GB2159808A - Process for conditioning organic sludge - Google Patents

Abstract

The process relates to the conditioning of organic sludges, for example, from effluent treatment, in order to improve the water extraction capability, to reduce the mass and the volume, and to prevent decomposition during further application. The conditioning is achieved with low energy input and without the use of chemicals. The sludge, containing at least 0.5 % organic dry substance, is excited constantly or at predetermined intervals, to a frequency of at least 0.3 s<-1>, while at the same time injecting oxygen. To induce oscillation/vibration, the use of rotating bodies or vibrators has been found particularly advantageous. As a result, the dry substance is reduced by more than 80 % at ambient temperature. The sludge volume reduces to less than 5 %.

Description

SPECIFICATION Process for conditioning organic sludge The invention relates to the conditioning of sludges which contain a substantial proportion of organic constituents. These are mainly sludges deriving from water-treatment and from biotechnology. The process according to invention is particularly suitable for improving the filtration behaviour, for preventing decomposition during the period of further treatment, and for reducing the mass and volume of the sludge.

In the treatment of organic sludges, the following objectives are set: 1. Increasing the sludge concentration to reduce the volume for subsequent treatment processes; 2. Improving the hydroextraction capacity to achieve a solid product, e.g. for the preparation for treatment at sludge hydroextraction sites, the artificial filtration or the centrifuging action; 3. Preventing the decomposition during the period of further processing, or constantly (stabilisation).

4. Disinfection of the sludge To increase the sludge concentration, particular use is made of sedimentation in settling tanks or of flotation. In so doing, the mass of the dry substance is not reduced, but the volume is reduced by about 50%. Through sedimentation, only low solids contents can be achieved. To increase the rate of concentration, the use of raking mechanisms in the thickener is known. It is also known to use vibrators. In this way, there is a shakingtogether of the solids and the water is released. Further reaching effects of this mode of operation are not known.

Besides the stabilisation processes, an improvement in the hydroextraction behaviour is achieved through the addition of flocking additives and by the thermal treatment. It is also known that a homogeneously disperse colloid mass can be achieved by treating activated sludge by high-frequency vibrations (ultrasonic). This process can be used only in conjunction with a following chemico-physical flocculation, because after destruction of the activated sludge flocks, a renewed flock formation is necessary.

Currently, stabilisation takes place mainly by the anaerobic or aerobic processes. A notable reduction in the treatment time is achieved by the enzymatic sludge stabilisation system develped by the applicant. The energy input for this is about 7.5 kWh/m3 sludge.

Aerobic conditioning processes which act under psychrophilic and mesophilic conditions, have a duration of 10 to 17 days; under thermophilic conditions, times of 3.5 to 5 days are achieved.

The object of the invention is to find a process with which, under good hydroextraction facility of sludges, the conditioning time is greatly reduced. The necessary energy input should be low and the use of chemicals and ferments is to be dispensed with. The sludge should not decompose in the following hydroextraction stage, and its effectiveness should be increased by significant reduction in volume and weight.

The object of the invention is to develop a process which, by ensuring optimum conditions for the development of micro-organisms in hitherto unknown sequence, and through combination of process stages, which have hitherto not been possible to combine industrially, would bring about a change in the distribution of the frequency of individual systematic groups of micro-organisms and by removal of inhibitors formed under natural conditions through over production to achieve an increase in their activity, which would lead to a radical reduction in the overall processing time of the aerobic stabilisation of organic sludges, and to a significant reduction in the volume of the sludges with simultaneous reduction in the mass of the dry substance.By conducting the process under psychrophilic conditions (preferred temperature 285-291 K) and providing for electrokinetic, physicochemical and phsyico-mechanical fringe conditions, as well as through the action of catalytic activators and dispensing of inhibitors, a significant saving in energy is to be achieved, together with a reduction in the necessary reactor volume.

The applicant has developed a process in which suspensions are filtered through a fine grain filter layer. The removal of the filteredoff substance takes place continuously without interruption of the filtering process, by means of a soiled-water pump discharging the substance from the apparatus which demarcates a certain area, and generates within this, a flow direction counter to the filtration water, and thereby cleanses or washes the filter layer. By means of a rotating roller, the cleansing process of the sand is promoted, as the result of which, a mechanical-hydraulic regeneration of the filter surface is achieved.With the objective of hydroextraction, the following procedure is adopted for charging the filtering mechanism with sludge containing organic substances: Excess sludge from activated sludge installations (alone and in mixtures with primary sludge, untreated and pre-thickened) is charged on to the filter. The filter is regenerated at given intervals by moving the cleansing drum to and fro. In this manner, filtrate is removed in the anticipated amount from the sludge during filtration. The specific filter resistance does not change significantly during the first 2 to 3.5 hours, depending on the sludge temperature.

