Soil Chemical and Biological
Properties Relative to
Wastewater Treatment
John R. Buchanan, Ph. D., P. E. Associate Professor
Biosystems Engineering and Soil Science Department
Soil Chemical and Biological
Properties Relative to
Wastewater Treatment • Ultimate goal
– To convert wastewater back to water
• Intermediate goal
– To protect environmental and public health
– We separate humans from their wastes
• by putting wastewater in the soil
– And we separate the wastes out of the water
• by putting wastewater in the soil
Important Note
• We are very dependent on the soil
– to provide treatment
• This is not subsurface wastewater disposal
– This is wastewater renovation that disperses
water back into the hydrologic cycle
• recharges groundwater
• baseflow to streams
• evapotranspiration
With this Dependence on the
Soil
• We have to understand that all soils are
not created equal
– We have to understand the renovation
potential provided by different soils
– and quantify these differences by
understanding the
• physical properties of soil
• chemical properties of soil
• and the biological properties of soil
When using the Soil for
Wastewater Treatment
• We have to:
– Understand the quantity of waste constituents
in the wastewater
• let’s call it wastewater strength
– Understand how much wastewater we are
working with
• wastewater volume
– Understand how wastes are removed from
water
• wastewater treatment
This Session
• This session will focus on
– soil chemical properties
– soil biological properties
• All with the intention of converting
– wastewater back to water
Wastewater Defined
• Wastewater is water that has been used to
collect and transport waste
– Water that carries waste constituents
• suspended – easy to remove
• dissolved – difficult to remove
Water is the Universal Solvent
nDepartment of Biochemistry and Molecular Biophysics
The University of Arizona
When Wastewater…
• Infiltrates into the soil
– constituents dissolved in the water also
infiltrate into the soil and becomes part of the
soil solution
• This is where soil-based wastewater
treatment takes place
The Soil Solution
• Solution meaning
“a liquid that
contains dissolved
compounds”
• Water in soil
– there is a dynamic
equilibrium between
the liquid phase and
the solid phase
nhttps://dl.sciencesocieties.org/publications/nse/abstracts/36/1/45?access=0&view=article
So, our Chore is to Get Wastes
out of Water
• Is it difficult to get waste out of water?
– Yes, but we have a lot of help available to us
– Our team includes
• Gravity
• The sun
• Billions of microorganisms
• And, the soil
Drivers of the hydrologic Cycle
Ultimate Decomposers
The basis for all wastewater treatment
Wastewater
• By weight
– Is 99.9% water
– It is the 0.1% that we have to remove
• That 0.1% contains
– Organic matter
– Microorganisms (a few of which are pathogenic)
– Inorganics compounds
Wastewater Strength
• Typical wastewater constituents
– Solids
– Suspended and dissolved organic matter
– Pathogens
– Nutrients
– Personal care products and pharmaceuticals
We Need this Information
• So that we can design a soil-based
wastewater renovation system
– than can remove the wastes from water
– protect groundwater resource
– protect drinking water supply
The Soil is a Treatment Media
PROCESSES AT WORK
MEDIA
BIOLOGICAL
MASS
LIQUID WASTES
ORGANICS
END PRODUCTS
AIR
PATHOGENS
EXCESS
CELL MASS
B.O.D.
