor R comprises a bottom 2, a top 3, a first inlet opening 4, and a first outlet opening 5.

[0118] The first inlet opening 4 is positioned at the bottom 2 and the first outlet opening 5 is positioned at the top 3.

[0119] The first inlet opening 4 is designed for the inlet of flow i). The first outlet opening 5 is designed for the expulsion of flow ii) .

[0120] When the possible first-reactor stirring means are in a deactivated state, the granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities reside on the bottom 2. The first reactor R also comprises a second inlet opening 6 and a second outlet opening 7, wherein the second inlet opening 6 is positioned at the bottom 2 and the second outlet opening 7 is positioned at the top 3. The second inlet opening 6 is designed for the inlet of flow iii) . The second outlet opening 7 is designed for the expulsion of flow iv) .

[0121] The plant 1 further comprises a first manifold 8, which is in fluid communication with the first outlet opening 5 and with the second reactor R’. The first manifold 8 is designed to send flow ii) from the first reactor R to the second reactor R’.

[0122] The plant 1 also comprises a second manifold 9, which is in fluid communication with the second reactor R’ and with the second inlet opening 6. The second manifold 9 is designed to send flow iii) from the second reactor R’ to the first reactor R.

[0123] The plant 1 comprises a control system, not shown, where said control system is configured to control the movement of flows i) to iv), and to control the actuation of the possible first-reactor mixing means and of the second-reactor mixing means, as provided according to the preferred embodiment according to the present invention, herein described.

[0124] In general, the flow rates are variable and adjustable by the overseer ofthe system according to wastewater composition.

[0125] In general, in reactors with granular biomass, there is an exchange of wastewater volume varying between 20 and 40% of the total water volume of the reactor itself, although, in extreme cases, it is possible to increase feeding in the short term up to 50% or more for particular treatment needs (for example, in the case of a too-diluted pollutant load and / or of peaks of flow rate coming from the sewer) .

[0126] With reference to Figure 2, a flow chart in which the noteworthy steps of the process of the invention is shown.

[0127] At first, the feeding step a) is carried out, during which wastewater containing pollutants (organic carbon or COD, nitrogen, phosphorus) is fed to the first reactor from the bottom of the same, thereby exposing the bed of granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities initially sedimented at a high gradient of concentration of the compounds present in the influent, namely in the COD-loaded wastewater batch.

[0128] At the same time, through a taking-out location placed in the upper part of the reactor, namely through a second outlet opening, the wastewater treated by the preceding cycle and still present in the first-reactor body is discharged.

[0129] During this step, which is carried out without mechanical mixing, a piston vertical-rising flow is maintained by feeding the first COD-rich wastewater batch and avoiding the resuspension of the bed of granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities. During this step, mixing between the fed water volume and the already present volume is also prevented.

[0130] Step b) of transformation under anaerobic conditions is then carried out, during which granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities are mechanically mixed inside the first reactor so as to homogenize the entirevolume and to promote the intimate contact of the biomass with the components in liquid phase, in other words with the nutrients introduced through the COD-rich wastewater batch fed to the first reactor.

[0131] In this step, COD is sequestered by bacteria that accumulate under anaerobiosis conditions, and phosphates previously accumulated by the phosphorus-accumulating microorganisms (PAO) are generally released into solution.

[0132] Step c) of sedimentation follows, during which a first sedimentation of the activated sludge through the interruption of mixing takes place. The densified / granular activated sludge accumulates at the bottom due to gravity to form a compact bed.

[0133] Thereafter, step d) of discharging and step e) of treatment under aerobic conditions take place inside the second reactor.

[0134] During step e) of treatment under aerobic conditions, wastewater is fed to the second reactor and is processed in the nitritation / nitratation reactor, which is operated in series with respect to the first reactor.

[0135] During step e), nitritation of organic and / or ammonia nitrogen takes place, possibly combined with a step of nitratation, to generate nitrates (NO3 ) and nitrites (NO2 ). To obtain this transformation, wastewater is aerated, thus providing oxygen to bacteria.

[0136] Step f) of discharging and step g) of treatment under anoxic conditions follow, wherein, during step g), loading of wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen into the first reactor takes place in the same modalities previously described for step a) of feeding.

[0137] In this step, degradation of the soluble organic carbon sequestered during step b) of treatment under anaerobic conditions takes place. Said carbon degradation takes place through denitrification processes carried out by denitrificating accumulating organisms, including phosphorus-accumulating bacteria (PAO), which, during oxidation of the stored organic compounds, absorb the previously released phosphates.

