Method and installation for purifying water using variably agitated denitrifying physical-chemical sludge

To purify untreated water, for example waste water, containing organic pollution, materials in suspension and nitrogen-containing pollution, reagents are added under conditions adapted to coagulate all of the materials in suspension, including colloidal materials, to form granular physical/chemical floc. Diluting water is added, preferably after the reagents, in a flowrate ratio of at least 2/1. The dilute flocculated water is caused to flow through a bed of sludge in the form of such dense and granular floc and denitrifying bacteria. The bed is subject to turbulent but intermittent agitation. A denitrified effluent is recovered. This denitrified effluent is caused to flow through a biological filter or preferably through a fluidized bed containing nitrifying bacteria and into which oxygen or air is injected. A clarified effluent is obtained, some of which is recycled as the diluting water.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns the treatment of waste water or drinking water to 
eliminate therefrom mineral or organic particulate and colloidal materials 
in suspension and nitrogen-containing and phosphorus-containing 
pollutants. 
2. Description of the Prior Art 
There are two normally contradictory water treatment techniques: 
physical-chemical treatment and biological treatment. 
Physical-chemical treatment uses chemical reagents adapted to cause 
coagulation of colloidal impurity particles by binding them to form floc 
from which bulky and heavy clumps are then obtained. The coagulant 
reagents are hydrolyzed and precipitate in the form of discrete particles 
whose structure depends on the physics/chemistry of the reagents and which 
have a very high specific surface area, often in the order of 1 000 
m.sup.2 /g. Turbulence induced in the water causes the particles to 
interact with the organic materials and the materials in suspension to 
neutralize their surface charge and absorb them to form a microfloc, 
constituting seed particles from which a future floc (i.e. a future 
visible clump) is obtained by growth of the microfloc. The water laden 
with floc is then transferred to a separator reactor from which a 
clarified effluent and sludge made up of this floc are recovered 
separately. A particularly high-performance example of such 
physical/chemical treatment is described in French Patent No. 2,631,021 
which describes a method of purifying water with a prolonged coagulation 
phase enabling, with moderate concentrations of reagents and appropriate 
agitation, fast elimination of materials in suspension, including 
colloidal materials. The floc can be fixed or free, depending on whether 
it is grown on granular supports or in the free state. Separation of the 
effluent from the sludge can follow on immediately from 
coagulation/flocculation or be separated from the latter by intermediate 
stages in which a greater or lesser degree of turbulence is maintained. 
The coagulation/flocculation reagents can be of highly varied types 
including iron salts (such as ferric chloride), aluminum salts (especially 
chlorides and sulfates) and salts of other metals; they can equally well 
be polymers. 
There are various known methods for determining the quantity and the nature 
of the reagents to be added to a given water in order to 
coagulate/flocculate the organic pollution thereof (C, usually defined by 
the Biological Oxygen Demand BOD, by the Chemical Oxygen Demand COD or 
even by the Total Organic Carbon TOC) or particulate pollution (Materials 
In Suspension: MIS). One of these methods is the JAR-TEST. The previously 
mentioned French Patent No. 2,631,021 describes how to adapt the 
conditions of the coagulation/flocculation process (with particular 
reference to agitation) to obtain the required flocculation using the 
minimum quantities of reagents. 
Biological treatments use bacteria (sometimes referred to collectively as 
"biomass") which can be free bacteria (in "activated sludge" systems) or 
fixed bacteria (in reactors containing granular supports on which the 
bacteria are grown and which constitute filters). These bacteria obtain 
their nourishment from the water to be treated. In practice steps are 
taken to promote the growth of a particular bacteria population, chosen 
for its ability to break down a given type of pollution. 
Biological treatments are used in particular to eliminate 
nitrogen-containing pollution from the water to be treated. This pollution 
is eliminated in two stages, using two different families of bacteria. 
Liquid ammonia (in practice the NH.sub.4.sup.+ ion) is eliminated by 
autotrophic nitrifying bacteria. These bacteria must be kept in an aerobic 
medium, which means that aeration is required. They oxidize the 
NH.sub.4.sup.+ molecule using the injected oxygen to form nitrites 
(NO.sub.2, Nitrosomonas bacteria) and then nitrates (NO.sub.3, Nitrobacter 
bacteria). 
The nitrates are eliminated by heterotrophic denitrifying bacteria which, 
using a carbon-containing substrate and in the absence of free oxygen (the 
medium must be anoxic), take oxygen from the nitrates, releasing nitrogen 
in gaseous form. 
Maintaining a given bacteria population obviously entails maintaining a 
certain number of operating parameters (aeration or no aeration, provision 
of suitable substrates in a form that can be assimilated) and avoiding any 
phenomena which could have toxic effects on the bacteria, with the risk of 
a reduction in biological yield or even poisoning of the bacteria. This is 
why water treatment agencies are disinclined to envisage the injection of 
reagents in substantial quantities at the entry to a biological reactor. 
