SE508478C2 - Ways to improve wastewater treatment in biological aerated ponds - Google Patents

Abstract

The invention relates to a method of improving the purification result in treating wastewater in biological aerated ponds by introducing after the aerated pond (2) and ahead of a following after-sedimentation (4), if any, a biofilm process (3) having a carrier material for microbial growth which is kept completely or partly in suspension and movement by blowing air into the process.

Description

The pollutants are often incomplete, and this is especially true of many toxic and degradable compounds, which are of great importance from the point of view of emissions. Furthermore, the microbial biomass formed in an aerated pond usually has a very poor separability. In most cases, the treatment in an aerated pond is followed by a separation step, usually in a sedimentation basin, whereby the biomass formed must be separated from the wastewater and taken out as an excess sludge. If this separation does not work, it leads to large emissions of suspended material in the form of biomass, which contains both high levels of organic matter and significant amounts of nutrients. This is a very common problem in connection with wastewater treatment in that it is very easy to operate and that the need for supervision and aerated ponds. The advantage of the aerated pond years, control is minimal.

In order to obtain a more far-reaching degradation of the organic pollutants, biological purification processes are often used, in which the active microorganisms are retained in the process of maintaining a high content of active biomass. There are two main principles for how detention works. Either the biomass is allowed to grow in suspended form in the process, after which it is separated from the water in a separation step after the reactor and returned to it (eg the activated sludge process) or the biomass is retained by allowing the microorganisms to grow as a biofilm on some carrier material. , placed in the process (cinematographic processes). Both in activated sludge processes and in film processes, significantly better treatment results are usually obtained than in aerated ponds. The most far-reaching degradation is obtained in low-load activated sludge processes and a common way to improve the purification effect in existing aerated ponds is to rebuild these into activated sludge processes. This often requires extensive rebuilding of the dam and construction of a new post-sedimentation, which means large 3 sos'47s investment costs. The process also has the disadvantage of requiring significantly more supervision and control than the aerated dam. For hot wastewater, such as many forest industrial wastewater, it is also required that incoming water for the process is cooled down to a temperature below 40 ° C to avoid operational disruptions. For many industrial wastewater, it is also required that extra nutrients are added to the water before treatment in an activated sludge process. A common problem in low-load activated sludge processes is further, that sludge separation problems in the form of so-called sludge swelling occurs, which can give reduced treatment results and difficulties with the dewatering of the excess sludge taken out of the process. The redevelopment also results in a significant increase in the amount of surplus sludge that must be disposed of.

Biofilm processes are usually run as more high-load processes, which do not reach quite as far in degree of purification as a low-load activated sludge process. Biofilm processes are often used as pre-treatment before a subsequent activated sludge process, but biofilm processes are also operated as the only biological treatment, sometimes in combination with a subsequent chemical precipitation. A biofilm process is much easier to operate than an activated sludge process and the need for supervision is small. As with the activated sludge process, however, cooling of hot wastewater before treatment and the addition of extra nutrients are required in the treatment of many industrial wastewater, which is usually not required for treatment in aerated ponds. It has previously been shown that the purification results in aerated ponds can be improved by converting the ponds into a biofilm process by introducing a carrier material directly into the aerated pond (SE-A-9501744-8). It has also been proposed to introduce into non-aerated ponds a device containing carrier material, through which the water is circulated (JP-A-1-51193). Compared with rebuilding an aerated pond into a activated sludge process, these solutions have the advantage that the simple operation and the low soot 478, the need for supervision are maintained and that the risk of sludge swelling is avoided. The disadvantage is, however, that there is a risk of overgrowth of the carrier material and that bio-sludge, which detaches from the carrier material, forms sludge deposits on the bottom of the dam.

A new type of biofilm process that has come into widespread application in recent years is the so-called the biofilm process with suspended carrier material (EP-A-0575 314, WO-A-95/25072). In this process, carrier materials in the form of plastic filler bodies are used, which are kept suspended and in motion in the process by blowing air in at the bottom of the reactor tank. For the treatment of industrial wastewater, the process has primarily been used as a highly loaded pre-treatment before a subsequent treatment in the activated sludge process or as the only biological treatment at a somewhat lower load and in combination with chemical precipitation.

The method proposed according to the present invention to improve the purification of wastewater in biological aerated ponds has obtained the features which appear from claim 1 and has been found to give great advantages compared with the conventional way of improving the purification. It has been shown that through the combination of the two processes - the treatment in the aerated pond and the biofilm process - a significantly better result is achieved than the sum of the effects obtained through the processes separately.

Furthermore, the invention further comprises ways of supplementing the said process combination in order to reduce the emissions of suspended material, nitrogen and phosphorus.

