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  Introduction	1
      Alternatives to Septic Systems	2
      How to Use This Brochure	2

  Modify Existing Septic System	4
      Water Conservation	6
      Alternating Drain Fields	7
      Septic Tank Mound Systems	8
'      Sand Filter Systems	10
r;     Separate Graywater/Blackwater System	11
      Capillary Seepage Trench (CST)	12

  Lagoons	14
      Lagoon Systems	16
\         Anaerobic Lagoons	17
?         Facultative Lagoons	18
 >         Aerobic Lagoons	18
          Aerated Lagoons	18

?, Mechanical Treatment Systems	20
S.     Oxidation Ditch	22
;f     Package Treatment Plants	23
      Sequencing Batch Reactor (SBR)	24

^Natural (Land-Based) Treatment Systems	26
„<     Constructed Wetlands	28
\i     Rapid Infiltration	30
;\5     Slow Rate Land Application	31
x     Overland Flow	32
  Glossary	33

  Bibliography	37

Wastewater Treatment: Alternatives to Septic
Systems, guidance document.

EPA 909-K-96-001, Region 9 Drinking Water Program,
June 1996.
Additional copies may be obtained from:

Public Outreach Materials (EPA W-6-3)
75 Hawthorne Street
San Francisco, California 94105

Or by calling the Water Resource Center at (202) 260-

This document is intended to illustrate -waste-water treatment options.
It does not represent agency policy. Any reference to private
companies does not constitute an endorsement of those companies or
their services.

~W ~\ Then properly sited, constructed,
  %/%/ and maintained, septic systems
   T  V  can provide a low-cost,
environmentally responsible method of
waste disposal. Improperly sited,
constructed, operated, or maintained septic
systems can, however, lead to water quality
degradation and threats to public health.

This brochure will help in deciding what to
do when a septic system fails or is
overloaded.  Options include:

   •  Rehabilitating the septic system;
   •  Expanding or adding to it; or
   •  Properly abandoning the septic
      system and replacing it with an
      alternative system.

No brochure can provide a definitive answer
to every situation. This brochure provides
an overview of the types of alternative
systems available for use in specific

Septic systems can fail for a number of
reasons, including:

   •   Under-design, including faulty design
      of the septic  tank, or a leachfield that
      is too  small;
   •   Faulty installation, including plugged
      lines, not enough stone in trenches,
      smeared soil interface, or uneven
   •  Soil conditions, including high
      groundwater (i.e., within 4 feet of the
      surface), less than 6 feet of
      unsaturated soil cover over bedrock,
      or relatively impervious soils;
   •  Hydraulic overload; and
   •  Improper maintenance (i.e.,
      inadequate pumping of tank).

A broad range of proven alternatives to the
traditional septic tank/leachfield system are
available. The selection of a particular
treatment technology depends on a number
of site-specific factors such as climate,  soil
conditions, terrain, wastewater flow rate and
characteristics, local surface water discharge
limitations, land availability, economic
resources and constraints, and availability of
skilled operating personnel. In general,
small wastewater treatment and disposal
systems should be simple, reliable,
economical, and easily maintained. Systems
that discharge to surface waters may need
National Pollutant Discharge Elimination
System (NPDES) permits.

If treated effluent is to be discharged
directly to a surface water, an application
for a permit must be made to the appropriate
permitting authority 180 days before the
actual discharge is to begin. Effluent
limitations in the permit serve to control the
discharge of pollutants to the surface
waters. Effluent limitations may be based on
either technology or receiving water quality
factors, depending on local conditions.  The

permitting authority will generally require
monitoring of effluent and receiving water
to demonstrate compliance with permit
requirements. NPDES permits are usually in
effect for a period of five years.

It should be noted that this brochure is
intended for domestic wastewaters only;
treatment of industrial wastewaters is
beyond the scope of this brochure, although
some of the technologies described could be
utilized for certain industrial wastewaters.

Alternatives to Septic

The basic options available are upgrading
the existing system or replacing it with an
alternative technology. Technologies for
wastewater treatment can be divided into
natural systems and
mechanically assisted systems;
lagoon systems are considered
a combination of natural and
mechanically assisted systems.

Natural systems use the waste-
removal properties of soil,
vegetation, and aquatic
environments to treat
wastewater. They include
lagoons, constructed wetlands,
and various land-application
methods. Natural systems tend
to be inexpensive, have low
operation and maintenance
(O&M) requirements, and may
provide added benefits such as non-food
crop production, ground water recharge, or
wetland habitat. However, natural systems
provide less control of treatment
performance, and require relatively large
land areas for implementation; the amount
of land required for a given situation will
vary from a few acres to several tens of
acres, or more, depending on the site-
specific conditions.

For situations where there is a shortage of
available land suited for natural treatment
systems, mechanical treatment systems
should be considered. Mechanically assisted
systems are highly  engineered facilities that
treat large volumes of wastewater in
relatively small areas (generally less than
one acre). These systems require high
energy inputs and skilled operator attention;
pical Two-Compartment
Septic Tank
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therefore, they have higher operating costs
than natural systems. Additionally, although
mechanically assisted systems do not
require large land areas, they tend to
produce a greater sludge volume than
natural systems, which further increases
operating costs.

Two options not covered in this guidance
include incinerating toilets and biological,
or composting, toilets.

How to Use This Guidance

Before proceeding with a more complex, or
expensive, alternative to a septic system, it
is important to understand why the system
failed in the first place. Simpler, or less
expensive, solutions to the problem should
be considered. This brochure provides four
matrices to help with the decision-making
process. The matrices are:

   •  Modify Existing Septic System.

   •  Replace with Lagoon System.

   •  Replace with a Mechanically Assisted
      Treatment System.

   •  Replace with Natural (Land-Based)
      Treatment System.

Each of these matrices provides
comparative information regarding cost,
operator skill required, land requirements,
O&M requirements, and opportunities and
limitations of the technology. Brief
descriptions of the alternative technologies
are provided after each matrix. EPA
recommends that, before any alternative
technology is selected, additional technical
assistance be obtained to evaluate the
conditions at a site.

