ABE 325 Soil and Water Conservation Engineering Wetlandsabe325/Lectures/wetlands.pdf · Soil and Water Conservation Engineering Wetlands Rabi H. Mohtar Introduction Objectives The

ABE 325 Soil and Water Conservation Engineering

Wetlands Rabi H. Mohtar

Introduction Objectives The objective of this module us to describe natural and constructed wetland processes and how they operate towards purifying runoff or wastewater. What is a wetland? US EPA (1993) - a land where the water surface is near the ground long enough each year to maintain saturated soil conditions along with related vegetation. Wetland vegetation is the main factor in wetland classification. Processes and design factors that are part of a wetland system:

1. Detention of runoff water -- outflow 2. Retention of runoff water – bottom losses 3. Sedimentation – depth:length:sediment texture interaction 4. Adsorption – chemical constituent:soil texture interaction 5. Microbial degradation – oxygen flow, contaminant type, nutrient availability 6. Plant uptake – plant species, temperature 7. Chemical reactions – chemical composition, pH, temperature

All of the above processes remove chemical and biological impurities and reduce runoff flooding impact. Weather and temperature in particular play an important role in the feasibility, efficiency, and design of a wetland. The natural wetlands, also known as phytoremediation processes, are constantly working for treatment of wastewater. Typical or example vegetation includes various aquatic macrophytes:

• Pharagmites karka • Typha latifolia • Scripus • Eichhornia crassipers • Duckweed

Indicators/parameters in a wetland system include:

• pH • Electrical conductivity • Total solids dried at 103-105±C • Total dissolved solids dried at 103-105±C • Total suspended solids dried at 103-105±C • BOD – Biochemical Oxygen Demand 5 days test

COD: chemical oxygen demand TKN = Total Kjeldhal nitrogen TP = Total phosphorous FC = Fecal coliform Hydraulic Detention time (days) =

3

3volume of wetland (m )

mDischarge ( )day

Hydraulic loading rate = ( ) day

lmg =•

2

discharge daywetland area (m )

l

Organic loading rate 3(g m day)=•

3 3

2

Average BOD (g/m ) Discharge (m / day)Wetland area (m )

∗

BOD Removal BOD removal is due to microbial growth attached to the plant roots, stem and leaf litter fallen into the wastewater since algae are typically not present if the plant coverage is complete. The major sources of oxygen for these reactions are re-aeration at water surface and plant translocation of oxygen from the leaves to the rhizosphere. Most of the removal activities occur at the interface of water-soil. What is the optimum design for:

1. Runoff control 2. Sediment control 3. Nitrate removal 4. Toxic hazardous chemicals

The Water Budget One such analysis of constructed wetland hydrology is the development of a water budget. In short, it is an accounting system of the wetland’s water transfers over time. The dynamic overall water budget is (Kadlec, 1996):

i o c b qw smQ Q Q Q Q Q PA - ETA = dV/dt− + − − + +

where A = wetland top surface area, m2 ET = evapotranspiration rate, m/d P = precipitation rate, m/d Qb = bank loss rate, m3/d Qc = catchment runoff rate, m3/d Qgw = infiltration to groundwater, m3/d

Qi = input wastewater flow rate, m3/d Qo = output wastewater flow rate, m3/d Qsm = snowmelt rate, m3/d t = time, d V = water storage in wetland, m3

Example Problems – J.F. New, Indianapolis