University of California Water Resources Center

Microbial denitrification, a frequently used and relatively inexpensive method of removing nitrate from wastewater, has been applied to the treatment of potable water supplies, on a limited scale, using packed bed reactors. However, two significant drawbacks exist in transferring wastewater denitrification technology to the treatment of domestic water supplies: (1) the water is intimately mixed with microbial cultures and (2) organic compounds must be supplied as an energy source to drive the denitrification reactions and residual organics can be a water quality problem. Process configurations used experimentally have included both packed beds and fluidized beds. Denitrifying microbial cultures have been supported on sand, ceramics, polymers, clay, alginate gel, and agar gel. Work with conventional support materials (sand, ceramics, polymers, clay) has been relatively straightforward in that the microbial cultures are grown on support surfaces and water containing nitrate is passed through the fixed or expanded/fluidized bed. Carbon and energy sources, nearly always organic compounds, are added to the water. Thus the problem outlined above - introduction of bacteria and organics - is characteristic of systems used to date.

The current work utilizes microporous membranes to separate the water being treated from the microorganisms carrying out the denitrification reactions. Nitrate passes through the 0.02 urn membrane pores by molecular diffusion. Water does not move through the pores and therefore contamination of the product water does not occur. Operation of microporous membrane systems can incorporate a biofilm on the reaction side of the membrane or utilize a suspended growth culture. Transport, and hence denitrification rates appear to be greater using suspended growth systems. In addition, suspended growth systems will have advantages in terms of minimization of biofouling of hollow fiber continuous flow units.

Measured nitrate diffusivities through the membrane pores was 3.5 x 10-6 cm2/s for biofilm systems and 5.0x 10-6 cm2/s for suspended growth systems. Nitrate flux is dependent on the concentration gradient. Potential fluxes for concentration differences of 20 mgIL are in the range of 10 g/m2-day.