In the matrices, "land requirements" are
classified as low, medium, or high; while
the exact land areas would be subject to
site-specific conditions, "low" generally
refers to less than one acre, "medium" refers
to one to 10 acres, and "high" refers to over
10 acres. Specific requirements for operator
certification vary by state, but in general are
determined by the design flow and
complexity of the system.

A well-functioning septic system provides
both treatment and disposal of wastewater.
However, many of the alternative
technologies described in this brochure
(lagoon systems and mechanical treatment
systems) only provide wastewater treatment;
effluent disposal also needs to be
considered in the overall system design, and
can be either by surface or subsurface
methods. Surface discharge normally
requires an NPDES permit; therefore, it is
recommended that a subsurface disposal
method (rapid infiltration, slow rate land
application, or overland flow) be evaluated
first in connection with the selection of a
lagoon or mechanical treatment system. If a
subsurface disposal method is infeasible,
then a surface discharge needs to be
considered. •

Modify Existing Septic System
Operator Skill Disposal
Alternative Capital Cost Required Required?
Drain Fields
Sand Filters
$1 0-$500 or None No
Leachline: None No
$30 -$100
per foot
1 .5 - 2 times None No
$2 - $5 per Low Yes
Similar to Low No
septic system
Additional $1 None NO
- $2 per foot

None, beyond
general "house-
Switching the flow 1
- 3 times per year
Normal septic tank
maintenance; pump
Sand regeneration;
pump O&M
Same as normal
septic tank; pump
O&M for reuse
Same as
conventional septic
Can be a simple, economical
method of hydraulic load reduction
for a septic system
Provides resting period to allow the
regeneration of subsurface disposal
systems; most beneficial for soils
with slow percolation rates and high
Can permit septic systems in areas
with slow absorbing soils, shallow
bedrock, or high water table;
operates in all climates, minimal
Provides a stable, high-quality
effluent, possibly allowing reuse;
modular design allows for
expansion; allows for higher loading
Allows for the recycling of
wastewater, reduces hydraulic
loading on leachfield; advantageous
when disposal area or fresh water is
Provides more uniform distribution
of effluent for rapidly permeable, or
slowly permeable soils
Effective only if system
failure due to slowly
permeable soil, or
organic clogging of soil
Higher capital cost than
standard leachfield
Greater space
requirement, slope limits,
and pumping O&M;
limited to soils with
percolation rates below
120 min/in and slopes
less than 15%
Pumping requirements of
about 3 ft; odor
generation possible; may
need to protect from
Higher capital and
operating costs than for
conventional septic
Bottom of CST not used
as an effective
absorption area, possibly
increasing size of
disposal area

Modify Existing Septic System
Water Conservation
         Witer conservation can be a simple,
         economical way to correct the
         hydraulic overload of an existing
septic system. It can be especially effective
where the failure is caused by soils with low
permeability or by organic clogging of soils
with otherwise acceptable porosity. The
success of water conservation depends on
the severity of the malfunction to be
corrected; to be effective, water
conservation devices must reduce effluent
volumes below the capacity of the soil
absorption system.

Installation of low-flow fixtures (faucet
aerators and toilets) can reduce wastewater
flows by more than 50 percent. User
acceptance of such devices is generally very
good, and minimal maintenance problems
are encountered. The cost of a water
conservation refit program is normally less
than that of replacing the entire soil
absorption area or constructing a smaller
alternate soil absorption area. However, if
water conservation alone does not
sufficiently reduce the hydraulic loading on
the septic system, a combination of low-
flow fixture refit and the construction of an
alternative soil absorption system can be an
effective method  of correcting

Additional housekeeping aspects of water
conservation include eliminating leaks and
drips, maintaining proper water pressures,
eliminating the use of garbage disposals,
having laundry done off-site, and installing
a greywater treatment system. A public
information and awareness program should
also be included to inform and remind users
of the importance of water conservation. •

                                                           Modify Existing  Septic System
Alternating Drain  Fields
   A  Iternating fields are simply two or
  /\  more full-size or smaller drain fields
4.   \Jhat are alternatively put into service
and taken out of service by means of a valve
or diversion box. A resting period is critical
to the long-term operation of subsurface
disposal systems. Without a resting period,
soil absorption systems will progressively
clog. Adequate resting sometimes can be
accomplished by having a resting period
between doses from the septic tank. Resting
for at least four months often regenerates
the disposal field by allowing the clogging
mat to break down aerobically. The resting
drain field can also serve as a back-up
system if the operational drain field needs
repairs. When replacing a drain
field that has become clogged, the
clogged field can be kept as an

When a drain field that has built
up a thick biological mat along its
surfaces is allowed to rest,
incoming air provides oxygen for
bacteria to decompose the
remaining organic matter under
aerobic conditions. This process
continues for several months,
depending on the thickness of the
clogging layer and the climate.
Systems left to rest for 9 to 12
months are considered to be
rejuvenated. Two full-sized drain
fields, if properly designed,
constructed, operated, and
maintained, may last indefinitely if
  used only in alternate years.

