Determining a WWTP's treatment capacity (COD)

Home

Address/
Contact information/
How to find us

Scope of operations

Dynamical simulation of WWTP's

STOAT usage examples

• Achievable quality of simulation
• Additional highly loaded flow stream
• Control of balancing tanks
• Optimizing pre-treatment
• Topical side
• Optimization of aeration

Assistance in starting dynamic simulation

• Input required
• General approach
• Essentials of «ASM»
• Fractionating of COD
• STOAT «levels»
• «Processes toolbox», model building

Practical tips for using STOAT

• «Build»-Menü
• «Chart control»
• Data handling

Wastewater
register

References

Downloads

The beauty of rivers and lakes

Task definition, simulation approach

The operating license of a treatment facility at a chemical industry site limited the COD load at the influent to 15 t per day. This practically ruled out any capacity increase of existing production facilities or the installation of new ones. The task was to examine, using STOAT for dynamical simulation, to what extent the existing treatment facility would be able to accept higher influx load without extensive investments and with no danger of violating permit values.

Based on information supplied by the customer a model of the facility including all relevant treatment stages, their interconnection and their dimensioning (e.g. reactor geometry) was created.

In a second step computations were run using this model, retracing facility operation over the course of a whole year. After consulting with the competent authority a year with data adequate for the purposes of the examination was chosen.

Preparing the computation runs was the most time-consuming task, as the input files as well as necessary settings (e.g. oxygen transfer, sludge return and wastage of surplus sludge) for the model had to be generated from measurements of the self-monitoring and logs of the process control systems/PLC resp. online measuring instruments. For the input files, the necessary distribution of COD and total nitrogen in the influx to the different fractions according to the requirements of the underlying model deserved special attention.

The third step was to calibrate the model using measurements from facility operation (comparison of STOAT simulation results and measurements from the self-monitoring of the company). Certain presettings, like growth rates of heterotrophic and autotrophic organisms, were adjusted in order to bring the simulation results into agreement with measured efficiency of degradation of the facility.

Subsequently simulations were run with this calibrated model, gradually increasing COD inlet load in order to determine the highest acceptable influent load.

Explanation of the model

At the core the treatment facility is composed of three stages:

1. removal of carbonaceous compounds
2. nitrification
3. postdenitrifaction

The following figure shows the STOAT model of the examined processes.

Fig. 1 STOAT model of the investigations on COD treatment capacity of a treatment facility

The wastewater from the industrial site is routed to the first biological step, the removal of carbonaceous compounds. The municipal wastewater also treated by the same facility carries a smaller load and is therefore routed into the second biological step, the nitrification, directly. Some wastewater streams of the industrial wastewater are sent through pre-treatment stages not included in the model, before entering the biological treatment stages of the treatment facility. The downstream denitrification is also not included in the model, as the COD is not changed within this stage.

The gradual increase in COD was classified in scenarios. In close coordination with the company operating the treatment facility, the scenarios were chosen to preserve the dynamics within the COD measurement series for the corresponding time period. This was achieved by scaling measured COD values by the ratio of the scenario load over the measured load. Expressed mathematically this becomes:

COD _scenario = measured COD x chosen scenario mean load ⁄ mean measured load of underlying data set

For better understanding a concrete example for determination of a single COD value within a scenario is given:

The computation runs for those scenarios showed, that the biological treatment stages by themselves were not capable of coping with load peaks during the scenarios with high COD loads. However, the treatment facility included sufficiently large volume of balancing tanks, which could temporarily accomodate wastewater in case of a load peak. With consent of the authorities, this was taken into account during the simulations for determining the COD treatment capacity of the facility.

The «overflow» “bit” is used to model filling the balancing tank for operational reasons during the underlying time period. The relevant authority had righty demanded, that this usage of the balancing tank was to be deducted, as it reduces the volume available for load balancing. The corresponding events were accordingly taken from the operating log and fed into STOAT (see following figure).

Fig. 2 input data «overflow»

The data above is to be read in the following way:

Control of balancing tank filling

Depending on how high the load of the first biological step efflux is, the wastewater is entered into the second biological step either completely or only partially. If the COD load downstream the first biological step is higher than the value acceptable for the nitrification stage, the excessive COD load is stored in the balancing tank. Control of the splitter is modeled with a «Programmable Logic Controller».

Fig. 3 Configuration PLC 1, «Connectivity»

PLC 1 only has one input: COD load upstream the splitter. Based on this the percentage of splitter inflow sent to the second biological step is determined. The remainder is sent to the balancing tank.

Fig. 4 Configuration PLC 1, «Operation data»

These settings are more easily understood from the Excel spreadsheet «Operation data» PLC splitter.

