Separation Necessary to Protect Groundwater Quality;  Greywater vs. Septic Tank Effluent 

Art Ludwig, Greywater Researcher and Educator 

Jan 11th, 2010 

 

Experience with groundwater contamination from septics does not translate to greywater.  

In over 20 years of working in the greywater field, I have never heard of groundwater contamination from any of the 8 million greywater systems in the US.  

It is not likely that this has ever, or will ever occur. Because, multiplied together, these factors make a greywater system several orders of magnitude groundwater‐safer than a septic:   1) greywater has on the order of 1/1000th as many pathogens as combined sewage, and a fraction of the nutrients. 

1) Unlike a simple mechanical filter, the treatment capacity per inch of soil increases logarithmically with proximity to the surface.  The first foot of soil, has on the order of 100 times more treatment capacity than the fourth foot of soil. The first inch has thousands to hundreds of thousands of times more capacity than the 37th inch. (See reverse and below).  3) greywater application is typically diffuse as compared to septic application. Instead of all the septic effluent being added at one point in the leachfield, greywater systems typically have 4 to 30 or more outlets, and thus much less intense point loading. 

1000 x 100 x 4 = approximately 400,000 times less bacteria reaching groundwater from a greywater system than a septic at the same average loading rate and separation. 

This is a very rough calculation, however there is plenty of extra margin here to defend the decision to allow greywater to be applied with 3/5th  or 60% of the separation.  

In areas with high groundwater, the option for local jurisdictions to allow diversion of greywater to treatment higher in the soil profile is an important tool to safeguard groundwater quality. 

42

FIG

URE

4: H

OW

TRE

ATM

ENT C

APA

CIT

Y A

ND

CO

NTA

MIN

ATI

ON

PLU

MES

CH

AN

GE

WIT

H L

OC

ATI

ON

OF W

AST

EWA

TER A

PPLI

CA

TIO

N

Plum

e ex

tent

s do

not e

qual

reco

mm

ende

d se

para

tion

dist

ance

s. If

a se

ptic

le

achfi

eld

plum

e ge

nera

lly e

xten

ds 5

0’, t

he se

para

tion

dist

ance

shou

ld b

e a

mul

tiple

of t

hat t

o al

low

a sa

fety

fact

or. T

he U

PC re

quire

s 100

’ of s

epar

atio

n fr

om a

wel

l to

a le

achfi

eld,

for e

xam

ple—

a 2x

safe

ty fa

ctor

.Th

e C

ode

is n

ot e

spec

ially

ratio

nal.

It on

ly re

quire

s 150

’ for

a d

ryw

ell,

whi

ch p

rovi

des l

ittle

or n

o sa

fety

fact

or. T

he c

ode

sepa

ratio

n fr

om a

gre

ywat

er

outle

t to

a w

ell i

s 100

’, ab

out a

10x

safe

ty fa

ctor

, but

onl

y 50

’ fro

m n

atur

al

surf

ace

wat

ers,

abou

t a 3

x sa

fety

fact

or (p

atho

gens

can

trav

el fa

rthe

r ove

r the

su

rfac

e th

an th

roug

h th

e so

il).

The

optim

um a

pplic

atio

n po

int i

s a b

alan

ce b

etw

een

vario

us c

onsi

der-

atio

ns. Y

ou w

ant t

o ap

ply

was

tew

ater

hig

h to

get

the

best

trea

tmen

t, sa

fe-

guar

d gr

ound

wat

er, a

nd im

prov

e re

use,

but

not

so h

igh

that

it d

aylig

hts.

Thou

gh e

xcel

lent

trea

tmen

t can

be

atta

ined

on

the

surf

ace

(see

Ove

rland

Fl

ow. p

. 43)

, was

tew

ater

may

be

cont

acte

d by

chi

ldre

n or

ani

mal

s, or

run

off

into

surf

ace

wat

er b

efor

e it

is tr

eate

d.

In b

ulk

flow

ing

wat

er, t

here

is a

lmos

t no

natu

ral t

reat

men

t oth

er th

an

dilu

tion,

the

trea

tmen

t mod

ality

con

side

red

leas

t des

irabl

e by

the

Wor

ld

Hea

lth O

rgan

izat

ion.

Con

tam

inat

ion

can

spre

ad fo

r mile

s dow

nstr

eam

. Thi

s is

a m

ajor

wea

knes

s of c

urre

nt w

aste

wat

er tr

eatm

ent t

hink

ing

in th

e U

S,

whi

ch fa

vors

a p

ipe

dum

ping

art

ifici

ally

trea

ted

efflu

ent d

irect

ly in

to n

atur

al

wat

ers.

If th

ere

is a

ny p

robl

em —

the

plan

t run

s out

of c

hlor

ine,

the

pow

er

goes

off—

or fo

r tha

t mat

ter,

even

if th

ere

is n

ot, t

he w

aste

is in

ject

ed in

to th

e w

orst

pla

ce fo

r it i

n th

e en

tire

wat

er c

ycle

. Bul

k w

ater

flow

s hav

e th

ousa

nds

of ti

mes

less

pur

ifica

tion

capa

city

than

wat

er in

soil,

and

they

are

the

poin

t in

the

wat

er c

ycle

whe

re it

is m

ost c

ritic

al th

at th

e w

ater

be

pure

.

Wel

l 1

Soil

mic

ro-

orga

nism

s v

s.de

pth

bene

ath

surf

ace .

The

uppe

rmos

tso

il ha

sm

uch

grea

ter

purif

icat

ion

capa

city

per

unit

of s

oil

dept

h.

0’ /

0 m

3’ /

1 m10

’ / 3

m

50’ /

15 m

100,000,000Bene

ficia

l bac

teria

per

gram

of s

oil 15

0’/ 4

5 m

10,000,000

1,000,000

1,000

Dept

h

Wel

l for

dom

esti

c po

tabl

e wa

ter

supp

ly. In

the

wel

l’s “d

raw

down

”zo

ne, g

roun

dwat

er fl

ows

towa

r dth

e we

ll fr

om e

very

dire

ctio

n.Se

ptic

tan

k

Sept

ic e

fflu

ent

adde

d to

18” d

eep

leach

- fi

eld

is ra

pidl

y pu

rifie

d by

larg

e nu

mbe

r sof

ben

efici

al b

acte

ria n

ear s

urfa

ce o

f soi

l.Bi

olog

ical

con

tam

inat

ion

plum

eat

tenu

ates

rapi

dly;

pat

hoge

n le

vels

are

near

zer

o wi

thin

50

’ in m

ost

soils

.

Bedr

ock

fissu

res,

lim

esto

ne c

aves

, or l

ava

tube

swa

stew

ater

can

flow

rapi

dly

long

dis

tanc

es w

ith

little

tre

atm

ent.

Con

tam

inat

ion

plum

es c

an e

xten

dfo

r the

full

leng

th o

f the

fiss

ures

: a m

ile o

r mor

e .A

lmos

t th

e on

ly pu

rific

atio

n is

by

pass

age

thro

ugh

soil

befo

re t

he p

atho

gens

reac

h th

e fis

sure

.

Disp

osal

in 5

0’ (

15 m

) dee

pdr

ywel

l byp

asse

s th

e m

ost

effe

ctive

pur

ifica

tion

in t

heup

per r

each

es o

f the

soi

l. Th

ebi

olog

ical c

onta

min

atio

n pl

ume

star

ts d

eepe

r and

can

ext

end

a hu

ndre

d fe

et o

r mor

e, a

nd is

thus

mor

e lik

ely

to re

ach

grou

ndwa

ter.

Gre

ywat

er a

dded

at

or w

ithi

n6

” of t

he s

urfa

ce re

ceive

s ve

ryra

pid,

eff

ecti

ve t

reat

men

t in

this

mos

t bi

olog

ical

ly a

ctiv e

zone

. Als

o, t

he p

atho

gen

leve

lsar

e lo

wer t

han

blac

kwat

er t

obe

gin

with

. The

con

tam

inat

ion

plum

e do

es n

ot e

xten

d do

wnm

ore

than

a fo

ot o

r two

.

Bedr

ock

Sprin

g (w

her e

grou

ndwa

ter

reac

hes

the

surf

ace)

Efflu

ent

from

a fa

iled

sept

ic o

rgr

eywa

ter s

yste

m ru

nnin

g ov

er t

hesu

rfac

e wo

uld

be a

haz

ard

unti

l it

was

adeq

uate

ly t

reat

ed. D

epen

ding

on

cond

itio

ns, 5

0’ o

f ove

rland

flow

cou

ldre

mov

e 10

0% o

f pat

hoge

ns (s

ee p

age

41).

Tops

oil

Subs

oil

Wel

l sea

lag

ains

tsu

rfac

ewa

ter

infil

trat

ion

Gro

undw

ater

leve

lwi

thou

t we

ll dr

aw d

own G

roun

dwat

er le

vel

Path

ogen

leve

ls in

was

tewa

ter a

pplie

d to

soi

l dro

p on

the

orde

r of 9

0%

per f

oot

(30

cm

) of t

rave

l. G

roun

dwat

erge

nera

lly m

oves

muc

h le

ss t

han

a fo

ot a

day

. In

star

kco

ntra

st, e

fflu

ent

disc

harg

ed t

o flo

wing

sur

face

or

subs

urfa

ce w

ater

s, w

here

pat

hoge

n le

vels

real

ly m

atte

r ,re

ceive

s ne

ar z

ero

natu

ral t

reat

men

t in

a fo

ot, a

ndco

ntam

inat

ion

plum

es c

an s

prea

d fo

r tho

usan

ds o

f fee

t.

Rive

r

Perf

orat

edpa

rt o

f wel

lca

sing