United States
Environmental Protection •'
Agency
September
1987
9-7/0?
v>EPA Natural
Cold Climates
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Natural Systems for Wastewi
The Concepts
The term natural systems refers to wastewater
treatment systems which have minimal dependence on
mechanical elements to support the wastewater
treatment process.ln contrast, activated sludge utilizes
a natural biological sludge but depends on mechanical
elements for mixing, aeration, and sludge pressing.
Natural systems such as facultative ponds, rapid
infiltration, or wetlands treatment, depend directly on
natural components for treatment with minimal
mechanical assistance or energy inputs.
These natural systems are especially well suited for
small communities in rural areas where land may be
available and where it is important to keep construction
and O&M (operating and maintenance) costs as low as
possible! A potential concern with these systems,
however, is their susceptibility to operating problems
and icing during the winter. In natural systems, dormant
vegetation, the slow reaction rate for soil or aquatic
microbes at low temperatures, and/or the presence of
an ice, cover, may reduce both physical and biological
activity, and thus affect system performance on a
seasonal basis. Therefore, it is important to include
consideration of winter conditions when evaluating
these systems for use in a cold climate.
Pond Systems
Facultative ponds are used for wastewater treatment
throughout the United States. They depend on surface
reaeration and algal photosynthesis for oxygen, natural
die-off of pathogens, and on the contained biota for
treatment. A number of design models are available 1,
most of which include rate constant adjustments for low
temperature conditions. However, the effect of long-
term ice cover on the pond or lagoon is not always
given adequate consideration. The presence of an ice
cover for long periods eliminates significant surface
reaeration and, since algae are also dormant, the
oxygen levels in the liquid can decrease to zero. The
effluent quality during such periods can deteriorate to
primary treatment levels.In some cases, supplemental
winter aeration may resolve the problem.
A "controlled discharge" approach is used in Canada
and the North-Central United States to provide
sufficient detention time to eliminate the need to
discharge during the critical winter months. The
discharge is programmed for once or twice per year.
Each cell is isolated in turn and prepared for discharge.
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Overland Flow (OF) Land Treatment
In overland flow, partially treated wastewater is applied
to relatively impermeable soils at the top of a gentle,,..:
(2-8%) grass cov,ered slope. Treatment, by vegetation
and surface microbial bjota occurs as the wastewater.
moves via sheet flow down the slope. The treated
effluent is then collected at the toe^of the slope and
typically discharged to surface water. The hydraulic '
loading of an OF system can be 2 to 4 times higher
than an SR system, depending on treatment goals and
influent characteristics. Design details can be found in
References 2 and 3. ]
The vegetation is a critical treatment component in OF
systems; since perennial grasses are commonly used,
the operating season can usually be longer than the
typical SR system. The length of the OF operating
season can also be, influenced by the level of nitrogen .
removal required. If nitrogen is not a parameter of
concern, then the operating season may extend well '
beyond the growing season for the project area. A.
detailed map showing suggested wastewater storage
needs for OF can be found in' Reference 3. Figure 1 "j
presents a simplified approximation of storage i
requirements. Note the storage requirements north of
the 40 day line on Figure 1 are essentially the same for
both OF and SR systems.
40 65
Figure 1. Winter Wastewater Storage for OF Systems,
Days.
Constructed Wetland Treatment Systems
Constructed wetlands are a relatively new concept for
wastewater treatment 4>5. Yet a variety of systems
involving natural and constructed wetlands have been
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investigated and sucessfully utilized as a part of
wastewater treatment systems. Constructed wetlands
are typically installed for wastewater treatment at a site
where a wetland did not previously exist. There are
several advantages to this approach since greater
control of flow, therefore improved treatment efficiency^ :
' involving a smaller area is possible'with a "constructed :
- .toetland, rather than with a naturally; occurring w6trand> - '
Additionally, natural wetlands are'oftehJcbnsfclered'to ; ^
' be part "of the receiving waters rather than a part of the,;
treatfrierit system. Asa result, regulations.may restrict;','
; the allowable inputs.to a natural;wetland. r\ - s\'-(. •'
Pgrtiially treated wastewaterJslntraducedX^;^^^
end ,o'f the shallow constructed wetland and treatment;;/,
*• 'occurs during the several days of residence in the ;<;,.
