The role of rheological properties of sludge in anaerobic digestion process: biogas production and dewaterability

Anaerobic digestion (AD) is an environmentally sustainable technology that stabilises the pathogens present in the sludge whilst generating energy for heat, electricity and vehicle fuel. It helps to reduce large volumes of sludge that have been generated from wastewater treatment processes and contributes to reducing greenhouse gas emissions. Recently, high-solid anaerobic digestion has attracted enormous attention due to the size of smaller digesters and their higher organic loading, which usually results in relatively high volumetric biogas production. However, the yield of biogas (volumetric biogas production per gram added organic matter) does not significantly increase as a result of higher organic loading. Moreover, increasing the solid content of organic matter, such as sludge, significantly increases its rheological properties, which limits the accessibility of microorganisms to food and reduces biogas extraction due to the reduction in homogenisation. This whole study (consisting of four sub-studies) has identified the role of substrate rheology in anaerobic digestion performance when operating below inhibition levels. Study 1 analysed and compared the performance of four mesophilic anaerobic digesters that had been fed by waste-activated sludge (WAS) at 3.5% TS (total solids). The WAS's rheology was altered by mixing it with different doses of inorganic material to produce the substrates of different rheological properties but the same organic matter content while operating below the ammonia inhibition levels. Modifying the particle interaction of WAS using the inorganic material at different doses (0, 0.03, 0.07, 0.11, and 0.20 g) per g of WAS separated the role or rheology from its chemistry. The inorganic material altered the rheology of the sludge by changing the repulsion forces between the sludge flocs. The degradability of the substrate within the digester of the substrate with higher yield stress, apparent viscosity, flow consistency index, and storage and loss modulus was poor due to less homogenisation. Whereas, the substrate of the lowest viscosity increased the biogas yield and volatile solids removal efficiency by 19.29% and 12.5%, respectively, compared to the control digester. Furthermore, the amount of released soluble oxygen demand (sCOD) within the initial days of digestion from this digester was 22.5% more than that produced from the control digester, which also consumed faster during the process. The improvement in AD performance was explained by the easier access to the food for microorganisms, the better distribution of inhibitory by-products, and the reduction of stagnant regions within the digester due to better flowability. Moreover, the digestate of the mesophilic digesters, with 12% lower substrate apparent viscosity, also had 12% lower bound water (BW) on the final day compared to that of the control digester, which indicates a higher release of water from the intracellular substances during the flocs digestion. The impact of the rheological properties of the substrate on the final products of AD (cumulative biogas yield and digestate water content) was statistically assessed. In addition, periodical analysis of the degradability, physico-chemical and rheological properties during the digestion period revealed a proportional relationship between the flow consistency index (indicator of viscosity of media) and the loss modulus of digester media with the biogas yield, which demonstrates performing the rheological measurements as a promising indicator for biogas production. Due to economic considerations, it is almost impossible to encourage wastewater treatment plants to use dilute organic waste of high fluidity as a feedstock for anaerobic digesters. Although feeding thick organic waste increases the organic matter content in these digesters, at the same time it lowers the digesters¿ media flowability. Hence, there is ongoing contention regarding the trade-off between providing high organic matter content and the sufficient distribution of materials within a digester. Thus, Study 2 compared the performance of five mesophilic anaerobic digesters fed by WAS of different total solids (TS≤12%) to evaluate the effects of both rheological properties and organic matter content. The results revealed that the yield stress and consistency index of WAS exponentially increased, while the organic matter content linearly increased by thickening. In short, concentrating the substrate enhanced the biogas yield by 35% (in the digester of 10%TS WAS) as a result of providing more organic matter. However, the digester with the highest organic matter content (12%TS WAS) resulted in inefficient degradation as the biogas yield increased by only 25%. This fact highlights the point that a high concentration could be beneficial if enough fluidity exists to provide a sufficient mass transfer. Study 3 involved answering the question of why and how higher rheological properties hindered or delayed biogas production by focusing on hydrolysis rates and organic matter removal efficiency. To confirm the validity of the trend observed in Study 2, another set of anaerobic digestion of different sludge types (primary and waste-activated sludge), collected from two wastewater treatment plants with different food to inoculum ratios was also carried out. The low-viscous digester media (2%TS) revealed a hydrolysis rate that was five times higher and a 23% greater organic matter removal efficiency rate when compared to the high-viscous digester media (5%TS). As a result of this better digestion rate, a higher reduction in the rheological properties of the digester media (2%TS) was observed on day 3. For example, the τy, k, G´, and G´´ of materials within the digester on day 3 had reduced by 87%, 82%, 44%, and 23% respectively when compared to the rheological properties prior to the digestion process commencing. This association between the higher hydrolysis rate and the greater reduction in the rheological properties after digestion begins has shed some light on the link between rheological measurements and hydrolysis rates. Statistical analysis has revealed a linear correlation between the hydrolysis rate (calculated as the percentage change in soluble oxygen demand) and the digester media's rheological characteristics changes (calculated as the percentage change in flow consistency index). As hydrolysis is the first step in the AD process, this developed model enables engineers to recognise when the hydrolysis stage is completed, which is when the maximum observed changes in the rheological properties of sludge appear in the first three days of digestion. Similarly, a simple model was developed to link the rheological properties' evolution and the organic matter removal efficiency, which could be used for monitoring degradability as the digestion proceeds. This developed equation can assist in finding the rheological properties from the VS (volatile solids) data, or vice versa, depending which one is available and easier to measure. Furthermore, for the first time, the relationship between specific resistance to filtration (SRF) and yield stress of digestate on the last day of digestion was established. Study 4 explored the impact of the addition of organic waste as co-substrate to sludge on the rheological properties and degradation of sludge during anaerobic co-digestion. The experiment was carried out by mixing three different types of organic matter - cellulose, protein, and lipids - with sludge at different ratios of the co-substrate to the main substrate (2-8% wt.). The results revealed the combined effects of the co-substrate's rheology and degradability on biogas production, VS removal efficiency, and the kinetics of the methane yield. By adding 2% cellulose, the τy and k of the sludge increased by 153% and 174% respectively, which resulted in poor fluidity and insufficient homogenisation of the digester content and consequently an 18% fall in the biogas yield and a 13% fall in VS removal efficiency was observed. In contrast, by adding 2% protein, the biogas yield increased by 20% compared to the sludge mono-digestion, while ¿y and k of the sludge increased by 76% and 95% respectively. More interestingly, a 6% protein addition resulted in the τy and k of the sludge increasing by 125% and 240% respectively, while the biogas yield increased by 75%. This behaviour demonstrated that the optimal values for τy and k depend on the digestibility of the substrate and its degradation during the digestion process. For example, at relatively similar rheological properties of the digester media, the biogas yield increased by 75% with the addition of 6% protein and reduced by 18% with the addition of 2% cellulose because protein is more digestible than cellulose. In relation to lipids, there was an insignificant impact on the flow properties of the sludge after the 2-8% wt. addition of the lipids, which resulted in a biogas yield that increased by up to 250% (at 8% addition of lipid). The effect of flow properties on the kinetics of the anaerobic digestion process was also evaluated, using the first-order kinetic reaction model, and the results revealed that an increase in the τy and k is likely to reduce the degradation rate constant. The findings of this research provide new and fundamental insights into the refinement and improvement of the AD process as the optimised loading rate and biogas yield, with good digestate dewaterability, are intimately linked to the rheological characteristics of digester media.