IMPROVING AMMONIA REMOVAL IN MUNICIPAL CONSTRUCTED WETLANDS

摘要

The City of Mandeville, LA was one of the first recipients of an Innovative and Alternative Construction Grant in 1988 for a constructed wetlands (Rock Reed Filter). The City had been operating under Administrative Order because of a failure to meet an ammonia limit of 5 mg/L. In order to meet an ammonia discharge permit limit of 5 mg/L, improvements would have to be made to the existing 2 MGD WWTF that included additional aeration, rearrangement of existing aerators, and the construction of a recirculating gravel trickling filter. Beginning in 1997, the engineering team began with a review of previous historical water quality data. Construction of the recommended improvements was completed in December of 2000. Data presented represent 24 months of operating history since completion of construction. These improvements represent a classic restatement of basic wastewater engineering principles but nevertheless incorporate a new and innovative combination of aerated lagoons, constructed wetlands, and trickling filters. Discussion of capital costs includes comparisons with conventional alternatives. Analysis of the fundamental problems associated with nitrification is an essential first requirement. Discussion of the basic principles with an analysis of the data (2 years of composite samples and DMR’s) is used to evaluate the basic design criteria with the goal of comparing textbook formulas with actual performance. Particular attention is made to the design and performance of the gravel trickling filter, with comparison of design principles in the WEF Manual of Practice. Data presented show an average effluent ammonia concentration of 1.74 mg/L, and TSS and BOD reductions of 99% and 97% respectively. Data presented also include the performance of the various elements of the treatment process. An important element of the design required that the aerated lagoons reduce ammonia to a maximum level of 20 mg/L prior to discharge into the combined constructed wetlands trickling filter. Ammonia levels are presented at the end of the first and third cell of the aerated lagoons and at the end of the constructed wetlands/trickling filter. As this is an actual system, events such as the illegal dumping of large quantities of septage, temperature variations, and hurricanes, are all included in the presentation of data. These events demonstrate the robustness of this type of technology, and the importance of relatively long detention times. Statistical analysis of the standard deviation of influent and effluent data are presented as a confirmation of this observation. Graphical presentation of data clearly shows the damping effects of this combination of treatment technologies.