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As they are efficient and cost-effective methods of treating
wastewater, though, it is important to determine the optimum
conditions for their use. To this end, research is being done
to determine the optimum soil profile for wastewater treatment.
The basics of how such a wastewater treatment system works
have been established, as have the effects of the soil's
physical and biological characteristics on removing contaminants.
But ongoing study has resulted in a proposed code of rules
by the National Onsite Wastewater Recycling Association (NOWRA)
governing the design, construction and operation of soil treatment
of wastewater.
The Basics of Septic Systems and Wastewater Treatment with
Soil Media
Not every wastewater treatment system requires significant
capital investments in infrastructure such as sewers, pumps,
lift stations, large wastewater treatment plants, or lagoons.
Not every wastewater treatment system is sized for entire
communities. At the small scale of the individual user is
the septic system that relies on natural soils to treat wastewater
discharge. Septic systems primarily treat household wastewater,
but they can also manage black water discharges from larger
institutions such as restaurants, hotels, offices and small
businesses. These systems are mostly installed in rural areas,
where houses and other buildings are separated by great distances,
making the cost sanitary sewer systems prohibitive.
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A septic system utilizes an underground tank and a series
of discharge pipes arranged in a drainage field. Wastewater
flows into the septic tank, which typically holds 1,000 gallons.
In most states the size of the tank is legally determined
by the number of bedrooms in the home. The size of the tank
is calculated according to the specifications in Table 1.
For large-flow capacity septic systems, between 500 and
1,500 gallons per day, the volume of the septic tank should
be at least 1.5 times the daily wastewater flow. For very
large systems, more than 1,500 gallons per day, the tank volume
should be at least 1,125 gallons plus 75% of the anticipated
daily waste water flow.
Wastewater is detained in the septic tank and settles out
into three layers. Floating materials lighter than water (grease
and fatty solids) rise to the surface and form the "scum
layer." Solid material heavier than water sinks to the
bottom to form the "sludge layer." Sludge builds
up in the tank and must be removed at regular intervals by
a qualified contractor. Between them is the middle "water
layer" which contains bacteria as well as chemicals
like nitrogen and phosphorus, all of which can cause health
problems. Therefore, household wastewater must have adequate
treatment to prevent water contamination. This treatment is
provided by the filtering soils and soil bacteria beneath
the drainage field. These chemicals are also effective fertilizers,
which is why "the grass is always greener over the septic
tank." The tank's size ensures enough storage
capacity to provide sufficient detention time for the separation
of each of the three layers.
The operation of a septic system is entirely by gravity;
no mechanical pumping is required. With each discharge, additional
wastewater enters the tank. As the water is displaced, the
scum and water flow out of the discharge pipe located near
the top of the tank at the opposite end from where wastewater
enters the tank. From here the discharge enters a drainage
field. This field carries discharges to the soil absorption
system, where most of the treatment process occurs. A drainage
field operates as a French drain in reverse, with perforated
pipes set in trenches backfilled with gravel. The pipes are
typically schedule 40 perforated PVC, SDR-11 (or thinner)
perforated HDPE, or equivalent perforated and corrugated ABS.
Pipe diameters vary between 4 and 8 inches, depending on anticipated
flows, and are set with a positive flow gradient of at least
2% (an elevation drop of 1 foot for every 50 feet of pipe
length) away from the septic tank. Multiple branch pipes,
in parallel arrangement 5 to 15 feet apart, make up the field.
These branch pipelines connect to a distribution box that
evens out the flows from the septic tank. The pipelines are
usually in a circuit, forming connected loops in case an individual
pipe segment clogs.
The pipes are set in trenches 4-6 feet deep, depending on
local topography, anticipated flows, and local soil conditions,
with each trench being 2-3 feet wide, depending on the pipe
diameter. The drainage pipes are bedded in about 3 inches
of gravel with gravel used to backfill the bottom 2-3 feet
(depending on the pipe diameter) of the trench. The rest of
the trench is backfilled with previously excavated soil. A
separation medium, such as a geotextile filter, is often used
between the gravel and the backfilled soil to keep the trench's
backfill from migrating downward into the gravel and clogging
its pore spaces.
The discharged wastewater exits the perforated pipes, flows
through the gravel and is absorbed and filtered by the soil
outside and beneath the trench. Bacteria, chemicals and other
contaminants are removed through filtration, adsorption, and
other processes before reaching groundwater or surface water.
Biological growth (scum) occurs on the bottom and side walls
of the trench, providing the primary treatment process of
the system. In addition to providing a flow medium for the
discharge from the perforated pipes, the gravel supports the
trench sidewalls, preventing collapse. This treatment soil
can be either natural soil with appropriate characteristics
or, if necessary, engineered soil. The hydraulic conductivity
of the soil will determine the relative size of the drainage
field. Fine soils, such as hard clays with low permeability
will require larger drainage fields. Coarse, sandy soils with
high permeability, conversely, will need smaller drainage
fields.
