NZ727654B2 - Plant for treatment of liquid as well as method for controlling such a plant - Google Patents

Abstract

The invention relates to a plant and a method for controlling such a plant suitable for treatment of waste water. The plant (1) comprises a basin (2), at least one flow generating machine (6) adapted to generate a liquid flow in the basin (2), at least one equipment (7) in the basin (2) that effects the momentum of the liquid flow, and a control unit (8). The method being characterized by the steps of in the control unit (8) storing a predetermined relationship between the operational speed N of the flow generating machine (6) and an operational parameter P from which the torque M of the flow generating machine (6) may be derived, which relationship depends on a predetermined liquid flow speed V in the basin (2) by the flow generating machine (6), determining the operational speed N of the flow generating machine (6), from the determined operational speed N determining a set value of the operational parameter P of the flow generating machine (6) based on said relationship between the operational speed N of the flow generating machine (6) and the operational parameter P of the flow generating machine (6), by means of the control unit (8) determining a real value of the operational parameter P of the flow generating machine (6), and by means of the control unit (8) adjusting the operational speed N of the flow generating machine (6) if the real value of the operational parameter P of the flow generating machine (6) is different than the set value of the operational parameter P of the flow generating machine (6). the momentum of the liquid flow, and a control unit (8). The method being characterized by the steps of in the control unit (8) storing a predetermined relationship between the operational speed N of the flow generating machine (6) and an operational parameter P from which the torque M of the flow generating machine (6) may be derived, which relationship depends on a predetermined liquid flow speed V in the basin (2) by the flow generating machine (6), determining the operational speed N of the flow generating machine (6), from the determined operational speed N determining a set value of the operational parameter P of the flow generating machine (6) based on said relationship between the operational speed N of the flow generating machine (6) and the operational parameter P of the flow generating machine (6), by means of the control unit (8) determining a real value of the operational parameter P of the flow generating machine (6), and by means of the control unit (8) adjusting the operational speed N of the flow generating machine (6) if the real value of the operational parameter P of the flow generating machine (6) is different than the set value of the operational parameter P of the flow generating machine (6).

Description

PLANT FOR TRiZATMiZNT Oh' T. QU 3 AS WELL AS METHOD FOR CONTROLLING SUCH A RLANT Technica' field 0" ,he Invention The present invention relates in general to a plant and a method for controlling a plant for treatment of liqiid, especia'ly biologica' treatment 0 'iquid. The present invention relates especially to a plant and a method for controlling a plant suitab'e or treatment ol liquid such as waSte water, wherein the plant comprises a basin configured to house a liquid, at leaSt one flow ting machine arranged in the basin and configured to generate a liquid ow in the basin, at leaSt one equipment that is ed in the basin and thaw e "ects the momentum ol the liquid ow, and a control init that is operatively connected to said at leaSt one flow generating machine.

Background of ,he Invention and prior art A ation channel, or a ring channel, is usually an upwardly open endless basin that is used during biological treatment or oxidation 0" , ally waste water.

The waSte water/liquid is made to flow along the circulation channel and is thereby made to pass di "erent zones in the circulation channel.

During such ical ent the waste water is usual'y purified from nitrogen and biological material by having micro organisms breaking down the biological material into carbon dioxide and water, and by having baCteria transforming the water bound en to aerial nitrogen. ?uri; ied waste water is released into the nature and in the case the water bo;nd nitrogen is nOt eliminated there is a risk for eutrophication in the natural watercourses, and due to the fact that the biological material is consuming considerable amounts 0: oxygen watercourses deficient in oxygen are generaced i" insu "iciently purified water is released. The ng down 0: the biological material is VV()2015/193783 stimulated by adding large amounts 0: oxygen to the waSte water by means 0: one or more aeration sectors, and the ation o: the water bound nitrogen takes place in the circulation channel in areas without added oxygen or in separate basins without added oxygen and/or in areas/basins in which the dissolved oxygen leve' is 'ow enough.

