Report No FR0382



Jul 1993



The performance of aerobic and anaerobic two-stage sludge digesters at pilot-scale has been examined.


To find if the successful laboratory scale work on two-stage sewage sludge digestion could be replicated at a larger scale and to gain experience of operation at that larger scale. To obtain sufficient volumes of sludge treated using the two-stage treatment method to enable realistic settling and consolidation measurements.


Interest has increased in processes which are capable of producing a sludge which has an increased stability, and which provide additional pathogen destruction to anaerobic digestion, but which may be operated at little extra cost. Laboratory scale work on multi-stage aerobic/anaerobic sludge digestion processes led to the design and construction of a small pilot-scale two-stage apparatus. The increase in scale was designed to enable production of larger quantities of treated sewage sludge for physical tests than was possible using the laboratory-scale equipment and also to validate laboratory-scale work using conditions which more nearly resembled full-scale conditions. The work was expected to assist in determining under what circumstances multi-stage processes could be useful sludge treatment methods.


The period of successful operation of the pilot-scale equipment which was achieved after adjustment of the operating methods provided confirmation of the laboratory-scale results. The process became stable and the volatile solids destruction averaged 48% over the whole process, split between the thermophilic aerobic and mesophilic anaerobic stages.

Tests on the sludge consolidation properties gave variable results, for which the thermophilic aerobic sludge settled and consolidated better than the sludge from the anaerobic digestion stage. There was some evidence that the rate of settlement and final sludge concentration was improved, but that the resultant supernatant carried suspended solids concentrations of up to 10 kg/m3.

Pathogen destruction in the pilot-scale was not compared directly against an equivalent single stage process, although previous work has done so at laboratory-scale. Pathogen destruction was adequate.

It became apparent that the large pipework diameter which enabled trouble free automatic operation hindered accurate assessment of sludge flows through the system- Other difficulties indicated that the size of scale-up was not ideally suited to the process.

It is possible to use batch-test methods to estimate the effectiveness of a proposed two-stage sludge treatment method.


The industry may be confident in using this technology at sites where it is assessed as being appropriate to upgrade existing treatment, or to provide a new treatment method for which the sludge has enhanced product qualities. Caution would be required if the produced sludge was required in a dewatered form as the resulting liquor may require additional separation methods.

Larger scale equipment would present fewer transfer, operational, and monitoring difficulties.


Commissioning and initial calibration is described together with operational results. Solutions found for operational difficulties are described together with the final operational success. Figures and tables are included. Appendices cover the equipment design and selection, and results of the consolidation tests. Reference is made to the accompanying report which summarises the laboratory-scale work carried out for this project.

Copies of the report are available from FWR, price 25.00, less 20% to FWR Members.