TRAINING SYLLABUS

BASIC ON-SITE SYSTEMS

I. Expectations and Objectives

A Disease Prevention

Diseasesspread through contact with untreated or inadequately treated sewage are many and
are well documented. They include but are not limited to hepatitis, shigellosis, poliomyelitis,
infestations with various round and flat worms, cholera, typhoid, bacillary dysentery and amoebic
dysentery. On-site sewage systems, when properly designed, installed, operated and maintained
treat domestic sewage to a high level of quality and thus eliminate the potential for disease
transmission through preventing contamination of ground water, surface water, shellfish and ground
surfaces.

Onsite systems, in addition to providing good treatment, are also expected to provide good
disposal, minimal exposure of the public to the harmful components and long-term performance,
all at a reasonable cost.

B. Treatment Potentials and Levels Normally Attained

The quality of sewage treatment in on-site systems varies only slightly according to the type
of system installed. In almost all cases,the final and highest degree of treatment occurs in the soil
or soil substitute under unsaturated flow conditions over a vertical distance of 2 to 3 feet. Under
these conditions, treatment levels are often better than those achieved in all but the most advanced
municipal systems. Typically, the following levels of treatment can be expected from a properly
functioning onsite system: biochemical oxygen demand (BODs) -10 mg/l; total suspendedsolids
(fSS) -10 mg/l; and fecal coliforms -less than 200 per 100 ml. These treatment levels are
jeopardized when the when the flows in the drainfield are saturated, the unsaturated flow is for less
than 2-3 feet, the strength of the sewage is too high, toxic chemicals are being put into the system,
or the system is too deep in the ground for air to reach the soil beneath the infiltrative surface.
Proper siting, design, installation, operation and maintenancewill promote optimal treatment levels.

c. Onsite Sewage Flow Regimes

On-site systems must have the capacity to handle sewage loads that are not steady but rather
are in distinct peaks. The volume of the septic tank is designed to accept the peak flows through
the system while providing a minimum retention time of 36 hours for separation of solids from
the liquid. The drainfield must also have the capacity to accept the peak daily and weekly flows,
not just the average daily flow.

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n. Factors Mfecting System Performance

A. Soil

The soil in an on-site sewage system performs two major functions: (1) treating the
wastewater to eliminate pollutants before releasing it to the groundwater, and (2) disposing the
treated wastewater so that it moves away from the site, making room for more treated wastewater.
In order to assure that the particular soil is capable of performing these valuable and necess~ry
functions, it must be examined for its capacity to treat and dispose of wastewater.

Several properties of the soil are important to proper design and functioning of an on-site
system. These include soil texture, structure, depth, compaction, and landscape position.

Texture is a function of the relative amounts of sand, silt and clay particles, and determines
the pore size, surface area and capacity for unsaturated flow. Soils with coarse texture have large
void spacesand a high capacity to move water away from the site as long as a water table or other
barrier is not present. However, the limited surface area and potential for rapid flow through soils
reduce their ability to treat effluent. Finer textured soils have about the same volume of void space
as other soils, but have smaller pore sizes and larger surface areas.Therefore, water moves through
finer textured soils at a much slower pace, and thus these soils have a much slower disposal
capacity. These same soils can move wastewater by unsaturated flow and therefore can provide
a high level of treatment. Soils with a mixture of particle sizes such as loams, sandy loams, loamy
sands, and silt loams usually provide the best balance between treatment and disposal.

The presence and degree of structure (tendency to form distinct aggregates,with lines of
separation between) has an important impact on the soil's ability to treat and dispose of
wastewater. The structure will create channels to move water away from the site ( disposal), but
in some cases can act to short circuit the treatment process.

Depth of soil of unsaturated soil is important to ensure adequate contact time and distance
for treatment to occur. There needs to be 2-3 vertical feet of unsaturated flow for adequate
treatment to occur.

Compaction of the soil, even loams and sandy loams, can create a barrier to water flow, and
therefore adversely affect treatment and disposal.

Landscape position often determines the moisture regime of the soil, such as a high water
table, a well drained soil, or somewhere in between. For example, soils at the bottom of slopes
tend to accumulate water, whereas soils at the upper end of the same slope will tend to release
their water and therefore be drier. Sites near riverbeds and in flood planes will also likely have
higher water tables.

B. Wastewater characteristics

Most on-site sewage systems are designed to treat and dispose of sewage with the strength
and characteristics of normal domestic wastewater (see Table 1). As the BODs of the wastewater
approaches 230 mg/L or exceeds this level, the treatment efficiency and longevity of a standard
 3
on-site system is reduced, and other technology should be incorporated ( e.g. aerobic treatment
device) prior to discharge to a soil component. Alternatively, a lower wastewater strength can be
accomplished by altering habits and practices in the home, such as not using garbage grinders and
not dumping grease down the drain (see section II.D.). Introduction of biologically active
chemicals (e.g. large quantities of chlorine bleach or antibiotics) will also affect the performance
of an on-site system. Therefore if the facility being served by an on-site system normally would
introduce such materials into their wastewater, a pre-treatment will likely be necessary for the
system to perform correctly. Chemicals such as petroleum products, degreasers,pesticides, and
cleaning solvents, will not only harm the system but will also degrade groundwater and therefore
should never be put down the drain.

Table 1. Characteristi~ of raw sewage and septic tank effluent

(Note that the numbers for the Raw Sewage and Septic Tank Effluent are from different sets of data.)

Raw Sewagel Septic Tank Effluenf

Characteristics
Range Range for 95% of Average
Samples

pH 7.2
Total suspended solids (mg/l) 200-290 47-62 54
BODs (mg/l) 200-290 142-174 158
Total Nitrogen 35-100 48.9-61.6 55.3

Total Phosphorus 18-29 11.4-17.7 14.6

Fecal colifonns (MF) per lOOml lW-lO9 2.88xlOS -6.l6xlOS 4.2lxlOs

1 From EPA Design Manual, Onsite Wastewater Treatment and Disposal Systems

2From Otis, RJ, W.C. Boyle and D.K, Sager, 1974, 'The Perfomlance or Household Wastewater Treatment Units Under Field Conditions', Home Sewage r,-t, American Society
or Agricultural Engineers Publication, St. Joseph, MI.

c. Loading rates

The design flow from a house is estimated based on the number of people in the house
(usually 2 per bedroom). Sixty gallons per person per day is a common quantity used for
calculating design flows. These estimates of flows can be grossly in error if the number of
occupants increases,if they take in outsiders' laundry, throw large parties every week, have a large
jacuzzi bathtub which empties after each fill, etc. Loading rates are the amount of septic tank
effluent that is applied to each square foot of infiltrative surface per day. If the system design were
based on a loading rate of .6 gpd per square foot, but the actual flows resulted in a design flow
of .8 or 1.2 gpd per square foot, the drainfield would likely become hydraulically overloaded and
be headed for early failure. Loading rates are established in Washington according to the type of
soil, which is based largely on the texture of the soil receiving the septic tank effluent.

D. Users' lifestyle

Flows into the onsite sewage system that are in excess of the design flows can lead to
hydraulic overloading of a system. Examples of situations which could lead to excesswater flows
were mentioned in the preceding paragraph. Other causes are leaking fixtures, excessive flushing
4

of the toilet, and roof downspouts plumbed into the house drain. Use of a garbage grinder can
shorten the interval between pumpings of the septic tank, and could produce high strength effluent
which the system is not designed to handle, and thereby lead to excessive thickness of the biomat
(see section V.A.b.3.) or anaerobic (absence of air) conditions in the drainfield. Excessive use of
cooking fats and oils which find their way into the sewage system can clog pipes, overload the
septic tank and finally the drainfield.

