141 In the control raceway with no microalgae, no significant

copper or phosphate removal occurred (copper < 1.6% ± 0.5, phosphate < 0.8% ± 0.1,), although between 5 and 20% of the

ammonia was removed. The ammonia removal did not show any trend of increase or decrease over time and followed no obvious pattern. However, it appeared to be temperature related, increasing on hotter days and reducing on cooler ones.

6.4 Discussion

6.4.1 Copper removal by dried biomass

Dried Tetraselmis sp. (TSAW92) biomass removed copper from solution to a final loading of 28 ±0.3% by weight (Table 6.1). There is little published work in this area, in some cases due to commercial interests. A small scale industrial metal removal process based on dried algal biomass has been developed by B.V. Sorbex Inc. of Montreal, however no hard data on its performance is available, a similar process called Algasorb has also been developed (Anon.). It is therefore very difficult to say how this system

compares to others in the field, although the favourable

comparability with Chelex 100, a commercial metal removal resin, is encouraging. The results for macroalgal biomass were somewhat disappointing with very low levels of copper loading being

achieved (Table 6.1). One possible explanation for this result is that

the dried microalgal biomass had a far higher specific surface area than the dried macroalgal biomass. The microalgal biomass was

i

initially composed of individual cells which were dried to a cake and then ground to uniform grain size, but the macroalgal biomass was dried tissue, subsequently ground to the same size. This

potential difference could be checked by the use of microscopy, or by repeating the experiment with macroalgal biomass which had been ground very finely prior to caking.

The final grinding of the dried biomass also had a major impact on the practicality of the column method. If the biomass was ground too finely it packed down hard and blocked the flow of liquid through the column. On the other hand, if the biomass was left too coarse the liquid poured rapidly through the column and was found to contain high levels of copper.

It is clear that a careful choice must be made between live and dead biomass for each individual application, and that great care must be taken in selecting the appropriate grain size for the dried biomass. It is possible that the column method with its inherent risk of blockage is not the best technique for metal removal using dried biomass. One possible alternative would be the use of a

fluidisôd bed in which the biomass granules were suspended in j solution by balancing their negative buoyancy with an up-flowing I

current of waste water. Using this system it would be possible to prevent blockages and to fine tune the flow rate/ residence time in response to the outflow quality during metal removal. Preliminary experiments were carried out using home-made equipment of this type but proved to require design and construction techniques out with our abilities.

143

6.4.2 Copper removal by live Tetraselmis sp. (TSAW92) biomass

There is a clear gradient in copper loading ability running | down from live to dry Tetraselmis sp. (TSAW92) cells (Table 6.1).

Although, the total reduction in copper loading from live to dried

Tetraselmis sp. (TSAW92) cells is only 9 ± 0.5%, and even at the lower levels it compares very well to commercial resins (eg 38 ± 1.9% for Chelex 100). Therefore, the compromise between the robustness of dried microalgal biomass and the higher metal loading potential of live biomass may not be of vital importance, especially if the production costs of the biomass are low.

The use of live microalgae on the raceway system removed

the need for potentially expensive biomass drying and processing, tj I and enabled the growth of biomass to be coupled with continuous

metal removal. The use of live algae also provided the opportunity I to remove nutrients from the waste stream (Oswald, 1988c).

Initially a dual raceway was set up using a non-metal accumulating alga Phaeodactylum tricornutum followed by the non-metal

tolerant alga Botryococcus hraunii. This system was used as a control for the presence of non-specific algae or for some type of bacterial or physical metal removal process. No significant metal removal was observed (< 2% ± 0.7) and the metal sensitive alga died out completely within the week. This suggests that a generic

microalgal presence, with associated high pH and leached organic compounds, does not cause any significant metal removal or state change. In other words, a specific metal removing alga is required for effective treatment. However, it also presents the possibility of

including Botryococcus hraunii. prior to the final outflow as a bio­ indicator of metal removal efficiency.

A dual raceway was then set up with the metal accumulating / green alga Tetraselmis sp. (TSAW92) on the upper level followed

by the nutrient removing green microalga Dunaliella tertiolecta on the lower level. This system was found to remove 100% of the applied copper and ammonia and 87% of the applied phosphate. In another study an artificial algal meander system was reported to

remove 99% of the heavy metals from a mine waste stream, | although specific details of the running conditions were not given

(Hassett et al., 1979). A laboratory scale batch culture wastewater treatment system based on Scenedesmus sp. was capable of