Anaerobic digestion of freshwater microalgae : effects of reactor type, operation and cultivation conditions

• Main Author: Edward, Stephen Robert
• Published: University of Newcastle upon Tyne 2016
• Subjects: 628.3
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.713809

This thesis evaluates the technical potential of using microalgae as a substrate for anaerobic digestion. Investigating the control and operation of different reactors, under different operating conditions (OLR, SRT, HRT) to determine potential of microalgae as a feedstock and determine whether improvements in performance can be achieved. Thermophillic digestion offers higher methane yields compared to mesophilic digestion in simple reactor systems at 25 day SRT, being able to cope with higher organic loading rates. Low C:N ratio in microalgae has the potential to result in high levels of ammoniacal nitrogen within anaerobic systems with levels as high at 754 mgTAN/L observed at maximum loading rates. No apparent inhibition was observed in any reactor, with free ammoniacal nitrogen levels of 100 mg/L achieved without any drop in methane yield. While a UAnMBR system offered improved yields compared to CSTR systems, its performance was still relatively poor compared to theoretical maximum yields. The UAnMBR system did however cope with high hydraulic throughput (low HRT) without a significant drop in methane yield demonstrating that this system is potentially suitable for simultaneous harvesting and digestion. The microalgal biomass was inherently resistant to degradation, and over the duration of a lengthened growth cycle, can change its intracellular and cell membrane structures, changing its susceptibility to enzymatic attack and subsequent methane yield. Nutrient depletion in batch microalgae culture results in intracellular lipid and carbohydrate accumulation, which potentially could have resulted in a higher methane yield of 0.283 LCH4/gVSin (equivalent to 0.184 - 0.201 iii LCH4/gCODin) when compared to microalgae harvested during nutrient replete conditions. Allowing cultures to mature for longer periods in the stationary phase of growth under nutrient depleted conditions resulted in a significant reduction in methane yield to 0.174 LCH4/gVSin (0.124LCH4/gCODin). The selection of microalgal species appears to significantly affect the methane potential and degradation rates, with methane yield as high as 0.313 LCH4/gVSin (0.222 LCH4/gCODin) and as low as 0.130L CH4/gVSin (0.092 LCH4/gCODin) found in different pure cultures. The difference in yield was considered to stem from a wide variability in intracellular and cell wall structures. Poor correlation existed between gross biochemical content (protein, lipid, carbohydrate) and the methane yield, and confirms that variability in methane yield is not solely dependent on the biochemical composition (e.g. lipid content).