Enhancing methane production in the UK WWTP via co-digestion of microalgae and sewage sludge

The study investigated the possible integration of microalgal digestion into existing WWTP configurations to benefit from the existing infrastructure. This was with the overall aim of increasing energy production. In particular, it was aimed at plants that utilise advance anaerobic digestion with the thermal hydrolysis process (THP) in place, as this is hypothesized to enhance algal degradability and overall CH4 yield while simultaneously producing biomass suitable for co-digestion with sewage sludge.
The effect of the existing THP was studied on the Chlorella vulgaris adopted for the study and results showed a 35% increase in methane yield from 0.265 to 0.357L CH4 /g VS added suggesting the possibility of this approach to upgrade microalgae to a competitive standard with the likes of food waste and FYM. It was then proposed for the first time, the co-digestion of pre-treated microalgae with sewage sludge thus, several co-digestions (0%, 25:75, 50:50, 75:25 and 100%) alga: sludge respectively were studied. Results showed the addition of pre-treated alga to sludge had a linear relationship to methane yield obtained up to a ratio of 75% algae with a corresponding methane yield of 0.369L CH4 /g VS added.
Based on this, laboratory scale CSTR were developed to identify the possible operational parameters and challenges that can favour and be encountered during the continuous running of these substrates. A co-digestion ratio of 75:25 with an OLR of 4 g VS L-1d-1 and HRT of 20 days produced the highest methane yield of 0.434L CH4 /g VS added suggesting a balance between substrate thus favouring methanogenic activities.
The study goes further to investigate alternative biofuel production using the carboxylate approach under which the influence of iodoform, pre-treatment and retention time on the possible VFA yield and production was studied. During the study, optimum iodoform for complete methanogenic inhibition in order to avoid conversion of the produced VFA into methane gas was reported at 10mg/L. Experiments were then carried out to investigate the respective co-digestion studied in this research under the carboxylate platform experiment, peak VFA concentrations ranged between 6.01 and 6.94 g/L, highest VFA concentration was produced in the 50:50 (alga:sludge) fermenter with a corresponding yield of 0.992 g total acid/g VS fed and a retention time of 11 days , this was seconded by 25:75 (alga:sludge) with a yield of 0.952 g total acid/g VS fed and a varying retention time of 17days.
Comparing both approaches investigated in this study, the carboxylate study proved a significant increase in VFA yield over the AD experiments with an increase between 92 and 166% for all tested co-digestion ratio. Conclusively, it could be inferred from the results in the study that the anaerobic fermentation of WAS and microalgae at several co-digestion ratios to produce VFAs may be an alternate option to methane production. While 75:25 proved to be the most optimal at an OLR of 4 g VS L-1d-1 and HRT of 20 days in the AD experiments, 50:50 (alga:sludge) proved to be the most effective co-digestion for carboxylate production. Retention time under this platform however ranged between 11 and 20 days.
The anaerobic digestion of microalgae using the existing facilities installed in the WWTP presents a positive energy balance for a WWTP about the size of Esholt UK, serving population of about 700,000 and currently treating about 80 tonnes dry sludge daily, where each possible tonne of microalgae produced and digested within the system would gain 1926 MJ (535 kWh). Not only will this implementation of technology help produce additional revenue from the extra biogas produced, it is also expected to offset some of the energy expended in wastewater treatment via utilization of the nutrients in the digestate and reducing nutrient load being recycled back for wastewater treatment.
The production of alternative products such as acetic acid and ethanol using a modified AD system will lead to an energy profit of 3536 MJ/tonne (982 kWh) and 6654 MJ (1848 kWh) respectively per ton of microalgae, suggesting an improvement by 1.8 – 3.4 times the methane production. The study concludes with some possible areas recommended for further study to achieve full exploitation of these techniques/resources at a commercial level.