WO2003106355A2

WO2003106355A2 - Reactor for thermophilic biological sludge degradation

Info

Publication number: WO2003106355A2
Application number: PCT/FR2003/001819
Authority: WO
Inventors: Vincent Savall; Stéphane Humbert
Original assignee: Ondeo Degremont
Priority date: 2002-06-18
Filing date: 2003-06-16
Publication date: 2003-12-24
Also published as: FR2840895A1; AU2003263240A1; FR2840895B1; WO2003106355A3

Abstract

The invention concerns a reactor for thermophilic biological degradation of sludge derived from biological treatment installations of urban or industrial waste water, by enzymatic degradation. The invention is characterized in that it comprises physical agitation (5, 6) by gas injection, of the effluent contained therein and means for regulating (8, 10) oxygen inputs based on specific requirements of the biomass ensuring degradation of residual organic matter, said oxygen input regulation being independent of the effluent stirring by gas injection.

Description

"Thermophilic biological degradation reactor for sludge"

The present invention relates to a thermophilic biological degradation reactor for sludge extracted from biological treatment plants for urban or industrial wastewater by enzymatic degradation.

We know that biological reactions are universally implemented to treat biological, domestic or industrial pollution, and that they lead to the production of significant quantities of biomass (biological sludge) resulting from the growth of the bacterial species used for degradation of residual organic matter. Recent regulations relating to restrictions on the landfill of this biological sludge and the obligation to treat and stabilize it, lead to the implementation of techniques for Reducing Sludge Production (abbreviated RPB), the importance of which in the technical and economic directions of the Environment industries is increasing.

Those skilled in the art have many technologies to reduce the production of sludge extracted from biological treatment facilities for urban or industrial wastewater, these technologies ranging from incineration techniques to endogenous enzymatic hydrolysis processes, including natural or thermal composting and oxidative hydrolysis with or without physical alteration of cell membranes.

Recent studies concerning BPS show that the processes using biological techniques of BPS are more and more favored by environmental professionals due, on the one hand to the effectiveness of the treatments and, on the other hand the low operating costs of said biological techniques.

Experience in this field, however, reveals that these techniques are still insufficiently optimized and that many problems remain to be resolved, in particular with regard to the operation of reactors. Among these technical problems to be solved, the following may be mentioned in particular:

- the thermal and mechanical energy consumption of agitation systems whose order of magnitude is

100 Watts / m ³ ;

- regulation of the quantity of oxygen necessary for bacteria ensuring the degradation of residual organic matter; - regulating the reactor temperature;

- control of foaming phenomena and

- the selection and activation of specific bacteria (for example Bacillus stearothermophilus) in microaerophilic conditions. The present invention therefore proposes to provide a reactor which in particular solves the technical problems thus mentioned. Consequently, this invention relates to a thermophilic biological degradation reactor for sludge extracted from biological treatment plants for urban or industrial waste water, by enzymatic degradation, characterized in that it comprises physical agitation, by injection of gas, of the effluent that it contains and means for regulating the oxygen supply according to the specific needs of the biomass ensuring the degradation of the residual organic matter, this regulation of the oxygen supply being independent of the agitation of the effluent by injection of gas .

According to a characteristic of the invention, the reactor comprises, at its upper part, a probe for measuring the oxygen content of the gas making it possible to regulate the quantity of oxygen supplied to the reaction system, by action on a valve connected to the oxygen supply.

According to another characteristic of the invention, the reactor includes a circuit for recycling the vent gases from the reactor, so as to ensure stirring of the latter as well as heating of the effluent that it contains.

Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate two embodiments thereof without any limiting character. In the drawings:

- Figure 1 of the drawing schematically shows a first embodiment of a reactor according to the invention and Figure 2 is also a schematic representation of a second embodiment. In the figures, the reference 1 designates the reactor in which the sludge suspension to be treated, for example coming from a conventional treatment plant using activated sludge, is introduced in 2, after prior thickening, by means of a heat exchanger 3, with cross circulation with the effluent from the reactor 1 admitted into the exchanger via a pipe 4.