After expiry of this time, it was surprisingly found that the structure of the sludge suddenly underwent a complete change. A distinctly visible flocculation occurred, which was structurally different from the amorphous flocks of the activated sludge. On closer examination a very sharp rise was observed in the Redox potential by 300-500 mV and a sudden fall in the specific oxygen consumption of the micro-organisms by 1.2 mg/(1 .min) oxygen. Thereafter, the Redox potential fell sharply by 200-400 mV to + 10 mV. Despite the very short treatment time, there was no decomposition of the sludge during the subsequent natural or mechanical hydroextraction.When continuing the treatment, these parameters deteriorated again during the following 3 to 4 hours, after which, the same effect reoccurred, i.e. sudden reduction in the specific filter resistance and onset of good hydroextraction properties.

The described effect occurred especially when the activated sludge, prior to treatment, was subjected briefly to anaerobic conditions.

It was also found that the effect also occurred when the drum did not enter into the sand layer and when no liquid is filtered off.

In this case, the intensity was however not so marked. The effect is also weaker if additional sludge is introduced during the process after the first 90 minutes.

When the drum did not function within the enclosed compartment, there was only a single change in the sludge structure.

On the basis of this discovery, the objective is thus achieved in that sludge, containing at least 0.5% organic dry substance is excited constantly or at given intervals to a frequency of at least 0.3 s-1, while at the same time injecting oxygen.

The excitation and oxygen injection may take place simultaneously throughout the whole reactor volume, but it is advantageous to demarcate a part volume to make full use of the discovered effect, and that this part volume should be continuously or intermittently moved through the conditioning reactor. In this way, at given cycles, the entire sludge volume is caused to oscillate and is aerated in successive sections. After two to five hours treatment time, it is necessary to maintan anaerobic conditions outside the demarcated region. By changing the locality of this demarcated region within the reaction, there is a constant change in aerobic and anaerobic conditions, which is necessary for the second phase of the sludge conditioning.

It is envisaged treating the sludge to achieve a Redox potential increase of 300-500 mV. After a treatment time of 90 to 180 minutes, there is no further sludge addition. Moreover, prior to aeration, the sludge is subjected to an anaerobic state for 30 to 120 minutes. A preferred frequency range of 0.6 to 1.25 s-1 is selected. The preferred content of organic dry substance is 16 to 40 g/l. The process is intensified by the fact that the sludge is subjected to continuous hydroextraction during the treatment. The gaseous metabolic products are removed from the sludge, particularly by the aeration. Dissolved products are constantly removed with the filtrate.

In addition, there is a sludge-washing. The hydroextraction properties of the sludge are greatly improved if, during the treatment, a granular material is added to the sludge, with a density different from that of water.

Various methods can be applied for inducing oscillation/vibration, for example, rotating bodies or vibrators.

All process conditions are particularly advantageously effected if the sludge is applied to a granular filter layer. In this case, excitation is induced by a rotating body. This is located within a demarcated area which is moved over the filter surface. By this means, several additional effects are achieved: ~Regeneration of the filter layer for sludge hydroextraction; ~Agitation of the sand for improving the degree of dispersion; ~Aeration in a demarcated, changing locality region.

The treatment, according to the method of the invention, leads initially to the consumption of the exogenic energy sources, after which the endogenic breathing takes place.

Due to the high concentration of organic substances, the above-average breathing activity of the micro-organisms, and the processrelated absence of natural inhibitors, the existing mass population of bacteria suddenly dies off, as the result of which, the resultant biopolymers, the high molecular protein compounds and the external cell polymers adsorbed on the cell outer walls, give rise to a flocculation process. The flock formation can be promoted in that a granular material (e.g sand) is agitated with the sludge, as the result of which, the mechanical effect of the excitation on the sludge degree of dispersion is greatly intensified.

By this time, because of the sharply increased Redox potential, the majority of pathogenic bacterial will have died off. The specific oxygen consumption suddenly drops to the value of endogenic breathing (AA = 1.2 mgO2 (1 . min)), with simultaneous fall-off in the pH-value. At this time, the sludge is technically stabilised, i.e. permanently incapable of putrefaction, odourless and hydroextractable, reduced in volume to more than 50% and the dry substance is reduced by more than 20% in relation to the starting amount.

If the process is continued beyond this stage in the same manner, such that outside the demarcated region, anaerobic conditions are maintained, then during the next 3 to 4 hours, a hihgly-effective, combined nitro- and denitrification of the nitrogen compounds takes place.

