Evaluating the feasibility of creating a zero waste discharge aquaculture system†

Abstract

Aquaculture operations are synonymous with high freshwater consumption, followed by the discharge of large volumes of heavily contaminated wastewater. The freshwater consumption stresses clean water resources, and the discharge of contaminated wastewater induces ecological imbalance. Therefore, there is a crucial need to promote the concept of resource efficiency via wastewater management practice that supports the purification of nutrient-rich aquaculture wastewater (AQW) for reuse and nutrient recovery for agricultural use. Herein, the synergistic effects of the combination of primary coagulants (i.e., Moringa oleifera (MO) and chitosan (CH)) with a coagulant aid (i.e., thermally treated shell of a gastropod (GS)) was evaluated for AQW purification and nutrient recovery. From the determination of the optimum coagulant and coagulant aid dosages, two different coagulant–coagulant aid (C–CA) combinations (i.e., MO–GS and CH–GS) were prepared and comparatively assessed with the respective precursors for the AQW wastewater purification. The settled flocs were harvested and evaluated for the fertilizer values and nutrient releasing profiles and patterns. The coagulant aid produced water with the highest turbidity removal efficiency (98.8%), but the need to moderate the extreme pH value (12.1) of the produced treated water prompted the combination with different coagulants to produce the C–CA combination. The physicochemical characteristics of the dried harvested flocs (DHF) were comparable with those of the commercial fertilizers. All DHF samples showed a delayed TP release, fast TN release, and different modes and rates of nutrient release. The mode of nutrient release was similar in the C–CA combination, as they were best described by the Baker Lonsdale nutrient release kinetic model. Field application simulation and process safety evaluation are recommended to ensure a smooth transition from the bench scale to real-life applications.