Abstrict
An anaerobic fermenter-decanter for the purification of residual
water from the sugar industry, with recovery of methane, consists
of a tank with inclined walls, with a central agitator on a vertical
shaft. A flexible cover anchored by its periphery to the walls of
the tank and totally submerged forms a collecting pocket for the
fermentation gases. The water to be purified is introduced, after
being heated to about 35.degree., towards the bottom of the tank
near the agitator. A metal collecting bell with submerged edges
and with the shaft of the agitator passing axially through it is
connected by its edges to a central opening of the cover. The purification
yields may exceed 90%.
Claims
I claim:
1. An anaerobic fermenter-decanter for digesting and clarifying
sludge from sugar manufacturing and refining plants and for recovering
fermentation gases, said anaerobic fermenter-decanter comprising
an upwardly opening inverted truncated pyramid shape tank having
a substantially flat bottom wall and a fluidtight skin covering
the sidewalls and the bottom wall thereof, an intake pipe passing
through a heater and delivering heated sludge to the bottom of said
tank adjacent an impeller carried by a vertical shaft of a rotary
agitator, a flexible cover anchored at its periphery midway along
the height of the side walls of the tank and extending inwardly
to a central opening, said cover being tensioned between the central
opening and its peripheral anchoring, a metal collecting bell having
free edges adapted to be immersed in the sludge and attached at
the central opening of the cover, whereby fermentation gas produced
during digestion is collected at the center of the cover, an evacuation
tube for removing the fermentation gas from the center of the cover,
the peripheral edges of the cover and the adjacent side walls of
the tank defining between them a marginal peripheral space for the
flow of digested sludge upwardly to a decantation area disposed
in the upper part of the tank above the cover so that the cover
is entirely immersed in said tank, means for removing the digested
sludge from the upper part of the tank, and means for removing clarified
liquid from the upper part of the tank.
2. A fermenter-decanter according to claim 1, wherein said collecting
bell comprises a downwardly opening annular collecting chamber defined
between inner and outer walls, said cover being attached to the
collecting bell at said outer wall, the inner wall of the collecting
chamber surrounding the agitator axis and defining with a lower
flange a sealed cavity.
3. A fermenter-decanter according to claim 2, wherein said means
for removing clarified liquid from the upper part of the tank comprises
gutter means surrounding the outer wall of the collecting bell and
forming an overflow lip, and an evacuation channel for carrying
the clarified liquid, said gutter means determining the level of
liquid in the upper part of the tank and thereby insuring that the
cover is constantly immersed.
4. A fermenter-decanter according to claim 1, wherein the periphery
of the cover has a hem with peripherally extending rods, and tensioning
members attached between the rods and horizontally extending beams
for tensioning the cover.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a fermenter-decanter intended for the
purification of residual waters, particularly from sugar refineries,
with recovery of the fermentation gases which consist for the most
part of combustible methane.
2. Description of the Prior Art
Conventional purification by storing the residual water from sugar
refineries in static water, this residual water containing various
sugars which are destroyed by fermentation during the storage in
the static water, requires very large tanks, causes the release
of disagreeable odours into the environment, does not permit precise
control of the effective purification level and, finally, means
that the energy contained in the sugar products is left to disperse
into the ambient medium.
The inventor and his fellow-workers have made studies in the purification
of residual water by fermentation under the effect of mesophilic
anaerobic micro-organisms (i.e. those which are active in a temperature
range around 35.degree. C.), the results of this type of fermentation
being mainly methane and carbon dioxide, the latter being largely
held in a carbonate combination, whilst the methane which is released
constitutes a useful fuel. In fact, fermentation occurs in two stages;
in the first stage, acidifying micro-organisms convert the sugars
into organic acids, whilst strictly anaerobic mesophilic micro-organisms
decompose the acids into methane, carbon dioxide and water.
The inventor's studies were based on laboratory tests, then tests
on pilot fermenters. A first pilot plant, comprising a 90 m.sup.3
tank, at the Escaudoeuvres sugar refinery, was followed by a second
with a 1580 m.sup.3 tank at the sugar refinery of Vauciennes. A
description of the work and the plants will be found in the journal
Sucrerie Francaise: "La depollution des eaux", by J. P.
