In this work, the anaerobic valorization of Category 2 Animal By-Products (ABP) to biogas is
assessed. Category 2 ABP other than manure, digestive tract content and milk, usually called
‘bone flour’ and hereafter designated as Category 2 ABP* have been considered as a
sustainable substrate for anaerobic digestion, due to their protein and grease content. Main
characteristics of Category 2 ABP* are as follows: total solids (TS) = 916 ± 1 mg g-1
residue,
volatile solids (VS) = 904 ± 5 mg g-1TS, total Kjeldahl nitrogen (TNK) = 84 ± 9 mg g-1TS,
total phosphorus (TP) = 6.2 ± 0.9 mg g-1TS, chemical oxygen demand (COD) = 1221 ± 173
mg O2 g-1TS and oil/grease = 278 ± 2 mg g-1TS. The ratio COD/N/P is 164 /8 /1. In this work,
anaerobic biodegradability tests were performed by ‘multiple flask’ reactor method. Initial
conditions of the experiment are: all flasks have the same content and it is assumed that the
progress of hydrolysis, acidogenesis and methanogenesis is similar in all flasks. Sampling was
based on the quantitative analysis of content of the flask withdrawn from the reactor system.
[1]. Three concentrations of substrate were tested: 2, 5 and 10 % TS (dry weight of substrate
per volume). To evaluate the performance of the reactors loaded with the three concentrations
of substrate, methane and soluble COD were measured. The concentration of 2 % TS resulted
in higher production of accumulated methane compared with the other concentrations. At
25th day of incubation soluble COD reached the same value for the three concentrations,
approximately 450 - 500 mg g-1VS_COD. After that period, the soluble COD concentration
started to decrease for 2 % TS while it continued to increase for 5 and 10 % TS. These results
suggest occurrence of methanogenic inhibition for the higher initial concentrations of
substrate. The pH varied between 7.00 – 7.95 for 2 % TS, 6.76 – 7.70 for 5 % TS and 6.69 -
7.18 for 10 % TS. Inhibitory Volatile Fatty Acids (VFA) were not at the limiting
concentrations for methanogenesis [2], however ammonia concentrations higher than 3700
mg NH4
+ L-1 were observed for both, 5 and 10 % TS, and thus its inhibitory effects were
observed [3]