E. Operation and maintenance

On-site sewage systems are designed for long-term operation, i.e. for the lifetime of the
house. In order for this to happen, proper operation and maintenance must be carried out. Only
materials appropriate for the system should be put down the drain. The following types of
materials should never be introduced into the system: disposable diapers, condoms, sanitary
napkins and tampons, grease, gasoline, paint, paint thinner, pesticides, and organic solvents, etc.
Performing necessary, routine maintenance is essential for systems to perform trouble-free for
many years. For example, check septic tank baffles for intactness and proper positioning, and have
the tank pumped when the solids have accumulated to the maximum level that is safe for the
system (See figure below).

F. Temperature

Treatment of sewage relies heavily on biological activity. Lower temperatures will reduce
the biological activity approximately one half for each lO°C drop in temperature until almost all
activity stops at about 2°C (35°F), providing little more treatment than physical filtering and
adsorption in the soil component and physical separation in the septic tank. Fortunately, most
drainfields, even in the winter, stay above the 2°C level becauseof heat from the incoming sewage,
heat from the biological activity, and heat from the surrounding soil. Temperature also affects the
flow and mixing characteristics in the septic tank as well as the efficiency of grease and oil
separation in the septic tank (the cooler the better).

G. Rainfall

Rainfall can adversely affect on-site system performance by placing an additional hydraulic
load on the soil. The greatest effect is when the groundwater table rises enough to decreasethe
vertical separation ( distance between the infiltrative surface and the impervious layer or water
table) to less than 2 or 3 feet, and thus decreasing the treatment of the wastewater. Rainfall has
a positive affect by diluting the nitrates that are released from an onsite sewage system, and
thereby reduces the pollution and potential health risks of nitrates in the treated effluent. At the
same time however, the additional water increasesthe rate at which these nitrates travel from the
site.

F. Surrounding Development

A parcel of land, when being considered for development using an onsite sewage system,
must be evaluated in relation to surrounding parcels and their accompanying onsite systems'
contribution to groundwater. Accumulative affects of onsite systems from developments can raise
the water table significantly in some casesand thereby degrade the performance of the system if
this factor was not adequately considered during the design and installation phases.
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ill. Water Movement in Soils
~\;
A Saturated and Unsaturated Flow

When sufficient water is applied to soil to fill all the pores, it flows rapidly downward with
the force of gravity. This movement is called saturated flow. When water is applied to the soil
without filling the larger pores, it is drawn along the soil particle surfaces by capillary (or matric)
forces, the same forces that draw water up the sides of a glass. This movement can be in the
horizontal (for short distances -up to a few feet) as well as the vertical directions. Capillary flow
occurs under unsaturated conditions, proceeds much slower than saturated flow, holds the water
close to the soil particles and allows air to enter through the open pores. It should be noted that
there is a gradual transition between the extremes of saturated and unsaturated flows. The more
saturated the flow, the larger are the pores that are filled with water, and the faster the water will
flow downward. In addition, the larger the pores that are filled with water, the more difficult it is
for air to move into the soil from the atmosphere.

B. Hydraulic conductivity

This term relates to how fast water can flow through the soil. Coarser soils have a higher
hydraulic conductivity than finer soils, and hydraulic conductivity decreasesas the soil becomes
drier.

C. Capillary or matric forces

As explained in "A" above, capillary or matric forces are what move water through soil
during unsaturated conditions. Finer textured soils have smaller pore sizes than coarser textured
soils and therefore, like smaller capillary tubes compared with larger ones, exert greater matric
forces to move water through them. See illustration.

Upward movement by capillarity in glass tubes as compared with soils (after Brady 10) .
6

D. Through layered soils

Unsaturated water movement through soil is interrupted when it encounters a layer with an
abrupt textural change. This delayed flow occurs both when moving from a finer texture to a
coarser texture, and vice versa. When water flows from a finer to a coarser-textured layer,
movement slows because the larger pore sizes of the coarser material do not exert as strong a
matric force on the water. Therefore the finer soil must reach near saturation at the interface in
order for water to pass into the coarser material. Likewise, when water encounters a layer that
consists of clay or compact material, unsaturated flow is interrupted because it cannot pass as
rapidly through the finer pores as water collects above it.

F. Horizontal movement

Water movement in the horizontal direction requires a horizontal water gradient and saturated
flow conditions if it is to proceed for much distance. Horizontal movement, as will be discussed
in more detail later, does not provide a known degree of treatment of wastewater. Its major
contribution to the functioning of on-site sewage systems is in disposal, i.e. carrying the treated
wastewater away from the site.

Go Entry to groundwater

The final fate of wastewater treated by an on-site system is discharge to, and dilution with,
groundwater. Properly sited, designed and functioning on-site systems provide a high degree of
treatment of the wastewater, and therefore do not degrade the groundwater to any significant
degree. However, the potential for contamination always exists due to poor siting, design,
installation or poor operation and maintenance.

H. Where does all that water (> 100,000 gal/yr) go?

100,000 gallons???!!! Yes, the typical three-bedroom household discharges 100,000 gallons
per year through their on-site system. As .mentioned above, this water is discharged to the
groundwater. The accumulative contribution from the thousands of homes in the state each year
is a significant quantity of groundwater recharge each year, and can be a positive asset when
adequate treatment precedes the discharge.

IV. Sewage Treatment

A. What is sewage?

1. Definition in on-site regulations

"Sewage" means any urine, feces, and the water carrying human wastes, including
kitchen, bath and laundry wastes from residences, buildings, industrial establishments or
other places.
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2. Domestic vs industrial?

Domestic sewage is all the human body and water-carried waste generated in a normal
household. Industrial waste may contain most of the same items as domestic waste, but also
contains waste from industrial processes, high strength waste and a whole variety of
chemical pollutants in quantities beyond the ability of onsite systems to handle. On-site
systems are designed specifically to treat and dispose of domestic sewage.

B. When is sewage no longer sewage?

Normal household sewage is represented in the following table at different points in the
onsite system.

Obviously this treated effluent does not meet drinking water standards,and could conceivably
still contain some factors that pose a risk to human health. At this point, the key contaminants of
public health concern are the nitrogen and the coliforms, the former being an indicator of possible
chemical contamination by soluble constituents, and the latter an indicator of microbial
contamination.

For nitrogen, very little removal treatment occurs in conventional systems, which must rely
on dilution with groundwater to render the concentration inconsequential to health. There are

lZiebell, W.A., D.H. Nero, J.F. Deininger and E. McCoy, 1974, Use of Bacteria in Assessing Waste
Treatment and Soil Disposal Systems." Home Sewage Treatment, American Society of Agricultural Engineers
Publication Proc-175, St. Joseph, MI, p.58.