In one, or a few, locations along the circulation l I) oxygen is supplied to the waste water by means 0; mechanical surface aerators, bottom located aerator sections, jet aerators, etc. The micro organisms in the so— 'ed activated sludge consume the oxygen in order to break the biological material t in the waste water, as 'Eor ni' tri ‘ication o incer alia ammonium Flow ting machines/mixer machines are used in circulation channels in order to mix the liquid/waste water in order to obtain an as nous :_iquid mixture as possible, keeping the biological al suspended, as well as generating a liquid ‘low circula,ing/"lowing along the circulation channel.

In some known processes 0: liqiid creacment it is requeSted that the liquid flow speed along the circulation channel is kept at a predetermined conscanc leve' "n a theoretical circulation channel having only one flow generating machine and homogenous liquid, this is easily "ul"illed by operating she llow ting machine at a constant operational speed. However, in reality the liquid is not homogenous over time and thereto she creacmenc plant comprises equipmenc thac are arranged in the circulation channel and that e""ect the momentum 0: she Slowing liquid in positive direction or in negative direCtion. A change in the momentum of the 'iquid ow entails chad she speed 0; the liquid tlow is e /changed. The e ecu ol these equipment may also be variable over time due to varying operation. Based on she lact that she status ol the incoming waste water is changed over time, the amount 0: added oxygen must also be modified over time, and modified ity s that the speed of the liquid flow is effected.

The changing speed of the liquid flow may be compensated by revising the operational speed of the flow generating machine, however, this demands for the speed of the liquid flow being monitored by means of expensive speed sensors that are tible to disturbances and require regular nance.

Object of the Invention The present invention aims at improving previously known methods for controlling treatment plants with the object of reducing the costs for the operation of the treatment plant and thereby providing an improved method for controlling a treatment plant suitable for treatment of liquid. A basic object of the invention is to e an ed method of initially d type, which entail that a constant speed of the liquid flow can be achieved without the need for external speed sensors. External speed sensors suffer from the drawbacks that they are ive and require monitoring and maintenance as they are susceptible to disturbances and thereby risk to provide incorrect information.

Brief description of the Invention According to the invention at least the basic object is ed by means of the initially d method, having the features defined in the independent claim. Preferred embodiments of the present invention are further defined in the dependent .

According to a first aspect of the present invention there is provided a method for controlling a plant for treatment of waste water, wherein the plant comprises: - a basin constituted by a circulation channel and configured to house a liquid, - at least one flow generating machine constituted by a submergible mixer machine and arranged in the basin and configured to generate a liquid flow along the circulation channel, wherein the operational speed N of the flow 40 generating machine is higher than a predetermined lowest allowable operational speed Nmin , preventing solid matter from accumulating on the bottom of the basin, and is lower than a predetermined highest allowable operational speed Nmax , preventing the flow generating machine from being overloaded, - at least one equipment that is arranged in the basin and that effects the momentum of the liquid flowing in the basin, and - a control unit that is operatively connected to said at least one flow generating machine, the method including the steps of: - in the control unit g a predetermined onship n the ional speed N of the flow generating machine and an operational parameter P from which the torque M of the flow ting machine may be derived, which onship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, - determining the operational speed N of the flow generating machine, - from the determined ional speed N determining a set value of the operational parameter P of the flow generating machine based on said onship between the operational speed N of the flow generating e and the operational parameter P of the flow generating machine, - by means of the control unit determining a real value of the operational parameter P of the flow generating machine, and - by means of the control unit adjusting the operational speed N of the flow generating machine if the real value of the operational parameter P of the flow generating machine is different than the set value of the operational parameter P of the flow generating e.

According to a second aspect of the present invention there is provided a plant for treatment of waste water, sing: - a basin constituted by a circulation channel and configured to house a liquid, - at least one flow generating machine constituted by a submergible mixer machine and arranged in the basin and configured to generate a liquid flow along the circulation channel, wherein the operational speed N of the flow generating machine is higher than a predetermined lowest allowable operational speed Nmin , preventing solid matter from accumulating on the bottom of the basin, and is lower than a predetermined highest allowable operational speed Nmax , 40 preventing the flow generating machine from being overloaded, - at least one equipment that is arranged in the basin and that effects the momentum of the liquid flowing in the basin, and - a control unit that is operatively connected to said at least one flow generating machine, the control unit sing a predetermined relationship between the operational speed N of the flow generating machine and an operational parameter P from which the torque M of the flow generating machine may be derived, which relationship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, the control unit being configured to determine from a given operational speed N a set value of the operational parameter P of the flow generating e based on said relationship between the operational speed N of the flow generating machine and the operational parameter P of the flow ting machine, determine a real value of the ional parameter P of the flow generating e, and adjust the operational speed N of the flow generating machine if the real value of the operational parameter P of the flow generating machine is different than the set value of the operational parameter P of the flow generating machine.

