METHOD OF CONTROLLING A WASTEWATER TREATMENT PLANT

Abstract

The present invention provides a method of controlling a wastewater treatment plant. The plant comprises at least one process tank and an aeration structure configured for aeration of the at least one process tank. The method comprises alternating steps of increasing aeration to the at least one process tank when a first event occurs, wherein the first event is one of when a first threshold value of a first tank parameter is reached and when a first predetermined time period has expired, and decreasing aeration to the at least one process tank when a second event occurs, wherein the second event is one of when a second threshold value of a second tank parameter is reached and when a second predetermined time period has expired. The first and second tank parameters are selected from a group consisting of nitrate concentration, nitrite concentration, and ammonium concentration determined in the water in the at least one process tank, and the steps of increasing aeration and decreasing aeration are carried out to reduce N2O emissions from the plant.

Claims

1-15. (canceled)

16. A method of controlling a wastewater treatment plant, the plant comprising at least one process tank and an aeration structure configured for aeration of the at least one process tank, the method comprising alternating steps of: increasing aeration to the at least one process tank during a nitrification phase when a first event occurs, wherein the first event occurs when a first threshold value of a first tank parameter is reached and when a first predetermined time period has expired, and decreasing aeration to the at least one process tank during a denitrification phase when a second event occurs, wherein the second event occurs when a second threshold value of a second tank parameter is reached and when a second predetermined time period has expired, whereby the length of different time periods for the nitrification phase and the denitrification phase is determined based on the first and second tank parameters and on the expiry of the first and second predetermined time periods, wherein the first and second tank parameters are selected from a group consisting of nitrate concentration, nitrite concentration, and ammonium concentration determined in the water in the at least one process tank, and wherein the steps of increasing aeration and decreasing aeration are carried out to reduce N2O emissions from the plant.

17. The method according to claim 16, wherein the first and second tank parameters are identical.

18. The method according to claim 16, wherein the step of decreasing aeration is continued until aeration is interrupted.

19. The method according to claim 16, wherein aeration of the at least one process tank is decreased when a third threshold value of a third tank parameter is reached, the third tank parameter being N2O determined in the water in the at least one process tank.

20. The method according to claim 19, wherein at least one of the first, second, and third tank parameters is measured in the water in one of the at least one process tanks.

21. The method according to claim 20, wherein at least one of the first, second, and third tank parameters is continuously measured at a predefined first time interval.

22. The method according to claim 19, wherein at least one of the first, second, and third tank parameters is determined by use of mathematical model.

23. The method according to claim 16, wherein at least one of the first, second, and third tank parameters is determined by use of forecast information.

24. The method according to claim 23, wherein the forecast information is based on information from a group consisting of: information from one or more rain gauges, weather radar, weather forecasts, temperature, satellite data, one or more flow measurements in the sewer system, one or more signals from one or more pumping stations arranged in the sewer system, and combinations thereof.

25. The method according to claim 1, wherein the aeration structure is continuously controlled based on at least one of the determined first, second, or third tank parameters.

26. The method according to claim 19, further comprising a step of changing at least one of the first, second, and third threshold values while energy consumption in the plant is monitored, and wherein at least one of the first, second, and third threshold values is changed until the energy consumption is decreased.

27. The method according to claim 19, further comprising a step of monitoring a substance value for at least one of: Total-N, NH4, NO3, NO2, N2O, Total-P, PO.sub.4—P, COD (Chemical Oxygen Demand), BOD (Biological Oxygen Demand), and DO (Dissolved Oxygen) in the water in the process tank, and wherein at least one of the first, second, and third threshold values is changed in dependency of the monitored substance value until the substance value is decreased.

28. The method according to claim 27, wherein at least one of: Total-N, NH4, NO3, NO2, N2O, Total-P, PO.sub.4—P, COD (Chemical Oxygen Demand), BOD (Biological Oxygen Demand), and DO (Dissolved Oxygen) is continuously measured at a predefined second time interval.

