7    INHIBITION OF MESOPHILIC SOLID SUBSTRATE
ANAEROBIC DIGESTION (DASS) BY
AMMONIA-RICH WASTES
Hector M. Poggi-Varaldo
Ecatepec Institute of Technology-TESE and P3 Consulting Engineers, P.O. Box 75-202,
Mexico D.F., 07300, MEXICO
E. Arce-Medina
ESIQIE del IPN, Graduate School of Chem. Engng., Mexico City, MEXICO
G. Fernandez-Villagomez
Centre for Disaster and Hazards Prevention (CENAPRED), Mexico City, MEXICO
S. Caffarel-Mendez
Ecatepec Institute of Technology-TESE and P3 Consulting Engineers, P.O. Box 75-202,
Dexico D.F., 07300, MEXICO
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% (1C50) was determined based on SAM. Second, bench-scale, semi-
continuous, mesophilic reactors were operated at 21 days mass retention time and dosed with
NH4C1 such that the corresponding COD/N ratios in their feeds were 90, 80, 65, and 50 (reactors
Rl 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 bio-
gas 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.
INTRODUCTION
Solid substrate anaerobic digestion (DASS, digestion at high total solids content), has become
an attractive and demonstrated technology for stabilizing agricultural, food, yard and municipal
solid wastes '"". However, little is known regarding the effects of potential inhibitors or toxi-
52nd Purdue Industrial Waste Conference Proceedings, 1997. Ann Arbor Press, Chelsea, Michigan 48118. Printed in
U.S.A.
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