Method of purifying waste water biologically

A method of purifying waste water biologically, in which at least one biological reaction is performed in at least one biological reactor containing microorganisms in activated sludge, and the waste water is separated from the activated sludge in a clarification basin, with at least a fraction of the activated sludge from said biological reactor being recirculated, wherein, prior to admitting the waste water to be treated into said biological reactor, at least a fraction of the polluting particles in suspension therein are separated out therefrom, the separated polluting particles are sent to a sludge activation reactor that is separate and disposed in parallel, the separating polluting particles are used as the main substrate for the development of microorganisms in strict aerobiosis, and the superactivated sludge obtained in this way is injected into the biological reactor(s).

The invention relates to a method of purifying waste water biologically, in 
particular domestic, urban, or industrial waste water or any water 
containing pollutants or impurities in solution and in suspension, said 
pollutants and impurities being suitable for removal biologically. 
BACKGROUND OF THE INVENTION 
At present, waste water or sewage is purified essentially by a method 
consisting in causing a culture of bacteria dispersed in treatment basins 
to develop in the presence of oxygen (with such a culture also being known 
as a free culture or activated sludge), then, after sufficient contact, 
the purified water is separated from the sludge by sedimentation in a 
clarifier, with a fraction of the sludge being recirculated to one of the 
treatment basins in order to maintain a sufficient concentration of 
purifying bacteria therein while the remainder of the sludge, representing 
excess activated sludge, is removed from the installation. Such a 
purification method seeks both to eliminate organic carbon pollution and 
to oxidize nitrogen pollution by nitrification. By including periods of 
contact between the activated sludge and the water to be treated in the 
absence of aeration (anoxic contact) and in the presence of carbon, 
heterotrophic microorganisms are caused to degrade nitrates into gaseous 
nitrogen in order to perform denitrification. In addition, by subjecting 
the microorganisms to systemic alternation between anaerobic and aerobic 
conditions, it is possible to cause phosphor-containing compounds to 
accumulate in excess in the microorganisms, thereby giving rise to 
biological dephosphatization of the water being treated. Over the last 20 
years,numerous variants of that method including various possible 
dispositions of anaerobic, anoxic, and aerobic zones have been developed 
and have given rise to numerous patents and publications. Unfortunately, 
it has been observed that in methods using activated sludge, the parameter 
limiting nitrification is not the reaction kinetics of transforming 
ammonia or organic nitrogen into nitrate, but the age (or real retention 
time) of the sludge which is necessary for conserving nitrifying 
microorganisms in the installation. As is known, the times required for 
forming such nitrifying microorganisms are very long and vary strongly 
with temperature: thus, at a temperature of 11.degree. C., the sludge must 
be 10 days old, and it may be assumed that for temperatures below 
11.degree. C., the growth of such microorganisms is slowed down very 
considerably. In addition, these microorganisms which are obligate aerobes 
are capable of growing in an aerated zone only, so the retention time that 
needs to be taken into consideration is the time spent in the aeration 
basin. As a result, it is necessary to provide aeration basins of large 
volume that are overdimensioned relative to reaction rate. 
If the age of the sludge is written A, the temperature of the biological 
basin in .degree.C. is written T, and the daily production of excess 
sludge is written Px kg of solids/day!, then the mass of activated 
sludge, being aerated for nitrification purposes, Mba, is given by the 
following equation: 
EQU Mba=A*Px (1) 
with: 
EQU A=4.5*0.914 .sup.(T-20) ( 2) 
which, according to the proposals of the German organization Abwasser 
Technischen Vereinigung E. V. (abbreviated below as ATV) gives rise to a 
daily sludge production Px of 
EQU Px=0.6*(MES+BOD.sub.5)-(0.072*0.6*K)/(1+0.08*K)!*BOD.sub.5( 3) 
where: 
MES=daily throughput of solids admitted to biological treatment (in 
kg/day); 
BOD.sub.5 =daily throughput of 5-day biological oxygen demand admitted for 
biological treatment (in kg/day) with: 
EQU K=A*1.072 .sup.(T-15) ( 4) 
The dimensions of the aeration volume are then given by the following 
equation: 
EQU V=Mba/Cba (5) 
where: 
V=aeration volume (m.sup.3) 
Mba=mass of activated sludge (kg) 
Cba=concentration of activated sludge (kg/m.sup.3). 