Control of balancing tank emptying

The balancing tank is dimensioned to allow storing the wastewater for extended periods of time. Installations for recirculation avoid the deposition of solids. Despite of this, the balancing tank of course has to be emptied as soon as possible. For this purpose the stored wastewater is entered into the step for removal of carbonaceous compounds for a second time. To avoid overburdening this first biological step, release from the balancing tank must only happen at times of low COD load from the production site. Tank effluent control is therefore also modeled using a «Programmable Logic Controller».

Fig. 5 Configuration PLC 2, «Connectivity»

Fig. 6 Configuration PLC 2, «Operation data»

PLC 2 has two inputs:

• COD load of wastewater flowing into the biological treatment step for removal of carbonaceous compounds
• fill level of the balancing tank

The chosen settings can be viewed in detail in Excel spreadsheet «Operation data» PLC balancing tank. They are to be understood in the following way:

1. As long as COD load of the industrial wastewater is < 800 000 g/h (= 800 kg/h) and tank fill level is < 11 000 m³, COD load released from the tank shall be 700 kg/h. The maximum load at the influent of biological step 1 then is 1,5 t/h COD. (setting 0).
2. If COD load is < 800 kg/h, but tank fill level is ≥ 11 000 m³, COD load released shall be 1000 kg/h. Maximum load then is 1,8 t/h COD. (setting 1).
3. If COD load is ≥ 800 and < 1000 kg/h and fill level is < 11 000 m³, COD load released shall be 500 kg/h. Maximum load then is 1,5 t/h COD. (setting 2).
4. If COD load is ≥ 800 and < 1000 kg/h and fill level is ≥ 11 000 m³, COD load released shall be 800 kg/h. Maximum load hen is 1,8 t/h COD. (setting 3).
5. If COD load is ≥ 1000 and < 1300 kg/h and fill level is < 11 000 m³, COD load released shall be 200 kg/h. Maximum load then is 1,5 t/h COD. (setting 4).
6. If COD load is ≥ 1000 and < 1300 kg/h and fill level is ≥ 11 000 m³, COD load released shall be 500 kg/h. Maximum load then is 1,8 t/h COD. (setting 5).
7. If COD load is ≥ 1300 and < 1500 kg/h and fill level is < 11 000 m³, COD load released shall be 100 kg/h. Maximum load then is 1,6 t/h COD. (setting 6).
8. If COD load is ≥ 1300 and < 1500 kg/h and fill level is ≥ 11 000 m³, COD load released shall be 350 kg/h. Maximum load then is 1,85 t/h COD. (setting 7).
9. If COD load is ≥ 1500 and < 1800 kg/h and fill level is < 11 000 m³, COD load released shall be 50 kg/h. Maximum load then is 1,85 t/h COD. (setting 8).
10. If COD load is ≥ 1500 and < 1800 kg/h and fill level is ≥ 11 000 m³, COD load released shall be 100 kg/h. Maximum load 1,9 t/h COD. (setting 9).
11. As soon as COD load of the industrial wastewater is ≥ 1800 kg/h (up to 9 999 999, i.e. ∞), nothing is to be released from the tank, independent of the tank fill level (> 0 and < 9 999 999 m³). (setting 10).

The «COD modifier»

The «COD modifier» between ZKB and the two-way mixer ahead of nitrification is present to adapt the inert soluble COD at the effluent of the first biological treatment step to the conditions within the nitrification, i.e. shift in favour of degradable soluble COD. This models the phenomenon, that a significant part of “refractory COD” in biological step 1 efflux is verifiably eliminated after mixing with municipal wastewater in the second biological step.

Results

STOAT offers several easily understandable ways of presenting results. One especially useful overview of the material flows is offered by Sankey diagrams. Fig. 7 serves as an example of this, depicting the scenario with the highest possible COD load. It is easy to discern, that the industrial wastewater carries a higher COD load than the municipal wastewater. Especially high COD loads circulate with the recycled activated sludge, but the wastage of both biological steps expectedly shows high COD loads.

The influent of the balancing tank consists of two flow streams, which are separated from the effluent of the 1^st biological step. The origin of the first flow stream is the «Overflow», the origin of the second flow stream is the «Splitter», which itself is controlled by the PLC 1. The flow stream from the «Overflow» to the balancing tank represents the filling of the balancing tank for operational reasons. This had to be taken into account, because it decreases the volume of the balancing tank, which is available for load control.
The COD load entering the balancing tank via the «Overflow» is significantly lower than that split from the first biological step effluent by the control logic. The COD load routed to the balancing tank is then entered into the first biological step influent. The control logic explained above is responsible for triggering release from the tank only if the load of the industrial wastewater influx is below a set threshold.

Fig. 7 Sankey diagram COD loads along the flow paths of the model

With the help of the STOAT simulation, it was possible to reliably and reproducibly demonstrate, that with only changes in the operating method (higher wastage of surplus sludge, buffering of peak loads in an existing detention basin) the facility was capable of coping with a significantly higher COD load than allowed by the then valid operating licence. Subsequently, the relevant authority adjusted the operating licence accordingly.