Recovery of organic nutrients from biogas slurry using decolorized ultra filtration membrane compared with nanofiltration membrane

Abstract

Membrane separation technology has played a crucial role in concentrating biogas slurry (BS) for efficient water discharge. However, a membrane that selectively separates organic molecules and inorganic ions for the recovery of organic nutrients from BS for its valorization has not yet been reported. In this study, we investigated the use of decolorized ultra filtration membrane (DUFM) for the separation of organic molecules and inorganic ions from BS, compared it with the commonly used commercial nanofiltration (NF). The results showed that DUFM concentrated approximately 63 % of the COD content, which was 24 % lower than that achieved by NF. However, the concentrated BS using DUFM exhibited higher levels of protein-like substances (9 %), aromatic humic-like substances (14 %), and pigments (8 %) compared to NF, as indicated by absorbance of λ254, λ420 and λ560. In terms of inorganic ions, NH4+, Ca2+, HCO3-, and PO43- constituated approximately 88 %, 90 %, 80 %, and 70 % in the permeate side of DUFM, respectively. In constrast, NF allowed only about 70 % of NH4+ and 67 % of HCO3- to pass through, with almost complete rejection of PO43- and Ca2+. The mix-charged DUFM (500 Da) demonstrated superior separation of organic molecules and inorganic salts with low energy consumption compared to NF (150 Da) with negative charges. The DUFM with mix-charged performance increases organic molecules rejection by Donnan effect via electrostatic exclusion and with 500 Da reinforces inorganic ions’ permeation by size exclusion. Additionally, DUFM exhibited tolerance for large molecular dissolved organic matter in BS due to its membrane’s high surface hydrophilicity and smoothness. In conclusion, this study suggests DUFM, with its mix-charged characteristics, could serve as a promising membrane for the selective separation of organic molecules and inorganic ions, facilitating the recovery of organic nutrients for BS valorization.