The invention provides as good a degradation of the organic pollutants in the wastewater as through a conversion to an activated sludge process, but with great operational advantages. The process does not require any rebuilding of the dam, but only the establishment of a very compact process after this. Furthermore, very little supervision, control and instrumentation is required. No cooling of the incoming soš 47s wastewater is required, and the sludge production is significantly lower than in an activated sludge process. Compared with previously proposed solutions regarding the application of carrier material directly in the aerated dam, the invention provides a more controllable process and eliminates the risk of clogging of the carrier material or storage of sludge in the dam. The invention also provides a significantly lower sludge production than an installation of carrier material in the dam. In a pilot-scale comparison between the invention and other alternatives for improving the purification results in aerated dust, the invention obtained a sludge production of 0.04 kg sludge / kg reduced COD, while an installation of carrier material directly in the pond gave a sludge production of 0.10 kg sludge / kg reduced COD and conversion to activated sludge process gave a sludge production of 0.14 kg sludge / kg reduced COD.

A comparison between the combination according to the invention and the treatment of the waste water in an aerated dust or biofilm process separately also showed that the combination according to the invention gives a significantly better result than the sum of the effects of the two processes separately. As shown in TABLE 1 below, the reduction of the nutrients nitrogen and phosphorus as well as the acute toxicity are dramatically improved while the sludge production and emissions of suspended material are greatly reduced by treatment in the process combination according to the invention compared to the two processes separately. sus 478 6 TABLE 1. Comparison between purification results in aerated dust, biofilm process and the combination of aerated dust and biofilm process according to the invention Luftnd dun 810211: Luttnd dlnnobiot.

Residence time (h) 70 2 70 + 2 COD reduction (i) 30 35 70 Sludge production (kg / kg red 0.29 0.32 0.04 COD) Reduction of acute toxicity 30 42 100 (t) Reduction of phosphorus (I ) 10 increases 70 Reduction of nitrogen (i) increases increases 60 Emissions of suspended material (50 50 25 mg / l) A comparison with other types of biofilm processes with stationary carrier materials has shown that these are much less suitable for use for the current application. Thus, the treatment of outgoing water from aerated dust in biofilm processes has been shown to give a very strong biofilm growth with clogging and impaired contact between wastewater and biomass as a result in biofilm processes with stationary carrier material, so that the desired effects could not be achieved with these. In the process of suspended carrier material, the movement of the material in the water means that overgrowth is avoided and the contact between the wastewater and the cinema film is always good.

For some wastewater, the extremely low sludge production obtained with a process combination according to the invention has been shown to cause problems in achieving high degrees of separation of nutrients in the process. This applies in particular to wastewater, which contains significant amounts of nutrients in relation to the organic material in the wastewater.

Nutrients can be separated from the wastewater mainly by being taken up by the microorganisms and built into the biomass formed by the growth of the microorganisms. By separating this biomass from the wastewater after the biological treatment process, the nutrients contained in the biomass are also separated. However, if the growth of biomass in the process, sludge production, is very low, it may happen that the uptake of nutrients becomes so low that the nutrient emission from the process becomes higher than desired. If this is the case, it has proved beneficial to direct a partial stream of untreated wastewater past the aerated dam directly into the biofilm process. This has been shown to increase sludge production in the biofilm process, so that a larger part of the nutrients in the wastewater is taken up and bound in the sludge.

The invention also relates to this method of improving the nutrient separation.

The effect on nutrient separation by passing untreated wastewater past the aerated dam directly into the biofilm process was clearly demonstrated in pilot experiments, where a combination of aerated dust and biofilm process according to the invention before a partial stream of untreated wastewater was passed past the aerated dam gave a separation of nitrogen and phosphorus of 10 and 25%, respectively, while the separation after 20% of the wastewater was led past the dam directly to the biofilm process increased to 55 and 67%, respectively.

For many wastewater, however, the nutrient separation is good even if untreated wastewater is not diverted to the biofilm process. In many cases, no nutrient separation is sought either, and in these cases it is relatively common not to have a separation step after the aerated pond but to discharge the treated wastewater with the biosludge contained therein directly to the recipient. For these cases, the invention can provide a significant reduction in the emissions of organic material and suspended substances by breaking them down in the biofilm process before release to the recipient. see 478 8 To further improve the purification, it has been found possible after the biofilm process to place an aerated tank, to which biosludge is returned in a subsequent separation step. This after-treatment is thus operated according to the activated sludge principle, but with a much shorter treatment time than would be required in an activated sludge process without prior aerated dust and biofilm process. The function of the subsequent activated sludge system is to further improve the separation of suspended material from the wastewater. Thus, it has been found possible in this way to reduce the starting content of suspended material from about 25 mg / l to about 10 mg / l. This reduction of outgoing suspended material also improves the separation of nutrients from the wastewater and furthermore a certain improvement of the decomposition of dissolved organic material can be achieved by introducing the actual after-treatment.

