The benefit of black water separation in energy consumption of wastewater treatment plants

Solar sludge drying — Bettembourg WWTPs

Introduction

Aeration in WastWater Treatment Plant (WWTP) is responsible for majority of energy consumption in the WWTPs. The amount of existing Nitrogen in wastewater can be noticeably reduced by separating black water from grey water. Basically, black water comes from household toilets including brown water (faeces) and yellow water (urine). The rest of household wastewater which is mainly considered as less polluted wastewater like from shower and doing dishes is referred as grey water.

Around 80 percent of Nitrogen in wastewater derived from black water which makes up a small proportion of wastewater total volume. Therefore, elimination of black would significantly lower the level of Nitrogen in urban wastewater.

Methodology

In this study, the effect of black water separation on aeriation demand in WWTPs is discussed. Initially, the aeriation demand for typical influent parameters is calculated in a way to satisfy effluent pollution limits including effluent requirements of COD maximum level of 90 mg/l, NH almost equal to zero levels and NO less than 10 mg/l. The amount of O2 in Nitri tank is also limited to 2 mg/l. The reference WWTP which is simulated by SIMBA software is shown in Figure 1.

Figure 1 — Reference WWTP model

The result of simulation for reference model indicates that the required air for treatment process is equal to 60,000 cubed meter per day. In the second phase of this study, it is considered that the inflow stream merely spring from grey wastewater. Therefore, the influent level of Nitrogen is reduced to zero as the most proportion of existing Nitrogen in residential wastewater comes from black water indeed. As it is shown in Figure 2, the amount of Nitrogen is considered to be zero and the volume of anoxic tank is reduced to 1 cubed meter.

Figure 2 — Schematic illustration of a WWTP for grey water treatment

Results and discussions

This model is analysed for a period of 100 days to indicates the steady state effluent levels. The amount aeration in each simulation altered to determine the maximum and minimum acceptable aeration levels. Figure 3 indicates the amount of effluent parameters including O2, NH and NO which are measured in the Nitri tank 5100 cubed meter aeration per day. This amount of aeration leads to O2 equals to 2 mg/l which is the maximum allowable O2 in Nitri and any level of aeration above this level will not fall in acceptable range of O2.

Figure 3 — Effluent parameters measured in Nitri tank for 5100 cubed meter aeration per day

The level of aeration was altered to meet all the effluent requirements. The results of simulation for different levels of aeration shows that the minimum acceptable amount of aeration is 15000 cubed meter per day which satisfy all above-mentioned effluent criteria. The levels of pollution for 15000 cubed meter aeration measured in outflow stream is shown in Figure 4 and measured in Nitri tank is shown in Figure 5.

Figure 4 — Effluent parameters measured at outflow stream

Figure 5 — Effluent parameters measured in Nitri tank

The summary of main parameters implemented in SIMBA simulation is listed in Table 1.

Conclusion

Black water constitutes major proportion of Nitrogen in urban wastewater. This chemical can be significantly reduced by separation of black water from residential wastewater; thereby, lowering the aeration demand in WWTPs. As it is shown in this study, the required aeration can be reduced from 60,000 to 15000 cubed meter air per day which leads to noticeable reduction in energy consumption. However, there are still challenges ahead; for instance, the separation should be commenced from households’ toilets and urban piping network must be altered accordingly. This procedure is not only time consuming, but it requires also significant investment in relevant infrastructure. The major changes should be made in WWTPs to prepare them with the new type of inflow stream.