Anaerobic Co-digestion Of Cow Manure Research Articles

Semi-continuous anaerobic co-digestion of cow manure and steam-exploded Salix with recirculation of liquid digestate

The effects of recirculating the liquid fraction of the digestate during mesophilic anaerobic co-digestion of steam-exploded Salix and cow manure were investigated in laboratory-scale continuously stirred tank reactors. An average organic loading rate of 2.6 g VS L−1 d−1 and a hydraulic retention time (HRT) of 30 days were employed. Co-digestion of Salix and manure gave better methane yields than digestion of manure alone. Also, a 16% increase in the methane yield was achieved when digestate was recirculated and used instead of water to dilute the feedstock (1:1 dilution ratio). The reactor in which the larger fraction of digestate was recirculated (1:3 dilution ratio) gave the highest methane yields. Ammonia and volatile fatty acids did not reach inhibitory levels, and some potentially inhibitory compounds released during steam explosion (i.e., furfural and 5-hydroxy methyl furfural) were only detected at trace levels throughout the entire study period. However, accumulation of solids, which was more pronounced in the recycling reactors, led to decreased methane yields in those systems after three HRTs. Refraining from the use of fresh water to dilute biomass with a high-solids content and obtaining a final digestate with increased dry matter content might offer important economic benefits in full-scale processes. To ensure long-term stability in such an approach, it would be necessary to optimize separation of the fraction of digestate to be recirculated and also perform proper monitoring to avoid accumulation of solids.

Development of simplified anaerobic digestion models (SADM's) for studying anaerobic biodegradability and kinetics of complex biomass

The anaerobic co-digestion of cow manure and waste paper at ambient temperature condition was observed to be optimized at a mix proportion of 75:25 respectively. The development and testing of a set of simplified anaerobic digestion models (SADM's) for this mixture revealed that the Hill's based biogas yield rate model was most appropriate in describing the kinetics of biogas production. Parameter estimation using non-linear regression revealed that the half saturation constants expressed as acidified substrate and volatile solids equivalents were 0.228g/L and 5.340gVS/L respectively, and the maximum specific biogas yield rate and biodegradability were 2.2mL/gVS/day and 0.313 respectively. The coefficients “n” and “m” indicative of acidogenic bacterial adaptation for degradation and acetogenic/methanogenic bacterial cooperativity were estimated to be 1.360 and 2.738 respectively, while hydrolysis/acidogenesis was considered the rate limiting step. The need of bacterial adaptation may be an important factor to consider during anaerobic modeling of complex biomass.

Batch anaerobic co-digestion of cow manure and waste milk in two-stage process for hydrogen and methane productions

Anaerobic co-digestion of cow manure (CM) and waste milk (WM), produced by sick cows during treatment with antibiotics, was evaluated in two-stage process under thermophilic condition (55°C) to determine the effect of WM addition on hydrogen (H2) and methane (CH4) production potentials, volatile solids (VS) removal, and energy recovery. Six CM to WM VS ratios of 100:0, 90:10, 70:30, 50:50, 30:70, and 10:90 were examined using 1-L batch digesters. The WM VS ratio of 30% was found to be the minimum limit for significant increases in specific H2 and CH4 yields, and VS removal as compared to digestion of manure alone (P<0.05). The highest specific H2 and CH4 yields, VS removal and energy yield were 38.2mL/g VS, 627.6mL/g VS, 78.4% and 25,459.8kJ/kg VS, respectively, in CM:WM 30:70. Lag phases to H2 and CH4 productions were observed in CM-WM mixtures, increased with increasing the amount of WM in the feedstock and were greater than 72h in CM:WM 50:50 and 30:70. The digestion system failed in CM:WM 10:90. The results suggest that CM:WM 30:70 was optimum, however, due to limited amount of WM usually generated and long lag phase at this ratio which may make the process uneconomical, CM:WM 70:30 is recommended in practice.

Mesophilic anaerobic co-digestion of cow manure and biogas crops in full scale German biogas plants: A model for calculating the effect of hydraulic retention time and VS crop proportion in the mixture on methane yield from digester and from digestate storage at different temperatures

Data from 24 full scale biogas plants in Germany digesting cow manure and crops were evaluated. Special emphasis was given to the effect of hydraulic retention time HRT and proportion of crops in the mixture (VS basis) pVS,CropsInp on the methane yield from the digester yCH4D,VSInp and the storage tank yCH4,TS,VSInp at 37 and 22°C. The evaluation has shown model parameters for maximal methane yield of manure and crops ymaxCH4eff,VSInp at 270 and 420 Lkg−1, respectively. For example, at HRT of 60days, maximum methane yield result to 249 and 388 Lkg−1 for a crop proportion in the input of 0.0 and 1.0, respectively. The calculation of yCH4,TS,VSInp considers first order reaction rates and a temperature term fT. Hence, at any arbitrary temperature in the range of 12°C<T<37°C the values of yCH4,TS,VSInp in the course of time can be calculated, which correspond to methane emissions for uncovered storage tanks.