PD

-Sw

20

5w

1

PD

-Sw

20

5w

1

STUMOD – A Practical Tool for Predicting Nitrogen Removal in Soil Treatment Units

2015 Convention & Tradeshow

March 5 – 8, 2015 Marriott at River Cree, Edmonton, AB

Mengistu Geza Civil and Environmental Engineering, Colorado School of Mines, Golden, CO

PD

-Sw

20

5w

2

Experimental studies

Treatment of organics, nutrients, and pathogens

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

Total NNO3-NNH4-N

Depth (cm)

Co

ncen

trati

on

(m

g/l)

STUMO

D

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

Total NNO3-NNH4-N

Depth (cm)

Co

ncen

trati

on

(m

g/l)

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350

Total NNO3-NNH4-N

Depth (cm)

Co

ncen

trati

on

(m

g/l)

STUMO

D

Colorado School of Mines Onsite Wastewater research Thrust

Mathematical modeling

• Analytical and numerical methods

e.g. Hydrus 2D, STUMOD

• Field testing using soil treatment units,

dispersal cells and networks in sandy

loam soil at the Mines Park Test Site

• Lab testing using porous media in 1-D

columns, 3-D tanks, 2-D microcells

PD

-Sw

20

5w

3

STUMOD

STUMOD-HPS

User friendly tools for modeling saturated and unsaturated zone nitrogen transport

1. Unsaturated zone- concentration at the water table - STUMOD

2. Saturated zone-concentration down gradient from STU (well or water body) - STUMOD-HPS

PD

-Sw

20

5w

4

Modeling the Expected Performance of Wastewater Soil Treatment Units

Development of tools to estimate performance of Soil Treatment Units

• Look-Up Tables developed using outputs from a numerical model that provide insight for treatment based on generalized site and operational conditions

• STUMOD: A vadose zone model; for fate and transport of nitrogen

A simple to use practical tool but detailed enough to account for major STU processes?

Development of quantitative tools WERF Funded Project Project No. DEC1R06

WERF FINAL REPORT - DEC1R06: DEVELOPMENT OF QUANTITATIVE TOOLS TO DETERMINE THE EXPECTED PERFORMANCE OF UNIT PROCESSES IN WASTEWATER SOIL TREATMENT UNITS.

PD

-Sw

20

5w

5

• Removal through sorption, reaction, plant uptake

• Steady soil moisture & concentration profiles

• Effect of degree of saturation on nitrification & denitrification

• The effect of temperature and concentration

• Carbon content depth function for denitrification

• Root water and nutrient uptake

• Heterogeneous layers, a biomat, capillary zone effects

• Accepts NH4, NO3 or both as an input (STE or after secondary treatment)

5

What can STUMOD do? Unsaturated zone

PD

-Sw

20

5w

6

6

• Simple for operation

• Physically based (Soil moisture profile is based on Darcy’s equation-

Chemical transport is based on Advection Dispersion Equation-ADE)

• Scenarios can be evaluated in a relatively short time

Advantages

2

2L x

dc C CR D v R C

dt x x

Geza, et al, 2013. STUMOD- a Tool for Predicting Fate and Transport of Nitrogen in Soil Treatment Units. Environ Model Assess DOI 10.1007/s10666-013-9392-0

STUMOD has since then been modified !

PD

-Sw

20

5w

7

Adjusted nitrification and denitrification rates

Degree of saturation (%)

Rate adjustment

factor

• Degree of saturation - Degree of saturation used to account for the

effect of aeration on nitrification and

denitrification

• Adjusted rate = Optimum/maximum rate x a factor for degree of saturation {0 to 1} x a factor soil temperature {0 to 1} x a factor for concentration {0 to 1} x a factor for carbon content

• Soil temperature

• Concentration

• Carbon content

Nitrification response function Denitrification response function

Temperature (oc)

25oC

Temperature response function Concentration response function

An exponential decrease away from optimum (25oC)

Concentration (mg/L)

