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

Ershad Ullah Khan (), Åke Nordberg and Peter Malmros
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Ershad Ullah Khan: HVR Water Purification AB, Vasagatan 7, 111 20 Stockholm, Sweden
Åke Nordberg: Department of Energy and Technology, SLU-Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden
Peter Malmros: Uppsala Vatten och Avfall AB, 751 44 Uppsala, Sweden

Sustainability, 2022, vol. 14, issue 20, 1-21

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 °C and 61 °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 €/m 3 and 4.1 €/m 3 at a digestate feed temperature of 66 °C and 61 °C (with heat recovery), and 7.6 €/m 3 and 9.1 €/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 (search for similar items in EconPapers)
JEL-codes: O13 Q Q0 Q2 Q3 Q5 Q56 (search for similar items in EconPapers)
Date: 2022
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