No abstract

Waste of apples, asparagus, carrots, green peas, French beans, spinach and strawberries from a canning factory have been screened on mesophilic anaerobic digestion in 90-day experiments at loading rates varying between 0.80 and 1.60 kg volatile solids (VS) m-3 day-l at a retention time of 32 days. Average biogas yields varied from 0.30 to 0.58 m3 kg-l VS day-1. High percentages of reduction in VS, carbohydrate and crude fibre were obtained in most experiments. Some waste materials showed unbalanced digestion, as might be expected from carbohydrate-rich substrates. In those cases alkali addition, feed interruption and mixing with a nitrogen-rich substrate were used to overcome unbalanced digestion. Residual solids in the digested sludges were removed by flocculation with a polyelectrolyte and centrifugation; liquids with lower CODs remained after flocculation.