Several solutions have been proposed for reducing the aeration volume. 
For example, it has been proposed to increase the concentration (Cba) of 
the biomass being aerated. If Qt is the through flow rate, Qr the 
recirculation flow rate, and Cr the concentration of the substance being 
recirculated, then the mass equilibrium equation for the sedimentation 
clarifier is as follows: 
EQU Qr*Cr=(Qt+Qr)*Cba (6) 
from which it can be deduced: 
EQU Cba=(Cr*Cr)/(Qt+Qr) (7) 
Since Cr is necessarily greater than Cba, the clarifier must therefore 
perform a thickening function to enable the concentration Cr to be 
achieved. However, the thickening concentration of a sludge is a function 
of thickening time (Tps) and of the mechanical characteristics of the 
sludge. Such mechanical characteristics can be expressed by the volume 
sedimentation index in ml/g (i.e. the volume occupied by 1 gram of sludge 
after 30 minutes of sedimentation), which mechanical characteristics are 
written below as ISV. As proposed by ATV, it is possible to write: 
EQU Tps=(Cr*ISV)/(1000*K)!.sup.3 ( 8) 
or: 
EQU Cr=K*(1000/ISV)*Tps .sup.1/3 ( 9) 
with: 
Tps=thickening time of sludge being clarified (hours) 
ISV=sludge volume index (ml/g) 
K is a constant equal to 0.6 or 0.7 depending on the sludge takeup 
technique. 
Unfortunately, since the thickening of sludge in clarification takes place 
in the absence of added oxygen, if it is desired to avoid degrading 
treatment performance and the mechanical quality of the sludge then Tps 
must necessarily be less than, or at most equal to, the time the actived 
sludge spends in anaerobiosis. A Tps of 3 h is generally considered as 
being the maximum value. Consequently, in the presence of a very good ISV 
corresponding to 100 ml/g, the recirculation Cr cannot exceed 10 g/l if 
equation (9) is applied. 
In addition, Qr is limited firstly by the volume of the clarifier allocated 
to thickening sludge, and secondly by problems of distributing energy at 
the inlet of the clarifier. For both of those reasons it is common to 
adopt a recycling ratio (where recycling ratio is defined as Qr/Qt) equal 
to 100% or rarely 150% of the raw water flow rate. Using the preceding 
example of an ISV of 100 ml/g and according to equation (7), the recycling 
ratio gives rise to a concentration of biomass in aeration Cba lying in 
the range 5 kg/m.sup.3 to 6 kg/m.sup.3, which is the concentration 
conventionally observed in water works. 
Another means of increasing the concentration of the biomass in aeration is 
to use a system comprising firstly a "contact" basin where nitrogen in the 
form of ammonia is transformed into nitrates (nitrification) and secondly, 
on the sludge return circuit, a "stabilization" basin which enables a high 
value of sludge age to be obtained. Under such circumstances, the 
above-described constraints are found again, i.e. activated sludge 
concentration is equal to Cba in the contact basin and the concentration 
in the stabilization basin is equal to Cr, which represents only a small 
overall reduction in the volume of the works. 
EP-A-0 309 352 describes a method in which obtaining the desired age for 
the sludge is performed in a generation basin located in a loop outside 
the reaction basins and wherein, prior to recycling the sludge through at 
least one purification process, a step of concentrating said sludge by 
floatation is performed upstream from the generation basin. That method 
makes it possible to achieve a considerable reduction in the volume of the 
Works in that the problem of obtaining aged sludge is treated in a basin 
that operates at high concentration (30 g/l) so the reaction basins can 
then be dimensioned as a function of reaction rates. 
