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Research article - Peer-reviewed, 2022

Waste Heat Driven Integrated Membrane Distillation for Concentrating Nutrients and Process Water Recovery at a Thermophilic Biogas Plant

Khan, Ershad Ullah; Nordberg, Ake; Malmros, Peter

Abstract

To efficiently utilize low-concentrate digestate nutrients, further treatment is needed to decrease their volume, recover process water, and increase nutrient concentrations. Membrane distillation (MD) is a thermally driven process that is advantageous due to its ability to harness low-grade waste heat to treat highly complex wastewater streams. This study assessed the techno-economic performance of integrating MD for two-fold concentrations of nutrients and the recovery of process water from digestate at a thermophilic biogas plant. Thermal assessment showed that the recovered waste heat from flue gas and digestate fully met the thermal energy demand of MD and saved 20% of boiler energy by heating incoming slurry. The permeate flux from MD was 3.5 L/(m(2)h) and 3.1 L/(m(2)h) at 66 degrees C and 61 degrees C digestate inlet temperatures during winter and summer, respectively. With internal heat recovery, the specific heat demand for MD was 80 kWh/m(3) and 100 kWh/m(3) in winter and summer, respectively. The unit cost of MD permeate was estimated to be 3.6 euro/m(3) and 4.1 euro/m(3) at a digestate feed temperature of 66 degrees C and 61 degrees C (with heat recovery), and 7.6 euro/m(3) and 9.1 euro/m(3) (without heat recovery) in winter and summer, respectively. However, cost sensitivity analyses showed that waste heat recovery and thermal energy cost variations had a significant impact on the MD permeate production cost. Nevertheless, the economic assessment indicated that the thermal integration of a biogas plant with industrial-scale MD digestate treatment capacity could be economically feasible, with winter being more economically favorable due to higher waste heat recovery.

Keywords

thermophilic biogas plant; membrane distillation; digestate effluent treatment; concentrated nutrients and water recovery; techno-economic analysis

Published in

Sustainability
2022, volume: 14, number: 20, article number: 13535
Publisher: MDPI

Authors' information

Ullah Khan, Ershad
HVR Water Purification AB
Swedish University of Agricultural Sciences, Department of Energy and Technology
Malmros, Peter
Uppsala Vatten och Avfall AB

UKÄ Subject classification

Bioenergy

Publication Identifiers

DOI: https://doi.org/10.3390/su142013535

URI (permanent link to this page)

https://res.slu.se/id/publ/119799