Proceedings

The recent years there has been an increasing interest in optimizing the anaerobic digestion
at wastewater treatment plants since the energy produced in the biogas has become more
valuable, and the cost for sludge disposal more expensive. The Käppala Wastewater
Treatment Plant is designed for a load of 700 000 p.e, receiving wastewater from eleven
municipalities in Stockholm. The gas production at Käppala has increased by 50% since
2001, 6,1 million Nm3 biogas was produced during 2007. In addition to measures taken to
improve the operation of the digesters, different methods for pre-treatment of sludge have
been tested. However, the tested methods have shown not to be economically feasible. A
breakthrough was made during 2007-2008 when a mechanical disintegration used for treating
screenings from the mechanical treatment at Käppala WWTP was tested for pre-treatment of
sludge. Full-scale batch tests has been made and evaluated in laboratory scale digestion.
Significant increase in specific methane production was observed in the case of biological
excess sludge and digested sludge, resulting in a 7% increase of the total gas production. A
cost-benefit analysis has been done of the system, showing a break-even below 5% increase
of the gas production. This would be equivalent to a pay-back time of approximately one year.