Evaluation of the subsurface vegetated bed form of constructed wetlands for domestic wastewater treatment

Abstract

Sixteen small scale Subsurface vegetated beds (SVB) were installed down slope of the residence at the Whitethorne Planation (WP) site. Treatments included three detention times (2.6, 3.9, and 5.9 days), two recirculation ratios (0 and 0.5), and two plant species [woolgrass (<i>Scirpus cyperinus</i>) and cattail (<i>Typha latifolia</i>)]. Twelve larger scale SVBs were constructed at the Powell River Project (PRP) site, Wise County, VA. Treatment consisted of three detention times (4, 8, 12 days) with the same plant species (cattail and woolgrass).

At the WP site, pollutant removal incresed with detention time for 5-day biochemical oxygen demand (BOD₅), total Kjeldahl nitrogen (TKN), ammonium (NH₄-N), phosphates (PO₄³⁻-P), redox potential (Eh), and total dissolved solids (IDS). The pollutant removal rates, based on 23 data sets, were: BOD₅ (54-70%); NH₄⁺-N (30-61%); TKN (33-52%); PO₄³⁻-P (7-28%); fecal coliforms (FC) (>99%) and coliphages (>95%). Recirculation appeared to have no apparent benefit. There were no differences in P, S and secondary and trace metals concentrations in the SVBs planted to cattail and woolgrass. Plants could remove 2 -10% of the total N applied and 13-57% of the total P applied to the SVBs if harvested once a year during the growing season.

At the PRP site, differences were observed among the detention times for BOD₅, TKN, NH₄-N, PO₄-P, TDS, pH, EC and Eh. The pollutant removal percentages were: FC (>99%); BOD₅ (30-75%); NH₄-N (27-88%); TKN (27-81%); PO₄³⁻-P (24-46%); and TDS (12-73%). Samples collected from shallow wastewater column exhibited a higher level of treatment. There were no differences in FC, BOD₅, TKN and NH₄⁺-N concentrations between s VB s planted to cattail and woolgrass, and there were no differences in tissue N content between cattail and woolgrass at both sites.

The average NH₃ volatilization rate was 236 mg N/m²/day, and accounted for 7-37% of total N applied to the SVBs. Average measured denitrification rates ranged from 3.87 to 6.69 mg N₂O /m²/ h, and accounted for 2-17% of total N applied.