Solid management is one of the most critical yet problematic issues in recirculating aquaculture systems (RAS). Dewatering or thickening solids is often needed to reduce the sludge volume by about 50 to 100 times to minimize the handling and disposal costs. Coagulation–flocculation process is commonly employed in municipal water treatment facilities to remove solids. However, most of the conventional metal-based or synthetic coagulants may not be ideal for RAS due to potential toxicity and incompatibility with downstream sludge processing methods. Therefore, a natural coagulant extracted from Moringa oleifera (MO) seed protein is proposed.
The aquaculture waste collected from the backwashed beadfilter discharge was collected from a Delta Smelt (Hypomesus transpacificus) hatchery in Byron, California, USA. The coagulation study was conducted using a jar test with the following conditions: flash mix at 200 rpm for 30 s, slow mix at 40 rpm for 15 min, and settling for 30 min. At the end of the settling period, samples were taken at 8 cm above the bottom (marked on the jar) using a pipette, and water quality was tested. The preliminary result showed that raw MO seed (Figure 1) was able to successfully remove the total suspended solids from the aquaculture waste.
Nonetheless, to increase the economic viability of the proposed method, defatted MO seed cake, a byproduct from MO oil industry, may be used in place of the raw seed. Further study is designed to explore different pretreatments of oil expression and to investigate the protein extractability and coagulant performance of the resulting MO seed cake. The oil from MO seeds is extracted using either thermomechanical (hot or cold press), solvent (hexane or ethanol), or the combination of the two. The protein is extracted by mixing the defatted samples with NaCl solution. The protein solubility is calculated by comparing the soluble protein (measured using BCA protein assay) with the initial protein in the samples (measured using combustion method). Finally, the coagulation efficiency is performed using jar test.