Kai-Chee Loh and Prashant Praveen
Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore
The treatment of biogas digestate, with high NH4+-N content, high COD, presence of metals and pathogens, is considered a major bottleneck in anaerobic digestion. In this research, a bacterial-algal sequential process was designed for treatment of biogas digestate.
Batch experiments conducted using Chlorella vulgaris in real digestate (pure and diluted) did not result in microalgae growth due to bacterial contamination and poor light penetration. Microalgae cultivation in synthetic digestate without organic carbon also resulted in low nutrient uptake due to NH4+-N inhibition. Further studies in synthetic medium indicated poor NH4+-N uptake by the microalgae at concentrations above 35 mg/L, and cell growth was adversely affected above NH4+-N concentration of 80 mg/L. In contrast, microalgae exhibited high growth and nutrients assimilation, when cultivated with NO3--N at concentrations as high as 360 mg/L.
In order to turn digestate more amenable to microalgae cultivation, the digestate was diluted with municipal wastewater in a ratio of 1:10, and the diluted wastewater was treated through activated sludge process for COD removal and nitrification. This was followed by microalgae-based autotrophic nutrients recovery. This sequential bacterial-algal batch process resulted in removal of 90% nitrogen and 80% phosphorus within a week. Microalgae contributed to removal of 40% nitrogen and 45% phosphorus, and microalgae concentration increased from 0.024 g/L to 0.3 g/L. Under continuous operation, a membrane bioreactor (MBR) operating with activated sludge was operated in tandem with a membrane photobioreator (MPBR) operating with microalgae. The integrated MBR-MPBR was able to remove COD by 95% and total nitrogen by 77%, while PO43--P was completely exhausted. Microalgae could recover 22% nitrogen and 48% phosphorus, and the biomass accumulation after 70 days of operation was about 5 g/L.
These results indicate that bacterial pre-treatment of digestate can improve nutrients assimilation by microalgae through nitrification and organic carbon removal. It can thus be concluded that a sequential bacterial-algal process may be promising in the treatment of biogas digestate.