| Abstract: | Using laboratory-scale daily fed digesters operating at steady-state on separated cattle slurry, the temperature was lowered in a similar manner to that which might be expected on a farm due to heating failure; three different recovery methods were then tested. Raising the operating temperature from 3–6°C to 35°C in a single day had only a transient effect on digester stability. Steady-state conditions were re-established within 8 days of the temperature rise when a 25-day retention time was employed and within 6 days at 10-day retention time. However, slowly raising the operating temperature (over several days) had a more deleterious effect on digester stability. When a 25-day retention time was employed 10 days were required from the initial temperature rise before steady-state conditions were re-established, whilst at a 10-day retention time the period was greater than 12 days. It is therefore proposed that the digester temperature should be raised back up to the normal operating temperature as soon as possible after a heating failure. Digesters which were not fed during the recovery period showed a rapid removal of potentially toxic volatile fatty acids, and this procedure is recommended in the period following temperature shock.The main indicator of digester instability was a dramatic but unequal rise in the concentration of the individual volatile fatty acids. An order of sensitivity to the temperature shock treatment was established: i-butyrate i-valerate i-caproate > propionate > n-valerate n-caproate > acetate n-butyrate. Those to the left accumulated most rapidly in the temperature stressed digester, and were removed least quickly during the recovery period. Thus the bacteria responsible for the breakdown of the higher volatile fatty acids (presumably the obligate proton-reducing bacteria) were more sensitive to the shock treatment applied than those catabolising acetate (presumably the acetate-utilizing methanogenic bacteria).The concentration of the branched volatile fatty acids and propionic acid were the most sensitive to the temperature shock treatment, and therefore represent more sensitive monitors of digester stability than acetic acid or the total volatile fatty acid concentration. |
