THE EFFECTS OF AMMONIA-N ON anSBR PERFORMANCE

Abstract

The effect ofNH3-N on anaerobic sequencing batch reactor (anSBR) performance was investigated in this study. Two bench-scale anSBRs with 8.5-L working volume were set up to treat synthetic wastewater containing sucrose. Best performance was achieved at the organic loading rate (OLR) of 2.5 gCOD/l·d with influent COD of 5000 mg/l and 2 days HRT. The COD removal efficiency of the system reached 92-95%, and the methane yield, was 0.34 1 CH4/gCODremoved· The system could not tolerate OLRs exceeding 4 gCOD/l·d because of the granular sludge deteriorated resulting in poor settleability. Results of anaerobic toxicity assays (ATA) at various pH levels using biomass cultivated at 2.5 gCOD/1,d OLR and C:N ratio of 35:1, indicated that NH3-N inhibition was closely dependant on pH. The inhibitory and toxic thresholds were: (i) 1850 and 2850 mgNH3-N/l at pH 7.0, (ii) 1950 and 3150 mgNH3-N/l at pH 7.5, (iii) 1100 and 2350 mgNH3-N/l at pH 7.8, and (iv) 700 and 1520 mgNH3-N/l at pH 8.1. Free ammonia (NH3) was considered more toxic than the ammonium ion (NH4+). At pH 8.1, free NH3-N concentration of around 100 mg/I caused 50% inhibition of methanogenic activity, while the total NH3-N concentration was only 850 mg/l. At high NH3-N concentrations, VFA was accumulated in the system. However, this phenomenon was not considered to be the cause of inhibition, but was the result of NH3-N inhibition. Total and un-ionised VF A concentrations were then still at concentrations far below toxic levels. Anaerobic bacteria were successfully acclimated to NH3-N concentrations of 800 and 1500 mg/l after 20 and 30 days acclimation. VFA accumulation, mainly in propionic acid form corresponding to high partial pressure of hydrogen, was observed during the acclimation period. This was due to the inhibition of H2-utilizing methanogens, which would have utilized hydrogen and carbon dioxide to produce methane. Results of AT A indicated that acclimated biomass could tolerate a higher NH3-N shock-load with inhibitory and toxic thresholds at 2540 and 3520 mgNH3-N/l for biomass acclimated to NH3-N concentration of 800 mg/I, and 3350 and 4600 mgNH3-N/l for biomass acclimated to NH3-N concentration of 1500 mg/I. Although high NH3-N concentrations strongly inhibited methanogenic activity, it was suggested that NH3-N toxicity have no influence on the morphology of the anaerobic culture, because SEM tests showed a similar morphology for the uninhibited and inhibited biomass. Track run data at various NH3-N concentrations were used to determine the kinetics of sucrose degradation. An uncompetitive inhibition model, in terms of COD, was proposed to describe the effect of NH3-N on sucrose degradation:

The maximum specific substrate utilization rate (k) and half-velocity constant (Ks) were estimated to be 0.94 l/d and 257.6 mg COD/I respectively. The average inhibition coefficient, K1avg, was 779.7 mg/I. This model could be used to predict the treatment of high NH3-N wastewater and provide information for design and operating parameters, such as HRT, F/M ratio.