A known and particularly effective process for the biological purification 
of nitrogen-containing pollution consists in causing the water to be 
treated to flow upwards through a denitrification reactor containing 
denitrifying bacteria in an anoxic medium and then through a nitrification 
reactor containing nitrifying bacteria in an aerobic medium, a substantial 
fraction of the effluent leaving the second reactor being recycled by 
mixing with the effluent to be treated before it enters the first reactor. 
The denitrifying bacteria therefore degrade the nitrates from the second 
reactor using the carbon-containing substrate constituted by the 
carbon-containing pollution in the effluent to be treated. The biological 
factor limiting the elimination of nitrates in this case is in practice 
the quantity of carbon present in the effluent to be treated in a form 
that can be assimilated by the bacteria. 
A process of this kind is described in French Patent No. 2,673,618. This 
patent describes a process for nitrifying and denitrifying polluted water 
by biological treatment in an aerobic medium and then an anaerobic (or 
anoxic) medium in which the untreated water, without preliminary settling, 
is caused to flow upwards through an anoxic first reactor with free 
biomass in the form of a highly concentrated sludge bed with a high upward 
flowrate. The effluent from the first reactor is transferred to an aerobic 
second reactor. Some of the treated water, overflowing from the second 
reactor, is recycled to the bottom of the first reactor. The total 
flowrate at the entry to the first reactor (water to be treated plus 
recycled water) preferably represents an upflow velocity of at least 3 
m/h. The flow entering the bottom of the first reactor produces a granular 
sludge made heavier by the material in suspension in the untreated 
influent. The concentration of this sludge in the first reactor is 
preferably between 30 g/l and 100 g/l. Agitation at the bottom of the 
first reactor improves distribution and enables bubbles of nitrogen to 
escape. The upper part of the first reactor is advantageously equipped 
with a lamellar (parallel plate) settling tank. The untreated water has 
usually been screened and had sand oil removed from it beforehand (but 
without any primary settling being necessary) and can have reagents added 
to it adapted to increase the settling rate of the sludge, for example 
flocculating agents such as alum, ferric chloride or polymers. Lime can 
also be added to form carbonates and precipitate the phosphorus. 
Another denitrification-nitrification process is described in European 
Patent No. 0,522,966 (TAMBO) which proposes a flocculation stage to form 
floc laden with phosphorus, a stage of mixing with a recycled fraction of 
nitrified water, a denitrification stage, a solid separation stage and a 
nitrification stage at the outlet from which the recycled fraction is 
taken. Polymer flocculating agents are used, for example, at a 
concentration of about 0.1 ppm-20 ppm (equivalent to 0.1 mg/l-20 mg/l). 
Further polymer flocculating agents can be added prior to the 
denitrification stage. 
The idea of a denitrification reactor with no granular material had already 
been described by A. KLAPWJIC, in particular in "The Application of an 
upflow reactor in the denitrification step of biological sewage 
treatment", KLAPWJIC, JOL and DONKER, published in Water Research Vol 13, 
pp 1009-1015, Pergammon Press Limited 1979. Tests have been carried out 
with a small reactor (14.7 l) and sludge containing denitrifying bacteria 
agitated intermittently (10 minute intervals between agitation for 3 
seconds at 120 rpm). The water to be treated had been allowed to settle 
beforehand, the upflow velocity was 0.12 m/h and the mass concentration of 
the sludge was 30 g/l. 
An object of the invention is to eliminate organic and nitrogen-containing 
(even phosphorus-containing) particulate and colloidal (materials in 
suspension) pollution, offering purification performance levels exceeding 
the prior art levels. In particular, it is directed to purification of 
urban or industrial waste water (and drinking water) in conformance with 
the E standards (MIS&lt;30 mg/l, COD&lt;90 mg/l, nitrogen-containing pollution 
in the form NH.sub.4.sup.+ and NO.sub.3.sup.- &lt;20 mg/l), or even the F 
standards (MIS&lt;15 mg/l, COD&lt;M 50 mg/l, nitrogen-containing pollution in 
NH.sub.4.sup.+ and NO.sub.3.sup.- form&lt;10 mg/l), and preferably the PT1 
standards (phosphorus&lt;2 mg/l) or even the PT2 standards (phosphorus&lt;1 
mg/l), using a process that is fast, efficient and economical and which 
does not require a costly and large-size installation. 
To this end the invention teaches combining a specific physical/chemical 
treatment (thorough coagulation/flocculation) with a specific biological 
treatment (denitrification with dilution in the nitrified water obtained 
in a second stage). 