Lescure and P. Bourlet, March 1977, pages 103-109; "Nouvelles
perspectives pour le traitement des eaux en sucrerie; la fermentation
methanique mesophile", P. Devillers, J. P. Lescure and P. Bourlet,
April 1977, pages 173-183; and "Traitement des eaux residuaires
par fermentation methanique mesophile", J. P. Lescure, P. Bourlet,
March 1978, pages 107-114. In the plants described, fermentation
took place at about 35.degree. C. by the passage of the affluent
into a heater, from which this affluent was carried along piping
to the bottom and near the centre of the tank, where it was mixed
up by a rotary agitator. A cover made of reinforced butyl rubber
was anchored by its periphery and submerged in the tank and covered
the fermentation zone so as to form a collection pocket for the
methane released, which was evacuated via tubing fixed to the upper
part of the cover pocket. This arrangement, which resulted in rapid
completion of the fermentation process, nevertheless had some disadvantages
which affected the safety of use and the reliability of the installation.
In fact, the cover was not tensioned and partially emerged from
the water. The agitation of the residual water below the cover was
transmitted to the cover, which then became subject to premature
cracking. Leakages through the cover, through the beginnings of
cracks, created a danger of explosions or fire in the pocket inflated
by the gas.
SUMMARY OF THE INVENTION
To remedy these disadvantages, the invention provides an anaerobic
fermenter-decanter intended for the purification of residual water,
especially from the sugar industry, with recovery of the fermentation
gases, which consist for the most part of combustible methane and
comprising a tank excavated in the ground with a fluid-tight skin,
in the form of a truncated pyramid with walls widening upwards and
a substantially flat bottom and adapted to be substantially filled
with residual water to be purified, towards the centre of the tank
a rotary agitator with a vertical shaft having an impeller near
the bottom, a flexible cover extending in the central zone of the
tank, anchored by its periphery substantially halfway up the walls
and forming a pocket for collecting the fermentation gases, with
a tube for the evacuation of the fermentation gases at the top and
pipes for the intake of residual water passing through a heater
and opening out near the impeller, wherein a collecting bell made
of metal, with submerged edges and having the shaft of the agitator
passing axially therethrough, is connected by its edges to a central
opening of the cover, the latter being tensioned between its central
opening and the peripheral anchoring points.
With the metal collecting bell alone emerging from the tank, the
fermentation gases are not present in a substantial quantity, in
the undispersed state in the residual water, except in the bell
and the evacuation tubing. Thus, the risk of leakages of combustible
gas is virtually eliminated. The cover, being supported between
the bell and the anchoring points, is not susceptible to any great
movements; moreover, the movements of the cover are suppressed by
the presence of water on both its surfaces. The risk of cracking
is therefore greatly reduced.
Preferably, the bell comprises a collecting chamber which is open
at the bottom and laterally bounded by inner and outer edges, the
latter forming a connection with the central opening of the cover,
whilst the inner edge surrounds the agitator axis, defining a sealed
cavity with a lower flange. Thus, gas leaks along the agitator axis
are reduced.
Preferably, the outer edge of the bell is surrounded by an annular
spout open at the top, with a horizontal peripheral rim forming
an overflow lip and a channel for the evacuation of purified residual
water. As the level of residual water in the tank is determined
by the overflow lip over which the purified residual water being
evacuated through the channel flows, the depth of immersion of the
edges of the bell is maintained constant.
According to a preferred arrangement, the periphery of the cover
is braced by rods passing through lap-joints and fixed by tensioning
means to the anchoring points, the latter being fixed in a frame
formed by beams extending horizontally substantially halfway up
the tank. This ensures satisfactory rigidity of the cover's edges
without subjecting the cover to excessive local stresses. The beam-type
frame ensures good performance of the anchoring points, thanks to
its weight and structure, without the need to bury these anchoring
points in the underlying ground, piercing the fluidtight skin.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general sectional view of a fermenter-decanter according
to the invention;
FIG. 2 shows a collecting bell in more detail;
FIG. 3 is a schematic perspective view of a collecting cover;
FIG. 4 is a detailed view of the arrangement for peripherally anchoring
the cover.
