Cleaning method and cleaning fluid for aeration bodies

The present invention relates to a method for cleaning aerator elements in aerated basins of wastewater purification or water treatment plants according to the preamble of claim 1 and a cleaning agent according to claim 9.

Systems for wastewater treatment essentially consist of the following process steps or components, for which in turn there are many possible embodiments:

Coarse cleaning of the wastewater with rake, sieve and sand trap Primary clarifier for sedimentation of easily settable substances (optional)

Plant for biological phosphate elimination (optional)

Activation processes in the form of continuous systems (with separate activation stage and secondary clarifier), sewage ponds or impounding systems ("Sequencing Batch Reactor", SBR)

Various peripheral systems, for example for sludge stabilization, sludge treatment, or digester gas utilization

The wastewater treatment takes place mainly in the context of the activation process. Sufficient ventilation of the wastewater is ensured here, as a result of which activated sludge flakes develop. The activated sludge consists of bacteria and protozoa, these convert the organic substances into mineral degradation products, CO₂ and water and multiply, ie new biomass is created. Special types of bacteria (“nitrifying agents”) also convert ammonium into nitrite and then into nitrate. These material conversion processes require the presence of oxygen, which in ventilated pools is usually supplied by blowing air with pressure ventilation systems.

Pressure ventilation systems for introducing oxygen into the ventilated basin include, in particular, aerator elements which are arranged at the bottom of the ventilated basins and into which air is blown via compressed air blowers and corresponding pipelines. In special cases, technically pure oxygen is also blown into the ventilated pool. The aerator elements are provided with pores or holes (e.g. perforated plastic membrane) through which the blown air is introduced in the form of small bubbles into the medium to be ventilated in the pool.

When these aerator elements are in operation, deposits form on the surfaces and in the pores or bores after an average of 1 to 4 years, depending on the running time, aerator model and hydrochemical conditions, which, according to current teaching, mainly consist of calcium carbonate and organic substances. The increasing clogging of the pores or bores causes an increasing pressure loss and thus a higher energy expenditure for blowing in the air. This reduces profitability (the oxygen input in kg O₂/ kWh) of the ventilation system. In particular, the back pressure can become so high due to clogged pores or bores that the aerator membranes slip out of their holder or tear or the blowers fail due to overload and the cleaning performance of the aeration stage is severely impaired. The aerator elements must therefore be cleaned regularly.

One possible cleaning process provides for emptying the aerated basin, removing the aerator elements and washing each individual aerator element, for example in an acid bath made from hydrochloric acid. The aerator elements can then be reinstalled and the aerated basin can be put into operation. However, the resulting shutdown and downtime of the ventilated basin have a decisive impact on the availability and cost-effectiveness of the overall system.

There are therefore also known methods in which chlorine or hydrogen chloride is blown in gaseous form into the pressure ventilation system with built-in aerator elements, DE 33 33 602 A1 discloses a method in which formic acid is metered into the compressed air at periodic intervals to remove the calcium deposits in the pores or holes or also partially loosens on the water-side membrane surfaces.

US Pat. No. 5,597,491 A and US Pat. No. 5,378,355 A provide for the aerator field to be provided with a separate pipe system via which a cleaning fluid is supplied to the aerator elements. Acids are mentioned here as the preferred cleaning liquid, in particular hydrogen chloride. However, this solution for cleaning the aerator elements is complex and expensive.

In US Pat. No. 5,051,193 A, it is proposed to briefly fill the aerator field with an acidic solution, preferably a solution of hydrogen chloride, in order to clean aerator elements.

AT 41 1 359 B

When using acidic cleaning agents, in particular chlorine or hydrogen chloride, which is introduced into the pressure ventilation system in gaseous form or as a liquid, however, increased corrosion of the pipes and the distributors was found. In addition, with these processes there is a risk that bacterial cultures in the ventilated basin will be destroyed and thus the biological purification of the water will be impaired. These processes also prove to be costly in practice because of the necessary safety measures for handling chlorine or hydrogen chloride.

Furthermore, recent studies on several plants have shown that, contrary to current teaching, it is not alkaline earth carbonates (especially lime) and / or organic substances (e.g. the biomass built up by microorganisms) that are primarily responsible for the counterpressure increasing clogging in the pores or bores, but rather felled resp. In some cases crystallized inorganic compounds such as (earth) alkali (alumino) silicates and alkaline earth orthophosphates as well as silicic acid hydrate. An acidic cleaning agent cannot dissolve such deposits.

