The filtration characteristics of anaerobic digester effluents employing cross flow ceramic membrane microfiltration for nutrient recovery

• Digested effluent filterability was tested by a ceramic MF system of processing volume 140 L/m2 h.
• Pretreatment scheme reduced TS by 20.75% and 48.58% coarse particles (PDS 27.17 to 13.97 μm).
• Diafiltration and dewatering schemes were tested for fluid processability at TMP 15 psi.
• A compressible permeable cake layer was formed allowing continuous operation in DF.
• Enhanced filterability of fluids and varying concentrations in nutrients are found using DF

In the present study, a monolithic alumina coated microfiltration ceramic membrane was used for solid particulate removal and nutrient recovery from anaerobic digester complex effluent streams. The aim was to test the effect of the cake layer developed by the solids, on the surface of the membrane channels, to the filterability of these materials. The solid content ranged between 2.6 g/L to 15.1 g/L. During practical application, two processing techniques targeting the enhanced recovery of the materials of interest including ammonia, phosphate, calcium bicarbonate and volatile fatty acids, namely dewatering and diafiltration, were used. These had an immediate effect on the solid content (PDS 13 μm to 3.97 μm) enhancing the filterability of the effluents. Their processability was evaluated in terms of flux, cross flow velocity, membrane resistance and cake resistance. An important finding of this study is the nonalignment of the flux rates to the cake resistance, explained by the formation of a compressible, permeable cake layer that allowed the continuous operation of the system, under constant low pressure conditions (TMP 15 psi). Permeate flux remained constant to 120 L/m2 h when applying diafiltration, while when dewatering process is used the permeate flux remained constant at 115.4 L/m2 h.

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