It is important to emphasize that water treatment agencies have been 
reluctant to accept any such combination, for the following reasons: 
the short-term and long term effects of physical/chemical reagents on 
bacteria and on their biological yield is unknown or at best only poorly 
understood; all the more so in that there may be negative synergistic 
effects (with attendant risks of toxicity) between reagents and/or 
breakdown products which do not in isolation have deleterious effects, 
it is logical to think that the carbon which can be assimilated by the 
bacteria and which is therefore needed for the denitrification process (in 
theory this is only part of the BOD which is itself only part of the COD), 
is mainly in soluble form. It is clear that flocculation tends to reduce 
the quantity of soluble carbon available in the water, due to adsorption, 
and therefore tends to prevent the carbon that can be assimilated reaching 
the bacteria freely. It is therefore logical to fear that 
physical/chemical coagulation/flocculation treatment could reduce the 
quantity of carbon available in the water in a form that can be 
assimilated by the bacteria and therefore at least inhibit to a greater or 
lesser degree the biological denitrification process. It has therefore 
seemed unthinkable to envisage physical/chemical treatment pushed to the 
extent of virtually total elimination of the carbon-containing pollution 
that could be assimilated immediately before biological treatment 
employing heterotrophic bacteria using the carbon-containing substrate to 
break down the nitrates. 
SUMMARY OF THE INVENTION 
The present invention consists in a process for purifying untreated water 
containing organic pollution, materials in suspension and 
nitrogen-containing pollution, in which process: 
the untreated water, subject to given agitation conditions, has added to it 
coagulation/flocculation reagents in a given proportion and diluting water 
containing nitrates injected at a flowrate ratio of at least 2/1, to 
produce a dilute flocculated mixture, 
the dilute flocculated mixture is passed through a denitrification reactor 
via a bed of sludge containing denitrifying bacteria, subjected to 
agitation and maintained under anoxic conditions, to flow upwards therein 
at an upflow velocity sufficient to maintain expansion of the bed of 
sludge, clarified denitrified effluent overflowing from this reactor, 
the clarified denitrified effluent is caused to flow upwards in a 
nitrification reactor maintained under aerobic conditions and containing 
nitrifying bacteria fixed to a granular material, clarified nitrified 
effluent overflowing from this reactor, and 
the water containing nitrates with which the mixture is diluted is removed 
from the clarified nitrified effluent, wherein: 
the given agitation conditions and the given proportion of reagents are 
chosen such as to coagulate all the materials in suspension, including 
colloidal materials, contained in the untreated water in the form of dense 
and granular floc constituting, with the denitrifying bacteria, the bed of 
sludge, 
the agitation to which the layer of sludge is subjected in the 
denitrification reactor is turbulent but intermittent, with stop periods 
substantially longer than the agitation periods, adapted to shear and 
degas the bed of sludge. 
The invention can be regarded as a coagulation/flocculation/sedimentation 
process with the following specific features: 
to consume the organic pollution of the water to be treated, denitrifying 
bacteria in the sedimentation area are further nourished with water 
containing nitrates, 
the water containing nitrates comes from a nitrification area on the 
downstream side of the sedimentation area, 
the flocculated water flows upwards through the sludge, and 
the sedimentation area is agitated intermittently. 
As defined above, the invention is distinguished from French Patent No. 
2,673,618 and European Patent No. 0,522,966 by the extent of the 
coagulation/flocculation process involved and by the intermittent 
agitation of the denitrifying sludge. 
With regard to the first distinguishing feature, it has been mentioned 
above that although the above mentioned patents disclose the injection of 
reagents into water intended to pass through a bed of sludge containing 
denitrifying bacteria, one skilled in the art has been logically obliged 
to restrict this injection of reagents so as not to unduly reduce the 
allowable quantity of carbon available to the bacteria in a form they can 
assimilate; one skilled in the art was logically obliged to consider the 
quantity of reagents as a compromise between the requirement to increase 
the weight of the floc and that to maintain sufficient carbon available to 
the bacteria. 
The inventors have surprisingly discovered that this concept of a 
compromise was illusory and that, going against all expectations, the 
carbon which can be assimilated by the bacteria is not neutralized by 
coagulating it and incorporating it into the floc. In other words, the 
inventors have found that the carbon which can be assimilated by the 
bacteria (i.e. which is biodegradable) initially contained in the 
untreated water continues to be available after thorough 
coagulation/flocculation. 
In the present context, thorough coagulation/flocculation means 
coagulation/flocculation carried out with concentrations of reagents and 
agitation conditions adapted to eliminate as completely as possible all 
materials in suspension contained in the influent, including colloidal 
materials. A typical example of such thorough coagulation/flocculation is 
the prolonged coagulation process described in the previously mentioned 
French Patent No. 2,631,021. 
This is routine practice in the treatment of surface water: the correct 
concentration of coagulant is determined by a laboratory test, called the 
JAR-TEST, carried out with flocculation jars of given capacity (typically 
1 l) and given agitation conditions (typically 120 rpm) in which various 
concentrations of reagents are added to water representative of the water 
to be treated, in order to identify the concentration yielding the lowest 
possible turbidity (content of colloids). This thorough coagulation 
process is not used much in the case of waste water, however (this was one 
of the novel features of French Patent No. 2,631,021). 
It is important to realize that the sludge laden with biomass that the 
invention uses in the denitrification reactor is special, as compared with 
the sludge of French Patent No. 2,673,618 and European Patent No. 
