FR2519965A1 - Digestion of sewage sludge to produce methane and fertiliser - passes incoming sludge through steam ejector:mixer to preheat to about 95 deg. C - Google Patents

Abstract

Appts. for the digestion and thickening of sewage sludge obtd. from purificn. treatment plants. Upstream of the digestion chamber, incoming sludge is preheated by being passed through an ejector/mixer together with steam at high temp. and pressure, e.g. 185 deg.C and 10 bars. This results in sludge leaving the ejector/mixer at a temp. between 80 and 100 deg.C, pref. about 95 deg.C, which is sufficient to reduce the level of pathogens without completely sterilising the sludge. The digester is equipped with a mechanical agitator to homogenise the sludge. Digested material is discharged to a storage basin where the material is allowed to thicken. Used to produce methane and agricultural fertiliser by digestion and thickening of sewage sludge. The sludge is drawn from purificn. plants treating domestic, industrial or agricultural effluent. The controllable, high temp. injection of incoming sludge reduces the level of pathogens and enables the user to regulate fermentation rate.

Description

n Installation for conditioning sludge from water
waste for reuse in the field
agricultural ".

 The present invention relates to an installation for packaging, with a view to their reuse in the agricultural field, of sludges from domestic, industrial or agricultural waste water and obtained after a succession of operations implementing a primary treatment. involving screening, desanding, de-oiling, primary decantation providing primary sludge, secondary treatment including activation with oxygenation and secondary decantation providing secondary sludge, said primary and secondary sludge then being combined and providing, after a phase of thickening, fresh mixed sludge which is conveyed to a biochemical digester. In addition, the treated liquor can be concentrated pollution, as described above, or direct waste water if the organic load is low and fermentable.

 Today, the time has passed when everyone could source spring water. The user is no longer satisfied with clear and limpid water and the increasing requirements of modern hygiene ~ vis-à-vis viruses, micropollutants in particular, will require in a few years, the treatment of almost all the water drinkable worldwide by sophisticated techniques such as clarification, ozonization, use of activated carbon, etc.

 The industry consumes more and more water, because many are the processes which need this element at one or the other of the stages of their implementation.

Water is then a manufacturing agent, cooling agent or energetic fluid. Each requirement corresponds to one or more specific treatment techniques for industrial water, such as clarification, decarbonation, softening, demineralization, reverse osmosis, etc.

 Of all the natural environments disturbed by human activities, water is the first attack.

The purification of wastewater, an essential goal in the fight against pollution, has two distinct aspects depending on whether it is urban or industrial wastewater.

 Urban wastewater requires various treatments: pre-treatments such as screenings, sand removal, de-oiling, treatments by biological or physico-chemical purification. These different operations produce waste, sludge, which must be conditioned, eliminated by digestion after concentration, drying and sometimes incineration.

 Industrial wastewater requires complex treatment depending on the nature of the pollutants. No industry, or even any factory, gives identical pollution. That is to say that there are multiple techniques to be implemented: some, usual such as biological purification or lagooning; others, complex such as ultrafiltration, electrocoagulation or chemical detoxification.

 In the current technical context, there are several conventional methods for treating fresh mixed sludge and organic liquors.

 According to a first process, the thickened liquors are sent to an anaerobic digester, of the large concrete structure type, in which they are stirred and heated to around 350 ° C. to initiate the fermentation which gives off methane serving, via a boiler and a conventional heat exchanger, to heat the sludge, after the start of the process. These sludges are then treated in a device carrying out static or mechanical dewatering (pressing, centrifugation), an operation for which additives (flocculants) can optionally be used to facilitate their use.

This treatment provides on the one hand recycled water in the treatment plant and on the other hand, sludge in the form of a pelletizable product, which can possibly be used as fertilizer. This process makes it possible to recover the sludge as soon as it leaves the digester by transit in a sludge silo and to recover the pelletable product. However, this process can only be validly applied to large treatment plants because of its high implementation costs and the significant investments which prove necessary.

 According to a second method, the sludge is treated by aeration (aerobic stabilization). This process treats sludge by supplying large amounts of oxygen and reducing the amount of organic matter due to the development of the activity of microorganisms (aerobic). Then, the sludge is directed to drying beds in order to dehydrate them. This applies especially to small and medium-sized wastewater treatment plants and the operation of this second process has certain drawbacks: on the one hand, the quality of the disposal leaves something to be desired because the drying of the sludge is a function of their constitution, the climate and season. Furthermore, the irregularity of the drying time mobilizes the beds and leaves the basins upstream of the water treatment congested.

