Abstract
Combined microalgal and membrane filtration could effectively treat aquaculture wastewater; however, the membrane fouling induced by extracellular organic matter (EOM) during the dewatering process is an issue. This study investigated diatomite dynamic membrane (DDM) fouling behaviour during the dewatering of Chlorella pyrenoidosa under the influence of copper ions. The results indicate that copper ion heavy metals in aquaculture wastewater significantly affected purification and algae dewatering by DDM. Aquaculture wastewater with a high copper concentration (1 and 0.5 mg/L) could induce serious DDM fluxes and cake layer filtration resistance (Rc), whereas fewer filtration fluxes were induced when aquaculture wastewater had a low copper concentration, particularly that of 0.1 mg/L, at which the Rc was lowest and the concentration effect was highest. Macromolecular organics of EOM, such as biopolymers, polysaccharides, and proteins, were responsible for DDM fouling and accumulated mostly in the slime layer, whereas only a small amount of them accumulated in the diatomite layer. The DDM rejected more protein-like organics of EOM in the slime layer when dewatering algae at low copper concentrations (<0.1 mg/L); however, when using the DDM to dewater algae at high copper concentrations, more polysaccharides of EOM were rejected (0.5 < Cu2+ < 5 mg/L). This result has significant ramifications for aquaculture wastewater treatment as well as algae separation and concentration by the DDM.
Highlights
Aquaculture water quality has seriously deteriorated due to bait residues and excrement accumulation during the breeding process with the rapid global expansion of aquaculture in recent years
Algae exposed to 0.1 mg/L Cu2+ had the smallest decline in filtration flux, whereas the filtration flux declined severely with increasing copper concentration, especially at Cu2+ 1 mg/L, which can be interpretated as the presence of copper ions in aquaculture wastewater influencing microalgal growth during the algal purification of aquaculture wastewater (Figure S1)
The effects of copper ions in aquaculture wastewater on diatomite dynamic membrane (DDM) fouling and behaviour during dewatering of C. pyrenoidosa were investigated, and the key findings were as follows: Copper ions in aquaculture wastewater had significant effects on water purification and algae dewatering by DDM filtration
Summary
Aquaculture water quality has seriously deteriorated due to bait residues and excrement accumulation during the breeding process with the rapid global expansion of aquaculture in recent years. Traditional treatment for aquaculture water mainly includes mechanical filtration, foam separation, ozone oxidation, and chlorine dioxide oxidation [2]. These processes could effectively remove suspensions, there were some problems, such as high treatment costs, a low reduction in nutrients, and sludge generation [3]; the development of an efficient, low-cost, and environmentally friendly aquaculture wastewater treatment has become very important. Using microalgae to treat sewage could overcome the disadvantages of traditional treatment such as potential losses of nutrients, incomplete utilization of resources, and secondary pollution [5]; the microalgae produced can be utilized as aquaculture bait [6]. Low biomass concentrations and the excretion of algogenic organic matter (AOM) make this harvesting process more difficult, whereas without algae harvesting [9], algae is discharged into the environment, such as with sludge, leading to secondary pollution and significant losses in the economic benefits of water treatment
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