Leachfield Area Required for Greywater Systems
Subject: Required leachfield area for greywater systems
Date: Tue, 30 Jan 2001
From: Art Ludwig <Oasis Design>
> I've purchased all your greywater guides, which are excellent, but still
> have a question that I think is important, and that I don't find addressed in
> your guides or website. The reason I'm inquiring is that I'm helping a neighbor
> to get approval for a greywater system, probably branched drain, as the site
> is well suited for it. He's under the eye of the local authorities, so there's no
> avoiding the legal route.
> One of the important considerations in getting greywater systems through
> the legal process is determining an acceptable ratio of greywater discharge to
> infiltrative area of soil. The Administrative Authority is more likely to approve
> such systems if ratios are based on well-documented practice or research. In septic
> system leach field design, the ratio is typically determined by the absorbency of
> the soil, based on percolation tests or soil analysis. This is a well-documented
> ratio, but, in my experience, results in an over-sized infiltration area for greywater
> systems. (I suspect this is largely because biologically active, mulched greywater
> basins don't tend to form a biomat the way leachfields do).
> By contrast, Table J-2 of the California Greywater Law, and page 9 of the Branched
> Drain guide, recommend ratios that result in a much more reasonably sized
> infiltration area.
> The discrepancy between the the ratios recommended for septic leach fields and
> those recommended for greywater systems is considerable. In regard to septic
> leachfield design, Texas and Arizona, the two states whose on-site disposal law I
> have some familiarity with, are in approximate agreement, recommending a ratio
> of about 1-3 sq. ft. per gallon per day, depending on soil type.
> In regard to greywater design, Table J-2 of the California Greywater Law
> recommends from 0.2 - 1.2 sq. ft. per gallon per day, and the rough rule of thumb
> given on page 9 of the Branched Drain guide, suggests a ratio of 0.14-0.29 sq. ft. per
> gallon per day (for ease of comparison, I've converted all figures to sq. ft. per gallon
> per day).
> The greywater ratios consequently result in infiltration areas of about 1/2-1/7 of
> those dictated by septic leachfield ratios. Unfortunately, Administrative Authorities
> are often more comfortable applying the standard leachfield ratios to greywater
> systems since those ratios are known to work well over long periods of time.
> In my neighbor's case, the area available for greywater basins is quite limited, so we
> would obviously prefer to use either the California Greywater Law ratios or those in
> the Branched Drain guide. Is there documentation for either of those ratios that an
> Administrative Authority could reasonably be expected to accept?
> David Omick
Your points are well-taken.
This is an area which is not well-researched at all.
Tables J2 and G2 (CA and UPC greywater irrigation areas, respectively) are both nearly
exact copies of table K2, the uniform plumbing code standard for leachfield area. No
thought or research has gone into distinguishing greywater and clarified septic tank acceptance rates in these laws, that I know of.
You are correct that mulch basins do not seem to form a biomat they way leachfields
do. In fact, the tilling action of worms and beetles at the soil interface, and the
increase in soil organic matter over time seem to *increase* the LTAR (long term
acceptance rate) of a properly designed and maintained mulch basin, in stark contrast
to a septic leachfield, where a biomat can lower the LTAR to 1/100th of the initial
On the other hand, I would guess that the LTAR vs. loading rate curve climbs very
steeply when the aerobic capacity is exceeded at the soil interface; this would argue
for conservative design, as does the extreme variability of greywater characteristics
relative to clarified septic tank effluent (consider the BOD of raw kitchen sink water,
for example). Also, mulch basins seem to be more sensitive to initial perk rate
than leachfields. If your soil has good perk, probably it would be difficult to overload it.
Fortunately, the CA greywater law has dropped the requirement for multiple
redundant irrigation zones, cutting the required area by half.
I hope this answers at least as many questions as it raises...good luck!
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