Organic compounds organized into structurally complex parts of cells are significantly less biodegradable compared to pure compounds and simple com­pound mixtures, possibly due to lower accessibility to enzymes [207]. Many studies have shown that thermal pretreatment increases solubilization of particulate organic fractions and partially hydrolyzes polymeric organic molecules. The products formed during pretreatment of pure macromolecules, as well as components of pri­mary sludge (PS) and WAS, at temperatures ranging from 130 to 220°C, was recently studied [209] . The main findings of this study are: (1) no caramelization (pyrolysis) or significant hydrolysis of starch and cellulose to mono — or dimeric reducing sugars occurred at temperatures lower than 220°C; (2) breakdown of pro­teins to smaller peptides is accompanied by significant ammonia release at tempera­tures higher than 150°C, that can lead to inhibition of methanogens; (3) unsaturated lipids are hydrolyzed mostly to VFA (acetic and propionic) while saturated lipids form long fatty acids (LFA) (valeric, capronic, heptanic); (4) amount of LCFA and products with different degrees of oxidation (aldehydes, ketones, alkanes, alkenes, alcohols) increases with increasing treatment temperature (especially for tempera­tures higher than 170°C). Some of these compounds can be toxic to microorgan­isms, especially to methanogens [62, 234].

Bougrier and colleagues classified thermal pretreatment regimes into two groups, based on their temperature range, duration, and the impact on methane (biogas) yield [196]. The first group is characterized by moderate temperature in the range from 70 to 120°C and treatment duration time from 30 min to several days. The outcome is a 20-30% increase in methane (biogas) yield. The second group is characterized by

higher temperatures in the range of 160-180°C and shorter treatment time of 1-60 min with a 40-100% increase in biogas yield. Increasing the pretreatment tem­perature to 200°C reduced the bioconvertibility of all tested pure nitrogenous com­pounds (amino acids mix, RNA, DNA, collagen) and all tested carbohydrates (ribose, deoxyribose, glucose) and increased the toxicity. Stuckey and McCarty viewed ther­mal pretreatment as the sum of two separate processes: the first is the hydrolysis of complex polymeric compounds to soluble biodegradable molecules; and the second is the formation of refractory and toxic compounds from simple degradable mole­cules [207]. This trend was found to be true for tested amino acids, nucleotides and sugars with few exceptions (arginine, guanine, thymine). The pretreatment tempera­ture around 170-175°C was found to be optimal for biodegradation of WAS due to formation of less biodegradable and toxic compounds at temperatures 200°C and higher [207, 235, 236].

Samson and LeDuy studied the influence of thermal pretreatment on solubiliza­tion and methane yield from cyanobacterium A. maxima at 50, 100, and 150°C for a 1-h contact time [233] . Thermal pretreatment at all temperatures was favorable for COD solubilization. The COD values increased by 1.8, 1.7, and 2.3 fold com­pared to nontreated samples (Fig. 12a). Solubilization of the COD reached 36.2%, 40%, and 50%, respectively. The levels of solubilization of thermal pretreated sam­ples were somewhat higher than values determined for WAS. Solubilization of WAS increased from 8.1 to 17.6% (from 2.25 to 4.9 g/L) after thermal pretreatment dur­ing 30 min at 121°C [201].

The thermal treatment resulted in no benefit or inhibited the yield of methane and VS reduction (Fig. 12b). Similarly, thermal treatment of WAS during 30 min at 121°C resulted in a significant increase of COD (more than 100%). The methane yield and VS reduction increased as well and reached 135.2% and 132.1%, respec­tively, compared to the untreated control [201]. Pretreatment of Chlorella at 80°C decreased the methane production by 19% [237].

In contrast, thermal pretreatment of unicellular algae harvested from the effluent of a high-rate sewage oxidation pond at temperatures from 40 to 120°C and treat­ment duration of 30 min increased the methane yield [238]. The methane yield increased from approximately 0.14 to 0.17 L/gVS at 50°C and to approximately 0.25 L/gVS at 100-120°C. The methane yield increased with temperature and reached a maximum value at 100°C. The authors also reported the influence of treat­ment duration (from 1 to 120 h) and biomass concentration (from 3.7 to 22.5%) on methane yield at 100°C. The maximum methane yields were observed for a pre­treatment duration of 8 h (0.3 L/gVS, 15% increase) and with a pretreated algae concentration of 3.7%.

Drying

The water content of macroalgae is in the range of 80-95% [79, 124, 144, 239], and centrifuged microalgae have a water content of 90-95% [157]. Biomass drying is energy intensive, but is considered if storage or transportation is necessary before

AD. Drying of biomass by heating at 105°C for 24 h had a negative influence on methane yield with C. kessleri and C. reinhardtii [157]. The decrease in biogas yield from C. kessleri and C. reinhardtii compared to a nondried control were 23 ± 2.8% and 20 ± 2.7%, respectively. The authors hypothesized that easily digestible VS were lost during the drying process or became inaccessible for enzymatic attack. Consequently, AD of nondried biomass is more feasible and recommended. Technologically, it is beneficial to couple algae cultivating and AD facilities.