"system. Extensive work in Ontario, Canada has w,' •:;:£•:
"' -develope'd'engiheering criteria for year-round operatior).;j
"of consKructed wetlands in cold" climate's 4f P,. "\ /.-,..; •£
A system designed for BOD, SS, and significant
nitrogen removal would typically consist of an 8 to 10
day detention time in a partial-mix aerated lagoon with
a settling zone, followed by a wetland area constructed
as long, narrow channels (e.g., L:W = 10:1). The
design detention time in the summer is typically 7 days
with a 10 cm water depth, at a hydraulic loading of
about 200 m3/ha/yr. The water depth is increased to
about 30 cm at the onset of winter to allow for an
adequate treatment zone beneath the winter ice cover.
Most of the nitrogen in the effluent will be in the
ammonia form and additional pre-or-post treatment
may be required if the discharge permit requires low
ammonia levels. Similarly, if phosphorus removal is
required, pre-or-post treatment may be required.
Emergent vegetation (e.g., cattails, reeds, and rushes)
is planted in constructed wetlands to form a dense
cover. The plants themselves provide little treatment
but the stalks and root nodules provide a substrate for
the extensive microbial growth which is thought to be
responsible for most of the treatment activity. Since the
plants are not the major factor in treatment, year-round
operation of the system without plant harvest is
possible. Performance can be sustained at consistently
high levels as shown in Table 3, which presents results
from a system in Canada.4'5
The performance, and many of the treatment
responses in constructed wetlands are similar to those
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Parameter mg/L
BOD5 SS Total N
.9 '
ibli 3.;;Quality of Wpstewater following Wetland '••-'"' : ^ -?/
•-.."• ^Treatment '"»', • •"'*• ••'•' 'y**-- • T • ;;%:;*
!* 'V^ -'" ^Vj-^ '. ''• ' vV " - ' V '".«:• ••"•'•.'",' '
I' ff - „ "" ' •* "V • "- ' " ** '^ '
bccurfing'lri OF systems. The wateV depth is greater in
Ihe wetland and'this,, plus the insulating Ice cover;:
Mlqws operation under condition's that .would; cause
•q.mpletejurface''freezing1 of an OF slope. ', ,v "'*" -
, yniformflow under the wetlandice cover is essential
lo.avpid development of anaerobic conditions and the
|esulting deterioration in performance.
tie major factor influencing cost effectiveness of a
anstructed wetland versus OF is the amount of land
lequired for each of the concepts. If extensive winter
•storage is required for an OF system then the wetland
3 likely to be the more economical system. The 65 day
Ine on Figure 1 is the approximate dividing line, with
lespect to land requirements, for the two concepts.
•Dverland flow systems will usually require less land
irea south of that line, while constructed wetlands may
pe more cost effective to the north. It should also be
emembered that OF systems have a proven record of
eliable performance while constructed wetlands are
•~t emerging from the developmental stage.
Conclusions
he natural wastewater treatment systems described in
his text can be successfully used in cold climates. The
hajor winter concerns are dormant vegetation, low
baction rates, freezing of equipment, and ice formation
|n the surface of the treatment system.
j non-forested natural systems where vegetation is a
ritical treatment component, it may be necessary to
lore wastewater during the winter months. Forested
low rate and rapid infiltration systems and constructed
•etlands, however, can all operate through the winter
lith proper care of the wastewater distribution network.
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Forested systems can also be operated successfully
through the winter if freezing is prevented in the .
mechanical equipment (pumps, pipes, sprinklers, etc).
The ground surface in forests freezes more slowly than
exposed agricultural soils and once a deep snow fall
occurs, the soil might not freeze at all, permitting
wastewater to infiltrate all winter long. As shown in
Table 2, performance can be excellent on a year-round
basis in a forested SR system.
Time BOD5 SS Total P Total N Fecal Coli
mg/L mg/L mg/L mg/L #/100 ml
Winter 3 3 0.3 7 A 60
(Nov-Mar)
Summer 11 0.2 2.5 57
(April-Oct)
Note: percolate samples obtained in cut-off ditch at
toe of treatment slope.
Table 2. Winter and Summer Performance of a
Forested SR System in Central Vermont.
