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