Optimization of biogas production from manure: Final project report EFP-04

The main objective of the project was to improve biogas production from manures. This objective was
addressed by investigating 1) the effect of different reactor configurations, 2) operational procedures,
aiming to selectively retain/return degradable material in the reactor and 3) different posttreatments to
improve the degradability of the undegraded material. Both lab-scale and pilot-scale experiments were
carried out at the Institute of Environment & Resources, Technical University of Denmark. In the first
experiment, the effect of serial digestion on process performance and methane production was compared to
a conventional single CSTR process at 55°C. The total working volume (5 l) between the two
methanogenic reactors of serial CSTR process was varied by distributing the volume between the first and
second reactor at 90/10, 80/20, 70/30, 50/50, 30/70 or 13/87%. Results showed that serial CSTR process at
90/10, 80/20, 70/30, 50/50 or 30/70% volume distribution could produce 11-17.8% more biogas compared
to single CSTR process under similar operating conditions. The increased biogas production was mainly
from the second reactor of the serial process, which accounted for 16-18% of the total biogas production.
At 13/87 ratio, no significant increase in biogas production was noticed. Both single and serial CSTR
processes were stable when operated 90/10, 80/20, 70/30 or 50/50% volume distributions and also during
an organic pulse load (19.6 to 65.3 g/l reactor volume). Results from pilot-scale studies showed that serial
digestion with 77/23% volume distribution produced 1.9-6.1% more biogas compared to that obtained
during one-step CSTR operation. However, temperature was found to have a strong influence on the
methane production and process performance of the second reactor of a serial CSTR process.
In the second experiment, the effect of temperature (10 & 55°C) and microbial activity on passive
separation of digested cow manure was investigated in vertical columns (100 cm) with an aim to improve
solids retention time within the reactor and improve biogas production. Results showed that greatest degree
and rates of passive separation took place when the temperature during the settling was maintained at 55°C
and after 24 hrs of incubation. Higher temperature and lower viscosity, and probable higher biological
activity aided the separation process at 55°C than at 10°C. The effect of continuous mixing (control),
mixing for 10 min per day (minimal mixing) and withholding mixing for 2 hrs prior to feeding (intermittent
mixing) on biogas production was evaluated in three lab-scale CSTRs. Results showed that minimal and
intermittent mixing improved biogas productions by 12.5% and 1.3% respectively over continuous mixing.
Intermittent mixing also resulted in stratification of solids with higher solids content in the top and bottom
layers compared to middle layer. Similar result was also noticed in pilot-scale plant when intermittent
mixing was sequenced with continuous mixing. Biogas yields improved from 2.5 to 14.6% when the
reactor was operated under intermittent mixing compared to continuous mixing. The effect of mixing
intensities (minimal, gentle or vigorous) in batch assays at 55°C showed that when the process was
overloaded by high substrate to inoculum ratio (40/60), gentle (35 times per minute) or minimal mixing (10
minutes mixing before feeding) was advantageous compared to vigorous mixing (110 times per minute).
On the other hand, under low substrate to inoculum ratio (10/90), gentle mixing was the best. The study
thus indicated that mixing schemes and intensities have some effect on anaerobic digestion of manures.
In the third experiment, the effect of eight different post-treatments on improving the biogas
production of fibres separated from thermophilically digested cow manure was studied in batches at 55ºC.
Results showed that only partial aerobic treatment (air flow rate at 0.28 l/min g total solids) and grinding
(mortar and pestle) improved methane yields while chemical treatments (NaOH or CaO at 40g/kgVS)
resulted in more or less similar methane yields to that of untreated fibres. Treatments such as microwave
irradiation (300-700 W), conventional boiling and wet oxidation (195ºC, 12 bar of O2 for 10 min.)
improved the soluble chemical oxygen demand content while ultrasound irradiation did not affect the
fibres’ SCOD content. Thus, the present results showed that biogas production from manure can be
improved from 7 to 18% by either adopting serial digestion of manure, mixing the reactor intermittently
with a 2 hour mixer blocking prior to feeding or by partial aerobic treatment or grinding of the fibers
separated from the digested manure. However, the optimal volume distributed between the two
methanogenic reactors in serial digestion could be either 70/30 or 50/50% and/or effluent should be
removed from the middle layer after 2 hrs of settling. The improved methane recovery with the tested
posttreatment needs further investigation with respect to the costs, efforts and energy inputs.