DETAILED DESCRIPTION OF THE INVENTION
According to the preferred embodiment of the invention shown in
FIG. 1, a fermenter-decanter comprises, excavated in the ground,
a tank 1 in the general form of a straight truncated pyramid with
a square base, with edges opening outwardly in the upward direction
and inclined at about 45.degree.. A fluidtight skin 2, formed in
known manner by assembling plastics sheets, is fixed at its periphery
by being buried in trenches 3, 3' and is weighted with blocks of
masonry. Halfway up the inclined edges are provided horizontal set-offs
4, 4' to which the fluidtight skin is adapted in form. On these
set-offs 4, 4' are placed concrete beams 4a, 4'a which form a frame
in which are fixed piles or anchoring points 11, 11' for a submerged
flexible cover 10 in the form of a pyramid with a square base. At
the top of the cover 10, this cover is connected to a collecting
bell 12 which partially emerges from the water and which has in
its centre a motor 13 for a rotary agitator with a vertical shaft
14 ending near the bottom of the tank with a helical impeller 15.
The bell 12 is held to the bottom of the tank by a tripod support
17 and is connected to the periphery of the tank by a platform 20
which forms a support for a tube 21 for the evacuation of fermentation
gases which comes out of the annular chamber 18, opening towards
the bottom of the bell 12 and for a channel 22 for evacuating the
purified waste water, said channel starting from the peripheral
spout 16. The waste water to be purified, or affluent, is introduced
into the tank 1 through piping 23 the end 23a of which opens out
near the bottom of the tank, in the vicinity of the impeller 15;
this is after it has passed through a heater 24 intended to bring
the temperature of the affluent to about 35.degree., as this temperature
promotes the activity of mesophilic anaerobic micro-organisms.
The bell 12 with the shaft 14 of the agitator passing vertically
through it is more clearly shown in FIG. 2. The bell consists of
an annular chamber 18 bounded by inner and outer cylindrical walls
30 and 32, respectively, which are coaxial with the shaft 14 and
this chamber is open at the bottom. The inner wall 30 has a flange
31 on its lower part so as to define a sealed cavity 19 through
which the shaft 14 passes. Surrounding the outer wall 30, the annular
spout 16 is provided with a peripheral rim 16a which forms an overflow
lip for the purified waste water or effluent, so as to define the
level in the tank whilst ensuring that the walls 30 and 32 are permanently
partly submerged. The cover 10 is clamped between the base of the
spout 16 and a circular crown 33 which terminates the tripod support
17. It will be appreciated that the fermentation gases formed below
the cover 10 collect in the chamber 18 to be evacuated through the
tubing 21 without penetrating to an appreciable extent into the
cavity 19, as the submerged flange 31 obstructs the passage of the
gases.
As can be seen from FIG. 3, the cover 10 is made up of four triangular
sections 41, 42, 43 and 44 converging towards the top circular connecting
aperture 45 with the collecting bell. The bases 41a, 42a, 43a and
44a of the sections are reinforced by rods to form a square.
FIG. 4 shows, in more detail, how the edges of the cover 10 are
stiffened and anchored. The cover edge forms a lap-joint 49, by
the bending and fastening of the border 50 over the cover 10. A
rod (metal tube) 51 is passed through the lap-joint 49. The anchoring
on the pile 11 fixed in the beam 4a is obtained by means of a tensioning
member 52 of the nature of a long nut with a reversed thread engaging
with two threaded rods, the rod located nearest the cover 10 ending
in a yoke 53 which fits over the rod 51. Controlled tightening of
the tensioning nuts makes it possible to tension the cover uniformly
over its entire periphery, whilst the tensile stresses of the tensioning
members 52 are transmitted uniformly to the cover via the reinforcing
rod 51 and lap-joint 49.
In one embodiment, the tank, with a fluidtight skin of reinforced
polyvinyl chloride, had an opening, at ground level, with sides
22 meters long and with a depth of 5 meters, with walls inclined
at 45.degree.. The agitator was a helical mixer (S E M H M 2500)
with variable speeds, permitting circulation flow rates of 3000
to 17600 m.sup.3 /h, at operating cycles of between 3.2 and 22 revolutions
per minute, whilst the motor had a power of 10 h.p.
