FR2844787A1 - Reducing sludge production in wastewater treatment plants comprises using separate means dedicated to sludge reduction and phosphorus precipitation - Google Patents

Abstract

Reducing sludge production in municipal or industrial wastewater treatment plants comprises decoupling sludge reduction from the biological channel of the treatment plant by using separate means dedicated to sludge reduction and phosphorus precipitation. An Independent claim is also included for apparatus for carrying out the above process, comprising a low-, medium- or heavy-duty biological treatment unit in the form of an activated sludge reactor, mixed culture reactor, fixed biomass reactor or membrane bioreactor, a sludge separator, and sludge reduction means receiving sludge from the biological treatment unit.

Description

The present invention relates to a method and a device

  ensuring the reduction of the production of biological sludge from water treatment plants

  urban or industrial waste.

  It is known that the purification of waste water by biological means consists in using organic, nitrogen and phosphorus pollution as a nutritive substrate for the purifying biomass, said carbon and nitrogen pollution being transformed into carbon dioxide and nitrogen, while the biomass proliferates in proportion to the quantity of pollution degraded. The excess of biomass formed constitutes what is called

  commonly excess organic sludge.

  The treatment and final disposal of this sludge is a major environmental and economic issue. With an annual production forecast, in 2002, of more than 1,200,000 tonnes of dry matter per year for the treatment of urban wastewater in France, and more than 10 million tonnes for the European Community, we realize that the management of the production and treatment of the sludge produced by the 15 treatment plants is a growing concern, amplified by new regulatory, environmental, health, economic and even

political and psychological.

  Conventional techniques for the disposal of this waste include

  including landfill, agricultural development and incineration. These techniques have serious drawbacks, however.

  Thus the ban on landfilling of non-ultimate waste, the

  severe constraints (notably with regard to heavy metals and certain suspect organic compounds) for agricultural recovery, the control of costs and the risks of atmospheric pollution from incineration will weigh heavily on the elimination of this waste.

  In this context, new technologies have been considered, with the objective of reducing the production of sludge (abbreviated RPB) from biological treatment facilities for urban or industrial wastewater. Some of these RPB techniques are based on the coupling between a conventional activated sludge process and a sludge treatment installed in a loop on the aeration tank. The associated treatment can call upon mechanical, physicochemical, thermal or biological techniques and often relies on the coupling of several of these techniques. In addition, the standards in force increasingly require the implementation of phosphorus removal processes in the wastewater treatment sector. In the case of a conventional biological treatment, the phosphorus treatment can be carried out according to two main routes:> By physicochemical elimination: the phosphates are made insoluble in the form of precipitates obtained by the addition of inorganic reagent, such as aluminum, iron or lime salts. The addition of reagent can be carried out directly in the aeration tank (simultaneous dephosphating) or can intervene in a specific step, after clarification (tertiary dephosphating). In all cases, this elimination technique leads to a significant production of physico-chemical sludge and the consumption of reagents is greater than the stoichiometric quantity due to the existence of side reactions.

  especially with organic matter.

  > By biological elimination: the phosphorus derivatives are "superaccumulated" by the biomass, as a result of alternation of anaerobic and aerobic phases. The extraction of excess biological sludge therefore allows the extraction of phosphorus combined with biomass. A combination of these phosphorus removal pathways can be used when the required level of removal is high or when complete biological phosphate removal is not possible. The combination of these two techniques makes it possible to greatly reduce the consumption of reagent in the physico-chemical precipitation stages, which has the consequence of reducing the production of sludge

mineral phosphating.

  The adoption of a stage of treatment of RPB by thermophobic enzymatic degradation (one can in particular in this respect refer to EP-A-924 168 and 1 008 558), installed in parallel and in loop on the biological basin is very efficient and leads, for certain processes, to sludge production reductions

significant, exceeding 80%.

  FIG. 2 of the appended drawings represents an exemplary embodiment of an activated sludge treatment installation comprising, in a loop on an aeration tank, an aerobic thermophilic degradation reactor. In this FIG. 2, the reference 1 designates the introduction of the waste water to be treated (ER), the latter passing respectively through an anaerobic zone R, an anoxic zone X, an aerobic zone A before being treated in a clarifier C. The RPB reactor designated as a whole by the reference B is mounted in parallel and in a loop on the activated sludge treatment installation. Despite their interest in terms of reducing the production of sludge, such installations nevertheless have various drawbacks, namely:> The implementation of a RPB process, looped over an aeration tank, leads to numerous repercussions on the design, performance and operation of the entire treatment plant (modification of sludge concentrations,

  their settling speed, the quality of treated water).

