Published on January 3, 2008
Nitrogen and phosphorus removal in constructed wetlands: Nitrogen and phosphorus removal in constructed wetlands Sasha Hafner 3/31/2005 Outline: Outline The problem: Nutrient excess N and P cycling in wetlands N and P removal in treatment wetlands Models to predict N and P removal First-order model Litter model Applications Conclusions N and P excess in ecosystems: N and P excess in ecosystems Anthropogenic activity has had major impacts on N and P cycling (Galloway et al. 2002, Carpenter et al. 1998). Rate of N2 fixation doubled (Vitousek et al. 1997) Pollution of aquatic systems substantial (Driscoll et al. 2003, Carpenter et al. 1998) Pollution of terrestrial ecosystems increasing (Aber et al. 1998) (Driscoll et al. 2003) Engineering solutions: Engineering solutions In general terms: Reduce formation rate of reactive N, increase destruction of reactive N, reduce transfer to ecosystems, increase efficiency of reactive N cycling Increase efficiency of P use, reduce transfer of P to ecosystems Major specific challenges: Reduce gaseous emissions (NOx) Improve removal of N and P in wastewater treatment Increase nutrient use efficiency in agriculture Constructed wetlands: Constructed wetlands Ecosystems for wastewater treatment Can remove SS, BOD, & nutrients (EPA 2000) Constructed wetlands: N cycling: Constructed wetlands: N cycling NH4+ DON PON NH4+ Mineralization Sediment Uptake & assimilation Sedimentation Nitrification NO3- N2, N2O, NO Denitrification Death/litterfall Death/sloughing Immobilization/mineralization Sorption Constructed wetlands: P cycling: Constructed wetlands: P cycling PO43- DOP PP PO43- Mineralization Sediment Uptake & assimilation Sedimentation Death/litterfall Death/sloughing Immobilization/mineralization Sorption Precipitation N and P removal mechanisms: N and P removal mechanisms N removal Sorption short-term and reversible Denitrification rates can be very high, but requires aerobic & anoxic areas Plant assimilation and litter accretion can be an important long-term removal mechanism P removal Precipitation specific to waste (Fe, Ca) Soil sorption saturates, short term Plant assimilation and litter accretion can be an important long-term removal mechanism Different opinions on N and P removal: Different opinions on N and P removal EPA: constructed wetlands cannot remove significant amounts of N or P (EPA 2000: 5), plant assimilation is unimportant and largely reversible Other researchers: constructed wetlands can remove significant amounts of N & P Entire issue of Ecological Engineering dedicated to N & P removal in constructed wetlands (Mitsch et al. 2000, and rest of issue) Plant assimilation and litter accretion can be an important long-term sink for N & P (Heliotis & Dewitt 1983, Kadlec & Knight 1996, Kadlec 1997) Modeling N and P removal: Modeling N and P removal Single-parameter models First-order removal model, plug flow reactor Problem: empirical, requires determination of k, often determined from calibration Modeling N and P removal: Modeling N and P removal First-order removal model Effluent concentration Influent concentration Hydraulic loading rate Rate constant Modeling N removal with the first-order model: Modeling N removal with the first-order model To predict required area, solve for hydraulic loading, q: Substitute expression into an expression for A: Final equation: Modeling N removal with the first-order model: Modeling N removal with the first-order model Example: 10,000 people, Q = 0.4 m3 person-1 d-1, TN = 40 mg L-1, removal efficiency = 80%, k = 0.04 m d-1 Hydraulic loading, q = ? Required area = ? Modeling N removal with the first-order model: Modeling N removal with the first-order model Example: 10,000 people, Q = 0.4 m3 person-1 d-1, TN = 40 mg L-1, removal efficiency = 80%, k = 0.04 m d-1 Hydraulic loading, q = 0.025 m d-1 Required area = 161,000 m2 = 16 ha Modeling N and P removal : Modeling N and P removal Mechanistic numerical ecosystem simulation models General, explicit simulation of nutrient removal mechanisms Ideal approach, but insufficient data for development, parameter determination, and validation, and difficult to apply Other approaches Many models lie between these two extremes (Kadlec 1997, Wynn & Liehr 2001) For example, include some ecosystem components, use empirical parameters (Wang & Mitsch 2000) Modeling N and P removal: litter model: Modeling N and P removal: litter model Jewell’s experiments on aquatic plant (and algal) decay, 1960s & 1970s (Jewell 1971, Jewell & McCarty 1971) Conclusions: Mass loss essentially ceased after 50 days Nutrient retention could be predicted based on remaining plant and decomposer biomass Up to 100 d Litter model: Litter model Starting with one cohort of plant litter, wait until decomposition is complete Nutrient retention is equal to refractory organic matter masses times nutrient concentrations Can predict refractory organic matter masses from data on plant biodegradability and decomposer yields Litter model: Litter model What’s the idea? Start with one cohort of litter: Fresh Decomposed Biodegradable plant biomass Refractory plant biomass Decomposing Fresh decomposer biomass Gaseous waste Gaseous waste Refractory plant biomass Refractory plant biomass Refractory decomposer biomass Decomposers: Decomposers Decomposers assimilate what is needed for growth, maintenance and reproduction Nutrients that do not go toward making decomposer biomass contribute to net mineralization Nutrients that are incorporated into decomposer biomass contribute to net immobilization Pseudomonas aeruginosa Litter model: Litter model With continuous plant growth and litter production: continuous N & P retention Plants Labile Refract- ory N & P Decom- posers Litter model: equations: Litter model: equations