This results in both cases, a malfunction of the treatment plant due to jolts, there is loss of sludge to the receiving environment. In addition, the activation of sludge is expensive in energy.

 In some major treatment plants, the operations of the first process mentioned are implemented, then the mud is also sent to a drying bed, which causes the same drawbacks as above, but makes it possible to dispense with the dehydration mechanical. However, in these stations, the surface area mobilized is significant.

 It turns out that in the various solutions currently existing, the systems used are complex and / or require significant installation and / or numerous handling of dehydration, and always involve high investment and operating costs (labor work and energy).

 This is why, the present invention aims to greatly reduce the costs indicated, to obtain a final product of easy implementation with good agronomic qualities, without harmånce or health risk, and to eliminate the jolts of very harmful functioning to a complete purification, by replacing them a great regularity, and this as well in the course of digestion as in the production of methane.

 This problem is solved in an installation of the type described below thanks to the fact that it comprises, upstream of the digester, a simple apparatus for heating sludge of the type for ejection by steam under high pressure and temperature and comprising a nozzle coaxial with the sludge supply line, the circulation of steam and sludge taking place at an equal current and that the digester is confined by an elongated closed enclosure provided with a stirring device intended to homogenize the sludge during its passage through the enclosure, and the outlet of which is optionally connected to a storage and thickening tank for digested sludge. Thus fresh mixed sludge undergoes very brutal reheating, at high temperature, aimed at reducing the number of pathogenic germs without completely sterilizing the medium, while providing the calories necessary for the digestion phase, during which stabilization occurs, that is to say, a transformation of matter s organic which ferments producing methane, in a covered digester where a mixing of sludge takes place. Then comes a storage of the digested sludge, which causes it to thicken. These provisions clearly simplify the procedure by eliminating, separately or simultaneously, the conventional steps either of mechanical dewatering, or aerobic stabilization of sludge, or of the use of drying beds and by reducing the apparatus, the personnel and the necessary energy.

 According to a characteristic of the invention, the enclosure of the digester consists of a rustic earth digestion basin, the lower part of which is made watertight by clay or by a synthetic diaphragm and the upper zone of which is covered by a synthetic tarpaulin, market garden type, mounted on transverse arches and longitudinal reinforcement and blocked at the periphery, at ground level, by earth. We see that the investment costs are reduced compared to the prior art and that the installation according to the invention is adaptable to small or large scale and this with the same yield.

 According to another advantageous characteristic of the invention, the sludge heating apparatus, formed by a central sludge supply pipe, surrounded by the high pressure steam injection nozzle (for example 10 bars) and at high temperature (for example around 1850 C) intended to entrain the sludge by heating it to a sufficiently high temperature (between 80 - 100 C, preferably 950 C) to reduce the number of pathogenic germs without completely sterilizing the medium, consists of '' a cylindrical box partially crossed by the sludge supply pipe to a removable inner ring defining with this pipe an annular slot for the passage of steam towards the digester, the end of the pipe being secured to a ring perforated stop, fixed in the box upstream of the removable ring and that part downstream of the box in which is fixed an inner wear ring, bearing against the removable ring, is connected pa r flanges to the mud-steam mixture drive line to the digester.

Other characteristics and advantages of the invention will emerge from the description which follows by way of nonlimiting example and with regard to the appended drawings in which
- Fig. 1 shows a schematic assembly view, partially in longitudinal section of the sludge conditioning installation, according to the invention
- Fig. 2 shows a cross-sectional view of the installation taken along line 2 2 of Figure i
- Fig. 3 shows a view in longitudinal section of the sludge storage tank of the installation of FIG. 1; and
- Fig. 4 shows a partial sectional view of the sludge heating device of the installation according to the invention.

 The entire installation according to the invention consists of a sludge reheating device, generally designated by the reference i, to which a biochemical digester 2 (Fig. 1) is connected, the digested sludge of which is sent to a tank 3 for storing and thickening the sludge (fig. 4).