2Brown, K.W., J.F. Slowey & H.W. Wolf, 1977. "The Movement of Salts, Nutrients, Fecal Coliform and
Virus Below Septic l.ead1 Fields in Three Soils", Home Sewage Treatment, American Society of Agricultural
Engineers Publication 5-77, St. Joseph, MI.
8

certain alternative systems that can reduce significantly the nitrogen concentration. For the
coliforms, the little research that has been done in this area indicates that coliforms will not
migrate more than 3 to 4 feet in unsaturatedsoil, and this is the key. There are plenty of examples
in the literature where coliforms and pathogens have migrated long distances from drainfields
having saturated conditions under them. It is reasonableto assumethat sewage is no longer sewage
when it has received a degree of treatment that yields the above quality wastewater, even though
it does not meet drinking water quality .Pathogen removal in soil is dependent on vertical
separation, loading rates appropriate for the soil conditions, distribution methods, clogging mat,
and soil temperature.

c. How is sewage treated?

Reactions and processesthat treat sewage occur on the surfaces of soil particles. Treatment
dependson the accessibility and types of surfaces available. Very coarse-texturedsoils have limited
surface area and water travels through so fast as to limit the level of treatment. Very fine-textured
soils on the other hand have vast amounts of surface area and retain water for long periods of time.
Their problems are: releasing the water so that new water can have contact with the surfaces and
therefore be treated and, diffusing adequateoxygen through the soil pores. As a consequencethese
fine-textured soils cannot process water as fast as a household will deliver it. Most other textured
soils have enough surface area while maintaining enough permeability for good disposal. Note:
A discussion on texture vs void space and surface area appears in section II.A.

In the soil with aerobic conditions, bacteria use for food the organic material that produces
the BOD (literally, they break down the BOD and solids, and incorporate them). Pathogens are
trapped in the soil, either by being adsorbedonto soil particles (electrical and chemical interactions
between the soil particles and the surfaces of the sewage microbes), or by becoming stuck to the
microbial slimes laid down by soil bacteria. Once trapped, some pathogens die because of
unfavorable temperature, lack of moisture and food, and other causes.Others are inhibited or killed
by antibiotics given off naturally by soil fungi and other organisms. Still others are actually preyed
upon by soil bacteria and are literally eaten. This process proceeds faster with warmer soil
temperatures and slower with cooler temperatures. A total vertical distance of unsaturated (and
therefore aerobic and slow) wastewater flow of 2 to 4 feet is sufficient to produce high quality
effluent with very low disease potential.

Nitrogen enters the drainfield soil from a septic tank largely as ammonia but is quickly
oxidized to nitrate when it encounters unsaturated, aerobic conditions. From aerobic devices, most
of the nitrogen will enter the drainfield already converted to nitrate. Nitrates are very soluble and
therefore move with the water as it passesout of the drainfield and mixes with the groundwater.

Phosphates are normally tightly bound to the soil clays and hydrous oxides, and therefore
do not migrate far from the drainfield area, even under saturated flow conditions, until all the
binding sites are utilized (a very long-term process). Few soils are completely devoid of clay and
hydrous oxides, but the less they contain, the sooner the time when phosphates will begin to
migrate a distance from the drainfield area. Phosphatesare only a problem where septic systems
are located in coarse-textured soils surrounding a lake. In these casesthe phosphatewill eventually
exceed the soil's binding capacity and leach into the lake, where it will cause excess growth of
algae and aquatic plants.
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D. Vertical vs Horizontal Separation

Assurance of public health protection traditionally has relied on a combination of vertical

separation from the water table and horizontal separation from a drinking water source, surface
water or some other sensitive area. Vertical
separation means the depth of unsaturated soil
between the infiltrative surface of a drainfield and
an impervious layer or water table. It is in this
unsaturated zone that treatment of the sewage in
the soil occurs.

Horizontal separation is the horizontal

distance between the infiltrative surface and the
feature needing protection from sewage. As noted
before, vertical separation furnishes the treatment
of sewage, and if adequately provided for, will
render sewage nearly harmless. On the other hand, horizontal movement across the horizontal
separation occurs almost exclusively under saturated conditions and therefore may not yield much
treatment except perhaps dilution, and for certain, does not yield consistent, known levels of
treatment. Since we want to maximize the known treatment before releasing the effluent into the
environment, it is far superior to rely on the known treatment capability of vertical separation for
public health protection than to hope for adequate dilution during horizontal flow. For the most
part, if adequatetreatment is provided by vertical separation, horizontal distances are not of great
concern.

E. Basis for loading rates.

Loading rates are based on long-term experimental observations of installed systems,

comparing loading rates in the drainfields with the soil texture in which they were placed. The
outcome of this research is that these rates vary with the soil texture. The rates and corresponding
soil types and textures used for systems installed in Washington State are shown in the table
below.

The rate at which water will infiltrate into undisturbed, native soil is many times greater than
the loading rates in the state regulations. As a soil treatment system matures, a clogging mat forms
across the face of the soil where the wastewater is applied. In gravity distribution systems, this
mat, together with the loading rates, regulates the rate at which the effluent is applied to the
receiving soil. It is the soil that provides the matric potential ("sucking force") to draw the fluid
through the mat. This matric potential will depend on the texture and on the degree of saturation
in these soils. The intent is to add effluent into the infiltration network no faster than it can move
through the clogging mat and into the soil under unsaturated flow conditions.

In pressure distribution systems, the pressure dosing, which provides relatively equal
distribution, regulates the rate at which the effluent is applied to the soil, even before a mat forms.
In this case, the loading rates are set to allow for unsaturatedflow in the receiving soil. A clogging
mat will likely form eventually and slow the passage of effluent into the soil. The loading rates
account for these conditions.
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F. Life expectancy

Well designed, constructed, operated and maintained on-site sewage treatment and disposal
systems are capable of very long term service. They should be able to perform their intended
functions for the lifetime of the dwelling they serve. Unfortunately, there are many places where
errors can be made during the development of a system. Lack of maintenance and misuse can also
lead to failure. Therefore it is prudent to include a replacement area into the design of a property.

Go What is a Failure?

"Failure" means a condition of an on-site sewage system that threatens the public health by
failing to adequately treat the sewage or by creating a potential for the public to come in direct
contact with sewage. Examples of failure include:

1. Sewage on the surface of the ground;

2. Sewage backing up into a structure caused by slow soil absorption of sewage;

3. Sewage leaking from a septic tank, holding tank, pump chamber, or collection system;

4. Cesspools or seepage pits where concerns exist for the quality of the ground water or
surface water; or

5. Sewage contaminating ground water or surface water.

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v. Components of On-site systems

A Conventional system

1. Septic tank

a. Anatomy of a Septic Tank

(1) Inlet baffle or tee

This device directs the incoming waste stream downward and allows
venting of the tank through the house plumbing stack with a minimum of
disturbance of the tank contents.

(2) Compartment divider

The divider is a barrier to inhibit the movement of solid material

from the Ist compartment to the 2nd compartment. In Washington State,
2 compartments are required to provide improved solids separation and to
provide a safety margin if there is a failure to pump the tank.

(3) Outlet baffle or tee

The outlet tee inhibits movement of suspended and floating solids

from the septic tank to the drainfield by drawing liquid from the most
clarified zone.. .

( 4) Outlet filter

Sometimes a filter is placed on or used in place of the outlet tee. It

serves to prevent all but the very fine suspendedsolids from entering the
drainfield or pump chamber.

