supported positive microalgae biodegradability increases, further studies should
evaluate the risk of contamination in continuous bench and pilot-scale reactors. An
alternative cost-effective microalgae pretreatment method may be the use of environmentally friendly and low-cost chemicals such as lime (CaO). A recent study
found that the methane yield increased by 25% in BMP tests after pretreating
microalgae at 72 °C with CaO (Solé-Bundó et al. 2017). Biological pretreatments
constitute another promising pretreatment technology. Experiments conducted so
far have still not elucidated the best pretreatment conditions, resulting in lower
biogas production increases compared to thermal and thermochemical methods. In
Table 4 Comparison of pretreatment methods for increasing microalgae anaerobic biodegradability (Passos et al. 2014a)
Pretreatment
Control
parameters
Anaerobic
biodegradability
increase
Pros
Cons
Thermal (<100 °
C)
Temperature;
exposure time
√ √
Lower
energy
demand;
scalability
High exposure time
Hydrothermal
(>100 °C)
Temperature;
exposure time
√ √
Scalability High heat demand; need
for thickened or
dewatered biomass; risk
of formation of refractory
compounds
Thermal with
steam explosion
Temperature;
exposure
time; pressure
√ √ √
Scalability High heat demand; Need
for thickened or
dewatered biomass; risk
of formation of refractory
compounds
Investment cost
Microwave
Power;
exposure time
√ √
╶
High electricity demand;
scalability; need for
biomass dewatering
Ultrasound
Power;
exposure time
√
Scalability High electricity demand;
need for biomass
dewatering
Chemical
Chemical
dose;
exposure time
√
Low
energy
demand
Chemical contamination;
risk of formation of
inhibitors; high cost
Thermochemical Chemical
dose;
exposure
time;
temperature
√ √
Low
energy
demand
Chemical contamination;
risk of formation of
inhibitors; high cost
Enzymatic
Enzyme dose;
exposure
time; pH,
temperature
√
Low
energy
demand
Cost, sterile conditions
254
F. Passos et al.