Co-Digestion of Cattle Slurry and Waste Milk under Shock Loading Conditions

Anaerobic digestion of cattle slurry has become a well established practice. In recent years disposal of waste milk in the UK has been the cause of many pollution incidents. This paper describes the co-digestion of cattle slurry and waste milk under shock loading conditions in 1 dm³ laboratory digesters. Methane concentration in the biogas of all digesters receiving waste milk additions initially dropped, but quickly recovered to above previous levels. The methane production rate (dm³ methane day⁻¹) was most elevated in those digesters receiving the greatest loading of waste milk; however, initial depression of biogas methane concentration was also greatest in this system. © 1997 SCI.     

The influence of gas inlet design on gas hold-up values for water and various solutions in a loop-type air-lift fermenter

Gas hold-up was measured in an external loop air-lift fermenter for air in water, in sodium chloride and sodium sulphate solutions (concentrations between 0.025 and 1.0 kmol m⁻³), in glucose solutions (0.025 to 0.25 kmol m⁻³), sodium carboxymethyl cellulose (CMC) solutions (0.1, 0.2 and 0.5 mass %) and in suspensions of cellulose powder in water. Gas superficial velocities were up to 0.03ms⁻¹. Two types of gas spargers were used: one had six holes each of 1.6mm diameter, the other was of porous metal, which produced very small bubbles. Porous metal spargers with different diameters were also used, to alter the fraction of the cross-sectional area of the riser which was aerated (the ‘fractional aerated area’). The use of dimensionless gas velocities, obtained by dividing the superficial gas velocities by a characteristic velocity for each of the non-Newtonian liquids and solid suspensions, made possible a general correlation between gas hold-up, dimensionless gas velocity and the fractional aerated area, in bubbly flow conditions. The characteristic velocities for the electrolyte solutions could be related to the concentrations of the electrolytes. No general correlation could be obtained for gas slug-flow conditions, although theoretically-based relationships represent the results for each liquid. Gas slug-flow was observed for the non-Newtonian CMC solutions for most of the gas rates used at the lower concentrations; only at the highest concentration was bubbly flow found over a wide range of gas rates.     

Effect of Immobilisation on the Production of α-Amylase by an Industrial Strain of Bacillus amyloliquefaciens

The effect of immobilisation of an industrial strain of Bacillus amyloliquefaciens in calcium alginate beads on production of α-amylase was investigated using lactose-based media in shake flasks and in a 0.3 dm³ glass fermenter. Although the microorganism was a good α-amylase producer in batch cultures of free cells, it was unable to produce the enzyme for extended periods either in repeated batch cultures, or in continuous cultivation. In each case, parallel tests with cells immobilised in calcium alginate beads gave still further reduced enzyme yields, and the free cells released into the broth from these beads probably contributed substantially to any amylase produced during these extended fermentations. After prolonged use, the core of the alginate beads accumulated hard insoluble material, with viable immobilised cells confined to a surface layer.