Anaerobic co-digestion of sewage sludge with cellulose, protein, and lipids: Role of rheology and digestibility.

Rheology is known to have an impact on the performance of digesters, but the effect of additional substrates (co-digestion) is poorly understood. The main objective of this study was to investigate the effects of the addition of cellulose, protein and lipids to substrates on the rheological behaviour and biogas production of the mixture of primary sludge (PS) and waste-activated sludge (WAS) in a batch system. A mixture of PS and WAS to form the main substrate was anaerobically co-digested with different types of organic matter (cellulose, protein and lipids) as co-substrates at different co-substrate to main substrate ratios of 2-8 (wt%) under mesophilic conditions and below ammonia inhibition levels. Yield stress (τy) and the flow consistency index (k) of the combined feed in the case of cellulose and protein were significantly dependent on the amount of co-substrate added, while there was an insignificant impact on these properties when lipids were added. Cellulose significantly increased τy and k in the feed, which resulted in poor fluidity and the improper homogenisation of the digester content, and consequently decreased the biogas yield. In contrast, the biogas yield was improved through the addition of 2% to 6% protein despite an increase in τy and k of the feed, but the methane yield decreased at 7% and 8% levels of protein concentration. This observation indicates that the threshold for τy and k of the digester media depends on the organic nature and digestibility of the substrate. There was no significant impact on the flow properties of the initial mixture when lipids were added, and their addition increased the biogas yield. A first-order kinetic reaction model was used for predicting the yield of methane from these digesters. The rate constant values revealed an increasing trend, with the highest for protein then lipids then cellulose.

Impact of rheological properties of substrate on anaerobic digestion and digestate dewaterability: New insights through rheological and physico-chemical interaction.

Mesophilic batch anaerobic digesters fed by different substrates were set up to identify the role of substrate rheology in anaerobic digestion performance while operating below the toxic level. Five substrates of different rheological behaviour but at the same amount of organic matters were prepared by addition of different amount of an inert material (0, 0.03, 0.07, 0.11, and 0.20 g) per g of waste activated sludge (WAS). To gain a comprehensive insight, the interactive relationship between substrate rheology, physico-chemical properties and biogas production as well as digestate dewaterability was investigated. The results proved that better access of microorganisms to organic matters improved the digester performance and led to 19.29% and 12.5% increase in biogas yield and VS removal efficiency, respectively. Moreover, the statistical analysis showed that consistency index and loss modulus of sludge could be employed as promising indications for biogas yield while yield stress could predict dewaterability of digestate as far as the other physico-chemical properties remained unchanged. During digestion measurement of consistency index and loss modulus of digestate could be performed as a reliable tool to monitor biogas production.