Process for nutrient recycling from intensive aquaculture through microalgae-bacteria consortium.

This work studies a biological process based on a microalgae-bacteria consortium for recycling nutrients in a recirculating aquaculture system (RAS) implanted in an intensive marine aquaculture farm. Additionally, some techniques were used for microalgae biomass harvesting and tested the effectiveness of filtration by a column with multi-layer sand to reduce the solids concentrations in the effluent. The consortium was grown in photobioreactors in batch and semi-continuous operation modes using the solids concentrated stream generated in the RAS system. The semi-continuous operation showed a high percentage of TDN and TDP removal, achieving final concentrations of 1.09 ± 0.02 mg·L-1 and 0.01 ± 0.01 mg·L-1, respectively, while DOC was reduced to 3.87 ± 0.06 mg·L-1. The values of productivity 44 ± 9 mg TSS·L-1 indicated that the studied stream is a suitable culture medium for the growth of the microalgae-bacteria consortium. A combination of harvesting techniques was studied, coagulation-flocculation-settling and coagulation-flocculation-flotation. The first step was to optimise the dose of FeCl3 through the coagulation-flocculation test to pre-concentrate the biomass generated, achieving an optimal dose of 0.106 mg Fe·mg TSS-1. Then, two separation processes were applied to the stream and compared: settling and flotation. The maximum removal efficiency (90.2 ± 0.3 %) was obtained in the settling process, so the coagulation-flocculation-settling was select as the best combination of harvesting techniques. Finally, sand filtration was studied as an effluent refining process to improve solids reduction of the water obtained in the harvesting step resulting in an effluent with 17.18 ± 1.49 mg TSS·L-1. The proposed sequence process is capable of recycling nutrients from an intensive marine aquaculture farm by using these resources via transformation into microalgae biomass and generating quality effluent.

Recycling "waste" nutrients back into RAS and FTS marine aquaculture facilities from the perspective of the circular economy.

The feasibility of use microalgae biotechnology to improve water quality together with the production of biomass to replace fish meal or fish oil in two marine fish farms with different production systems were studied. The samples were taken from a flow-through system (FTS) and a recirculating aquaculture system (RAS) with sea bass cultures of 300 g and 120 g, respectively. The most suitable stream for microalgae cultivation was that from RAS as the concentration of N in the microalgae reactor influent should be ≥8 mg TN L-1 to operate at the same hydraulic retention time than the solids retention time, independently of the productivity of the reactor. Tetraselmis chuii were cultured in 18 L bubble column reactors under batch and semi-continuous operation in media that mimic a RAS stream. The results showed that RAS systems enriched with trace metals generate viable streams for microalgae growth with average biomass productivity under semi-continuous operation of 69 mg TSS L-1 d-1. Nutrients concentrations at the end of the experiment under semi-continuous operation were 0.76 mg TDN L-1 and 0.01 mg TDP L-1, similar to those in the make-up water of the RAS. The composition of microalgae biomass obtained shows that it could be optimal as a substitute for fish meal in sea bass feed.

Lipid production of microalga Ankistrodesmus falcatus increased by nutrient and light starvation in a two-stage cultivation process.

The aim of this work was to study the stimulation of lipid production on the microalga Ankistrodesmus falcatus by varying cultivation conditions during the stationary phase. The effect of three factors (presence and absence of nitrogen, phosphorus, and light) has been tested once the cultures reached the stationary phase with the aim to increase the value of the biomass for further applications. Lipid content, elemental composition, Nile red fluorescence evolution, and calorific value of microalgal biomass were studied as well as biomass growth. Biomass presented a lipid content of 36.54 % at the end of the first stage, while at the end of the second stage, the experiments with the absence of phosphorus increased their lipid content until 45.94 and 44.55 %, the first with nitrogen and light presence and the second with absence of all factors. The combination of phosphorus absence and nitrogen and light presence achieved the highest lipid productivity (20.27 mg/L/day). The two-stage strategy to culture microalgae is a feasible option to increase the economic or energetic value of biomass.

Capability of different microalgae species for phytoremediation processes: wastewater tertiary treatment, CO2 bio-fixation and low cost biofuels production.

Scenedesmus obliquus, Chlorella vulgaris, Chlorella kessleri and a natural Bloom were cultivated in batch experiments, under controlled conditions, in urban wastewater (WW) and synthetic wastewater (SW) under 5% CO2 in air, with the object of estimating their capacity for nutrient removal, carbon dioxide biofixation, and generation of valuable biomass. In both culture media, the Bloom (Bl) and Scenedesmus (Sc) showed higher final biomass concentration (dried weight, dw) than the other species; the maximum yield obtained was 1950 ± 243 mg L(-1) for Bl and the minimum 821 ± 88 mg L(-1) for Cv, both in synthetic wastewater. Maximum specific growth rate values do not show significant differences between any of the 4 strains tested (p ≤ 0.05), nor between the 2 culture media. A new homogeneous method of calculating productivities has been proposed. Nitrogen removal in all the reactors was higher than 90%, except for BlSW (79%), and for phosphorus, the removal was higher than 98% in all trials. Maximum CO2 consumption rates reached were 424.4 and 436.7 mg L(-1) d(-1) for ScSW and ScWW respectively.

Comparing the use of different domestic wastewaters for coupling microalgal production and nutrient removal.

The streams from municipal wastewater treatment plants (WWTP) have been considered a valuable medium for mass cultivation of algal biomass. The aim of this work is to test and compare the performance of Chlorella vulgaris on several streams from five stages, from two different WWTP. The results showed biomass yields ranging from 39 to 195mg dry-weightl(-1)days(-1). The best performance as biomass production was obtained with the centrate (effluent from drying the anaerobic sludge). After testing a wide range of N/P ratios with centrate, the highest productivity and growth rates were obtained with the original N/P ratio (2.0) of this stream. The highest removal rates were of 9.8 (N) and 3.0 (P) mgl(-1)days(-1), in the centrate. Finally, this research also suggests that microalgal production seems to be a promising process when coupled to wastewater treatment.