29. A wastewater treatment plant comprising at least one process tank, an aeration structure configured for aeration of the at least one process tank, at least one sensor for detecting a first tank parameter and a second tank parameter, and a control unit for controlling the aeration structure, wherein the control unit is configured to determining a first event wherein the first even occurs when a first threshold value of the first tank parameter is reached and when a first predetermined time period has expired, determine a second event wherein the second event occurs when a second threshold value of the second tank parameter is reached and when a second predetermined time period has expired, control the aeration structure to increase aeration to the at least one process tank in dependency of the first event during a nitrification phase, and control the aeration structure to decrease aeration to the at least one process tank in dependency of the second event during a denitrification phase, whereby the length of different time periods for the nitrification phase and the denitrification phase is determined based on the first and second tank parameter and on the expiry of the first and second predetermined time periods, wherein the first and second tank parameters are selected from a group consisting of nitrate concentration, nitrite concentration, and ammonium concentration determined in the water in the at least one process tank, and wherein the control unit control the aeration to reduce N2O emissions from the plant.

30. A process for reducing the omissions of nitrous oxide (N2O) while performing a nitrification and denitrification process on an ammonium-containing wastewater stream, the process comprising: directing the ammonium containing wastewater to at least one process tank having an aeration system; performing nitrification phases and denitrification phases on the wastewater in the process tank; in the course of performing the nitrification and denitrification phases, reducing the omission of N2O by controlling and varying the cycle time for the nitrification and denitrification phases by: (i) determining one or more parameters of the wastewater in the process tank wherein the parameters are selected from ammonium concentration, nitrate concentration and nitrite concentration; (ii) increasing the aeration in the process tank during the nitrification phase when a first event occurs and wherein the first event occurs when a first threshold value of one of the parameters is reached and when a first predetermined time period has expired; (iii) decreasing the aeration in the process tank during the denitrification phase when a second event occurs and wherein the second event occurs when a second threshold value of one of the parameters is reached and when a second predetermined time period has expired; (iv) wherein the length of different time periods for the nitrification and denitrification phases is determined based on the first and second parameters; and after the wastewater has been subjected to the nitrification and denitrification phases, directing the wastewater to a clarifier which produces a clarified effluent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Embodiments of the invention will now be further described with reference to the drawings, in which:

[0069] FIG. 1 schematically illustrates an embodiment of a wastewater treatment plant in the form of an activated sludge plant,

[0070] FIG. 2 graphically illustrates of a traditional control strategy for an activated sludge plant,

[0071] FIG. 3 graphically illustrates measured values from a plant operated by a traditional control strategy,

[0072] FIG. 4 graphically illustrates of a control strategy for an activated sludge plant according to the invention, and

[0073] FIG. 5 graphically illustrates measured values from a plant operated according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0074] It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

[0075] FIG. 1 schematically illustrates a wastewater treatment plant in the form of an activated sludge plant 1. It should be understood, that an activated sludge plant is only one example of a wastewater treatment plant in which the claimed method can be used for control.

[0076] The plant 1 comprises at least one process tank 2 (of which only one is illustrated) comprising activated sludge. Each process tank 2 has an inlet 3 for receiving wastewater and an outlet 4 for supplying treated water to a clarification tank 5 for separation into a water fraction and a sludge fraction. The clarification tank 5 of the illustrated embodiment comprises a first outlet 6 for water and a second outlet 7 for water comprising sludge.

[0077] One fraction of the water comprising sludge may be returned to the inlet 3 via the return inlet 7a whereas another fraction comprising excess sludge may be removed via the excess outlet 7b.

[0078] The plant 1 comprises an aeration structure 8 configured for aeration of the process tank 2 to ensure a sufficient amount of oxygen for the micro-organism for the biological treatment of the wastewater.

[0079] Furthermore, the illustrated plant 1 comprises a stirring structure 9 configured for stirring of the process tank 2 to improve the conditions for micro-organisms.