For the same reason as described above for the cinema film process, it may in some cases be an advantage to direct a partial stream of untreated wastewater past the aerated dam and the biofilm process directly into the activated sludge system.

This leads to an increased sludge production in the activated sludge stage and thus an increased uptake of nutrients. In order to increase the nutrient separation, untreated wastewater can thus be led past the aerated pond to the biofilm process or to the activated sludge process or to both processes.

For a more detailed explanation of the invention, exemplary embodiments are given in the following with reference to the accompanying drawings, of which Fig. 1 shows an embodiment of the invention with post-sedimentation steps, Fig. 2 shows the same process combination as Fig. 1 but without post-sedimentation steps.

Fig. 3 shows the same process combination as Fig. 1 but by bypassing a partial stream of untreated wastewater, 9 soa 478 Fig. 4 shows a process combination with activated sludge process, Fig. 5 shows the same process combination as Fig. 4 but by bypassing a partial stream of untreated wastewater, and Fig. 6 shows the same process combination as Fig. 4 but by bypassing two substreams of untreated wastewater.

In the plant according to FIG. 1, the untreated waste water is led to a pre-sedimentation device 1 and from there to a biologically aerated pond 2. From there the waste water is led to a tank 3 with carrier material for growing biofilm, which is kept completely or partially suspended and in motion by blowing air into the tank, to undergo a biofilm process therein. From the tank 3 the treated wastewater is diverted via a post-sedimentation device 3.

The plant in Fig. 2 lacks the sedimentation device 3 but is otherwise in accordance with the plant in Fig. 1. Fig. 3 also shows a plant in accordance with that in Fig. 1, but in this case a partial stream 5 of waste water from the pre-sedimentation device 1 is led past the aerated dam 2 directly to the tank 3 to undergo the cinema film process.

The plant in FIG. 4 is also in accordance with that in FIG. 1 but is supplemented with an aerated reactor 6 for activated sludge process between the tank 3 and the post-sedimentation device 4, whereby sludge from the latter is returned to the reactor 6.

According to Fig. 5, in a plant of the embodiment in Fig. 4, a partial stream 8 of waste water from the pre-sedimentation device 1 is led directly to the reactor 6 for the activated sludge process while passing the aerated pond 2 and the tank 3 for the biofilm process, while the plant according to Fig. 6 is 508 478 10 a combination of the plants in FIGS. 3 and 5, in that the waste water from the pre-sedimentation device 1 is led directly partly to the tank 3 and partly to the reactor 6 with the passage of the aerated dam 2.

Claims (10)

W U 20 25 30 35 11 509 478 PATENT REQUIREMENTS Method of improving the purification of waste water in biological aerated ponds, characterized in that the waste water after treatment in the aerated pond (2) is led to a (3) in which active microorganisms grow on a carrier material which is kept wholly or partly in suspension and movement through air injection in the process. Method according to claim 1, characterized in that the residence time of the waste water in the biofilm process (3) is between 0.3 and 10 hours, in particular between 0.5 and 5 hours, preferably between 1 and 4 hours. Method according to claim 1 or 2, characterized in that a partial stream (5) of biologically untreated wastewater is led past the aerated pond (2) directly into the biofilm process (3). Method according to claim 3, the partial stream (5) characterized in that of biologically untreated wastewater, which is passed past the aerated dam (2) directly into the biofilm- (3), constitutes between 5 and 70%, preferably between 20 and 40 %, the process especially between 10 and 50%, of the total wastewater flow to the biofilm process. 5. A method according to any one of claims 1-4, characterized in that an aerated reactor (6) is supplied with a subsequent separator (4), A method according to claim 5, characterized in that the effluent from the biofilm process (3) steps from which sludge is returned to the reactor. the residence time of the waste water in the aerated reactor (6) is between 0.5 and 10 hours, respectively between 1 and 5 hours. In particular between 0.7 and 7 hours, preferably A method according to claim 5 or 6, characterized in that a partial stream (8) of biologically untreated wastewater is led past the aerated dam (2) and the biofilm process (3) directly into the aerated reactor (6). Method according to claim 7, characterized in that the partial stream (8) of biologically untreated wastewater, which is passed past the aerated dam (2) and the biofilm process (3) directly into the aerated reactor (6) constitutes between 5 and 70%, in particular between 10 and 50%, 20 and 40% of the total wastewater flow to the aerated reactor. preferably between Method according to claim 5 or 6, characterized in that two partial streams (5, 8) of biologically untreated wastewater are led past the aerated dam (2) directly to the biofilm process (3) and the aerated reactor (6), respectively. Method according to claim 9, characterized in that the two partial streams (5, 8) of biologically untreated wastewater, which are passed past the aerated dam (2) together constitute between 5 and 70%, in particular between 10 and 50%, before - between 20 and 40%, of the total waste water flow to the aerated reactor (6).