The subject matter of the invention is thus a method which also allows the use of alternative cleaning agents that are tailored to the deposits that occur in each case. It is provided here that the entire aerator field consisting of the aerator elements and the pipelines connecting them is completely filled with a cleaning agent in liquid form, with sufficient pressure being generated by the hydrostatic pressure, the cleaning agent pumps or the air blower to allow the cleaning agent to pass through the pores or to press holes in the aerator membrane. After a variable exposure time, the aerator field is emptied again using compressed air. This process can be repeated several times even when the basin is full. According to claim 1 it is provided that the cleaning liquid contains an agent for ensuring an alkaline environment with a pH of at least 10, preferably 12. According to the state of the art, no attention was paid to the use of alkaline cleaning agents because, due to insufficient knowledge about the composition of the deposits in the pores or bores, the use of gaseous acids or oxidizing agents appeared to be completely sufficient. In addition, it is common practice to keep the aerator field largely dry, for example to avoid damage to the pipelines such as corrosion.

In the case of cleaning fluids according to claim 1 of the invention, it has furthermore been shown that the risk of impairment of the pipeline system is negligible. After all, it is sufficient to flood the aerator field with the cleaning agent according to the invention for only about an hour, after which the pores or bores of the aerator membrane are largely cleaned and the system can be used again for years. This is possible in particular because the composition of the cleaning fluid according to the invention takes into account the latest findings about the nature of the deposits and is also able to dissolve deposits such as (earth) alkali (alumino) silicates and alkaline earth orthophosphates as well as silicic acid hydrate.

Claims 2 to 7 relate to advantageous embodiments of the method according to the invention and claims 9 and 10 to advantageous embodiments of the cleaning agent according to the invention. Claim 8 claims the use of a cleaning liquid containing an agent for ensuring an alkaline environment with a pH of at least 10, preferably 12, for cleaning aerator elements provided with pores or bores in aerated basins of wastewater purification or water treatment plants.

The invention will now be explained in more detail with reference to the accompanying FIG.

FIG. 1 shows a schematic sketch of an aerated basin 2, near the bottom of which the aerator field 4 is located. The aerator field 4 consists of the aerator elements 1 and the pipes 3 connecting them. In Fig. 1, only one aerator field with a number of aerator elements 1 and a pipe 3 is shown, but there can be several aerator fields with parallel pipes 3 and the through them supplied aerator elements 1 be available. The aerator elements 1 are equipped with a perforated membrane or frits with pores or bores.

The aerator field 4 is connected to a fan 7 via additional pipelines, whereby the specific embodiment and the routing of the pipelines can of course vary.

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One possible construction of the piping is shown in FIG. A main air line 17 with a shut-off valve 18 leads from the fan 7 to the ventilated basin 2. The distribution line 6 is guided along the basin 2 at the top of the basin. There the downpipes 5 branch off to the aerator field 4. The downpipes 5 open into a distribution pipe 15, which is parallel to the pool wall, where the branch lines 3 branch off at right angles. The aerator elements 1 are mounted on the branch lines 3.

The type of pipe routing can, however, have different designs and will depend on the technical requirements of the specific system. The crucial point is to connect one or more blowers 7 to the aerator elements 1 arranged in the aerated basin 2 via pipelines in an expedient manner.

Furthermore, drainage lines 8 can be provided which, during normal operation of the ventilated basin, serve to divert the water that has penetrated into the aerator elements 1 and the pipelines 3. The drainage lines 8 can branch off, for example, from a collecting pipe 16 into which the parallel pipes 3 open.

When the system is in operation, there is 2 wastewater 19 in the aerated basin, where it is subjected to microbiological activity. The ventilation of the wastewater required for this is accomplished by a fan 7 which presses air or oxygen into a pipeline system so that the gas introduced is distributed to the aerator elements 1. With increasing clogging of the pores or bores of the aerator elements 1, the counter pressure rises and the efficiency of the aerator system drops significantly. According to current teaching, the deposits were identified as alkaline earth carbonates and the cleaning process was therefore also tailored to the removal of alkaline earth carbonates. As recent chemical analyzes and investigations with electron beam microprobes have shown, precipitated or crystallized inorganic compounds such as (earth) alkali (alumino) silicates and alkaline earth orthophosphates as well as silicic acid hydrate can be found on and in the pores or bores of the aerator membranes . This explains why the cleaning of the aerator membranes with the help of conventional methods was only inadequate and therefore a renewed blockage occurred relatively quickly after cleaning. The pores or bores can, however, be cleaned with the aid of the method according to the invention and the cleaning fluid according to the invention, quickly and without the need to empty the ventilated basin 2.

