WO2015022709A1

WO2015022709A1 - Process for aerobic sludge treatment

Info

Publication number: WO2015022709A1
Application number: PCT/IT2013/000226
Authority: WO
Inventors: Marcello BONDESAN
Original assignee: Hera S.P.A.
Priority date: 2013-08-12
Filing date: 2013-08-12
Publication date: 2015-02-19

Abstract

A process (1) for aerobic treatment of sludge and the like, which consists of a first step of sludge thickening and a second step of sludge dehydration. The first step of sludge thickening is constituted by four substeps. In a first substep a), the excess sludge to be treated, which originates from a waste treatment plant, is conveyed into a stabilization tank (2) provided with an overflow channel (3). In a subsequent substep b), air is dispensed inside the tank (2) so as to reduce the volatile component of the excess sludge that is present inside the tank (2). In a different substep c), the stabilized sludge that is present in the overflow channel (3) is conveyed to the inlet of a thickener (3). The thickening is intended to reduce its humidity; this entails a strong volume reduction and therefore a significant saving in terms of dimensions in the subsequent steps and therefore a reduction in the operating costs of the plant. In the subsequent substep d), the thickened sludge of the thickener (3) is transferred to the inlet of the stabilization tank (2) by means of a recirculation pump (5). The second sludge dehydration step is constituted by three substeps. In a first substep e), the mixture of the excess sludge and of the thickened sludge is transferred from the stabilization tank (2) to a dehydration unit (6). In an additional substep f), the dehydrated sludge is drawn from said dehydration unit (6) for subsequent operations such as disposal, agronomic reuse and the like. In substep g), the mother liquors in output are drawn from the dehydration unit (6) and conveyed upstream of the waste treatment plant.

Description

PROCESS FOR AEROBIC SLUDGE TREATMENT

The present invention relates to a process for aerobic sludge treatment.

Plants are known which perform aerobic sludge treatments and are generally provided with an aerobic stabilization section, a thickening section and a sludge dehydration section.

As is known, the excess sludge to be treated, which has a density of approximately 10/12 kg/m³ and a volatile component of approximately 75- 80%, originating from a waste treatment plant, is introduced initially in the aerobic stabilization compartment, usually a tank, where, by injection of air, the volatile component is reduced to 50-60%, thus rendering the sludge less putrescible.

In particular, the higher the level of the tank 2, and therefore the transit time of the air bubbles toward the surface, the higher the utilization of said air by microorganisms.

Moreover, the higher the level of the tank 2, and therefore the time of transit of the sludge through it (called retention time), the longer the time available to cause the biological stabilization reactions.

The stabilized sludge is then conveyed into a thickener, which separates part of the water from the sludge, producing as precipitate a more concentrated sludge and as supernatant a clarified current or mother liquors.

This sludge, which is generally termed thickened, has a density of approximately 30-40 kg/m³.

The mother liquors are then transferred upstream of the waste treatment plant, while the thickened sludge is transferred to the dehydration section by means of a feeder pump.

The activation of the feeder pump allows in fact the drawing, by gravity or by means of the operation of an additional pump whose activation is coordinated with the preceding one, of the sludge from the thickener and in turn from the stabilization compartment. Due to the dehydration mechanisms (filtration, centrifugation and the like), the sludge thus reaches a degree of dryness that is suitable for the subsequent treatments for disposal or agronomic reuse.

In this dehydration section, too, mother liquors are produced which are transferred upstream of the waste treatment plant.

In smaller plants, in which the dehydration section is absent, there is instead a small tank for storing the sludge in the liquid state: in any case, this is sludge that is concentrated (34/40 kg/m³) and stabilized (MV% = 50/60%).

Various devices are known which are used to inject air and are constituted generally by apparatuses that compress air and introduce it in the stabilization compartment by means of a system of perforated plates, porous diffusers and the like.