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Physical Soil Characteristics and Their Effect on Wastewater
Treatment
Percolation testing is the primary method for determining
the physical compatibility of potential drainage-field soils.
While full-scale analyses of the natural soils can be carried
out, these are often not necessary, since percolation tests
provide the rule-of-thumb information for a layout of the
drainage field. Percolation directly correlates with the absorptive
capacity of the underlying soils. Since these are in-situ
tests, they provide valuable information on the actual performance
of the soil. At least six evenly spaced percolation tests
should be performed. A hole is first dug with a width of at
least 4 inches to the depth of the proposed trench line. The
exposed surfaces should be scarified to restore a natural
soil interface. The hole is then filled with water to a minimum
depth of 12 inches. Water should stay in the hole at least
4 hours. Soils with high hydraulic conductivities (like sandy
soils) will drain more rapidly and may need additional water.
Measurements (measured in minutes) are taken of the time it
takes the water to fall 1 inch. The size of the leaching system
depends on the measured percolation rate. The required area
is shown in Table 2.
Soils with percolation rates slower than 60 minutes per
inch are not considered suitable for soil treatment of wastewater.
The absorption area is equal to the sum of the lengths of
the trenches times the width of the trench, 3 feet. Therefore,
a three-bedroom house located on soil with a percolation rate
of three minutes per inch would require 300 square feet of
absorption area, or 100 feet of trenches. The arrangement
and distribution of the pipes depend on the property limits
and site topography.
Given these percolation test results, what other physical
characteristics make soil suitable for wastewater treatment?
Soils consist of four basic components: solids, organics,
air and water. Solids are typical small particles of minerals.
Air and water are collectively called the soil's voids.
The ratio of these components determines the soil's
classification and its suitability for wastewater treatment.
Research performed at Ohio State University has yielded a
general profile of the ideal soil for wastewater treatment.
The most ideal soils for septic systems are those soil layers
that are gently sloping to allow for positive flow away from
the drainage field. The color of the soil layer should be
uniform (reddish-brown, reddish, yellow-brown, or yellowish).
Soils with gray mottling should be avoided since the grayness
indicates a drainage problem. Gray soils indicate that they
have been waterlogged for a significant portion of the year,
with water displacing air in the pore spaces of the soil.
The soil texture should have neither excessive sand (with
too high of a permeability) nor excessive clay (with too low
of a permeability) and should be well-graded with good aggregation.
This is important for the structural stability of the soil
that is being trenched for the drainage field. Areas with
rock, impermeable clay strata or other aqua barriers within
3 feet of the surface are not suitable for drainage fields.
The blocking layers result in perched water that saturates
the soil and prevents downward migration (the actual treatment)
of the wastewater. Note that while the higher permeability
soils will result in smaller drainage fields, this does not
automatically mean that high permeability soils are always
the best for treating the wastewater.
Another key characteristic is the soil's porosity,
the ratio of the volume of voids to the total volume of the
soil. Different soil types have differing porosities. Soils
with low hydraulic conductivity, such as clays and silts,
have a greater amount of smaller pores compared with high-conductivity
soils such as sands. These structural differences result in
sands having an overall lower porosity (and higher bulk density)
than clays and silts. Because of the clinging force of capillary
action, water is held more tightly in soil with smaller pores.
This explains the low hydraulic conductivity of clays and
silts. Therefore, as measured by their field capacities, clays
and sands retain more water than sand over time.
So how does soil treat wastewater effluent? In fact, most
of the treatment preformed by a septic system is performed
by the soils underlying the drainage field. Most of the chemical
reactions and entrapment of microbes occur on the surfaces
of soil solids immediately adjacent to soil pores. The first
thing that soil does is filter out pathogens, bacteria and
viruses. Often the larger microbes are bigger than the soil's
pore space and are unable to pass through. Viruses have a
positive charge, and can be held by the negatively charged
soil particles. In addition to filtering out larger microbes,
the soil absorbs viruses until they are destroyed.
Soil also has a high capacity to retain certain chemicals.
In septic discharge, the primary chemicals of concern are
phosphorus and various forms of nitrogen. Proper soil will
retain phosphorus and most forms of nitrogen. These usually
act as soil amendments, fertilizing plant roots systems immediately
above the drainage field. Soil will not retain the nitrate
form of nitrogen, which moves downward with the septic discharge
water. In fact, the only significant chemical that is transported
beyond the drainage field to groundwater or possibly surface
water is nitrate.
Pure sand, with its high hydraulic conductivity, may appear
at first to be an ideal soil medium for wastewater treatment.