[0138] Nitrate and nitrite in the wastewater are used to degrade the COD stored during step b), converting it into gaseous N2. Phosphate is stored by PAO bacteria during their metabolism. In step g), from the point of view of the granular biomass, there are consumption of the COD stored in the biomass in the step b), and a concurrent phosphate absorption and storage from the liquid phase.

[0139] Finally, step h) of sedimentation is carried out, carrying out a second sedimentation of the granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities inside the first reactor and through the interruption of mixing.

[0140] The granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities accumulate at the bottom due to gravity to form a compact bed.

[0141] Then, step i) of discharging can be carried out, which can occur at the same time of the step a) of feeding a new COD-loaded wastewater batch during a successive execution cycle of the process of the invention.

[0142] Figure 3 schematically shows a plant 1’ for carrying out the process according to a particular embodiment of the invention, wherein the granular and / or densified biomass contained in the first reactor also contains acidogenic fermentative microorganisms and methanogenic fermentative microorganisms.

[0143] In addition to a first reactor R” and a second reactor R’”, all the components already described with reference to plant 1 are comprised in plant 1’ and are indicated in Figure 3 with corresponding reference numbers 2’, 3’, 4’, 5’, 6’, 7’, 8’ and 9’.

[0144] In particular, the first reactor R” comprises means 10 designed to carry out an operation of separation of the solubilized gaseous mixture rich in volatile fatty acids and / or methane which was obtained by carrying outa step of conversion of a fraction of organic carbon into volatile fatty acids and / or methane.

[0145] The first reactor R” also comprises a third outlet opening 11 , which is designed to discharge the gaseous mixture rich in volatile fatty acids and / or methane from the first reactor R”, once said mixture has been separated from the wastewater contained in said first reactor R” and accumulated at the top 3’, just above the waterline P, which is also shown schematically.

[0146] The plant 1’ further comprises a collecting reservoir 13, as well as a manifold 12, wherein the manifold 12 is in fluid communication with the first reactor R” through the third outlet opening 11.

[0147] The second reactor R’” is designed to carry out a step of feeding the wastewater batch depleted of COD and possibly enriched with phosphates, during which the wastewater batch depleted of COD and possibly enriched with phosphates is subjected to a nitritation step combined with a nitratation step under aerobic conditions, thereby obtaining a wastewater batch that is enriched in nitrates, substantially devoid of nitrites, and depleted of organic and / or ammonia nitrogen. The present invention is further described with reference to some examples of carrying out the process, which are only provided by way of non-limiting illustration.

[0148] Said examples can be applied to any other type of COD-loaded wastewater.

[0149] EXAMPLE 1

[0150] In the modalities described above in connection with the detailed description, the process of the invention was carried out in a pilot plant at laboratory scale wherein the first reactor was an AnoxGS reactor (reactor R) with an operative capacity of 5 L.

[0151] The execution of the step of transformation under aerobic conditions inside the reactor of nitritation possibly combined with a nitratation process under aerobic conditions was simulated based on the dosage ofnitrate in the flow fed to the first reactor before the execution of the transformation under anoxic conditions.

[0152] The wastewater fed to the system consisted of a recipe of synthetic wastewater prepared with laboratory reagents.

[0153] According to the present example, during the step of feeding the wastewater into the first reactor, it was calculated that the exchange of wastewater volume was 40% of the operative capacity of the first reactor. The operating conditions calculated in terms of concentrations of chemicals within the flows of the system (with reference to Figure 1) are shown in Table 1 below.

[0154] COD N-NH4+N-NO3- / N- P-PO43- [mg / L] [mg / L] NO2- [mg / L] [mg / L]

[0155] Flow i) - Influent 150 ± 25 + 5 0 10 ± 1

[0156] 25

[0157] Flow ii) - First-reactor 20 ± 10 23 + 5 0 15 ± 2 effluent

[0158] Flow iii) - Second-reactor 5 ± 2 2 ± 1 20 ± 5 14 ± 2 effluent

[0159] Flow iv) - Final effluent 3 ± 1 2 ± 1 0 1 ± 0.5

[0160]

[0161] Table 1

[0162] With reference to Figures 4 and 5, the typical profile of the process parameters is shown.

[0163] The parameters pH (Figure 4) and oxidation-reduction potential (ORP, Figure 5) are essential to understand the biochemical processes occurring in the different treatment steps.Figures 4 and 5 show the typical profile of an AnoxGS reactor (reactor R) during step b) and step g), recorded during its operation under stability conditions. To promote COD storage by the biomass (step b), negative ORP values, advantageously lower than -50 mV, which are associable to anaerobic conditions of the treated wastewater, are necessary.

