Water from recirculating aquaculture systems (RAS) has been shown to be a suitable growth medium for microalgae and their cultivation can, therefore, be used to reduce RAS emissions. However, while efficient wastewater treatment is possible, the nutrient content of RAS water limits attainable microalgae biomass densities to 1-2 g l-1 at best, which requires frequent harvesting of microalgae. We have taken advantage of the constant evaporation of water from an open thinlayer photobioreactor (200 l volume, 18 m 2 illuminated surface, artificial supply of CO2) to continuously add water from RAS to a microalgae culture and thereby provide nutrients for continued growth while evaporating all water. To test for a possible inhibitory effect of RAS water on microalgae growth, components of mineral medium were omitted stepwise in subsequent cultivations and replaced by RAS water as the only source of nutrients. This approach showed that microalgae can be grown successfully for up to three weeks in RAS water without additional nutrients and that high (20 g l-1) biomass densities can be attained. While growth in wastewater did not reach productivities measured in mineral medium, analysis of growth data suggested that this reduction was not due to an inhibitory effect of the RAS water but due to an insufficient supply rate of nutrients, even though RAS water contained up to 158 mg l-1 NO3-N. It is, therefore, concluded that this method can be used to fully treat the wastewater discharge of a RAS. Furthermore, because both water evaporation from and microalgae growth in the photobioreactor correlated positively with each other due to their shared dependency on solar radiation, supply of nutrients continuously adjusts to changes in demand. It is estimated that the area of a photobioreactor required to treat all emissions of a RAS requires approximately 6.5 times the area of the latter.

A liquid digestate rich in ammonium nitrogen (8.3 g L −1 ) was collected from an agricultural biogas plant and supplied to microalgae cultures as their only nitrogen source. Chlorella vulgaris was cultivated for up to 21 days, both under controlled conditions in laboratory-scale glass-column photobioreactors as well as outdoors in an open pilot-scale thin-layer photobioreactor. By systematically addressing issues associated with the use of liquid digestate (i.e., turbidity, nutrient imbalance, ammonium toxicity, and acidification), microalgae were robustly cultivated at a high density and cultures achieved a net biomass dry weight of between 10 and 14 g L −1 , and a productivity of up to 0.93 g L −1 d −1 (93% of maximum expectation). Cultivation in the thin-layer photobioreactor achieved areal productivities between 7 and 10 g m −2 d −1 . Water acidification due to the uptake of ammonium by microalgae was prevented by a controlled addition of NaOH. A detailed mass balance showed that, despite high removal efficiencies (approximately 3% of the supplied nitrogen remained in the medium), microalgae assimilated only 40-60% of the supplied nitrogen and, consequently, a large amount of nitrogen was lost to the atmosphere.

Ethoxyquin (EQ) is an antioxidant that has, to date, been commonly used in feed production. Reports on the detrimental effects of this substance on vertebrates are growing, but effects in aquatic systems have rarely been described. Therefore, the present study was conducted using serial concentrations of EQ ranging from 0.03 to 16.5 mg l −1 to determine effects on 3 types of aquatic organisms. In zebrafish, 5 mg l −1 EQ caused mortality (25%) and a further 62.5% of the embryos showed yolk sac edema as well as deformed bodies or missing eyes. Furthermore, all the investigated EQ concentrations decreased the heart rate of the embryos. The lowest observed effect level was 0.31 mg l −1 . In addition to zebrafish, the study also used water fleas Daphnia magna and green algae (Scenedesmus obliquus and Chlorella vulgaris). These treatments revealed that daphnids are also sensitive to EQ, exhibiting detrimental effects with a half-maximal effective concentration (EC 50 ) of 2.65 mg l −1 after 48 h of exposure. The algae appeared to be at least 2 times less sensitive to EQ than fish embryos or daphnids. The results were used to calculate the risk for aquatic life resulting in a maximum tolerable level of 1 µg l −1 for fish embryos and daphnids, with a safety factor of 300. According to current knowledge, this does not exceed environmental concentrations of this substance. However, this study raises further concern about the (until recently) legal maximum tolerable EQ levels in fish feeding and the rather slow pace at which authorization to use EQ as a feed additive for diverse animals in Europe is being suspended.

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