Ce = Wetland effluent concentration (mg/L). C0 = Wetland influent concentration (mg/L). Kt = Rate constant at temperature T (d-1). 8 = Temperature coefficient at 20°C.
Tw = Average water temperature in wetland during period of concern (°C). A, = Treatment area (bottom area) of wetland (m2). Qa = Average flow in the wetland (m3/d) = (QIN + QOUT)/2. y = Average depth of water in the wetland (m). n = Porosity of the wetland (% as a decimal).
Note: (1) The effluent concentration (Ce) cannot be less than the background concentrations listed below. (2) The average flow (QA) accounts for water gains and losses from precipitation, evapotranspiration, seepage, etc.
Parameter FWS Wetland SSF
Background concentration (mg/L) = 6 TSS Removal:
where HLR = hydraulic loading rate (mm/d x 0.1), and TSS removal is not dependent on temperature. Background concentration (mg/L) = 6
At 0°C, KT (d-1) = 0. At 1°C+, K20 = 0.2187. 8 = 1.048.
KNH is a rate constant at 20°C for FWS wetlands (d-1). Background concentration (mg/L) = 0.2 Note: It is prudent to assume that all TKN (from municipal wastewater) entering the wetland can appear as ammonia, so assume C0 for ammonia is equal to influent TKN.
At 0°C, KT (d-1) = 0. At 1°C+, K20 = 1.000. 8 = 1.15.
Background concentration (mg/L) = 0.2.
Note: It is conservative to assume that all ammonia removed in the previous step can appear as nitrate, so C0 for nitrate removal design equals Ce from ammonia removal plus any nitrate present in the influent.
Total Nitrogen Removal:
Effluent TN = C,(/ + ((/ - C,(/) Background concentration (mg/L) = 0.4. Note: A specific model for total nitrogen removal is not available in this set. The effluent total nitrogen (TN) can be estimated as the sum of residual ammonia and remaining nitrate (C0 - Ce).
Total Phosphorus Removal:
where HLR is the average hydraulic loading rate (cm/d), and total phosphorus removal is not dependent on temperature.
Background concentration (mg/L) = 0.05.
Fecal Coliform Removal:
where t, d = HRT in the system, and x = number of wetland cells in series. Kw (d-1) = 2.6. e = 1.19.
Background concentration (cfu/100 mL) = 2000. Note: This model was developed for facultative ponds and is believed to give a conservative estimate for fecal coliform removal in both FWS and SF wetlands.
The background concentration is given for each of the parameters listed above. These values represent an external boundary condition on the design models in this set. None of the models should ever be solved for a concentration less than these background levels.
The parameter (BOD5, etc.) that requires the largest treatment area for removal is the limiting design factor, and that area should be selected for the intended project. The wetland should then provide acceptable treatment for all other parameters of concern.
It is typical in all engineering design projects to apply a safety factor. In most cases, the final safety factor is applied after the preliminary calculations are completed. In this case, the safety factor is applied after the wetland size has been determined and ranges from 15 to 25% depending on the uncertainty of available data and on the stringency of performance expectations. The selection of a safety factor is an engineering judgment and represents a comparable increase in the calculated treatment area.
Source: Adapted from Reed, S.C. et al., Natural Systems for Waste Management and Treatment, 2nd ed., McGraw-Hill, New York, 1995. With permission.
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