System dynamics and controller development for denitrification using a packed column bioreactor

Abstract

The nursery/floral industry is looking for ways to reduce the amount of nitrate present in their runoff so that they may discharge it without violating Environmental Protection Agency guidelines for nitrate discharge. One such method being studied is biological denitrification. For this study, two 1.5 m x 0.3 m columns were built and placed in parallel. The columns were filled with lava rock to provide sites for denitrifying bacteria to attach themselves. The system was inoculated with water from a golf course pond. The bacterial colony was established by recirculating this pond water through the system, adding nitrate and methanol as needed. Once a colony was well established, effluent and influent nitrate/nitrite concentrations were monitored for four different influent nitrate concentrations and four different hydraulic residence times, with the intent of determining the removal characteristics. The system attained removal percentages between 62 and 96 percent, with low (< I mg N02-N/L) effluent nitrite concentrations. All effluent produced by the system met or exceeded Environmental Protection Agency guidelines for nitrate discharge, suggesting that the limits of the system were not reached. The system's response to both positive and negative step changes in the influent nitrate concentration was also studied. In general, the system was able to attain equilibrium within 6 hours of the negative step change. The positive step change was not as clear, but effluent nitrate. levels did not exceed Environmental Protection Agency guidelines for nitrate discharge, again suggesting the limits of the system were not reached. The combination of these two studies was to result in a controller equation for an automated system. However, it was determined that automation was not necessary for this type of system under a very controlled environment. A simple monitoring strategy is sufficient for producing effluent that meets Environmental Protection Agency guidelines. However, for a system that does not have tight environmental control, automatic control would be very useful.