Investigations into control of balancing tanks

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The beauty of rivers and lakes

The wastewater systems of all but the smallest industrial sites include balancing tanks for buffering of flow and concentration peaks and leveling of the flow streams. This happens in the vicinity of the wastewater generating processes as well as ahead of or in the course of wastewater treatment facilities End-of-the-Pipe.

STOAT with its building block «Balancing Tank» (see Icon «Balancing Tank») provides everything necessary to examine the effects of buffering tanks on the load at important points within the wastewater system . If you additionally utilize one of the universally employable control components, like «PID controller» or «Programmable Logic Controller» (see Icons «Control functions»), control and regulation of filling and emptying of the tanks can be modelled and optimized according to practical requirements. The big advantage of this “toy” is the possibility to test all imaginable cases through simulation – especially extreme situations. The wastewater facilities themselves are spared any dangerous conditions.

By means of a practical example a brief introduction to the sheer unlimited possibilities for optimizing tank control is given. The example is a follow-up to the Investigations on whether a wastewater treatment facility can accomodate an additional highly loaded Teilstrom. The basic task is therefore not explained in detail again.

The previous example included two balancing tanks (see STOAT model for example “Additional Abwasserstrom”), namely

• balancing tank for condensate wastes
• balancing tank before wastewater is entering the aeration tank

Purpose of the condensate wastewater tank to be built was to balance the discontinuity in the flow of this stream, that was to be treated at the facility in addition to the wastewaters already dealt with. Additionally, peaks in ammonia concentration were to be reduced as much as possible.

Fig. 1 Flow and NH4-N of the condensate wastes at the point of discharge (= influent of balancing tank for condensate wastes)

The already existant balancing tank before the aeration step was previously used mainly to balance the COD load at the influent of the treatment facility. New task with the additional wastewater stream is to balance the ammonia load coming from the condensate wastewater tank in order to ensure a manageable NH₄-N concentration at the influent of the activated sludge treatment.

Determination of the necessary volume of the balancing tank for condensate wastes

Before control of the balancing tanks can be examined in detail, the approximate volume of the condensate wastes tank has to be known. STOAT provides a fairly easy tool to acquire a first estimate: the assumption of an infinitely large volume. In order to determine the necessary tank volume a simulation run of the calibrated model is done with the setting «Infinite volume» (∞). This allows the tank to hold all incoming wastewater without any overflow occuring. The input file for the condensate wastewater stream is prepared to model the worst-case expected influx. The tank effluent also has to be set. For now a reasonable value is the average influent (here 4.5 m³/h). The following figure shows these settings.

Fig. 2 Balancing tank for condensate wastes, «Process calibration data»

Fig. 3 Balancing tank for condensate wastes, «Operation data»

The «Minimum volume» was set to 100 m³. This at all times ensures a minimum fill level, enabling mixing in of freshly inflowing wastewater. Should the influx be smaller then the preset effluent for an extended period of time, STOAT will close the tank effluent as soon as the set «Minimum volume» is reached. Note: The minimal volume of existing tanks will generally depend on operational or geometric givens, e.g. installation height of the effluent pipe.

That concludes the preparations for having STOAT calculate the necessary minimal volume. The simulation run is started and the results are evaluated. A plot of the condensate wastewater tank volume actually used over time results in the following figure:

Fig. 4 Fill level of the balancing tank for condensate wastes (non-regulated)

At the beginning of the simulation period - up to approximately 1200 h simulated time - the tank is emptied down to minimal volume several times. This results from the large intervals between influx events during this timespan (compare Fig. 1). Following this the tank fills up to a maximum of ca. 4000 m³. The fill level then decreases again. This implies, that without dedicated control the tank would need a volume of at least 4000 m³ in order to avoid overflow. This will be used as a starting value for subsequent calculations.

Before we start optimizing the control procedure, let us have a look at the tank effluent without control:

Fig. 5 Effluent of the balancing tank for condensate wastewater (without control)

A comparison with fig. 4 shows, that the effluent is indeed cut off whenever the set minimal volume is reached. After about 1200 h simulated time the fill level is continually high enough to ensure a constant effluent.

Control of the condensate wastewater tank

The effluent of the condensate wastewater tank shall be controlled by a «Programmable Logic Controller» (PLC). This “bit” allows the consideration of up to five input signals. This example only requires two, though:

• filled volume of the tank («Volume», input variable 1)
• NH₄-N concentration («Ammonia», input variable 2)

Output of the PLC is the «Pump rate» (= regulated effluent from the condensate wastewater tank). Input and output of the PLC handed over to the model via the socalled «Connectivity» of the «Ladder Logic Controllers».

Fig. 6 Configuration of the PLC for condensate wastewater tank, «Connectivity», input variables 1 and 2

How the control logic is to react to different input signals in detail, is determined by the «Operation data».

Fig. 7 Configuration of the PLC for condensate wastewater tank, «Operation data»

You will be better able to look at and understand these settings using the Excel spreadsheet «Operation data» condensate wastewater tank.