The essential problem in operating conventional activated sludge methods is 
that it is essential to obtain sludge having at least a minimum age while 
seeking high reaction rates, and this must be done in a reaction context 
which is favorable to neither of those two parameters. Studies have shown 
that those parameters and objectives are partially incompatible because 
the specific activity of sludge decreases with its age. 
OBJECT AND SUMMARY OF THE INVENTION 
The method of the invention enables the above incapability to be mitigated 
in that prior to inserting waste water to be treated into the biological 
reactor, at least a fraction of the polluting particles in suspension is 
separated therefrom, and the separated polluting particles are applied to 
a sludge superactivation reactor which is separate and disposed in 
parallel, the separated polluting particles being used as the main 
substrate for the development of microorganisms in strict aerobiosis, and 
the superactivated sludge obtained in this way is injected into the 
biological reactor. 
The development of microorganisms in strict aerobiosis in the sludge 
superactivation reactor makes it possible to obtain superactivated sludge 
having high specific activity, and particularly suitable for eliminating 
carbon and nitrogen pollution and for biological dephosphatization in the 
reaction basins of conventional equipment for treating waste water by 
means of activated sludge. A particular advantage of the invention lies in 
obtaining operation of the purification installation using activated 
sludge under conditions limited only by the biological reaction kinetics 
of the reactions used in the various biological reactors, with this being 
independent of the constraints of maintaining sludge age, since that is 
achieved in a specific reactor that is independent from the water path and 
from the sludge path. 
This independence of the reactor for superactivating sludge also sets it 
apart from the hydraulic conditions applied to the station, thus 
guaranteeing in said reactor, a) maintenance of high concentrations of 
substrates, and b) good control over the transit time of microorganisms 
under aerobic conditions, thereby having the following consequences: 
a) The high concentration of substrate in the superactivation reactor makes 
it possible, because of the exothermal nature of the oxidizing reactions, 
to obtain temperatures (25.degree. C. to 40.degree. C.) in the reactor 
which are highly favorable to the development of microorganisms in general 
and in particular to the development of the nitrification microorganisms. 
Thus, if equation (2) is applied, the sludge age that needs theoretically 
to be maintained in the reactor in order to obtain development of the 
nitrification microorganisms lies in the range 2.8 days to 1 day for 
temperatures lying in the range 25.degree. C. to 35.degree. C. 
b) By keeping the biological culture permanently in strict aerobiosis in 
the superactivation reactor, it is possible to obtain optimum conditions 
for growth of the strict aerobic microorganisms (such as the autotrophic 
microorganisms that perform nitrification), and to exert selection on 
microorganisms that are obligate anaerobes, or optional anaerobes. In 
addition, the power of the biological reactions implemented gives rise to 
considerable degradation of the substrate (about 35% to 50% of organic 
matter is eliminated) and thus of polluting particles, thereby reducing 
sludge production by 20% to 30%. 
A method of purifying water biologically may be implemented in various 
different ways, of which the following are cited by way of example: 
a mere step of oxidizing the carbon pollution, performed in a biological 
reactor operating under aerobic conditions; 
a step of oxidizing the carbon pollution plus a step of nitrification which 
take place under aerobic conditions and generally in the same biological 
reactor; 
a step of oxidizing the carbon pollution, a step of nitrification, and a 
step of denitrification which can be performed in the same biological 
reactor, the first two steps taking place under aerobic conditions while 
the last step takes place under anoxic conditions, the reactor operating 
alternately in aerobiosis and in anoxia; 
a step of oxidizing the carbon pollution, a step of nitrification, and a 
step of denitrification, the denitrification step taking place in anoxia 
in a biological reactor that is distinct from the reactor for oxidizing 
and for nitrification, and that is situated upstream therefrom and through 
which a fraction of the contents of the oxidizing and nitrification 
reactor is recycled; and 
a step of oxidizing the carbon pollution, a nitrification step, a 
denitrification step, and a dephosphating step, the dephosphating step 
taking place in an anaerobic biological reactor which is situated upstream 
from the denitrification reactor and in which there is recycled the waste 
water separated from the activated sludge which is collected at the end of 
the system. 
