The present invention relates to improvements to the treatment of wastewater using activated sludge processes. It relates more particularly to a process and to a plant for controlling the retention time of the sludge undergoing clarification in a process for the treatment of wastewater by activated sludge comprising a recirculation of the activated sludge from the clarification stage to the aeration stage.
It is known that in such activated sludge processes, the recirculation of the sludge is a significant component in the overall operation of purification plants. The role of the secondary clarifiers consists in separating the sludge from the treated water, and to do this a sludge-concentrating stage is essential in the structure. This concentrating phase requires a retention time imposed on the sludge in the clarifier, and, if this retention time is not controlled, the process drifts towards malfunction logic.
Too long a retention time firstly results in anoxia conditions, with an immediate effect of potential denitrification and the appearance of froth on the surface of the clarifiers. Secondly, the sludge comes into anaerobic conditions, the repercussions of which are often disastrous on the water system and the treatment of the sludge since the development of filamentous microorganisms is favoured, which causes, over the entire plant, foaming problems and problems of poor flock settling (a rise in the Mohlman index). The risk of sludge egress is t hen increased in the event of a hydraulic surge. In addition, anaerobic conditions are completely incompatible with biological dephosphatizing processes, and in this case the release of phosphorous into the clarifier results in the discharges being immediately off-specification. These events are thus characteristic of a major malfunction of the water system. In parallel, and from a direct economic standpoint, the sludge treatment is not optimized when the flock-settling properties are poor, whatever the treatment system involved. The operating times of the equipment of the sludge treatment system are lengthened, the solids contents are reduced and, immediately, the volumes of sludge extracted are increased for the same mass of treated dry matter.
In the clarification stage, the retention time of the sludge must therefore remain limited, with the risk of causing malfunctions in the water treatment. This limit is about 2 hours in the case of activated sludge processes operating with prolonged aeration. It is less in the case of moderate or high load conditions.
On the other hand, too short or variable a retention time for sludge under clarification may also be a limiting factor in the case of sludge treatment systems. This is because it gives rise to a sludge concentration not suitable for certain thickening and dehydrating structures when the sludge is extracted from the recirculation line. This is generally the case for small-sized and medium-sized plants equipped with structures for thickening by draining. The dilution of the sludge entering this type of equipment results in insufficient capture rates or in the need for excessive dosing with flocculating polymer, resulting in risks of blockage. variations in load on these conditioning apparatuses often result in malfunctions, such as creep or blockage, which are connected with the modifications in the operating conditions predefined during the initial set-up operations. The minimum permitted concentration on the draining screens or tables is about 6 g/l. The variations in loads withstood by this type of equipment are about 10 to 20%. These conditions on the concentration, which corresponds to the concentration encountered in the sludge well, are directly connected with a constraint on the minimum retention time in the clarification stage. This minimum retention time itself depends on the sludge settleability properties.
Maintaining and controlling a mean retention time of the sludge undergoing clarification are therefore of prime importance for ensuring quality, reliability and economy of the treatment. This is indeed the technical problem that the present invention aims to solve.
In the technology of water treatment with regard to the activated-sludge recirculation function, two types of recirculation in a purification plant may be identified:
recirculation which returns the sludge from the clarifier into the aeration basin: the objective is to recycle part of the biological sludge so as not to impoverish the purifying mass of the aeration basin;
recirculation of mixed liquors which exist in the plants having an anoxia basin and an aeration basin; they recover the sludge from the aeration basin rich in nitrates in order to bring it into the anoxia region so that the denitrification can reduce the contents of the discharges.
The process forming the subject-matter of the present invention relates to the recirculation of activated sludge from the clarifier into the aeration basin. Given that the various arrangements for the activated sludge to be recirculated from the clarifier into the aeration basin form part of the conventional technical knowledge of those skilled in the art in the water treatment field, they will not be described. The reader may refer for this purpose to Mxc3xa9mento Technique de l""Eau, Ninth Edition (1989).
The general principle of managing the recirculation of activated sludge in water treatment will now be explained.