It is possible, according to the invention, to provide on the effluent supply circuit, an additional exchanger 17 supplied with heating steam, which can be used during the start-up and / or back-up phases.

According to the invention, the reactor 1 is subjected to agitation by gas injection using a device shown diagrammatically in 5 which is equipped with distributor elements 6 and which is supplied from a blower pump 7 .

A valve 8, connected to an oxygen storage means 9 (liquid oxygen or compressed air), provides the oxygen supply. The reactor further comprises, at its upper part, a probe 10 for measuring the oxygen content of the gas which makes it possible to regulate, as a function of specific needs of the biomass contained in the reactor, the quantity of oxygen supplied to the reaction system. via valve 8.

The quantity of oxygen which is supplied to the reaction system via the valve 8 can also be regulated by the following means: - a probe for measuring the pH of the liquid fraction, a measurement which makes it possible to detect the variation in the concentration in carbon dioxide (when this concentration increases, the pH decreases and vice versa), - a probe for measuring the redox potential of the liquid fraction in order to maintain it at a desired reference level, and

- a probe for measuring the concentration of dissolved oxygen in the liquid fraction, in order to maintain it at a desired set level.

The two exemplary embodiments illustrated respectively in FIGS. 1 and 2 differ only in the mode of oxygen supply, as is clearly shown in these figures.

The process for the reduction of sludge production by the enzymatic route implemented in this reactor is of the type described in publications EP 0 924 168 and 1 008 558. This process is based on the development of a specific bacterium Bacillus stearothermophilus SPT 2-1 . In its non-sporulated form, this bacterium produces under aerobic or micro-aerobic conditions enzymes such as proteases which act on the walls of mesophilic organisms contained in activated sludge. The enzymatic reactor 1 must be maintained at a set temperature (between 50 and 70 ° C) fixed so as to allow optimal development of the specific bacterial population.

Although according to the invention, energy recovery is carried out by virtue of the presence of the cross-current exchanger 3, which makes it possible to preheat the sludge supplying the reactor 1 using the sludge extracted from the reactor through line 4, the system thermal balance is in deficit and it is therefore necessary to provide additional heat.

In fact, the thermal losses of the system are broken down as follows: • losses by convection of the reactor and by the pipes of the installation;

• losses by water saturation of the gas introduced into the reactor and • temperature rise of the feed sludges preheated to the set temperature set. At the present time, the enzymatic reactors implementing such a RPB process have been designed as follows: - heat addition by injection of steam into the sludge supplying the reactor and injection of compressed air by means of a distribution means (floor) or a hydro-ejector ensuring the recirculation of the reactor sludge. In these known techniques, the reactor vents are either sent to a deodorization unit (local deodorization unit or unit of the treatment station) or in the aeration tank, or discharged into the atmosphere. Such a solution is not satisfactory both technically and economically.

According to the present invention, the limitation of the heat losses induced by the injection of gas is obtained by recycling the vents of the reactor, a solution which also ensures mixing of the latter, while maintaining, independently, the necessary supply of oxygen. to the smooth running of the process.

To this end, the reactor 1 according to the invention comprises, in the upper part, a gas purge 15, the flow rate of which is regulated using a valve 11, by means of a pressure measurement probe 12 gas from the reactor. A circuit 13 for recycling the vent gases is provided, this circuit comprising a demister 14 intended to trap aerosols and droplets. The vent gases are then recycled to reactor 1 via line 16.

The characteristics of the enzymatic reactors to which the present invention applies are as follows: hydraulic residence time of the sludge in the reactor: from 24 to 48 hours;

- useful volume of the reactor: 50 to 5000 m ³ ; - operating temperature: from around 50 ° C to around 70 ° C.

