Macrophyte Density in Constructed Wetlands


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A little study of emergent plant density and surface area in constructed wetlands for wastewater treatment.

Why do we care? Because when free surface wetlands are used for water treatment, the volume is an important parameter and some of that volume is taken up by the emergent plants; and because these plants offer a large, fractal surface area for the growth of helpful microorganisms, just like in a trickling filter.

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  • Macrophyte Density in Constructed Wetlands

    1. 1. Emergent Macrophyte Density In Constructed Wetlands for Wastewater Treatment Sophie Lagacé, Curtis Miller, and Robert Gearheart
    2. 2. Problem Statement <ul><li>How much plant material is there in a constructed wetland? </li></ul><ul><li>How much surface do the plants offer for microbial growth? </li></ul><ul><li>How much volume do they occupy? </li></ul>
    3. 3. Why are we interested? <ul><li>Design volume calculations </li></ul><ul><li>Growth surface for microorganisms </li></ul>
    4. 4. Site Conditions <ul><li>Arcata Wastewater Treatment Plant </li></ul><ul><ul><li>Treatment marshes </li></ul></ul><ul><ul><li>Pilot marshes </li></ul></ul><ul><li>Municipal wastewater effluent from oxidation ponds </li></ul><ul><li>Cattail-bulrush marshes </li></ul><ul><li>Dormant period (January) </li></ul>
    5. 5. Studied Plant Species <ul><li>Hardstem bulrush (Scirpus acutus) </li></ul><ul><ul><li>Obligate wetland species </li></ul></ul><ul><ul><li>Usually 1-3m (3-10 ft) tall </li></ul></ul><ul><ul><li>Stout, firm, gray-green round stem </li></ul></ul><ul><ul><li>Up to 30 mm (1¼ in.) at base, tapering to 2-4 mm ( 1 / 8 - 1 / 16 in.) at top </li></ul></ul><ul><ul><li>Little leaf and stem litter </li></ul></ul>
    6. 6. Studied Plant Species <ul><li>Wide-leaved cattail (Typha latifolia) </li></ul><ul><ul><li>Obligate wetland species </li></ul></ul><ul><ul><li>Usually 1-3m (3-10 ft) tall </li></ul></ul><ul><ul><li>Stout, smooth cylindrical stem with spongy core </li></ul></ul><ul><ul><li>8+ alternate, elongated, leaves per stem, > 12 mm (½ in.) wide </li></ul></ul><ul><ul><li>Accumulates leaf and stem litter </li></ul></ul>
    7. 7. Field Work - I <ul><li>Select a location and note the species (monoculture plots) </li></ul><ul><li>Stake and mark a 1m x 1m plot </li></ul><ul><li>Ten plots in all: five for Typha , five for Scirpus </li></ul>
    8. 8. Field Work - II <ul><li>Cut stems to about 30-40 cm above water level in and around the plot </li></ul><ul><li>Remove the litter </li></ul>
    9. 9. Field Work - III <ul><li>Measure water depth at each corner and in center of plot </li></ul><ul><li>Count every stem within the plot </li></ul><ul><li>As a stem is counted, cut it just above water level </li></ul>
    10. 10. Field Work - IV <ul><li>For two Scirpus plots, measure each stem’s average diameter after cutting (round stems) </li></ul><ul><li>For two Typha plots, measure each stem’s maximum and minimum diameters after cutting (elliptical stems) </li></ul><ul><li>Pull five randomly selected stems of Scirpus, whole* </li></ul><ul><li>Measure the diameter along the Scirpus stems every 50 mm (2 in.), starting at the base and up to 1m </li></ul>* Typha stems were too decomposed to sample
    11. 11. Calculations <ul><li>Mean areal stem density for each species </li></ul><ul><li>Mean stem diameter and cross-section for each species </li></ul><ul><li>Scirpus typical taper shape equation </li></ul><ul><li>Wet area for varying water depth and stem density for each species </li></ul><ul><li>Submersed volume for varying water depth and stem density for each species </li></ul>
    12. 12. Results Areal Density: Stems per m 2
    13. 13. Stem Diameter and Cross-Section <ul><li>Scirpus acutus: </li></ul><ul><ul><li>Maximum diameter along the stem varied between 20 and 30 mm </li></ul></ul><ul><ul><li>Typical diameter at water level: ~15 mm </li></ul></ul><ul><ul><li>Typical cross-section at water level: 190mm 2 </li></ul></ul><ul><li>Typha latifolia: </li></ul><ul><ul><li>Assumed constant cross-section along the stem </li></ul></ul><ul><ul><li>Typical diameters at water level: 30mm x 40mm </li></ul></ul><ul><ul><li>Typical cross-section at water level: 1000mm 2 </li></ul></ul>
    14. 14. Scirpus Typical Taper Shape
    15. 15. Wet Area: Scirpus as m 2 of surface area per m 2 of wetland
    16. 16. Wet Area: Typha as m 2 of surface area per m 2 of wetland
    17. 17. Submersed Volume: Scirpus as m 3 per m 2 of wetland
    18. 18. Submersed Volume: Scirpus as % of volume in water column
    19. 19. Submersed Volume: Typha as m 3 per m 2 of wetland
    20. 20. Wet Area
    21. 21. Submersed Volume
    22. 22. Implications for Design <ul><li>Emergent vegetation typically occupies 4 to 10% of the available volume </li></ul><ul><li>Emergent vegetation stems typically offer 1.3 to 10 m 2 of surface area for attached growth per m 2 of wetland (57,000 to 436,000 ft 2 /acre!) </li></ul><ul><li>Next question for research: Surface area and volume of plant litter? </li></ul>