Membrane distillation for algal supernatant reuse from hydrothermal carbonization aqueous product

ABSTRACT

With growing demand for more sustainable dairy manure management methods, a system involving hydrothermal carbonization (HTC), membrane distillation (MD), and algae cultivation for resource recovery from dairy manure was partially investigated in this study. Two algal species, Arthrospira maxima and Chlamydomonas reinhardtii, were cultivated on the hydrothermal aqueous product (HAP) of dairy manure, but the former requires a carbonate buffer for successful cultivation. The spent HAP after algae cultivation was treated using MD where the operational efficacy and distillate quality were assessed. Comparing the MD performance of the buffered HAP to the unbuffered HAP revealed the addition of the buffer decreased water flux and distillate quality, but cultivation with A. maxima reversed these effects and produced distillates with reduced COD, TN, and NH₃ concentrations. C. reinhardtii cultivation resulted in increased distillate COD and NH₃ concentrations compared to the unbuffered HAP. Fluorescence was used to broadly characterize the organic components of the feed and distillate streams. Regrowth experiments with A. maxima revealed the spent buffer can successfully be recycled, but fresh nutrients are required.

KEYWORDS:

• Membrane distillation
• algae cultivation
• hydrothermal carbonization
• wastewater treatment
• waste valorization

Disclosure statement

No potential conflict of interest was reported by the author(s).

Statement of novelty

This work uses membrane distillation (MD) to treat the supernatant from algae grown on the aqueous product of hydrothermal carbonization (HTC) of dairy manure. HTC converts the manure into a low-grade solid coal and an aqueous stream with nitrogen and phosphorus that can be used to cultivate Arthrospira maxima and Chlamydomonas reinhardtii. A. maxima media also requires a buffer to maintain an alkaline pH for successful cultivation. After growth, the algal supernatant was treated with MD; transmembrane flux and distillate quality were assessed. The buffer alone decreased MD operating potential; however, after cultivation with A. maxima the distillate quality was improved. The distillate from the C. reinhardtii supernatant had higher COD and NH₃ concentrations. The flux of both supernatants resembled the performance of the initial, unbuffered aqueous product. A regrowth experiment for A. maxima was conducted to demonstrate the potential of recycling the MD concentrate and buffer for cultivation.

Additional information

Funding

This material is based on work supported by the National Science Foundation under NSF Award #1856009. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation.