Pilot Study on Treatment of Wastewater from an Ethylene Plant with Membrane Bioreactor Technology
Pilot studies were conducted with an anoxic/aerobic concept membrane bioreactor (MBR) technology and a hollow fiber Petro ® MBR system with capacity of 12 m 3 /d was operated continuously (24-hour) during the study. Trials on different membrane fluxes were conducted to obtain the sustainable flux while mixed liquor suspended solid (MLSS) was maintained at 9-11 g/L. The results of the MBR pilot trials showed that no obvious fouling of the membrane was found when the plant was operated at the flux of 12 L /m 2 /h (LMH) over 3 months and 15 LMH over one month during the pilot study. Design guidelines such as hydraulic retention time (HRT), sludge retention time (SRT), anoxic and aera- tion volume ratio, re-circulation flow rate and air scouring were obtained for a full-scale plant. It was concluded that treatment of wastewater from an ethylene plant without addition of any chemicals using MBR technology is feasible. The product quality consistently met the requirement for discharge and was suitable for the feed of further reverse osmosis (RO) post-treatment.
Reduced sludge production in a two-stage membrane-assisted bioreactor
Abstract The performance of different two-stage systems was compared for the treatment of synthetic wastewater. The first stage was a completely mixed reactor without sludge retention for the stimulation of dispersed bacterial growth. The second stage was an activated sludge system in which growth of protozoa and metazoa was stimulated. Solid-liquid separation was achieved either by sedimentation (conventional activated sludge (CAS) system) or submerged membrane filtration (membrane-assisted bioreactor (MBR) system). Some 80% of the chemical oxygen demand (COD) was removed in the first stage at volumetric loading rates up to 1.6 g COD l −1 d −1 . In case the subsequent stage was conventional activated sludge, a further COD decrease was noticed. In case of a subsequent MBR system, however, the dissolved COD concentration tended to increase due to the retention of organics by the membrane. In the first stage of both systems, most of the nitrogen and phosphate was used for biomass incorporation. Increased mineralization by protozoa and metazoa in the second stage resulted in a partial release of nitrogen and phosphate to the effluent. The MBR system yielded a 20–30% lower sludge production than the CAS system under similar conditions of solids retention time and organic loading rate. This was attributed to the higher amount of predators in the second stage of the MBR configuration. However, the increased grazing of predators on nitrifying bacteria can result in overgrazing of the latter population. Overall, the two-phase system based on conventional activated sludge had as major point of weakness the wash-out of suspended solids, while the one based on the MBR was hampered by too intensive grazing on the nitrifiers, increased N and P concentrations and wash-out of soluble but humified COD.
Influence of Predators on Nitrification in Aerobic Biofilm Processes
The influence of predators on nitrification in aerobic biofilm processes was investigated in a laboratory study carried out using two aerobic continuous-flow suspended-carrier biofilm reactors operating in parallel on the same synthetic waste water. After nitrification was established in the reactors and stable operating conditions at 3 h hydraulic retention time were achieved, nystatin and cycloheximide, substances inhibitory to eucaryotic organisms, were added to one of the reactors to selectively inhibit the predators. The other reactor was operated as a reference, without inhibitors being added. Adding the inhibitors to the test reactor led to a rapid decrease in the quantity of biofilm-consuming predators, most of them rotifers and nematodes, and a simultaneous increase in nitrification, which finally stabilized at a level twice as high as in the reference reactor. The addition of inhibitors was then switched between the reactors, resulting in a rapid increase in nitrification in what had been the reference reactor, and a slow decrease in nitrification in the reactor in which the adding of inhibitors had been stopped. Again, approximately twice as high a nitrification rate was achieved with addition of inhibitors than without. The results clearly show that biofilm predators can have a strong negative effect on nitrification in aerobic biofilm processes.