# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 7 Full-Scale Application of Biogas Production from Halophytes  
For more than 20 years, anaerobic digestion has become a mature full-scale technology for the treatment of agricultural residues and livestock waste, energy crops and municipal and industrial organic waste and wastewaters. Depending on feedstock availability and economic frameworks, full-scale application of AD can be very versatile, from high-tech large-scale centralized plants to farm-scale plants and low-tech plants on a household scale. Depending on the feedstock characteristics, full-scale technology can generally be distinguished into three different systems:  
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* CSTR (continuous stirred tank reactor) systems for the treatment of slurries, such as manure or sewage sludge, with an optional admixing of other solid organic substrates;
* Dry AD systems for mono digestion of solid biomass or organic waste, such as OFMSW (organic fraction of municipal solid waste), with a total solids (TS) content of more than 15%;
* High-rate reactor systems of the UASB (up-flow anaerobic sludge bed) or EGSB (expanded granular sludge bed) type for the anaerobic treatment of wastewater.  
Accordingly, biogas production from halophytes can be applied in full scale with different sizes and different technology levels using AD technology suited for the treatment of solid biomass, i.e., CSTR systems for co-digestion of halophyte material together with other liquid biomass or as mono digestion in dry AD systems [114; 115; 116].  
Co-digestion of halophytes in CSTR systems has the advantage of direct dilution of the salt concentration through the addition of the liquid waste, thereby reducing inhibition effects, but this requires the availability of at least twice the amount of liquid organic waste streams in the proximity of the plant to reduce the TS concentration of the feed mixture to max 15% to enable mixing and pumping.  
Mono digestion, on the other hand, can be directly applied on-site, where the halophyte biomass or plant residues are produced, reducing the need of transportation of other biomass feedstocks. However, in this case, measures must be taken to reduce the salt concentration prior to feeding to the biogas reactor to avoid inhibiting effects in the AD process. This could be implemented by combining percolation of the biomass for dissolving the salt in the percolate and subsequent salt removal from the percolate before its recirculation on the biomass. This requires advanced separation techniques, for example membrane filtration and a subsequent treatment of the salt effluent [117].  
Process efficiency wise, higher specific methane yields were generally achieved for co-digestion of solid organic waste in wet digestion systems at organic loading rates (OLR) below 6 kg-VS m\({}^{-3}\) d\({}^{-1}\), while dry mono-digestion systems showed to be favorable at higher OLR [117]. Consequently, the decision of co-digestion vs. mono-digestion for large-scale applications rather depends on the availability of liquid biomass streams in the proximity of the biogas plant treating halophyte biomass. Simply speaking, co-digestion of halophytes is the first choice if other biomass is available; otherwise, mono digestion can be applied, but this needs the integration of solid/liquid separation and desalination processes.