 The apparatus 1 for reheating fresh mixed sludge arriving via a supply line 4, coming from conventional and not shown installations, primary treatment, primary and secondary settling and prior thickening, consists essentially of a unit 5 described in more detail with reference to FIG. 4 and serving both to entrain, by heating, the mixed fresh sludge by means of ejection by steam under high pressure and temperature, by a pipe 6 opening into the unit 5 , in a pipe 7 connected to the inlet of the biochemical digester 2.

 In addition, the sludge heating device 1 comprises an automatic adjustment system for the flow rate of the sludge entrained in the pipe 7 to the digester 2 and which consists of a thermostatic probe 8 disposed in the pipe 7 and connected by a conductor 9 to a box 10 for supplying electrical energy from a network symbolized by two poles 11 for connection and regulation.

This box allows you to position the desired treatment temperature (setting the displayed setpoint value). This control box 10 is connected by a conductor 12 to an electro-mechanical modulator 13 connected to a modulation valve symbolized at 14 in the form of a movable flap placed in the conduit 4 for supplying fresh mixed sludge.

 Line 7 opens horizontally at the top of the biochemical digester 2, shown in longitudinal section in FIG. 1 and in cross section in FIG. 2. The digester 2 is in the form of an elongated sealed enclosure, constituted by a basin rustic in earth 15, whose cross-section and longitudinal section is trapezoidal. This basin can be coated on its bottom and its lateral walls inclined by a waterproof synthetic diaphragm 16 rising up to the horizontal lateral edges 17 of the basin and engaging in recesses 18 formed in these edges 17, if the tightness of the clay is not enough.

 Furthermore, the upper zone 19 situated above the basin proper, is covered by a synthetic retaining tarpaulin 20 of the market gardening type, supported by horizontal longitudinal reinforcement 21 and by transverse arches 22 and delimiting a sort of tunnel. The lower lateral edges of the sheet 20 also engage in the recesses 18 mentioned above and arranged in the earth of the edges 17 of the basin and held by blocking with earth. The lower zone 19 is maintained in slight depression in order to permanently suck up the gas produced.

 The digester 2 further comprises, for example in its central part, a sludge mixing device, generally designated by the reference 23 used to stir the sludge 24 in transit in the basin, by homogenizing them. This device 23 consists essentially of a propeller agitator block 25 suspended by rigid supports by supports 26, which can be adjusted in height and transversely in a conventional manner not shown in detail, to a horizontal upright or support suitable 27, which may be constituted by rails, a gangway, etc., and bearing laterally on the horizontal edges 17 of the basin, inside the sealed tunnel, above the level 28 of the sludge in the basin.

 The digester 2 comprises a pipe 29 for sludge outlet 24, which starts from a level located approximately halfway up in the basin 15, has a part along a side wall of the latter and exits out of the tunnel through the diaphragm 16 , in the form of a horizontal pipe element 30 disposed underground and opening into the storage tank 3. This pipe 29 for the outlet of digested sludge also plays the role of regulator of the constant level 28 of the sludge 24, ensuring emptying permanent and automatic sludge 24 by overflow and sealing.

 There is further provided a pipe 31 sealingly passing through the cover 20 of the tunnel and serving for the permanent extraction of the methane released by fermentation of the sludge 24 located in the basin, and which is connected to conventional installations not shown of storage and direct or secondary use of the gas produced.

 As can be seen in FIG. 3, the horizontal element 30 of the pipe 29 for leaving the digester basin 2, opens into the basin 3 for the storage and thickening of sludge, which is a rustic basin made of natural soil, of which a side wall, located on the outlet side, is formed by a dike 32. This basin also has a trapezoidal cross-section in longitudinal and longitudinal section and the pipe 30 opens into it descending via a pipe element 33 along the inclined wall, located entry side almost to the horizontal bottom 34 of the basin.

 This storage tank 3 comprises, on the outlet side, in its bottom 34, in the immediate vicinity of the dike 32, a drawing-off tank 35, from which the digested sludge 36, contained in this tank can be drawn up to a maximum upper level 37 , by means of a horizontal conduit 38 ending in a vertical part 39 fitted with a closing valve 40 and a connection fitting 41.

 In addition, there is also provided in this storage tank 3 a stirring device designated as a whole by the reference 42, similar to the stirring device 23 of the digester tank 2 and consisting of a propeller agitator block 43 mounted, with the possibility of vertical and lateral adjustments, on a vertical support 44 fixed in the bottom 34 of the basin 3.