The cleaning liquid according to the invention is based on a strong alkali with pH values ​​of at least 10, but preferably above 12. This is achieved by adding potassium hydroxide in an amount of 0.1 to 15% by weight (percent by weight), preferably 2.5% by weight. It preferably also contains an agent which forms complexes dissolved in water with alkaline earth ions and thus prevents the precipitation of alkaline earth (alumino) silicates, alkaline earth orthophosphates, but also alkaline earth carbonates. Inorganic and organic complexing agents, such as EDTA (ethylenediaminetetraacetic acid), which are added in the form of EDTA sodium salt in amounts of 0.05 to 5% by weight, preferably 3%, are suitable for this purpose.

The deposits in the pores or bores of the membrane are often hard crusts that would take a long time to decompose through chemical processes alone. Therefore, a further embodiment of the cleaning agent according to the invention provides an additional mechanical cleaning effect, which is achieved by adding hydrogen peroxide (H.₂O₂) is accomplished in amounts of 0.1 to 10% by weight, preferably 4% by weight. Under the alkaline conditions within the cleaning liquid, the hydrogen peroxide decomposes with the release of molecular oxygen. Since the cleaning agent according to the invention is pressed under pressure into the aerator elements and through the pores or bores, the cleaning liquid and the hydrogen peroxide contained therein enter free spaces between membrane pores or bores and the deposits, which is made easier by adding a surfactant to reduce the surface tension. In contact with the surfaces of the deposits, there is a strong decomposition of the hydrogen peroxide and a strong release of oxygen, which leads to effective convection. In the course of the convection movement, parts of the deposits are torn off and the pores or bores are thus quickly cleaned. A corresponding catalyst, which is added in amounts of 0.001 to 0.5% by weight, preferably 0.02% by weight, accelerates the decomposition of the hydrogen peroxide. The surfactant is preferably non-foaming and is added in an amount of 0.01 to 2% by weight, preferably 0.1% by weight. In order to be able to store the cleaning liquid for a longer period of time,

AT 411 359 Β the hydrogen peroxide must be separated from the potassium hydroxide, the complexing agent, the surfactant and the H₂O₂Alkaline composition containing catalyst are stored. The above-mentioned quantities in% by weight relate in this case to the respective total weight of the two individual components.

For the application of the cleaning method according to the invention, it is now necessary to provide two containers 9, 10, one of which has the above-described alkaline composition with the potassium hydroxide, complexing agent, surfactant and H.₂O₂-Catalyst and the second the hydrogen peroxide. The two components are only mixed within the pipeline system leading to the aerator elements 1. For this purpose, a Y connection piece, for example, which opens into the distribution line 6, can be provided for the discharge lines 13, 14 from the containers 9, 10. It will be useful to equip the drain lines 13, 14 with shut-off valves 11 and pumps 12.

If the aerator elements 1 are to be cleaned, the compressed air supply from the blower 7 is interrupted when the aerated basin 2 is filled or emptied, the shut-off valves 11 to the containers 9, 10 are opened and the pumps 12 are put into operation. As a result, the two components of the cleaning fluid according to the invention are pumped from their respective containers 9, 10 into the drainage lines 13, 14 and, depending on the type of field piping, into the distribution line 6, for example. In the following, the two components of the cleaning fluid according to the invention are mixed and transported to the pipelines 3 and the aerator elements 1. The pressure conditions caused by the pumps 12 are selected so that the cleaning fluid passes through the pores or bores the aerator membrane comes where the cleaning effect can now unfold. In order to ensure a suitable contact and a suitable flow through the pores or bores with the cleaning fluid, provision can also be made for the compressed air supply to the aerator field 4 from the blower 7 to be opened at intervals of about 5 to 10 minutes.

The required exposure time depends on the degree of contamination of the membrane pores or holes, but is usually less than an hour. The aerator field 4 can then be emptied again by opening the compressed air supply from the blower 7, wherein the emptying can take place from the drainage lines 8 or exclusively from the aerator elements 1. The substances contained in the cleaning liquid are used in a concentration that, especially after dilution to the entire volume of the wastewater 19 in the aerated basin 2, is non-toxic to the microorganisms living there and also does not impair the quality of the runoff of the cleaned wastewater.