These devices are distinguished mainly in the different degrees of efficiency, which can be measured in kWh/kgVSS (units of energy expended per unit of volatile suspended solids in the stabilization tank).

For an equal result, in terms of sludge stabilization, the cheapest process is the one that makes it possible to minimize the energy units expended per unit of treated sludge.

This dutiful goal of economy in operation must be achieved if possible in combination with additional operating conditions dictated by reasons of economy and management, and in particular a certain flexibility in the management of the stabilization section and of the sludge line in general must be possible.

It must in fact be possible to suspend the operation of the sludge dehydration section for longer or shorter periods.

It may in fact be temporarily impossible to transfer the dehydrated sludge to the reuse/disposal plants due to the unavailability of these plants to receive, or more simply due to the temporary blocking of the road for access to the plants by heavy vehicles (due to reasons linked to public hygiene, wastewater treatment plants are located far from inhabited areas: in mountain regions, in case of snow, it may be difficult to reach the plants with the heavy vehicles dedicated to sludge transport).

The need is observed, moreover, to be able to achieve an extraction of the excess sludge to be treated with the highest possible regularity and in any operating condition.

Continuous transfer of small amounts of excess sludge from the biological treatment section (waste treatment plant) is an indispensable condition for regular operation of the oxidation compartment (which corresponds to the area of sludge exposure to the air stream) and therefore for a good quality of the treated water.

Therefore, in order to achieve continuous and calibrated extraction, it is necessary to perform continuous dehydration of small quantities of sludge, which requires considerable operating commitment unless the stabilization compartment is managed at a variable level, to the detriment however of the energy used for air injection.

A further need is to limit as much as possible loading irregularities at the waste treatment plant that derive from the transfer and consequent treatment of the supernatants that arrive from the thickener.

The mother liquors obtained in the thickening and dehydration process in fact entail a load of pollutants and nutrients that is added to that of the sewer wastewater that normally enters the waste treatment plant.

It is therefore convenient to ensure their treatment at periods of lower load, due both to reasons of biological load at the waste treatment plant and due to reasons of operating energy cost.

During the dehydration steps, the thickener in fact receives a load of solids that is at least equal to the density of the sludge of the stabilization section multiplied by the flow-rate of sludge in input to said section, to which the produced mother liquors are to be added.

The load of solids, therefore, can have even rather high instantaneous values (for example in case of the first step of filtration of filter press machines), compelling the designer to oversize the thickener.

This causes a high return of mother liquors to the waste treatment plant, which can cause a consequent organic overload.

Further critical elements of the plant of the known type are therefore that the density of the sludge in the aerobic stabilization section is similar to that of the excess sludge to be treated (10/12 kg/m³). This entails non- maximal use of the oxygen supplied by means of the air injection machines.

In some plants, in order to be able to achieve the considerable operating versatility (reduction of the amount of dehydrated sludge for certain periods of time) it is necessary to manage the stabilization section in a variable manner, utilizing the latter as an operating buffer.

However, this operating mode is detrimental to the energy used for air injection, reducing both the transit times of the air bubbles toward the surface and the retention times of the sludge in the tank.

Moreover, if "filamentous bulking" phenomena occur, in the oxidation compartment of the waste treatment plant, the density in the stabilization compartment is reduced further, causing degradation of the energy efficiency of the plant.

Bulking or sludge bulking or literally sludge swelling is a phenomenon that can occur in waste treatment plants and causes an increase in the volume of the sludge in the stabilization section.

Bulking therefore reduces the settling rate and the compaction of the sludge on the bottom, becoming unsuitable for its waste treatment function and losing its adsorbent and oxidizing qualities.

It is due generally to the excessive proliferation of filamentous bacteria that are normally present in sludge together with flock-forming bacteria. In this case the sludge flock appears very large and very rich in filamentous bacteria.

The aim of the present invention is to solve the problems described above, proposing a process for aerobic sludge treatment with high energy efficiency.

