Microalgae production in wastewater treatment systems, anaerobic digestion and modelling using ADM1

• An experimental HRAP was evaluated for microalgae and biogas production over one year.
• Microalgal biomass production showed a seasonal pattern (average of 10 g TSS/m2·day).
• Anaerobic biodegradability was influenced by HRT and microalgae species.
• ADM1 was able to predict anaerobic digestion process by fitting inert COD fraction.
• Average biomass methane yield was 0.16 L CH4/g COD with 20 days HRT in the digester.

An integrated microalgae-based system for urban wastewater treatment, microalgae production and bioenergy generation through anaerobic digestion was evaluated over a period of one year. The pilot HRAP was effective at removing COD (~ 80%) and ammonium (~ 95%) and robust, despite common variations in wastewater composition and weather conditions in the Mediterranean region. Biomass production showed a strong seasonality, reaching an annual average of 10 g TSS/m2·day and the highest values in spring (23 g TSS/m2·day). Conversely, the macromolecular composition was fairly constant (58% proteins, 22% carbohydrates and 20% lipids). Predominant microalgae species varied throughout the year, influencing biogas production. Indeed, the anaerobic biodegradability of harvested biomass was 20–25% in July–October 2012 and May–July 2013 and 25–38% in November 2012–April 2013. Adapting the content of particulate inert COD in Anaerobic Digestion Model No. 1 (ADM1) was crucial for model calibration. After adjustment, ADM1 was able to predict microalgae anaerobic digestion performance, which showed an average methane yield of 0.09 L CH4/g COD at 15 days HRT and 0.16 L CH4/g COD at 20 days HRT.