In all cases, the superactivation reactor is placed in parallel between the 
separator and the first biological reactor, and the superactivated sludge 
it produces is injected into at least one of the subsequent biological 
reactors, either continuously or discontinuously. 
In order to improve the efficiency with which polluting particles are 
captured, a biosorption reactor is used situated upstream from the 
polluting particles separator, with the water to be treated being put into 
contact in said biosorption reactor with the superactivated sludge coming 
from the superactivation reactor. In another variant, the nitrates formed 
in the superactivation reactor can be eliminated by implementing an anoxic 
reactor situated between the polluting particle separator and the 
superactivation reactor. 
In yet another variant, additional substrate may be added in the super 
activation reactor in order to acclimatize the superactivated sludge to 
eliminating particular pollution, or for the purpose of increasing the 
specific activity of said sludge. 
The superactivation reactor may also be used as a storage tank, and the 
various biological reaction basins are then fed with superactivated sludge 
only as a function of requirements, e.g. during peak periods, so as to 
obtain a better ratio between pollution flow and biomass. 
One of the advantages of implementing the method of the invention is that 
it is possible to increase very significantly the retention time of 
activated sludge through the clarification reactor without fear of 
degrading the quality of the sludge by anaerobiosis. This is due firstly 
to the thorough elimination of organic matter from the activated sludge 
due to the use of superactivated sludge, and secondly to the strictly 
aerobic conditions that are maintained in the superactivation reactor. As 
a result, the following are obtained: ISV=80 and Tps=6. If equation (9) is 
applied, a concentration of recirculation product Cr is found of the order 
of 15 kg/m.sup.3, which for a recirculation ratio of 150% makes it 
possible, according to equation (7), to maintain a concentration Cba of 
the order of 9 kg/m.sup.3 in the reaction basins.

In the figures, identical reference numerals relate to identical elements. 
MORE DETAILED DESCRIPTION 
In FIG. 1, there can be seen a scheme for a water purification station 
using a superactivation reactor of the present invention. 
All or a fraction of the effluent to be treated A is admitted into a phase 
separator 1. When a fraction is admitted, then the remaining portion Al of 
the effluent does not pass into the separator 1. In the separator 1, 
polluting particles are recovered using various possible systems, 
dynamically or statically. The effluent to be treated, with at least some 
of its polluting particles removed therefrom, is sent to a reaction basin 
5 where it is put into contact with activated sludge and from which, after 
the retention time necessary for obtaining the biological reaction(s), it 
is sent to the clarifier 6 where it is separated into an effluent A2 that 
satisfies the looked-for discharge standards and activated sludge which is 
recirculated to the reaction basin 5. The polluting particles B leave the 
phase separator 1 and are directed to a superactivation reactor 2 in which 
the microorganisms develop in aerobiosis. The superactivated sludge C 
coming from the reactor 2 is injected into the reaction basin 5 in 
continuous or discontinuous manner. 
The reaction basin 5 can operate 
under aerobic conditions so as only to oxidize the carbon pollution; 
under aerobic conditions so as both to oxidize the carbon pollution and 
simultaneously to nitrify the ammonia nitrogen and the organic nitrogen; 
and 
under aerobic conditions both to oxidize the carbon pollution and to 
nitrify, followed by operation under anoxic conditions to eliminate 
nitrogen by denitrification. 
The scheme shown in FIG. 2 differs from the scheme shown in FIG. 1 by the 
addition of a denitrification reaction basin 4 disposed upstream from the 
reaction basin 5 in Which the carbon pollution is then oxidized and the 
nitrogen is nitrified, together possibly with additional denitrification 
by switching to anoxic conditions. The fraction of the contents of the 
basin 5 that has been subjected to oxidizing of the carbon pollution and 
to nitrification is recirculated to the basin 4 for denitrification. The 
superactivated sludge coming from the superactivation basin 2 can be 
injected in continuous or discontinuous manner into the reaction basin 5 
(sludge C) or into the denitrification basin 4 (sludge C1 ). 
