Cc. Tanner et al., RELATIONSHIPS BETWEEN LOADING RATES AND POLLUTANT REMOVAL DURING MATURATION OF GRAVEL-BED CONSTRUCTED WETLANDS, Journal of environmental quality, 27(2), 1998, pp. 448-458
Relationships between pollutant mass loading and removal are reported
for a series of five pilot-scale constructed wetlands in their 4th and
5th years of operation. The wetlands received different hydraulic loa
dings ranging between similar to 15 and 70 mm d(-1) (9.5-2 d theoretic
al retention, respectively), Effluent concentrations of 5-d carbonaceo
us biochemical oxygen demand (CBOD), total nitrogen (TN), ammonium N (
NH4N), and fecal coliforms (FC) broadly followed seasonal patterns in
influent wastewater strength, Mean annual mass removals of 58 to 78% s
uspended solids (SS), 73 to 91% CBOD, 48 to 65% TN, 34 to 60% NH4-N, 1
5 to 38% total phosphorus (TP), and 93 to 99.6% of FC were recorded, w
ith removal efficiencies inversely related to loadings, Mass removal r
ates were monotonically related to loading rates, and could be modeled
using a simple plug-flow, first-order approach accounting for removal
down to nonzero background concentrations, Comparisons with treatment
performance recorded for the wetlands soon after commissioning showed
relatively constant relationships between mass loading and removal of
CBOD, TN, and FC, In contract, SS and TP removal declined significant
ly over the same period, Reduced SS removal efficiency appeared to res
ult from clogging of the gravel substratum by refractory organic solid
s, and reduced TP removal from saturation of substratum sorption capac
ity and filling of plant storage pools. To improve N removal predictio
ns for wetlands treating ammonium-rich wastewaters, the use of a combi
ned carbonaceous and nitrogenous BOD term is proposed, which addresses
the oxygen dependence of microbial nitrification, the principal rate-
controlling process.