A novel option for reducing the optical density of liquid digestate to achieve a more productive microalgal culturing

The liquid fraction of digestate produced by agricultural biogas plants is rich in macro and micronutrients that are valuable for the culturing of microalgae. Nonetheless, the high ammonium concentration may cause toxicity and the high optical density may reduce light penetration, negatively affecting the biomass production rate. Dilution with fresh water has been frequently suggested as a mean for improving the digestate characteristics in view of microalgal culturing. In this paper, the feasibility of culturing microalgae on undiluted raw digestate or on digestate after pretreatment by stripping and adsorption was investigated. First, adsorption tests were performed using commercial activated carbon from wood in order to identify appropriate conditions for optical density (OD) reduction. Up to 88% reduction was obtained by dosing 40 g L− 1 after 24 h of contact time. Then, culturing tests were performed on a microalgal inoculum including mainly Chlorella spp. and Scenedesmus spp. under controlled temperature and light conditions during 6–14 weeks. Raw, stripped, and stripped and adsorbed digestate samples were tested. The biomass production rate increased from 27 ± 13 mg TSS L− 1 d− 1 on raw digestate, to 82 ± 18 mg TSS L− 1 d− 1 by using stripped digestate, and to 220 ± 78 mg TSS L− 1 d− 1 by using the stripped and adsorbed digestate. Moreover, nitrification was constantly suppressed when using the stripped and adsorbed digestate, while relevant nitrite built-up was observed when using raw and stripped digestate. These results suggest that microalgae are able to grow on the raw digestate, provided that long hydraulic retention times are applied. A much faster growth (up to 10 times) can be obtained by pretreating the liquid fraction of digestate by stripping and adsorption, which may be an effective means of improving the areal productivity of microalgal culturing on digestates.