A simple method for estimating design hydraulic loading rates in ...

A Simple Method for EstimatingDesign Hydraulic Loading Ratesin OWSs 1David Radcliffe and Larry WestUniversity of Georgia1Presented at the CSREES National Water Conference,February 3-7, 32008, Reno NV

Onsite Wastewater Systems75% of new homes in Georgia are on OWSs

Absorption Field Trench

Design Hydraulic Loading Rate• Wastewater infiltration rates decline over timedue to formation of a biomat• Estimate of final infiltration rate needed todetermine design hydraulic loading rate (HLR D )• Trench line length based on estimate of HLR D• Also referred to as Long Term Acceptance Rate(LTAR)• LTAR includes effect of organic loading rate• Do not consider organic loading rate in this analysis

Design Hydraulic Loading Rate• Siegrist (2007)• HLR D varies widely among states & often based onempirical evidence• Need more rational and uniform approach to estimateHLR D• HLR D should be based on trench bottom flux undershallow ponding conditions (normal loading)• Reserve trench volume and sidewall flux for peak flow (peakloading)• Use computer models to aide design

Bouma Equation• Bouma (1975) developed simple equation forestimating trench bottom fluxK h h Z0−s+bsZb= Kh ( )• K bs = biomat saturated hydraulic conductivity• Z b = biomat thickness• h 0 = height of ponding in trench• h s = soil matric potential below biomat• K(h s ) = soil unsaturated hydraulic conductivity belowbiomatbs

Bouma EquationKh ( s)Hydraulic Conductivity or Biomat Flux (cm/d)1 . 1001010.10.010 20 40 60 80 10010 3 Absolute Value of Matric Potential Head (cm)?

Hydraulic Conductivity or Biomat Flux (cm/d)Bouma EquationK h h Z sbsZ0 − + ?1 . b1001010.10.010 20 40 60 80 10010 3 Absolute Value of Matric Potential Head (cm)b

Bouma EquationHydraulic Conductivity or Biomat Flux (cm/d)K h − h + Z0 s bbs= Kh (s)Z1 . b1001010.10.010 20 40 60 80 10010 3 Absolute Value of Matric Potential Head (cm)

Bouma EquationHydraulic Conductivity or Biomat Flux (cm/d)K h − h + Z0 s bbs= Kh (s)Z1 . b1001010.10.010 20 40 60 80 10010 3 Absolute Value of Matric Potential Head (cm)

Objectives• Develop simple method for estimating HLR Dbased on soil and biomat hydraulic properties• Use Bouma equation to estimate trench bottom fluxunder shallow ponding for 12 soil textural classes• Compare Bouma equation estimates to 2D computermodel (HYDRUS)• Take ½ flux as estimate of HLR D (safety factor of 2)• Compare estimates of HLR D to classification systemsthat have been proposed

Soil textural class K s n α θ r θ scm d -1cm -1cm 3 cm -3Sand 642.98 3.18 0.0353 0.053 0.375Loamy sand 105.12 1.75 0.0347 0.049 0.390Silt 43.74 1.68 0.0066 0.050 0.489Sandy loam 38.25 1.45 0.0267 0.039 0.387Silt loam 18.26 1.66 0.0051 0.065 0.439Clay 14.75 1.25 0.0150 0.098 0.459Sandy clay loam 13.19 1.33 0.0211 0.063 0.384Loam 12.04 1.47 0.0111 0.061 0.399Sandy clay 11.35 1.21 0.0334 0.117 0.385Silty clay loam 11.11 1.52 0.0084 0.090 0.482Silty clay 9.61 1.32 0.0162 0.111 0.481Clay loam 8.18 1.41 0.0158 0.079 0.442From Rosetta Lite database in HYDRUS

Biomat Properties• Finch et al (2007)• Measurements from 6 OWS sites in Georgia• Biomat average thickness ( (Z b ) = 0.5 cm• Biomat average saturated hydraulicconductivity ( (K bs ) = 0.23 cm/day• We assumed a loam texture for biomat inall simulations

0.055 0.086 0.117 0.149 0.180 0.211 0.243 0.274 0.305 0.337 0.368 0.399Water Content - th[-], Min=0.055, Max=0.399

Deep water table201816Bouma bottom flux (cm/d)14121086y = 2.032x - 4.1472R 2 = 0.82234200 2 4 6 8 10 12 14 16 18 20HYDRUS bottom flux (cm/d)

Modified Bouma Equation• Original equation• Modified equationk∑i=0⎡K h h s−b ⎢ 2⎣ Zb0ZbZ⋅k⎛⎜⎝K h − h + Z0 s bbs= Kh (s)Zb+ 12Zb⎞⋅ i⎟ +k + 1 ⎠bh0⎤⎥⎦h − h + Z0Zsbb=Kh ( )s

Deep water table2018Modified Bouma bottom flux (cm/d)16141210864y = 1.089x - 0.2718R 2 = 0.9583200 2 4 6 8 10 12 14 16 18 20HYDRUS bottom flux (cm/d)

Soil textural classModified Bouma EquationBottom flux Bottom flux / K scm d -1 %Sand 10.31 2Loamy sand 8.88 8Silt 10.80 25Sandy loam 6.62 17Silt loam 9.41 52Clay 4.04 27Sandy clay loam 4.16 32Loam 5.59 46Sandy clay 2.97 26Silty clay loam 5.93 53Silty clay 3.82 40Clay loam 4.00 49

Hydraulic Conductivity or Biomat Flux (cm/d)100101 . 10 3 Pressure Head (cm)Loamy sandSandSilt loamModified Bouma biomat fluxSiltSandy clay10 10 20 30 40 50 60 70 80

Soil textural classcm/dayHLR Dgpd/ft 2ClassSilt 5.40 1.32 ISand 5.16 1.26 ISilt loam 4.71 1.15 ILoamy sand 4.44 1.09 ISandy loam 3.31 0.81 IISilty clay loam 2.97 0.73 IILoam 2.79 0.68 IISandy clay loam 2.08 0.51 IIIClay loam 2.00 0.49 IIIClay 2.02 0.49 IIISilty clay 1.91 0.47 IIISandy clay 1.48 0.36 IV

Conclusions• Modified Bouma equation can be used to develop HLR Dfor OWSs• Based entirely on soil and biomat hydraulic properties• Not recommending a particular set of HLR D but rather aprocess• Users can make many choices to tailor system to their needs• Ponding depth, biomat thickness, biomat conductivity, safetyfactor, soil groupings, etc.• Unsaturated soil and biomat properties are important• Bouma Calculator spreadsheet available at• www. cropsoil.uga.edu/soilphysics/publications.html