Influence of the cationic flocculant praestol K233L on the mesophilic anaerobic digestion of waste activated sludge

Bolzonella, David, Pavan, P, Battistoni, P and Cecchi, F (2005) Influence of the cationic flocculant praestol K233L on the mesophilic anaerobic digestion of waste activated sludge. Journal of Residuals Science and Technology, 2 (3), 133-139.

Abstract

In order to better understand the influence of cationic flocculants (polyelectrolytes) on the performances of the anaerobic digestion process, a lab-scale experimentation was carried out using waste activated sludge conditioned with a cationic flocculant as substrate. The selected flocculant was a cationic organic polyelectrolyte, based on polyacrylamide: the PRAESTOL K233L by Stockhausen GmbH. A 15 litres stirred reactor was fed once a day with flocculated activated sludge originated from a large full-scale wastewater treatment plant. After a first trial (reference period), where only activated sludge fed the reactor, the activated sludge was then conditioned with increasing amounts of the cationic polymer dosed at 5, 15 and 30 g per kg of sludge (dry weight). The performances of the reactor were then studied in terms of biogas production, volatile matter removal and parameters indicating the process stability (pH, alkalinity, biogas composition). These were compared to the results obtained in the reference period. The digester worked in the mesophilic range of temperature (35–37°C) with a hydraulic retention time of 20 days and an organic loading rate of some 1 kgVS/m3reactor per day. These were chosen as typical operational conditions for anaerobic digesters working with waste activated sludge. The experimental set up enabled to observe that the addition of the cationic flocculant did not affect the digester performances in terms of biogas production, the specific biogas production was 0.15–0.23 l/gVSfed in all the tested conditions, and that the flocculant was partially biodegradable. Also kinetic studies confirmed this situation and a value of the first order reaction kinetic constant, K, equal to some 0.10 day-1 was observed, no matter the flocculant concentration. On the other hand, the presence of the typical functional groups of the flocculant was observed in the digester supernatants by spectro-photometric analysis. This topic should be more investigated in future studies to verify the final fate of persistent organic pollutants like polyacrilamide or polyacrylate.

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