 Hereinafter with reference to FIG. 4, we will describe in more detail the unit 5 serving simultaneously to heat and entrain the fresh mixed sludge towards the digester 2. This unit is constituted by a cylindrical box 45, in the closed end of which penetrates the pipe 4 sludge conveyance, which extends to the center and inside of the box 45 being integral with a fixed annular ring 46 mounted in the box. The latter has perforations 47 parallel to each other and to the axis of the box 45 and allowing the passage of steam from the pipe 6 in the direction of the outlet pipe 7, connected by a connection by flanges 48 to the outlet open from the box.

 In addition, a removable ring 49 slid into the box 45 is positioned around the free end of the pipe 4 and against the fixed ring 46, so that the downstream transverse faces of the ring 49 and the downstream end of the line 4, are located in a common plane. The ring 49 is formed by an annular block whose upstream face 50 is funnel-shaped and which has an internal cylindrical surface coaxial with the pipe 4 and delimiting with the latter, an annular passage 51 for the passage of steam arriving from the line 6, the box assembly 45 line 4 - rings 46 and 49 thus constituting a nozzle. Furthermore, downstream of the ring 49 and against the latter is arranged a ring or a cylindrical wear sleeve 52, slid into the box 45 mobilized in translation by the joint between flanges, this ring serving to protect the box at the nozzle ejection outlet.

 We will now describe the operation of the entire installation according to the invention.

 The fresh mixed sludge arrives in line 4 ending in the sludge heating and entrainment apparatus with a view, on the one hand, to undergoing an abrupt or thermal reheating there and being drawn towards the digester 2 in line 7 The purpose of reheating according to the invention is to reduce the number of pathogenic germs contained in the sludge, without completely eliminating them, by providing them with the calories necessary for the next stage of digestion. For this purpose steam at a temperature of about 185 C and a high pressure of 106 Pa is sent through line 6 into the box 45, where it passes through the perforations 47, and penetrates at high speed, after crossing the 'funnel 50 and the annular passage 51, in the space downstream and enclosed by the wear ring 52.

There, the depression created by the very high speed of the steam causes, by spraying and heating them, the fresh sludge delivered in the central position of the pipe 4 and this, according to an equicurrent circulation, in the direction of the pipe 7.

 The sludge entrained by the steam in the pipe 7 is thus brought to a temperature of between 80 and 100 ° C., preferably 950 ° C., and thus arrives in the covered biochemical digester basin 2 where they accumulate while being homogenized permanently by the stirring device 23. In this basin they are in the required temperature conditions of 32 to 35 ° C. favoring their fermentation, causing in the upper zone 19 of the sealed tunnel, a release of methane withdrawn in the pipe 31. In the digester 2, the hot sludge undergoes stabilization.

 As the sludge arrives via the pipe 7, the hot sludge circulates (from left to right in FIG. 1) to be finally transformed into digested sludge 24 at the outlet pipe 29, by which they are evacuated by siphoning towards the storage tank 3 (fig. 3), where they are stored and provide thickened sludge 36. It is then possible to withdraw sludge 36 continuously or discontinuously by means of the pipe 38 and of the shut-off valve 40. The tank 3 ensures the storage of sludge, essential for difficult spreading periods in the field of agriculture.

 It will also be noted that the device for automatic adjustment of the sludge supply rate in line 4 serves jointly to provide for a fixed vapor flow rate in line 6 (adjusted by the annular orifice of the nozzle) an adjustment thermostatic. In fact, if the flow rate of the sludge sent in the pipe 4 to the unit 5 is too high, the thermal probe 8 disposed in the pipe 7 leading to the digester 2 detects a value lower than the setpoint, with the effect that the box of regulation 10 controls the electromechanical modulator 13 which actuates the modulating valve 14, reducing the free passage in the pipe 4. As a result, the reduction in the flow of sludge causes, for the same thermal supply of steam in the pipe 6, heating more intense sludge entrained in the pipe 7.

 To summarize, it can be said that with this installation according to the invention, the installed power is very low, for example 26 kw instead of 330 kw in the case of known conventional installations with mechanical dehydration, drying and incineration, or a reduction of the factor 10, that is to say a significant energy saving.

 In addition, the amount of investments increases, for example, from approximately 9 million francs for a conventional installation of the prior art to a total of less than one million francs for the installation according to the invention, or a reduction factor equal to approximately 10. The operating cost is very low, given the elimination of certain operations previously planned, and operations carried out automatically and the low energy consumption. Finally, we obtain a final mud which is very easy to spread, which does not present any nuisance (odors) and has good sanitary characteristics.