(5) Access ports and covers

These ports, or openings, are essential for monitoring the condition

of the baffles, tees, sludge depth and the scum layer. They also are
required when it is necessaryto pump the septic tank. The standard access
ports currently used in tanks installed in Washington (see previous
illustration) have several important drawbacks. Because they do not rise to
the surface they are difficult to locate after several years of being buried.
In addition, they require considerable work to dig up when the tank is to
be inspected or pumped, and then allow dirt and debris to fall in while the
12

cover is off. Finally, these standard ports are not water tight and therefore
may allow water to leak in. There are septic tank designs that can
accommodate watertight risers to the surface or just below the surface to
overcome the drawbacks listed here (see illustration below).
TankCovers~

--
Inlet Frcxn
HD.- O~letTo
Dr.inlield

b. Water tightness

(1) Why it is important

There are at least three important reasonsfor watertightness in septic tanks

and pump chambers. Liquid in the tanks is only partly treated and therefore
should be releasedinto the environment only after passing through the drainfield.
Hence, it is undesirable for liquid to leak out of the tank. Another consequence
of a leaky tank is that water may enter into it, resulting in reduced retention
time, upset of temperature and oxygen levels. Thirdly, leaking tanks can result
in infiltration of water into the system, thereby leading to hydraulic overloading
of the drainfield, sand filter or mound.

(2) How to achieve it

Leaks in concrete septic tanks can occur most readily around the inlet and
outlet holes, in the seams of a 2-piece tank, in the comers between sides and
floor, and through cracks that develop in other places. Pouring a water-tight
septic tank requires a more expensive set of molds and considerably greater care
in the manufacturing process than now commonly used. These tanks therefore,
are somewhat more costly, but can be built with current technology.

Plastic and fiberglass septic tanks, although constructed of water-tight

material, are not necessarily water-tight. Unless properly reinforced and installed,
they can deform, over time, when filled with water or when backfilled. When a
plastic or fiberglass tank deforms it can leak at the inlet and outlet ports, at
welded joints and at the joints between the upper and lower halves (2-piece
tanks). Major advantages of these tanks are impervious to water, corrosion
resistent materials, and lighter weight for installation and transportation.

To assure watertight tanks, a system for certifying tanks needs to be

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developed, as has been done already with many other construction materials. In
addition, a properly designed and constructed tank may leak if installed
improperly. Thus, tanks will continue to need a final inspection for damage
during transportation and for correct installation.

(3) Current status in Washington

No accurate figures are available for the percentage of water-tight septic

tanks in the state. Many regulators in the local health departments report that
many tanks they see are clearly not water-tight (water level coincides with
groundwater, or is below the outlet invert). It is likely that the manufacturers
supplying concrete septic tanks in Washington are not using the molds, correct
concrete mix, or quality control necessary for water tight construction. Other
problems are likely behind the leaking fiberglass and plastic tanks.

c. Size

The sizing of septic tanks is based on a 36 hour retention time and storage
volume for sludge. A longer retention time may have some merit, as evidenced by the
higher quality effluent produced by tanks receiving waste from homes equipped with
water conservation fixtures resulting in significant flow reductions. Currently in
Washington, the minimum size tank is 750 gallons for less than 3 bedrooms, 900
gallons for 3 bedrooms, 1000 for 4 bedrooms, and 250 gallons more for each
additional bedroom.

d. Function

The primary purposes of a septic tank are to separatethe solids from the liquid
fraction, provide a retention time adequateto achieve this separation, and an anaerobic
treatment process to reduce the volume of solids. Retention of solids is crucial to long-
term functioning of the drainfield.

2. Drainfield

Overview of Drainfield
14

Cutaway of Drainfield Section

a. Piping

A network of piping carries the septic tank effluent to the treatment and disposal
area. In gravity distribution systems, the network consists of 4 inch diameter pipe
which is solid from the tank to the drainfield and then perforated with 1/2 inch holes
in the drainfield. It should be noted that research has shown that the liquid exits the
pipe through just a few holes, usually those closest to the septic tank, and then flows
along the bottom of the trench until absorbed. Eventually the requirement for pipe in
gravity drainfields may be removed, but more experience and testing will be needed.

In pressure systems, the pipes are 1 to 2 inches in diameter with the drainfield
portion perforated with 3/16 inch diameter, or larger, holes. The septic tank effluent
is distributed relatively evenly throughout the drainfield area under low pressure (1 to
4 psi). Pressure is usually supplied by a pump with its associated float switches.
Occasionally a dosing siphon is used.

b. Trenches/Bed

The standard infiltrative surface is created by excavating a level trench ( < 3 feet
wide) or bed (> 3 feet wide), placing a minimum of 6 inches of washed gravel (3/4
to 2 1/2 inch diameter) in the excavation, laying the piping, and then placing enough
additional gravel to cover the piping by 2 inches. The trench bottom must be level to
avoid ponding of the effluent in one area.

The gravel functions (1) to provide access to the infiltrative surface, (2) to
provide storage for effluent, and (3) to maintain the trench or bed and hold back the
backfill.

c. Replacement Area

The part of the onsite system that occupies the largest area is the drainfield.
Development of properties with onsite systems includes a 100% replacement area for
the drainfield. This practice insures a viable option for repair if the primary drainfield
fails due to biological, hydraulic or particulate overload. All new lots in Washington
 15

State are required to have a 100% replacement area. This area must be preserved from
physical damage ( e.g. excavation, vehicle traffic, construction over the area, large
animal grazing, etc. )

3. Soil

The soil is where the wastewater receives its final treatment and where it is returned
to the environment. In order for the soil to do the required job, it must have a texture of type
2-6, have an acceptable structure, and have sufficient unsaturated depth. The several features
of the soil treatment system are discussed in the following paragraphs.

a. Infiltrative surface

The place where the septic tank effluent begins soaking into the soil is the
infiltrative surface. The rate at which this occurs depends on the texture,
structure, compactnessand moisture content of the soil. It is also influenced by
the degree of development of a biological "clogging mat" at this surface. This
surface is where treatment of the sewage begins in earnest.

b. Clogging mat

The clogging mat is a complex mat of microorganisms and polysaccharide

secretions, filtered suspended solids and even inorganic precipitants. It effectively
slows the infiltration of liquid into the soil so that flows beneath the drainfield are
unsaturated. It also provides some direct treatment of the sewage, removing much of
the BOD and sewage microbes. This mat can form as quickly as 1 week, but usually
requires months or even several years.

The clogging mat usually forms in a gravity system only after several months
of continuous use. It can only fonn under continuous flooding which will create
anaerobic conditions.

c. Treatment zone

This part of the system begins at the clog (infiltrative surface if there is no clog)
and includes the unsaturated zone that extends for a minimum of two feet downward.
It is in this unsaturated zone where treatment occurs and the minimum of 2 feet in
depth provides treatment to an acceptable level. As mentioned previously unsaturated
flow occurs when water moves through the micro pores and along surfaces of the soil
particles by capillary forces (matric potential). Water moves from the wetter to drier
areas and moves much slower than in saturatedflow conditions. In addition, the larger
pores are filled with air, thus promoting aerobic conditions in the soil. It should be
noted that there is a continuum from unsaturated to saturated flow, and the definitions
here are the extremes of the continuum.

d. Disposal

Disposal means the return of the treated effluent to the environment, usually to
16

the groundwater. In conventional systems, this is also accomplished in the drainfield

area. Properly designed and installed systems treat the wastewater to a degree that will
not cause disease or environmental degradation, and then place the treated liquid back
in the natural environment. The treated wastewater must move away from the site in
order to make room for more treated wastewater.

