Sludge removal frequency influences heavy metal accumulation, water quality, and growth in Nile tilapia ( <scp> <i>Oreochromis niloticus</i> </scp> ) biofloc systems

Heavy metals (HMs) are one of the emerging micropollutants, which have been found a significant toxicant to the fish fauna; therefore, the current study aimed to examine HMs including Cd, Cr, Pb, and Cu concentrations in sludge and their accumulation in Oreochromis niloticus tissues along with growth and water quality in Biofloc Technology (BFT). Tilapia, initially weighing 0.40 ± 0.15 g, were divided into three groups (G1, G2, and G3) with three replicates each. Sludge was removed weekly for G1, biweekly for G2, and monthly for G3 over 90 days. Water quality parameters were monitored daily, and (HMs)in sludge and fish tissues were analyzed. One-way ANOVA with multiple comparisons was used to compare the results among groups. Results showed that sludge removal frequency significantly (p < 0.05) affected water quality. G3 had higher ammonia, nitrate, and nitrite levels, and a slightly acidic pH. G1 had higher dissolved oxygen (6.72 mg/l) compared to G3 (5.32 mg/l) and better growth performance with a lower feed conversion ratio (FCR) of 1.44 versus 1.67 for G3. The HM analysis showed that G3 had higher metal concentrations in both fish tissues and sludge, with a strong correlation between metal levels in sludge and fish tissues. The health index (HI) values confirmed that fish from all groups were safe for human consumption. The study concludes that regular sludge management is crucial for optimizing BFT production and safety; therefore, G1 has optimum growth and water quality with less HM accumulation. PRACTITIONER POINTS: Study examines Cd, Cr, Pb, and Cu accumulation in Nile tilapia tissues and sludge under varying sludge removal frequencies. Weekly sludge removal improves water quality, dissolved oxygen, and fish growth, with better FCR. Monthly sludge removal shows higher ammonia, nitrate, nitrite, acidic pH, and metal concentrations in sludge and fish tissues. Strong correlation found between sludge metal levels and fish tissue accumulation. Findings emphasize regular sludge management for enhanced BFT production, safety, and reduced heavy metal bioaccumulation.