[0164] EXAMPLE 2

[0165] The test carried out in Example 2 was carried out again inside the same pilot plant at laboratory scale, carrying out a step of COD removal through methanogenesis by means of methanogenic fermentative microorganisms present in the granular biomass.

[0166] The execution of the step of transformation under aerobic conditions inside the reactor of nitritation possibly combined with a nitratation process under aerobic conditions was simulated based on the dosage of nitrate in the flow fed to the first reactor before the anoxic step.

[0167] In this case too, the nitratation step was simulated through the use of a laboratory-created artificial solution. Although Table 1 shows both the presence of nitrites and of nitrates, it is possible to simplify by essentially considering only the presence of nitrates, since the presence of nitrites was always close to zero.

[0168] The wastewater fed to the system consisted of a recipe of synthetic wastewater prepared with laboratory reagents.

[0169] According to the present example, during the step of feeding the wastewater into the first reactor, the exchange of wastewater volume was imposed to be 40% of the operative capacity of the first reactor.

[0170] The operating conditions calculated in terms of concentrations of chemicals within the flows of the system correspond to those already shown in Table 1.

[0171] The relative contributions to COD removal that are attributable todenitrification and methanogenesis processes are shown in Table 2 below, in which they are expressed as COD mass removed per treatment cycle. _

[0172] Mass Percentage of [mg / cycle] total [%]

[0173] Total removed COD 240 ± 38 100 COD removed by 61 ± 30 76 ± 11 denitrification

[0174] COD removed by 179 ± 26 24 ± 11

[0175]

[0176] methanogenesis

[0177] Table 2

[0178] The typical profile of the process parameters, recorded during an operation thereof under stability conditions, corresponds to the one previously discussed in connection with Figures 4 and 5.

Claims

CLAIMS1. A process for the treatment of wastewater, comprising the following steps:a) feeding a COD-loaded wastewater batch into a first reactor (R; R”), in which there are granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities;b) in said first reactor (R; R”), putting said COD-loaded wastewater batch in contact with said granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities, subjecting said COD-loaded wastewater batch to a step of transformation under anaerobic conditions through the action of said PAO and / or GAO bacteria, COD being sequestered by the same, preferably said first reactor (R; R’j having an internal temperature between 10-30°C, more preferably between 18-22°C;c) separating said COD-containing granular and / or densified activated sludges by precipitation, thereby obtaining a wastewater batch depleted of COD and possibly enriched with phosphates, said phosphates being released by said PAO bacteria;d) discharging said wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor (R; R’j;e) feeding said wastewater batch depleted of COD and possibly enriched with phosphates into a second reactor (R’; R”j and subjecting said wastewater batch depleted of COD and possibly enriched with phosphates to a nitritation step possibly combined with a nitratation step under aerobic conditions, thereby obtaining a wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly obtaining a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, preferably said second reactor (R’; R”j having an internal temperature between 10-30°C, more preferablybetween 18-22°C;f) discharging said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, from said second reactor (R’; R’”);g) feeding said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, to said first reactor and, in said first reactor, putting said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, in contact with said COD-containing granular and / or densified activated sludges, so as to bring about a step of transformation under anoxic conditions through the action of said PAO / GAO bacteria, phosphorus being possibly sequestered by the same, preferably said first reactor (R; R”) having an internal temperature between 10-30°C, more preferably between 18-22°C;h) separating said granular and / or densified activated sludges possibly containing phosphorus by precipitation, thereby obtaining a purified wastewater batch, said purified wastewater batch being depleted of COD, nitrogen, and possibly phosphorus.

2. The process for the treatment of wastewater according to claim 1 comprises the following additional step:i) discharging said purified wastewater batch from said first reactor (R; R”).

3. The process for the treatment of wastewater according to claim 1 or 2, wherein said process is carried out repeatedly and according to a plurality of successive cycles, and wherein, during said feeding step a) of each successive cycle, a COD-loaded wastewater batch is fed to the firstreactor (R; R”), said COD-loaded wastewater batch being different from the COD-loaded wastewater batch fed to the first reactor (R; R”) during the feeding step a) carried out in the preceding cycle or in the preceding cycles.

4. The process for the treatment of wastewater according to claim 3, wherein, in said feeding step a) of each successive cycle, said densified and / or granular activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities correspond to granular and / or densified activated sludges, possibly containing phosphorus, that have been precipitated during the step h) of separation by precipitation carried out in the preceding cycle.