Nutrient nitrogen- or phosphorus-containing products may optionally
be added to the affluent introduced under the cover, so as to obtain
a C/N/P equilibrium of .about.300/5/1. The pH is maintained constant
by the addition of lime and scum (CaCO.sub.3) from the clarification
of juices. After fermentation, the waste water passes round the
periphery of the cover in order to be clarified by decantation in
the part of the tank located above the cover (decantation surface
area approximately 400 m.sup.2). The slurry is evacuated discontinuously
by means of a submerged pump, whilst the clarified effluent passes
into the spout and is evacuated as fast as the affluent is introduced.
For the test results, the following abbreviations are used:
COT total organic carbon content;
DCO chemical oxygen requirement;
DBO.sub.5 biological oxygen requirement, up to day 5.
______________________________________ Purification Results Average
DCO Charge by Gas flow rate taken volume kg production Yield Week
m.sup.3 /h Kg/day DCO/m.sup.3 /day m.sup.3 /day % ______________________________________
43 27.0 2430 3.04 490 83 45 37.6 5022 6.28 960 83 47 31.9 3926 4.91
550 88 48 .about.30 3000 3.75 518 70 49 18.3 2020 2.53 700 77 50
19.5 2915 3.64 706 97 ______________________________________ Analysis
minimum mean maximum ______________________________________ Affluent
COT mg/l (C) 1145 1956 2280 DCO mg/l (O) -- 5896 6900 DBO.sub.5
mg/l (O) -- 3983 4920 Kjeldahl nitrogen mg/l (N) 37 49.7 62 Total
Phosphorus ng/l (P) 6.3 8.4 9.9 Total sulphur 0.4 6.5 19.3 Effluent
COT mg/l 62 414 807 DCO mg/l 340 1107 2400 DBO.sub.5 mg/l 95 720
1600 N Kjeldahl mg/l 14 36.7 41 P total mg/l 2.5 4.3 6.7 S total
mg/l 0 3.0 7.5 Gas % CO.sub.2 9.96 12.30 15.8 H.sub.2 traces 0.22
0.42 O.sub.2 0.8 1.11 1.76 N.sub.2 3.18 3.95 6.14 CH.sub.4 80.00
82.41 85.95 ______________________________________
By recovering 700 m.sup.3 /day of gas containing 82.5% methane,
i.e. 420 kg of methane per day, one has calorific energy of about
5000 therms, i.e. approximately 5800 kW/h, whilst the purification
of waste water attains a valuable level.
Obviously, from the calorific energy recoverable, one must deduct
the energy required to heat the affluent, but the balance sheet
still remains positive and can be improved by passing the effluent
and affluent in counterflow through a heat exchanger, so that the
losses of heating energy are limited to the losses on the surface
of the tank. Moreover, in absolute terms, these losses increase
more slowly than the capacity of the tank.
Although the fermenter-decanter described above has been produced
within the framework of the sugar refining industry, which produce
large quantities of fermentable effluent requiring purification
before it is disposed of in the environment, it is plain that there
is no obligatory connection between the structures described and
the origin of the fermentable effluent and consequently the fermenter-decanter
can be used for the treatment of effluent from any origin, provided
that this effluent is capable of anaerobic fermentation under the
effect of specific micro-organisms, with methane as the end product.
In fact, methane is the chief end product of all reducing degradation
of organic matter and anaerobic micro-organisms select themselves
under conditions in which their activity is optimum, these conditions
including the nutrient medium (effluent which has optionally been
adjusted), the temperature and an adequate dispersion of the micro-organisms
in the effluent. From this point of view, the arrangements used
in this fermenter-decanter are generally suitable and lend themselves
to the particular adjustments required. Moreover, the structure
of the fully submerged collecting pocket is suitable for collecting
any combustible gas formed within an agitated liquid filling a tank
under conditions of safety and reliability, for a relatively low
installation cost, whatever the formation process of the combustible
gas. However, the formation process of the combustible gas is specifically
fermentation.
It is obvious that the invention is not limited to the examples
described, but covers all variants.
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