glycerol, activated sludge and others wastes have consistently showed
(Fernandez-Rodriguez et al. 2014; Herrmann et al. 2016; Neumann et al. 2015).
Continuous stirred tank reactors like those used for sewage sludge digestion or
other organic substrates are the most popular bioreactor configuration for the
conversion of microalgae biomass into biogas. Indeed, most of the reported studies
used that configuration with hydraulic retention times ranging from 20 to 40 days
(Jankowska et al. 2017). However, other alternative bioreactor configurations such
as UASB reactors have been proposed (Tartakovsky et al. 2015). Unfortunately, the
low solids retention times of granular-based reactors may not provide an efficient
conversion of microalgae. The use of membrane bioreactors has also been proposed, which represents an interesting opportunity to provide the required solids
retention for effective microalgae digestion (Zamalloa et al. 2012b) and to tackle the
problem of ammonia inhibition. Hence, medium exchange without biomass
washout can be implemented in membrane bioreactors to reduce the toxicity
mediated by NH 3 built-up, although the operating costs associated to this operational strategy still need to be evaluated under full-scale implementation.
6 Process Modelling
Process modelling is defined as the mathematical representation of a certain process
or system, which could be either based on the underlying mechanisms or phenomena (model-based) or on the experimentally generated input/output data
(data-based). Mathematical modelling is being increasingly used as a tool for
diagnosis, hypothesis formulation, prototyping, scenarios evaluation, process
design and optimisation. Thus, the anaerobic degradation of organic biomass,
including microalgae, can be modelled using different models. In this context, the
Anaerobic Digestion Model 1 (ADM1) has been the most popular, accepted and
applied model in research and industrial applications (Batstone and Keller 2002).
Nonetheless, there are plenty of other modelling approaches that have been
reviewed in the literature (Batstone 2006; Donoso-Bravo et al. 2011; Tomei et al.
2009).
Regardless of the model used to describe the methane production from
microalgae, the most important issue is the proper selection of the model parameters. In the specific case of microalgae, as discussed in the above sections, the
disruption of microalgae cell wall is considered the limiting reaction step, especially
if non-pretreated microalgae are fed to the anaerobic digester. Therefore, both the
disintegration and the hydrolysis coefficients, required in ADM1, have to be
carefully estimated. However, it is worth to point out that the elimination of the
disintegration step has been recommended due to the fact that the use of a
two-hydrolysis step possesses some correlation and identification problems, especially for sewage sludge approaches (Batstone et al. 2015). A description of how the
modelling of methane production from microalgae has been addressed in different
operation modes is given and discussed below.
258
F. Passos et al.
(Fernandez-Rodriguez et al. 2014; Herrmann et al. 2016; Neumann et al. 2015).
Continuous stirred tank reactors like those used for sewage sludge digestion or
other organic substrates are the most popular bioreactor configuration for the
conversion of microalgae biomass into biogas. Indeed, most of the reported studies
used that configuration with hydraulic retention times ranging from 20 to 40 days
(Jankowska et al. 2017). However, other alternative bioreactor configurations such
as UASB reactors have been proposed (Tartakovsky et al. 2015). Unfortunately, the
low solids retention times of granular-based reactors may not provide an efficient
conversion of microalgae. The use of membrane bioreactors has also been proposed, which represents an interesting opportunity to provide the required solids
retention for effective microalgae digestion (Zamalloa et al. 2012b) and to tackle the
problem of ammonia inhibition. Hence, medium exchange without biomass
washout can be implemented in membrane bioreactors to reduce the toxicity
mediated by NH 3 built-up, although the operating costs associated to this operational strategy still need to be evaluated under full-scale implementation.
6 Process Modelling
Process modelling is defined as the mathematical representation of a certain process
or system, which could be either based on the underlying mechanisms or phenomena (model-based) or on the experimentally generated input/output data
(data-based). Mathematical modelling is being increasingly used as a tool for
diagnosis, hypothesis formulation, prototyping, scenarios evaluation, process
design and optimisation. Thus, the anaerobic degradation of organic biomass,
including microalgae, can be modelled using different models. In this context, the
Anaerobic Digestion Model 1 (ADM1) has been the most popular, accepted and
applied model in research and industrial applications (Batstone and Keller 2002).
Nonetheless, there are plenty of other modelling approaches that have been
reviewed in the literature (Batstone 2006; Donoso-Bravo et al. 2011; Tomei et al.
2009).
Regardless of the model used to describe the methane production from
microalgae, the most important issue is the proper selection of the model parameters. In the specific case of microalgae, as discussed in the above sections, the
disruption of microalgae cell wall is considered the limiting reaction step, especially
if non-pretreated microalgae are fed to the anaerobic digester. Therefore, both the
disintegration and the hydrolysis coefficients, required in ADM1, have to be
carefully estimated. However, it is worth to point out that the elimination of the
disintegration step has been recommended due to the fact that the use of a
two-hydrolysis step possesses some correlation and identification problems, especially for sewage sludge approaches (Batstone et al. 2015). A description of how the
modelling of methane production from microalgae has been addressed in different
operation modes is given and discussed below.
258
F. Passos et al.