SS
NUTRIENTS
A Conundrum
• Homeowners can add chemicals to their
wastewater that can damage the soil
– especially clayey soils
• salts - water softeners
• strong acids – drain cleaners
• strong bases – ammonia cleaners
It is very Interesting
• That the “Alberta Onsite Manual”
specifically talks about
– dispersion of clay particles
– and you look at the Sodium Adsorption Ratio
of the source water
• We don’t want our clay to disperse and
limit the permeability
– this is a soil chemistry issue
Clays
• The reactive portion of the soil
– sand and silt are just part of the soil skeleton
• For onsite wastewater system design
– we often look at clay as being bad
– it increases the size of our system
– but if you can get water to go through it
• it is a fantastic wastewater treatment media
Clays attract Cations which
attract Anions and so on
Not all Clays are Bad
But, If you have Smectitic Soils
• If you want to locate
a onsite system in
– high shrink/swell clay
– then clay is bad
nhttp://faculty.yc.edu/ycfaculty/ags105/week08/soil_colloids/soil_colloid
s_print.html
Cation Exchange Capacity
• A soil fertility measurement
– total number of cations the soil can hold and
make available for exchange with the soil
solution
– can provide insight into soil quality and site
characteristics
• higher CEC – clayey and limited internal drainage
• lower CEC – sandy with low water holding
A Swarm of Ions
There are a Lot of Ions in
Wastewater
n“Next to photosynthesis and respiration, probably no
process in nature is as vital to plant and animal life
as the exchange of ions between soil particles and growing
plant roots.” nNyle C. Brady
If Ions in Wastewater
• Can react with ions with the clay surfaces
– then those ions are removed from the
wastewater
• sodium
• calcium
– positively-charged organic compounds
• trace organics (pharmaceuticals)
• pesticides
If the Ions in Wastewater
• Cannot react with the clay surfaces
– then ions can move with the percolating water
• most notably is Nitrate (NO3-)
• nitrate is an ion and does not bind to the soil
– This is why there is so much discussion about
nitrates in groundwater
• from wastewater
• from fertilizer
• some toxicity to infants
Phosphorus
• We excrete phosphorus in a form bound to
organic compounds
• Released to environment during
degradation of organic compounds
• Treatment process
– WWTP uses chemical precipitation
– onsite systems use the soil
Phosphorus
• Orthophosphate
– is the reactive form of
phosphorus typically found
dissolved in water
– an anion
• PO4 -3
– can be a limiting nutrient
• causing eutrophic conditions
nhttp://www.chemspider.com/Chemical-Structure.1032.html
Phosphorus Fixation
• Cations in the soil bind with phosphate
– Ca2+
– Al3+
– Fe3+
– Each forms an insoluble precipitant
• Naturally occurs in clayey soil systems • not in sandy soils
• not much iron available
Phosphorus Movement
nhttp://faculty.yc.edu/ycfaculty/ags105/week12/biogeochemical_cycles_information/biogeochemical_cycles3.html
If Phosphorus Removal is
Required
• If the soil has sufficient
– iron, aluminum, calcium available,
• then phosphorus removal is assured
• However,
– in sandy soils
– phosphorus removal may have to be
conducted before effluent is applied to soil
Soil Biological Properties
• The soil is teeming with life
– dirt is not
• Bacteria, virus, fungi, nematodes, rotifers
– and many other critters
• All these life forms need nutrients
– and we provide them with plenty of food
– the real workhorses of wastewater treatment
Organic Compounds
• The waste portion of
wastewater is
largely organic –
carbon based
– needless to say
• so are the soil
microbes
– they need the carbon
and the other
nutrients
Energy
• As complex organic carbon is degraded to
the less complex inorganic form
– Energy is released
• think about a self-heating compost pile
• Becomes building block to create more for
soil-borne microorganisms
Breakdown of Organics
• Organic carbon is an energy source to
most microorganisms
2 2 2aerobic
microorganismsOrganic Carbon + O Energy + CO + H O + Residue
2 2 2new aerobic
microorganisms+ O Energy + CO + H O + Residue
2 2 2new aerobic
microorganisms+ O Energy + CO + H O + Residue
Organic Biodegradation • Fixed Film treatment on soil particle surfaces
Septic Tank Effluent
Microbial film
Soil Particle
Air in
unsaturated pore
space
Aerobic Biotransformation
• Dissolved oxygen is consumed in the
process of converting organic matter into
inorganic matter
2 2 2aerobic
microorganismsOrganic Carbon + O Energy + CO + H O + Residue
2 2 2new aerobic
microorganisms+ O Energy + CO + H O + Residue
2 2 2new aerobic
microorganisms+ O Energy + CO + H O + Residue
Aerobic or Anaerobic
• If aerobic conditions,
– organic carbon can be oxidized to carbon
dioxide gas and water
• If anaerobic conditions,
– much of the organic matter remains as dead
cells and forms the clogging layer
Degradation of Organic Matter
• Releases other compounds
– Typically in an inorganic form
• For example
– Nitrogen becomes ammonia/ammonium
• Creates an additional oxygen demand
– Phosphorus becomes ortho-phosphate
Nitrogen
• Urea and other nitrogenous compounds in
wastewater
• Nitrates and ammonium are plant available
• Treatment Process:
– WWTP use anaerobic digestion
– onsite systems use the soil
Biological Nitrification • Ammonia/ammonium is then converted to
nitrite and nitrate
– Nitrification
– Oxygen demand
• Nitrification is a two-step autotrophic process
– the conversion from ammonium to nitrate
Nitrosomonas
Step 1: NH4+ + 3/2O2 NO2
2- + 2H+ + H2O
Nitrobacter
Step 2: NO2- + 1/2O2 NO3-
Biological Nitrification • During this energy yielding reaction
– some of the NH4+ is synthesized into cell tissue
giving the following overall oxidation and
synthesis reaction:
– Nitrifiers use CO2 instead of organic carbon as
their carbon source for cell synthesis and for the
conversion of NH4+ to NO3
--N.