Predicted mass of N & P retained Decomposer N Decomposer yield Decomposer refractory Plant refractory Plant productivity Plant N Plant refractory Plant productivity Litter model: equations: Litter model: equations Decomposer contribution Plant contribution Predicted mass of N & P retained Predicted mass of N & P retained Litter model: equations: Litter model: equations Can predict total litter accumulation, with assumptions regarding ash Assuming ash is not redissolved: Decomposer ash Plant ash Model application: Model application Nutrient film technique (NFT) hydroponic system (Jewell et al. 1993) No interferences from sediment (sorption) Model application: predicted N & P removal in NFT system: Model application: predicted N & P removal in NFT system Net primary productivity: NPP = 8,000 g m-2 yr-1 Plant ash content = 15% N concentration in plant: fno = 0.024 P concentration in plant: fpo = 0.0047 N concentration in decomposers: fnd = 0.12 P concentration in decomposers: fpd = 0.02 Decomposition yield: f '1 = 0.5 Effective refractory content of decomposers: f '2 = 0.2 Plant refractory content: f3 = 0.5 Model testing: predicted N & P removal in NFT system: Model testing: predicted N & P removal in NFT system N removal Plant contribution = 0.22 g N m-2 d-1 Decomposer contribution = 0.11 g N m-2 d-1 Total N retention = 0.33 g N m-2 d-1 P removal Plant contribution = 0.044 g P m-2 d-1 Decomposer contribution = 0.019 g P m-2 d-1 Total N retention = 0.063 g P m-2 d-1 Model testing: predicted N & P removal in NFT system: Model testing: predicted N & P removal in NFT system Model predictions: Model predictions Nitrogen 0.05 Model predictions: Model predictions Phosphorus Calculating area requirements: Calculating area requirements Predicted area requirements needed to size system Express area required as a function of waste water concentrations, flows, and nutrient retention rates for P people Q = L person-1 d-1 TN = mg L-1 = g m-3 Nr = g m-2 d-1 E = proportion of 1 Area requirements: Area requirements Area required = A = nutrients removed (g d-1)/nutrient retention rate (g m-2 d-1) x 1 ha/10,000 m2 Area required = A = P people x Q m3 person-1 d-1 x E x TN g m-3 x 1/(Nr g m-2 d-1) x 1 ha/10,000 m2 Example application: Example application 10,000 people, Q = 0.4 m3 person-1 d-1, TN = 40 mg L-1 If N removal = 0.33 g N m-2 d-1, what area of wetland is required for 80% removal? A = 10,000 people x Q m3 person-1 d-1 E x TN g m-3 x 1/(Nr g m-2 d-1) x 1 ha/10,000 m2 = ? Example application: Example application 10,000 people, Q = 0.4 m3 person-1 d-1, TN = 40 mg L-1, removal efficiency = 80% If N removal = 0.33 g N m-2 d-1, what area of wetland is required? A = 10,000 people x 0.4 m3 person-1 d-1 x 0.8 x 40 g m-3 x 1/(0.33 g m-2 d-1) x 1 ha/10,000 m2 = 39 ha Conclusions: Conclusions Constructed wetlands are capable of N, & P removal Several mechanisms are responsible for N & P removal Plant assimilation and litter accretion is an important long-term mechanism The first-order model lumps all removal processes into one term, but can successfully predict N & P removal in wetlands The litter model successfully predicts N & P removal due to plant assimilation and litter accretion using a mechanistic approach References: References Aber, J., McDowell, W., Nadelhoffer, K., Magill, A., Berntson, G., Kamakea, M., McNulty, S., Currie, W., Rustad, L., Fernandez, I. 1998. Nitrogen saturation in temperate forest ecosystems: Hypotheses revisited. Bioscience 48: 921-934. Carpenter, S.R., Caraco, N.F., Correll, D.L., HOwarth, R.W., Sharpley, A.N., Smith, V.H. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8: 559-568. Galloway, J.N., Cowling, E.B. 2002. Reactive nitrogen and the world: 200 years of change. Ambio 31: 64-71 Heliotis, F.D., DeWitt, C.B. 1983. A conceptual model of nutrient cycling in wetlands used for wastewater treatment: a literature analysis. Wetlands 3: 134-152. Jewell, W.J. 1968. Aerobic decomposition of algae and nutrient regeneration. Ph.D. thesis, Stanford University, Palo Alto, CA. Jewell, W.J. 1971. Aquatic weed decay: dissolved oxygen utilization and nitrogen and phosphorus regeneration. Journal Water Pollution Control Federation 43: 1457-1467. Jewell, W.J, McCarty, P.L. 1971. Aerobic decomposition of algae. Environmental Science and Technology 5: 1023-031. Kadlec, R.H. 1997. An autobiotic wetland phosphorus model. Ecological Engineering 8: 145-172. Kadlec, R.H., Knight, R.L. 1996. Treatment Wetlands. CRC Press, Boca Raton. 893 pp. Mitsch, W.J., Horne, A.J., Nairn, R.W. 2000. Nitrogen and phosphorus retention in wetlands–ecological approaches to solving excess nutrient problems. Ecological Engineering 14: 1-7 United States EPA. 2000. Constructed Wetlands Treatment of Municipal Wastewater. United States Environmental Protection Agency, Office of Research and Development, Cincinnati, OH. Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H., Tilman, D.G. 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7: 737-750. Wang, N., Mitsch, W.J. 2000. A detailed ecosystem model of phosphorus dynamics in created riparian wetlands. Ecological modeling 126: 101-130. Litter model: Litter model Problem in going from one litter cohort to continuous production Can address with estimates of decomposition rates Litter model: Litter model Set up as differential equations Litter model: Litter model Set up more complicated version that includes decomposition rate, using differential equations, solve in Matlab Conclude that simple model is sufficient for most decomposition rates Total N storage Rate of N accumulation Rate predicted by simple model Litter model: Litter model Problem with seasonal patterns: plant assimilation occurs during the growing season, decomposition? Difficult to evaluate
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