Method and device for treating wastewater in a clarifier

Abstract

Method for treating wastewater in a settling tank (3), the bottom (4) of which is covered with a plurality of individual plate-shaped aeration elements (6) which have an elastic membrane with air outlet holes on their upper side, wherein an operating air stream (BL) is continuously blown into the wastewater during an aeration phase (B), characterized in that during the aeration phase (B) additional air is blown in with the aid of aeration pulses (22) to stir up the wastewater.

Description

The invention relates to a method and an apparatus for treating wastewater in a clarifier, wherein air is continuously blown into the wastewater during a aeration phase with the aid of aeration elements arranged in the clarifier.Such a method can be taken from DE 199 03 035 C2, for example. In sewage treatment plants, the waste water located in the sewage treatment basin is regularly supplied with oxygen with the aid of aeration elements arranged on the bottom side, by blowing in air via the aeration elements. For this purpose, the ventilation elements typically have elastic membranes which have pore-like air outlet openings via which the air can emerge. These ventilation elements are regularly connected to an air supply system, via which the required compressed air is provided.Further methods and apparatuses for waste water treatment can be found, for example, in DE 30 34 763 A1 and in DE 12 46 602 A.DE 199 43 853 A1 discloses the use of special bioreactores for a technically different application, namely for the fermentation of solid substances, in which compressed air is introduced into a closed pressure vessel via a nozzle arrangement with conduits arranged on a pressure cover, via which compressed air a mixing of the material to be fermented is aimed at.The oxygen supply is required for the wastewater purification by means of the microorganisms present in the wastewater. For the wastewater purification, a certain oxygen content is required in the wastewater. Ventilation is used in particular to regulate to a desired setpoint value for the oxygen content. During a ventilation phase, an at least largely constant operating air flow is typically established, which is blown in.Such a aeration phase is also referred to as a denitrification phase, since during this phase the microorganisms convert the ammonium (NH4) contained in the waste water into nitrate (NO3) with the consumption of oxygen. For the clarification process, a denitrification phase is additionally required, which follows the denitrification phase and in which the microorganisms react the oxygen from the nitrate in the absence of free dissolved oxygen. These two modes of operation, i.e. the denitrification and the denitrification, can be carried out in separate basins or also in a common basin. In the latter case, aeration phases with continuous aeration and denitrification phases alternate intermittently, in which the continuous aeration is shut off, so that in each case the aeration phase is followed by an unaeratored phase. When two basins are used, the denitrification takes place in the one basin. For this purpose, this basin is permanently ventilated, i.e. the ventilation phase takes place permanently without interruption during normal operation. In the other basin, usually connected upstream, denitrification takes place. This basin is not aerated.Adequate mixing is generally ensured by aeration. Such an aerated clarifier is also referred to as an activated basin.During the clarification process, biological and / or mineral deposits regularly occur on the surface of the membrane, which affect the air discharge from the air discharge openings. The problem of deposition occurs especially during the unvented phases. To avoid at least excessive deposits during such unvented phases, according to DE 199 03 035 C2 a so-called impulse ventilation is provided during the unvented phases, in which air is blown in with the aid of short-term ventilation impulses. The aeration pulses swirl up the waste water, as a result of which the deposits are counteracted.The sufficient thorough mixing and supply with the required oxygen requires a high air input into the wastewater. For energy saving reasons, more recent developments provide a large occupancy density, in which as large as possible areas in the clarifier are occupied by such ventilation elements, so that the air flow emitted via a respective ventilation element can be reduced compared to a lower degree of coverage. This leads to lower operating pressures, smaller air bubbles, longer residence times of the air bubbles in the wastewater and thus to the desired energy savings. The occupancy density in conventional solutions is typically about 10-20%, i.e. 10-20% of the area of the floor is covered with the ventilation elements.The operating parameters for ventilation changed with a higher occupancy density result in new requirements for ventilation.Proceeding from this, the object of the present invention is to reliably ensure a permanent and sufficient mixing of the activation tank even at high occupancy densities.The object is achieved according to the invention by a method according to claim 1 and an apparatus according to claim 12 for treating wastewater in a clarifier, in which an operating air stream is blown into the wastewater continuously, i.e. without interruption, during a aeration phase with the aid of aeration elements arranged in the clarifier. For a permanently reliable wastewater treatment, provision is now also made during the aeration phase for additional air to be blown in with the aid of aeration pulses for additional fluidization of the wastewater. Thus, during the aeration phase for normal aeration, an additional impulse aeration takes place.The advantages and preferred embodiments listed below in connection with the method are also to be transferred analogously to the device and vice versa.As explained at the beginning, air is permanently blown in during the aeration phase, i.e. the air stream referred to in the present case as the operating air stream is blown into the waste water without interruption via the aeration elements. The air flow, i.e. the air volume blown into the waste water per unit time, is adjusted, in particular regulated, as described as a function of the desired oxygen content. The operating air flow is therefore the (target) air flow which is specifically specified by a control unit and is set in order to achieve the target oxygen content.The operating air flow is typically subject to fluctuations which are caused, for example, by conditions in the clarifier which vary over the course of the day. For example, time-of-day variations in the operating air flow occur as a result of inflow variations in the wastewater fed to the clarifier. Under constant conditions and in particular for shorter periods of time, for example in the range of several 10 min, a constant or an at least largely constant operating air flow is typically established.If a ventilation pulse is mentioned here, this is understood to mean that, in addition to the operating air stream blown in continuously, air is blown in for a short period of time. That is, for the duration