A method for reclaiming nutrients from aquacultural waste for use in soilless growth systems

Abstract

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.

Hydroponic Lettuce Cultivation Using Organic Nutrient Solution from Aerobic Digested Aquacultural Sludge

“…There was only a small decrease in TP, while an increase in TN of 30.1% was indicated. Such an increase has also been observed during AD in other studies [16], and can partly be explained by nitrogen fixation. It has been reported that some free-living heterotrophs can fix significant levels of nitrogen without the direct interaction with other organisms [21].…”

“…For removal of residual solids from the clear phase after AD, flocculation and sedimentation, using chitosan as coagulant, was conducted. According to experiences from previous research with similar sludge [16], a commercial chitosan (Kitoflokk High from Teta Vannrensing AS, Norway) at a dosage of 15 mg L −1 was applied. The chitosan had a molecular weight of 161 kD and a degree of deacetylation of 80%.…”

“…Aquacultural sludge contains high amounts of nutrients, the majority of which are bound to organic matter [13,14]. In order to increase the ratio of plants' available nutrients in aquacultural sludge, aerobic digestion (AD) has been used to solubilize nutrients from organic compounds while breaking them down [15][16][17]. The solubilization step should be followed by a solid precipitation step in order to obtain a good quality solids-free phase, in compliance with water quality guidelines for hydroponic and aquaponic systems.…”

“…The solubilization step should be followed by a solid precipitation step in order to obtain a good quality solids-free phase, in compliance with water quality guidelines for hydroponic and aquaponic systems. Chitosan, which is a natural polymer made from shrimp and crab shells, has been shown to be an effective flocculant for solids precipitation from aquacultural sludge [16]. Unlike other metal-based coagulants and synthetic polymers for the flocculation and precipitation of solids, chitosan does not combine chemically with dissolved phosphate and has no health concerns [18].…”

“…The aim of this study was to demonstrate how aquacultural waste can be processed and utilized as a valuable nutrient solution for hydroponic growth. Aquacultural sludge was processed by AD, and clarified by chitosan flocculation and sedimentation, as described by Ezziddine et al, 2020 [16]. The performance of the recovered ONS solution on lettuce growth was assessed by cultivation experiments where the fresh weight of lettuce, yield, water consumption, and nutrient and heavy metal content in the leaf tissue were measured.…”