NITRITE ACCUMULATION FROM SWINE WASTEWATER IN A SEQUENCING BATCH REACTOR
Shortcut biological nitrogen removal is a process based on nitrification and denitrification via nitrite, rather than nitrate, so both the aeration needed in nitrification and the organic carbon required by denitrifiers can be reduced. This research investigated the possibility of accumulating nitrite in swine wastewater treated in an activated sludge sequencing batch reactor (SBR). The SBR was operated under hydraulic retention times (HRTs) of 3 days and 1.5 days for one month. Ammonium loadings were increased from 0.04 to 0.7 kg NH4-N m-3 d-1. The nitrite/nitrate ratio (NO2/NO3) in the effluent lay mainly within the range of 3 to 4, generating an effluent with 13% to 23% NH4-N, 15% to 21% NO3-N, and 56% to 72% NO2-N. Lower HRT means larger daily feeding volume, which is always preferred if the same treatment efficiency can be obtained. Our results showed that HRT did not affect the composition of effluent nitrogenous compounds, so a longer feeding time would lead to a greater nitrite production rate. Since either or both free ammonia (FA) and free nitrous acid (FNA) were always present at inhibitory levels in the system, a nitrite-dominant effluent could thus be produced from the SBR used in this study with swine wastewater, regardless of the HRT used. The cycle comparison between the two modes with different HRTs with the same ammonium loading rate, 0.058 g NH4+ g-1 MLSS (mixed liquid suspended solid) per day, shows that there is not much difference with regard to the whole nitritation process, which is characterized by continuous conversion of loaded ammonium to nitrite and nitrate in the aerobic feeding period and no further conversion after the loading was terminated, resulting in relatively stable levels for all three nitrogen components in the entire cycle.