Providing Base Line Data for the Treatment of Mauritian Sugar Factory Wastewater by the Upflow Anaerobic Sludge Blanket (UASB) Process from Pilot-plant Study

Abstract

Although the Mauritian sugar factories have to comply with the relevant norms applicable to discharge of  effluents, they are not yet equipped with a conventional secondary biological wastewater treatment  method. There is thus a need to find an appropriate and cost-effective treatment system for the factories.  The upflow anaerobic sludge blanket (UASB) process was compared with other anaerobic treatment systems and has been found to be more advantageous and appropriate for the Mauritian sugar industry. In that context, a pilot study was carried out to determine the performance of the UASB process in treating sugar mill effluent and to provide base-line operational and design data for the reactor  on a large scale. A sugar factory wastewater was characterised and the data were used to prepare the molasses-based synthetic influent involved in the pilot plant, thereby simulating real effluent. It was found that the real factory wastewater had an average chemical oxygen demand (COD) of 1065 mg/L and was deficient in nutrients and alkalinity. The investigations indicated that there was an influence of the  hydraulic retention time (HRT), the only operational parameter, on the steady state COD removal efficiency of the reactor. At a HRT above 4 hours and an average organic loading rate of below 6.7 kg COD/m3.day, the COD removal efficiency was found to be almost independent of the HRT and  systematically remained above 76%. At shorter HRT, the COD removal efficiency decreased sharply. The optimum HRT of a UASB reactor was found to be between 4 and 6 hours. The inert soluble COD of the synthetic wastewater was found to be 16% of the total COD, which passed through the reactor  unaffected. It was concluded that it is feasible to design and operate an UASB reactor to treat Mauritian sugar mill wastewater up to a COD removal efficiency of 90% at a HRT of 6 hours. However, the effluent should be pre-treated in terms of addition of alkalinity and nutrients. Also, in order to achieve compliance
with the effluent discharge standards using a UASB reactor, a post-treatment should be applied.

Keywords: sugar factory effluents, anaerobic treatment, UASB process.