B. Alternatives to the conventional systems

1. Types

There are a number of approachesto on-site sewagetreatment and disposal other

than the conventional system. These alternative systems are utilized where site
conditions would prevent a conventional system from working properly. The three
major alternatives in Washington are: pressuredistribution to standard trenches, mound
systems, and sand filters. Other alternatives include: gravelless drainfields, aerobic
treatment devices, incineration toilets, composting toilets, and vault and pit privies.

2. How they accomplish treatment and disposal

a. Pressure distribution

Pressure provides fairly even distribution of the effluent throughout the

drainfield from the first use, without having to wait for a mature biomat to form.
Conditions favoring the use of pressure distribution are: medium sands that are
on the coarse end of the spectrum, shallow soils or high water table that reduce
the vertical separation to less than 3 feet, during part or all of the year, or where
the drainfield is a single trench (> 100 feet) on a slope.

Pressuredistribution accomplishes treatment under the above conditions by

spreading the septic tank effluent over the entire infiltrative surface and in doses
that promote unsaturated flow conditions. This regime promotes aerobic
conditions, unsaturated flow and extended contact between the soil and the
wastewater. Pressure distribution enhances the treatment and tends to slow
formation of a clogging mat (This latter item not necessarily beneficial).

b. Sand filter

At present there are two basic types of sand filters, single-pass and
multiple pass. The single-
pass sand filter is used Gr~ C\Ner InspocfIJn
p~
where the soil is too
shallow to provide
adequate treatment or
where the soils are
excessively permeable.
Treatment is accom-
plished by applying doses
of wastewater to a bed of specified media, collecting the treated wastewater at
 17

the bottom and sending it to disposal. The effluent from a sand filter must still
be discharged to a subsurface soil absorption system. A special type of sand
filter is a sand-lined trench or bed. For these special sand filters, the filtrate is
not collected for disposal, but instead infiltrates into the soil at the bottom of the
trench. These systems provide a high degree of treatment of BOD, TSS,
microorganisms, and some nitrate reduction.

The multiple pass sand filter is used when the wastewater strength is
higher than typical household sewage, i.e. a BODs > 230 mg/l or where the daily
flows are high. Treatment is accomplished by dosing the wastewater through a
specified medium (coarser than single-pass filter media) multiple times. The
biological activity in the media treats the higher strength effluent to a level that
can be handled in a soil absorption system. These filters can produce effluent
quality similar to single pass sand filters except for fecal coliform bacteria,
which will be in significantly higher numbers compared to a single-pass filter.

c. Mound system

Mound systems are used when the soil depth is insufficient to provide 2
feet of vertical separation or where the soil is excessively permeable. Mound
systems are sometimes selected to meet other concerns on a site, such as a repair
on a small lot. Treatment is accomplished in a manner similar to intermittent
sand filters, except the effluent is discharged directly to the upper layer of the
native soil.

d. Graveless drainfields

This system usesplastic or concrete channels or large diameter pipe instead

of gravel to provide accessto the infiltrative surface, storage volume above the
infiltrative surface and to keep the overburden from collapsing into the trench.
Treatment occurs in the biomat and the underlying soil, as it would in a standard
drainfield.
18

VII. Subsurface Absorption Fields

A. Trenches vs beds

The difference between a trench and a bed is the width. Trenches are 3 feet or less wide and
beds are greater than 3 feet wide. Good design practice limits beds to 10 feet wide, to allow air
to diffuse into the soil beneath the center of the bed. In Washington, beds can be used only with
soil types 1, 2, and 3 (soil type 1 will require enhanced treatment, such as a sand filter or a sand-
lined bed). Beds are often used in order to provide the required bottom area on lots with limited
space.

B. Sidewall vs Bottom Area

Sizing a drainfield in Washington is based on the bottom area only, except for some eastern
Washington counties, where some consideration is given to sidewall area. Where credit for part
or all of the sidewall is allowed, there can be a reduction in the overall length of trench required.
Such reductions are not useful in Western Washington because of the long periods of very damp
soil, which reduces the matric potential available for drawing the water through the sidewalls.

c. Deep vs Shallow vs At-grade Infiltrative Surfaces,

Deep trench bottoms are sometimes proposed in order to reach more permeable soils.
Optimum treatment occurs in the upper layers of the soil, as they have greater aeration. Standard
practice should place the bottom of the trench no deeper than 3 feet from the final surface grade.
Use of deep trenches is advised only when the trenches are lined with medium sand up to within
3 feet of the surface, where treatment can readily occur. Final disposal can then ensue at the
deeper level after the treatment. When sand-lined trenchesare proposed in Washington, they should
follow the guidelines for them in the Washington State Sand Filter Guidelines. To achieve the
required vertical separation under limiting site conditions, trenches can be placed as shallow as 6
inches, with final cover mounded over the top. At-grade infiltrative surfaces are sometimes
proposed when the soil is shallow, but has an acceptable texture. These systems are not currently
permitted in Washington.
 19

VllI. Elements of Proper Design ~

A. Site evaluation

Proper design begins with a thorough and accurate site evaluation. This activity includes
measurement and preparation of a plot plan which shows the property boundaries, wells, springs,
surface water, roads, buildings, cuts, banks, fills, topography, drainage patterns, and any other
feature which would affect the location, performance, design and installation of an on-site
treatment and disposal system. The site evaluation also includes soil logs in all areas being
considered for the drainfield in order to evaluate the suitability of the soil for an on-site system.
In addition, there is a need for area-wide knowledge of groundwater flows and direction of flows
so some calculations can be made as to short term and long term accumulative effects. The site
also needs to be evaluated for its ability to handle any effects of groundwater mounding beneath
drainfield, and to handle the ultimate disposal and dispersal of the wastewater.

B. System sizing

On the basis of the proposed size of the house (and therefore the design flows the system
would need to handle) and on the soil texture in the proposed drainfield area, the amount of
drainfield area is calculated. It should be noted that the design flow is not to be considered the
average daily flow, but rather an attempt to factor in peak flows on an occasional basis. The
system should be designed using a minimum design flow of 360 gallons per day. A plot plan
showing a silt loam at the infiltrative surface and proposing a 4 bedroom house should show a
minimum 1067 sq. ft. of drainfield area (120 gal per bedroom x 4 bedrooms divided by .45 gal
per sq. ft. per day).

c. Systemlocation

Once the size of system is determined, an area must be chosen in which to place this size
of system. It also must meet all the necessary setbacks from water supplies, property lines,
buildings, etc. The soil texture and depth in this area should be confirmed by assuring that soil
logs were performed there. A 100% replacement area is also a part of the system design and
should be included in the design and located where acceptable soil logs were performed.