FIG. 3 is a scheme similar to that of FIG. 2, to which there is added a 
biological dephosphating basin 3 disposed upstream from the 
denitrification basin 4, itself disposed upstream from the reaction basin 
5. As before, the reaction basin 5 oxidizes the carbon pollution and 
performs nitrification of the nitrogen by operating under aerobic 
conditions, and possibly also additional denitrification by operating 
under anoxic conditions. The superactivated sludge formed in the 
superactivation reactor 2 may be injected in continuous or discontinuous 
manner into the reaction basin 5 (sludge C), into the denitrification 
basin (sludge C1) or into the dephosphating basin 3 (sludge C2). 
The scheme in FIG. 4 shows a variant of the method shown in the scheme of 
FIG. 1, however the variant shown in FIG. 4 could also be applied to the 
treatment scheme of FIGS. 2 and 3. 
Prior to insertion of waste water to be treated A in separator 1, with or 
without a bypass fraction A1, the water A is treated in a biosorption 
reactor O in which it is put into contact with superactivated sludge D 
coming from the superactivation reactor 2. This makes it possible to 
improve the efficiency with which polluting particles are captured because 
of the bioflocculation effect that occurs in the reactor O. 
FIG. 5 shows a variant of the method shown in FIG. 1, however this variant 
could equally well be applied to the schemes shown in FIGS. 2, 3, and 4. 
The variant in Figure 5 consists in providing a predenitrification step in 
a predenitrification basin 7 situated between the phase separator 1 and 
the superactivation reactor 2. 
In the reactor 7, the polluting particles B are put into contact with 
superactivated sludge E containing nitrates and coming from the 
superactivation reactor 2. This variant makes it possible to eliminate at 
least a fraction of the nitrates formed while the microorganisms are 
growing in the superactivation reactor 2. 
The following examples serve to illustrate the advantages provided by the 
present invention, and in particular the percentage volume saving obtained 
in the works when implementing a sludge superactivation reactor situated 
in parallel with the water-plus-sludge path. 
The examples apply to an installation corresponding to the scheme of FIG. 1 
(only one reaction basin in which the carbon pollution is oxidized, and 
also a nitrification/denitrification step), and the waste water treated 
had the following basic parameters: 
______________________________________ 
flow of water to be treated 
10,000 m.sup.3 /day 
daily throughput of BOD.sub.5 
3,000 kg/day 
daily throughput of solids 
3,500 kg/day 
daily throughput of NTK 
600 kg/day 
minimum temperature 11.degree. C. 
______________________________________ 
where NTK is total nitrogen measured using the Kjeldahl technique. 
EXAMPLE 1A 
Conventional treatment without primary sedimentation 
Sludge age from equation (2): 10 days. 
Production of excess sludge from equation (3): 3,725 kg/day. 
Mass of activated sludge in aeration: 3,725 * 10=37,250 kg 
Maximum possible IVS 150 ml/g and Tps=3 hours. 
Maximum recirculation concentration from equation (9): 6.3 kg/m.sup.3. 
Maximum recirculation ratio 150% 630 m.sup.3 /h. 
Maximum clarification speed 0.7 m/h 1,100 m.sup.2. 
Clarification volume 1,100+(630*3)=3,000 m.sup.3. 
Activated sludge concentration from equation (7): 3.8 kg/m.sup.3. 
Aeration volume from equation (1).apprxeq.10,000 m.sup.3. 
Volume for denitrification, 30% of aeration volume=3,000 m.sup.3. 
Total volume of reaction basin 10,000+3,000=13,000 m.sup.3. 
Total volume of works 13,000+3,000=16,000 m.sup.3. 
EXAMPLE 1B 
Conventional treatment with primary sedimentation 
Average speed applied to primary sedimentation means: 0.9 m/h. 
Primary sedimentation volume.apprxeq.1,400 m.sup.3. 
Reduction of solids due to primary sedimentation.apprxeq.50%. 
Reduction of BOD.sub.5 .apprxeq.35%. 
Reduction of NTK.apprxeq.9%. 
Throughput of solids admitted to biological treatment: 1,750 kg/day. 
