Moving municipal WWTP towards circular economy: Cellulose recovery from primary sludge with ionic liquid

Abstract Tetrakis (hydroxymethyl) phosponium chloride ionic liquid was evaluated to recover carbohydrates, especially cellulose, from municipal wastewater treatment plant (WWTP) primary sludge (96 % water content). Different process operative conditions were evaluated: temperature (35, 60 and 80 ℃), time (6, 13 and 20 h) and sludge weight:ionic liquid volume ratio (1 g TS of SLUDGE: 7, 12 and 17 mL). A three levels, three factors central design was applied to optimize the procedure of recovery of the cellulose. Precipitated solids were analysed to quantify the content of cellulose, hemicellulose, lignin, proteins and ashes. The ionic liquid showed good performances in the recovery of cellulose. The average amount of carbohydrates recovered using the ionic liquid method was comparable with the amount of carbohydrates measured by the Dubois method in the raw sludge. All cellulose was recovered in the precipitated solid, but also part of the protein and ashes. According to the statistical tests, two interactions were found: between temperature and sludge:IL ratio for recovery of hemicellulose; between temperature and time for recovery of proteins. Experimental design was not useful to find optimized conditions of the process. Experiments realised with central values (60 °C; 13 h; 1:12 mL) gave very good results, allowing the recovery of high quantities of cellulose with the best removal of ashes but maintaining high quantities of proteins in the precipitate. For these reasons central values can be considered optimal reaction conditions. The design of novel ionic liquids will be necessary to fully purify the cellulose in a single dissolution step.