Maintenance of the wastewater distribution network for
continuous winter operations of an SR system can be
labor intensive. In some situations, where low cost land
is available, it may be more cost effective to provide for
winter wastewater storage. This, however, will also
require a larger treatment area for summer application
of the total flow.
Rapid Infiltration (Rl) Land Treatment
Rapid infiltration involves the application of partially
treated wastewaters to shallow basins in highly
permeable soils. The hydraulic loading rates are usually
at least an order of magnitude greater than SR
systems, but it is still possible to achieve ,a very high
quality percolate. Design criteria can be found in
References 2 and 3. Since vegetation is not used as a
treatment component there are no seasonal limitations
and an Rl system can operate on a year-round basis.
Freezing must be prevented in the piping system1 but
ice formation in the basins is not usually a problem
when the applied wastewater is relatively warm. Icing
may become a problem if very cold effluent from a long
detention time lagoon is applied to an Rl basin in a
very cold climate. Reference 3 provides a detailed
discussion on this topic.
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Treatment in Cold Climates
When water quality in the cell and in the receiving
stream are compatible, the entire cell is discharged over
a period of several days to several weeks. Excellent
performance is possible. Table 1 presents results from
a controlled discharge pond in Michigan.
Discharge
Effluent Quality, mg/L
Period
Date
May
Oct.
Nov.
Days
••• 5-
12
8
BOD5
7.7
1.2
0.6
SS
18.4
3.5
1.6
NH3-N
2.1
0.6
3.8
.NO3-N :
1.0
0.3
0.5 '
Table 1. Performance, Controlled Discharge Ponds.
Pond systems combined with various aquatic plants
(water hyacinths, duck weed, etc.) in aquaculture
systems are capable of high levels of treatment during
the growing season. Use of these concepts in colder
climates would probably require covers and additional
heat for sustained winter performance. It is not likely
that these extra protective elements will be cost
effective. -|.
Slow Rate (SR) Land Treatment j
In SR systems, partially treated wastewater is applied
to a vegetated soil surface, followed by infiltration and
percolation through the soil.Sometimes it is followed by
gravity drain recovery and surface discharge of the
treated effluent. The vegetation, soil, and microbial i
biota all contribute significantly to treatment. If properly'
designed and managed it is possible to achieve i
drinking water quality in the treated wastewater after a
few feet of travel in the soil. Complete design details
can be found in Reference 2. ' ,
The operational period for a particular system will
depend on the type of vegetation selected for the
system. The use of agricultural crops (grasses, hay, ;
corn and small grains) typically requires wastewater
storage during the winter months. Storage time might
range from 20 days in North Carolina and northern .
Arizona, to 160 days in northern Wisconsin and Maine.
Reference 2 provides complete details.
Grass-covered systems can typically be operated for a
longer period than row crop systems. If nitrogen is not
a critical parameter, grass covered systems might be
operated into the winter as long as general ice build-up
on the ground surface does not interfere with infiltratiori.
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References
1. US EPA. 1983. Process Design Manual for
Municipal. Wastewater Stabilization Ponds. EPA
625/1-83-015. US EPA CERI, Cincinnati, OH.
2. US,EPA^i98t Process Design Manual,Land
Treatment of Municipal Wastewater. EPA : ' -!
, / 625/1:81-0|3: US EPA/QERI,..Cincinnati,' OH. >1 '--^
S.'.US EPA."1984, Process Design /Wanua/ for Land
-, -Treatment of Municipal Wastewater Supplemejit on
• 'f?ap/c( Infiltration -and Overland Flow, EPA 625/1 -811
- ?Oi3arU%EPACEra,Cincinnati/OH.'" " . V'''".
4. Reed, S.C., R. Bastion, S! Black, R. khettry. 1985.'
..' Wetlands .for Wastewater Treatment in Cold
-', Cljmates. pp 962-972: in: Proceedings AWWA Water
.; .Reuse HI.AWWA, Denver,. CO. , ' .-,;-;
'5. Herskowitz; "J.1986. Town of Listowel Artificial
''Mars/7 Project,Project Report 128RR. Ontario ' ' "
Ministry of the Environment, Toronto, Ontario.
Text prepared by Sherwood C. Reed, USACQE-CRREL,
Hanover, NH. under EPA IAG No. DW 969361.
Prepared by Environmental Resources Management, Inc.
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