PD

-Sw

20

5w

8

STUMOD Improvements

Florida Onsite Sewage Nitrogen Reduction Strategies Study

PD

-Sw

20

5w

9

9

Co

C

qz Dz

• Outflow concentration from a segment above becomes inflow or boundary concentration to segment below

Multiple layers

• Removal may vary from segment to segment based on soil moisture content and concentration

• Allows up to 4 soil layers including a biomat

• Soil hydraulic properties can be varied by layer

• Each layer is further divided into segments

Biomat

PD

-Sw

20

5w

10

• Nutrient uptake in STUMOD is driven by actual

Evapotranspiration (ET) and nutrient availability

• Nutrient uptake = Concentration X upward water flux (ET)

= < crop nutrient demand

Plant nutrient uptake

• Both NO3 and NH4 species are assumed to be equally

available to plants (no preference by plants)

• Important factors

- Crop nutrient demand (kg/ha/day) - a user input

- Soil water nutrient concentration calculated by the model

- ET calculated by the model

PD

-Sw

20

5w

11

Boundary conditions

• Shallow water table - soil moisture affected by capillary rise; a pressure head of zero at the water table

• Deep water table - no effect on soil moisture due to capillary (free drainage) -mimicked by setting the water table to a greater depth

• Thickness of unsaturated zone; Two options available in STUMOD

- User input water table depth

- Water table depth calculated by the model

• Analytical model for water table fluctuations in response to

precipitation was implemented and applies to Florida

conditions only

PD

-Sw

20

5w

12

STUMOD inputs and outputs

• Effluent concentration (NH4, NO3)

• Hydraulic Loading rate

• Parameters for soil moisture characteristic curve

- [n, a, qs, qr , Ksat, l]

• Denitrification rate

- Parameters for soil moisture response curve

• Nitrification rate

- Parameters for soil moisture response curve

• Soil Temperature

- Parameters for soil moisture response curve

• Sorption rate

• Concentration @ water table or any depth from infiltrative surface

(NH4, NO3 and total N)

• Mass loading

• Soil moisture distribution

Outputs

PD

-Sw

20

5w

13

User interface

PD

-Sw

20

5w

14

NO3

NH4 NH4

NO3

• Faster nitrification in sandy soil due to relatively low steady state moisture content • Higher moisture content in clays slows nitrification but improves denitrification

33 %

46%

STUMOD outputs- Sandy and Clay soil

PD

-Sw

20

5w

15

STUMOD outputs – effect of heterogeneity - A homogeneous layer = 60 cm sand layer - Heterogeneous layer, top 30 cm- sandy soil bottom 30 cm-clay soil

Effect of plant nutrient uptake

STUMOD outputs – Effect of heterogeneity & plant uptake

Improved removal with heterogeneous layer, improved nitrification in the top sandy layer followed by improved denitrification in the bottom clay layer

33 %

40 %

33 %

40 %

PD

-Sw

20

5w

16

Higher hydraulic loading rate • Increased in soil moisture content • Limits nitrification in some soil textures • Enhances denitrification • Reduces travel time limiting removal

Interaction among factors affecting treatment

Soil texture • Soil moisture content differs by texture for a given loading • Travel times differ by texture for a given loading rate • Capillary rise differs by soil texture

Depth to water table • Increased soil moisture at the capillary zone may enhance denitrification • Increased soil moisture due to capillary rise may limit nitrification for shallow water

table conditions

Effluent quality STE vs Nitrified effluent • Benefits from nitrifying for clayey textures and shallow water table conditions

Nomographs demonstrating the interactions among several factors for a site can be developed using STUMOD.

