Anaerobic digestion of waste activated sludge can be improved if hydrolysis of particulate substrates is enhanced and available substrate is made more accessible by both breakup of the sludge matrix floc and rupture of the cell wall. Microwave (MW) pretreatment was suggested and studied as a way to improve digestion efficiency. The work done focuses on the effects of MW pretreatment on the characteristics of the sludge, due to thermal and athermal effects. It also evaluates the effects some process variables in the activated sludge process have on the pretreatment efficiency as well as the effect operating conditions in the downstream anaerobic digestion process have on the biodegradability efficiency of those sludges. Effects of athermal and thermal MW radiation were measured by use of a customized MW oven capable of providing MW radiation with uncoupled thermal and athermal effects. Athermal radiation was capable of increasing substrate present in the soluble phase of sludge, and had a positive effect in the digestion of athermal samples. The increases in biogas production and substrate solubilisation were smaller in magnitude than the increases measured for MW thermal tests. Further refining of the tests with athermal and thermal sludge, involved separation by size class of the solubilized substrate by means of ultrafiltration (UF), and revealed that changes in particle size distribution were significant not only for MW thermal tests, but also for athermal tests, with a particular emphasis in proteins in athermal tests. These changes had an effect on the biodegradability of the sludges by class size, with thermally pretreated sludge producing more biogas for smaller particles size classes but also exhibiting more inhibition. Tests were made with several combinations of sludge with different ages and subject to different MW pretreatment temperatures. The work showed that sludge age or solids retention time (SRT) has a significant effect on the pretreatment efficiency with maximum biogas improvements measured at different MW pretreatment temperatures depending on the SRT of the sludge tested, and with different behaviour for mesophilic and thermophilic digestion. Mesophilic tests showed greater improvements in terms of digestion effiency on average, but thermophilic tests showed more uniform performance, with a higher baseline efficiency. The presence of an optimum of MW pretreatment temperature and sludge SRT for maximal biogas production is more defined for mesophilic conditions than for thermophilic conditions. Semi-continuous studies were conducted with several combinations of single and two stage mesophilic and thermophilic digestors treating MW pretreated sludge and non-pretreated sludge. Staging and thermophilic digestion allowed the maintenance of a stable digestion process with high biogas productions and high solids removal efficiencies with production of sludge with good bacteriological characteristics for an very low residence time (5 d).

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The long-term kinetic rates of solubilization/utilization of proteins, carbohydrates, and lipids in secondary solids from three municipal wastewater treatment plants were measured. Temperature, input sludge retention time (SRT₀), and terminal electron acceptor were used as control variables for two four-month digestion studies employing seven mixed batch reactors. Three types of adenosine triphosphate (ATP) measurements were used to differentiate between viable endogenous biomass, viable anaerobic/facultative biomass and intact non-viable solids. Solids solubilization was significantly affected by all three control variables. Temperature and terminal electron acceptor showed greater effects on solubilization than SRT₀. The effect of temperature was time-dependent and differed for the two types of secondary solids evaluated. Proteins were removed to the greatest extent, lipids to an extent comparable to that of volatile suspended solids (VSS), and carbohydrates were the most resistant component. Direct sample ATP or VSS measurements were poor indicators of the time-dependent potential of mixed microbial population for further waste stabilization. However, the analysis of the ratios of recovery ATP to sample ATP (ATP recovery ratio) provided more insights into the microbial behavior during sludge digestion. Under aerobic conditions, ATP recovery ratios generally increased with time. This result implied that during the initial phase of digestion the volatile solids consisted of a higher fraction of intact non-viable material, but after prolonged digestion the solids included a greater fraction of viable endogenous biomass. Under anaerobic conditions, ATP recovery ratios increased and then decreased with time, implying that the fermentative microbial fraction was highest at some intermediate point in the time frame employed. Four phase degradation patterns were observed in all cases, indicating an inability for any single equation to effectively model the long-term solubilization process.

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