The use of cavitation to improve the methane production and dewatering efficiency of waste water sludge

Annually, large amounts wasted activated sludge are produced by municipal waste water treatment plants (WWTP’s). In the Netherlands annually ± 353 thousand tons of sludge is produced (Henze, et al. 2008). Due to strict regulations regarding the disposal of heavy metals on agricultural lands and corresponding risks of diseases spreading, it is not allowed to use wastewater sludge as fertilizer for agricultural purposes (Cimochowicz-Rybicka, 2013). Therefore sludge has to be incinerated for which the costs could rise up to 500 € per ton of wet sludge (Henze, et al. 2008). In order to reduce sludge volumes and therefore reduce sludge disposal costs, sludge has to be pre-treated. In the Netherlands, the most common sludge pre-treatment technique is thermal pressure hydrolysis (TH). TH is a costly process since large amounts of energy are consumed by heating the sludge (Schepers and Elzen, 2013). In addition, the TH system requires a lot of maintenance and is hardly scalable which in the future may cause problems regarding the growing world population, with the corresponding growing sludge production levels (Schepers and Elzen, 2013). In order to be able to handle the expected future sludge amounts new techniques have to be developed based on the improvement of sludge reduction, pathogen removal and increase of potential sludge benefits. In this study a newly developed sludge treatment technique is examined. The technique is developed by the company Whirlwind International and is based, on “active hydrodynamic cavitation” (Bakker, 2012). Literature describes ‘active hydrodynamic cavitation’ as a situation in which passive hydrodynamic cavitation is accompanied by external forces (Bakker, 2012). In this research ‘hydrodynamic cavitation’ was generated by using a constriction (venturi pipe) which caused a sudden pressure drop within a stream flow. A vibrating ball was placed within the constriction to create the ‘active hydrodynamic cavitation’ event. In this research it was chosen to use secondary sludge from WWTP Houtrust during the ‘active hydrodynamic cavitation’ treatments since literature shows that an active hydrodynamic cavitation especially has a positive effect on the conversion of secondary sludge during the digestion phase (Rens, et al. 2005) and has less effect on primary sludge (Nickel, 2002). The cavitation treated sludge was assessed on three parameters: 1. improvement of the methane production levels, 2. improvement of the dewatering efficiency and 3. improvement of the total suspended solids (TSS) reduction levels. Based on the results it can be concluded that an active hydrodynamic cavitation treatment can result in the improvement of the methane production levels up to ~20%, improvement of the TSS reduction levels by 0-3% and the improvement/decline of the dewatering efficiency by ±3%. Profitability results shows that profits related to extra methane production levels are of significant less importance than the profits that can be achieved by the improvement of TSS reduction levels and dewatering efficiencies. However due to the large data ranges in TSS reduction levels and dewatering efficiencies both profits and losses can be generated which on average can rise up to ± €350,000 per year which accounts for ± 17% difference on the total yearly sludge treatment costs. Based on the current study it is not likely that profits can be generated since these calculations are mostly based on assumptions. Overall can be concluded that the effect of an active hydrodynamic cavitation treatment on the improvement of the profitability of waste water sludge is still doubtful since results show broad data ranges and several assumptions were made for the calculations. Further research is needed to obtain a dataset without these data ranges and should especially be focused on the improvement of dewatering efficiencies