Removal of Nitrate from Contaminated Water Sources Using Membrane Bioreactor/Membrane Contactor Systems

The use of membrane bioreactor/membrane contactor (MBR/MC) systems is proposed as a promising alternative far nitrate removal from drinking water. The results of an experimental study, developed on a MBR/MC pilot plant, are presented and discussed, and a new mathematical model is introduced for the design of these systems. The model is then applied to the pilot plant to simulate its performance, and the obtained results are compared with the experimental data. Uncontrolled activities related to agriculture and farming are causing a wide contamination of groundwater sources used far drinking supply. The most dramatic situations are related to nitrate contamination, being well know the risk of nitrate assumption in high doses. To face the problem and treat the water, membrane separation processes can be used, but the resulting operational costs are usually very high. Biological denitrification is much cheaper, but it is usually regarded with great suspicion because of the possible contamination caused by the contact between the water and the biomass. The special configuration of the bioreactor adopted in the experimental study described in the paper, allows to avoid this contact, while maintaining low the operational costs. An ultrafiltration membrane, in fact, is used to keep separated the biomass and the water, while the biological process takes place. Hollow fibres are chosen as membrane units for the pilot plant, and the biological process is demanded to the biofilm developed on the exterior of the fibres. The experimental results, obtained varying the influent flow and the nitrate influent concentration, confirm the system potentiality and show the possibility of full-scale applications of the system. The trend of nitrate removal rate and the corresponding values of carbon source consumption operated by the heterotrophic biomass, revel also the risk of parasite processes development. These processes compete with the denitrification one and reduce the overall efficiency of the treatment. The model, proposed for the design and the simulation of MBR/MC systems and presented in the paper, is based on simple mass balances in the flow direction and through the membrane. To obtain a closed solution of these equations, steady-state conditions are supposed, and each fibre is considered as a plug-flow reactor. Moreover it is hypothesed that the biofilm is able to remove, instantaneously, all the nitrates diffusing through the membrane, so that nitrate concentration outside the fibres is always zero. Data obtained with the pilot plant do not contradict the formulated hypotheses and the experimental data are almost always in good agreement with the results obtained applying the simulation model, which results to be an useful tool for future full scale application of MBRIMC systems.