[0080] The illustrated plant 1 further comprises a sludge scraper structure 10 configured for scraping of the clarification tank 5, and a sludge pump 11 configured for pumping sludge from the clarification tank 5.

[0081] Treatment of wastewater in the plant 1 comprises at least the steps of increasing aeration to the at least one process tank 2 when a first event occurs, where the first event is one of when a first threshold value of a first tank parameter is reached and when a first predetermined time period has expired, and decreasing aeration to the at least one process tank 2 when a second event occurs, where the second event is one of when a second threshold value of a second tank parameter is reached and when a second predetermined time period has expired, where the first and second tank parameters are selected from a group consisting of nitrate concentration, nitrite concentration, and ammonium concentration determined in the water in the at least one process tank 2, and wherein the steps of increasing aeration and decreasing aeration are carried out to reduce N2O emissions from the plant.

[0082] Increasing and decreasing aeration in the process tank 2 may be controlled by a control unit 12 which is configured for communication with the aeration structure 8.

[0083] Furthermore, the control unit 12 may be in communication with a number of valves 13, 14 configured to interrupt supply to the process tank 2 and the clarification tank 5, respectively, by closing said valve. Supply may be resumed by opening of the valve 13, 14 and may be controlled by the control unit 12.

[0084] In the illustrated embodiment, the activated sludge plant 1 further comprises a pumping structure 15 configured for recirculation of at least a part of the treated water from the process tank 2.

[0085] FIG. 2 graphically illustrates of a traditional control strategy for an activated sludge plant. The plant is controlled based on two criteria “end N (nitrification) phase” and “end DN (denitrification) phase”, corresponding to interrupting and starting aeration, respectively. During nitrification, ammonium concentrations change from high to low, while nitrate concentrations change from low to high. The nitrification phase ends at a point where the coordinates of ammonium and nitrate cross the curve of the criteria function for the nitrification phase.

[0086] The opposite situation applies to the denitrification phase. The ammonium and nitrate concentrations are maintained in the area between the two criteria functions and move in the direction of the arrow either towards “end N” or towards “end DN”.

[0087] FIG. 3 graphically illustrates measured values from a wastewater treatment plant operated by a traditional control strategy as illustrated in FIG. 2. FIG. 3 illustrates effluent nitrogen concentrations (middle plot), and corresponding N2O production and emissions (bottom plot).

[0088] By application of a traditional control strategy, alternations between the nitrification and the denitrification phase occurred 24 times per day. Thus, a cycle comprising both a nitrification phase and a denitrification phase lasted in average 120 minutes.

[0089] FIG. 4 graphically illustrates of a control strategy for an activated sludge plant according to the invention, where the limitation of N2O production and emission is obtained by setting upper values to the ammonium concentrations (of the criteria function DN phase) to a threshold value and by operating the process at a narrow interval between the criteria functions. This control results in fast phase shifts since i) denitrification phase comes to an end at low ammonium concentrations; ii) low ammonium concentrations at the start of the aerated phase shorten the duration of the aerated phase.

[0090] Since N2O is mainly produced and substantially only emitted during the aerated nitrification phase and since emissions increase when the aeration phase starts at high ammonium concentrations, these actions reduce the rate at which N2O is produced, the period in which N2O is produced and emitted, and thus overall decrease the magnitude of N2O emissions.

[0091] FIG. 5 graphically illustrates measured values from a plant operated according to the invention using the control illustrated in FIG. 4. FIG. 5 illustrates effluent nitrogen concentrations (middle plot), and corresponding N2O production and emissions (bottom plot).

[0092] By application of the control according to the first aspect of the invention, alternation between nitrification and denitrification phase occurs 35 times per day in the illustrated embodiment. A cycle comprising both a nitrification phase and a denitrification phase lasted in average 80 minutes, which is 50% shorter than operation with traditional control.