The objectives of management suitable for sludge recirculation must satisfy the criteria below:
to prevent the sludge in the clarifier from undergoing anaerobiosis in order to reduce any risk of malfunction;
to adapt the operating mode of the plant to the conditions encountered in operation (sludge masses in the system, sludge-settling properties, rain showers, etc.), so as in particular to prevent sludge bed egress;
to manage the distribution of the total mass of sludge between the biological basin and the clarifier in order to make the biological treatment reliable.
Reference is made to FIG. 1 of the appended drawings, which is a diagram illustrating the construction of the sludge flow balance and in this diagram the aeration basin is represented by 1 and the clarifier by 2.
The amount of recirculation of the sludge from the clarifier 2 into the aeration basin 1 is by definition the ratio of the rate of recirculation to the throughput of the plant:
xcfx84rec=Qr/Qt
where xcfx84rec denotes the rate of recirculation (0 less than xcfx84rec less than 1)
Qr denotes the rate of recirculation (m3/h) and
Qt denotes the throughput (m3/h).
This amount of recirculation is defined by the conditions encountered on the site. There must be an overall balance between the sludge flows into and out of the clarifier:
(Qt+Qr)xc3x97Cas=Qrxc3x97Cr
where Cas is the sludge concentration (g/l) in the aeration basin and
Cr is the sludge concentration (g/l) in the recirculation line.
The storage term is neglected here, just like the treated-water discharge outflow.
This equation is used to calculate the degree of recirculation xcfx84rec:
xe2x80x83xcfx84rec=Qr/Qt=Cas/(Crxe2x88x92Cas)
Cas is an operational parameter that can be measured directly in the aeration basin. As regards Cr, this must be controlled so as to meet the requirements on the retention time of the sludge in the clarifier since it has been seen, empirically, that the sludge retention time is linked to the Mohlman index, representative of the sludge-settling and thickening properties, and to the sludge concentrations in the aeration basin and in the recirculation line. The following correlation has been established between these variables:
RT/60=(CrMI/1000)3xe2x88x92(CasMI/1000)3
where RT is the retention time (in minutes) of the sludge undergoing clarification and
MI is the Mohlman index (ml/g).
This formula is the reference relationship for the automated recirculation management logic tool. It is based on a formula of the same type published by ATV (ATV Standard, A 131. (1991); xe2x80x9cDimensioning of single-stage activated sludge plants upwards from 5 000 total inhabitants and population equivalentsxe2x80x9d Abwassertechnische Vereinigung e.V., St. Augustin):       C    r    =            1000      ⁢                        RT          /          60                3              MI  
If the measurement of the Mohlman index is not representative of the fact of the lack of sludge settling, a settling index Is may be defined for safety protection, and then used in these formulae.
Is is the undiluted settling index (ml/g).
In practice, the maximum permitted retention time of the sludge undergoing clarification is only dependent on the mass charge of the plant. As an indication, it is about 120 minutes for a low charge. It may be reduced to 40 minutes for high charges.
For a given charge, since the settling index and the sludge concentration in the aeration basin are operating parameters, it is thus possible to define a setpoint for the sludge concentration in the recirculation line which overall satisfies, with respect to the maximum permitted retention time in the clarifier, the constraint associated with making the water treatment reliable.
The amount of recirculation defined will thus be a function of the sludge concentration in the aeration basin and, indirectly, of the sludge settling index and of the mass charge of the plant, that is to say of the operating conditions encountered on the site. This amount of recirculation can therefore be expressed as:
xcfx84rec=Qr/Qt=Cas/(Cr,setxe2x88x92Cas)
where
Cr,set=1000/MI(RTmax/60+(CasMI/1000)3)⅓;
Cr,set is the setpoint for the sludge concentration (g/l) in the recirculation line; and
RTmax is the maximum permitted retention time (in minutes) of the sludge undergoing clarification.
In fact, to be rigorous it would be necessary to complete the balance equation with the flow of extracted sludge into the sludge treatment system. This quantity may sometimes be predominant for the outflows in small plants:
(Qt+Qr)xc3x97Cas=(Qr+Qext)xc3x97Cr,set
where Qext is the extraction flow rate (m3/h).