Sand, in fact, is very good at removing organic matter (greatly
reducing the effluent's five-day BOD), total suspended
soil (inorganic matter), and nitrogen in the form of ammonia.
What sand doesn't do well is filter out microbes and
pathogens. Most of these slip rather easily through the sand's
large pore spaces. Viruses do not readily adhere to the surface
of sand particles either. Sand by itself is not sufficient
for treatment of wastewater. Either a mixture of other soil
gradations in a natural formation or a secondary layer in
a man-made drainage field is required.
What is needed for proper wastewater treatment is a mixture
of coarse and fine soils. While coarse soils rapidly dispose
of wastewater, their treatment of the wastewater may be insufficient.
However, soils consisting entirely of fines, such as heavy
clays, are unsuitable for drainage field discharges because
of their low permeability. A mixture of soils with a relatively
small percentage of fines can provide superior wastewater
treatment. With these mixed soils the best discharge conditions,
resulting in the best treatment, are conditions of unsaturated
flows. In saturated flows, 100% of the pore space is filled
with water, and flow through the soil is purely by gravity,
occurring primarily in the larger pores adjacent to the coarse
soil particles. Saturated flows therefore avoid contact with
the fine soil particle where most of the treatment occurs.
Biological Characteristics of Soils and Their Effect on
Wastewater Treatment
The biological characteristics affecting wastewater treatment
by soils are affected by two primary factors: organic material
(mostly parts of dead plants and living root systems) and
soil bacteria. Organic matter is anything that contains carbon
compounds that were formed by living organisms. Most organic
soil material (more than 85%) is dead plant matter; living
roots are about 10%, with all other types being less than
5%. Partly or completely decayed organic material is referred
to as "humus." Organic matter does not directly
treat or interact with sewage discharges, but it does provide
the foundation for the real work performed by the soil microbes.
As chemically complicated material, organic matter provides
a source of nutrients for the growth of soil microbes. It
also tends to have relatively large surfaces and complicated
structures and can bind many substances, chemicals, pathogens
and some viruses. While it does not directly treat or destroy
these contaminants, it provides an excellent stage for soil
microbes to do their work.
And there is no shortage of naturally occurring soil microbes.
One tablespoon of soil may contain more than 1 million microscopic
organisms. Nor is there a limit on variety with soil organisms,
including fungi, bacteria, protozoa and larger multicellular
organisms ranging from earthworms to moles. Their importance
to the sewage treatment process is that many act as soil and
moisture predators and grazers. Some of these organisms feed
on the organic matter in wastewater, and some prey on bacteria
found in the wastewater. The best soil bacteria for these
tasks are aerobic (needing oxygen) bacteria. They are far
more efficient at breaking down and consuming wastewater organics
than are anaerobic bacteria (organisms actually poisoned by
oxygen). This brings us back to the need to avoid waterlogged
and saturated "gray" soils. Where water has displaced
air in the soil's voids and pore spaces, oxygen is no
longer present and aerobic bacteria are displaced.
While sandy soils in themselves have a hard time trapping
viruses, viruses can be captured in the microbial slime generated
by soil bacteria. This slime often forms at the interface
between the trench and its aggregate backfill and the adjacent
natural soil. Other microbes are held in the soil and die
from temperature extremes, lack of moisture, or lack of nutrients.
Soil fungi do their part by secreting natural antibiotics
that poison wastewater microbes. Other soil bacteria directly
prey on wastewater organisms.
So what possible chance does a wastewater pathogen or microbe
have of surviving such a hostile environment? Well, that depends
on the soil, especially its oxygen content. In aerobic soil
environments, wastewater microbes are quickly killed or die
off. Under anaerobic soil conditions, however, there is little
threat or competition, and wastewater organisms can survive.
Temperature also determines the potential for survival. In
low soil temperatures, natural soil bacteria are less active
and the wastewater organisms have a better chance to survive.
This is true of both bacteria and viruses.
Results of Recent Soils Studies
Larry D. Stephens, P.E. of Haslett, MI-based Stephens Consulting
Services, has made a study of the current state of research
in this field. One interesting point he makes is that, in
addition to soil characteristics, separation between the drainage
field and the seasonal high groundwater table (or "zone
of seasonal saturation") is important. The reasons for
maintaining a proper vertical separation between the two are
the need to protect the groundwater from contamination and
the need to protect the drainage field from saturation. But
research has shown, contrary to previous belief that little
or no treatment of wastewater occurs once it reaches the groundwater
table, some treatment occurs even in saturated zones at significant
depth. "While treatment efficiency may be diminished
for some parameters, there is still some treatment and removal
that occurs in the saturated zones," Stephens wrote
in a 2004 paper, Wastewater Treatment Over Fine-Textured Soils.
"Physical filtration is still occurring, as well as
some ionic exchange. Biological activity may be diminished
with depth, but not completely eliminated."
