ABSTRACT
It is known from the anaerobic digestion practice with liquid wastes that ammonia could be toxic and inhibitory to methanogenic bacteria and the process. However, no similar information is available for the solid substrate anaerobic digestion of municipal and industrial wastes (DASS). Therefore, this work focused on determining the effects of ammonia-nitrogen concentration (TAN) on the specific methanogenic activity of microbial consortia from DASS and the stability of DASS continous process.
First, specific methanogenic activity tests (SMA) were carried out with DASS mesophilic and thermophilic inocula at several doses of ammonia nitrogen (range 500 to 7000 mgTAN/kg). TAN inhibition concentration 50% (IC50) was determined based on SAM. Second, bench-scale, semi-continuous, mesophilic reactors were operated at 21 days mass retention time and dosed with NH4Cl such that the corresponding COD/N ratios in their feeds were 90, 80, 65, and 50 (reactors R1 or control, R2, R3, and R4, respectively).
Based on the IC59 from the SMA tests, mesophilic DASS consortia seemed to be more resistant to ammonia-nitrogen than thermophilic cultures. Also, results indicated that aceticlastic cultures were affected less than the thermophilic ones. Regarding the mesophilic continuous DASS, the process deteriorated with increasing dosages of ammonia-N, with practically process cessation at COD/N=50 (R4). Inhibition was characterized by a gradual decline of efficiency and biogas productivity and a more sudden drop of methane content in biogas and pH. A significant increase of propionic, butyric, and valeric acid was found in reactors receiving the highest doses of ammonia-N (R3 and R4), suggesting the inhibition of syntrophic bacteria. Luong and Pearson inhibition models adequately represented the acute effects of N-supplementation on DASS; however, the Luong model could also account for the process cessation at a critical ammonia-N concentration of 2800 mg/kg mixed solids.