The invention will now be described in more detail using two exemplary embodiments.

Example 1:

The wastewater treatment plant in a large city by the sea has been expanded to accommodate around 4 million population equivalents. The wastewater comes from households as well as industrial and commercial operations. The wastewater composition largely corresponds to that of domestic wastewater. The high salinity caused by the entry of salty groundwater (seawater) is striking. The system consists of three lines. Each line consists of two double pools, 28 meters wide and 145 meters long. The total volume is approx. 280,000 m³. Around 22,000 aeration elements are installed in the 12 basins for ventilation. The pools are continuously ventilated, there is no provision for stopping the ventilation. Due to power failures, however, the blower fails again and again and thus unventilated phases. After line 1 was put into operation, after about 1.5 years of operation there was a massive deterioration in the bubble pattern. When the basin was emptied, it was found that the membranes had partially slipped out of the aerator elements (specifically from the retaining rings).

An examination of the membranes using wet chemical methods, powder diffractometry and electron beam microprobe studies has shown that mineral deposits occur in the membrane pores and on the membrane surfaces. The partially crystalline substances are assigned to the alkaline earth-aluminum-orthophosphate-silicate system. The operator of the plant had the membranes of this line with a mechanical cleaning and a washing

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Subjected to hydrochloric acid. An electron beam microprobe examination of these membranes cleaned in this way had shown that alkaline earth phosphates could be removed, but the silicates or amorphous silica remained in the pores. The basin in which the membranes cleaned in this way were installed was filled with water about half a meter by the operator and aerated for 3 weeks. This resulted in massive calcium deposits due to algae growth and an additional blockage of the membrane pores with calcium (magnesium) carbonates. The task of a trial cleaning of an aerator field or several aerator fields of this basin was to remove both the newly added calcium deposits and the remaining silicate blockages. The cleaning was carried out with the basin empty. The components of the cleaning solution were determined in preliminary tests and the resistance of the materials that came into contact with the cleaning solution was tested. To measure the cleaning success, a measuring device for the air mass flow and the pressure was installed between the distribution line and the downpipe. It became a field with 170 aerators with around 6 m³Air applied per aerator and hour. The back pressure at the top of the downcomer was measured to be 9.5 kPa. Now the aerator field in free slope with acidic cleaning solution (1.4 mol per liter of nitric acid , ENT₃) filled. During a 40-minute exposure time, the air supply was opened several times in order to push the cleaning solution through the pores and thus allow it to develop its effect there. The ventilation field is emptied by opening the air supply via the open drainage line and, in some cases, also via the aerators. Now the air throughput was again about 6 m³ set per hour and aerator and the counter pressure measured at 8.0 kPa. In the next step of the cleaning process, a cleaning solution consisting of 2 components was fed into the ventilation field piping in a free slope. The first component of the cleaning solution consisted of 25 kg of potassium hydroxide, approx. 30 kg of BASF Trilon B (Na-EDTA), approx. 1 kg of a non-foaming surfactant (BASF Plurafac LF 431) and approx. 200 g of a catalyst for the controlled decomposition of hydrogen peroxide. These components were partly pre-resolved, partly added directly to a total volume of 1000 liters. The second component was 75 kg hydrogen peroxide (50%) made up to 1000 liters. During the exposure time of approx. 50 minutes, the air supply was opened several times and then new cleaning solutions were refilled. This enabled the cleaning solution to be pressed through the pores or bores and brought into contact with the deposits. The cleaning solutions were then displaced from the system by opening the air supply via the drainage pipes and the aerator elements. After cleaning, the aerator field was filled with rinsing water in a free gradient and the air supply was opened several times for 1 to 5 minutes and then rinsing water was added. The ventilation field was emptied again by opening the air supply, partly via the drainage pipes, but also via the ventilation elements. The aerator elements were then operated for about 1 hour with an air throughput of 6 m³ charged per aerator and hour. The back pressure was now 5.6 kPa.

The cleaning solutions were in 1 m³ Receiving containers attached. A 110 liter vessel with a circulation pump was used to dissolve the partially solid constituents. The connection between the storage tanks and the pipework was made with hoses.

According to the manufacturer, the pressure loss of a new aerator element is at an air throughput of approx. 6 m³ per aerator and hour, at approx. 4.0-4.5kPa, whereby it must be taken into account that when measuring the pressure for the entire aerator field, the counterpressure of the measuring section (air mass measurement, with two 90 ° and one 180 ° bends), the downpipe (approx. 8 m) and the horizontal distribution pipes are also recorded. (The back pressure could be reduced by 41% by cleaning.)