  For new leachfield systems, it is
  recommended that 150 percent of the
  required area be constructed, in three 50
  percent sections. In this way, two of the
  three leachfield areas are on line at a time,
  and one field can be rotated in or out of
  service every four months, allowing a
  resting period for each area of four months
  per year. Other than an occasional
  inspection to ensure that no wastewater is
  collecting in the inspection vent at the end
  of the leachline(s), switching the flow one
  to three times per year is the only O&M
  requirement. •
Alternating Drain Fields
    Leach Une
    SerJng Disposal
    Field 1
                                 Leach Line
                              Serving Disposal
                                   Field 2

Modify Existing Septic System
Septic Tank Mound Systems
       Tound systems are used where there
       | is a thin soil layer above the
          oundwater table or above an
impermeable layer, and in areas with slowly
permeable soils or shallow bedrock. For a
mound system, effluent is discharged under
pressure through perforated pipes located in
an absorption system that is elevated above
the natural soil surface in a sand fill. Mound
systems require more space than do
conventional septic systems because of their
sand fill requirements and because slope
limitations (12 percent maximum) are more
restrictive for mound systems than for
conventional systems. Mound systems
require the importing of fill and provision
of a pump or siphon to uniformly load the
mound; electrical power  is therefore an
ongoing expense. It should be noted that
many regulatory agencies no longer approve
          the use of community-sized mound systems
          because of the high rate of failure and
          because other disposal systems are more

          Septic tank effluent is first conveyed to a
          dosing chamber, where it is stored
          temporarily, and then pumped or siphoned
          to the elevated absorption area and
          distributed through a distribution network
          located in coarse aggregate at the top of the
          mound. The effluent then passes through the
          aggregate and filters into the  sand fill, thus
          permitting the effluent to spread over a
          large area of native soil. A biological mat
          develops in this area that consumes
          biodegradable materials and filters out
          pathogens and parasites. There should be at
          least two feet of natural soil below the
          mound to accept the effluent. Mounds
                              Mound System
                                                        NOT TO SCAtE
              Septic Tank
Wet Well or Dosing

                                                          Modify Existing Septic System
designed to dispose of water through
evapotranspiration are called
evapotranspiration/absorption (ETA)
mounds. ETA mounds must have no more
than 24 inches of wicking sand between the
distribution laterals and the top of the
mound; the wicking sand allows water to be
transported to the mound surface to be
disposed of by evapotranspiration.

Mounds should not be located in swales or
depressions where rainfall runoff can
collect. Locating a mound at the top of a
slight rise will maximize the spread of
effluent in all directions beneath the soil.
Long, narrow mounds along gentle slopes
appear to work very well because the
effluent is allowed to flow through the
mound and down gradient. Siting mounds
so that they are exposed to sun and wind
will result in the evaporation and plant
transpiration of a portion of the effluent,
thus reducing the load on the soil below. •

Modify Existing Septic  System
Sand Filter Systems
A       sand filtration system is particularly
       well suited to treating septic tank
       effluent because it is capable of
producing a high-quality secondary effluent
with a minimum of O&M requirements.
There are three types of sand filters:  buried
sand filters, open (single pass) intermittent
sand filters, and recirculating sand filters.
While all three are somewhat similar in
design, they may differ in method of
operation, performance, access, and  filter
media specifications.  Selection of a
particular type of sand filter will depend
upon site-specific conditions. Capacity can
be easily expanded through modular design.
The high-quality effluent may allow for
disinfection with ultraviolet light, thus
opening up disposal alternatives such as
irrigation, surface discharge, and reuse of
water for toilet flushing.

Sand filters typically consist of one or more
beds of granular material, typically graded
              Sand Filter System
sand, 2 to 3 feet deep, underlain with
collection drains imbedded in gravel. Septic
tank effluent is intermittently applied to the
surface of the sand bed and allowed to
percolate through the bed. A biological mat
forms on the sand surface that allows for the
decomposition of the biodegradable
materials; the rate of flow through the mat is
limited, while rapid flow occurs beneath the
mat. The filter media thus remains
unsaturated and is vented to the atmosphere
to maintain an aerobic environment. The
treated effluent is  collected by the
underdrains for disposal.

Sand filters require relatively little
operational control or maintenance. The
surfaces of the sand  filters eventually clog,
thus requiring sand regeneration that is
typically accomplished either by surface
tilling or by removing the top layer and
replacing it with clean sand. Sand filters
require somewhat more land area than do
               mechanical treatment units,
               but their land requirements
               are lower than those of
               other systems, such as
               lagoons. Because the head
               required by the filters
               typically exceeds 3 feet,
               pumping may be required
               for effluent disposal. Odors
               can  occur from single-pass
               filters treating septic tank
               effluent, and buffer zones
               between the system and
               adjacent dwellings may be
               required. •

                                                         Modify Existing Septic System
Separate Graywater/Blackwater System
      Blackwater is toilet wastewater, and
      graywater is the wastewater from all
      other sources, including sinks, baths,
showers, and laundry. Normally, the pipes
collecting graywater and blackwater are
joined, and both types of waste are
combined before reaching the septic tank.
However, blackwater and graywater can be
segregated and treated separately.

Graywater contains a lower and more
readily biodegradable BOD loading than
does blackwater, considerably lower
bacterial loading, and contains less
settleable solids; however, graywater
typically contains more grease than does

While graywater tends to be more readily
usable than blackwater or combined
wastewater, it still needs treatment prior to
reuse. Typically, a separate septic system is
installed for the treatment of graywater,
with a parallel treatment system for
blackwater treatment. Kitchen graywater is
often routed to the blackwater system
because of its relatively high loading of
BOD, TSS, and grease. The graywater
treatment system can employ a conventional
septic tank and leachfield system, but is
usually designed to allow the reuse of the

The most common uses of graywater are for
irrigation and toilet flushing. The latter use
requires a dual piping system to recycle the
graywater used to flush toilets. Retrofitting
a building for this purpose, however, is not
always possible or economically feasible.

Graywater should be treated and
disinfected, even when used for flushing.
Disinfection can be avoided, however, if the
graywater is used for landscape irrigation or
on non-food crops, especially if access to
the irrigated areas is restricted.