of the ventilation pulse, the total air flow blown in is increased compared to the operating air flow which is set for the desired oxygen content. The total air flow is thus composed of the operating air flow and an additional impulse air flow.These ventilation pulses also during the ventilation phase provide protection against deposits and thus adverse effects on ventilation.Studies have shown that deposits can also occur during the aeration phases. This problem can occur in particular in sewage treatment plants with a high occupancy density, since in these plants--due to the desired energy saving--the energy introduced into the waste water during aeration is reduced, whereby a reduced fluidization of the waste water takes place, so that the risk of deposits is increased.A further advantage of the aeration pulses is that the desired mixing is improved and / or ensured by these. In the case of a high occupancy density of the clarifier with aeration elements in order to achieve an energy-efficient design, the problem may arise that a desired (complete) mixing is not achieved even in the case of an overall high air volume flow.The device and in particular the control unit are generally designed to carry out the following, preferred steps:In order to reliably prevent deposits, it is provided in a preferred embodiment that the aeration pulses take place during the entire aeration phase.As mentioned at the beginning, there are plant concepts in which the denitrification and the denitrification are divided into two basins. In this case, the basin provided for the denitrification is permanently aerated, i.e. the aeration phase is permanently applied during normal operation of the plant. In the present case, normal operation is generally understood to mean the operation of the plant for treating the wastewater without further service or revision measures.In a plant concept with a common tank, in which aeration phases and denitrification phases alternate intermittently, the aeration phases typically take 1⁄2 hour to several hours and are preferably in the range between 30 min and 4 hours and in particular between 30 min and 2 hours.In both plant concepts, therefore, the aeration pulses are carried out during the entire time duration of the respective aeration phase, in which air is additionally blown in in order to ensure the desired fluidization and mixing of the wastewater.The ventilation pulses are preferably carried out periodically in a recurring manner, i.e. identical time intervals are in each case situated between successive ventilation pulses. The duration of the individual aeration pulses is preferably also the same in each case. These periodically recurring aeration pulses ensure a reliable fluidization.During a ventilation pulse, the total air flow blown in is preferably increased by at least 50% and in particular by at least 100% with respect to the operating air flow, that is to say with respect to the (instantaneous) air flow before the start of the ventilation pulse and thus in particular also with respect to the air flow as is predetermined in order to achieve the desired oxygen content. This clearly increased amount of air per unit time achieves a distinct swirling.The duration of a ventilation pulse is preferably in the range between 10 seconds and 120 seconds and in particular in the range between 20 seconds and 60 seconds. This time has been found to be particularly suitable.Preferably, furthermore, the time interval between two successive ventilation pulses is in the range between 5 minutes and 45 minutes and in particular in the range between 10 minutes and 30 minutes. These values have also proved to be particularly suitable.As has already been described above, in a plant concept with intermittent operation, unvented phases, i.e. so-called denitrification phases, also take place in addition to the aeration phases. It is preferably provided that air is blown in with the aid of further ventilation pulses even during the unvented phases. Overall, therefore, aeration pulses are produced both during the aeration phases and during the unapped phases and thus virtually continuously over the entire normal operation of the sewage treatment plant. Alternatively, there are also plant concepts in which mixing is ensured during the unvented phase, for example by stirring mechanisms.In an expedient embodiment, during a respective further ventilation pulse, a higher air flow is blown in compared to the operating air flow during the ventilation phase. This embodiment is based on the consideration that the operating air flow is not sufficient to avoid the deposits, so that a reliable swirling during the unvented phases is achieved by the increased air flow in comparison therewith during the further ventilation pulses. The air flow during the further ventilation pulses is increased by at least 50%, for example, compared to the operating air flow.Preferably, it is further provided that the total air flow blown in during a ventilation pulse during the ventilation phase is equal to the air flow blown in during a further ventilation pulse. This is advantageous, for example, from the control point of view, since during the ventilation phases and the unvented phases, the same air flow is set for the respective duration of the impulse ventilation and therefore does not have to be differentiated between the different phases. This is achieved, for example, by a defined, predefined rotational speed of a blower arrangement for generating the air flow.In the intermittent operating mode, the aeration phases and the unapped phases repeat, i.e. they alternate. For example, this takes place periodically after predetermined time intervals. The time duration of a respective aeration phase is, as stated above, typically in the range from half an hour to several hours, preferably in the range between 0.5 and 4 hours and in particular in the range between 0.5 hours and 2 hours.In each case, an unvented phase is arranged therebetween. Their duration is typically likewise in the range of several hours, wherein the duration is preferably less than that of the respective aeration phase. Alternatively, the time duration of the unvented phase is equal to that of the respective ventilation phase. Preferably, the time durations of the individual unvented phases are each identical and / or the time durations of the individual ventilation phases are each identical.The time durations of the different phases are set in particular as required. The time duration, aerated or unapped, is set dynamically, for example, as a function of the load and the degradation behavior. The values of oxygen, ammonium and nitrate required for this purpose are measured and evaluated continuously, for example, by means of measurement elements (probes).In a preferred embodiment, all ventilation pulses, i.e. both the ventilation pulses during the ventilation phases and the further ventilation pulses during the unvented phases, take place without interruption over the entire duration of these ventilation phases and unvented phases, i.e. during normal operation of the sewage treatment plant. Normal operation is understood here to mean alternating, intermittent operation between the aeration phases