The cleaning of all aerator elements located in a partial basin during operation using cleaning solutions takes approx. 14 days. In the case of mechanical cleaning or cleaning of the membranes in the dismantled state using cleaning solutions, approx. 5 weeks are required for cleaning the basin and another 5 - 8 weeks for dismantling, cleaning and reassembling the membranes. So in total at least 2-3 months.

Example 2:

In the activation stage of an industrial wastewater treatment plant, mineral oil components become

AT 411 359 B of the wastewater reduced using the activated sludge process: The activated sludge stage consists of two sub-basins of equal size ("East" and "West") which are alternately charged with wastewater. The amount of waste water is approx. 1000 m³/H. The inlet has a particularly high salt concentration of approx. 12500 mg / l chloride. The BOD5 (biochemical oxygen demand in 5 days) in the inflow is approx. 150 mg / l, that in the outflow approx. 30 mg / l. There is sufficient carbon and nitrogen in the wastewater, phosphorus has to be added. The total volume of the two sub-basins is 7400 m³, the ventilation takes place with 4420 part fans. 13 fields with 170 partial aerators each are installed for each partial basin. The pools are operated according to the SBR principle. One cycle lasts 180 minutes. The "East" basin is first aerated for 75 minutes, then the aeration is switched off and the activated sludge is sedimented for 15 minutes. Now the loading phase follows, which also lasts 90 minutes. During the aeration and settling phase in the "East" basin, the "West" basin is filled; when the "East" basin begins the loading phase, the "West" basin is aerated. The inflow is not interrupted but only directed to either the "East" or "West" basin by means of a slide valve.

In preliminary tests in a pilot plant, which lasted approx. 18 months, no negative influence on the aerator membranes, ie no increasing counter pressure, was found. However, after installing the aerator elements in the basin and operating for 6 months, the system pressure had risen by approx. 8.0 kPa.

After emptying the basin and dismantling the aerator elements, salt crystals (99% sodium chloride) were found in the throttle holes in the plastic bodies. An examination of whether and which blockages were present in the pores of the membrane has not been carried out. The presence of the zeolites added to the wastewater has been proven, a proportion of precipitated silicates is probable based on previous experience (with comparable systems).

To test the procedure for clearing the blockages, an aerator field in the “East” sub-basin was cleaned. The aeration basin "East" was full at this second point. Before cleaning, the resistance of the materials of the aerator elements and the piping to the cleaning solutions was tested in laboratory tests. Since the entire field piping is made of plastic, special attention was paid to the joints in the pipe system. To determine the success (reduction of the pressure loss), a separate air supply was installed for the aerator field to be cleaned (1.9 m³/ h / aerator). After draining the field, the differential pressure was measured to be 47.1 kPa. When new, the pressure should be approx. 40.0 kPa under these conditions.

The cleaning solutions were mixed in two containers. In one, 24 kg of potassium hydroxide (approx. 35% strength, hardness stabilized) and 1 liter of catalyst solution were added to approx. 500 l of usable water and made up to 900 l. In the second container, approx. 500 l of usable water were also placed, 60 kg of hydrogen peroxide (30%) were added and made up to 900 l. The field piping (with pumps) was filled via a Y connection piece into the distribution line and from there into the downpipe. The two pumps in the tanks were put into operation at the same time, making sure that the addition was as even as possible. Approx. 600 l were pumped out of each container. The pumps were then switched off, the air flap in the downpipe and the taps on the Y-piece of the pump lines closed. Just a few minutes after the introduction, rising gas bubbles could be observed on the pool surface (released oxygen from the peroxide). At the beginning the bubble pattern was very uneven. After 10 minutes, approx. 50 I were pumped up from each container. Here it is important that the pumps are switched on first and only then the air flap and the pump line are opened, otherwise gas will escape through the pump lines. The post-pumping was repeated a total of 5 times. After an hour of exposure, the field piping was emptied via the drainage line. A sampling of the detergent solution revealed a dissolved salt load of approx. 2 kg, which corresponds to approx. 12 g per aerator. After the cleaning solution had been drained off, the piping was flooded again with service water and emptied via the drainage line (rewashing). The air volume flow of the external air supply was now applied to the ventilation field again and the differential pressure was measured to be 42.0 kPa. The comparison of the two differential pressures measured with the same exposure to air shows a reduction in pressure loss of 5.1 kPa.