Impact of pre-ozonation on distillery effluent degradation in a constructed wetland system
Thesis (Msc Food Sc (Food Science))--University of Stellenbosch, 2007.
Distilleries are an example of an agricultural industry that generates large volumes of wastewater. These wastewaters are heavily polluted, and due to the seasonal nature of the product, the amount and composition of the wastewater may exhibit major daily and seasonal variations. Wine-distillery wastewaters (WDWWs) typically are acidic (pH 3.5 - 5.0) and have a high organic content (sugars, alcohol, proteins, carbohydrates and lipids), a COD range of 10 000 – 60 000 mg.L-1, have a high suspended solids content as well as containing various inorganic compounds. Additionally refractory compounds present in these wastewaters, such as polyphenols, can be toxic for biological processes, making the selection of a suitable treatment process problematic. Wetlands have been shown to be a feasible treatment for effluent originating from wine, however, they are normally used as a secondary treatment method and not well suited for high volume, high COD (> 5 000 mg.L-1) wastewaters. Ozone has been successfully used as a pre-treatment for WDWW due to its oxidising capabilities to partially biodegrade organics and non-biodegradable organics, and reduce polyphenols, which results in an increase in biodegradability. Currently a wetland system is being used on its own at a distillery to treat wastewater from a series of stabilisation dams, but the legal requirement for discharge into a natural resource (COD < 75 mg.L-1) is not being met. Additional treatments suited for WDWW are therefore being considered. Wine-distillery wastewater was characterised and found to show a large variation over time (COD ranging from 12 609 - 21 150 mg.L-1). Ozonation of WDWWs was found to be effective in decreasing COD over a wide range of organic loads. For pre-wetland wastewater from the distillery, an average COD reduction of 271 mg COD.g O3-1 was found, and for post-wetland effluent, an average of 103 mg COD.g O3-1. The effect of ozone on the biodegradability of the wastewater was monitored by activity tests, and a low ozone dose (200 - 400 mg O3.L-1) was found to increase activity in terms of biogas, methane and cumulative gas volumes. By showing an increase in the biodegradability of WDWW, it was concluded that ozone has potential as a pre-treatment step to increase the effectiveness of a biological wetland system. Lab-scale wetlands were used in trials to determine the effect of pre- and post-ozonation on WDWW. It was found that the efficiency of the wetland receiving the pre-ozonated “off-season” WDWW (2 200 mg COD.L-1) had a higher COD reduction (73%) than the wetland fed with untreated (62% COD reduction) WDWW, and the total polyphenol content was reduced by 40 and 31%, respectively. Treatment efficiency in terms of the reduction of colour, total solids, suspended solids and phosphates were also greatly improved for the pre-ozonated WDWW. Similar results were found when treating high COD “peak season” (7 000 mg COD.L-1) WDWW, with higher reduction rates for the wetland treating pre-ozonated WDWW (84% COD reduction) than for the wetland fed with untreated WDWW (74% COD reduction), and the total polyphenol content was reduced by 76 and 72%, respectively. Post-ozonation was also shown to be beneficial in that it improved the final effluent quality leaving the wetland system. Increasing the hydraulic retention time (HRT) of the wetlands from 9 days to 12 days resulted in similar COD reductions for the control and experimental wetland, highlighting the benefits that pre-ozonation has on reducing the acclimatisation period. Therefore using ozone as a pre-treatment could help in reducing the wetland size, HRT and allow increased volumes of wastewater to be treated. In this study ozone was successfully utilised to reduce COD levels in wine-distillery wastewater, and increase the biodegradability of the wastewater. This study also showed that ozone, used as a pre-treatment to a wetland system, can contribute to improving the performance of a wetland system in terms of higher removal efficiencies. Wetlands are, however, unsuited for treating high strength COD wastewater, and the final effluent was still well above the South African legal limit for direct discharge into a natural resource. The results obtained during this study contributed to developing a method to achieve a more efficient treatment system utilising wetlands for the distillery industry, and can be of value in facilitating efficient environmental management.