and unvented phases, if appropriate without further service or revision measures.In a particularly preferred embodiment, the ventilation pulses and further ventilation pulses are periodically repeated, i.e. in each case after fixed time intervals during normal operation. In conjunction with the setting of an identical air flow of a respective pulse, this can be implemented particularly easily in terms of control technology.An exemplary embodiment of the invention is explained in more detail below with reference to the figures. These are shown in simplified representations: FIG. 1 is a schematic representation of a device for treating waste water, and FIG. 2 shows a ventilation scheme for ventilating the wastewater.FIG. 1 shows a detail and greatly simplified of a sewage treatment plant 2 as a device for treating wastewater. A clarifier is shown whose bottom 4 is covered with a plurality of individual aeration elements 6. The ventilation elements 6 are plate-shaped ventilation elements, which are therefore typically rectangular and have an elastic membrane with air outlet holes on their upper side, from which air can emerge during operation.The individual ventilation elements 6 are connected to an air supply system 8. This comprises a pipe system 10, a blower arrangement 12, a control unit 14 and also further supply and control units, such as control valves 16, for example. In the exemplary embodiment, the pipe system 10 comprises a distributor pipe 18, from which in each case a supply line 20 leads, via which the individual ventilation elements 6 are connected via branch lines. The blower arrangement 12 and, if required, also the control unit 14, in particular an PLC controller, are typically accommodated in a common operating building 21.By way of the control valves 16, for example, the air flow for the respective supply line 20 can be regulated.During operation of the sewage treatment plant 2, a controlled aeration of the wastewater takes place with the aid of the air supply system 8 and the aeration elements 6. This is therefore configured such that the respective components of the sewage treatment plant 2 are suitably controlled in order to achieve the desired aeration.An example of a ventilation scheme according to the invention is shown in FIG. 2. This shows the time profile of an air flow L introduced via the ventilation elements 6 versus time t.During operation, a periodic return switching is made between a ventilation phase B and an unvented phase U. The duration of a respective aeration phase B is in this case, for example, in the range between 60 minutes and 120 minutes, and the duration of an unasperated phase U is typically somewhat less than this and, for example, in the range between 50 minutes and 100 minutes. During a respective aeration phase B, continuous continuous aeration takes place with an aeration air flow BL, which is typically constant or largely constant, at least after a settling process after switching on the aeration at the beginning of an aeration phase B until a desired desired oxygen content in the wastewater is reached. By means of this aeration air flow BL, the oxygen content in the waste water is generally adjusted to a desired setpoint value. In contrast, during the unvented phase, the continuous air supply is switched off.It can also be seen from FIG. 2 that during the aeration phases B aeration pulses 22 take place and during the unapped phases U further aeration pulses 24 take place.The concept of impulse aeration during unasperated phases is already known.The present sewage treatment plant 2 is a plant with a comparatively high occupancy density, in which a large part of the area of the floor 4 is thus covered with ventilation elements 6. The occupancy density is preferably above 15% and is, for example, in the range between 20% and 40%.Especially at high loading densities, there is the risk that biomass particles floating in the wastewater will settle out even during aeration phases B.In order to prevent this, pulse ventilation with the aid of the ventilation pulses 22 is now also provided during the ventilation phases B. During the ventilation pulses 22, the air flow L is increased compared to the set operating air flow BL, namely to a so-called total air flow GL. This is preferably at least 20% and preferably 50% above the operating air flow BL.The duration of a respective ventilation pulse 22 is, for example, 30 seconds. A distance between successive ventilation pulses 22 is, for example, 15 minutes.The further aeration pulses 24 also have comparable and preferably identical time durations and time intervals. In the exemplary embodiment, the air flow L of the further ventilation pulses 24 preferably corresponds to the total air flow GL during the ventilation pulses 22.The required air flow L is generated and provided via the blower arrangement 12, which can have a plurality of individual blowers, for example. To set different air flows L, for example, fans are switched on and off or the rotational speed of the individual fans is set and regulated.The ventilation scheme shown in FIG. 2 is characterized by the periodic sequence of ventilation pulses 22, 24 over the entire time duration during normal operation, i.e. during intermittent operation with the ventilation phases B and the unvented phases U. In addition, it is preferably provided that the air flow L is identical during a respective pulse 22, 24. This means that the air flow L during the further ventilation pulses 24 corresponds to the total air flow GL.As already mentioned, pressure relief typically takes place during a revision phase, so that an overpressure can escape in the pipe system 10. This has the result that the elastic membrane of a respective ventilation element 6, which is stretched during ventilation due to the overpressure, is relaxed again. This repeated stretching and relaxing removes a coating forming on the surface of the membrane.Such revision phases take place with the air supply switched off, that is to say during the unvented phases U and additionally at the times at which no further ventilation pulses 24 take place. Due to the time interval between the further ventilation pulses 24 of typically more than 10 minutes, sufficient time remains for the revision phases, which preferably take place immediately after a switching off of the air supply and at the beginning of an unvented phase U.The invention is not limited to the embodiment described above. Rather, other variants of the invention can also be derived from this by the person skilled in the art without departing from the subject matter of the invention. In particular, all the individual features described in connection with the exemplary embodiment can also be combined with one another in another manner without departing from the subject matter of the invention.List of reference characters2 Sewage treatment plant 3 Sewage treatment tank 4 Floor 6 Ventilation element 8 Air supply system 10 Pipe system 12 Blower arrangement 14 Control unit 16 Control valve 18 Distributor pipe 20 Supply line 21 Operating building 22 Ventilation pulse 24 Further ventilation pulse B Ventilation phase U Unvented phase L Air stream BL Operating air stream GL Sankt air stream t Time

