Investigation On Nitrate Removal From Groundwater By The Composite Membrane Bioreactor

Groundwater is the most important source of drinking water in northern China; in some rural areas, it is the only readily available source of drinking water. As a result of excessive use of urea and/or other nitrogen fertilizers, wastewater irrigation and leakage, nitrate contamination of the groundwater has become a common issue. Once consumed, nitrate may be converted to nitrite which is responsible for methemoglobinemia (the blue baby syndrome) in infants and other health problems including cancer.The high NO3--N concentration can be reduced by ion exchange (Ⅸ), reverse osmosis (RO) and biological denitrification. AlthoughⅨor RO treatment may be effective, it however will result in a concentrate stream of waste by-product that requires further treatment or final disposal at a high cost. Biological denitrification is the most cost-effective process for nitrate removal in groundwater. However, the unresolved issues of poor retention of both the microbial biomass and the electron donor have delayed their full-scale applications.To overcome such stated disadvantages of the groundwater denitrification processes, research was conducted in our lab to develop three types of composite membrane bioreactors which integrates immobilized cells technique with membrane separation technology for groundwater denitrification, including double-compartments composite membrane bioreactor (DCMBR), three-compartments composite membrane bioreactor (TCMBR) and multiple-compartments composite membrane bioreactor (MCMBR). The groundwater compartment (water compartment for short) and carbon source compartment (carbon compartment for short) were separated by the composite membrane consisting of a microporous membrane facing the influent and an immobilized cells membrane facing the ethanol solution. Molecules of nitrate and ethanol diffused from the respective compartments into the immobilized cells membrane where nitrate was reduced to gaseous nitrogen (N2) by the denitrifying bacteria present there with ethanol as carbon source. On the one hand, the tabulate immobilized cells can prevent product water contaminated by carbon source through separating groundwater and carbon source; On the other hand, the microporous membrane attached to one side of immobilized cells to separate product water from an immobilized cells or to provide effective retention of the biomass.The main objectives of this research were therefore to investigate the possibility of DCMBR for groundwater denitrification and the efficiency of controlling contamination of the product water by added organic carbon source. The optimal preparation conditions of immobilized cells and denitrification conditions were obtained by orthogonal and single factor experiments, respectively. The results showed that the optimum conditions of immobilized cells preparation were 4 mm thickness, embedding denitrifying bacteria in 15% PVA solution was 20g (wet weight per litter.; The optimum carbon to nitrogen ratio, temperature and pH were 6:1,30℃and 7-8, respectively. And ethanol was a proper carbon source.The denitrification rate of DCMBR was proportional to the initial concentration of NO3--N. The effect of dissolved oxygen in groundwater on the denitrification of immobilized cells was little than that of free cells.The three-compartments composite membrane bioreactor (TCMBR) and multiple-compartments composite membrane bioreactor (MCMBR) were developed and their characteristics of denitrification for groundwater were investigated. The results showed that the denitrification rate of TCMBR was low than that of DCMBR, but the producing water quantity per reactor volume was higher than that of DCMBR. For the MCMBR, the maximum influent loading reached up to 4.97 gN·m2·d-1 as the influent nitrate nitrogen concentration was lower than 100 mg·L-1. The MCMBR demonstrated the excellent operational stability without decreasing of denitrificaton activity after running over 6 months. The quality of the product water was fit to standards for drinking water quality.The study showed that the immobilized cells could obviously block the diffusion process. The diffusion coefficients of nitrate and ethanol were 0.707×10-9m2·s-1 and 0.640×10-9m2·s-1 in PVA gel without bacteria respectively, which were 37.2% and 58.2% of their diffusion coefficients in the pure water. And embedding cells in PVA gel, number of freezing and thawing, PVA solution concentration and use of microporous membrane would affect the diffusion coefficient. The groundwater denitrification rate was controlled by the internal mass transfer as the immobilized cells was prepared with 15% PVA contain 20 g·L-1 bacteria, and the immobilized cells was 4 mm thickness.The determination of molecular weight distribution and fluorescence properties of the dissolved organic matter (DOM) in product water showed that the small moleculars below 1000 Da were the main part in the product water using fluorescence excitation-matrix(EEM) spectroscopy. Most of the small molecular were humus without biotoxicity in product water.In order to investigate the microbial diversity during denitrification process of MCMBR, four microbial samples were taken to total microbial DNA extraction and polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) analysis from the initial inoculation sludge, acclimation sludge and immobilized cells running for two and six months. The DGGE profile analysis showed that the same and different species existed in inoculation sludge and immobilized cells in different stage. Sequencing and phylogenetic analysis of 10 dominant bands in the DGGE profile indicated that 2 of strains belonged to Diaphorobacter sp. and Bacillus sp. with function of denitrification and other 8 strains were new strains. The dominant status of same microbial species in different stage was different and special microbial community was due to the succession of long term. The total change trend of microbial diversity tended to simplifying.