From: rjsalvad@iastate.edu (Ricardo J Salvador)
Subject: Re: Land Application of Waste Water Sludge
Organization: Iowa State University, Ames, IA
Date: Mon, 23 Nov 1992 17:27:40 GMT

In article <1992Nov21.182407.17137@samba.oit.unc.edu> Michael.Lowry@launchpad.unc.edu (Michael Lowry) writes:

>One proposed, readily available alternative is the application of sewage
>sludge, a byproduct of standard waste water treatment.
[...]
>Potential problems include nitrate pollution of ground and surface water 
>and oversaturation of the soil with heavy metals--chromium, lead, etc.
>It is my understanding that soils accumulate all heavy metals applied,
>which can lead to plant toxicity.  

"Heavy metals" may accumulate in soil depending on the metals'  state of
charge and the state of saturation of the complimentary charge matrix
of soil. E.g., soil organic matter is a potential "sink" for cationic
metals (whether "heavy" or not, e.g., magnesium) and for hydrous oxides.
Humic materials in general tend to have high cation exchange capacities
(in the neighborhood of 200 m.e./100 g), and so serve as effective
"buffers" for toxic heavy metals.  As you state, however, such buffering
may be effective in preventing heavy metal transport, but may in fact
result in plant toxicity because the stability constants of metal-humic
substances are considerably lower than those for complexes formed between
the same metal ions and synthetic complexing agents such as EDTA. There-
fore, the ions may be immobilised, but readily available to plant roots
and soil microbes.

>Thus, land application of sewage sludge can provide much needed soil
>organic matter, which certainly would be a component of a sustainable
>agricultural system. However, the costs include the accumulation
>of heavy metals, which does not seem to be regenerative at all. 

You present the issue in complete fashion: a management goal, a practice
that might help attain the goal, the benefits of the practice, the dis-
advantages of the practice.  We have an immediate case study of the factors
you describe on the Richard and Sharon Thompson farm near us.  The
Thompsons farm 300 acres and attempt to manage their resources in a
regenerative fashion.  For a number of years the Thompsons have used
municipal sewage sludge from the nearby town of Boone, mixed with manure
from their livestock herds (cattle and hogs), to manage the fertility of
their crops (a five year rotation of corn, soybeans, corn, oats-hay, hay.) 
At first the city of Boone provided the sewage free of charge, but now the
Thompsons are actually paid to receive the sludge.

The sludge is analyzed in order to determine its fertilizer value.  It
averages 3.5 to 4.0% nitrogen, 1% phosphorus and 1.3-1.6% potassium (on
dry weight basis). City trucks deliver about 200 tons (at 80% moisture)
annually.  The Thompsons have had regenerative management and ecological
goals , but have had to learn to work through several problems of the
sort you ask about.  There is a tannery in the city of Boone that 
contributes to the chromium load of the sludge.  The city monitors the
chromium concentration, and this hovers around 2,000 ppm (EPA guidelines
establish 1,000 ppm as a safe annual concentration for application to
soil).  The Thompsons dilute the chromium content of the sludge by
mixing with manure and other organic residues before application.  They
calculate that the sludge (applied at 0.8 dry tons/A annually) contributes
3 lbs/A of chromium.

An additional problem encountered by the Thompsons concerns the logistics
of sludge and manure handling, an important detail that would-be
regenerative farmers should not overlook.  To handle the tons of manure
and sludge that the Thompsons accumulate before they are ready to spread
in the Spring of the year, they built a large open bunker (at cost of
$25,000).  I remember taking Dr. Pedro Sanchez (North Carolina State
University) to see the Thompson farm sometime in the middle 80s.  Sanchez
noted the effluent collecting on one side of the bunker (the bunker was
purposefully built with a sloping floor to collect this effluent) and
pointed out that the leaching of the liquid fraction from the mass of
manure was probably removing potassium ion and so reducing the fertilizer
value of the composting dry fraction. This caused the Thompsons to 
rething the way they were storing, composting and handling their manure/
sludge (it turned out that through volatilization and leaching, nitrogen
was also being lost).  In fact, in Spring 1987 an analysis of the liquid
fraction showed that it contained 18 lbs. N, 2 Lbs. K2O, 33 Lbs P2O5 per
1,000 gallons.  The effluent itself is therefore an important nutrient
resource as well, and the Thompsons have used it as "starter" fertilizer.

If you are further interested in the evolution of the Thompson management
practices, they are fairly well documented in a string of articles published
in Rodale's "New Farm" magazine, and in a chapter (Case Study: A Resource-
Efficient Farm With Livestock) in Chuck Francis et al.'s text "Sustainable
Agriculture in Temperate Zones (Wiley Interscience, 1990), and in the
National Academy of Science's "Alternative Agriculture" (National Research
Council, 1989; see pp. 308-323 "Crop-Livestock Farming in Iowa: The
Thompson Farm.)  By the way, FYI, there exist a couple of interesting
rebuttals to the last work, and you may be interested in consulting
them.  One is published by the Potash and Phosphate Institute (A Review of
the National Research Council's Report on Alternative Agriculture, by
PPI Scientists), and can be obtained through any of their salesmen.
Another is published by the Council for Agricultural Science and Technology
(CAST), and can be obtained for $15 by requesting to: CAST, 137 Lynn Ave.,
Ames, IA, 50010-7197.

-Ricardo



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