Within this aim, an object of the invention is to propose a process for aerobic sludge treatment with high versatility.

Another object of the present invention is to provide a process for aerobic sludge treatment that requires equipment with reduced space occupation.

A further object of the present invention is to provide a process for aerobic sludge treatment that is low-cost, is relatively simple to provide in practice and safe in application.

This aim, as well as these and other objects that will become more apparent hereinafter, are achieved by a process for aerobic treatment of sludge and the like, which consists of a first step of sludge thickening and a second step of sludge dehydration, said first step consisting in:

a) conveying excess sludge to be treated, which originates from a waste treatment plant, into a stabilization tank provided with an overflow channel;

b) dispensing air inside said tank in order to reduce the volatile component of said excess sludge that is present inside said tank; c) conveying said stabilized sludge that is present in said overflow channel to the inlet of a thickener;

d) transferring the thickened sludge of said thickener to the inlet of said stabilization tank by means of a recirculation pump;

said second step consisting in:

e) transferring a mixture of said excess sludge and of said thickened sludge from said stabilization tank to a dehydration unit;

f) drawing from said dehydration unit the dehydrated sludge for subsequent operations such as disposal, agronomic reuse and the like;

g) drawing from said dehydration unit the mother liquors in output and conveying them upstream of said waste treatment plant.

This aim, as well as these and other objects that will become more apparent hereinafter, are achieved furthermore by a plant for performing aerobic treatments of sludge and the like, characterized in that it comprises a first circuit for thickening sludge and a second sludge dehydration circuit, said first circuit comprising a stabilization tank, provided with at least one inlet that is connected to a duct for supplying excess sludge to be treated, means for injecting air, an overflow channel, a thickener connected to said overflow channel, and a thickened sludge recirculation pump, which is connected to an outlet of said thickener, leading through a respective pipe to said at least one inlet of said tank, said second circuit comprising a dehydration unit, which is connected to an output channel of said tank that conveys a mixture of said excess sludge to be treated and said thickened sludge.

Further characteristics and advantages of the invention will become more apparent from the description of a preferred but not exclusive embodiment of the process for the aerobic treatment of sludge and the like, according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

Figure 1 is a diagram of a plant of a known type for the aerobic treatment of sludge and the like;

Figure 2 is view of a method for the aerobic treatment of sludge and the like of a known type.

With particular reference to the figures cited above, the numeral 1 generally designates a process for the aerobic treatment of sludge and the like.

Next to the waste treatment plant one can see a sludge treatment line, to which the excess sludge is transferred.

Said line is generally constituted by a stabilization tank a, inside which, by injection of air, the volatile component is reduced to 50-60%, thus making the sludge less putrescible, a thickener β, which separates part of the water from the sludge, producing as precipitate a more concentrated sludge that is transferred to the dehydration section δ by means of a feeder pump γ.

The excess sludge in fact must be drawn and removed periodically in order to prevent the concentrations of suspended solids from increasing until they compromise the correct operation of the plant.

The sludge must be treated because it contains putrescible substances and pathogenic bacteria and has a very low content of dry substance in a high volume, which would make its direct disposal extremely expensive.

This process is used generally for sludges originating from the treatment of municipal wastewater, but it can also be applied to sludge of industrial origin containing organic substances capable of biological degradation.

The process 1 consists of a first step of sludge thickening and a second step of sludge dehydration.

This first sludge thickening step is constituted by four substeps.

In a first substep a), the excess sludge to be treated, which arrives from a waste treatment plant, is conveyed into a stabilization tank 2 provided with an overflow channel 3.

Said overflow channel 3 consists of a barrier that has the purpose of keeping the level of the stabilization tank 2 at a predefined value.

The introduction of new excess sludge into the stabilization tank 2 causes the level of the sludge inside said tank to rise.

The stabilization tank 2 in fact is managed at a fixed level that is permanently equal to the maximum hydraulic volume allowed for regular operation.