PD

-Sw

20

5w

17

PD

-Sw

20

5w

17

Sandy soil - STE

Depth to Water Table (ft)

Rem

ain

ing

co

nce

ntr

ati

on

(%

)

Rem

ain

ing

co

nce

ntr

ati

on

(%

)

Depth to Water Table (ft)

Rem

ain

ing

co

nce

ntr

ati

on

(%

)

Rem

ain

ing

co

nce

ntr

ati

on

(%

)

Depth to Water Table (ft) Depth to Water Table (ft)

Clayey soil - STE

Sandy soil – Nitrified effluent Clayey soil – Nitrified effluent

PD

-Sw

20

5w

18

STUMOD performance evaluation

Soil moisture profile-Hydrus Vs STUMOD

Loading rate = 2 cm/day (0.5 gal/day/sq.ft)

PD

-Sw

20

5w

19

STUMOD Evaluation

Comparison to numerical model and field data

• Field data collected during six sample

events

• Samples collected inlude the outflow from the septic tank

• Depths: 1, 2, 3.5 ft below infiltrative surface

• Analyzed for TN, NO3-, NH4

+, Organic N

• STUMOD run with default parameters

SE1 SE2 SE3 SE4 SE5 SE6

2012 June

Aug Oct Jan Mar 2013 June

• SE2, SE4, SE6 had a considerable rainfall events in the days preceding the sampling events and were not applicable to STUMOD steady state model

PD

-Sw

20

5w

20

STUMOD Evaluation

0

10

20

30

40

50

60

70

80

90

100

SE1 SE2 SE3 SE4 SE5 SE6

To

tal N

itro

ge

n (

mg

-N/l

-N)

Sample Events

Predictions vs Observations at 2 ft Depth

STUMOD-FL

HYDRUS 2D

Observation

PD

-Sw

20

5w

21

Saturated zone modeling

STUMOD-FL

Aquifer

Model

Introductio

n

• A saturated zone module has been developed • STUMOD is linked to a saturated zone module. Outputs from the unsaturated zone model

(STUMOD) are used as inputs to the saturated zone module. • Boundary concentration (concentartion at the water table) estimated using STUMOD

PD

-Sw

20

5w

22

Groundwater transport models are based on the advective-dispersive-reactive equations.

rate of change in conc. at any point

=

net rate of advective transport to that point

+

net rate of dispersive transport to that point

net rate of denitrification at that point

-

Transport at time t advection only

Transport at time t with dispersion

Transport at time t with dispersion & denitrification

Saturated zone transport processes

PD

-Sw

20

5w

23

Analytical Solutions

Vertical Plane Source Solution-Domenico & Robbins, 1985; Wexler 1992

• Considers a vertical contaminant source plane Solution exists in closed form • Requires a few assumptions and estimations in order to be

used to model an OWS

• The geometry of the contaminant source for HPS is consistent with that of an OWS –horizontal plane like

OWS trench, not vertical • No closed form solution available

Horizontal Plane Source Solution- Galya, 1987

Horizontal Plane Source model inputs

• Source Plane Dimensions-Width, Length

• Hydraulic gradient, Hydraulic conductivity, Porosity

Groundwater Seepage Velocity

• Mass loading-Concentration, Volumetric flux

• 3D Dispersivity Coefficients

• Retardation (NO3- , R = 1)

• 1st Order Decay (λ=0, no decay)

PD

-Sw

20

5w

24

PD

-Sw

20

5w

25

HPS outputs

• Mass Flux passing through a vertical or horizontal plane

• Plume delineation (depth, width)

• Centerline concentration along the flow path

PD

-Sw

20

5w

26

Comparison to numerical model and field

observations

R² = 0.66

0

5

10

15

20

25

30

0 5 10 15 20 25 30

HP

S P

red

icte

d N

O3

[m

g-N

/L]

Observed NO3 [mg-N/L]

Observed NO3 vs HPS NO3

R² = 1.00

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.05 0.1 0.15 0.2 0.25 0.3

HP

S R

esu

lts

[mg

/l]

MODFLOW/MT3DMS Results [mg/l]

PD

-Sw

20

5w

27

STUMOD Hypothetical Example

Site location: Florida Soil texture: sandy soil Treatment goal: 50% reduction of total nitrogen Soil depth: 60 cm below the trench bottom

