# Halophyte Plants and Their Residues as Feedstock for Biogas Production--Chances and Challenges
## 6 Conversion of Halophytes into Biogas
### Inhibiting Effects of Halophytes on the AD Process
#### 6.3.2 Co-Digestion of Halophytes  
The method of choice to overcome adverse properties of certain biomass feedstocks for AD, i.e., C/N ratio, pH, moisture content, the concentration of nutrients, trace metals and toxic compounds, is to co-digest different biomass feedstocks with synergistic characteristics [82; 96; 108; 109]. The C/N ratio is one of the most important process parameters for AD, which can be adjusted through co-digestion of different feedstocks to an optimum range between 10-30 [85]. Low C/N ratios of biomass leads to a relatively high release of ammonia during decomposition, which will eventually inhibit methanogenic archaea, leading to a decrease in methane production and an accumulation of VFAs within the anaerobic reactor. On the other hand, a high C/N ratio can cause nitrogen deficiency in the whole anaerobic microbial consortium, thus decreasing the methane production potential [74; 96; 110]. Many studies indicate that low C/N ratios were overcome through co-determination of microalgae with other waste streams or biomass, such as pig manure, cow manure, corn stalks, municipal wastewater solids and lipid-rich fats, oils and grease, though other sources are also possible. The utilization of two substrates in co-digestion showed a significant increase in methane yield and a more balanced C/N and VFA/alkalinity ratios compared to mono substrate digestion. For example, [PERSON] et al. [111] investigated the co-fermentation of fish waste and sisal pulp and stated that an anaerobic co-digestion of 67% sisal pulp and 33% fish waste showed an increase in methane yield of 59-94% in comparison to mono-substrate digestion. Some research has been performed on halophyte biomass used as mono substrate, but little on co-substrates, and this may vary the C/N ratio. For example, the research by [PERSON] et al. [96] and [PERSON] et al. [74] showed a C/N ratio of less than 10 for mono-substrate AD of halophilic microalgae and halophyte _Chenopodium quinoa_, respectively. Low C/N ratios of biomass can lead to a relatively high release of ammonia during decomposition, as explained above. Many studies indicate that low C/N ratios were overcome by co-fermentation of microalgae with other waste streams or biomass, such as cow manure, corn stalks, wastewater sludge and lipid-rich waste such as fats, oils and grease. The utilization of two substrates in co-digestion showed a significant increase in methane yield and a more balanced C/N and VFA/alkalinity ratios compared to mono-substrate digestion. However, there is still limited research on the anaerobic co-digestion of halophytic biomass with biodegradable materials to enhance biogas/biomethane production. As previously mentioned, halophytes are able to accumulate high salt contents in their tissues without affecting drastically their biomass production, but they are capable of decreasing the rate of digestion of biomass substrate to produce biofuel by hindering the enzymatic degradation of lignocellulosic and accelerating the corrosion of reactor components [112]. One approach to overcome such undesirable consequences is co-digestion with biodegradable substrates that act as energy supplies. [PERSON] et al. [82] investigated the co-digestion of the halophyte _Spartina alterniflora_ with varying co-digestion ratios of cow manure at an organic loading of 7% VS. With the addition of cow manure, the C/N ratio dropped from 21.82 to 14.19, of which the optimum range is between 10-30 [85], and co-digestion improved the biodegradation of _Spartina alterniflora_, with lower VFAs concentrations and higher methane yields of 7 to 44%. [PERSON] et al. [113] also investigated the lignocellulosic structural changes of _Spartina alterniflora_ after anaerobic co-digestion with potato, and reported that co-digestion had more favorable effects on improving the hemicellulose degradation in comparison to mono-substrate digestion. [PERSON] et al.