Thus, the present invention is based on the understanding that by ring the operational speed of the flow generating machine and an operational parameter P from which the torque M may be derived, a predetermined speed of the liquid flow may be obtained with the need of external speed sensors.

According to a preferred embodiment of the present invention, the operational speed N of the flow generating machine shall be increased if the true value of the ional parameter P of the flow ting machine is greater than the set value of the ional parameter P of the flow generating machine, and be decreased if the true value of the ional parameter P of the flow generating e is less than the set value of the operational parameter P of the flow generating machine.

According to a preferred embodiment of the present invention, said at least one equipment is constituted by a mechanical surface aerator, especially a mechanical surface aerator comprising a horizontal rotational axis.

According to a preferred embodiment of the t invention, the operational parameter P is constituted by the current I that the flow generating machine consume.

Other advantages with and features of the invention will 40 be apparent from the other dependent claims as well as from the following, ed description of preferred embodiments. arie" description 0: the drawings A more complete understanding 0: abovementioned and other features and advantages 0: the present invention will be apparent from the following, detailed ption 0; red embodiments with reference to the appended drawings, in which: Fig. l is a schematic i"uscra,ion 0' an inventive plan'( according so a lirst embodiment, Fig. 2 is a schematic i"uscra,ion 0' an inventive plan'( according to a second ment, and Fig. 3 is a schematic i"uscra,ion 0“ an inventive plan'( according to a third embodiment.

Detailed description 0: the Invention Reference is initially made so figure 1. The present invention relates to a plant, genera"y designated 1, suitable for treatmenc/puri "icavion 0" , such as waste water, comprising biological matter. The plant 1 comprises a basin 2, configured to house the liquid to be treated.

In the disclosed embodiments the plant is constituted by a treatment plant and the description hereinbelow is written using the term enc plant, but it shall be ed chac other equivalent plants are included if nothing else is stated. Thereto the basin 2 is conStituted by a circ1lation l in the disclosed embodiments and the description below is written using she cerm circulation channel, but it shall be realized that also basins that are not circulation channels are to be seen as equivalents and be inclided if nothing else is stated.

This, she treatmenc plant 1 comprises an endless circulation channel 2, or race track, configured to house the liqiid to be treated. In the disclosed embodiment the ation channel 2 is conscicuted by an oblong basin having rounded ends and comprising a lly located VV()2015/193783 longitudina' r 3, the circulation channel 2 obtaining two parallel straight channel segments that are ted to each other by means 0:: two direction changing/semi circilar channel segments. The direction changing channel segments comprises in the disclosed embodiment guide walls 4 that faci' itates the change 0“ direction 0'‘ the liquid flow. "" shall be pointed out that circulation channels may present any other conceivable shape, for instance annular shape or serpentine shape, and this the circulation channel may comprise several straigh': and direction changing channel segments, tively, or may present an entirely circular/elliptic track shape.

The circulation channel 2 is adapted to house liquid/waste water up to a predetermined filling height/liquid level, even thus the actual liquid level during operation may be below as well as above said filling height without the present invention being e "ected appreciably. The waste wa':er is puri ‘ied either continuously or batch wise in an invencive Lrea tment plant l and by having a waste water volume reaching up '0 said predetermined "i' ling height an op':imal iti'ization of the treatment plant is obtained. A typical fi'ling height is about 3—8 meters. During continuous ,rea ,ment liqiid is supplied continuously to the circula':ion channel 2 at the same time as liquid is removed uous'y "rori the circulation channel 2. The input and the output diring continuous treatment is a fraction o the circu'ating "low, usually about 1/30—1/20 of the circu'ating "low. The ation channel 2 is con:figured to have a predetermined f'ow direCtion, schematically shown by means of the arrow 5, along which the liquid is intended to flow. o th inv ntiv tr atm no p'ant ses at least one flow generating machine 6 arranged in the ation channel 2, y two or more flow generating machines are located neX': to each other. The f'ow generating machine 6 is configured to generate a 'iquid "'ow "lowing along said circulation channel 2, and may be constituted by one or more submergible mixer machines, often so—called slowly operated mixer machines having a propeller rotating at a rpm in the area less than 100 rpm, usually in the area —50 rpm. In some installations the treatment plant 1 comprises ow generating machines 6 at two or more locations, which preferab'y are mutua' 'y equidistantly arranged along the circulation channe' 7. Preferably the flow generating machine 6 shall be located at a diStance from a direction changing l segment 0: the circulation channel 2 such that a councer "orce from the walls of she ation channel 7 having e "ect on the flow tion machine 6 and negatively e""eccing she tion of the 'iquid flow does not arise.

