Wanhe Qi, Jouni Vielma
This study aims to enrich potassium (K) content in sludge from recirculating aquaculture systems (RAS) by replacing sodium (Na)-based alkalinity-controlling chemicals with K-based derivates. Nine identical pilot-scale RAS rearing rainbow trout ( Oncorhynchus mykiss ) were operated for 10 weeks in triplicate treatments with K 2 CO 3 , NaHCO 3 , and NaOH as alkalinity-controlling chemicals. Water and sludge samples were collected weekly from the RAS for water quality and sludge property measurements. Results showed that K 2 CO 3 significantly increased K concentrations, reaching average values of 190 mg/L in water samples and 180 mg/kg in sludge samples. These levels were 17–20 times and 14 times higher than those observed in NaHCO 3 and NaOH treatments, respectively. All treatments achieved satisfactory biofilter performance (TAN < 1 mg/L and NO 2 - -N < 0.2 mg/L), and good fish production performance (FCR=0.96–1.17, survival > 95%). Significant accumulation of Na in rearing water from NaHCO 3 and NaOH treatments was observed with 133 ± 7 mg/L and 136 ± 2 mg/L at the end of experiment, respectively, which are far above phytotoxic Na threshold of 50 mg/L. In contrast, K 2 CO 3 treatment maintained low and stable Na levels over sampling period, ranging from 4.0 ± 1.3 mg/L to 6.3 ± 0.6 mg/L. No difference observed among treatments on other sludge (TN, TP, DM, TSS and SVI 30 ) and water (TN, TP, Ca, Mg, S, and Si) parameters. This study confirms that K 2 CO 3 as an alkalinity-controlling chemical in RAS is a feasible and synergistic solution in unifying alkalinity adjustment and producing a K-rich fertilizer from RAS sludge, which enhances the sustainability and circularity of aquaculture production.