O&M for a graywater system is the same as
for a conventional leachfield system.
However, graywater systems that reuse the
water will have pumps, which increases the
O&M costs. The use of graywater systems is
particularly advantageous in locations where
the disposal area is limited or where water
shortages exist. A dual graywater/
blackwater system may be possible on a lot
that would be too small for a conventional
septic system. •

Modify Existing Septic System
Capillary Seepage  Trench (CST)
~W "IT Then expanding an existing leach
 \/\/ field or constructing a new one,
   T T consideration should be given to a
modified design that appears to resist
clogging better than conventional systems.
This design modification is known as the
capillary seepage trench, or CST, and is
widely used in Japan. The CST is similar to
a conventional trench, but includes an
impermeable liner at the bottom of the
trench and part way up the sidewalls.  The
effluent collects along the entire length of
the trench, moving upward and horizontally
by capillary action, before percolating
downward. This modified design utilizes a
larger area of unsaturated soil matrix
surrounding the trench, and results in a
fairly uniform distribution of effluent along
the trench.

Because effluent comes in contact with a
large area of soil, seepage velocity is
decreased while the amount of time that the
effluent is in contact with the biomat is
increased. This design avoids the
phenomenon of progressive failure, where
portions of the system are overloaded due to
an uneven distribution of effluent over the
length of the trench. In addition, the aerobic
conditions necessary for optimum removal
of organic matter seem better maintained
because the effluent is evenly distributed
throughout the soil matrix. Fly ash can be
used as trench fill to allow for the rapid
movement of water by capillary forces, and
for the increased surface area available for
microbial growth. This type of trench has
also demonstrated greater nitrogen removal
than conventional trench design.
  Capillary Seepage Trench Design
                                BACKFILLED SOIL

                                 DISTRIBUTION PIPE

                                2-3 INCHES OF STONE

                                FLY ASH

                                 IMPERMEABLE TROUGH

                                                            Modify Existing Septic System
For soils with low permeability, the CST
system may access the more permeable
sidewalls, and allow the sidewalls to rest
after each dose from the septic tank. For
soils with a higher permeability, the CST
system provides a more uniform distribution
of applied effluent, compared to
conventional  trenches. However, since the
CST system does not use the bottom of the
trench as an effective absorption area,
required size  of the trench will usually need
to be longer than conventional trenches. •

Capital Operator Skill Capacity Disposal
Alternative Cost Required Limits Required?
Largely Low
Largely Low
Largely Low
<$1/gpd, Medium
plus land
Not suitable Yes
for low
Not suitable Yes
for tow
Not suitable Yes
for low
None Yes

Routine inspection
and maintenance;
occasional sludge
Simplest and least expensive
lagoon to operate; minimal
power requirements or
operator expertise
Potentially high level of odor
generation during periodic
cleaning; additional treatment
required prior to discharge;
biological activity adversely
affected by cold and hydraulic
or organic overload
Routine inspection
and maintenance;
occasional sludge
Effluent can be infiltrated into
the soil; minimal power
requirements or operator
expertise; algae can be mixed
with animal feed or used as a
soil conditioner
Need to maintain 200-foot
buffer zone; biological activity
adversely affected by cold and
hydraulic or organic overload
Routine inspection
and maintenance;
occasional sludge
Effluent can be infiltrated into
the soV; minimal power
requirements or opertor
Limited to warm climates; high
algae loading can clog
infiltration  systems
Regular preventive
maintenance on
aerators; sludge
Minimal odor generation; can
increase organic loading on
existing lagoons by adding
High energy costs; will not
solve hydraulic overloading

Lagoon Systems
"Y "T There large tracts of land are
 \/V/ readily available, lagoons are the
  V V  most commonly used wastewater
treatment system, especially for small
communities. Lagoons can provide a simple
and economical means of wastewater
treatment, but they may require additional
downstream treatment (e.g., sand filtration)
to polish the lagoon effluent prior to surface
discharge. Lagoons may be used alone or in
combination with other treatment processes.
They can also be used to pretreat and store
wastewater prior to release to constructed
wetlands or land-application systems.

Lagoon systems are natural treatment
processes in which bacteria and algae
reduce the organic content of the
wastewater. A healthy lagoon will exhibit a
green color from the large algae population
that develops. Lagoons act as extended
oxidation or waste stabilization systems, hi
addition to physical treatment such as
settling and floatation, the lagoon provides
storage time, which permits bacteria to
metabolize the biodegradable portion of the
wastewater, thereby stabilizing the effluent.

Lagoons are usually earth-diked ponds that
vary in size and shape, and are typically
constructed using on-site soils. Topsoil is
removed from the lagoon area, and the dikes
are compacted in layers to a height that
allows some freeboard for waves.
Depending on the soil characteristics, it may
be necessary to line the lagoon with rubber,
plastic, or clay. Dike slopes are normally
between 3:1 and 6:1, and top widths can be
up to 10 feet to allow for maintenance
vehicles. A hydrologic budget must be
calculated for the lagoon so that water will
remain at the desired depth;  inflow and
precipitation must be balanced with
evaporation, outflow, and infiltration.
Lagoons are characterized according to the
dissolved oxygen (DO) source, and can be
anaerobic, facultative, aerobic, or aerated.

For efficiency of maintenance, operation,
and treatment, a multicell lagoon system is
better than a single-cell lagoon; normally, a
minimum of three separate cells in series are
constructed. The first cell can be larger than
the other cells, thereby receiving the bulk of
the organic loading, with the remaining
cells acting as polishing ponds or settling
basins for  TSS removal, hi aerated lagoon
systems, the final cell is usually left
unaerated to allow for TSS removal.

Lagoons generally have few O&M
requirements because of their simple design,
and usually require daily site inspection,
routine site maintenance, and periodic
sample collection and testing. Lagoons
should be checked at least once a year for
sludge accumulation.  Sludge may need to be
removed every 5 to 10 years, depending on
site conditions, and disposed of in an
approved manner.