D. System selection and design

The type of system selected will depend on the site conditions. Where 3 feet of vertical
separation and adequatehorizontal setbackscan be maintained, the system of choice is the standard
septic tank and drainfield. Shallow soils, high water tables, small lot size and excessively
permeable soils dictate the use of an alternative system. The system design should specify the
location and size of the septic tank and the location, depth and length of each drainfield trench.
The design should specify in detail the location and size of all components of an alternative
system. Construction details such as proper orientation and leveling of the septic tank, assurance
of level drainfield trench bottoms, etc. should also be included. The system that is selected should
be compatible with the site conditions.
20

IX. Elements of Proper Installation

In order for a properly designed system to provide long-term service, it must also be installed
properly. Correct installation practices include construction only when soil moisture conditions are
right and excavating trench bottoms level. Other correct installation practices are: not compacting
the soil, not scraping away the upper layers of the soil during lot preparation, and not smearing
the infiltrative surfaces. They also include using washed gravel, using certified ASTM C-33 sand
when specified, and installing water tight septic tanks and leveling and orienting them properly.

x. Elementsof ProperUse

Users of on-site sewagesystems are well advised to use a number of system-saving practices.
These include avoiding excessive water usage and spreading out over several days heavy water
using activities such as laundry .They also include avoiding use of garbage grinders, emptying
grease or other high strength waste down the drain, not adding solvents or other chemicals which
will kill the biological action, and not putting items like disposable diapers, tampons, or paper
towels into the system. Another element of proper use is not building over, driving over or grazing
animals on the drainfield or downslope of a mound.

XI. Elements of Proper Maintenance

Maintenance of on-site systems is often overlooked, but is just as important as the other
elements already discussed.The depth of sludge accumulating in the septic tank should be checked
each year or two, and at the same time the integrity of the baffles should be inspected. The septic
tank should be pumped when the solids are within 6 inches of the bottom of the outlet baffle in
the first compartment. (See illustration below. )

Pencil

"p ~? ~? '~p ..? ~? Scum Measuring Devices Sludge Measuring Device

House

~Inlet ~
Tee ~
Tee

1 It Compartment
2nd
Complrtment
Black C olor
I,~n!.
White ra 9
t
Dlatlngul8hea Sludge
~.ighl
or towel~
3'
i
Pump Out Tank When: ~~
6 ,
.A. is 3. or Less or
Layer From Liquid
.a. is 12. or less

If a filter is installed on the outlet of the septic tank, it should be cleaned whenever the tank is
inspected. Pump chambers should be checked regularly for accumulation of solids on the floats,
proper function of the alarm system, pump operation within the designed pumping period, and
evidence of water infiltration. Drainfields should be inspected for surfacing moisture, lush growth
of plants, and odors. Septic tank additives are of no known value and some can be very harmful
 21

to the system and the receiving groundwater. Such products can deceive homeowners into forgoing
the necessarymaintenance by use of a relatively cheap off-the-shelf item. Washington State Health
Department recommends against their use while urging that routine maintenance and repairs be
carried out to prolong and protect the life of onsite sewage systems.

With proper siting, design, installation, use and maintenance,onsite systemscan provide high
quality treatment to domestic sewage. The treatment and disposal furnished by these systems are
both long-term and cost-effective.
 APPENDIX A

WHY DO COARSE SOILS HA YE LARGER PORE SIZE AND LESS SURF ACE AREA?

WHY DO FINER SOILS HA VE SMALLER PORE SIZE AND MORE SURF ACE AREA?

By definition, coarse soils have larger particles. Imagine some objects of similar size and
having a round shape. When placed together in a container, they will touch each other in places
and leave voids or spaces in other places. The larger the particles, the larger these spaces.
Similarly the smaller the objects, the smaller the spaces that are left. Particles of soil lying
together in the ground also leave void spaces,with the coarser soil textures having the larger void
spaces.

A simple geometry example can illustrate how finer textured soils have larger surface
areas. Imagine a cube that measures 1 inch on each side. The surface area of that cube is 1"
x 1" to get the surface area of one face, time 6 faces in the cube, equals 6 square inches total
surface area. Now if that same cube were cut into 64 smaller, equal sized cubes of .25 inches
on a side, the total area of those smaller cubes is .25 x .25 x 6 x 64 = 24 square inches. "The
actual surface area of 1 cubic centimeter (about 1/4 teaspoon) of coarse sand is roughly
equivalent to the surface area of half dollar, while the surface area of 1 cm3 of fine clay is
equivalent to the area of a basketball court."l

For other relevant discussion on soils as they relate to onsite sewage disposal, see
Appendix B.

1 Cogger, C.G. 1987. "Septic System Waste Treatment in Soil", Washington State
University Cooperative Extension, Publication EB1475.
 APPENDIX B

Cooperative
Extension

College of Agriculture
& Home Economics

Washington
State
University

EB1475
Craig G. Cogger

This bulletin provides basic sci- rapidly under saturated or nearly to the area of a basketball court.
entific background on the role of saturated conditions, while micro- Because fine-textured soils have
soils in wastewater treatment. This pores transmit water more slowly more surface area, their chemical
information relates to the basic by capillary flow. Many chemical activity is generally much greater
principles behind Washington's and biological reactions in soil, in- than that of coarse-textured soils.
rules and guidelines on issuing per- cluding those which are important Soil surfaces playa role in
mits for and designing on-site in wastewater treatment, occur on wastewater treatment only when
wastewater treatment (septic) surfaces. adjacent to soil pores. wastewater contacts them. Massive
systems. The ability of a soil to treat clay soils, for example, often have
A household septic system pro- wastes depends on four factors: few pores that are readily perme-
perly designed, installed, and I) the amount of accessible soil ated by water, so the usable sur-
maintained on suitable soil is as particle surface area; face area is quite small. Heavy
effective as a sophisticated sewage 2) the chemical properties of the clay soils are not suitable for sep-
treatment plant. A conventional surfaces; tic systems because they are too
septic system consists of three 3) soil environmental conditions, impermeable to treat or dispose of
parts: septic tank, absorption such as temperature, the wastewater. In very coarse
trenches, and surrounding soil. moisture, and oxygen (OJ soils, water can travel so rapidly
Household wastes flow into the levels; and through the profile that it does
septic tank, where the wastewater 4) the nature of the particular not contact enough surfaces to
purification process begins. In the substances in the wastewater . provide good wastewater treat-
septic tank, solid wastes settle to ment. Coarse soils allow rapid
the bottom of the tank as sludge, Surface Area disposal of wastewater, but treat-
and grease floats to the top as ment may be inadequate. Soils
scum. Bacteria (called anaerobes The amount of surface area with even a small amount of fine
because they live without oxygen) depends on the texture or particle- particles can provide excellent
begin to slowly digest the solid size distribution of the soil. Clay waste treatment if the wastewater
wastes. The remaining wastes flow particles « 0.002 mm in diameter) contacts the particle surfaces.
out of the tank into the trenches have a much greater surface area Better contact occurs between
as liquid effluent. per unit volume than silt (0.002- wastewater and soil surfaces under
The absorption trenches distri- 0.05 mm) or sand (0.05-2.0 mm) conditions of unsaturated flow. The
bute the effluent to the soil, where particles. soil pores are partially filled with air ,
final treatment and disposal occur . To visualize the relative sizes of and wastewater moves by capillary
This bulletin will focus on waste- sand, silt, and clay particles, con- flow along soil particle surfaces. In
water treatment in the soil, the sider magnifying the largest clay saturated flow, gravity pulls water
most complex and limiting factor particle to the size of a penny. A through the macropores. Flow is
in septic system operation. A sys- silt particle would then range up- faster, but there is less contact
tem designed and installed to pro- ward to the size of a basketball, with the soil surfaces; wastewater
vide proper wastewater treatment and the largest sand particles treatment is less efficient.
in the soil will also provide ade- would approach the size of a
quate wastewater disposal. house. The actual surface area of Chemical Properties of
Soil contains roughly 500?0pore one cubic centimeter (cm3) of Surfaces
space. The pore space is broadly coarse sand (about ~ teaspoon) is
subdivided into macro pores (larger roughly equivalent to the area of a Chemically different soil sur-
pores) and micropores (smaller half dollar, while the surface area faces can be divided into four
pores). Macropores transmit water of 1 cm of fine clay is equivalent broad categories: 1) silicate clay
 APPENDIX B