As a general rule of thumb, the deeper into the underlying
soil strata the lower the hydraulic conductivity, at least
in the vertical direction. However, water can move orders
of magnitude faster in a horizontal direction than in a vertical
direction. Wastewater continues to be treated but in a different
direction vector, and not just within a low-flow strata. Clays
and other low hydraulic conductivity soils tend to shed large
flows of percolating precipitation but will tend to absorb
slowly applied wastewater discharge. Therefore, the upper
surface of low permeability strata can serve as an effective
treatment zone even if wastewater does not deeply penetrate
the strata.
It also turns out that groundwater saturation is not the
only mechanism for cutting off oxygen to the soil pores and
creating an anaerobic condition. If the discharge system is
buried too deep or the backfill is too tight, oxygen may not
make it into the bottom of the trench to replenish what is
used up by aerobic bacteria during the treatment process.
Highly compacted backfill, or backfill consisting of low permeability
soils, will effectively cut off air infiltration into the
soil where it is most needed.
Apparently we have also underestimated the importance of
capillary action on wastewater treatment systems. Depending
on the soil and the climate, soil at reasonable depths may
remain dry year-round. If rainfall is light, only the upper
soil strata and overlying topsoil may be wetted. Given the
slow vertical rate of hydraulic conductivity, percolation
may be removed by root action and desiccation prior to reaching
significant depth, keeping soil near the groundwater table
dry even during the wet season. As Stephens notes: "Capillary
movement of soil moisture in finer texture soils is stronger
than gravity, and therefore, may very well be more important
to treated effluent dispersion under soil absorption systems
than is gravity. Therefore, because of these combined processes
of slow downward water movement and soil treatment, the movement
of contaminants down to groundwater should be of much less
concern in finer textured soils than in coarse textured soils."
NOWRA Rules and Rationales
Since there are codes governing the construction of landfill,
highways, bridges, buildings, and electrical systems, it is
past time that a code was developed for septic systems and
the proper soils needed for their use. NOWRA has stepped up
top the challenge by drafting a proposed Model Performance
Code. The code will set definitive standards for the soil
types needed for a functioning soil treatment system. Leading
this effort is NOWRA's Soils Subcommittee headed by
Jerry Tyler, of the University of Wisconsin, and Del Mokma,
of Michigan State University. Their mission statement is as
follows: "The capability of the soil to treat wastewater
to the standard required by state and local codes defines
the level of pretreatment required before the wastewater enters
the soil component. The Soils Subcommittee is developing treatment
credit tables for all soil conditions in the country, ranging
from no credit to full credit for each of the following wastewater
constituents: fecal coliform, nitrogen and phosphorus. The
committee is considering a major shift in the method of analyzing
information collected by the site soil assessor. For pathogen
reduction, the subcommittee is concentrating on the time the
wastewater resides in the treatment zone and the access to
oxygen. For nitrate reduction they are focusing on the presence
of anaerobic zones and a carbon source. This means that some
saturated zones will be desired zones for nitrogen treatment.
For phosphorus, they are looking at soil properties that will
bind the ions."
The code defines the soil underlying the drainage system
as the "unconfined treatment component" and describes
it as "…the volumetric area of land and water,
not within a confining structure." This can include
both natural in situ and engineered soils. Included in this
definition are saturated soils and even surface water, as
recent research has shown that they both can perform limited
treatment functions in the form of nitrate reduction and dilution.
The designers of soil treatment systems may utilize this unconfined
treatment component only as far as its characteristics have
been evaluated.
The code recognizes the difficulty in monitoring effluent
from the soil component of the system. Monitoring is expensive
to begin with; trying to get statistically meaningful sample
results from the soil component may be prohibitively expensive.
This brings us to the NOWRA soil treatment credit tables and
associated calculations. The credits are intended to establish
that the designed system meets treatment standards without
effluent monitoring. These credits can be claimed based on
the results of a site evaluation. These credits apply to the
following treatment parameters: hydraulic conductivity, nitrogen,
phosphorus, bacteria, in-situ organics and dilution.
Interestingly, the code recommends that percolation tests
(which have usually been the standard for system design and
evaluation of site suitability) be performed only as a supplemental
source of information in areas where there are unresolved
questions concerning the local groundwater movement. In relegating
percolation tests to secondary importance, the code seeks
to require more specific assessment of the soil.
Conclusions
The proposed code is in draft form and is being shaped by
new knowledge and more reined treatment techniques. The final
code will establish uniform standards of analysis and evaluation
and design methodologies. The old rules of thumb may have
been adequate, but the new rules will establish standards
that can take their place alongside the other construction
codes and serve as the basis for state regulations.
DANIEL P. DUFFY, P.E., is an environmental
engineer in Cincinnati, OH.
OW - January/February 2006 |