Thereto th inv nciv or atm no plant 1 comprises at least one equipment 7 that is arranged in the ation channel 2 and that s she momencum of she flowing liquid. The equipment 7 may be e""ecced by the momentum of the liquid flow in a positive way, i.e. a momentum source adding speed to the liquid f'ow, or in a negative way, i.e. a momentim sink reducing tre speed of the liquid flow. The equipment 7 may be a immovable equipment or a movable equipment, an aCtive equipnent or a passive equipment.

Th inv ntiv tr atm rt plant 1 also ses a control init 8, cha c is operatively connected to said at least one flow genera ing n achine 6 and l the operational speed N a which said flow genera:ing machine 6 is to be , for instar ce by controlling she frequency O__ she current operating -r e flow generating machine 6. The COIltrol unit 8 may be constitu':ed by an external control unit or a control uni incegraced in the flow generating machine 6.

In the embodimen': disclosed in figure 1 the equipment 7 is conStituted by a mechanical sur:face aerator having a horizontal onal axis. Such a mechanical surface aerator having a horizontal rotational axis comprises VV()2015/193783 according to the embodiment disclosed in figure 1 a horizontal shaft 9 having blades/arms/brushes lO protruding therefrom in the radial direction. The shaft 9 is preferably located in flish with or above the liquid 'eve' in the circulation l 2. The mechanical surface aerator having horizontal rOtationa' axi s is in th e disclosed embodiment arranged in the midd'e o a straight channel t, however other locations are conceivable such as preferably in the beginning ol as traight l segment. Upon rotation o: the horizontal sha ft 9 the blades 10 bring the air above the liquid to be mixed with the liquid, in order to transfer oxygen (Ofl from tr e air to the waste water/liquid.

In the embodiment disclosed in figure 2 the equipment 7 is constituted by a mechanical sur: ace aerator having a vertical rOtational axis. Such a me chanical e aerator having a vertical rotational axis comprises ing to the disclosed embodiment ’ a vertical sha _:t ‘l having blades/arms l2 protruding therefrom in the radial direCtion, and/or having blades/arms ding there from in the axial direction. The shalt 11 extend from a level above the liquid 'eve' in the circulation channel 2 to a level below the 'iquid 'eve' in the ation channel 2 as in the disclosed embodiment, or is preferably located above the 'iquid 'eve' in the embodiment having axial blades/arms. The mechanical surface aerator having vertical rotational axis is in the disclosed embodiment arranged in a direction changing channel segment, however 0 ther locations are conceivable. Upon rotation ol the vertical shaft 11 the blades 12 bring the air above the l iquid to be mixed with the , in order to transfer oxygen (ON from the air to the waste water/liquid.

Such mechanical sur:face aerato rs, i.e. horizontal and vertical, can e "ecL Lhe momentum o f the flowing liquid in positive as we" as negative direction, and preferab'y the ical surface aerator is ively connected to the VV()2015/193783 control uni 8 whereupon the operational speed/rotational speed 0: the mechanical s1r face r can be adjusted/al ered, there so i is conceivable to adjus t/alter the submersible depth of she aerator. Other types 0; mechanical sur:face aerators are conceivable, but are not disclosed herein.