The overflow channel 3 therefore has the function of collecting the stabilized sludge propelled by the excess sludge in input out of the stabilization tank 2. In particular, according to the process 1 , the excess sludge, extracted continuously (24 hours a day) in small quantities from the recirculation current of the oxidation compartment of the waste treatment plant, can have a density comprised between 10 kg/m³ and 12 kg/m³ (depending on the effectiveness of the secondary settling and on the presence or absence of filamentous bulking phenomena in the waste treatment plant) and a component of volatile materials comprised between 75% and 80%.

In a subsequent substep b), air is dispensed into the tank 2 in order to reduce the volatile component of the excess sludge that is present inside said tank 2.

The excess sludge is in fact characterized by high putrescibility and contains many microorganisms, including pathogens. A stabilization step is therefore indispensable in order to obtain a sludge that is no longer putrescible, is safer to handle and easy to dehydrate.

With biological stabilization one in fact achieves a substantial reduction in the amount of suspended solids present initially in the sludge; this reduction entails the decrease by approximately one third of the amount of sludge that must be handled subsequently.

Aerobic digestion consists of a process of assimilation and biological degradation of the organic substances that are present in the sludge in an aerobic environment that is rich in oxygen, continuing the endogenous phase started in the biological oxidation process.

The process occurs inside the tank 2, in which the agitation used to keep the sludge particles suspended is provided by injection of air.

Moreover, the dispensed air mixes the sludge inside the stabilization tank 2, preventing density stratification.

In a different substep c), the stabilized sludge that is present in the overflow channel 3 is conveyed to the inlet of a thickener 4.

Thickening has the purpose of reducing its humidity; this entails a great reduction in volume and therefore a substantial saving in the sizing of the subsequent steps and therefore a reduction of the operating costs of the plant.

The thickener 4 allows increasing the density of the solid particles and expelling part of the water that is then transferred upstream of the waste treatment plant.

In the subsequent substep d), the thickened sludge of the thickener 4 is transferred to the inlet of the stabilization tank 2 by means of a recirculation pump 5.

The second sludge dehydration step is constituted by three substeps.

In a first substep e), the mixture of the excess sludge and of the thickened sludge is transferred from the stabilizing tank 2 to a dehydration assembly 6.

The purpose of dehydration is to further lower the content of water in the mixture so as to make the subsequent treatment and disposal operations cheaper and easier.

In a further substep f), the dehydrated sludge is drawn from said dehydration unit 6 for subsequent operations such as disposal, agronomic reuse and the like.

In substep g), the mother liquors in output are drawn from the dehydration unit 6 and conveyed upstream of the waste treatment plant.

It is specified that the expression "mother liquors" is used generally to designate the saturated residual liquids that remain after partial crystallization of the solute that is present in a given solution.

According to a further constructive solution, which is particularly simple and effective, the first step of sludge thickening and the second step of sludge dehydration can be performed alternately.

In periods in which the dehydration unit 6 is not operating, the thickened sludge is transferred back continuously to the stabilization tank 2 by means of the recirculation pump 5.

This allows a reduced dimension of the thickener 4 and a consequent reduced return of mother liquors to the waste treatment plant that is organized over all 24 hours of a day.

In this case, the thickener 4 is no longer linked to the flow-rate of dehydration but only to the level of thickening that is desired in the stabilization tank 2.

The density of the stabilized sludge that arrives at the thickener 4 is thus much higher than that of processes of the known type of Figure 1 (the density of suspended solids that is obtained is normally higher than 28/30 kg/m³) but the flow of the solids, on which the sizing parameter is based, is not.

The increase in sizing density is in fact more than compensated for by the low flow-rate to which the thickener 4 is subjected, which is equal to the flow-rate of the excess sludge and of the thickened sludge, thus leading to a reduced flow of solids and therefore to dimensions that in some cases are even smaller than the usual version.