The calculated rate of recirculation is then:
Qr=[(Qtxc3x97Cas)xe2x88x92(Qextxc3x97Cr,set)]/(Cr,setxe2x88x92Cas).
The management methods known at the present time for recirculating sludge in the field of water treatment by activated sludge will now be explained.
In general, two management methods are used:
the recirculated flow rate is constant and represents from 100 to 150% of the intended daily wastewater throughput of the plant. Above a limiting throughput threshold, the recirculation equipment may have to be forced;
the rate of recirculation is slaved in a proportional manner to the incoming flow rate.
In order to proportion the rates of recirculation, the operation of the equipment is continuous at a fixed rate, or syncopated by slaving to a clock or to a cyclic metering device. In large plants, specific regulations exist which use a combination of sensors for the throughput, the rate of recirculation, the weight of sludge in the aeration basin and in the recirculation line, the height of the sludge blanket and the output turbidity.
A well-managed recirculation favours good quality of the sludge and clarification of the water. The sludge must not be stored in the clarifier in order to prevent fermentation. The operating periods must always be longer than the rotation time of the scraper bridge of the clarifier or at least managed on the basis of non-harmonic frequencies.
If the sludge concentration is adapted and if the recirculation is well managed, the sludge blanket is not visible and cannot be detected with the Secchi disc: the sludge blanket is at most 1 m in depth.
These management methods according to the prior art have the following drawbacks:
1xe2x80x94The conventional recirculation management methodsxe2x80x94fixed or slaved flow ratexe2x80x94remain dependent on the definition of an average amount of recirculation and therefore on a control setting tied to the operating conditions. Too often, the actual amounts of recirculation are defined when designing and commissioning the plants. They are only rarely re-adjusted, with the exception of periods of malfunction (a typical example: a leak in the sludge bed is observed and the immediate reaction is to increase the recirculation. Unfortunately, this operating reaction occurs too late). One of the essential management rules might be to re-update the amounts of recirculation when a Mohlman index and/or sludge concentration measurement is made in the aeration basin and to modify the settings as a consequence. Apart from this main defect, managing the recirculation on the basis of a fixed flow rate has the drawback of not being good in taking account of the hydraulic events encountered. Peak management is rarely modified according to the daily regimes (working days or weekends) and the various seasonal regimes are treated in the same way (summer periods, rainy periods, temporary activities (grape harvesting, etc.)), and the forced operations actuated by rainy weather are blind to the history and to the environment of the event (sudden or uniform, short or persistent rainfall).
2xe2x80x94The management method slaved to the flow rate seems to be more suitable in this field. However, from a rigorous standpoint, it appears that the dynamics of sludge transfer in the aeration basin/clarifier system have not been taken into account hitherto. This is because the response of the system is slow and damped by buffer effects, and the reaction to a rapid event cannot be simply controlled by an action proportional to the excitation.
3xe2x80x94Finally, the method of controlling the recirculation equipment is ordinarily syncopated, and the modifications to the settings often result in a phase lag in the syncopating frequencies with the rotation time of the scraper bridge of the clarifier. The non-harmonic frequencies as they are known are no longer respected and the recirculated sludge always comes from the same regions of the bottom of the clarifier. This is because the scraper has a mainly destructuring action, since the relative compactness of the sludge blanket limits homogeneous recovery of the sludge by suction towards the sludge well. Passage of the scraper locally fluidifies the sludge blanket and encourages the recovery of freshly destructured sludge upon activating the recirculation pumps. For harmonic frequencies (15 minutes operation, 15 minutes stoppage for a bridge rotation time of 30 minutes for example), sludge recovery is effective only over part of the clarifier.