In the embodiment disclosed in figure 3 the treatment plant 1 comprises an aeration arrangement, generally designated 13, CODfigured to provide a gas flow Q comprising oxygen to the liquid. The on arrangement 13 comprises preferab' y at leaSt one aeration sector 14. The on seCtor 14 is in the sed embodiment arranged in the middle of a straight channel segment, however, also Other ons are conceivable such as pre:ferably in the beginning 0 : a straight channel segmen': or along the entire leng ch of a straight channel ': and/or in a direCtion changing channel segment. Said at leaSt one aeration sector ’4 is preferably arranged at the bo ctom ol the circulation channel 2 and is configured to provide a gas flow Q from the aeration arrangement 13 to the liquid, in order to transfer oxygen (Ofl from the gas to the waste water/liquid. The gas flow Q is consti:uted by oxygen containing gas such as air, other oxygen con':aining gas mixture or pure oxygen. The aeration seCtor ’4 i s for in stance conStituted by a large number " 0 di "usors or r members 15, p referab' y so— called fine bubble aerators, which together cover the entire or main part of the wid,h of she circulation l 2. The aeration arrangemen l3 thereco comprises at least 0ne blowing machine l6 that provides gas to the aeration sector 14 via a pipe system l7, the blowing machine l6 is pre ferably operatively ted to the l unit 8 whereupon the operational speed/rotational speed ol she blowing e 16 can be adjusted/al tered. The aeration arrangement 13 can e "ect the moment 1m Ol the flowing liquid in positive as well as negative direCtion.

VV()2015/193783 In an alternative nOt disclosed embodiment the equipment 7 is constituted by a so—called jet aerator/venture aerator. It sucg up liquid and pump it out h a nozzle whereupon air from above the liquid level in the circ1lation l 2 is sucked into the nozzle and is mixed wi,h she pumped liquid. A jet aerator e "ect the momentum o_ the flowing liquid in positive direCtion.

It shall be realized that an equipment 7 nOt necessarily need to be constituted by an aerator, but can for instance be constituted by a plate or Other stationary ve equipment 7 located in the ation channel 2.

The ent may for inStance also be conStituted by a pump device configured to raise/lower the liquid level in the basin.

Refernece is now made to figure ’, however it shall be realized that it correspondingly also applies to other embodimencs i _ nothing else is stated.

In an optimal embodiment a homogenous uniform liquid ow reach the mechanica' surface aerator 7 and the liquid ow reaching the flow generating machine 6 is ree "rom gas bubbles and gas flow induced c1rrents. ab'y the distance between the mechanical surface aerator 7 and the flow generating machine 6 is a : least as big as the distance between the flow generating machine 6 and the mechanical surface aerator 7 seen in the flow direction 5, such that the operation 0: the mechanica' surface r 7 e "ects she "'ow generating machine 6 as little as possible.

It is ial for the present invention that he mechod for controlling the plant l, comprises the steps I) in the control unit 8 Store a predetermined relationship between the operational speed N of she flow ting machine 6 and an operational parameter P from which the torque M o_ the flow generating machine 6 may be derived, which relationship depends on a predetermined liquid flow speed V in the basin 2 at the flow ting machine 6, determining the operational speed N o_ the flow generating VV()2015/193783 2015/054499 machine 6, from the determined operational speed N deter— mining a se't value ol the operational parame'ter P oj the flow genera'ting machine 6 based on said onship between the operational speed W ol the llow generating machine 6 and the operational parameter P oj the Slow generating machine 6, by means 0: the control unit 8 determining a real value of the operational ter P oj the Slow ting machine 6, and by means 0: the control unit 8 adjusting the operational speed N o: the "'ow generating machine 6 ij the real value of the operational parameter P oj the jlow generating machine 6 is ent than the set va'ue o" the operational parameter P o: the jlow generating machine 6.

The relationship between the operational speed n of the :'ow generating machine 6 and the operational parameter P of the llow generating machine 6, jor the respeCtive liq lid OW speed V, is preferably stored in the control unit 8. It( shall be pointed out that the opera'tional speed h o j the ow generating machine 6 in an arternative, completely equivalent, way can be sed as the operatior al rpm or the electrical drive arequency o: the llow ting machine 6 without e""ecting the present inventior.

