Soil as a Wastewater Treatment Medium

By

J.G. Mexal

Department of Agronomy & Horticulture

New Mexico State University

Soil as a Treatment Slow Rate Land Application System• “Slow rate land application for wastewater

treatment is a proven technology for municipal and other organic wastewaters. Used for over one hundred years, it has evolved from a ‘disposal’ method to one that can be used to recycle wastewater onto agricultural crops, forests, or park lands.”

• EPA 1992

Soil as a Treatment

Who Does It?Location Wastewater Size (ac) SpeciesJacksonville, FL secondary 6,000 ‘forest’

Clayton Co., GA secondary 3,600 Loblolly pine

Myrtle Beach, SC secondary 1,500 Hardwoods

Edenton, NC chlorinated 300 Sycamore

Woodburn, OR secondary 80 Poplar

Adelaide, So.Aust.

Secondary 35 Eucalyptus

Soil as a Treatment

Site Characteristics

Characteristic AcceptabilityPoor Fair Best

Depth <20” 20-40” >40”Slope (%) >8 3-8 <3Texture clay sand loamEC (dS/cm) >8 2-8 <2AWC (in/ft) <0.6 0.6-1.2 >1.2SAR >8 4-8 <4

Soil as a Treatment

Wastewater Loading Rates/ Tyler & Assoc.

Soil Texture Loading Rate (in/day)

Wastewater Water

Septic Sand Filter

Sand 1.3 10.0-20.0 400

Loam 0.7 2.5-5.0 20

Clay 0.3 0.8-2.4 2

Soil as a Treatment

Wastewater Infiltration

A = RestaurantB = SepticC = GrayD = Tap

Soil as a Treatment

Wastewater Composition/ Metcalf & Eddy

Contaminant Unit Weak Medium StrongTDS mg/L 250 500 850TOC “ 80 160 290BOD5 “ 110 220 400Nitrogen “ 20 40 85CaCO3 “ 50 100 200Coliforms #/100ml <107 <108 <109

Soil as a Treatment

Survival of Human Parasites

Parasite Time to Kill 90%

Protozoans 2-10 days

Viruses 6 days

Ascaris worms 17 days

E. coli 30 days

Soil as a Treatment Land Application Performance

Parameter Removal Comments

BOD5 94-99% Percolate < 1 mg/L

Nitrogen 65-95% Function of crop &management

Phosphorus 75-99% Plant uptake ~ 25%

Pathogens >99% --

Organics varies Removesbiodegragables

Soil as a Treatment Wastewater Treatment OptionsSystem Advantages Disadvantages

Conventional Small landrequirementReturn clean water

ExpensiveComplicated

ConstructedWetlands

Moderate landrequirementReturn clean waterInexpensive (?)

No source of revenue

Land Application(Slow Rate)

InexpensiveSource of revenueBest of LA systems

Large land requirementNo water return to riverWinter use/storageconcern

Soil as a Treatment Wastewater Treatment OptionsSystem Construction Costs

($/1,000 gal/day)O & M Costs

($/1,000 gal/day)

Conventional $3,000 $1.00

Constructed Wetlands $700 - $3,000 “negligible”

Land Application(Slow Rate)

$1,000 $0.20[$0.05]

Wastewater Treatment Options/ PSC

Soil as a Treatment

Conceptual Mass Balance Diagram

Soil as a Treatment

Water Characteristics--OjinagaParameter Wastewater Rio Grande Well Water

EC (dS/m) 2.9 2.3 2.5

Coliforms (mpn) 105 150 --

BOD5 ~35 3 14

Nitrate N (mg/L) 0.16 0.05 1.8

Kjeldahl N (mg/L) 25 11 0

Chloride (mg/L) 220 560 191

Soil as a Treatment Species Selection--Ojinaga / 2 yrs

Salt Content (%)Species Clone Survival(%)

Height(m) Ca++ Cl-

Populus 376 88 6.4 1.30 0.29

Populus 029 67 4.8 0.99 0.52

Populus 197 24 3.9 0.90 0.53

Eucalyptus SC5 98 5.2 0.80 0.49

Eucalyptus 4019 94 5.2 0.67 0.51

Eucalyptus 4016 97 5.2 0.61 0.75

Soil as a Treatment Groundwater Quality/ OjinagaParameter Before (8/97) After (8/98)Coliforms (mpn) 0 0Kjeldahl N (mg/L) 30.91 1.5Nitrate N (mg/L) 0.4 0.2EC (dS/cm) 3.5 3.9 (+12%)

TSS (mg/L) 18,1381 266Chloride (mg/L) 292 339 (+16%)

Land Application System/West Mesa

Pipes

H20

H20Wastewater(H20, OM, N, Salts)

N,P,Ca,Mg,K

N2

Soil Matrix/PrecipitantP,Ca,Mg

PCa

Mg Cl Na

Cl Cl

Cl

PP

P

Na

NaNaCa

Ca

Ca

Mg

Mg

MgMg

Mg

P

PP

Cl Cl

Cl

Cl

Caliche

CO2

Soil as a Treatment

Limitations of SRLA System

• Percolate < 10 mg NO3- -N

• Balance water- vs N-loading

• Salt loading limits plant productivity

• Growing season limits water use

Soil as a Treatment

Conclusions

• A functioning soil-plant ecosystem is an effective wastewater treatment and water reuse system.

• Arid environments create unique situations that preclude ‘off-the-shelf’ adoption.

• SRLA systems can be economical, sustainable treatment systems.