According to the present invention, the minimum gas flow rates necessary for mixing the enzyme reactors vary according to the dimensions of the latter from 4 to 10 Nm ³ / hm ² of reactor. The oxygen requirements to maintain the activity of the specific bacterial population are between 2 kg and 300 kg 0 ₂ / h per reactor depending on the size. The recirculation flow rate of the vents will thus be defined as a function of the minimum flow rate necessary for stirring and of the gas flow rate (air, air enriched with oxygen or oxygen) necessary to transfer the oxygen supplied to the reaction system into the reactor.

Among the advantages provided by the present invention, the following may be mentioned in particular:

- a limitation of the thermal deficit of the process thanks to the recirculation of the vents within the reactor, a separate regulation of the necessary oxygen supply, controlled by the measurement of physical parameters of the liquid fraction of the reactor (dissolved oxygen, pH, redox ) or recirculated gas (oxygen content), adapting thus the oxygen concentration in the gas to the needs of the process,

- the possibility of obtaining the transfer of the desired quantity of oxygen in the reactor regardless of the water height and the transfer performance of the distribution member, thus promoting a homogeneous distribution of the gas by means of robust systems ( injection canes, “medium bubble” floor) and limiting the creation of foam as it manifests in the case of air injection systems in the form of fine bubbles,

1 absence of moving mechanical parts inside the reactor.

It remains to be understood that the present invention is not limited to the exemplary embodiments described and shown above, but that it encompasses all variants thereof.

Claims

1 - Thermophilic biological degradation reactor for sludge extracted from biological treatment plants for urban or industrial wastewater, by enzymatic degradation, characterized in that it comprises physical agitation (5, 6), by gas injection, effluent it contains and means for regulating (8, 10) the oxygen supplies as a function of the specific needs of the biomass ensuring the degradation of the residual organic materials, this regulation of the oxygen supplies being independent of the agitation of the effluent by gas injection. 2 - Reactor according to claim 1, characterized in that it comprises, at its upper part, a probe (10) for measuring the oxygen content of the gas making it possible to regulate the quantity of oxygen supplied to the reaction system, by action on a valve (8) connected to the oxygen supply (9).

3 - Reactor according to one of claims 1 or 2, characterized in that the quantity of oxygen which is supplied to the reaction system is regulated by the following means: - a probe for measuring the pH of the liquid fraction, measurement which allows to detect the variation in the carbon dioxide concentration (when this concentration increases, the pH decreases and vice versa), - a probe for measuring the redox potential of the liquid fraction in order to maintain it at a desired reference level and - a probe for measuring the concentration of dissolved oxygen in the liquid fraction, in order to maintain it at a desired set level.

4 - Reactor according to claim 1, characterized in that it comprises a circuit (13) for recycling the vent gases from the reactor, so as to ensure mixing of the latter as well as heating of the effluent that it contains .

5 - Reactor according to claim 4, characterized in that it comprises, in the upper part, a gas purge (15) whose flow is regulated using a valve (11), via a probe (12) for measuring the gas pressure of the reactor, a demister (14) being provided on the recycling circuit (13), the vent gases being recycled into the reactor (1), from the demister, by the 'through a pipe (16).

6 - Reactor according to any one of the preceding claims, characterized in that it comprises a heat exchanger (3), with cross-circulation with the reactor effluent admitted into the exchanger, this heat exchanger ensuring heating of the effluent introduced into the reactor after prior thickening. 7 - Reactor according to any one of the preceding claims, characterized in that the minimum gas flow rates necessary for stirring vary according to the dimensions of the reactor from 4 to 10 Nm ³ / h. per m ² of reactor. 8 - Reactor according to any one of the preceding claims, characterized in that the oxygen requirements to maintain the activity of the specific bacterial population is between 2 kg and 300 kg 0 ₂ / h depending on the size of the reactor.

9 - Reactor according to any one of the preceding claims, characterized in that there is provided on the effluent supply circuit, a complementary exchanger (17) supplied by heating steam, which can be used during start-up and / or backup phases.