Hydrous oxides. The next im- in much the same way that iron
Si At Si Si Si Si portant constituents are the and aluminum oxides do.
hydrous oxides of iron, aluminum,
and manganese. These are clay- Organic matter. Organic matter
Al Al At Al AI sized minerals made of iron (III), i~.chemically very complex and has
aluminum, or manganese (IV) ions a large reactive surface. Soil
bonded to oxide ions. They often organic matter can provide an
Si Si Si Al Si Si occur as poorly formed crystals or energy source for microbial growth
as coatings on other particles. Iron and can bind many substances,
oxides are the source of the red- although its capacity to bind
Fig. 1. Simplified structure of a dish or brownish coloration char- viruses appears limited. Since most
silicate clay mineral, showing substitu- acteristic of well-drained soils. soil organic matter is confined to
tion in the silica layers and resulting The hydrous oxide surfaces com- the upper part of the soil profile,
negative charges on th~~surface. (0-2 bine with water molecules and form it is usually not of major impor-
and 08- ions in tht~ structure are a mixture of positively and nega- tance in septic waste treatment.
not shown.) tively charged sites. In acid soils
the positive sites predominate, and Soil Microorganisms and
minerals; 2) hydrous oxides of the oxides have a net positive Soil Environment
iron (Fe), aluminum (AI), and charge. Thus, oxides have the ability
manganese (Mn); 3) carbonates; to attract and hold anions. Nitrate Soil surfaces are also important
and 4) organic matter . (NO3) and chloride are weakly at- because they are the home for soil
tracted to these oxides, but their microorganisms which carry out
Silicate clay minerals often com- movement through soil is only slight- many wastewater treatment pro-'
prise much of the clay fraction of ly inhibited. Phosphate (H2PO4 and cesses. Soil microorganisms play
soils. They give soil its stickiness HPO~-), however, can bond di- important roles in the breakdown
and plasticity when moist, and rectly to iron and aluminum at oxide of organic matter, the treatment
hardness or resistance to crum- surfaces, resulting in rapid re- of nitrogen, and the removal of
bling when dry. The molecular moval of phosphate from solution. bacteria and viruses from waste-
structure of silicate clays is much The movement of phosphate through water. These microorganisms are
like a sandwich, with silica layers soils containing large amounts of sensitive to environmental condi-
(silicon (Si4+), oxygen (02-) and hydrous oxides is therefore limited. tions within the soil, including
hydroxyl (OH-» bonded to Oxide surfaces also adsorb and temperature, moisture levels, and
alumina layers (aluminum (Ar+) possibly inactivate some viruses. oxygen availability. Cold tempera-
and 02-) as shown in Figure I. The edges of silicate clay miner- tures will slow all biological reac-
Different types of clay minerals als are chemically similar to the tions in the soil, reducing the rate
vary in structure and properties, hydrous oxides, and the surfaces of wastewater treatment.
but all have some permanent of many soil particles may actually Oxygen availability affects
negative charge. This charge is be coated with oxides. In such microbial populations and waste
usually due to imperfections in the cases, particle interactions with treatment. Excess water (as from a
crystal structure (isomorphous wastewater may mimic those of high water table) saturates soil
substitution), where ions of lower the hydrous oxides of iron, pores and greatly decreases diffu-
charge replace silicon or aluminum aluminum, and manganese. sion of oxygen into the soil. Once
during crystal formation, so that the existing oxygen is depleted, the
the oxide ions are not completely Carbonates. Calcium and soil becomes anaerobic; the rates
balanced by positive charges. As a magnesium carbonates (lime) are and types of microbial processes
result of their negative charge, primarily important in arid regions that occur in the soil will change
silicate clay minerals can attract or in soils developed from lime- greatly.
and sometimes bond cations stone-rich parent materials. In Oxygen functions as a biochemi-
(positive ions) to their surfaces. humid regions, however, they are cal electron acceptor for aerobic
This surface bonding is called ad- dissolved and leached from soils. organisms. The biochemical pro-
sorption. Potassium is more per- Soils containing calcium carbonate cesseswhich convert food into
manently held by some clay miner- (CaCO3) will fizz (release carbon energy rely on the transfer of elec-
als through a stronger bonding dioxide) in the presence of acid, trons from one molecule to an-
mechanism. Since most of the in- and sometimes have visible car- other in the cells, and on the cap-
organic pollutants from septic bonate accumulations which look ture of energy released during
tanks are anionic (negatively like white threads. In extreme those transfers. At the end of the
charged), they are not attracted to cases, carbonates can form im- biochemical pathway, the elec-
clay minerals. These minerals do permeable hardpans called caliche. trons, at a very low energy level,
adsorb bacteria, viruses, and many The carbonates are important be- have essentially become a waste
organic compounds, however . cause they can adsorb phosphate product. They are removed by
 APPENDIX B

