Development and Optimization of a Denitrification System to Promote Biological Nutrient Removal in a Subtropical Estuary

Abstract

Eutrophication impacts coastal ecosystems globally and triggers phytoplankton and algal blooms, leading to an increase in oxygen demand in the water column and sediments due to various biogeochemical processes. Naturally, biological nitrification, denitrification and anammox processes help to remove nitrogen predominately by nitrifying and denitrifying bacteria in the sediments. Eutrophic systems have increased nutrient and organic matter loading and are prone to the accumulation of ‘muddy’, fine-grained, organic-rich sediments (locally termed ‘muck’) that may not be able to support high enough populations of these bacteria, and therefore, may not adequately support nitrification/denitrification processes that would otherwise remove nitrogen. To decrease nitrogen and phosphorus concentrations in brackish water and dredge material from the Indian River Lagoon, treatment bioreactors promoting biological nutrient removal processes were developed, in which different media (bioballs, biochar, sand, zeolite, perlite and corn maize) and environmental conditions (carbon sources – methanol, acetic acid and sugar; aerobic and anaerobic conditions) were tested. In contrast to results from many existing systems, accumulation of nitrate was not observed in any of the treatments during this study with no added benefit observed through the addition of anaerobic treatment (TDN: p = 0.66; TDP: p = 0.39) where anaerobic microenvironments within biofilms facilitated treatment in a single reactor cell. Following the success of plastic bioballs and a desire to find an eco-friendlier and more cost-effective alternative for potential use in a large system, repurposed plastic bottle caps were tested and added to the study. Overall, there was no significant difference in treatment efficiencies between systems using bioballs versus plastic bottle caps (TDN: p