  > The consumption of phosphorus linked to the synthesis of living matter from biomass is reduced in the same proportions as the reduction in sludge production. This fraction of phosphorus therefore remains in water-soluble form and requires specific treatment by physicochemical precipitation, prior to release to the natural environment. However, taking into account the low concentrations to be eliminated (a few mg / l in P), the reaction requires a very large excess of precipitation reagent, compared to stoichiometry. The production of mineral mud from physico-chemical phosphate removal is then significant and reduces the interest accordingly.

overview of the RPB process.

  > In accordance with the previous remark, the efficiency of a step for removing phosphorus by biological means is reduced in the same proportions as the production of sludge. We can thus compare the phosphorus removal yields of a conventional activated sludge treatment installation, as illustrated in Figure 1 (in which we find the different means of this installation described above with reference to the figure 2), with those of an installation 25 according to FIG. 2: In both cases, the operating conditions are as follows No. of inhabitants equivalent: 50,000 eq-Hab Daily flow rate: 7,500 m3 / d Sludge production: 3,000 kg MES / j Station volume: 15,000 m3 Phosphorus concentration: 8 g / m3 At the end of the treatment in the installation according to FIG. 1, treated water (ET) is obtained which has a phosphorus concentration of 1 mg / l and an extraction of the sludge in excess of: 3000 kg MES / d and phosphorus approximately 50 kg, whereas according to the treatment in the installation in accordance with FIG. 2, the treated water has a phosphorus concentration of 6 to 7 mg / l, the extraction of excess sludge being 600 kg MES / d (80% RPB and phosphorus approximately 10 kg), this installation requiring a physico-chemical treatment of phosphorus. > The carbon and nitrogen fluxes which are used for the synthesis of living matter,

  in the case of a conventional biological system, are subject to resolubilization during the RPB stage and therefore constitute an additional pollution charge, recycled to the biological reactor and which requires an adaptation of the capacities and of the phases ventilation.

  > The implementation of sludge production reduction processes on small treatment plants, less than 10,000 population equivalents, leads to difficulties in terms of operation (in particular, the passage sections of the pipes and heat exchangers, are not adapted to the hydraulic overload induced by the looping on the aeration tank and clogging problems can arise) and remains difficult to justify in terms of investment. It is precisely these stations which meet the most

  difficulties in disposing of their sludge, due to their dispersion over the territories.

  > Finally, on a more contractual level, which says reduction in the production of sludge, 20 supposes to relate it to a production reference of a conventional installation not equipped with RPB: now this reference becomes virtual

  that reduction technology is implemented.

  To solve the problems posed by the reduction in production of biological sludge, the present invention takes the opposite of the prior state of the art recalled above, by dissociating the method of Reducing the Production of Sludge from the sector. treatment plant, by designing a platform dedicated to BPS and precipitation treatment of phosphorus accumulated in sludge from a sanitation facility. Thus, according to the present invention, the treatment of the sludge is carried out in a plant which is physically and biologically independent of the treatment plant (s), thus making it possible to maximize the "biological phosphate removal" function on the "water" line, while ensuring Reduction of

Mud production.

  According to an embodiment of the present invention, the elimination of phosphorus pollution is carried out in an installation independent of the wastewater treatment system, the phosphorus pollution being able to be treated by precipitation with

  means of salts and / or mineral compounds.

  According to the invention, the phosphorus stored in excess biological sludge is treated in concentrated form (factor 10 to 100) separately on the installation of

reduction of mud production.

  According to the invention, the method is implemented in an installation which is

  geographically distant from the station or stations generating the mud.

  According to another embodiment of the present invention, a specific biological treatment is provided for the organic matter originating from the lysis of bacteria, (of the activated sludge type, SBR "Sequenced Batch Reactor", oxidation channel, bioreactor with "BRM" membrane) this treatment can be different or

  not the biological treatment that led to the production of the mud.

  The invention also relates to a device for implementing the method defined above. Other characteristics and advantages of the present invention will emerge from the

  description given below, with reference to the accompanying drawings which illustrate

  exemplary embodiments devoid of any limiting character. In the drawings: - Figures 1 and 2 are diagrams illustrating the prior art described above; FIG. 3 is a diagram illustrating an example of implementation of the method according to the invention and

  FIG. 4 is also a diagram illustrating the implementation of this method and the results which it makes it possible to obtain.

  Referring to FIG. 3, it can be seen that a method according to the present invention comprises: a biological treatment installation with low, medium or high load which can be carried out in the form of an activated sludge reactor (treatment C / N / P), a mixed culture or fixed culture reactor, or a membrane bio-reactor; - a sludge separation device and - dedicated means for reducing the production of sludge, these means receiving the sludge, after any thickening, coming from the installation of

biological treatment.

  These latter means can be produced in the form of a RPB 5 reactor by thermophilic enzymatic degradation (as shown in FIG. 3, these means possibly being of the type described in EP 924 168 and 1 008 558). The source of nutrient substrate for bacteria in biological treatment is organic matter from the lysis of bacterial cells resulting from the thermophilic enzymatic reaction. he

  it is useless in this configuration to resort to the organic matter of wastewater.

