THE RESULTS OF THE Mg ADDITION EXPERIMENT .1 The Effect of Mg Addition on Nutrient Removal

Algal beads without Mg addition were most effective in phosphate removal with their 76%

phosphate removal percentage. The lowest phosphate removal (64%) was achieved in low and high Mg concentration treatments (Table 5). The efficiency of blank beads in phosphate removal was comparable to the efficiency of treatments with algal beads and suspended algal cells. After the cultivation the phosphate concentration of control aquaculture wastewater was about 2.5 times higher compared to treatments with algae or beads (Figure 18).

Figure 18. Phosphate concentrations (mg/l) in the treatments of Mg addition experiment after 4.5 days of cultivation.

The treatments containing algal beads were equally effective in nitrate and nitrite removal.

After 4.5 days of cultivation the nitrate concentration was below the detection limit of the analysis method (1.3 mg/l) in all the treatments containing algal or blank beads as well as in the control aquaculture wastewater. The reduction percentage of nitrate is thus over 63 in these treatments (Table 5). The treatment with suspended algal cells did not remove nitrate from the wastewater. Instead, the nitrate concentration of this treatment increased 44% during the cultivation, reaching 5.1 mg/l after 4.5 days of cultivation. Also the nitrite concentration of all the treatments containing algal beads decreased under the detection limit of the analysis method (0.007 mg/l) during the cultivation, which represents over 78% reduction in nitrite concentration. The efficiency of suspended algal cells in the nitrite removal was lower, only 56% (Table 5). In the control aquaculture wastewater and the treatment with blank beads the nitrite concentration increased 16- and 37-fold during the cultivation.

As can be seen from Figure 19, the differences in ammonia concentrations between the treatments with low, medium, and high Mg concentration were minor after 4.5 days of cultivation. The treatments with Mg-enriched beads could remove 95-96% of ammonia from the aquaculture wastewater. Ammonia concentrations of the treatments with low, medium, and high Mg concentration cannot be compared with the other treatments of the Mg addition

13,52 11,44 13,60 9,04 12,72 12,88 32,60

0 5 10 15 20 25 30 35

Low Mg

concentration Medium Mg

concentration High Mg

concentration Algal beads Suspended

algal cells Blank beads Control aquaculture wastewater

mg/l

PO

concentration

experiment, since the concentrations were measured after different cultivation times due to issues in the ammonia analyses. After 6 days of cultivation, algal beads could reach the highest ammonia removal of 96% between the treatments of algal beads, suspended algal cells, and blank beads (Table 5). Also the other two treatments performed well, removing over 90% of ammonia from the aquaculture wastewater. The ammonia concentration of the control aquaculture wastewater declined to 2.06 mg/l in the course of the experiment, which represents 88% ammonia removal. However, the standard deviations of control aquaculture wastewater as well as blank beads treatment were high (Figure 19).

Figure 19. Ammonia concentrations (mg/l) in the treatments of Mg addition experiment. The concentrations for treatments with low, medium, and high Mg concentration were measured after 4.5 days of cultivation and the concentrations for other treatments after 6 days of cultivation.

5.5.2 The Effect of Mg Addition on Algal Growth

The culture of suspended algal cells exhibited higher algal growth than immobilized cultures after 6 days of cultivation (Figure 20). The difference between the two culture types was clear as the algal concentration in the treatment with suspended algal cells was as high as 102  10⁵ cells/ml of culture medium while most of the immobilized cultures only reached the concentration of about 60  10⁵ cells/ml of culture medium. In addition, some algae was growing attached to the bottom of the culture bottle in the suspended algal cell culture, which

0,64 0,80 0,68 0,63 1,17 1,46 2,06

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Medium Mg concentration

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Algal beads Suspended algal cells

Blank beads Control aquaculture

wastewater

mg/l

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makes the total algal concentration of the treatment even higher. Magnesium addition did not seem to have a positive impact on the algal growth as the algal concentrations of the treatments with beads with low and high Mg concentration as well as treatment with beads without Mg addition were very close to each other (57.8-62.9  10⁵ cells/ml of culture medium). The treatment with medium Mg concentration, however, had clearly lower algal concentration compared to the other treatments.

If the algal concentrations are presented in the unit of cells/bead the concentrations for the treatments with low Mg concentration, medium Mg concentration, high Mg concentration, and algal beads without Mg addition are 29.4, 26.3, 40 and 42.5  10⁵ cells/bead, respectively. Even though the cells/bead concentration of low Mg concentration treatment is lower than the algal concentrations in the treatments with high Mg concentration and beads without Mg addition, the amount of algal growth was similar in these treatments. The number of algal cells in the beads at the end of the cultivation was about 14-15-fold compared to the initial concentration in all of these treatments, due to the difference in the initial algal concentration between the treatments, which was resulting from the smaller size of the beads in the low Mg concentration treatment.

Figure 20. Algal concentrations in cells/ml of culture medium in the treatments of Mg addition experiment after 6 days of cultivation.

4,1 62,9 39,2 57,8 59,1 102,0

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Initial concentration

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Medium Mg concentration

High Mg concentration

Algal beads Suspended algal cells 105cells/ml of culture medium

Algal concentration

6. DISCUSSION

6.1 EVALUATION OF THE EXPERIMENTAL SET-UP AND THE SOURCES OF