Autotrophic
1.00NH+ + 1.89O2 + 0.08CO2 0.98NO3- + 0.016C5H7O2N + 0.95H2O + 1.98H+
Bacteria new bacterial cells
Denitrification
• NO3- can be reduced,
– under anoxic conditions, to N2 gas through
heterotrophic biological denitrification
– Two issues
• Anoxic conditions
– almost no dissolved oxygen
– nitrate is available
• Heterotrophic bacteria
Biological Denitrification
• Totally cool process
– Nitrate has chemically-bound oxygen
– Through reduction/oxidation processes
• Oxygen is pulled from nitrate ion
• Nitrogen evolves as a gas form
Biological Denitrification
• Heterotrophic
– organic carbon as the carbon source
– The following unbalanced equation illustrates the
process when wastewater or bacterial cell material is
used as the carbon source:
COHNS + NO3
- N2 + CO2 + C5H7O2N + OH- + H2O + end products
Limitation on Denitrification
• Here is the rub
– we consumed the most of the organic carbon
in the previous step
– under aerobic conditions
• Thus, available organic carbon in the soil
becomes a limiting factor
– for dependable denitrification, use a treatment
system that recirculates nitrified effluent back
through the septic tank
Pathogen Removal
• Disease carrying organisms
– the original causation for wastewater
treatment and disposal
• separate humans from their wastes
• minimize the transmission of diseases and illnesses
– treatment process
• WWTP use disinfection: chlorine, UV
• onsite systems use the soil
Soil Based Pathogen Removal
• Soil can hold some
pathogens
– many soils have a
negative charge
– microbes with a
positive charge will
bond to the soil
• Predation
– some pathogens
provide a snack to
natural soil biota
Soil Based Pathogen Removal
• We typically measure E. Coli and fecal
coliforms as indicators for the presence of
true pathogens
– we almost never test for specific pathogens in
the soil – too expensive
– we make the assumption that if the indicator
organisms are reduced, then the pathogens
will be reduced
Soil and Pathogens
• The soil is not sterile
– Most of the microorganisms in the soil are
beneficial
– we depend on these beneficial microbes for
our existence
• they help grow our crops
– the good news is that human pathogens prefer
warmer temperatures than found in the soil
What we Don’t Know
• We don’t know is the fate of
pharmaceuticals and personal care
products
– our bodies only uses about 20% of the active
ingredient and excretes the remainder
– what about the metabolites, what are the
breakdown products
• We do know that many of these products
can survive a WWTP
When we Synthesize Organic
Compounds
• We create an organic compound that was
not created by a natural process
– and thus may be difficult to degrade by natural
processes
– a lot of energy goes into the process
• For example
– Some drugs are designed to survive the liver
– in some situations, the break-down product is
the active ingredient
Okay, so what’s going on in
the Septic System
• In relation to pharmaceuticals
– Science is just starting to think about it
• Why, because until recently,
– we could not measure on the ppb and ppt
level
– If your equipment can only measure in the
magnitude of ppm
• then we could not find the trace organics
So,
• For many of the pharmaceuticals
– we really do not know if we are doing a good
job of protection water resources
• We can identify some pharmaceuticals
– that are readily biodegradable
– that will bind with the soil
A Potential Management Tool
• Source separation
– pharmaceuticals and
their metabolites
tend to be excreted
in the urine
• If we separate
– what do we do with
the urine?
http://www.chekhovskalashnikov.com/human-waste-disposal/
“No-Mix” Toilet
• Men
– are you ready to sit
down?
– may have to get a
waterless urinal
• Dual plumbing
– urine storage
– solids and flush
water move on to
treatment
http://inhabitat.com/nomix-toilets-separate-waste-are-super-eco-friendly/
Treatment of the Future
• Quaternary treatment for trace organics
– Add more energy to the system
• UV light
– even exposure to sunlight can degrade many
recalcitrant compounds
• Ozone
– very strong oxidizers
– breaks apart larger compounds
Using the Soil as a
Wastewater Treatment Plant
• Must think past simple infiltration
– Will the water be sufficiently renovated before
it re-enters the hydrologic cycle?
– What is “sufficient” treatment?
• Onsite systems must be managed
– Cannot be installed and forgotten
– These systems can have a tremendous
influence on our environmental quality
The Soil has a Tremendous
Capacity to Renovate
Wastewater
• We just have to work within the limitations
– Of the soil resource
– And the site
Questions