oxygen- an excellent electron phosphate, and in time their ef- from inorganic ammonium under
scavenger -which plays the role of fluent will contain nearly as much oxidizing conditions, it is usually
a garbage collector. When no ox- phosphate as the influent from the the end product of nitrogen me-
ygen remains to scavenge elec- septic tank. Once the mound or tabolism in a properly functioning
trons, the energy-producing path- sand filter effluent comes in con- septic system. Because nitrate is so
ways shut down, resulting in death or tact with native soil, however, the soluble in soil solution, it will
dormancy of the aerobic organisms. phosphate is usually removed often leach to ground water. These
Some organisms can survive un- quickly. factors have led to the great con-
der anaerobic conditions. They can cern about nitrate pollution from
function without oxygen by using Nitrogen. Nitrogen is much septic systems.
substances such as nitrate, iron more mobile than phosphate, and In order to prevent methemo-
(III), sulfate, or organic com- its reactions in the soil are con- globinemia, the Environmental
pounds as electron scavengers. siderably more complex. In raw Protection Agency has established
Since these other substances only wastewater, nitrogen is primarily a maximum acceptable level of 10
accept electrons from much higher associated with organic matter in ppm for nitrate-N in public drink-
energy levels than oxygen can, the substances such as proteins. Begin- ing water systems. Since nitrate
anaerobes must rely on much less ning in the septic tank, organic removal is incomplete using cur-
efficient biochemical pathways nitrogen compounds are broken rent septic system technologies, lot
than aerobes do. Less efficient and down (mineralized) and inorganic size restrictions have been the
less complete treatment of waste- ammonium (NHt) is released. main tool used to prevent nitrate
water occurs under anaerobic concentrations from exceeding
conditions. Org-N mineralizatio!\.. NHt standards in ground water beneath
nitrification;;.- NO 3 denitrificatio!\.. areas served by septic systems.
Chemical Components of Although recognition of the ef -
N2,N2O
Wastewater fects of nitrate, the establishment
Ammonium is soluble in water but of drinking water standards, and
Phosphate. The chemical sub- is weakly retained in soil by at- the increased use of breast feeding
stances of greatest concern in traction to negatively charged soil and liquid infant formula concen-
household wastewater are phos- surfaces. The persistence of large trates have almost eliminated
phate, nitrogen, and organic mat- amounts of inorganic ammonium reported cases of methemoglob-
ter. Phosphate ions are negatively in the soil usually indicates anaer- inemia in the United States, nitrate
charged (H2POi and HPO~-). obic conditions and an improperly will continue to be an important
Phosphate is adsorbed strongly, operating septic system. Under indicator of subsurface pollution.
although not completely irrever- aerobic conditions inorganic am-
sibly, to hydrous oxide and car- monium is rapidly oxidized to Organic compounds. Organic
bonate surfaces. It can also be nitrate (NO 3) through a microbial matter comprises the bulk of the
biologically incorporated into process called nitrification. Nitrate solids in wastewater. Chemical and
organic matter. Although phos- is very soluble in soil solution, and biological oxygen demand (COD
phate is not a toxic substance, ex- is often leached into the ground and BOD), total organic carbon,
cess levels in lake waters can pro- water. If nitrate is leached to an and suspended solid$ are water
mote eutrophication, the excessive anaerobic zone in the soil, it can quality analyses commonly used to
growth of aquatic plants and even- be used as an electron acceptor by indicate the amount of organic
tual depletion of oxygen in the denitrifying organisms. This pro- matter present in wastewater .
water. cess is called denitrification, which Nearly all organic matter in house-
The capacity of most soils to reduces nitrate to gaseous forms hold wastes is biodegradable, and
hold phosphate is large compared of nitrogen (N2 and N2O). Nitrifi- it does degrade readily in soil.
with the phosphate load from a cation and subsequent denitrifica- Aerobic conditions beneath the ab-
septic system, so there is usually tion can occur when aerobic and sorption field increase the rate of
little concern over this substance. anaerobic zones alternate in the degradation, while anaerobic con-
An important exception occurs soil. This provides a good mechan- ditions slow degradation.
when septic systems are located in ism for nitrogen removal, but un- Trace amounts of toxic, syn-
coarse-textured soils surrounding a fortunately the potential for deni- thetic organic compounds appear
lake. Because of limited surface trification is limited in many soils in the organic matter from house-
area, these soils may eventually which otherwise provide excellent hold wastewater. Existing research
become saturated with phosphate. wastewater treatment. indicates that levels of these com-
Phosphate will then move through Nitrate is considered a pollutant pounds are so low that they pose
the saturated soils, posing a threat in drinking water because elevated little threat to ground water qual-
of eutrophication to the lake. levels have caused methemoglo- ity. There is little data available,
Mounds and sand filters also binemia, or oxygen deprivation, in however; this subject merits addi-
have a limited capacity to adsorb infants. Since nitrate is produced tional research.
 APPENDIX B

Microorganisms, in when retention occurs by adsorp- A number of other factors also

Wastewater tion processes. Retention slows the affect the survival of bacteria and
movement of bacteria and viruses viruses in soil. Survival var.es
While most microorganisms in through the soil, but may also significantly among types of bac-
wastewater are harmless, many prolong their survival. teria and viruses, which show
pathogenic (disease-causing) organ- Viruses and bacteria are ad- widely varying resistance to hostile
isms may be present. The inter- sorbed by clay minerals and conditions. Soil adsorption tends
actions of these organisms with hydrous oxides, while organic mat- to protect microorganisms from
soil are much more complex and ter appears less effective at micro- destruction, although adsorption
less well understood than the reac- bial adsorption. Soil acidity affects to certain hydrous oxides may
tions of nitrogen and phosphate. virus adsorption; increased reten- speed the inactivation of some
Pathogenic organisms in waste- tion occurs in more acid soils viruses. Many bacteria die off
water can include bacteria, viruses, (lower pH). The.amount of viral more rapidly in acid soils, while
protozoa, and helminths (worms). adsorption to soil also depends on viral persistence often increases,
Helminths are approximately the the ability of the virus surface to probably due to increased adsorp-
size of sand particles, protozoa the bind to soil surfaces. Scientists tion under acid conditions. Both
size of silt particles, bacteria the have observed a wide range of ad- sunlight and drying decrease mi-
size of fine silt and coarse clay, sorption even among different crobial survival, but neither of
and viruses the size of very fine strains of the same virus, appar- these are likely to be factors in
clay. D1.1eto the relatively large ently due to differences in the septic systems.
size of helminths and protozoa, chemical charge on the protein In general, removal of viruses
their movement from septic sys- coat of the virus. and bacteria is rapid under aerobic
tems through soil pores is usually Retention is not necessarily per- conditions and unsaturated flow; a
limited. Bacteria and viruses have manent. During periods of heavy number of researchers have found
a much greater potential for move- rainfall, retained viruses become nearly total removal of bacteria
ment and have been the principal resuspended in the soil water, and and viruses in 2 feet of soil.
causes of disease outbreaks related are transported rapidly by satu- Several monitoring studies, how-
to ground water contamination by rated flow through large soil ever, have suggested bacteria and
septic systems. pores. When retention protects viruses can sometimes move many
To minimize the risk of disease viruses from destruction, they may feet, even under supposedly hostile
transmission, pathogenic organ- reach ground water by alternate conditions. It is probable that the
isms must be removed from the cycles of retention and resuspen- cases of extreme movement in-
soil before they reach drinking sion. volved factors such as improper
water aquifers. The transport of septic system design or operation
microorganisms through the soil Inactivation in Soil. Bacteria or inadequately protected wells.
depends on two main processes - and viruses need favorable envi- These occurrences underscore the
retention and inactivation (die- ronmental conditions and protec- importance of proper design, in-
off). Retention slows the move- tion from competition or preda- stallation, and maintenance of
ment of microorganisms while in- tion in order to persist. Bacteria in septic systems and wells in protect-
activation results in final removal. sewage are predominately faculta- ing ground water quality.
Soil properties, environmental con- tive-that is, they can survive
ditions, and the nature of the under either aerobic or anaerobic Practical Considerations
microorganisms themselves control conditions. Most soil bacteria are
both processes. obligate aerobes, active only in the The two factors most important
presence of oxygen. Sewage organ- in the treatment of septic system
Retention in Soil. Protozoa and isms are rapidly destroyed in aero- wastes are 1) maintaining adequate
helminths are retained in soil pri- bic soils, because they compete contact between the wastewater
marily by entrapment in soil pores poorly with the natural soil micro- and the surfaces of soil particles;
or settling from soil solution. oganisms, Under anaerobic condi- and 2) maintaining aerobic condi-
Viruses, which are small enough to tions most soil bacteria are inac- tions beneath the absorption field.
move easily through soil pores, are tivated, and the facultative sewage The rules and guidelines for de-
retained primarily by chemical or organisms can survive for a much signing and installing septic sys-
physical adsorption to clay or ox- 1onger time. Virus survival shows tems are based on these factors.
ide surfaces. Bacteria, which are similar patterns. Survival decreases To maintain aerobic conditions
intermediate in size, are retained under aerobic conditions and in- beneath the absorption field, the
by both entrapment and adsorp- creases under anaerobic condi- soil must remain unsaturated.
tion processes. Retained organisms tions. Both bacteria and viruses Under unsaturated conditions, ox-
are not necessarily inactivated, and from sewage survive longer at low ygen can diffuse through the soil
may even be protected from inac- soil temperatures, because natural air and replenish the oxygen con-
tivation. This is especially true soil microbial activity is reduced. sumed by microbial activity.