Claims (8)

 10) - Installation for packaging for reuse in the agricultural sector, either sludge from domestic, industrial or agricultural wastewater and obtained in particular after a succession of operations using a primary treatment involving screening , a sand removal, an oil removal, a primary decantation providing primary sludge, a secondary treatment including an activation with oxygenation and a secondary decantation providing secondary sludge, said primary and secondary sludge then being combined and providing after a thickening phase, fresh mixed sludge which is conveyed in a biochemical digester (2) or polluted effluents sufficiently organic to be directly fermentable in this same digester, characterized in that it comprises, upstream of the digester (2) a device (1) for reheating sludges or effluents of the type with ejection by steam under pressure and el eves and comprising a nozzle (45, 46, 49) coaxial with the pipe (4) for supplying the sludge, the circulation of steam and sludge being effected at an equal current, and that the digester (2) is constituted by an elongated closed enclosure (15, 16, 20) provided with a stirring device (23), intended to homogenize the sludge during their passage in the enclosure, and the outlet of which is optionally connected to a basin (3) for storage and d thickening of digested sludge (24).  20) - Installation according to claim 1, characterized in that the enclosure of the digester consists of a rustic basin (15) of earth digestion, the lower part of which is sealed and the upper zone (19) of which is covered by a synthetic tarpaulin, market gardening type (20) mounted on transverse arches (22) and longitudinal reinforcements (21) and blocked on the periphery at ground level by earth.  30) - Installation according to claim 2, characterized in that the basin (15) of the digester (2) is an elongated rectangular basin, the cross section of which and in longitudinal section is trapezoidal and in which said device is disposed stirring (23) formed in the form of a propeller agitator block (25), mounted by being adjustable in height and transversely on transverse supports (27) extending over the entire width of the basin.  40) - Installation according to any one of claims 1 to 3, characterized in that it comprises a system for maintaining the sludge during digestion at a constant level, constituted by a pipe (29, 30) for permanent emptying of overflow connecting the outlet of the digester (2) to the storage tank (3) and opening out of the tank (15), at the same level as the pipe (7) for supplying the sludge-vapor assembly.  50) - Installation according to any one of claims 1 to 4, characterized in that the sludge heating device (1), formed by a central pipe (4) for sludge conveyance, surrounded by the nozzle d steam ejection at high pressure (for example 10 bar) and at high temperature (for example around 185 C intended to entrain the sludge by heating it to a sufficiently high temperature (between 80 and 100 C, preferably # 950) C) to reduce the number of pathogenic germs without destroying them completely consists of a cylindrical box (45) partially crossed by the pipe (4) for supplying the sludge to a removable inner ring (49) delimiting with this pipe , an annular slot (51) for passing steam in the direction of the digester (2), the end of the pipe (4) being integral with a perforated stop ring (46) fixed in the box (45) in upstream of the removable ring (49), and that the downstream part of the box (45) in which is fixed a inner wear ring (52) bearing against the removable ring (49), is connected by flanges (48) to the pipe (7) for driving the slurry-vapor mixture towards the digester (2).  60) - Installation according to claim 5, characterized in that the removable inner ring (49) has the shape of an annular block whose upstream face, facing the fixed perforated stop ring (46), is in the form of 'funnel (50) and whose downstream transverse face, against which carries the wear ring (52), is level with the free end of the pipe (4) for the supply of sludge.  70) - Installation according to any one of claims 5 and 6, characterized in that it comprises an automatic device for regulating the flow of sludge leaving the heating device (1) constituted by a thermal probe (8) disposed in the pipe (7) supplying the sludge-vapor mixture to the digester (2) and connected to a cabinet (10) for energy supply and thermostatic regulation, itself connected by an electromechanical modulator (13) to a modulation valve (14) installed in the line (4) for the arrival of fresh mixed sludge.  80) - Installation according to any one of claims 1 to 6, characterized in that the basin (3) for storing and thickening the digested sludge (24) is a rustic basin in the sludge arrival zone, a stirring device (42) of the propeller agitator type (43) and, downstream in the sludge outlet zone (36), a draw-off tank (35) allowing the sludge to be withdrawn via a pipe (38) extraction in which is mounted a stop valve (40).