0,522,966, in the sense of being formed of cross-linked floc which is 
dense and compact (average size typically less than 3 mm-5 mm), calling 
for moderate coagulation (typically with low concentrations of reagents, 
often less than around 20 mg/l with conventional agitation 
conditions--other than those of the prolonged coagulation of French Patent 
No. 2,631,021), and produces loose, poorly aggregated and sometimes 
fibrous floc which is bulky and not very dense and which can release some 
of the materials in suspension, especially the colloids, if agitation 
produces high shear stresses. 
The dense and compact cross-linked floc advantageously generated by 
uniformly distributed high turbulence (the teaching of French Patent No. 
2,631,021) guarantees, by virtue of its strength and its chemical 
formation, the absence of any significant release of colloids in the case 
of fast agitation, so preventing any deterioration of the water leaving 
the layer of sludge. Such floc can therefore withstand, without releasing 
colloids, agitation conditions adapted to degas the sludge to evacuate 
therefrom the bubbles of nitrogen generated by the bacteria breaking down 
the nitrates. 
In other words, one novel feature of the invention stems from the fact that 
thorough coagulation/flocculation producing dense cross-linked floc leaves 
the carbon-containing pollution in a form that can be assimilated by the 
bacteria while resisting the tearing off of colloids by brusque agitation 
phases needed for degassing. It seems that the adsorption of the colloids 
is irreversible; it seems that there is a threshold beyond which this 
irreversibility occurs. 
One hypothesis is that it is the quantity of the ferric hydroxide in the 
floc (essentially otherwise comprising materials in suspension) that 
causes irreversible adsorption of the finest materials in suspension 
(colloids), this adsorption being, of course, also dependent on the 
specific surface area of the floc (small and granular), this parameter 
being related to the greater or lesser turbulence with which the floc was 
formed and the sludge kept fine. 
Release of colloids would deteriorate the water leaving the denitrification 
reactor and would be accompanied by significant leakage of heterotrophic 
bacteria to the nitrification reactor in which their short population 
doubling time (around 0.5 h) would oppose the growth and the assimilation 
kinetics of the nitrifying autotrophic bacteria which have a much longer 
population doubling time (around 20 h). 
The concentration of coagulant or flocculating agent naturally depends on 
the pollution content of the untreated water; in accordance with the 
invention, in the case of untreated waste water, it is usually in the 
range from 30 mg/m.sup.3 to 100 mg/m.sup.3 of Fe Cl.sub.3. 
Dense and granular floc in the sense of the invention can be recognized, 
for example, by virtue of the fact that in the JAR-TEST a sample of such 
sludge (approximately 1 liter of sludge agitated at 120 rpm) settles out 
in a time period between a few minutes and about one quarter-hour. 
The physical/chemical sludge laden with biomass used in accordance with the 
invention is therefore fundamentally different from the sludge obtained by 
limited and partial coagulation/flocculation which does not eliminate 
colloidal materials in suspension. 
As mentioned above, the principle of brusque but intermittent agitation had 
already been proposed by KALPWJIC; there is no description of the purpose 
of such agitation, however, but given the numerical values indicated, 
which yield a very high peripheral speed (1.5 m/s in a 20 cm diameter 
reactor), it seems impossible for such agitation to bring about degassing 
without substantial release of colloids and therefore of bacteria and even 
of floc. Consequently, and given that KLAPWJIC describes only laboratory 
tests (which are difficult to extrapolate to an industrial scale), with 
very low upflow speeds (0.12 m/h) and using primary settling (whence the 
formation of sludge very different from that disclosed in French Patent 
No. 2,673,618 and European Patent No. 0,522,966, it is hard to see what 
might have caused the man skilled in the art to consider this mode of 
agitation in combination with the teachings of French Patent No. 2,673,618 
in particular. 
In accordance with the present invention, the purpose of the brusque but 
intermittent agitation is to generate strong, short-range (a few 
centimeters) turbulence which, applied briefly, shears the floc (obtained 
by thorough coagulation/flocculation, see above) forming the bed of 
sludge, preventing the sludge developing, or even breaking up the sludge, 
without releasing particles (colloids or materials in suspension) able to 
escape from the layer of sludge and to be entrained out of the 
denitrification reactor: there is therefore good release of bubbles of 
nitrogen and stabilization of the floc size. This stabilization is an 
important aspect in that the invention uses the layer of sludge as a 
filter (adapted to hold back colloids mechanically and by adsorption); it 
is therefore beneficial to maintain the performance of this filter 
constant. Controlling the maximal size of the nitrogen bubbles released, 
by appropriate choice of the agitation frequency, has the advantage of 
guaranteeing that the bubbles are released before they are large enough to 
lift the floc. 
It is particularly advantageous if the nitrification reactor is a 
three-phase fluidized bed (water, fixed biomass and air or oxygen 
bubbles). This enables use of the invention continuously and within a 
compact overall size as, compared with the use of the filter, there is no 
need to wash the filter (and therefore to interrupt normal operation of 
the installation) or to store a large volume of good quality water 
specifically for carrying out such washing. The granular material on which 
the nitrifying bacteria are fixed is fine sand in practice, with a typical 
mean size between 100 .mu.m and 1 000 .mu.m. The range from 200 .mu.m to 
500 .mu.m has been found to yield entirely acceptable fluidized beds. 
