M. van de Ven1, A. Hol1, L. Luning1 and D. Traksel1
1Sustec BV, The Netherlands(free)
The thermal hydrolysis process (THP) has proven to be a valuable process to optimize anaerobic digestion by increasing the biogas production from waste activated sludge (WAS) and by substantially increasing the capacity of digester plants. The main drawback of the widely applied batch operation systems are the high operation costs, which are mainly related to pre-dewatering and energy consumption of the process. As an alternative, the continuous THP TurboTec® (cTHP) is available, which recovers its heat via heat exchangers and/or a mixing separation step. As cTHP allows lower pre-dewatering concentrations and steam consumptions, the costs are significantly reduced. To reduce the costs even further, a new cTHP version has been developed. In this concept, the final dewatering is added to the THP to separate the solids directly from the liquid fraction. This separation makes it possible to increase the total solids content from the solid fraction up to 45% with limited polymer (PE) use, while the liquid fraction contains most of the solubilized components. This liquid fraction can, depending on the obtained total suspended solids concentration, be treated in a UASB reactor to produce biogas. Nutrients (N & P) can be recovered from the UASB effluent with the NutriTec® process.
Continuous THP, direct dewatering, pilot experiment, TurboTec®, UASB.
Wastewater is a potential source for renewable energy and raw materials. For the water authorities, this is nowadays an important topic as can be seen from several initiatives, like in the Netherlands the “WWTP as energy and raw materials factory” initiative and the European “Full-scale demonstration of energy positive sewage treatment plant concepts towards market penetration, (Powerstep)” project. Next to improvements in the waterline, specifically the sludge line has great potential for recovery of renewable energy, nutrients and potential costs reduction.
Already for a long time mesophilic anaerobic digestion (MAD) is applied at Waste Water Treatment Plants (WWTP) to convert biosolids into biogas. The performance of the digester can be strongly enhanced by applying a thermal hydrolysis process (THP) as a pre-treatment prior to digestion. In a THP pre-treatment the sludge is treated at a high temperature (140 – 160 °C) and high pressure (4 – 6 bar). The advantages of applying THP are more biogas production, stronger volatile solids (VS) reduction, shorter digestion retention times and an increased total solids (TS) content of the final sludge cake. By THP it is also possible to apply higher loadings to the digester, resulting in more capacity in the same volume. These effects result in substantial lower costs for the sludge treatment, while at the same time more renewable energy is produced.
The potential of the THP-process was recognised almost a decade ago by the Dutch company Sustec BV, when it decided to develop a practical process of its own. The activities led from lab experiments to determine overall process parameters, to pilot plant development and operation and finally to full-scale design, construction and operation.