Effects of low temperature thermal pretreatments in solubility and co-digestion of waste activated sludge and microalgae mixtures

A low temperature thermal pretreatment was applied to three different ratios of waste activated sludge (WAS) and microalgae mixtures to increase their solubility. The performance of three organic flocculants was assessed in order to select the best flocculant for previous microalgae harvesting. The effect of the following variables in the solubilisation were evaluated: ratio of the mixture of WAS and (flocculated and non-flocculated) microalgae (50:1, 25:1 and 10:1, in v:v basis), incubation time (24, 48 and 72 h) and temperature (37 and 60 °C). A cationic polymer of diallyl dimethyl ammonium chloride free of acrylamide was selected for microalgae flocculation due to its high harvesting efficiency (95%). In pretreatments at 37 °C, effect of temperature and time in the solubilisation of the mixtures was negligible. However, results showed a high increase in solubilisation pretreatments at 60 °C using both flocculated and non-flocculated microalgae in the mixtures. Initial soluble chemical oxygen demand in the mixtures containing WAS and flocculated microalgae was higher in comparison with that in the mixtures of the same ratio without previous flocculation of the microalgae. According to the results, the optimal pretreatment conditions were incubation of the mixtures during 24 h at 60 °C in a 25:1 (WAS:microalgae, in v:v basis) mixing ratio. Thus, these optimal mixtures using flocculated microalgae and non-flocculated microalgae were chosen to assess the effect of the pretreatment in the biogas yield. The anaerobic co-digestion of the selected mixtures indicated a lack of proportionality between the soluble chemical oxygen demand attained during the pretreatment and the methane yield obtained (51 and 34% lower in comparison to the control, respectively). This behaviour could be explained by the high organic matter consumption in the low temperature thermal pretreatment before the anaerobic co-digestion, which could have been fostered by the enzymatic activity of the sludge.