Specific tests have been carried out with a size of around 300 .mu.m. 
Alternatively, however, the nitrification reactor can be a biological 
filter. 
According to preferred features of the invention, some of which may be 
combinable with others: 
the turbulent but intermittent agitation is made up of intervals of between 
0.5 minute and 6 minutes duration between turbulent agitation phases of 
between 1 second and 10 seconds duration, 
each agitation phase is obtained by rotating an agitator with sharp edges 
having peripheral speeds of at least 0.3 meters per second at 1 meter from 
the rotation axis, 
the agitation is obtained by intermittent pumping within the layer of 
sludge, 
in this case, the pumping is adapted to force internal circulation of the 
layer of sludge through mechanical obstacles fixed relative to the pumping 
members, 
the mechanical obstacles are grids or bars, 
sludge is extracted from the denitrification reactor so as to maintain a 
constant biologically active height of the bed of sludge in service, 
the height of the bed of sludge in service is such that the upper surface 
of the bed of sludge remains on average at least 0.03 m below the overflow 
level, 
sludge is extracted from the bottom part of the denitrification reactor, 
and/or 
sludge is extracted from the top part of the active area of expanded sludge 
of the denitrification reactor, 
the denitrification reactor includes a separator in its upper part, for 
example a lamellar (parallel plate) assembly, 
the dilute mixture is forced through the layer of sludge over a height of 
at least 1 meter, 
a separator member is disposed in the bed of sludge so as to separate the 
layer of sludge into a top sub-layer which is subjected to turbulent but 
intermittent agitation and a bottom sub-layer which is not agitated, the 
mixture entering the denitrification reactor between these sub-layers, 
the given agitation conditions chosen by the coagulation induces 
homogeneous turbulence in contact areas, 
the given proportion of reagents is between 30 mg/l and 100 mg/l, 
the reagents are included in a group containing iron and aluminum salts and 
synthetic polymers, 
the diluting water is added after the reagents are added and allowed to 
act, 
the ratio of the flowrates of the diluting water and the untreated water is 
between 2/1 and 8/1. 
The invention further proposes, for implementing the method, an 
installation for purification of untreated water containing organic 
pollution, materials in suspension and nitrogen-containing pollution, 
embodying: 
a first area having an inlet for untreated water, a reagent inlet, agitator 
means, a diluting water inlet and an outlet connected to a first pipe, 
a first reactor containing in its bottom part a bed of sludge containing 
denitrifying bacteria and having an inlet in the bottom part connected to 
the first pipe, a sludge agitator, a sludge extraction outlet and 
containing in its upper part a clarified effluent outlet, 
a second reactor containing an immersed and aerated granular material laden 
with nitrifying bacteria and having an inlet connected by a second pipe to 
the clarified effluent outlet, an oxygen inlet for injecting oxygen, and 
an outlet in the upper part connected to a treated water removal line, and 
a recycling line connected between the removal line and the diluting water 
inlet of the first area, 
wherein: 
the bed of sludge of the first reactor is in the form of dense and granular 
physical/chemical floc and free denitrifying bacteria, 
the sludge agitator of the first reactor is operated intermittently with 
intervals between nonagitation phases substantially longer than the 
agitation phases. 
According to other features of the invention, some of which may be 
combinable with others: 
the second reactor is a three-phase fluidized bed composed of water, fixed 
nitrifying bacteria and injected oxygen, 
the granular material carrying the nitrifying bacteria is sand, 
the nitrifying bacteria is fixed to sand with a mean grain size between 110 
.mu.m and 1 000 .mu.m, 
the mean grain size of the sand is between 200 .mu.m and 500 .mu.m, 
the mean grain size of the sand is approximately 300 .mu.m, 
the bed of sludge of the first reactor has a concentration between 3 g/l 
and 20 g/l, 
a level controller is provided to maintain the top surface of the bed of 
sludge of the first reactor substantially constant in service, 
the level in service of the top surface of the bed of sludge of the first 
reactor is maintained at least 0.3 m below the clarified water outlet, 
the inlet of the first reactor is at least 1 meter below the level in 
service of the top surface of the bed of sludge, 
the first reactor contains, inside the bed of sludge, a separator member 
below the agitator and the inlet connected to the first pipe, 
the intermittent operation of the agitator of the first reactor includes 
activation of the agitator for between 1 second and 10 seconds and 
deactivation thereof for between 0.5 minute and 6 minutes, 
the agitator of the first reactor includes a rotary shaft having mobile 
parts with sharp edges immersed in service in the bed of sludge, 
the mobile parts include bars with sharp edges and spaced by a distance 
between 2 centimeters and 20 centimeters, the spacing being either 
constant or increasing from the rotation axis towards the periphery, 
the mobile parts have a peripheral speed of at least 0.3 m/s at 1 meter 
from the axis, 
the agitator includes a pumping member immersed in service in the layer of 
sludge, 
the agitator includes obstacles which are stationary relative to the 
pumping member and are disposed in the bed of sludge in such a way as to 
intercept the agitated sludge, 
the mechanical obstacles are grids or bars, 
the diluting water inlet is downstream of the first area, 
the first area includes a plurality of successive contact chambers each 
having a water inlet, a water outlet, an agitator and a reagent inlet, 
each mechanical agitator including thin paddles provided with combs whose 
teeth have identical REYNOLDS numbers and, 
the first reactor has a separator in its top part.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagram showing the stages of a purification process according 
to the invention and FIG. 2 shows one embodiment of an installation for 
implementing this process. 
