This thesis investigates the use of combined technologies, specifically combined biological and granular activated carbon in an expanded bed reactor, and sequential oxidation and biological treatment, for the removal of colour from yeast manufacturing wastewater. The colour in the yeast manufacturing wastewater (YMW) is due to the molasses used in the fermentation media, and this colour is attributed to melanoidins.
Calgon Filtrasorb 400 was selected from a range of activated carbons and was characterised for colour and COD removal using the Freundlich isotherm. The Calgon Filtrasorb 400 was used in a continuous expanded bed reactor to determine the effect of biological activity on the removal of colour over time. Two 7.2 L expanded bed reactors were operated as a continuous system for two months. They were operated at a HRT of 2.3-2.8 days, OLR of 6.5-8 kg/m /day, and colour loading rate of 32-40 kg PtCo/m3 /day. One reactor was operated as an anaerobic biological system, while the second reactor operated as a non-biological control. The anaerobic biological system removed COD at a rate of 2.2 kg/m3/day, and produced an average of 0.32 m3 CH4 / kg COD at steady state.
Hydrogen peroxide was then investigated at low concentrations for its effect on colour, COD, molecular weight distribution and on both subsequent anaerobic and aerobic biodegradation. Concentrations of hydrogen peroxide in the range of 2.5 g/L to 60 g/L were used to treat anaerobically pretreated YMW, and anaerobically and aerobically pretreated YMW. The affect on molecular weight was measured with SEC, the affect on anaerobic biodegradation assessed using the biomethane potential assay, and the affect on aerobic biodegradation assessed with an assay to measure changes in COD over time.
The major findings from this work are:
• Calgon Filtrasorb 400 removes colour and COD from YMW and 90% of the adsorption occurs in the first 24 hours. The colour and COD adsorption could be adequately described by the Freundlich isotherm to give the equations qe= 5C0.33 in PtCo units for concentration of colour and qe= 9.9C0.66 for COD respectively.
With respect to the anaerobic expanded bed reactor system:
• When comparing the colour removal of the biological and control system, it was shown that both systems removed colour for a short period until the GAC was saturated.
• The two reactors removed between 17-19 % of colour from the feed during two months continuous operation, and a total of 25 % of the colour including the stock preparation phase. The colour capacity of the GAC used in the expanded bed reactors was calculated to be between 3.2 and 3.8 kg PtCo per kg GAC.
• The biological reactor removed 41 % of the total COD fed to the reactor over two months continuous operation and 42 % of the COD overall including the stock preparation. The non-biological reactor removed a total of 21 % of the total COD added over the two month continuous operation and 25 % of the COD overall including the stock preparation phase.
• The biological activity did not improve the removal of colour, as measured by absorbance at 475 nm and the operating life of the GAC for colour removal was not increased by biological activity
• The conclusion was that this was not an appropriate system for treating YMW for colour on a continuous basis.
With respect to work with hydrogen peroxide:
• Hydrogen peroxide has a varied effect depending on the biological pre-treatment of the YMW.
• Hydrogen peroxide addition will remove colour from YMW.
• Hydrogen peroxide had no effect on the COD of high strength anaerobic pre-treated YMW, but did reduce the COD of the low strength anaerobic and aerobic pre-treated YMW at pH 8.
• Hydrogen peroxide did impact on the concentration of high molecular weight compounds present in YMW. However, at the hydrogen peroxide concentrations used, the high molecular weight compounds were not completely removed.
• Hydrogen peroxide had no measurable effect on the anaerobic biodegradability of the high strength anaerobic pre-treated YMW, while it had a detrimental effect on the low strength anaerobic and aerobic pre-treated YMW.
• Hydrogen peroxide had a measurable and positive effect on the aerobic biodegradability of the high strength anaerobic pre-treated YMW, while it had no measurable effect on the low strength anaerobic and aerobic pre-treated YMW. The hydrogen peroxide treatment does marginally improve the aerobic degradation of anaerobic pre-treated YMW by up to 12 % at hydrogen peroxide concentrations of 60 g/L. The findings suggest that an intermediate oxidation step between the UASB reactor and activated sludge reactor deserves further investigation.
Overall the work showed that an economic solution to removing colour from high strength YMW is still not available. The anaerobic expanded bed reactor with GAC is not a suitable technology for treating colour. However, the use of an oxidation step between an anaerobic and aerobic biological treatment set-up has some potential.