The aim of this work was to develop a method that allows the recovery of nutrients from aquaculture sludge, not only to alleviate the disposal problem, but also to address the future scarcity of non-renewable fertilizers. This method includes two steps: Nutrient mobilization using aerobic digestion followed by solids precipitation using chitosan as the flocculant. The aerobic digestion experiments were conducted in aerated batch reactors, while a jar test apparatus was used to assess the capacity of chitosan to remove total suspended solids (TSS) and turbidity. During aerobic digestion, the concentration of soluble N (sum of NH4-N, NO2-N, NO3-N) increased from 181 mg/L at the start to 890 mg/L after three weeks, and to 903 mg/L after four weeks and solids removal by chitosan flocculation. The corresponding concentrations for soluble P were 8.2 mg/L at start, 110 mg/L after three weeks of aerobic digestion, and 160 mg/L after four weeks of aerobic digestion and chitosan flocculation. Other macronutrients (K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, B, Cu, Mo) were mobilized to concentrations close to or higher than levels recommended for hydroponic growth of vegetables. Chitosan flocculation and precipitation using a dose of 15 mg/L resulted in a reduction of the turbidity by 96% from 156 to 6.5 FNU. After chitosan precipitation, 80% of the sludge could be reclaimed as a nutrient-rich clear phase, low in TSS and turbidity.

The aim of this study was to demonstrate how aquacultural sludge can be processed and utilized as an organic nutrient solution (ONS) for hydroponic lettuce production. By using a previous developed method, approximately 80% of the processed sludge was reclaimed as a clear, nutrient-rich solution. The performance of the recovered nutrient solution on lettuce growth was assessed in a nutrient film hydroponic system. The results were compared to the results obtained using a conventional nutrient solution (CNS). Yield, fresh weight, water consumption, and nutrient and heavy metal content in leaf tissue were measured. In spite of a 16% lower average fresh weight obtained in ONS compared to the weight obtained in CNS, there was no statistical difference of the yield of lettuce among the two nutrient solutions. After the cultivation period, 90% of the lettuce heads grown in ONS exceeded the marked weight of 150 g. Foliar analysis revealed a similar or higher content of all nutrients, except of magnesium and molybdenum in the leaves of lettuce grown in the ONS compared to lettuce grown in the CNS. This study shows that nutrients recovered from aquacultural sludge can be utilized as fertilizer, thereby reducing the dependency on mineral fertilizer in hydroponic and aquaponic systems.

In this study, the yield and growth performance of lettuce in an open-air rooftop hydroponic system were investigated. Lettuce was grown in a closed recirculating nutrient film technique (NFT) unit using a standard nutrient solution (NS). Yield, fresh weight, and nutrient content in the leaf tissue of the harvested lettuce were measured. The results were compared with the results obtained in indoor hydroponic lettuce growth with artificial lightning. Despite strong winds during the growth period, 25% of the total lettuce heads weighed twice the marketable weight; however, 25% of the total lettuce heads were below the marketable weight. A more efficient nutrient uptake was indicated by the lettuces in the rooftop system compared with the uptake in the indoor system. Foliar analysis revealed a higher content of all nutrients in the leaves of rooftop hydroponic lettuce compared with indoor hydroponic lettuce. This study suggests that hydroponic rooftop-grown lettuce can be competitive with their indoor counterparts if the rooftop hydroponic system is protected from extreme weather conditions.

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