Following the consumption of the exogenic energy sources, the inner reserves are attacked (polysaccharides, fats). The endogenic breathing consumes these substances, after which, oxidation of the cell protein finally takes place. At the same time, the process of oxidation commences of the ammonium, released during the biochemical oxidation of the organic substances, to give NO2 and NO3.

Since this process is oxygen intensive, the specific oxygen consumption increases sharply. If the oxygen supply is restricted to a demarcated area moving over the liquid surface, then intensive nitrification processes take place in this area, with nitric acid being released. If it is not removed from the system, this leads to a reduction in the pH-value, and associated therewith, a delay in the process time.

If the demarcated, oxygen-enriched area moves further, then because of the very high specific oxygen consumption in the nitrification processes, the dissovled oxygen will be completely consumed within a few minutes.

By virtue of the property of the denitrifying bacteria as facultative anaerobic micro-organisms, they become active at this instant.

Through using the killed bacteria (due to endogenic breathing) as an organic substance which serves as a hydrogen denator, and the nitric acid formed during the nitrification, then under anaerobic conditions, the dissimilative nitrate reduction to N2 is brought about through these bacteria. In parallel with this, the assimilative nitrate reduction takes place to organic nitrogen (cellular protein).

After 3-4 hours, there is again a sudden change in the sludge properties. It becomes readily hydroextractable. Further addition of oxygen contributes to the fact that due to the renewed formation of micro-organisms, the still present biologically decomposable organic substances become oxidised. At this time, the sludge is reduced to about 9% of its starting volume and to about 70% of the mass of the dry substance. After a further 2 to 4 hours, the volume reduces to less than 5%, and the mass of the dry substance to less than 20%, with good hydroextraction properties.

Due to the ability of the bacteria, during times of shortage of life-important elements (carbon, nitrogen, sulphur, phosphorus, oxygen) to attack its internal reserves, there is an intracellular improverishment of important elements. If after these periods of shortage, the bacterial reach areas having an excess of these elements, then over-compensation takes place. By means of the process according to the invention, there is a hitherto unknown intensification of the biotechnique, without addition of enzymes or chemicals under psychrophilic conditions, as the result of which reaction times including sludge thickening, of 4 to 12 hours are achieved, compared with 3.5 to 5 days with the thermophilic and mesophilic aerobic sludge stabilisation with lower energy and capital costs.Through reducing the mass (weight) and the volume of the organic sludge and the lower specific filter resistance, the subsequent hydroextraction becomes more effective by 300 to 500%; the sludge thickeners for pre- and post-purification are completely superfluous.

The invention will now be described on the basis of an embodiment: In an effluent treatment plant with 210000 m3/d daily capacity, there occurs 1470 m3/d primary sludge with a dry substance content of 40 9/1 (of which 65% organic dry substance), 5026 m3/d activated sludge with a dry substance content of 15 g/1, and 1470 m3/d activated sludge with a dry substance content of 1%.

In two stacker pockets, the incident sludge, after intensive aeration, is subjected, in alternate direction for 0.5 to 2 hours to anaerobic conditions. The psychrophilic aerobic/anaerobic sludge stabilisation takes place in open basins of 1.5 m depth. On the sealed bottom there is a drainage layer of 0.1 m thickness with a drainage tube per running basin width in sand of grain size 0.6-1.2 mm. Above this is a fine sand layer of 30 cm of grain size 0.4-0.63 mm. The free basin depth is 1.1.

m. Mobile in the length direction, the basin is provided with a device in which, in an enclosed space, a rotating roller agitates both the upper layer of the fine sand filter, and injects oxygen into the sludge, while at the same time, expels the forming gaseous metabollic products of the micro-organisms. In addition, the oxygen injection into the apparatus during the process with very high specific oxygen requirement, can be supported by providing additional aerators.

The sludge, subjected to anaerobic conditions in the stacker vessel, is fed over a period of 20-60 minutes, via an intensive aeration mechanism, in a quantity of 1 m3/m2 filter area, into the basins for the psychrophilic aerobic/anaerobic sludge stabilisation.

From the commencement of filling, the apparatus is moved over the basin in the length direction, at a speed of 1.5 m/min, while the roller revolves at a speed of 60 s-l and penetrates into the fine sand layer to a depth of 4-6 cm.

The Redox potential rises from - 200 mV to + 100 to 400 mV and above. The specific oxygen requirement also rises to 1.8-2.0 mg 02/ (1 .min), and suddenly drops off after 90-120 minutes to 1.2-0.5 mg 02/(1 .min).

A similar trend is observed with the pH-value.