PD

-Sw

20

5w

28

1.Determine the quality of the septic tank effluent • Use look-up table or CFD • Your own data • May engineer to a certain value to

achieve desired output (iterative, using STUMOD)

Hypothetical Example

Median: NH4 = ~ 60 mg-N/L

PD

-Sw

20

5w

29

3. Estimate the average annual soil temperature and/or temperature zone

Note: temperatures are in °C

Hypothetical Example

2. Estimate an appropriate HLR (< 5% of KSAT) – used 2 cm/day

PD

-Sw

20

5w

30

Hypothetical Example 4. Use STUMOD

~40 mg/L

33%

~33 % removal

PD

-Sw

20

5w

31

Hypothetical Example

5. If goal is not met, refine parameters to achieve treatment goal

~50% removal

• Reduce HLR to 1 cm/day, what will happen?

• Goal met – 50% removal at 2 ft

• Reducing HLR could be achieved by increasing area of application, 2x the original size

• This means increasing cost

PD

-Sw

20

5w

32

Hypothetical Example

6.How about if denitrification is enhanced with added carbon and denitrification rate is 2x the median value of 2.58 obtained from literature?

50% removal at ~ 50 cm depth The 50% goal met at 50 cm

38%

PD

-Sw

20

5w

33

Evaluate effect of 1 cm/d loading vs. 2 cm/d loading on on

concentration and mass loading down gradient of the STU ?

40 mg/L input Mass flux at 90 m down gradient = 58.77 kg/yr 30 mg/L input

Mass flux at 90 m down gradient = 38.45 kg/yr

-

10

20

30

40

50

60

0 200 400 600 800 1000

Nit

rate

(m

g/L

)

Distance from Source (m)

Center line concentration (mg/L)

• 2 cm/d loading- concentration reaching the water table = 40 mg/L • 1 cm/d loading – concentration reaching the water table = 30 mg/L

Effect on center line concentration

40 mg/L input to water table

30 mg/L input to water table

• Use the Horizontal plane source solution • Evaluate effect on mass flux 90 m down gradient from source

PD

-Sw

20

5w

34

Using Models for Decision Making

Model uncertainty

The facts

• The “real world” is uncertain.

• Subsurface processes are complex.

• Predictive models are uncertain.

• Model uncertainty can be quantified – enables a more-informed decision.

PD

-Sw

20

5w

35

STUMOD Sensitivity Analysis Results – Unsaturated zone

van Genuchten n Threshold soil moisture Soil Temperature Denitrification rate Porosity Hydraulic loading rate Effluent concentration Van Genuchten alpha Denitrification exponent Gardner’s alpha parameter Saturated hydraulic conductivity Residual moisture content Nitrification rate Nitrification soil moisture function Temperature factor Sorption

Relative sensitivity ST

UM

OD

pa

ram

ete

r

ST

UM

OD

pa

ram

ete

r

Sandy soil

PD

-Sw

20

5w

36

Uncertainty analysis-STUMOD

Range of inputs Range of outputs

Knowledge of possible ranges in the out put can help understand the risks involved and enable a more informed decision making

For a treatment depth of 1 ft, there is a 60% probability that 50% of nitrogen is removed

For a treatment depth of 2ft, there is a 90% probability that 50% of nitrogen is removed

~60%

~90%

PD

-Sw

20

5w

37

Parameter sensitivity analysis results

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

λ [1/yr] n [-] R [-] grad

[m/m]

Ksat

[m/yr]

HLR

[m/yr]

Conc

[mg/l]

L [m] αx [m] αz [m] B [m] αy [m] H [m] NumT

No

rma

lize

d S

ensi

tiv

ity

Model Evaluation

Source Plane Dimensions

• Width, Length

Groundwater Seepage Velocity

• Hydraulic gradient

• Hydraulic conductivity

• Porosity

Mass loading

• Concentration

• Volumetric flux

3D dispersivity coefficients

Retardation (NO3- , R = 1)

1st Order Decay (λ=0, no decay)

