Report No FR0383

Jul 1993

I BENEFITS

It has been shown at a small-scale that a hybrid sludge digestion system can potentially provide quality improvements in the treated sludge without entailing increased energy costs.

II OBJECTIVES

To provide a summary of laboratory and pilot-scale work carried out during an investigation of the two-stage thermophilic aerobic/mesophilic anaerobic sludge digestion process, to link this with other investigations of the same process, and to draw conc lusions.

III REASONS

Sewage sludge treatment processes which may improve the performance of existing methods are being investigated either because they reduce or eliminate disposal difficulties, or because they are more economic in terms of capital or running costs. Earlier work at laboratory-scale suggested that two-stage thermophilic aerobic/mesophilic anaerobic digestion could achieve improvements in the quality of the treated sludge in terms of solids and pathogen concentrations. Estimations also suggested that the process could be economic under the existing UK disposal regulations. The laboratory-scale work was sufficiently able to confirm claims by two-stage process designers that a pilot-scale version was built to extend the scope of the investigations.

IV CONCLUSIONS

The investigations have found that partial thermophilic aerobic digestion followed by mesophilic anaerobic digestion can produce a sludge which is better stabilised, and with fewer pathogens present, than produced using mesophilic anaerobic digestion alone. These benefits can potentially be achieved by making better use of the energy initially present in the sludge, and therefore at little extra cost compared to competing processes.

V RECOMMENDATIONS

Two-stage processes should be considered for new works or for upgrading existing sludge treatment centres.

VI RESUME OF CONTENTS

International experiences of two-stage, thermophilic aerobic/mesophilic anaerobic digestion are summarised. The mechanisms involved in aerobic and anaerobic digestion are described as they relate to the two-stage digestion system. Laboratory-scale results found during the period of this work are considered and compared to the pilot-scale results, and the relevance of the results to any application at full-scale is discussed.

Copies of the report are available from FWR, price £15.00, less 20% to FWR Members.