Anaerobic Lagoons

An anaerobic lagoon is an extended, open-
air anaerobic digester. It is the simplest and
least expensive type of lagoon to construct
and operate, and has a detention time of 15
to 30 days. Anaerobic lagoons are normally
used for wastes with high BOD such as
manure, but they can also be used as an
alternative to septic systems. These lagoons
receive raw wastewater; solids with specific
gravities greater than the wastewater settle
to the bottom of the lagoon, where they are
stored and "digested," or biodegrade
anaerobically. The liquid portion of the
wastewater then flows out of the lagoon for
further treatment or disposal. An anaerobic
lagoon develops a thick floating crust that
absorbs odors produced by anaerobic
decomposition; odors must be expected
during cleaning. Because of their higher
                Typical Three-Cell Lagoon System
                                                      Ootl«t Sim dura
                                                                 Multiple Submerged
                                                                 OutM GalM

loading and greater depth (10 to 30 feet),
anaerobic lagoons use less land area than do
other types of lagoons.

Facultative Lagoons

Facultative lagoons, also known as
stabilization or oxidation ponds, operate
with very long detention times, ranging
from 30 to  180 days. A facultative lagoon
has two zones: the top layer of liquid is
aerobic (relying on either mechanical or
natural aeration), while the bottom layer,
which contains the settled matter, is

Facultative lagoons contain algae in the
surface layer, which assists with the
degradation of the wastewater, as well as
aeration of the liquid layer. Thus the main
difference between an anaerobic lagoon and
a facultative lagoon is the top surface,
which remains aerobic by a combination of
algal respiration and mechanical mixing.

During periods of adequate sunlight, algae
produce oxygen and consume carbon
dioxide in water to a depth at which
decreasing solar energy inhibits
photosynthesis; algal growth contributes to
the degradation of organic matter both
directly, by incorporating nutrients in their
growth, and indirectly by providing oxygen
for aerobic bacteria to metabolize organic
matter. Wind action or mechanical aeration
also increase the oxygen transfer rate from
the air to the liquid.

Facultative lagoons without mechanical
aeration use more land area than those with
aerators, but mechanical aerators are not
usually necessary for diluted wastes such as
domestic wastewater. Lagoon depths are
limited to 2 to 5 feet because algae are
needed to produce oxygen. The effluent
from a facultative lagoon can infiltrate into
the soil through seepage trenches and the
lagoon bottom.

Maintenance requirements are simple,
consisting mainly of dike maintenance and
occasional water level adjustment, and,
when appropriate, maintenance of the
mechanical aerators. A buffer zone of
approximately 200 feet should be
maintained around the lagoon. Algae can be
harvested and mixed with animal  feed or
used as a soil conditioner.

Aerobic Lagoons

Aerobic lagoons are very shallow water
ponds (maximum depth of two feet) that
allow oxygen production by photosynthesis
and by diffusion from the atmosphere.
Detention times should range from five to
20 days. Aerobic lagoons  are limited to
warm climates, where sufficient sunlight
permits algae production year-round. Again,
treated effluent from the lagoon infiltrates
into the soil, but the high algae loading of
the effluent can clog infiltration systems.

Aerated Lagoons

Aerated lagoons are deep, lined ponds with
mechanical aeration that can treat a much
higher organic load than other lagoon types.
Because of the mechanical aerators, aerated
lagoons can be as deep as structurally
feasible (normally 6 to 10 feet), and use less
land than facultative lagoons. Detention
times should range from five to 20 days.
The aerators provide a more uniform and
dependable source of energy than solar and
wind energy, allowing a greater flexibility
for variable waste loads and climatic
changes; potential odor problems are also
minimized. The disadvantages of aerated
lagoons, however, are the energy cost and
the potential for wastewater foaming caused
by the mechanical aerators. •

  Mechanical Treatment Systems
          Operator               Effluent
 Capital      Skill       Capacity   Disposal      Land
  Cost    Required      Limits    Required? Requirements
 $10 per
50,000 gpd
Approx. $5
 <1 mgd

(10,000 to
50,000 gpd
Sequencing  Approx. $7
Batch       per gpd
                       <1 mgd

                      (10,000 to
                     30,000 gpd

    O&M Requirements
Regular inspection,
monitoring, and
maintenance of mechanical
and electrical equipment;
dewatering and disposal of
sludge; disinfectant control
Excellent performance and
reliability; low rate of sludge
production; possible
nitrogen removal
Relatively high
requirements;  high
operator skill required;
need to protect aerators
from freezing
Require 2 to 4 hours per      Can be quickly installed with  High maintenance
day, plus regular
preventative maintenance
on pumps, blowers, and
minimum site preparation;
can achieve high-quality
effluent; low sludge
production rate
requirements and
operator skill; unsuitable
for high flow variations;
high power consumption;
noise and odor concerns;
upset in cold weather
Need daily checking and
regular preventative
maintenance on mechanical
and electrical equipment;
sludge stabilization and
Batch treatment process
suitable for wide flow
variations; relatively low
maintenance and  energy
requirements; relatively
simple and reliable, low
sludge production, and
high-quality effluent
More expensive than
package treatment
plants, but will treat
wastewaters with
variable flow rates;
sludge handling facilities
must be considered in
overall plant design

Mechanical Treatment Systems
Oxidation Ditch
       : oxidation ditch is a mechanical
      treatment facility that is a closed loop
      variation of the extended aeration and
activated sludge process. The oxidation
ditch is applicable in any situation where
activated sludge treatment is desired and
flows are greater than 50,000 gallons per
day (gpd). These plants are capable of
achieving consistently high levels of BOD
and TSS removal, even in extremely cold
climates. High levels of nitrification are
possible, with up to 80 percent removal of

The components of an oxidation ditch
system typically include screening, grit
removal, oxidation ditch, secondary
clarification, and sludge handling; primary
clarification is generally not necessary. The
oxidation ditch itself is normally oval in
shape, made of concrete, 4 to 6 feet deep,
with 45-degree sloping walls, and sized to
provide a hydraulic retention time of 18 to
24 hours. At least two mechanical surface
aerators are used for mixing, aeration, and
circulation of the activated sludge; a normal
velocity of 1 to 2 feet per second (ft/s)
keeps waste materials suspended. The
design life of an oxidation ditch plant can
be 20 years or more, but mechanical equip-
ment is likely to have a service life of 5 to
15 years, depending on its type and quality.

