Emerging Technologies For Nitrogen And Phosphorus Removal, And Brine Management
Annual Conference
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Treatment wetlands are a sustainable phosphorus and nitrogen removal technology which can provide cost-effective treatment, with low operational costs compared to conventional technologies. A common drawback to their application is the footprint required to support nitrification, denitrification, and phosphorus sequestration.
This paper summarises recent case studies describing four emerging approaches to reducing wetland area through process intensification: super-oxygenation for nitrification, zeolite-anammox media filtration for deammonification, biochemical reactors for denitrification, and geochemical augmentation in surface flow (SF) wetlands for phosphorus removal. These wetland applications can become very effective for application as unit processes within small wastewater treatment plants.
Super-oxygenation recirculates a side stream in SF wetlands through a downflow pure oxygen contactor. A supersaturated recirculation flow (40-80 mg O2/L) boosts nitrification rates by over an order of magnitude compared to passive wetlands. Using this approach, an 800 m3/d groundwater remediation wetland located in Michigan USA demonstrates sustained nitrification through winter. A super-oxygenated SF wetland would reduce NH3-N from 10 mg/L to 1 mg/L within a wetted area of 5.0 ha, a significant reduction from the 120-ha required for a passive wetland.
Zeolite anammox uses flood and drain (tidal flow) beds of clinoptilolite. Ammonium adsorbs to media during the flooded stage and oxidizes when the bed drains. Nitrification occurs at low ammonium loading and results in complete oxidation. Anammox occurs at high ammonium loading (>120 g NH4+/m3/d) and beds are just partially drained. A demonstration system (21 m3/d) in Oregon USA reduced ammonia from 1000 mg/L to 300 mg/L after one year of operation and stepping through a nitrification phase. It is currently going through operational optimization to overcome a nitrite limitation without stimulating nitrite oxidation.
Biochemical reactors use compostable media (e.g., wood chips, sawdust, manure) to create saturated anaerobic conditions conducive to denitrification, sulfate-reduction and metal sequestration. Denitrification rates are typically two orders of magnitude greater than SF wetlands. Recent case studies from the USA in RO brine management and mine-water treatment fully demonstrates this technology for application within a wide range of wastewaters.
Phosphorus removal in wetlands is sustained through sedimentation, sorption and predominantly biological uptake and burial. Consequently, passive SF wetland area requirements are typically the largest for wastewater contaminants. For example, treating 10,000 m3/d in an SF wetland that polishes wastewater TP from 0.5 mg/L to 0.05 mg/L would require approximately 135 ha. One intensification approach is to add soluble (non-flocculating) doses of aluminium or iron salts at a concentration below the chronic toxicity threshold. Using this method of geochemical augmentation, removal rates have been shown to increase by approximately an order of magnitude, reducing treatment areas by approximately 90%. The method is in early stages of development. Full-scale pilot projects in Oregon (12,000 m3/d) and Georgia (56,000 m3/d) USA demonstrate the efficacy of this method.
If traditional passive wetlands are considered low-rate, and conventional wastewater technologies high-rate, these emerging wetland technologies would be considered medium rate. Adopting a medium-rate wetland process strategy appears to show great potential as a nutrient management application for small-flow treatment plants (<10,000 m3/d).