Evaluation of a MBR for treating slaughterhouse wastewater in Montevideo, Uruguay

Abstract

In Uruguay, the problem of eutrophication of the rivers is every time more concerning. One of the biggest contributors are slaughterhouses, where they usually have ponds treatment system and do not comply with the discharge Standards, especially regarding to nutrients.

This research aims to evaluate the performance of a pilot scale membrane bioreactor (MBR) for slaughterhouse wastewater treatment, in order to minimize the impact of their effluent discharge in rivers. It was carried out at one of the main slaughterhouses in Uruguay namely Schneck. The MBR consists on an anoxic compartment followed by one aerobic that contains a recirculation pump in order to recycle to the anoxic.

The MBR was placed in order to take its influent from the first step of the treatment plant that was a homogenization basin. After some drawbacks, the MBR was inoculated with a domestic wastewater treatment plant and started operating. Some periods of trials were necessary until it reached a steady state, where it was operated with a recirculation ratio of 4 and an average dissolved oxygen of 3 mg/L, with aeration always on. The MLSS during this period was maintained between 10 and 12 g/L, with a waste flow of around 50 L/d. With these conditions, a total nitrification was achieved, with an average NH4 of 0.74 mgNH4-N/L, while the National discharge Standard limits this value at 5 mgNH4-N/L. Regarding to this parameter, the actual treatment plant was obtaining effluent values between 8 and 79 mgNH4-N/L. The COD and BOD removals in the MBR where higher than 95% with effluent values of BOD below the Standard limit. The only parameter above the Standard was the TP, which average was 14.7mg/L and the limit is 5mg/L (a chemical phosphorous removal should be carried out adding e.g. Ferric Chloride to the MBR). The Nitrate average in the effluent was 24 mg/L and the TN removal was 57.6%, meaning that the denitrification was not completed. Because of that, another trial conditions were investigated, with the same control parameters as the last one except the dissolved oxygen. The aeration was intermittent, turned “on” for 5 minutes and “off” for 15 minutes. The denitrification was enhanced and the total nitrogen removal efficiency reached a value of 78%. However, the NH4 increased to 6.2 mgNH4-N/L.

Furthermore, a BioWin model for estimating the optimal location for the MBR at the existing ponds system treatment plant was carried out, considering the N-removal potential in relation to the COD/N influent ratio. The results shows that the best place to situate the MBR inlet is before the homogenization tank, where the COD/TN ratio is 14.0 and the removal TN efficiency is 83.7 %. The second best point for the MBR is after the homogenization tank (COD/TN=11.1), with an efficiency of 72.8 % . For the other points of the treatment plant (after each treatment pond), the COD/N ratio decreases below 6, and the TN removal was reduced at values below 53 %.