The operational parameter P ol the Slow generating machine 6 comprises preferably the torque M o: ore "'ow ting machine 6 and/or the t that ore "'ow generating machine 6 consume during operation. It shall be realized that also 0'ther operational parameters P , from which the torque o j the jlow generating machine 6 can be derived, are conceivable and are included in the term operational parameter.

Thus, the step ol by means ol the control unit 8 determine a true value 0: the ional parameter P of the ow generating machine 6, preterab' y comprises measurement 0 t the current consumption 0: the OW generating machine 6, :rom which the torque M o_ the jlow generating machine 6 can be derived. The current/power consumption ol the Slow generating machine 6 is thus an equivalent expression :or 2015/054499 the torque M o_ the Slow generating machine 6. Upon measuremeno ol she current/power consumption the control unit 8 preterab'y comprises a "iltra,ion O“ the current signal in order tor the operational speed N o_ the 'ow generating machine 6 shall nOt be e "ected by quick load variations for instance originating from solid matter comprised in the , tirbulence, etc.

According so she disclosed embodiments the operational speed N o_ the jlow generating machine 6 shall be increased if the true value 0: she torque M o_ the llow generating machine 6 is greater chan she set va'ue O“ the torque M o; she jlow ting machine 6, and the operational speed N oj she jlow ting machine 6 shall be decreased i: the true va'ue o the corque M 0: she "'ow generating machine 6 is less than the set value 0: she corque M o_ the Slow generating machine 6.

According to one embodiment the liquid flow speed V along the circulation channel 2 is consoano independently o: the operational speed N o_ the Slow generating e 6.

According to an alternative embodiment the liquid flow speed V along the circulation channel 2 varies as a n O“ the operational speed N 0: she llow generating machine, :or instance the liquid “ ow speed V decrease when the operational speed N o_ the Slow generating machine 6 se. ?referably the operational speed N o_ the Slow generating machine 6 is always higher than a predetermined lowest allowable operational speed NMm. At operational speeds lower than the lowest allowable operational speed Nmin the liquid flow risk to be not nous enough but solid matter will accumilate on the bottom 0: the circulation channel 2 as she same time as the liqiid flow speed V 0: she liquid flow risk to be too low for achieving the requeSted process res1los lor she specilic treacmeno plant 1. o it is preferable chad the operational speed N o_ she llow generating machine 6 always is lower than a predetermined 2015/054499 highest allowable operational speed NW“, in order nOt to risk that the ow generating machine 6 is overloaded.

The treatment plan 1 can also, as a complement to the present invention, comprise direct or indirect measurement ol she oxygen transfer rate to the liquid and/or the level 0; dissolved oxygen in the liqiid, and thereby an tion whether the oxygen cransfer rate need to be increased or decreased. ing to the most preferred embodiment the dissolved oxygen level is meas1red directly. An increase in the oxygen transfer rate is applicable when the dissolved oxygen level in the liquid is too low, and a decrease o: the oxygen trans:er rate is applicable when the dissolved oxygen level in the liqiid is too high. In order to m asur /d t rmin th dissolved oxygen level in the liquid the treatment plant 1 preferab'y comprises an oxygen sensor l8 locaced ac a predetermined location in the circulation channel 2. The oxygen sensor 18 is operatively conneCted to the control init 8. The oxygen sensor 18 is ably located in the area directly ream the r seen in she flow direction 5 along the circulation l 2. {owever, other locations 0: the oxygen sensor 18 are conceivable.

Feasible modi:fications o: the Invention The invention is nOt limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is ed to cover all adjustments and variants o: the preferred embodiments described herein, thus the present invention is defined by the wording of she appended claims and thus, the equipment may be modified in all kinds 0: ways within the scope ol she ed claims.

It shall also be pointed out that all information about/concerning terms such as above, inder, upper, lower, etc., shall be interpreted/read having the equipment oriented according to the Ligures, having the drawings oriented such that the references can be properly read.

Thus, such terms only indicates mutual relations in the shown embodiments, which relations may be changed it the inventive equipment is ed with another struCture/design.

It shall also be pointed out that even thus it is nOt explicitly stated that "eatures trom a speci:fic ment may be combined with "eaLures trom another er1bodiment, the combination shall be considered obvious, i : the combination is possible.

Claims (13)

Claims

1.