The process starts with untreated water containing organic pollution, 
colloids and materials in suspension and nitrogen-containing pollution. 
This untreated water can have undergone preliminary treatment to eliminate 
large particles. 
This otherwise untreated water then undergoes in succession a 
coagulation/flocculation/dilution stage A, a denitrification stage B and a 
nitrification stage C. This produces a flow of treated water some of which 
is used as the diluting water for stage A. 
In the example shown the diluting water is injected at the end of stage A. 
As an alternative, shown in dashed line, it can be injected at the start 
of stage A. 
The dilution ratio (dilution flowrate/untreated water flowrate) is 
typically between 2 and 8. 
The purpose of coagulation/flocculation is to concentrate within the floc 
all particulate and colloidal pollution, as an ancillary result of which 
some of the dissolved pollution is adsorbed into the floc. 
The coagulation/flocculation is carried out in accordance with the teaching 
of French Patent No. 2,631,021, for example. To be more precise, the 
untreated water fed by a line 1 circulates through a plurality of contact 
chambers (three chambers 2A, 2B and 2C in this example) into which 
coagulation/flocculation reagents are injected and in which strong 
turbulence that is as homogeneous as possible is maintained. As mentioned 
above, it is preferably after this that the water (laden with floc) is 
mixed with the diluting water. 
The various chambers have agitators 4A, 4B and 4C rotated by a drive system 
5. 
The reagents are fed into the contact chambers via a distribution line 6. 
The total dose of coagulant is advantageously injected into the first 
chamber 2A. 
The total dose of reagents to be injected is determined by a JAR-TEST, for 
example, with the result corrected in accordance with the teachings of 
French Patent No. 2,631,021. 
In theory this total dose is between 30 mg/l and 100 mg/l; in other words, 
it is significantly greater than the doses disclosed in European Patent 
No. 0,522,966, for example. 
The reagents are in practice chosen from the following group: 
iron or aluminum salts, and 
synthetic polymers. 
The agitation conditions are chosen to suit the flowrate of water to be 
treated, advantageously so as to conform to an average retention time of 
two minutes in each of the chambers 2A through 2C. 
Quantitatively, the turbulence is preferably generated by dissipating in 
each chamber approximately 0.7 Wh/m.sup.3 of water to be treated. 
The turbulence is preferably identical in each of the three chambers, and 
advantageously generated by toothed mobile members in accordance with the 
teaching of French Patent No. 2,631,021. 
The agitator of one of the aforementioned chambers, that of the chamber 2A, 
for example, is shown in FIG. 3. It embodies a shaft 40 rotating a mobile 
member 41 provided with a comb. To be more precise, the teeth 42 of the 
comb have equal REYNOLDS numbers. To this end the teeth have dimensions 
parallel to the shaft which decrease from the center to the periphery of 
the mobile member. 
The diluted mixture obtained at the end of stage A is forced to flow 
upwards through a reactor 7 which is kept anoxic (no injection of free 
oxygen) and passes through a layer of sludge 8 containing denitrifying 
bacteria (typically heterotrophic type bacteria). The reactor 7 
advantageously includes a lamellar settling tank 9 at the top. The 
clarified denitrified water overflows from the reactor 7. 
The upflow velocity (typically 3 m/h or greater) is sufficient to maintain 
the bed of sludge (or at least the part thereof through which the diluted 
water passes) in a state of expansion. 
In the layer of sludge is an agitator member 10 controlled by an 
intermittent device 11 adapted to generate brusque but intermittent 
agitation in the layer of sludge 8, so as to shear periodically the floc 
forming the bed of sludge. 
This preferably involves intervals of between 0.5 minute and 5 to 6 minute 
duration between turbulent agitation phases of between 1 second and 10 
seconds duration. 
The turbulence is caused by the rotation of a mobile member in the form of 
a barred gate (vertical bars in this example) with sharp edges. 
In this example, with a rotating mechanical agitator, the (intermittent, 
see above) rotation speed is at least 5 rpm, i.e. a speed of at least 0.30 
m/s at a distance of 1 m from the agitator axis. 
As best shown in FIG. 2 the mechanical agitator member 10 includes vertical 
bars 10B which are substantially coplanar and fastened to the shaft 10A by 
horizontal crossbeams 10C. FIG. 4 shows an alternative agitator 10' in 
which the shaft 10'A carries not one but two rows of vertical bars 10'B in 
a quincunx arrangement and fastened by horizontal crossbeams 10'C. 