The recirculation current of the thickened sludge has the effect of increasing the density in the stabilization tank 2, which in theory will tend asymptotically to the same value as the thickened sludge.

Of course, this value will never be reached, since the operating density of the stabilization tank 2 is limited by the capacity of the thickener 4 to separate sludge from water and there is in any case a component of excess sludge of low density that originates from the waste treatment plant.

However, this entails a higher absolute quantity of solids suspended within the stabilizing tank 2 (given by the density multiplied by the useful volume, which is always at its maximum value) and therefore a more efficient use of the energy used to inject the air and to a greater retention of said sludge within the tank.

Inside the tank 2, the density of the excess sludge and thickened sludge mix can be comprised between 30 kg/m³ and 32 kg/m³, thus achieving complete stabilization (MV% = 50/55%) even in small tanks 2. With the process in the steady state, the dehydration unit 6 can be fed directly by the stabilizing tank 2 due to the high level of stability and density of the sludge mix, constituted by excess sludge and thickened sludge, contained in said tank 2.

During the dehydration step, the operation of the thickener 4 therefore is no longer necessary.

The flow-rate of the sludge mix, furthermore, is generally higher than the sum of the flow-rate of the excess sludge and of the thickened sludge in input to the tank 2 and therefore the thickener 4 cannot be fed through the overflow channel 3.

The power-on of the sludge dehydration unit 6 is controlled synchronously with the shutdown of the recirculation pump 5 of the thickened sludge and vice versa.

According to the method 1 , step g) can be performed simultaneously with step f).

In particular, step d) can comprise a step d2) that consists in transferring the mother liquors in output from the thickener 4 to the waste treatment plant.

If the dehydration unit 6 is operating, the waste treatment plant receives only the mother liquors that arrive from the dehydration unit 6 and not the mother liquors of the thickener 4, which is not in operation, and vice versa.

According to the invention, the plant for performing aerobic treatments of sludge and the like can comprise at least one first sludge thickening circuit and one second sludge dehydration circuit.

The first circuit comprises a stabilization tank 2, which is provided with at least one inlet connected to a duct for supplying excess sludge to be treated, air injection means and an overflow channel 3.

The air injection means are constituted generally by devices that compress the air and introduce it into the tanks by means of a system of perforated plates or porous diffusers and are arranged generally on the bottom of the stabilization tank.

The first circuit according to the invention also comprises a thickener 4, which is connected to the overflow channel 3, and a thickened sludge recirculation pump 5 that is connected to an outlet of the thickener 4.

Said pump 5 leads, through a respective pipe, to the at least one inlet of the tank 2.

The second circuit according to the invention comprises a dehydration unit 6 that is connected to an output channel of the tank 2, which conveys a mixture of the excess sludge to be treated and of the thickened sludge.

In particular, the dehydration unit is of the type selected preferably from a centrifugal pump, a filter press, belt presses and the like.

Moreover, the channel can be affected by a pump 7 for feeding the mixture of excess sludge to be treated and thickened sludge, arranged upstream of the dehydration unit 6.

According to the invention, the thickener 4 can be of the type selected preferably from a gravity thickener, a flotation thickener and the like.

More particularly, the pipe can be provided with respective air injectors.

The method 1 according to the invention can be implemented in new plants for performing aerobic sludge treatments.

The implementation of the process 1 allows in fact containing substantially the dimensions of the stabilization tank 2 and, in some cases, also of the thickener 4, with a considerable saving of space and of construction work, reducing investment costs.

The higher efficiency of the stabilizing tank 2 indeed makes it possible to obtain a sludge treatment cycle that is very economical in terms of energy costs and labor costs.

The process 1 according to the invention can be implemented also in existing plants, since it can be provided by means of simple additional hydraulic equipment or by insertion of a recirculation pump 5.