A method for controlling a plant for treatment of waste water, wherein the plant comprises: - a basin constituted by a circulation channel and 5 ured to house a liquid, - at least one flow ting machine constituted by a submergible mixer machine and arranged in the basin and configured to generate a liquid flow along the circulation channel, wherein the operational speed N of the flow 10 generating machine is higher than a predetermined lowest allowable operational speed Nmin , preventing solid matter from accumulating on the bottom of the basin, and is lower than a predetermined highest allowable operational speed Nmax , preventing the flow generating machine from being 15 aded, - at least one equipment that is arranged in the basin and that effects the momentum of the liquid flowing in the basin, and - a l unit that is operatively connected to said at 20 least one flow generating machine, the method including the steps of: - in the l unit storing a predetermined relationship between the operational speed N of the flow generating machine and an operational parameter P from which the 25 torque M of the flow generating machine may be derived, which onship depends on a predetermined liquid flow speed V in the basin at the flow generating machine, - determining the operational speed N of the flow generating machine, 30 - from the determined operational speed N ining a set value of the operational parameter P of the flow generating machine based on said relationship between the operational speed N of the flow generating machine and the operational parameter P of the flow generating machine, - by means of the control unit determining a real value of the operational ter P of the flow generating machine, - by means of the l unit adjusting the operational 5 speed N of the flow generating machine if the real value of the operational parameter P of the flow ting machine is different than the set value of the operational parameter P of the flow generating machine. 10 2. The method according to claim 1, wherein the plant is constituted by a treatment plant for treatment of liquid such as waste water.

3. The method according to claim 1 or claim 2, wherein the 15 operational parameter P is constituted by the torque M of the flow generating e.

4. The method according to claim 1 or claim 2, n the operational parameter P is constituted by the current I 20 consumed by the flow generating machine.

5. The method according to any one of claims 1-4, wherein the operational speed N of the flow generating machine shall be increased if the true value of the operational 25 parameter P of the flow generating e is greater than the set value of the operational parameter P of the flow generating e.

6. The method according to any one of claims 1-5, wherein 30 the operational speed N of the flow generating machine shall be decreased if the true value of the operational parameter P of the flow generating machine is less than the set value of the operational parameter P of the flow generating machine.

7. The method according to any one of claims 1-6, wherein said at least one equipment is constituted by a mechanical surface aerator. 5

8. The method according to claim 7, wherein the mechanical surface aerator comprises a horizontal rotational axis.

9. The method according to claim 7, wherein the mechanical e aerator comprises a vertical rotational axis.

10. The method according to any one of claims 1-6, wherein said at least one equipment is constituted by an aeration sector arranged at the bottom of the basin. 15

11. A plant for treatment of waste water, comprising: - a basin constituted by a circulation channel and configured to house a liquid, - at least one flow generating machine constituted by a submergible mixer machine and arranged in the basin and 20 configured to te a liquid flow along the circulation channel, wherein the operational speed N of the flow generating machine is higher than a ermined lowest allowable operational speed Nmin , preventing solid matter from accumulating on the bottom of the basin, and is lower 25 than a predetermined highest allowable operational speed Nmax , preventing the flow ting machine from being overloaded, - at least one equipment that is arranged in the basin and that s the momentum of the liquid flowing in the 30 basin, and - a control unit that is operatively connected to said at least one flow generating machine, the control unit sing a predetermined relationship between the operational speed N of the flow generating 35 e and an operational parameter P from which the torque M of the flow ting machine may be derived, which relationship depends on a predetermined liquid flow speed V in the basin at the flow ting machine, the control unit being configured to determine from a given operational speed N a set value of the operational parameter P of the 5 flow generating machine based on said relationship between the operational speed N of the flow generating machine and the operational ter P of the flow generating machine, determine a real value of the operational parameter P of the flow generating machine, and adjust the operational speed N 10 of the flow generating machine if the real value of the operational parameter P of the flow ting machine is different than the set value of the operational ter P of the flow generating machine. 15

12. A method according to claim 1, substantially as herein bed or exemplified with reference to the accompanying drawings.

13. A plant according to claim 11, substantially as herein 20 described or exemplified with reference to the accompanying drawings. WO 93783 WO 93783