26 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG In certain groundwater types, iron concentrations were high. Extended aeration followed by filtration was sufficient for iron removal at pH>8. Low salinity (5-15 ppt) or ‘odd’ ionic compositions did not appear to inhibit microalgal growth, except for centric diatoms. In the ‘worst case’ example of VAM groundwater (salinity 27 ppt), growth rates of nondiatom microalgae (Nannochloris sp., Nannochloropsis salina, Porphyridium cruentum and Rhodomonas sp.) were low, while limestone deposition continued after passing 0.2-µm filters (Fig. 4; 1Rijstenbil, unpublished results). Phosphate was required to initiate cell growth. Growth rates were low, probably as a result of a calcium phosphate precipitate (Narasarakju et al. 1976, Griffioen 2006). Zeolite may remove heavy metals and toxic organic substances but may adsorb ammonium and phosphate (Mumpton 1999), but this has not been demonstrated in saline groundwater. Zeolite addition had no positive effect on non-diatom microalgae. It may be a problem to grow ‘feed’ diatoms (Skeletonema costatum and Melosira nummuloides) in in hard groundwater (e.g. VAM). The groundwater at VAM was treated as follows: 1) the SO4:Cl balance was corrected to seawater average, 2) water was passed through Chelex® 100 resin or EDTA was added, both to chelate heavy metals, 3) more EDTA was added to reduce hardness (Mg and Ca complexation) and 4) water was autoclaved to reduce hardness and destroy possible heat-labile inhibitory substances. No growth of S. costatum occurred, even after adding nutrients, trace elements and vitamins. In a separate series, Na+, SO4 2-- and Mg2+ were added to seawater values of K:Na, Ca:Mg, (Ca+Mg):(Na+K) and SO4:Cl; also, extra phosphate was incorporated. In this VAM filtrate, the diatom M. nummuloides did not grow well, however cells remained intact. After dilution (1:1) of this balanced VAM medium with distilled water or municipal wastewater effluent, the diatom grew well. Because ‘non-diatom’ microalgae were not inhibited in VAM water, it was unlikely that a toxic compound was involved. As it looks now, balancing the ions is more important for diatoms than nondiatom microalgae. Saline Groundwater for Bivalve Aquaculture Juvenile stages of blue mussels Mytilus edulis or Manila clams Venerupis (Ruditapes) philippinarum have been used for bioassay experiments to determine suitability of saline groundwater types from the Delta Area of the southwest Netherlands. If salinity was low, groundwater was mixed with full-strength seawater, in which case groundwater nutrients supported algal growth; bivalves were fed with microalgae. If problems occurred, such as growth inhibition or mortality, groundwater was treated chemically or the selected groundwater source was abandoned. At La Solitude, with brackish groundwater (depth -160 m) used in open ponds, microalgae grew but mussels did not filter feed and died eventually. Initially the problem was solved by mixing groundwater with seawater. It was not known if the cause was a toxic compound or low salinity but it appeared to be related to hardness because the (Ca+Mg):(Na+K) ratio was twice as high as in normal seawater (Fig. 3). When Ca2+ and Mg2+ were bound by EDTA, or Na+ and K+ were supplemented to obtain concentrations of normal seawater, mussels started filtering again. Groundwater types (SEAFARM, Grovisco) that were good media for microalgae after nutrient enrichment sustained lower growth rates for Manila clam spat.2 At similar initial algal concentrations, the growth rates of clam spat fed with microalgae grown in the same groundwater were far below those fed with microalgae grown in natural seawater, 50 percent and 85 percent lower, respectively. Some substance(s) in the groundwater, incorporated by microalgae, might have affected the edibility, digestibility or palatability of the feed microalgae. Water itself was not a problem for organisms fed on microalgae grown in natural seawater. In groundwater from VAM, Manila clams had high mortality although they were fed with diatoms grown in seawater. Ion correction by exchanging Ca2+ for Mg2+ using zeolite and adding SO4 2-- to obtain ‘seawater average’ concentration only had a minor positive effect. As with diatoms, hardness may have been the major problem here. Based on experiences with microalgae, toxic inorganic substances in VAM water could be ruled out. Organic toxins can be bound by activated-carbon, heat or ozone, but this would also remove ‘probiotic’ substances. It may not be possible to remove one inhibitory factor selectively, even if identified, particularly in full-scale aquaculture systems. We could follow a water treatment scheme as drawn in Figure 5. If this treatment scheme does not help, another groundwater source must be drilled at a different place or depth to start again with new groundwater. Water Treatment in Large-Scale Facilities The treatment scheme may be an effective tool to tackle a groundwater problem on an experimental basis. However, because of the high costs, water treatments as outlined in Figure 5 are not always possible in a large-scale production facility. The first step of Figure 5, intensive aeration and filtration (iron removal), must be considered necessary in almost all cases in the southwest Netherlands. If a certain groundwater source lacks nutrients for sufficient microalgae growth, nutrient addition in low amounts can increase production, which can make use of groundwater in aquaculture economically feasible. If ion correction is necessary, treatment by adding salts is not a feasible option because it is too expensive. ’Natural’ ion exchangers (clays, zeolite) must be recharged regularly, which costs energy and chemicals, depending on the ion and the amount of correction and may not be effective at all in chloride-rich environments. If metals are the problem in a groundwater source, removal with Chelex® resin columns is too expensive to be applied on a large scale. If organic compounds are the disturbing factor, mild ozone or hydrogen peroxide treatment and active-carbon filtration can be used to treat water, depending on the volume required. Previous testing with bioassays ruled out harm to cultured organisms by such treatments. However, more than 50 percent of the groundwater sources in southwestern Netherlands do not need additional water treatment. If a groundwater source is assessed as suitable for aquaculture, another aspect has to be considered. Groundwater may contain components that do not directly cause growth inhibition to an organism, but may accumulate over time, such as
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