Wastewater from a biodiesel plant: characterization of streams and suggestions for treatment options

Abstract: Biodiesel is an important alternative to ordinary diesel when limiting the effects on the climate. However, the production path of biodiesel produces a wastewater with a high chemical oxygen demand (COD) and it needs to be cleaned. The aim of this study was to characterise biodiesel wastewater as well as to suggest treatment options. The biodiesel wastewater studied came from a biodiesel plant in the south of Sweden. The wastewater is generated when the biodiesel is washed to remove the by-product glycerol, the reactant methanol and a catalyst. Streams from different processing steps were investigated as well as the final mixed stream that today is sent for treatment to at plant in Kristianstad about 56 kilometers away. The wastewater was characterised by measuring general parameters such as total bound nitrogen, total phosphorus, COD, pH and density. The results varied between the streams but the results of the final mixed stream will be presented here. The mixed water was found to have a high COD of 241 g/L, higher than many compared studies. It also had a high total phosphorus content of 1480 mg/L and a nitrogen content of 57 mg/L. The pH was neutral around 6 and the density was 996 g/L, which is somewhat lower than the density of water. When the samples were visually examined it was found that the mixed streams were most cloudy whereas some of the non-mixed streams were clear. This was supported by analysing the turbidity of the samples, which also indicated that there were many particles in the water. To find out if the COD was dissolved or particulate the samples were microfiltrated. Prior to the microfiltration the particle size distribution was determined to decide the pore size of the membrane, which was chosen to 0.2 µm. The analyses of the permeate showed that the turbidity decreased but the COD remained virtually unchanged. This meant that the COD was mainly dissolved. To evaluate the origin of the COD the streams were characterised further with HPLC and gas chromatography (GC). From the HPLC it was found that the final mixed stream had high concentration of glycerol of 41.7 g/L. This is higher than the compared studies. The results from the GC showed that the streams contain residues of biodiesel and some other unidentified components. Anaerobic digestion was evaluated as a treatment method by testing the biochemical methane potential (BMP) for the wastewater. The BMP of the mixed stream was 185.5 NmL CH4/g COD. It was found that codigestion of the wastewater with the glycerol by-product would have a higher BMP of 263.8 NmL CH4/g COD. This means that not only would the addition of glycerol to the stream generate a larger volume for anaerobic digestion but the potential of that stream would be higher. The results from the BMP tests were used to estimate the biogas production. The biogas production was then compared to the mean yearly energy consumption of a Swedish house. Using only the mixed stream the yearly production could cover the consumption of 8 houses per year. With the codigestion of the streams the yearly production could cover the yearly consumption of 67 houses.