Biologiska behandlingsmetoder för rening av rejektvatten från biogasproduktion
In this literature review the characteristics of two free-floating macrophytes, water hyacinth (Eichhornia crassipes) and duckweed (Lemna sp.), and two microalgae, Chlorella and Scenedesmus, have been examined regarding their suitability as efficient nutrient removers in the treatment of wastewater with high levels of nutrients and suspended solids from a biogas plant in Loudden. The needs required for the plants to be able to grow in wastewater and the amounts of biomass they can produce have also been studied. The results show that Chlorella is capable of a very high uptake of nutrients in photobioreactors with high nutrient loadings. With an ammonia uptake maximum value at 10900mg/m2/d Chlorella outshines the other organisms in this study. The test organism that performed the closest to Chlorella in terms of nitrogen uptake was water hyacinth with an uptake about 1602mg/m2/d. One factor affecting nutrient uptake in a positive way is the growth rate. Free-floating macrophytes produce more biomass than algae do, and water hyacinth have been shown to be the most productive. It is important to conduct a regular harvest of the plants if a high production should be maintained. High quantity of biomass per unit area can inhibit the growth, and algae are more sensitive to this than the macrophytes often suffering from self-shading when the density is too high. The high level of nutrients in the wastewater prevents growth and dilution is required to achieve any growth at all. Therefore, conventional treatment methods might prove to be a better option.