Unsuitable management of the sludge recirculation runs the risks mentioned below:
a)xe2x80x94Excessive Recirculation:
The risks run by excessive recirculation are too high a velocity in the internal xe2x80x9cskirtxe2x80x9d of the settler (xe2x80x9ccliffordxe2x80x9d) acting as a distributor of the flux to be treated (impeded settling) and an imbalance at the water/sludge interface: the rate of sludge pumping will be much higher than the particle settling velocity, thus creating a hydraulic short-circuit. In addition, excessively high rates of recirculation reduce the sludge concentrations in the recirculation line, which may place a limit on the feed for the sludge treatment system.
b)xe2x80x94Insufficient Recirculation:
The risks run as a consequence of insufficient recirculation are linked with possible uncontrolled denitrification and to the sludge moving to anaerobic conditions in the clarifier (release of phosphorus, promotion of the development of filamentous microorganisms and of foam, etc.). The clarifier is periodically converted into a sludge storer and thickener.
c)xe2x80x94Syncopated Recirculation with a Non-harmonic Frequency:
The risks are identical to those of insufficient recirculation, but in a less pronounced manner, by the creation of regions or unrecovered pockets of sludge, the retention time of which is long.
In the case of clarifiers with sucked bridges, the rate of recirculation must never be
less than the rate of depriming of the siphon
such that there is an imbalance in the load losses on the suction tubes (this depends intimately on the distribution of the sludge concentrations on the bottom).
Given the fact that the existing strategies for managing the recirculation of activated sludge are not satisfactory, the objective of the present invention is to provide a process for the automated management of this recirculation, for the purpose of controlling the retention time of the sludge undergoing secondary clarification in activated-sludge processes. Its primary objective is to limit the retention time of the sludge in the clarifier to below a maximum limit, for the purpose of making the water treatment reliable. Its secondary objective is to facilitate the management of sludge treatment systems using, for example, draining equipment, by maintaining a constant concentration in the recirculation line.
Consequently, the subject of the present invention is a process for controlling the retention time of the sludge undergoing clarification in a process for treating wastewater by activated sludge comprising a recirculation of the activated sludge from the clarification stage into the biological basins, characterized in that the rate of sludge recirculation (Qr) is varied so as to maintain a constant recirculated sludge concentration (Cr) while at the same time guaranteeing a retention time (RT) of the sludge undergoing clarification of less than a critical value (RTmax).
According to one method of implementing the invention, the rate of sludge recirculation (Qr) is varied according to a signal representative of the throughput (Qt) of water passing through the plant, so as to limit the retention time of the sludge in the clarification stage.
According to another method of implementation, when the validity of the signal representative of the throughput of water passing through the plant is not recognized, the recirculation of the sludge is controlled according to the fall-back method of managing the water treatment plant.
According to another method of implementing the process according to the invention, the rate of sludge recirculation (Qr) is varied according to a signal representative of the sludge concentration (Cr) in the recirculation line, obtained from a sensor positioned in the recirculation line, this signal being delivered to an automated logic tool which varies the rate of recirculation (Qr) according to the variation in the signal received, so as to keep the sludge concentration (Cr) in the recirculation line constant; in the event of doubt about the representativeness of the said signal, the sludge recirculation is controlled automatically according to a slaved management method whose objective is to limit the retention time of the sludge in the clarification stage, the said slaved management method being managed on the basis of the analysis of the values of the sludge concentration (Cas) in the biological basin and on the basis of the throughput (Qt) of water passing through the plant.
According to another characteristic of the process according to the invention defined above, the volume of settled sludge is periodically measured so as to evaluate the sludge settling index (Is) and the sludge concentration in the biological basin and to determine the setpoint values for the management controller.
According to the present invention, the automated logic tool for managing the sludge recirculation is designed so as:
to periodically update the average amount of sludge recirculation (xcfx84rec) so as to adapt it to the conditions of the treatment plant, on the basis of the value of the sludge concentration (Cas) in the aeration basin and on the basis of the re-updated settling index (Is);
to respond instantly to the hydraulic events and manage the reaction delays according to the response time of the clarifier/aeration basin system by being based on the variation in the throughput (Qt); and
keeping the sludge concentration (Cr) in the recirculation line constant.
The invention also relates to apparatuses for implementing the process defined above.