The vertical bars have sharp edges, at least in the areas designed to 
rotate in the sludge, so as to shear the sludge to the maximum, with a low 
drag coefficient (very restricted wake effect). 
FIGS. 5 through 12 show various cross sections for these bars (this 
selection is not exhaustive), the arrows indicating the direction in which 
the sludge moves past the bars. 
The bars 105 in FIG. 5 have a square cross section; the bars 106 in FIG. 6 
have a rectangular cross section elongate parallel to the forced flow of 
sludge; the bars 107 in FIG. 7 have a transverse rectangular cross 
section; the bars 108 in FIG. 8 have a triangular cross section with the 
point to the rear; the bars 109 in FIG. 9 have a trapezoidal cross section 
with the larger side at the front; the bars 110 have a cross section with 
a flat side at the front and a rounded side at the rear; the bars 111 have 
a diabolo-shape cross section with flat sides at the front and at the 
rear; and the bars 112 in FIG. 12 have a cross section in the shape of two 
trapeziums joined together on their shorter side. 
The spacing of the bars is constant in this example. As an alternative, it 
can increase from the axis towards the periphery. It is advantageously 
between 5 cm and 15 cm and the transverse dimension of the bars 
(perpendicular to the forced flow of sludge) is typically between 0.5 cm 
and 2 cm, preferably between 10 and 30 times smaller than the distance 
between the bars. 
The bars have an active vertical dimension (i.e. with sharp edges capable 
of shearing the floc) of between 100 cm and 150 cm in practice. The same 
goes for the vertical dimension of the mobile members of an agitator with 
a plurality of mobile members. 
If the reactor has a square cross section the total transverse dimension of 
the mobile members is preferably between 90% and 99% of the side length of 
this square cross section. 
The top surface of the active part of the mobile member of the agitator is 
preferably between 5 cm and 30 cm below the average level of the layer of 
sludge in service. The level of the sludge layer obviously varies with the 
upflow flowrate. 
In the embodiment shown in FIG. 13 the intermittent agitation is obtained 
by pumping sludge inside the bed, so that the sludge is recirculated. The 
pumping produced by a pump 51 preferably causes the sludge to circulate 
through members 52 which are fixed relative to the pump, such as bars or 
vanes disposed in the flow of pumped sludge. 
Excess sludge is taken off through a line 12. This extraction is 
advantageously regulated by means of a level sensor 13 controlling an 
extractor pump 14, or even a simple purge valve (not shown) operating by 
gravity and disposed at the bottom of a chute at the bottom of the 
denitrification reactor. 
The level is chosen so that the top surface of the layer of sludge remains 
on average between 10 cm and 100 cm below the lamellar settling tank 9 (if 
present). The level is also chosen such that the mixture from stage A 
travels a distance through the layer of sludge dependent on the quantity 
of nitrates to be reduced, in practice at least 1 m. 
In this example the excess sludge is extracted from the bottom of the 
reactor (conventionally chute-shaped, in practice, and optionally provided 
with conventional scrapers). 
As shown in FIG. 1, extraction of excess sludge is conventionally obtained 
by sludge traps 60 in the upper part of the bed of sludge. 
In an embodiment shown in FIG. 14 a separator member 71 such a lamellar 
settling tank is disposed in the sludge layer and adapted to separate the 
layer into an upper area which is agitated (by an agitator with bars 72) 
and a calmer, sludge concentration lower area. The mixture is then fed in 
just above the separator member, by means of a distribution line 73, for 
example. The upper area/lower area height ratio is preferably between 0.3 
and 3.0. 
The average concentration of MIS in the sludge bed is from 2 g/l to 10 g/l 
and this sludge embodies: 
colonies of (living or dead) free bacteria constituting a biomass, 
physical/chemical floc (hydroxides+mineral materials in suspension). 
The clarified effluent leaves the lamellar settling tank 9 (merely a 
precaution against particles leaving the bed of sludge in service for any 
temporary reason) at a flowrate equal to the sum of the untreated water 
and diluting water flowrates, and therefore very much greater than the 
untreated water flowrate alone. 
The clarified effluent is conveyed by a line 20 to the bottom of a second 
reactor 21 containing nitrifying (in particular Nitrobacter and 
Nitrosomas) bacteria 26 fixed to a granular support (for example sand). 
The presence and depolluting activity of the bacteria are promoted by an 
input of free oxygen, in practice in the form of air or oxygen, via a line 
22 in the conventional manner. 
At the top of the second reactor 21 is a second separator 23 over which 
nitrified water overflows, most of this water being fed as diluting water 
to stage A via a line 24. The remainder, whose flowrate is equal to that 
of the untreated water, is taken off through a line 25 as treated water. 
In this example the second separator 23 is in the known form of channels 
with convergent upper edges along and at a short distance from which are 
deflectors 23A whereby the water, on the point of being recovered by 
overflowing, is calmed to remove any agitation likely to cause entrainment 
of particles by bubbles of oxygen or air which could degrade the quality 
of the water or the denitrifying bacteria. 