Throughput of BOD.sub.5 admitted to biological treatment: 1,950 kg/day. 
Throughput of NTK admitted to biological treatment: 546 kg/day. 
Sludge age from equation (2): 10 days. 
Production of excess sludge from equation (3): 1,822 kg/day. 
Mass of activated sludge in aeration 1,822*10=18,220 kg. 
Maximum possible IVS 180 ml/g and Tps=3 hours. 
Maximum recirculation concentration from equation (9): 5.3 kg/m.sup.3. 
Maximum recirculation ratio 150%, 630 m.sup.3 /h. 
Maximum clarification speed 0.7 m/h 1,100 m.sup.2. 
Clarification volume 1,100+(630*3)=3,000 m.sup.3. 
Activated sludge concentration from equation (7): 3.2 kg/m.sup.3. 
Aeration volume from equation (1).apprxeq.5,700 m.sup.3. 
Volume for denitrification, 35% of aeration volume=5,700*0.35.apprxeq.2,000 
m.sup.3. 
Total volume of reaction basis 2,000+5,700=7,700 m.sup.3. 
Total volume of works 7,700+3,000+1,400=12,100 m.sup.3. 
Total production of sludge 1,750+1,822=3,572 kg. 
EXAMPLE 1 C 
Treatment installation using a superactivation reactor of the present 
invention 
Average speed applied to primary sedimentation means: 0.9 M/h. 
Primary sedimentation volume.apprxeq.1,400 m.sup.3. 
Reduction of solids due to primary sedimentation.apprxeq.50%. 
Reduction of BOD.sub.5 .apprxeq.35%. 
Reduction of NTK.apprxeq.9%. 
Age of sludge from equation (2) at 25.degree. C.: 2.8 days (we use a value 
of 4 days). 
Production of superactivated sludge: 1,250 kg/day. 
Mass of superactivated sludge in the reactor 1,250 * 4=5,000 kg. 
Volume of superactivation reactor 5,000/20=250 m.sup.3. 
Throughput of solids admitted to biological treatment: 1,750 kg/day. 
Throughput of BOD.sub.5 admitted to biological treatment: 1,950 kg/day. 
Throughput of NTK admitted to biological treatment 546 kg/day. 
Throughput of superactivated sludge: 1,250 kg/day. 
Nitrification speed : 3.5 mg N-NO.sub.3 nitrified per gram of 
superactivited sludge per hour. 
Production of excess sludge from equation (3): 1,822 kg/day. 
Nitrogen in biological synthesis: (1,250+1,822)*5%=154 kg/day. 
Nitrogen to be nitrified : (600-154-10,000*0.05)=396 kg/day. 
Mass of superactivated sludge for nitrification 396/(0.0035 * 24)=4,715 kg. 
Retention time of superactivated sludge through biological treatment : 3.77 
days. 
Total mass of activated sludge: (1,250+1,822) * 3.77=11,581 kg. 
Denitrification speed : 2 mg N-NO.sub.3 /g of superactivated sludge/h. 
Mass of nitrogen to denitrify (396-10,000*0.010)=296 kg/day. 
Mass of activated sludge for denitrification 296/(0.002*24)=6,166 kg. 
Maximum possible IVS 100 ml/g and Tps=4 hours. 
Maximum recirculation concentration from equation (9): 10.4 kg/m.sup.3. 
Maximum recirculation ratio: 150%. 
Concentration of activated sludge from equation (7): 6.2 kg/m.sup.3. 
Maximum clarification speed 0.7 m/h 1,100 m.sup.2. 
Clarification volume 1.100 (630*4)=3,620 m.sup.3. 
Volume of reaction basin (1): 11,581/6.2.apprxeq.1,868 m.sup.3. 
Total volume of works: 1.868+3,620+250+1,400=7,138 m.sup.3. 
Total production of sludge: 1.250+1,822=3,072 kg/day. 
It can be seen that there is no difficulty in obtaining a reduction in the 
volume of the works that may be as great as 40% to 50% depending on the 
starting path used.