Besides the high oxygen input and the ejection of inhibitors of the metabollic process, the apparatus achieves a very good intermixing and dispersion of the sludge, with the result that very good living conditions are achieved for the micro-organisms which decompose organic substances.

After 90-180 minutes, about 40% of the liquid phase is removed as filtrate containing the dissolved, biochemically difficult to decompose substances, as well as other dissolved compounds. The exogenic breathing of the existing mass population comes to an end.

These bacteria die off-there begins the phase of endogenic breathing and the nitrification of the resultant ammonium. The waterrelease facility of the sludge diminishes significantly. In this stage, the sludge is permanently incapable of decomposing and can be fed for composting, hydroextraction or for other further processing.

If the treatment is continued, NH4+ is oxidized to HNO2 and HNO3 through nitrifying bacteria. In order to avoid a severe pH-value reduction which would occur under these conditions, (caused by the resultant nitric acid), HNO, and HNO3 are reduced in parallel, and simultaneously to the nitrification process in the anaerobic state, to gaseous N2 through facultative anaerobic denitrificants in the same water body.

This process is made possible by the fact that in the apparatus, a technically attainable oxygen saturation is established. Outside the appartus, because of the nitrification process, very high specific oxygen requirements (in excess of 3 mg 02/(1.min)) of the dissolved oxygen is consumed completely within a few minutes. During the now existing anaerobic state, the resultant nitric acid is immediately reduced. This process continues until the apparatus again produces water saturated with oxygen, after which, the nitrification and denitrification takes place alternatively.

Due to the constant agitation of the filter sand, in addition to the good dispersion of the sludge, there is a solids phase for growing bacteria, as the result of which, far more favourable conditions exist for the regeneration than in the water substance.

After 4-5 hours, this process attains its peak. The water-release capacity of the sludge suddenly improves again while the specific filter resistance of the output sludge falls from 2.66 x 10" cm/g to 1.98 . 1011 cm/g.

After 5-7 hours, 80-92% of the liquid phase of the sludge is released as filtrate. The weight of the whole dry substance introduced is reduced to 18%. The sludge is no longer capable of decomposing.

The pathogenic bacteria can no longer live, because of the high rise in the Redox potential during the treatment. The remaining sludge can be dehydrated in further stages and, because of the low specific filter resistance and the good hydroextraction properties, a high level of efficiency can be attained.

Moreover, because the nitrogen compounds are no longer present to disrupt the thickening and flocculation process, there is now a more favourable pH-value range for the colloid system combined with an advantageous electrokinetic potential.

Claims (15)

1. Process for conditioning organic sludge, wherein the sludge is excited with mechanical energy or vibrations/oscillations, characterised in that a sludge, containing at least 0.5% organic dry substance is introduced constantly or at predetermined intervals, excited to a frequency of at least 0.3 s- while at the same time, oxygen is injected.

2. Process according to Claim 1, wherein the excitation and the injection of oxygen takes place in a restricted area of the sludge located in a conditioning reactor, such that this area is moved through the conditioning reactor constantly or at predetermined intervals of time.

3. Process according to Claims 1 and 2, wherein, after 2-5 hours' treatment time, anearobic conditions are established outside the restricted area.

4. Process according to Claims 1 and 2, wherein the sludge is treated up to a point when the Redox potential is increased by 300-500 mV.

5. Process according to Claims 1 and 2, wherein no further sludge is added after 90-180 minutes following commencement of the treatment.

6. Process according to Claims 1 and 2, whereby the sludge prior to the injection of oxygen, is subjected for 30-120 minutes to an anaerobic condition.

7. Process according to Claim 1, wherein the sludge, with 16 to 40 9/1 organic dry substance, is excited constantly or at predetermined intervals, to a frequency of 0.6-1.2 s

8. Process according to Claims 1 and 2, wherein water is continuously extracted from the sludge during the treatment.

9. Process according to Claims 1 and 2, wherein the gaseous metabolic products are extracted from the sludge.

10. Process according to Claims 1 and 2, wherein the sludge is washed during the treatment.

11. Process according to Claims 1 and 2, wherein during the treatment, a granular material with a density differing from that of water, is added to the sludge.

12. Process according to Claims 1 and 2, wherein a rotating body is used to induce the oscillation /vibration .

13. Process according to Claims 1, 2, 8, 11, 12 wherein the sludge is applied to a granular filter layer, and excited by the rotating body, such that at predetermined intervals of time, the upper region of the filter layer is agitated by the rotating body, and the filter grain is brought into contact with the sludge.

14. Process according to Claims 1 and 2, wherein the oscillations/vibrations are induced by a vibrator.

15. Process for conditioning organic sludge substantially as herein described.