  Found in Figure 4 of the accompanying drawings the same means as those described above with reference to Figure 3: a biological treatment plant with activated sludge and an aerobic thermophilic degradation reactor. In this variant of the invention, it is also possible to provide for the treatment of liquid sludge coming from remote biological treatment plants, as shown on this

1 5 figure.

  In Figure 4, we mentioned the different technical data of an example

  installation implementing the invention, as well as the results obtained.

  In a RPB installation in a loop on the aeration tank, according to the prior art described above, the phosphorus solubilized during the step of enzymatic thermophilic degradation is found in 2, diluted in the flow rate "Q" (FIG. 2). In the case of the present invention (FIGS. 3 and 4), the same quantity of phosphorus is found in the flow rate "q" which is 10 to 50 times lower than the flow rate Q. The phosphorus concentration to be treated is therefore 10 to 50 times superior; this allows other elimination routes, a better precipitation yield compared to the stoichiometry, therefore a saving of reagent and a production of phosphorus sludge lower. It is also possible to use more compact precipitate separation structures. In addition, the collection of slurries concentrated in mineral phosphate at the outlet of a centralized installation allows to envisage a

specific valuation.

  Such a dedicated RPB biological sector can therefore be designed very differently from the conventional wastewater treatment sector with RPB looped over the aeration tank: it allows to gain in compactness, in efficiency without having to undergo the constraints of treatment. required for the purification of wastewater (constraint resulting, in the case of "RPB closed", from the return of the effluent at the top of the water treatment line). The dedicated RPB installation can also be geographically distant from the treatment plant itself, as illustrated

Figure 4.

  Finally, the present invention has the advantage of allowing the objective quantitative measurement of the efficiency of the reduction in the volume of sludge: the reduction in the production of sludge is in fact calculated by difference between the quantity of sludge extracted from the treatment stations. water in 3, at the outlet of the clarifier and the quantity

  sludge extracted from the dedicated RPB sector in 4 (Figures 3 and 4).

  Such an association makes it possible to respond to the drawbacks mentioned above of RPB installations directly integrated in a loop on the activated sludge pool.

from the treatment plant.

  The process which is the subject of the invention makes it possible to reduce the sludge from several treatment plants, in particular of small size, of less than 20,000 equivalent inhabitants, whatever their treatment channel. In addition, it makes it possible to adapt the oxygenation capacity and the mode of diffusion of air or oxygen independently of that of the station having

generated mud.

  It remains to be understood that the present invention is not limited to the examples

  of the embodiments described and shown above, but that it encompasses all the variants thereof.

Claims (9)

  1 - Treatment process ensuring the reduction of the production of biological sludge from one or more urban or industrial wastewater treatment plants, characterized in that the process of reducing the production of sludge is dissociated , from the biological sector of the treatment plant, by providing separate means, dedicated to reducing the production of sludge and to the precipitation treatment of the phosphorus accumulated in the sludge from a sanitation installation.  2 - Process according to claim 1, characterized in that the treatment of the sludge is carried out in a facility which is physically and biologically independent of the treatment plant (s), thus making it possible to maximize the "biological phosphating" function on the "water" line. while ensuring the reduction of sludge production on the independent line.   3 - Method according to one of claims 1 or 2, characterized in that   the elimination of phosphorus pollution is carried out in an independent installation   of the wastewater treatment sector.   4 - Method according to claim 3, characterized in that the pollution   phosphorus is treated by precipitation using salts and / or mineral compounds.   5 - Method according to any one of the preceding claims, characterized in   what phosphorus stored in excess biological sludge is processed under   form concentrated separately on the sludge production reduction installation.   6 - Method according to any one of the preceding claims, characterized in   which is implemented in an installation which is geographically distant from the 25 or stations generating the mud.   7 - Method according to any one of the preceding claims, characterized in   what it involves a specific biological treatment of the organic matter coming from the lysis of bacteria, this treatment being able to be different from the biological treatment   having led to the production of mud.   8 - Device for implementing a method according to any one of   previous claims, characterized in that it comprises:   - a low, medium or high load biological treatment installation which can be carried out in the form of an activated sludge reactor (C / N / P treatment), a mixed-culture or fixed-culture reactor, or a membrane bio-reactor; - a sludge separation device and - dedicated means for reducing the production of sludge, these means receiving the sludge, after any thickening, coming from the installation of biological treatment.   9 - Device according to claim 8, characterized in that the dedicated means for reducing the production of sludge are produced in the form of a reactor for reducing the production of sludge by themophilic enzymatic degradation, the organic material coming from the lysis of the bacterial cells resulting from the thermophilic enzymatic reaction constituting the source of nutritive substrate for the bacteria biological treatment.   - Device according to one of claims 8 or 9, characterized in that it also ensures the treatment of liquid sludge from installations for distant biological treatment.