# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 6 Conversion of Halophytes into Biogas
### The Biogas Potential of Halophytes  
The AD process is well-established for the conversion of agricultural residues, food waste, animal slurry and other organic waste into biogas, which can be directly used as renewable fuel for combined heat and power (CHP) generation or--as biomethane after upgrading--be injected into the natural gas grid. Generally, biogas/biomethane yields are expressed in terms of mL g\({}^{-1}\) volatile solids (VS), which reflects the yield of biogas/biomethane per g of VS, the organic matter fraction of total solids (TS) that can be maximally degraded. Numerous biomass species and biomass residues have been evaluated and proved feasible for their biogas/biomethane production, such as, clover, cereals, maize and sunflower, though other sources are also possible. However, there is limited research on the biogas production potential of halophytic biomass. In principle, it would be an advantageous source of feedstock for biogas production in saline arid lands, as these plants are able to optimally thrive in high-salt conditions and absorb substantial quantities of salt, and thus, provide an avenue for the remediation of saline lands. In fact, salt-tolerant plants will not contend with traditional crops for land and water reserves in those regions. [PERSON] et al. [79] produced biogas at a yield of 358 mL g\({}^{-1}\) of organic solids or volatile solids (VS) with aboveground biomass of _Spartina alterniflora_ through AD (60 d, 35 \({}^{\circ}\)C and 1 atm). However, based on the elemental composition of _Spartina alterniflora_, the theoretical biogas yield was calculated at 795 mL g\({}^{-1}\) VS. Hence, based on this theoretical value, only 45% biodegradability was achieved from the AD experiment. It was suggested that upon analysis of cations, the sustained concentrations of K\({}^{+}\) and Mg\({}^{2+}\) ions may have caused inhibitory effects during AD. _Spartina alterniflora_ was also found to contain a high lignocellulosic content: 10% lignin, 30% hemicellulose and 34% cellulose on a dry basis [79; 82]. Therefore, the hydrolysis of the lignocellulosic material content, which accounts for about 74% of the gross quantity, can be viewed as the rate-limiting step in the AD processes of this halophyte. Similarly, [PERSON] et al. [82] investigated the biogas yield of _Spartina alterniflora_, sampled from the same region as [PERSON] et al. [79], but in a later season under varying organic loadings of the batch reactors. Biogas yields of 290 and 299 mL g\({}^{-1}\) VS were achieved at an initial loading of 4%VS and 6%VS, respectively, while the yield decreased to 253 mL g\({}^{-1}\) VS at a loading of 8% VS. The lower biogas yields of _Spartina alterniflora_ compared to 358 mL g\({}^{-1}\) VS found by [PERSON] et al. [79] are presumably due to the maturity of the halophyte plant with a lower content of easily degradable organic matter. The assessment of biogas potential from AD of wild or cultivated halophytes plants were investigated by [PERSON] et al. [75] in a series of batch-test and continuous mode experiments. The highest biogas yields from the investigated halophyte plants were from _Suada paradoxa_ (Bunge) Bunge, _Atriplex saggitata_ (Borkh) and _Karelitia caspi_ (Pall. ), with values of 277-398 mL g\({}^{-1}\) DM, 291-366 mL g\({}^{-1}\) DM and 257-358 mL g\({}^{-1}\) DM, respectively, under mesophilic conditions. These species contained, on a dry matter basis, about 736 mg g\({}^{-1}\) VS, 765 mg g\({}^{-1}\) VS and 795 mg g\({}^{-1}\) VS, respectively, which were the highest organic fractions observed, and also contained the least amounts of mineral ions accumulated in their biomass. It was anticipated within this study that the halophyte with the highest biomass yield, Salicornia europaea agg., will exhibit the highest biogasyield, but in reality, it demonstrated the lowest biogas yield. High sodium and sulfate content present in the halophyte species investigated caused drawbacks in the production of biogas. Likewise, according to [PERSON] et al.