Investigation on whether a wastewater treatment plant can accomodate an additional highly loaded flow stream

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Dynamical simulation of WWTP's

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• Achievable quality of simulation
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• Control of balancing tanks
• Optimizing pre-treatment
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• Optimization of aeration

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At a large chemical industry site collected condensates from steam supply are processed using ion exchangers. The ion exchangers (cation, anion and mixed bed exchangers) have to be regenerated regularly. This results in a discontinous flow stream highly loaded with ammonia. Depending on use of steam within the chemical production, outside temperature and raw water quality, the time span between regenerations can range from one day to four weeks.

The given task was to determine with the help of dynamical wastewater treatment simulation, whether this flow stream could be dealt with using the existing activated sludge installation without exceeding the monitoring values. Most important among these was NH₄-N and N total anorganic (sum of NH₄-N and NOx-N). In addition the necessary volume of a balancing tank for this condensate wastewater  was to be calculated using STOAT.

The following image shows the STOAT model for the processes examined.

Fig. 1 STOAT model additional flow stream

The model shows connections between nodes not physically present at the time of the investigation in red. This concerns the pipelines between point of discharge of the condensate wastewater and the inlet tank of the biological treatment step with a volume of 10.000 m³. All other connections were already present and are therefore shown in black.

One very important question was, whether the high additional ammonium load could be accomodated. While originally the treatment facility had to cope with an average ammonia load of ca. 7.2 kg/h, this was to increase with intake of the condensate wastewater to – including a safety margin – on average roughly 18 kg/h, i.e. to 250%.

The increase in peak load was even larger – and therefore even more of a problem. NH₄-N peak load at the influent was originally only 0.85 g/s. The additional flow stream however carried a peak load of 14 g/s at the point of discharge (more than 16 times the original peak load!). This was mainly due to the high discontinuity of the flow. It was therefore clear from the start to everyone involved, that the condensate wastewater could only be taken on by the treatment facility after leveling within a large enough balancing tank – if possible at all. Therefore, from the beginning, the STOAT model included a condensate wastewater tank.

Result of the investigation was the proof, that the condensate wastewater could be treated in the existing facility — subject to the condition, that the additional flow stream would be delivered  as evenly distributed as possible and the oxygen transfer capacity of the aeration tank would be increased by ca. 50 kg/h (consumption of oxygen for oxidation of ammonia nitrogen). The necessary volume of the condensate wastewater tank to be erected amounts to 1 200 m³.

The following figures are intended to show the gradual leveling and reduction of the ammonia concentration of the condensate wastewater by balancing tanks from the point of discharge up to the influent of the aeration step. Plotted is the concentration of NH₄-N in mg/l over simulation time in hours. The simulation is based on representative operational data taken over a timespan of seven months. Discontinuities in the plots are due to a zero flow, i.e. there was no flux at those points.

Fig. 2 NH4-N concentration of the condensate wastes at the point of discharge

Fig. 3 NH4-N at the effluent of the balancing tank for condensate wastes

Fig. 4 NH4-N at the effluent of the balancing tank before aeration step

Fig. 5 Flow, COD, NH4-N und NOx-N at the effluent of the WWTP

Fig. 5 shows fairly large fluctuations in flow at the effluent. This is a result of the control principles for the balancing tank before the aeration step: it is tuned for smallest possible variations in COD load fed to the activated sludge aeration tank. Since the load is given by multiplying the flow with the concentration, the flow is throttled for high and increased for low COD concentrations.

Fig. 5 also shows, that the desired reliable cleansing of the additional flow stream from high ammonia loads is in fact achieved. NH₄-N concentration at the effluent is so low during the whole simulation period, that the corresponding curve almost coincides with the abscissa. The highest value calculated by STOAT was a concentration of 1.6 mg/l. Accordingly there is a sufficiently large safety margin with regard to to the permit value.

Looking at the nitrate curve, it is justified to dispense with an additional denitrification step. Should the nitrogen load unexpectedly show a significant increase during regular operation, this option can still be realised.

The procedure of control implied by the simulation results has been realised. By now sufficent experience has been acquired from operation of the new resp. improved facilities (put into operation July 2006) to so far impressively confirm all simulation results.

Proportioning the Stapeltank for condensate wastewater, which was an important task for a successful conclusion of the measure, and calibration of the installations for control and regulation of the Stapeltank is explained in detail in the next example.