Maximising resource recovery from wastewater grown microalgae and primary sludge in an anaerobic membrane co-digestion pilot plant coupled to a composting process

A pilot-scale microalgae (Chlorella spp.) and primary sludge anaerobic co-digestion (ACoD) plant was run for one year in an anaerobic membrane bioreactor (AnMBR) at 35 degrees C, 70 d solids retention time and 30 d hydraulic retention time, showing high stability in terms of pH and VFA concentration. The plant achieved a high degree of microalgae and primary sludge substrate degradation, resulting in a methane yield of 370 mLCH(4).gV(inf)(-1). Nutrient-rich effluent streams (685 mgN.L-1 and 145 mgP.L-1 in digestate and 395 mgNH(4)-N.L-1 and 37 mgPO(4)-P.L-1 in permeate) were obtained, allowing posterior nutrient recovery. Ammonium was recovered from the permeate as ammonia sulphate through a hydrophobic polypropylene hollow fibre membrane contactor, achieving 99% nitrogen recovery efficiency. However, phosphorus recovery through processes such as struvite precipitation was not applied since only 26% of the phosphate was available in the effluent. Composting process of the digestate coming from the ACoD pilot plant was assessed on laboratory-scale Dewar reactors, as was the conventional sludge compost from an industrial WWTP digestion process, obtaining similar values from both. Sanitised (free of Escherichia coli and Salmonella spp.) and stable compost (respirometric index at 37 degrees C below 0.5 mgO(2) g organic matter(-1).h(-1)) was obtained from both sludges.

Automated summary

A pilot-scale microalgae and primary sludge anaerobic co-digestion plant was successfully operated for one year in an anaerobic membrane bioreactor, achieving high stability and substrate degradation efficiency. Nutrient-rich effluent streams were obtained, allowing nitrogen and phosphorus recovery. Sanitised and stable compost was successfully produced through composting process.