At the beginning of week 1, the BD Tank and the Control Tank had similar organic carbon (measured as COD) removal rates. As the biofilm developed on the surfaces of the Bio-Domes, the performance began to diverge. By weeks 3 and 4, the Control Tank effluent was around 100 mg/L, whereas the BD Tank effluent was around 50 mg/L. During the steady state weeks 12 – 15, the Control Tank effluent increased to around 150 mg/L, but the BD Tank Effluent remained at 50 mg/L. The presence of the aerated biofilm in the domes removed most of the biologically oxidizable material from the wastewater at winter temperatures.
Again, performance at the beginning of week 1 for the BD vs Control Tanks was similar for TSS removal. By week 4 the BD Tank was removing almost all the TSS, while the Control Tank effluent was around 20 mg/L. During weeks 12 – 15, the difference was even more dramatic, with the average BD Tank effluent less than 10 mg/L and the Control Tank effluent between 50 to 60 mg/L. The main reason for the difference is that the Control Tank was full of suspended growth, whereas the BD Tank biomass was fixed inside the domes and didn’t wash out with the effluent.
Biological nitrification is the desired removal mechanism to get rid of ammonium in wastewater, but for suspended growth, the necessary bacteria are suppressed at cold temperatures. The aerated fixed film biomass inside the domes allows nitrifiers to remain active at temperatures down to near freezing. At the start of the experiment the nitrifying biofilm was not well established, but by week 3, there was a dramatic increase in the removal rate, and by week 4, most of the ammonium was removed from the BD Tank effluent. This occurred despite temperatures less than 10 degrees C. Note that the Control Tank effluent showed no removal during week 3 and a slight removal during week 4. Because nitrification has a high oxygen demand, cycling aeration off decreases the amount of ammonia removed. This is apparent during the steady state period of weeks 12 to 15 when the air off period was increased from 2 to 3 to 4 hours per day. The stair-step effect on the ammonia concentrations in the BD Tank effluent is apparent. Week 12 shows the strength of the Bio-Dome design with moderate air cycling. At temperatures around 1 to 1.5 degrees C, with influent ammonia at 25 mg/L, the BD Tank effluent was 2 mg/L and the Control Tank effluent was 20 mg/L.
The purpose of air cycling in the Bio-Domes is to increase the de-nitrification rates. The bacteria that accomplish de-nitrification are suppressed by the presence of oxygen, so the air-off periods allow them to increase their metabolic rate. However, during air-off periods, the metabolic rate of the oxygen dependent nitrifying bacteria is suppressed. The removal of both ammonium and nitrate/nitrite can be accomplished by finding the right balance between the air on and air off periods. Since the Total Nitrogen (TN) value measures ammonium plus nitrate/nitrite (as well as organic N), a minimum value of TN would indicate the optimum air cycling. During weeks 1 – 4 the air was on 24 hours per day and by weeks 3 – 4 the BD Tank effluent was around 17 to 18 mg/L. For weeks 12 – 15, the air off period was gradually increased from 2 hours off to 4 hours off per day. As the air off period increased, the BD Tank effluent ammonium levels increased 2 to 3 mg/L, but the nitrate/nitrite concentrations dropped 5 to 10 mg/L. The overall effect on TN was that the best removal occurred during week 15, with TN levels of around 14 mg/L. The Control Tank was not responsive to air cycling.