The second reactor does not have any mechanical agitator means. 
The second reactor is preferably a three-phase fluidized bed, containing 
nitrifying bacteria fixed to an aerated and mobile granular material. 
The granular material is advantageously sand with a mean grain size between 
100 .mu.m and 1 000 .mu.m, preferably between 200 .mu.m and 500 .mu.m; 300 
.mu.m grains were chosen for the trials. The advantages of this choice 
emerge below. 
In an alternative embodiment the second reactor is a biological filter 
containing nitrifying bacteria fixed to an immobile granular material. 
In the FIG. 2 example the nitrifying second reactor has an elongate 
rectangular shape and runs along one side of the first reactor. The 
clarified water conveyed by the line 20 is injected along two lower 
longitudinal sides of the second separator 23 which in this example are 
the reactor. It is recovered at the top by means of troughs 23. 
In FIG. 2 the line 27 is a diagrammatic representation of internal 
recirculation between the outlet troughs 23 and the bottom feed of the 
nitrifying device, which contributes to expansion of the granular material 
to which the bacteria is fixed. 
For a mean flowrate of 208 m.sup.3 /h, for example, and untreated water 
containing on average: 
COD: 450 g/m.sup.3, 
of which BOD: 220 g/m.sup.3, 
MIS: 200 g/m.sup.3, 
nitrogen-containing pollution: 70 g/m.sup.3 NTK (Nitrogen Total KJEDAL), 
the choice can be as follows: 
* nature of reagents: FeCl.sub.3, 
* proportion of reagents: 50 g/m.sup.3, 
* recycling rate: 300%, 
* volume of chambers 2A to AC: 7 m.sup.3, 
* denitrification reactor cross section: 180 m.sup.2, 
* denitrification reactor height: 4.5 m approx, 
* dilute flocculated water upflow: 3.5 m/h, 
* height of sludge: 1.3 m, 
* sludge concentration: 4-6 kg/m.sup.3, 
* second reactor cross section: 70 m.sup.2, 
* second reactor height: 4.5 m. 
Table 1 shows the results of analysis of the water at the inlet and outlet 
of the denitrification reactor (phase B, FIG. 1) after, firstly, moderate 
injection of reagents without specific precautions and, secondly, with 
greater quantities injected, matched to the agitation conditions indicated 
above to achieve thorough coagulation. 
Table 1 also shows the results of analysis of the water at the exit from 
nitrification stage C. 
The table shows that the physical/chemical treatment proposed by the 
invention (addition of reagents in quantities and under conditions adapted 
to achieve thorough coagulation), combined with circulation of the water 
through a bed of denitrifying sludge (with additional COD consumption for 
denitrification and with adsorption of colloidal material) produces a 
water quality at the outlet from the denitrification reactor such that, 
when recycled, it achieves dilution of the untreated water almost as good 
as if the dilution were effected using water from the main supply. 
Accordingly, the quality of the decanted-denitrified water finally 
entering the nitrification reactor means that the latter can be a 
fluidized bed requiring no washing or regeneration of the material, whence 
great simplification of the installation. The biological yield of the 
fluidized bed is significantly greater than is obtained conventionally in 
treatment of waste water (approximately 1.5 times that of the reactors 
usually used to treat waste water). 
The invention provides a high capacity as two reactors 
(decanter-denitrifier and nitrifying three-phase fluidized bed) are 
sufficient to achieve very high levels of depollution. No ancillary 
capacity is required, either to store treated water needed for washing as 
in the case of a nitrification reactor with filter bed and fixed granular 
material, or for intermediate storage of dirty water resulting from the 
washing of any such filter. 
Obviously the foregoing description is given by way of non-limiting example 
and numerous variations thereon can be put forward by one skilled in the 
art without departing from the scope of the invention. 
For example, the flocculation can be carried out in the denitrification 
reactor (separated from the bed of sludge in any appropriate manner), and 
even in the entry pipes to this reactor. The agitation speeds and the 
agitation times and intervals can be varied in service. 
TABLE 1 
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WATER with conventional flocculation 
prolonged coagulation 
______________________________________ 
entry to 
MIS = 100 mg/l MIS = 60 mg/l 
phase B 
COD = 200 mg/l COD = 140 mg/l 
NTK = 28 mg/l NTK = 17 mg/l 
NO3 = 10 mg/l NO3 = 10 mg/l 
exit MIS = 70-80 mg/l MIS = 8-10 mg/l 
from COD = 120-130 mg/l COD = 40-50 mg/l 
phase B 
NTK = 28 mg/l NTK = 17 mg/l 
NO3 = 0.0 mg/l NO3 = 0.0 mg/l 
exit MIS = 50-60 mg/l MIS = 6-8 mg/l 
from COD = 100-110 mg/l COD = 40 mg/l 
phase C 
NTK = 15-20 mg/l NTK = 2-3 mg/l 
(final NO3 = 7-12 mg/l NO3 = 12-15 mg/l 
quality) 
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