5. The process for the treatment of wastewater according to any one of the previous claims, wherein the first reactor (R, R”) comprises a bottom (2; 2’) and a top (3; 3’) and wherein, during the feeding step a), said densified and / or granular activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities reside on the bottom (2; 2’) of said first reactor (R; R”) and said COD-loaded wastewater batch is fed at the bottom (2; 2’) of said first reactor (R; R”), preferably during the discharging step i) said purified wastewater batch being discharged from the top (3; 3’) of said first reactor (R; R”).

6. The process for the treatment of wastewater according to claim 5, wherein, during step g) of transformation under anoxic conditions, said wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, is fed at the bottom (2) of said first reactor (R; R”), preferably during the discharging step d), said wastewater batch depleted of COD and possibly enriched with phosphates being discharged from the top (3; 3’) of said first reactor (R; R”).

7. The process for the treatment of wastewater according to any one of the previous claims, wherein, during the feeding steps a) and f), saidwastewater batch loaded with COD and said wastewater batch loaded with nitrates and nitrites and depleted of organic and / or ammonia nitrogen, or possibly said wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, have a volume between 10% and 50%, preferably between 20% and 40%, of the operative capacity of said first reactor (R; R”) .

8. The process for the treatment of wastewater according to any one of the previous claims, wherein a reduction of the fraction of organic carbon that is mineralized during the oxidation process takes place with conversion of a fraction into methane that can be valorized through fermentative processes.

9. A plant (1; 1’) designed to carry out the process according to any one of claims 1 to 8, said plant (1; 1’) comprising:- a first reactor (R; R”), in which there are granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities, said first reactor (R; R”) being possibly equipped with first-reactor stirring means and being designed to carry out the transformation, under anaerobic conditions, of COD-loaded wastewater into wastewater depleted of COD and possibly enriched with phosphates and the transformation, under anoxic conditions, of wastewater enriched in nitrates and nitrites and depleted of organic and / or ammonia nitrogen, possibly enriched in nitrates and depleted of organic and / or ammonia nitrogen, into purified wastewater;- a second reactor (R’; R’”), in fluid communication with said first reactor, said second reactor (R’; R’”) being possibly equipped with second-reactor stirring means and being designed to carry out the transformation, under aerobic conditions, of wastewater depleted of COD and enriched with phosphates into wastewater enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, possibly enriched in nitrates and depleted of organic and / or ammonia nitrogen.

10. The plant (1; 1’) according to claim 9, wherein said second reactor (R’; R’”) being an aerobic reactor of nitritation / nitratation.

11. The plant (1; 1’) according to claim 9 or 10, wherein said first reactor (R; R”) comprises a bottom (2; 2’), a top (3: S’), a first inlet opening (4; 4’), and a first outlet opening (5; 5’), wherein said first inlet opening (4; 4’) is positioned at said bottom (2; 2’) and said first outlet opening (5; S’) is positioned at said top (3; 3’), said granular and / or densified activated sludges comprising PAO and / or GAO bacteria in mixed microbial communities residing on the bottom (2; 2’) of said first reactor (R) when said stirring means are in a deactivated state, said first inlet opening (4; 4’) being designed to feed a COD-loaded wastewater batch into said first reactor (R; R”) and said first outlet opening (5; 5’) being designed to discharge a wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor (R; R”).

12. The plant (1; 1’) according to claim 11, wherein said first reactor (R; R”) comprises a second inlet opening (6; 6’) and a second outlet opening (7; 7’) and wherein said second inlet opening (6; 6’) is positioned at said bottom (2; 2’) and said second outlet opening (7; 7’) is positioned at said top (3; 3’), said second inlet opening (6; 6’) being designed to feed a wastewater batch loaded with nitrites and nitrates, possibly a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, into said first reactor (R; R”), and said second outlet opening (7; 7’) being designed to discharge a purified wastewater batch from said first reactor (R; R”).

13. The plant (1; 1’) according to claim 12, wherein said plant (1; l’) comprises a first manifold (8; 8’), said first manifold (8; 8’) being in fluid communication with said first outlet opening (5; 5’) and with said second reactor (R’; R’”) and said first manifold (8; 8’) being designed to send a wastewater batch depleted of COD and possibly enriched with phosphates from said first reactor (R; R”) to said second reactor (R’; R’”), and wherein said plant (1; 1’) comprises a second manifold (9; 9’), said second manifold (9; 9’) being in fluid communication with said secondreactor (R’; R’”) and with said second inlet opening (6; 6’) and said second manifold (9; 9’) being designed to send a wastewater batch enriched in nitrites and nitrates and depleted of organic and / or ammonia nitrogen, or possibly a wastewater batch enriched in nitrates and depleted of organic and / or ammonia nitrogen, from said second reactor (R’; R’”) to said first reactor (R; R”).

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