By using stabilization tanks that are sized with the above cited criteria, this process, in addition to significant operating savings in terms of energy and labor, provides the plant with higher versatility (the removal of the dehydrated sludge can be suspended for the entire winter season) and excellent values of MV% of the stabilized sludge.

As an alternative, the compartmentalization of the stabilization tanks 2 to the new reduced values required with this process makes it possible to free spaces to provide, for example, denitrification compartments.

Advantageously, the process for aerobic treatment of sludge 1 has high energy efficiency.

Positively, the process 1 according to the invention is highly versatile.

Usefully, the process for aerobic sludge treatment 1 requires equipment with reduced space occupation.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the accompanying claims; all the details may furthermore be replaced with other technically equivalent elements.

In the exemplary embodiments shown, individual characteristics, given in relation to specific examples, may actually be interchanged with other different characteristics that exist in other exemplary embodiments.

Moreover, it is noted that anything found to be already known during the patenting process is understood not to be claimed and to be the subject of a disclaimer.

In practice, the materials used, as well as the dimensions, may be any according to the requirements and the state of the art.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A process for aerobic treatment of sludge and the like, which consists of a first step of sludge thickening and a second step of sludge dehydration, said first step consisting in:

a) conveying excess sludge to be treated, which originates from a waste treatment plant, into a stabilization tank (2) provided with an overflow channel (3);

b) dispensing air inside said tank (2) in order to reduce the volatile component of said excess sludge that is present inside said tank

(2);

c) conveying said stabilized sludge that is present in said overflow channel (3) to the inlet of a thickener (3);

d) transferring the thickened sludge of said thickener (3) to the inlet of said stabilization tank (2) by means of a recirculation pump

(5);

said second step consisting in:

e) transferring a mixture of said excess sludge and of said thickened sludge from said stabilization tank (2) to a dehydration unit (6); f) drawing from said dehydration unit (6) the dehydrated sludge for subsequent operations such as disposal, agronomic reuse and the like;

g) drawing from said dehydration unit (6) the mother liquors in output and conveying them upstream of said waste treatment plant.

2. The process for the aerobic treatment of sludge and the like according to claim 1 , characterized in that said first and second steps are performed alternately.

3. The process for aerobic treatment of sludge and the like according to claim 1 , characterized in that said step g) is performed simultaneously with said step f).

4. The process for aerobic treatment of sludge and the like according to claim 1 , characterized in that said step d) comprises a step d2) that consists in transferring the mother liquors in output from said thickener (3) to said waste treatment plant.

5. A plant for performing aerobic treatments of sludge and the like, characterized in that it comprises a first sludge thickening circuit and a second sludge dehydration circuit, said first circuit comprising a stabilizing tank (2), which is provided with at least one inlet connected to a duct for feeding excess sludge to be treated, air injection means, an overflow channel (3), a thickener (3) connected to said overflow channel (3), and a thickened sludge recirculation pump (5), which is connected to an outlet of said thickener (3), which leads, through a respective pipe, to said at least one inlet of said tank (2), said second circuit comprising a dehydration unit (6) that is connected to an output channel of said tank (2) that conveys a mixture of said excess sludge to be treated and said thickened sludge.

6. The plant for performing aerobic treatments of sludge and the like according to claim 5, characterized in that said dehydration unit (6) is of the type selected preferably from a centrifugal pump, a filter press and the like.

7. The plant for performing aerobic treatments of sludge and the like according to one or more of the preceding claims, characterized in that said channel is affected by a pump (7) for feeding said mixture of said excess sludge to be treated and said thickened sludge, arranged upstream of said dehydration unit (6).

8. The plant for performing aerobic treatments of sludge and the like according to one or more of the preceding claims, characterized in that said thickener (3) is of the type selected preferably from a gravity thickener (3), a flotation thickener (3) and the like.

9. A plant for performing aerobic treatments of sludge and the like according to one or more of the preceding claims, characterized in that said pipe is provided with respective air injectors.