Oxidation ditches require more land, have a
higher capital cost, and are generally used
for higher flows than package treatment
plants. Successful performance requires
skilled operators, regular inspections and
monitoring, and regular maintenance. The
oxidation ditch provides excellent
performance, high reliability, and a low rate
of sludge production. •
                    Oxidation  Ditch Flow Diagram

                                               .-^RATION ROTOR
          SCREENED RAW

                                                        Mechanical Treatment Systems
Package  Treatment Plants
Trackage plants are prefabricated and
 I—'pre-engineered treatment plants that
.A.  have been widely used to provide
activated sludge treatment to flows
generally between 10,000 and 50,000
gallons per day (gpd), although package
plant capacities up to one million gpd are
available. These plants are designed to
operate with a relatively constant influent
flow rate, and should not be used for
situations with large daily flow variations.
Package plants can either be used on their
own or to treat effluent from a septic tank,
which provides flow equalization and
sludge reduction. Package plants are  largely
preassembled to allow for rapid installation
with minimal site preparation, and they are
often assembled in modular form. These
plants generally discharge to surface waters
(after disinfection), although they can be
used for treatment prior to land application
or subsurface disposal.

The components of a package plant include
an aeration tank followed by a clarifier,
from which excess sludge is removed. A
portion of the settled sludge is returned to
the aeration tank to maintain a sufficient
population of bacteria to treat the
wastewater; some of the sludge must be
periodically removed from the system. The
aeration tanks are usually sized to provide
an average hydraulic detention time of 18 to
36 hours. A relatively high level of operator
skill is required to operate package plants,
which have a high power  consumption and
require regular preventive maintenance. •
Package Treatment Plant Activated Sludge Process
ATOM | 	
«* 	 ^

— _ 	 4 	 1
' ^ 1 	 1




Mechanical Treatment Systems
Sequencing Batch Reactor (SBR)
f I Vie SBR is an activated sludge batch
  I  process that is capable of producing a
 JL high-quality effluent, and is relatively
simple and reliable. Because wastewater is
treated in a batch process (rather than a
continuous flow process), SBR is ideally
suited to situations with wide variations in
flow rates. Other positive features include
easy installation and lower maintenance and
energy consumption than most mechanical
treatment processes. SBRs can be operated
to achieve BOD reduction, nitrogen
reduction, and phosphorus removal;
removal of these pollutants can be
accomplished with or without chemical

The SBR process typically consists of
screening, grit removal, SBR cycling,
disinfection, and discharge; primary
clarification is not usually included. In the
SBR process, aeration, sedimentation, and
decant functions are combined in a single
reactor. Most SBRs consist of two or more
tanks operated in parallel. The process treats
wastewater in a batch process with a five-
           Schematic Diagram for the SBR Process
                                                         SBR TANK PROCESS

                  WSTE SLUDGE TO
                 "SLUDGE HANDLING

                                                           Mechanical Treatment  Systems
stage cycle: fill, react, settle, draw, and idle.
Process control is provided by a
programmable logic controller, which is
typically provided by the manufacturer. The
disinfection system must either
accommodate the high periodic flow during
decanting or be preceded by flow

Residuals generated from an SBR facility
consist of screenings, grit, and secondary
waste sludge. Grit and screenings must be
removed and  treated or disposed of
promptly because of their putrescible
nature. Sludge production rates are
relatively low because of the lack of primary
clarification and the long solids retention
time. Sludge may be wasted to a sludge
holding tank or stabilization process such as
an aerobic digester and removed from the
plant as a liquid, or may be dewatered by a
sand bed or other means and hauled away as
a solid.

O&M requirements for SBRs are minimal
compared to other conventional activated
sludge treatment systems, although SBR
plants need to be checked daily by a skilled
operator. Sludge stabilization and
dewatering will likely constitute a
substantial portion of the O&M costs, and
regular preventive maintenance is required
on mechanical equipment.  Sludge handling
must be considered during project planning
and design, and sludge drying beds or other
sludge handling facility must be included in
the overall plant design. •

  Natural  (Land-Based) Treatment Sys-
  Alternative    Cost
          Operator Skill  Capacity  Disposal     Land
           Required     Limits   Required?  Requirements
  Constructed   Largely
  Wetlands     Land
$1 per gpd
 plus land
  Slow Rate    Largely
  Land        Land
  Application  Dependent

Weekly inspection;
periodic harvesting
of vegetation
Provides reliable, low-cost
BOD and TSS removal;
can provide wetland
habitat benefits
Require large land areas; lack of
generally agreed-upon design
factors; can take 2 years to
establish vegetation; plants subject
to shock loadings of toxicants;
removal rates for pollutants other
than BOD and TSS unknown;
cannot be used to treat raw
Basin cycling;
infiltration surface
management; pump
and dike
Good removal of
conventional pollutants;
simple to operate; lower
land requirement; year-
round operation; water
Limited by site and soil
characteristics; potential nitrogen
contamination of ground water
Crop management
and maintenance of
taifwater return
system (pump and
Nutrient removal and
recovery; cash crop
production; ground water
Limited by climate and nutrient
requirements of the vegetation;
storage is required during wet or
freezing weather; dissolved salts
may decrease soil permeability
Harvesting cover
crop; O&M for
distribution and
collection systems;
pest control;
periodic mowing
Relatively simple and
inexpensive to operate;
works well with impervious
soils; vegetative cover can
be harvested and sold
Limited primarily by climate, crop
water tolerances, and land slope;
not suitable for flat or steeply
sloping terrain; storage required
during wet or freezing weather;
disinfection required prior to

Natural (Land-Based) Treatment/Disposal  Systems
Constructed Wetlands
      Constructed wetlands are defined as
      those systems specifically designed
      for wastewater treatment and located
at a site where natural wetlands did not exist
at the time of construction. Most natural
wetlands are considered receiving waters
and are therefore subject to regulations
regarding discharge, while constructed
wetlands are considered to be part of the
wastewater treatment system. Constructed
wetlands can be designed and operated to
provide a range of treatment from effluent
polishing (a reduction in BOD and TSS), to
total retention systems that provide zero
discharge. The influent to a constructed
wetland could come from a number of
treatment systems, including septic tanks,
lagoons, or extended aeration systems;
constructed wetlands should not be used to
treat raw wastewater.

