Biogas Pre-treatment: Liquid Hot Water or Alkaline for Arundo?

There are various techniques and catalysts that improve the anaerobic digestion process for the Arundo. Biogas pre-treatment can often be combined or distributed separately. This article explores the use of liquid hot water and alkaline as biogas pre-treatments, and the effect they have on the yield produced from Arundo.

More than one biogas pre-treatment to choose from

Methane production from Arundo via anaerobic is a regularly suggested and utilised method due to its simple process, reliable performance, and low greenhouse gas emissions. Giant reed is a lignocellulosic biomass with a complex structure formed by cellulose, hemicellulose and lignin, which makes it relatively difficult to digest. However, in order to enhance the digestibility of Arundo and thereby subsequent improving the methane yield by anaerobic digestion (AD), various biogas pre-treatments have been analysed using different methods. These included diluted acid, steam explosion, cellulose solvent- based lignocellulosic fractionation (CSLF), liquid hot water (LHW), and alkaline.

Biogas Pre-treatment: LHW (Liquid Hot Water)

Liquid hot water (LHW) is one of the pre-treatment methods that increase the accessibility of cellulose in the network of lignocellulose by solubilising hemicellulose using elevated temperature and pressure. Organic acids, such as acetic acid, generated from the LHW process has the capacity to further catalyse the hydrolysis of hemicellulose into mono sugars.

In a report by Di Girolamo et al. (2013), it was revealed that LHW pre-treatment of Arundo at 180 degrees Celsius for 10 minutes caused a 23% increase in methane yield.

In comparison to other leading pre- treatments – such as dilute acid or ammonia – the biggest benefit of LHW is that it does not involve chemicals and ultimately, does not require corrosion-resistant materials for reactors. Furthermore, LHW generates fewer inhibitory compounds than steam explosion, which also uses high temperature and pressure for pre- treatment.

Biogas Pre-treatment: Alkaline

Alkaline pre-treatment has been argued to be the most promising pre-treatment methods to boost Arundo’s biogas yield. Other than splitting lignin-carbohydrate bonds, Alkaline can dissolve lignin and modify the construction of lignin in lignocellulosic biomass. As it has been established that lignin is the most stubborn element of lignocellulose, alkaline pre-treatment is mostly successful in refining digestibility of lignocellulosic biomass, particularly agricultural remains and herbaceous crops.

It has been previously observed, that even a mild alkali (6 g/L NaOH) of treatment for dissimilar biomass crops and residues, Arundo displayed the biggest rise in methane yield at 30% increase.

The benefit of the alkaline pre-treatment is that it can be managed without a need for a complicated reactor, at room temperature. Thus, making it a more appealing choice for on-farm application. Perhaps the downfall of alkaline – or other chemical – pre-treatment is the need for neutralising the biomass and washing it before the next stage of enzymatic hydrolysis. This leads to additional wastewater that needs to be dealt with, however, it could be collected, stored, and used for irrigation for the Arundo plantation. Furthermore, any leftover alkali is recycled and used again, thereby reducing the overall impact on the environment and minimising the costs for chemicals.

Figure 1 – Effects of LHW and alkaline pre-treatment on cumulative CG4 yield of Arundo

Overall, Jiang’s (2016) report can be utilised as the leading authority on the use of LHW and alkali as pre-treatment for Arundo; whereby it is established that alkaline pre- treatment produced a higher dry matter yield and accomplished a 25% higher glucose yield than LHW. Alkaline pre-treated Arundo achieved higher methane yields (up to 355 L/kg VS) than those from giant reed untreated or pre-treated with LHW (Figure 1). In addition, utilising AD with CHP for electricity generation from Arundo allows for a positive net biogas energy production and net electricity generation benefit. In conclusion, alkaline pre-treatment is more appropriate than LHW for enhancing electrical production, whilst alkaline salvaging is essential in order to advance the overall value.