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The aim of the study was to determine the changes in suspension particle size identified in biologically treated wastewater, which was then treated in hydroponic system with use of engineering lighting by the light-emitting diodes (LED). The study was subjected to wastewater purified under laboratory conditions, in a hydroponic system using the effect of macrophytes Pistia stratiotes and growing algae. Measurement of particle size was made using a laser granulometer. Analysis of the results showed that the additional lighting of the hydroponic system with LED can significantly influence the ability of the suspension particles to agglomerate and, consequently, determine their sedimentation properties. In hydroponic system supported by additional lighting, more particles were observed with equivalent diameter D(3.2) smaller than 10 μm than those in the tank without additional lighting, indicating a higher reactivity of the particles. Determining the size of equivalent diameters D(4.3) allowed us to observe that in hydroponic system, particles of relatively small size predominate, which negatively affects the sedimentation process of the suspensions. Determination of particle size of suspensions consisting mainly of algae and the dynamics of their changes are the basis for specification of an effective method of removing particles from the system to protect the receiver from excessive suspension concentrations.

The haptophyte Pavlova lutheri was cultivated in a 4-l flat alveolar photobioreactor (FAP) in semi-continuous mode during 35 days, achieving a mean volumetric productivity of 0.4 g l-1 day-1. The microalgal culture was harvested from the photobioreactor at an average concentration of 3.5 g l-1 and subjected to different storage conditions in which the effects of temperature (1 °C, 4 °C and 25 °C), air supply and preservative (ascorbic or propionic acid) addition on the viability of the stored suspensions were investigated. Low temperatures (1 and 4 °C) and air-bubbling extended the shelf-life of the stored suspension, while no beneficial effects of ascorbic or propionic acid were found. The influence of temperature and air-bubbling on fatty acid content and profile was also evaluated over a 23-day period of storage. Air-bubbled suspensions exhibited, on the average, a 70% decrease of total fatty acid content (TFA) during storage. Non-bubbled suspensions showed a much lower decrease. The decrease mainly involved the saturated 14:0 and 16:0 and the monounsaturated 16:1n-7 and 18:1n-9. The polyunsaturated eicosapentaenoic acid (20:5n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) decreased significantly only at the end of the storage period. A feeding trial on Pacific oyster (Crassostrea gigas) larvae was carried out with fresh and preserved P. lutheri biomass kept under the best storage conditions previously defined (1 °C; darkness; air-bubbling). After 14 days of rearing no significant differences in growth or survival were detected when fresh or preserved P. lutheri was used in a monospecific diet or in association with the diatom Chaetoceros calcitrans forma pumilum.