Land Treatment
(Natural Wastewater Treatment)
Uses plant-soil processes
to clean wastewater
Land Treatment
• Usually falls into three categories
– Irrigation (sometimes referred to as “slow
rate” or “SR”)
– Overland flow
– Rapid infiltration (soil aquifer treatment,
“SAT”)
Also
– Septic systems
– Constructed wetlands
Irrigation
or Slow
Rate
Overland
Flow
Rapid
Infiltration
Irrigation (slow rate), general description
• Most widely employed
• Application of effluent to land for treatment
and to meet growth needs of plants, i.e.,
grasses
• Applied by sprinkling or other surface
technique
• Application rate dictated by crop needs or by
need to protect surface and ground water
Spray Irrigation
Municipal, commercial,
cluster residential, or
industrial
Forest or agricultural
crops (Bermudagrass/rye)
Forest: less management,
but more land
How NOT to do it!
Effluent or Wastewater
Slow Rate Irrigation
Irrigation onto land to
support vegetative growth,
with no direct discharge to
surface water
Spray Irrigation
With filtering
and possibly
disinfection
first, can be
used to irrigate
parks or golf
courses.
Drip Irrigation
•Commercial or cluster
residential
•Some surface drip lines,
mostly buried 8-12 in.
•Have to have good
filters for particulates
•Usually septic tank then
ATU (aerobic) package
plant
Drip Irrigation
•Anecdotal evidence:
-Larger systems >10
acres tend to
experience more
problems
-Shopping center
systems tend to have
more problems
Overland flow, general description
• Basically involves the use of a sloped land
surface as a fixed film treatment system
• Needs impervious tight soils as infiltration is
usually kept to a minimum
• Flow is provided by land slope
• Treated waste is collected at the bottom of
the system to be further managed/discharged
Overland Flow
Discharge system in which
wastewater is treated as it flows
down grass-covered slopes. Soils
must have low permeability to
minimize percolation.
Rapid Infiltration, general description
• Application to a small area at very high rates
so that most of the water moves into the soil
• Involves treatment but also groundwater
recharge
• Wastewater may eventually recharge
groundwater
• Pumped or flows onto surface
• Generally uses unlined basins
Drainfields
Commercial, cluster
residential, single
residential
2,000–150,000 gpd
Septic tank and trenches
What Makes
It All Work!
Annual Rye
Bermudagrass
Plants!
Forests
Plant/Soil System
• Soil organisms and plants –
absorb nutrients, breakdown
organics
• Soil chemical characteristics
– hold metals
• Removing aboveground
plants - removes nutrients
What Makes It All Work
Actinomycetes
Protozoa
Mycorhizzae
Roots
Mites
Soils
How is treatment accomplished?
• Removal of organics (BOD)
– Aerobic fixed-film biological treatment
(soluble BOD)
– Filtration of organic solids by vegetation
and soil (particulate BOD)
How is treatment accomplished?
• Removal of nitrogenous compounds
– Two basic mechanisms
• Plant uptake and subsequent harvesting
• Nitification and Denitrification
NH3 → NO3 → N
Note: Denitification requires anaerobic conditions as
associated with rapid infiltration. Nitrogen not removed
may have important groundwater quality implications
Removal of phosphorus compounds
• Most removal by chemical means, namely
precipitation
• Adsorption onto soil particles also possible
• Uptake by plants also occurs
Important Design Criteria
•
•
•
•
•
•
•
Hydraulic loading
Organic loading
Nitrogen loading
Heavy metals loading
Cation concentrations
Crop type (corn v. sod)
Climate
Effects of Exchangeable Cations
• Sodium is typically present in all wastewaters,
along with Mg and Ca. Some crops may
exhibit specific ion toxicity.
• Concerns relate to potential for deflocculation
of soil and swelling of clay particles in the
presence of excess sodium.
Sodium Adsorption Ratio (SAR)
SAR = (Na)/[(Ca + Mg)/2]0.5
Where:
SAR = Sodium adsorption ratio
Na = Sodium concentration, milliequivalents/L
Ca = Calcium concentration, milliequivalents/L
Mg = Magnesium concentration, milliequivalents/L
A SAR of 10 or less should be acceptable on soils with
significant clay content (15 percent clay or greater).
Soils with little clay, or non-swelling clays can
tolerate an SAR up to 20.
What Makes It
All Work!
Soil!
•Habitat (mixture of
solid, water, and air
•Holds minerals and
metals
•Acts as filter
Adapted from Tisdale et al, 1993
Natural systems affected by
the environment
•
W eather
•
I nsect pests
•
Stresses
-too much water,
-not enough of certain nutrients
Constructed Wetlands
Discharge system where
wastewater treated by
plant/soil system then
discharged to stream.
Non-discharge system
where treated water
infiltrates or evaporates.
Constructed Wetlands
• Municipal, commercial,
cluster residential, single
residential, or industrial
• Septic tank or other
treatment, then wetland
• Free water surface and
vegetated submerged bed.
Choosing the Right Land-based System
Site Characteristics
Capital Costs
Waste strength
Land
Waste Flow
Equipment
Soils & Geology
Construction
Topography
Sensitive areas
Operating Costs
Electricity
Labor
Maintenance