The settings shown above were preceded by several intermediate steps, showing a tank volume of 600 m³ to suffice. These intermediate steps will not be discussed here. The final configuration of the PLC is explained below:

1. As long as the tank is filled no more than half (= 300 m³) and NH₄-N concentration is less than 150 mg/l, efflux shall be 2.5 m³/h (setting 0).
2. If the fill level is more than 300, but less than 500 m³ and NH₄-N concentration is less than 150 mg/l, efflux shall be 5.0 m³/h (setting1).
3. As long as the tank is filled no more than half (= 300 m³) and NH₄-N concentration lies above 150, but below 300 mg/l, efflux shall be 2.0 m³/h (setting 2).
4. If the fill level is more than 300, but less than 500 m³ and NH₄-N concentration lies above 150, but below 300 mg/l, efflux shall be 4.0 m³/h (setting3).
5. As long as the tank is filled no more than half (= 300 m³) and NH₄-N concentration is more than 300 up to a maximum of 9 999 999 mg/l (the PLC cannot process a higher numerical value), efflux shall be 1.0 m³/h (setting 4).
6. If the fill level is more than 300, but less than 500 m³ and NH₄-N concentration lies between 300 and 9 999 999 mg/l, efflux shall be 2.0 m³/h (setting 5).
7. If the fill level is more than 500, but less than 550 m³, efflux shall be 10.0 m³/h, independent of NH₄-N concentration (greater than 0, but less than 9 999 999 mg/l, setting 6).
8. If the fill level is more than 550 up to a maximum of 9 999 999 m³, i.e. shortly before overflow, efflux shall be 90.0 m³/h, independent of NH₄-N concentration (greater than 0, but less than 9 999 999 mg/l, setting 7).

The resulting effects of this regulation can be clearly seen from the time behaviour of volume and NH4-N concentration in the tank as well as at its effluent.

Fig. 8 Time behaviour of volume and NH4-N concentration within the condensate wastewater tank (controlled)

Fig. 9 Time behaviour of volume and NH4-N concentration of the effluent of the condensate wastewater tank (controlled)

Control of the balancing tank before the biological treatment train

The balancing tank before the biological treatment train had already been in use at the time of the investigation. It has a volume of 10 000 m³. The tank was regulated to permanently maintain a fill level of ca. 50% if possible. In exceptional situations manual interventions into filling and emptying of the tank occured.

The control logic for this tank was tested in analogy to the procedure outlined above for the condensate wastewater tank. Here too, the fill level of the tank is the first input quantity. The second input quantity is the COD concentration, though. The regulation again affects the effluent. The «Connectivity» of the «Ladder Logic Controllers» is configured accordingly.

Fig. 10 Configuration of the PLC of the balancing tank before the biological treatment train, «Connectivity», input variables 1 and 2

Fig. 11 Configuration of the PLC of the balancing tank before the biological treatment train, «Operation data»

For easy viewing the settings have been put into the Excel spreadsheet «Operation data» control of the balancing tank before the biological treatment train .

The following explanations are given to ease understanding of the different columns:

1. As long as the tank contains no more than 7000 m³ and COD concentration lies below 200 mg/l, efflux shall be 120 m³/h (setting 0).
2. For a fill level above 7000, but below 9000 m³ and a COD concentration below 200 mg/l, efflux shall be 150 m³/h (setting 1).
3. As long as the tank is filled with no more than 7000 m³ and COD concentration is above 200, but below 300 mg/l, efflux shall be 100 m³/h (setting 2).
4. If the fill level is more than 7000, but less than 9000 m³ and COD concentration lies between 200 and 300 mg/l, efflux shall be 130 m³/h (setting 3).
5. As long as the tank contains no more than 7000 m³ and COD concentration is from 300 up to a maximum of 1000 mg/l, efflux shall be 80 m³/h (setting 4).
6. If the fill level is more than 7000, but less than 9000 m³ and COD concentration is from 300 up to a maximum of 1000 mg/l, efflux shall be 100 m³/h (setting 5).
7. For a fill level of more than 9000 up to a maximum of 9 999 999 m³, i.e. shortly before overflow, efflux shall be 200.0 m³/h, unless COD concentration is above 1000 mg/l (setting 6).
8. In case of COD concentration exceeds 1000 mg/l and the tank contains no more than 7000 m³, efflux shall be only 20 m³/h (setting 7).
9. In case of COD concentration exceeds 1000 mg/l and the tank contains more than 7000 but less than 9000 m³, efflux shall be 35 m³/h (setting 8).
10. In case of COD concentration exceeds 1000 mg/l and the tank contains more than 9000 up to a maximum of 9 999 999 m³, efflux shall be 50.0 m³/h (setting 9).

The effects of this regulation can be readily seen from time behaviour of volume, soluble BOD and NH₄-N concentration in the balancing tank before the biological treatment train and at its effluent.

Fig. 12 Time behaviour of volume, soluble BOD and NH4-N concentration within the balancing tank before the biological treatment train (controlled)

Fig. 13 Time behaviour of flow, COD and NH4-N concentration of the effluent of the balancing tank before the biological treatment train (controlled)

The quality at the efflux of the treatment facility achieved with this regulation has already been demonstrated in the course of the previous example. Here it shall only be mentioned once again, that the simulation results including the control logic outlined above could be successfully transferred to practical operation.