Full scale experience of retrofitting thermal hydrolysis to an existing anaerobic digester for the digestion of waste activated sludge

Naestved central WWTP is a 5 MGD facility that was originally designed as a Bio DeNiPho plant following primary sedimentation. In 1998 the primary tank was bypassed in order to reduce organic micro-pollutants by aerobic treatment in the aeration basins. This reduced the micro-pollutants but led to a bulky undigestible sludge and foaming due to Microthrix. The Microthrix problem was stabilised using chemicals and an anoxic selector. Various options were studied to reduce the volume of sludge and it was decided to install Cambi thermal hydrolysis as a digestion pre-treatment. The project was completed in the spring of 2000 and has been operating for over nearly two years.

The process claims were an improvement of VS destruction by 60%, a doubling in dewaterability, doubling of digester loading, elimination of foaming and production of Class A Biosolids. The results have been very satisfactory and met these claims. Comparative data is presented which shows that there is about half the cake from before and now it can be used on farmland. The process has proven to be very reliable and runs automatically. The modular construction allowed an easy retrofit into existing buildings. Odour control has proven to be relatively simple. The filtrate has had little effect on final effluent, which has to meet the highest standards for COD, N& P.