Extracting Humic Acids from Spend Mushroom Compost (SMC) by Alkaline Treatment and Membrane Ultrafiltration

Production of high purity humic acid with desirable quality properties for different uses is dependent on extraction and purification methods in its production process. The research literature showed that, in order to achieve this, separation of fulvate compounds and inorganic elements from alkaline humate extracted from raw materials is paramount significant. The main purpose of this study was the separation of alkaline humates from fulvates and inorganic elements by membrane separation technology. Spent mushroom compost (SMC) was used as organic waste for extraction of humic compounds including humates and fulvates with alkaline treatment method. Then, a membrane separation process with a mini-pilot system equipped with frame and plate module and polysulfone membrane with molecular weight cut-off 5 kDa was tested. The effect of transmembrane pressure (TMP) with four levels of 50, 150, 250 and 350 kPa on separation performance including permeate flux of fulvates, predominant fouling mechanism, fouling index (i) and fouling resistances as well as chemical and spectral properties and purity percentage of the resulting humic acid ultrafiltered from humate retentates were investigated. The results of this study showed that by increasing of pressure, permeate flux of fulvate compounds increased by 41.8%. Based on the Hermia model, the predominant fouling mechanism in all pressure levels were cake layer formation. The results of the fouling index (i) showed that all of the fouling mechanisms occurred in the membrane system, and by increasing pressure, complete pores blocking and cake layer formation were occurred faster and earlier respectively. Scanning electron microscopy (SEM) analysis in order to confirm of experimental data of fouling phenomenon showed that by increasing pressure from 50 to 350 kPa, layer thickness accumulated on the membrane surface was increased 3-fold. Evaluation of ultrafiltered humic acid characteristics showed that membrane separation technology played an effective role on chemical and spectral properties of humic acid compared to other purification methods, and increased final purity of humic acid to 87.8%.