An Evaluation Of Freshwater Microalgae For The Treatment Of Wastewater Centrate And Production Of Biofuel Feedstock

Abstract

The continued use of fossil fuels is now widely regarded as unsustainable both economically and environmentally; as such, there has recently been much interest in biofuels derived from alternative and renewable sources. One of the most promising alternatives is microalgae because they can grow almost anywhere given CO2 and a nutrient source. However, limited water and nutrient supplies add significant costs to microalgae production. Centrate is a nutrient-rich liquid byproduct from the dewatering of wastewater solids that treatment facilities currently spend millions of dollars each year to treat. The aim of this study was to determine how well three strains of freshwater microalgae could grow on centrate and what level of nutrient removal could be achieved.Neochloris pseudostigmata, Neochloris conjuncta, and Scenedesmus dimorphus were studied using three different growth systems; the selection of these strains was based off their ability to grow in the presence of high concentrations of centrate. Two small-scale sterile systems were constructed and operated: a batch system and a continuous flow system. Additionally, a large-scale, non-sterile system was operated to verify that a microalgae species could survive in an environment that more closely mimics a real world scenario. Batch experiments were carried out in triplicate at 4 and 25% centrate concentrations for a duration of 49 days on N. pseudostigmata and N. conjuncta. The best results were observed using N. conjuncta in 4% centrate. N. conjuncta reduced the ammonia concentration by 70.1 ± 3.0% and the orthophosphate concentration by 42.2 ± 7.5% with a final dry weight algal density of 252 mg/L. The continuous flow experiments were run in duplicate at an HRT ranging from 4 to 9 days using N. conjuncta in 4% centrate. At a 9-day HRT, removal efficiencies of 41.2 ± 0.0% and 29.5 ± 0.1% were observed for ammonia and orthophosphate, respectively. The maximum dry weight algal density was 151.6 mg/L. Scenedesmus dimorphus was the strain examined in the large-scale experiment and results indicated that this strain could survive and remove nutrients in this type of growth system.