There are two basic types of constructed
wetlands, characterized by the flow paths of
water in the system. The first is called a free
water surface (FWS) wetland. It contains
aquatic vegetation in a relatively shallow
bed or channel, and its water surface is
exposed to the atmosphere. The FWS is the
most common type of constructed wetland
in the arid areas of California and Arizona.
The second type is called a subsurface flow
(SF) system. It contains a foot or more of
             Subsurface Flow Constructed Wetland
       Slotted Pipe for
  Inlet Stone
         1% slope-
                                         Soil or

                                          Natural (Land-Based) Treatment/Disposal Systems
permeable media such as rock, gravel, sand,
or soil that supports the root system of the
aquatic vegetation. The water level in the
bed is maintained below the top of the
media, which avoids problems with public
exposure, odor, and animals. SF-type
systems are most appropriate for small-scale
use. Both of these wetlands typically
include some type of barrier (compacted
clay or membrane liners) to prevent ground
water contamination beneath the bed.

About one-third of the systems in operation
use a mixture of plant species; the
remaining systems use bulrush, cattail, or
reeds alone. The primary removal
mechanism for nutrients in wetland systems
is the filtering and settling of inorganic and
organic particulate matter as the wastewater
passes through the plant community; settled
organic matter can then be degraded and
decomposed through a complex series of
plant-microbe reactions. Oxygen is
transported by aquatic plants to their rooting
zones, allowing aerobic organisms to live
on the plant stems in the submerged soil.
This produces a symbiotic relationship
whereby the products of microbial
degradation are used as a food source by the
plants, while the microorganisms use plant
metabolites for growth. This produces a
synergistic effect resulting in increased
degradation rates and removal of organic
chemicals from the wastewater.

Constructed wetlands require a relatively
low level of expertise to operate; only a
weekly inspection of the system is
necessary. Periodic harvesting of the
vegetation may be necessary, depending on
the design. The treatment process provides
excellent, low-cost removal of BOD and
TSS  from primary or secondary effluent.
The process is reliable, although subject to
the seasonality of vegetation growth. It also
can provide incidental wetland wildlife
habitat benefits. However, it can take up to
two years for newly planted vegetation to
reach maximum density and for the wetland
to achieve maximum efficiency. •

Natural (Land-Based)  Treatment/Disposal  Systems
Rapid  Infiltration
      apid infiltration (RI) is a soil-based
      treatment method that typically
         tisists of a series of earthen basins
with exposed soil surfaces, designed for a
repetitive cycle of flooding, infiltration/
percolation, and drying. The treatment
process depends on a relatively high rate of
infiltration into the soil, and percolation
through an unsaturated soil zone, prior to
recharging the water table. RI provides year-
round treatment for pretreated domestic
wastewaters.  Septic tank effluent generally
has a high TSS loading and can clog the
basins; therefore, RI is recommended as a
disposal method, to be combined with
another treatment process.

The hydraulic loading rate is determined by
soil characteristics, ground water mounding
potential, treatment requirements, applied
effluent quality, and climate. Typically, sites
             Rapid  Infiltration
with highly permeable soils such as sand to
sandy loam are selected. A minimum of five
to eight feet of unsaturated soil is required
beneath the infiltrative surface. Primary
treatment is the minimum level of
pretreatment required, but secondary
treatment is required if any public access
occurs in the vicinity of the wastewater
application. If nitrogen contributions to
ground water are of concern, the nitrogen
level in the effluent may control the
hydraulic loading. Underdrains or extraction
wells may be used to lower the water table
(thereby increasing the unsaturated depth of
soil) or to reclaim the water. A biological
mat may form on the surface of the
infiltration areas; it should be broken up
periodically by disking or harrowing.

RI provides favorable removal of
conventional pollutants (including
        ammonia), is simple to operate, can
        be operated year-round, requires
        less land than other land-
        application systems, and provides
        ground water recharge rather than a
        surface water discharge. The
        treated wastewater can be reused
        for landscape irrigation through the
        use of extraction wells located a
        sufficient distance down-gradient.
        However, the use of RI is limited
        by the suitability of the site to
        accept and treat the applied
        wastewater, and by the potential
        effects on ground water from
        nitrates. •

                                         Natural (Land-Based) Treatment/Disposal Systems
Slow Rate  Land Application
   A   land application system can be
  /\  considered both a treatment method
J-  \-and a disposal method for treated
effluent. Slow rate land application is a soil-
based wastewater treatment process in
which unchlorinated primary or secondary
effluent is applied, intermittently and at a
controlled rate, to a vegetated soil surface.
Wastewater effluent is usually applied using
sprinklers or through the flooding of
furrows to a soil surface of moderate-to-low
permeability. The applied wastewater
percolates through the soil to the water
table, and a tailwater return system contains
and recycles runoff from the site from
precipitation or over-application. The return
system typically includes a collection pond,
pump, and return pipeline. A storage
reservoir is required for periods of wet or
freezing weather, crop harvesting, or
With land application
systems, the highest
treatment levels are
attained by low
application rates on
natural vegetated soil
surfaces. Wastewater
constituents are removed
by filtration, adsorption,
ion exchange,
precipitation, and plant
uptake. Part of the water
is lost to evaporation and
plant uptake, while the
remainder percolates
through to the water
          table. Vegetation is an important component
          that serves to extract nutrients, control
          erosion, and maintain soil permeability.
          Either cultivated or forested sites can be
          used. The use of this type of system is
          limited primarily by land suitability, land
          costs, and climate. Generally, loamy soils
          (loamy sands to clay loams) are best suited
          for slow rate land application.