Claims

Method for treating wastewater in a clarifier (3) whose base (4) is covered with a multiplicity of individual plate-shaped ventilation elements (6) which have an elastic membrane with air outlet holes on their upper side, an operating air stream (BL) being blown continuously into the wastewater during a ventilation phase (B), characterized in that, during the ventilation phase (B), air is additionally blown in in in order to swirl the wastewater with the aid of ventilation pulses (22).Method according to the preceding claim, characterized in that the aeration pulses (22) are applied during the whole aeration phase (B).Method according to one of the preceding claims, characterized in that the aeration pulses (22) take place periodically in a recurrent manner.Method according to one of the preceding claims, characterized in that during a ventilation pulse (22) a total air flow (GL) is blown in, which is increased by at least 50% and in particular by 100% compared to the operating air flow (BL).Method according to one of the preceding claims, characterized in that the duration of a ventilation pulse (22) is in the range between 10 s and 120 s and in particular in the range between 20 s and 60 s.Method according to one of the preceding claims, characterized in that the time interval between two successive ventilation pulses (22) is in the range between 5 min and 45 min, in particular in the range between 10 min and 30 min.Method according to one of the preceding claims, characterized in that, in addition to the aeration phases (B), unaeratored phases (U) are provided, during which no continuous aeration takes place.Method according to the preceding claim, characterized in that air is injected during the unvented phases (U) with the aid of further ventilation pulses (24).Method according to the preceding claim, characterized in that during a further ventilation pulse (24), a higher air flow is injected compared to the operating air flow (BL) during the ventilation phase (B).Method according to one of the two preceding claims, characterized in that during the ventilation pulse (22) a total air flow (GL) is blown in which is equal to the air flow of the further ventilation pulse (24).Method according to one of Claims 8 to 10, characterized in that aeration phases (B) and unvented phases (U) repeat and the aeration pulses (22) and the further aeration pulses (24) take place without interruption and preferably periodically in a recurring manner.Device for treating wastewater in a clarifier (3) whose base (4) is covered with a plurality of individual plate-shaped ventilation elements (6) which have an elastic membrane with air outlet holes on their upper side and which are connected to an air supply system (8) which has a control unit (14) and with the aid of which air can be blown into the wastewater via the ventilation elements (6), characterized in that the control unit (14) is set up in such a way that air is blown in during a ventilation phase (B) during which air is blown continuously into the wastewater for swirling the wastewater with the aid of ventilation pulses (22).

Images