**Table 24.**

*Production of SCP.*

For methane fermentation in a sequencing batch bioreactor, see Ref. [154], for batch fermentation, see Ref. [155], and for a cascade of fermenters, see Ref. [156].

The historic Norferm process (see above) used a loop reactor, which can be seen as a PFR (plug flow reactor) comparable to an airlift fermenter only with a different agitation mode of the fluid, using a pump instead of the injected gasses to circulate the medium. **Table 26** assesses the relative production costs of different SCP processes.

It is estimated that methane fermentation comes with the highest investment costs (CAPEX), but will allow the lowest operational costs (OPEX), making that feedstock attractive for large-scale operations. When we look at the relative cots for substrate and utilities, there are also marked differences in the various processes. A waste stream has the advantage of low costs, but stable quality and quantity have to be ensured. Utility costs will depend on the geographic location of the site, apart from the unit operations chosen. Cooling costs are determined by the process temperature, and labor costs can be controlled by the degree of plant automation and the complexity of the process. Costs in SCP production are further discussed in Ref. [157].

*Value-Added Products from Natural Gas Using Fermentation Processes: Products… DOI: http://dx.doi.org/10.5772/intechopen.104643*


#### **Table 25.**

*Fermenter comparison for SCP production.*


#### **Table 26.**

*Comparative costs of various SCP processes.*

For engineering factors in the production of SCP, see Refs. [158, 159]. For process development, see Refs. [160].

Downstream processing depends on the exact target product(s). The "classic" separation of cells from the fermentation medium is centrifugation, followed by spray drying. Imasaka et al. have suggested cross-flow filtration of the fermentation broth with ceramic membranes [161], and Yang et al. the continuous methane fermentation in a fixed-bed reactor packed with loofah [69]. Heat treatment or enzymatic treatment [132, 162] can be used for nucleic acid reduction and digestability improvements.