According to a first embodiment, this apparatus comprises:
sensors making it possible, respectively, to measure the throughput (Qt);
means ensuring modulated control of the said rate of activated sludge recirculation (Qr);
means for measuring the volume of the sludge that has settled and for evaluating the settling index (Is) and the sludge concentration (Cas) in the biological basin;
a controller ensuring automated management of the recirculation, which comprises two main modules served by a common signal-input stage:
a first module providing the interface between the user and the controller and making it possible to display a concentration setpoint (Cr);
a second module forming the unit for controlling the recirculation flow rates and comprising:
a control logic tool for controlling the retention time of the sludge in the clarifier on the basis of the throughput (Qt), in order to calculate a signal for controlling the rate of recirculation (Qr);
a module for managing the safety actions of the logic tool for managing the recirculation means; and
a stage involving the parameterization, reception, processing, analysis and validation of the signals coming from the sensors and logic indicators of the operation of the equipment in order to supply the said modules.
According to a second embodiment, this apparatus comprises:
sensors making it possible, respectively, to measure:
the sludge concentration (Cr) in the line for recirculating the activated sludge from the clarifier into the aeration basin;
the sludge concentration (Cas) in the aeration basin; and
the throughput (Qt);
means ensuring modulated control of the said rate of activated sludge recirculation (Qr);
means making it possible to measure the volume of the sludge that has settled and to evaluate the settling index (Is);
a controller ensuring automated management of the recirculation, which comprises three main modules served by a common signal-input stage:
a first module providing the interface between the user and the controller and making it possible to display a concentration setpoint (Cr);
a second module forming the unit for controlling the recirculation rates and which comprises:
a control logic tool ensuring control and regulation of the rate of recirculation (Qr) on the basis of the measurement of the sludge concentration (Cr) in order to keep the recirculation concentration constant;
a control logic tool ensuring control of the retention time of the sludge in the clarifier on the basis of measurements of the sludge concentration (Cas) in the aeration basin and of the throughput (Qt), in order to calculate a signal for controlling the rate of recirculation (Qr);
a module for managing the safety actions of the logic tool for managing the recirculation means; and
a third module for calculating the mass of sludge present in the clarifier/biological basin system for the purpose of fixing and setting the rate of extraction of the sludge; and
a stage involving the parameterization, reception, processing, analysis and validation of the signals coming from the sensors and logic indicators of the operation of the equipment in order to supply the said modules.
As will have been understood, the process forming the subject-matter of the present invention allows the rate of recirculation to be varied so as to maintain a constant concentration of recirculated sludge, while at the same time guaranteeing a retention time of less than the critical value. One of its functions furthermore makes it possible to evaluate the total mass of sludge present in the system, so as to assist the management of the extractions. Finally, the safety detection provided by the controller prevents any possible drift in the quality of the sludge or in the components providing the recirculation function, and informs when exceptional maintenance operations are to be carried out.
The process forming the subject-matter of the invention makes it possible in particular to eliminate the risk of filamentous bacteria appearing from the sole fact of managing the secondary clarifiers. Thus, it makes the water treatment system reliable and makes it possible to optimize the performance of the sludge system by maintaining the quality of the sludge. Finally, the constancy of the extraction concentration and the possibility of renewing the predefined settings or of operating the equipment at any time make it possible to assist the operations relating to thickening by draining. The advantage of applying this automated management may be expressed in terms of operating costs:
Improvement in or maintenance of sludge quality, manifested by a low Mohlman index (increase in the capacity and reduction in the operating time of the sludge treatment equipment, improvement in the solids content and reduction in the volumes and in the costs of extracting the sludge produced, reactant metering, reduction in returns to the top, etc.).
Assistance in sludge treatment and elimination of draining malfunctions (organization of operating labour).
Conformity of the discharges (Water Agency Agreements and Premiums).
Prevention of malfunctions, especially under extreme operating conditions and during rain showers (organization of labour and elimination of malfunction handling costs).
Visual appearance of the clarifier.
Further features and advantages of the present invention will become apparent from reading the description given below with reference to the appended drawings which illustrate various methods of implementation and embodiments. In the drawings: