Renewable energy-assessment of pre-treatment and co-digestion strategies to optimize methane production from microalgae

Abstract

Anaerobic digestion (AD) of microalgae is an option to generate renewable energy in the form of methane-rich biogas. However, the high resistance of the cell wall, and unbalanced carbon to nitrogen (C/N) ratios can cause low methane production and an unstable AD process when using microalgae as the feedstock. Therefore, this research aims to optimize methane production from microalgae following pre-treatment and co-digestion strategies. Pre-treatment experiments were conducted in batch biochemical methane potential (BMP) tests, and the results showed that enzymatic hydrolysis offered a higher solubilisation of organic matters and methane yields compared to low-temperature thermo-alkaline pretreatment. However, both pre-treatments were considered energetically efficient in pretreating microalgae. Further BMP tests evaluated the feasibility of using potato processing waste (PPW) as a co-substrate with microalgae. The results showed that the addition of PPW to microalgae increased C/N ratios, methane production rates and final methane yields. BMP tests found that the addition of relatively high proportions of PPW could reduce the concentration of free ammonia nitrogen (FAN), and improve digestion performance and stability by reducing the likelihood of ammonia toxicity. The feasibility of using PPW as a cosubstrate for co-digestion with microalgae was further evaluated in the semi-continuous codigestion studies. The start-up strategy with immediate feeding enhanced methane yields significantly, and reduced the risk of ammonia toxicity for the methanogens. Co-digestion of microalgae with potato discarded parts (PPWdp) produced higher methane yields than codigestion with potato peel (PPWp). This start-up strategy and co-digestion of microalgae with PPWdp supported a high relative abundance of Methanosarcina. A second semi-continuous co-digestion study was carried out to optimize methane production from microalgae by codigestion with PPW and glycerol. Results showed that glycerol added to mixtures of 25:75 microalgae: PPW enhanced methane production significantly. Glycerol dosage of 1% v/v could be the optimal dosage with highest specific methane production and stable digestion process. However, glycerol dosage of 2% v/v was more likely to accumulate volatile fatty acids (VFA), leading to an unstable digestion process. Methaneosaeta was abundant in the digesters employing co-digestion of microalgae and PPWdp with or without glycerol. Methaneosaeta also predominated in the digester of co-digestion microalgae and PPWp with glycerol, however a higher relative abundance of Methanosarcina was detected due to the accumulation of VFA. Overall, this research shows that pre-treatment and co-digestion strategies can improve methane production from microalgae, and the results are encouraging for the future use of microalgae as a sustainable AD feedstock.