114
However, in most cases, the most inexpensive raw material options are from wastes
and by-products making organic-rich wastewaters a particularly attractive feedstock
option (Castilho et al. 2009).
Sugars as a carbon source produce a low yield of polyhydroxyalkanoates. For
example, the highest yield of 0.4 g of polyhydroxyalkanoates per g sugar was produced using glucose as a carbon source (Chee et al. 2010). Experiments conducted
on glucose as the only carbon source in a two-step fermenter accumulated a polyhydroxyalkanoates content of 17.2% of cell dry weight (Gonzalez-Garcia et al. 2008).
Agroindustrial wastes such as cheese whey, beet molasses, and plant oils are cheap
substrates for the production of polyhydroxyalkanoates. Plant oils are usually cheaper
than sugar sources and are great carbon sources for the production of polyhydroxyalkanoates and give a high yield of polyhydroxyalkanoates around 0.6 to 0.8 g of
polyhydroxyalkanoates per g of oil used (Chee et al. 2010). Moreover, cheese whey
is among the cheapest polyhydroxyalkanoates carbon sources, costing $0.22/kg,
and produces a polyhydroxyalkanoates yield of 0.33 g-polyhydroxybutyrate/g.
In comparison, petrochemical polypropylene substrate costs are around $0.185/kg
(Salehizadeh and van Loosdrecht 2004). Using cheap agroindustrial wastes as a substrate will result in a decrease of costs. However, to be truly cost competitive, there
is a need to use even lower cost, or free, substrates. Wastewaters provide an opportunity in this regard as not only is the substrate free of cost, but cost offsetting against
their treatment can be realized.
The next major cost influencer is productivity, which is related to substrate. In
experiments conducted by Lee et al., it was revealed that the cost of polyhydroxybutyrate production by Azohydromonas lata (previously known as Alcaligenes
latus) decreased as the polyhydroxybutyrate production rate increased. The costs
decreased from $4–5/kg polyhydroxybutyrate to $2.6/kg-polyhydroxybutyrate as
the polyhydroxybutyrate productivity increased from 1.98 g/L/h to 3.2 g/L/h
(Salehizadeh and van Loosdrecht 2004; Lee and Choi 1998).
The polyhydroxyalkanoates content of the microbial biomass also impacts the
efficiency of the recovery process. For instance, a polyhydroxybutyrate content of
50% results in a high recovery cost in the order of $4.8/kg-polyhydroxybutyrate,
while obtaining a high polyhydroxybutyrate content of 88% would reduce the
recovery cost to only $0.92/kg-polyhydroxybutyrate. This is related to the large use
of digesting agents for breaking the cell walls and the increased cost of waste disposal (Salehizadeh and van Loosdrecht 2004).
In an effort to reduce the production costs of polyhydroxyalkanoates, a focus on
identification and genetic engineering of bacterial strains and improvements in efficiency of fermentation/recovery processes have been undertaken (Salehizadeh and
van Loosdrecht 2004). The price of industrial polyhydroxyalkanoates was 15–17
times the price of petrochemical polymers back in 2004. Thanks to further research,
the price has reduced to three times that of petroleum-based polymers (Dietrich
et al. 2017). However, the cost of production is still a challenge that needs to be
overcome, and in general the consumer is not ready to pay a significantly higher
cost for a material with biodegradable properties. Further research on the effects of
S. Sali and H. R. Mackey
However, in most cases, the most inexpensive raw material options are from wastes
and by-products making organic-rich wastewaters a particularly attractive feedstock
option (Castilho et al. 2009).
Sugars as a carbon source produce a low yield of polyhydroxyalkanoates. For
example, the highest yield of 0.4 g of polyhydroxyalkanoates per g sugar was produced using glucose as a carbon source (Chee et al. 2010). Experiments conducted
on glucose as the only carbon source in a two-step fermenter accumulated a polyhydroxyalkanoates content of 17.2% of cell dry weight (Gonzalez-Garcia et al. 2008).
Agroindustrial wastes such as cheese whey, beet molasses, and plant oils are cheap
substrates for the production of polyhydroxyalkanoates. Plant oils are usually cheaper
than sugar sources and are great carbon sources for the production of polyhydroxyalkanoates and give a high yield of polyhydroxyalkanoates around 0.6 to 0.8 g of
polyhydroxyalkanoates per g of oil used (Chee et al. 2010). Moreover, cheese whey
is among the cheapest polyhydroxyalkanoates carbon sources, costing $0.22/kg,
and produces a polyhydroxyalkanoates yield of 0.33 g-polyhydroxybutyrate/g.
In comparison, petrochemical polypropylene substrate costs are around $0.185/kg
(Salehizadeh and van Loosdrecht 2004). Using cheap agroindustrial wastes as a substrate will result in a decrease of costs. However, to be truly cost competitive, there
is a need to use even lower cost, or free, substrates. Wastewaters provide an opportunity in this regard as not only is the substrate free of cost, but cost offsetting against
their treatment can be realized.
The next major cost influencer is productivity, which is related to substrate. In
experiments conducted by Lee et al., it was revealed that the cost of polyhydroxybutyrate production by Azohydromonas lata (previously known as Alcaligenes
latus) decreased as the polyhydroxybutyrate production rate increased. The costs
decreased from $4–5/kg polyhydroxybutyrate to $2.6/kg-polyhydroxybutyrate as
the polyhydroxybutyrate productivity increased from 1.98 g/L/h to 3.2 g/L/h
(Salehizadeh and van Loosdrecht 2004; Lee and Choi 1998).
The polyhydroxyalkanoates content of the microbial biomass also impacts the
efficiency of the recovery process. For instance, a polyhydroxybutyrate content of
50% results in a high recovery cost in the order of $4.8/kg-polyhydroxybutyrate,
while obtaining a high polyhydroxybutyrate content of 88% would reduce the
recovery cost to only $0.92/kg-polyhydroxybutyrate. This is related to the large use
of digesting agents for breaking the cell walls and the increased cost of waste disposal (Salehizadeh and van Loosdrecht 2004).
In an effort to reduce the production costs of polyhydroxyalkanoates, a focus on
identification and genetic engineering of bacterial strains and improvements in efficiency of fermentation/recovery processes have been undertaken (Salehizadeh and
van Loosdrecht 2004). The price of industrial polyhydroxyalkanoates was 15–17
times the price of petrochemical polymers back in 2004. Thanks to further research,
the price has reduced to three times that of petroleum-based polymers (Dietrich
et al. 2017). However, the cost of production is still a challenge that needs to be
overcome, and in general the consumer is not ready to pay a significantly higher
cost for a material with biodegradable properties. Further research on the effects of
S. Sali and H. R. Mackey