Nitrogen and phosphorus removal in constructed wetlands

1
Nitrogen and phosphorus removal in constructed
wetlands

• Sasha Hafner
• 3/31/2005

2
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

3
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)
4
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

5
Constructed wetlands

• Ecosystems for wastewater treatment
• Can remove SS, BOD, nutrients

(EPA 2000)
6
Constructed wetlands N cycling
Death/litterfall
N2, N2O, NO
Denitrification
Nitrification
NH4
NO3-
Mineralization
Immobilization/mineralization
DON
NH4
PON
Uptake assimilation
Sedimentation
Sediment
Sorption
Death/sloughing
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Constructed wetlands P cycling
Death/litterfall
PO43-
Mineralization
Immobilization/mineralization
DOP
PO43-
PP
Uptake assimilation
Precipitation
Sedimentation
Sediment
Sorption
Death/sloughing
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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

9
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)

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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
  

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Modeling N and P removal

• First-order removal model

Hydraulic loading rate
Rate constant
Influent concentration
Effluent concentration
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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

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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 ?

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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

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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)
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Modeling N and P removal litter model

• Jewells 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
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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
  

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Litter model

• Whats the idea? Start with one cohort of litter

Fresh
Decomposed
Decomposing
Gaseous waste
Gaseous waste
Biodegradable plant biomass
Fresh decomposer biomass
Refractory decomposer biomass
Refractory plant biomass
Refractory plant biomass
Refractory plant biomass
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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
20
Litter model

• With continuous plant growth and litter
  production continuous N P retention

N P
Decom- posers
Plants
Labile
Refract- ory
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Litter model equations

• Predicted mass of N P retained

Decomposer refractory
Plant productivity
Decomposer N
Plant refractory
Decomposer yield
Plant productivity
Plant N
Plant refractory
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Litter model equations

• Predicted mass of N P retained

Predicted mass of N P retained
Decomposer contribution
Plant contribution
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Litter model equations

• Can predict total litter accumulation, with
  assumptions regarding ash
  
• Assuming ash is not redissolved

Decomposer ash
Plant ash
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Model application

• Nutrient film technique (NFT) hydroponic system
  (Jewell et al. 1993)
  
• No interferences from sediment (sorption)

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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

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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

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Model testing predicted N P removal in NFT
system
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Model predictions

• Nitrogen

0.05
29
Model predictions

• Phosphorus

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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

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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
  

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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 ?
  

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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
  

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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
  

35
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
  wetlandsecological 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.

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Litter model

• Problem in going from one litter cohort to
  continuous production
  
• Can address with estimates of decomposition rates

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Litter model

• Set up as differential equations

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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 predicted by simple model
Rate of N accumulation
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Litter model

• Problem with seasonal patterns plant
  assimilation occurs during the growing season,
  decomposition?
  
• Difficult to evaluate