Integration time

B, L

V

𝛂𝐱, 𝛂𝐲, 𝛂𝐳

R

λ

T

𝐧

𝐂𝐨

𝐇𝐋𝐑 ∗ 𝐋 ∗ 𝐁

𝐌𝐨

i

Ksat

PD

-Sw

20

5w

38

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Cu

mu

lati

ve

Pro

ba

bil

ity

[%

]

Mass Remaining, M/Mo [g/g]

Uncertainty Analysis Results (McCray et al [2005]

reported values)

SCL

SLP

SMP

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Cu

mu

lati

ve

Pro

ba

bil

ity

[%

] Mass Remaining, M/Mo [g/g]

Uncertainty Analysis Results (50th percentile and

below denitrification rates)

SCL

SLP

SMP

uncertainty analysis results for a Sandy Clay Loam (SCL),

Less Permeable Sand (SLP) and More Permeable Sand

(SMP) utilizing denitrification up to 0.025 (1/day) which is the

50th percentile value reported by McCray et al. [2005].

uncertainty analysis results for a Sandy Clay Loam (SCL), Less

Permeable Sand (SLP) and More Permeable Sand (SMP) utilizing

denitrification values reported by McCray et al. [2005]. These results

do not consider the case of no denitrification or little denitrification. The

minimum denitrification value used is 0.004 (1/day).

Uncertainty analysis-STUMOD-HPS

Range of inputs Range of outputs

PD

-Sw

20

5w

39

Practical use

Scenario Evaluation • Practitioners can perform rapid evaluations of scenarios that may influence

nitrogen treatment such as the influence of nitrogen concentrations in septic tank effluent, pretreatment, soil properties, loading rates, and temperature

System Design • Hydraulic loading rates, effluent quality, trench size, or loading area can be

evaluated in light of nitrogen-treatment performance goals

Rigorous Assessment • STUMOD can be calibrated to actual site and subsequently used to optimize

operation of OWTS or to design new OWTS in a similar soil setting

PD

-Sw

20

5w

40

• STUMOD is a user friendly practical tool that can provide cost-effective evaluations on nitrogen removal strategies in STUs

• The model is relatively simple to use but detailed enough to account for

important fate and transport processes. • The input parameters for nitrogen transport and transformation were

derived based on a thorough literature review and statistical analysis of the available data

• The model accounts for the effect of aeration on nitrification and denitrification rates via soil moisture content. It also accounts for the effect of temperature & concentration.

Summary and Conclusion

PD

-Sw

20

5w

41

• Both the measured data and STUMOD output show a relatively higher removal in clayey soils compared to sandy soils.

• For sandy soils STUMOD predicted ammonium conversion to nitrate within

the first foot below the trench infiltrative consistent with field data. • STUMOD results showed that ammonium persisted relatively deeper below

the trench in finer grained soils and high loading rates due to low nitrification rates caused by high predicted water content

• Interactions between several factors including loading rate, distance to water table, effluent quality (STE vs nitrified effluent), capillary rise effects, soil texture, plant nutrient uptake can be evaluated in a relatively short time as demonstrated by the nomographs

Summary and Conclusion

PD

-Sw

20

5w

42

• The unsaturated zone model is linked to the saturated zone model • The saturated zone model provides the user with an estimate of

nitrate concentration or mass flux down gradient of an OWS • Its user friendly design will make it available to a large user group

that has not had access to such a tool • The capability of the aquifer model has been demonstrated by

calibration to observation data.

• The HPS solution is desirable because it considers the geometry of an OWS compared to VPS Domenico Solution

Summary and Conclusion

PD

-Sw

20

5w

43

Future plans

• Additional wastewater contaminants DOC, CEC and pathogens,

phosphorus

• Site specific parameterization

PD

-Sw

20

5w

44

PD

-Sw

20

5w

44

QUESTIONS? Contact Info

Dr. Mengistu Geza: mgezanis@mines.edu

Dr. John McCray: jmccray@mines.edu