          O&M requirements are nominal and
          typically include crop management and
          maintenance of the tailwater return system.
          Benefits of land application include nutrient
          recovery, cash crop production, ground
          water recharge, and water conservation.
          These systems are highly desirable in areas
          where surface water discharge requirements
          are strict and land is relatively inexpensive.
Slow-Rate  Land  Application

Natural (Land-Based) Treatment/Disposal Systems
Overland Flow
        airland flow is a land application
        ethod of wastewater treatment that
        eludes a point discharge to a
surface water. While overland flow can
provide relatively simple and reliable
secondary or advanced secondary treatment
for BOD and TSS, pathogen removal rates
are unknown and disinfection is required
prior to discharge.

For this method, primary-treated wastewater
is applied at the top of a gently sloping hill
and allowed to flow over the ground surface
to the bottom of the hill, where it is
collected, disinfected, and discharged to the
receiving water. The most common method
of pretreatment is a lagoon system with a
one- to two-day retention time to remove
grit and other solids. A series of uniformly
sloped, vegetated terraces are constructed;
the size and number of terraces depends on
the wastewater flow rate. A wastewater
           Overland Flow Application
distribution system is located at the top of
the terrace, and a runoff collection channel
is located at the bottom. Wastewater is
applied intermittently across the top of the
terraces, and it is allowed to flow over the
vegetated surface to the runoff collection
channel. The system is not designed for soil
percolation, but some incidental percolation
may occur. Wastewater storage facilities are
required during wet or freezing weather,
when wastewater application is halted.

Topography is the most important site-
selection criterion because of its effect on
earthwork costs. Terrace slopes should be
between two and eight percent, and fairly
uniform; they should be long enough to
provide adequate treatment, typically 100 to
200 feet. Soil type and permeability are of
limited importance, because overland flow
was developed for use on soils with low
permeability; for permeable soils, the design
                   should meet the same
                   criteria used for slow-
                   rate land application if
                   ground water
                   protection is to be
                   provided. Vegetation
                   planted along the
                   slopes is typically
                   perennial, water-
                   tolerant grasses such
                   as canary, fescue, rye,
                   orchard, bermuda, or
                   bahia grasses. •
                                       Front Slop*

activated sludge

a continuous flow secondary wastewater treatment process
characterized by the suspension of microorganisms
(maintained by mixing induced by an aeration system);
results in the digestion of organic matter by the bacterial

the process by which a soluble substance is taken up and
becomes attached to the surface of a solid

in wastewater treatment, aeration is the process of supplying
the required quantity of oxygen to (and adequate mixing of)
the wastewater to allow for aerobic decomposition of
organic matter; aeration can be accomplished by natural
(reaeration) or mechanical means

a wastewater treatment process by which microorganisms
consume organic waste, in the presence of oxygen, to
produce cell growth and end products of carbon dioxide and

a biological process where microorganisms decompose
organic and inorganic compounds to methane, carbon
dioxide, cellular materials, and other organic compounds in
the absence of oxygen; anaerobic conditions are often
associated with the production  of objectionable odors

a biological mat that develops in disposal field systems that
removes bacteria and suspended solids by mechanical
filtration, consumes biodegradable materials, and promotes
nitrification/denitrification under aerobic conditions



ion exchange






Biochemical Oxygen Demand; the amount of oxygen
consumed by microbes in a given volume of wastewater in
the biochemical oxidization of organic matter (the higher the
BOD of a wastewater, the greater the concentration of
biodegradable organic material present)

Dissolved Oxygen; in wastewater treatment, adequate DO
must be present to maintain aerobic conditions; this is
generally accomplished through aeration

a type of wastewater treatment lagoon that contains a two-
zone sludge layer with aerobic conditions on the top layer
and anaerobic conditions on the bottom

free water surface; a type of constructed wetland where the
water surface is exposed to the atmosphere

gallons per day; a measure of fluid flow rate

a measure of the specific energy contained in a fluid, with
the dimension of length; in wastewater treatment, head
generally refers to elevation above a certain point

a process whereby certain ions in a waste stream are
substituted by other ions

a soil absorption system that receives clarified sewage from
a septic tank and discharges it underground to the soil

the amount of pollutants (e.g., BOD or TSS) contained in a
unit volume of wastewater

together with denitrification, this is a biological process
resulting in the removal of organic, inorganic, or gaseous
nitrogen compounds from a wastewater

National Pollutant Discharge Elimination System; a
permitting process for point-source discharges

operation and maintenance

refers to post-secondary treatment of effluent by routing the
flow through a constructed wetland to further reduce or
balance fluctuations in BOD, TSS,  and nutrient

primary treatment
secondary treatment
septic system

septic tank



a wastewater treatment process involving physical treatment
(floatation and gravity settling) that is generally the first step
in the treatment process; it provides approximately 50 to 70
percent removal of BOD and 25 to 40 percent removal of
TSS (also known as primary clarification)

often the initial step in the treatment process, screening is
used to remove debris and other large objects, as well as
suspended solids; "screenings" are the materials removed by
this process

a wastewater treatment process utilizing biological activity
in the treatment of wastewater, generally the second step in
the treatment process (after primary treatment) that results in
the removal of approximately 90 to 95 percent of influent

an on-site wastewater treatment method, typically consisting
of a septic tank and leachfield

a subsurface tank that receives raw wastewater and stores it
temporarily, allowing the separation of settlable and
floatable solids, prior to discharge of the liquid portion;
solids need to be removed from the tank periodically

Subsurface Flow; a type of constructed wetland where the
effluent to be treated is not exposed to the atmosphere, but
rather maintained below grade

waste by-product generated during the wastewater treatment
process, normally consisting of inorganic grit and putrescible
organic matter

Total Suspended Solids; a measure of the pollutant loading
of a wastewater, important because solids can degrade water


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                                                 «U.S. GOVERNMENT PRINTING OFFICE : 1996-785-829