United States       September
Environmental Protection    1986
Agency
Operation of
Conventional
Wastewater
Treatment
Facilities in
Cold Weather

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Background
Cold weather can have significant and often severe
effects on the operation and performance of a
wastewater treatment facility. Because the system’s
components and the physical, chemical and biological
treatment responses are temperature dependent,
performance efficiency can suffer under low
temperature conditions However, cold weather
conditions can also offer beneficial opportunities for
heat recovery and sludge dewatering
A national survey reported that cold weather problems
were a major cause of poor performance and often
resulted in higher operation and maintenance costs. In
some cases, it appears that cold weather conditions
were not fully considered in the onginal facility design.
As a result, treatment plant operators have developed
solutions to the problems caused by cold weather.
Their solutions should be helpful to other operators and
also to design engineers so that these problems might
be avoided in future systems.
The Problems and Solutions
The survey identified the cold weather problems most
commonly encountered among conventional treatment
facilities and their solutions These solutions can be
implemented by either improved design, equipment or
by simple operation and maintenance procedures.
The majority of cold weather problems can be grouped
into the following categories
• Ice formation in vanous process components.
• Viscosity changes in the wastewater and in lubricants
for mechanical equipment.
• Reaction rate changes in physical, chemical and
biological reactions.
Ice formation in process components is the most
troublesome, since the entire plant’s operation will be
disrupted if one unit has to be shut down even
temporarily. A survey of facilities in northern New
England indicated significant problems with preliminary
treatment, clariflers (both primary and secondary), and
biological reactors. Tables 1, 2, and 3 list these
problems and the solutions developed by the operators

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Problem Solution
ice in headworks and Cover inlet channel; flush rack
on bar racks and grit with water, clean more
screen frequently, duct kerosene heater
into unit
Screened rags freeze Remove very frequently
Gnt conveyor freezes Enclose the unit.
Collected grit freezes Store containers in heated
shelter before emptying
Screw pumps freeze Install timer to “bump” screw
once per hour; run warm water
on unit.
Valves and hoses Drain lines, keep flow on (only if
freeze plant water system)
Automatic samplers Place inside heated shelter, build
freeze temporary winter shelter with light
bulb for heat; insulate suction
lines, purge lines after sampling,
suction lines should be vertical
for quick drainage
Flow measurement Use heat tape and insulation on
devices freeze transmitter, heat (with light bulb)
and insulate chamber, use heat
tape on Parshall flume linkage;
add antifreeze to float box.
Bypass channel at gnt Switch flow to bypass for 30
removal freezes mm/day
Ice formation in the Build temporary structure.
comminutor chamber
Access doors freeze Put heat tape around door
due to condensation penmeter
Table 1. Problems with Preliminary Treatment in Cold
Weather
Grit handling seems to be one of the most difficult
preliminary treatment activities in cold weather.
Temporary wind screens erected dunng cold months
will reduce heat losses and freezing problems, but in
extreme climates future designs should enclose the
entire grit removal operation including conveyors and
dumpster storage areas

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Operation of Conventional Wa
Problem Solution
Surface icing Cover the tank, shorten detention
times; remove skimmer arms;
operate 24 hrs Iday; suspend
rock filled plastic bucket in tank
to reduce ice stresses on walls
Icing in idle units Pump units routinely, remove ice
before restart
Icing in gear units Install heat tapes; drain water
after rain and after thaw periods.
Scum trough freezing Cover unit, hand remove ice;
install automatic flushing unit, use
hot water, stop scum removal in
winter, decrease exposed plate
area if adjustable
Ice on controls for Build enclosure.
traveling bridge
Condensation icing on Install hot air gun
electncal bus bars
Hydrants and hoses Drain, leave flow on (only if plant
freeze water system)
Snow accumulation Shut down during storm.
stops monorake wheels
Samplers freeze Build box with light bulb for heat.
Waste activated sludge Place lines deeper in ground;
lines freeze insure drainage between uses.
Scum line freezes Flush with hot water; use sewer
pig to clean.
Table 2. Problems with Clanfiers in Cold Weather
The most serious problem with both pnmary and
secondary clarifiers is ice build-up and scum freezing
on the beach plate. This can damage the skimmer
mechanism The response in many locations has
been to totally remove the scum skimmer arm in the
cold weather months.

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stewater Treatment Facilities in
Problem Solution
Ice build-up on surface Steam ice off , bump unit on and
aerator off, turn off for 1/2 hour to allow
liquid to warm unit then run at
high speed
Ice build-up on support Use hose and hot water, shorten
towers or aerator detention time or run unit on
shroud timers, remove shroud to avoid
impact damage from ice thrown
up by aerator
Cooling of liquid affects Use timers on aerators, use
performance diffused air instead of surface
units, remove some aerator
blades; increase MCRT;
decrease F/M ratio
Icing in idle units Fill tank and circulate water with
small sump pump, remove ice
prior to restart to avoid structural
damage to rakes, etc. on restart
Icing on oxidation ditch House brush, use heat lamps;
brushes use floating log or screen to keep
ice from brush.
Complete ice cover on Remove ice; cover tank
tanks
Icing on tnckling filter Stop flow from outer nozzles,
walls cover whole unit
Poor performance for Cover unit, close some air vents
trickling filter
Table 3 Problems with Biological Units in Cold
Weather
Figure 1 illustrates a system developed at Hampton,
NH which flushes the solids out of the scum box after
each pass of the skimmer arm The flush bar is tripped
by the skimmer arm and allows tank effluent to enter
the box and wash out the remaining floating solids The
amount of flush water is adjusted with counterweights
on the bar. The device works well, but freezing still
occurs dunng prolonged penods of extremely cold
weather.

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Cold Weather
Some Opportunities
Most manufacturers recommend seasonal changes of
oil in gear boxes and similar equipment A number of
systems have changed successfully to the use of the
synthetic, multiviscosity year-round lubricants These
lubricants are analyzed twice a year for metals and
water content to determine excessive wear or the
presence of moisture Under normal conditions the
lubricant may last three years or more Since it can
take 20 to 25 gallons of oil for each change, the
savings for materials and labor can be significant
The wastewater entering the system represents a heat
source which can be beneficially utilized in the cold
months The possibilities range from simple temporary
covers over tanks for passive heat conservation to
active efforts u ng heat exchangers for direct heat
recovery Heat exchangers in the system at Fort
Greely, Alaska extract enough heat from the
wastewater to heat the main building and the
chlorination station Heat exchangers are also used at
Wilton, Maine and Delafield, Wisconsin to extract heat
from the wastewater
Freezing as a Benefit
Freezing temperatures can be beneficially utilized for
sludge dewatering Freezing and then thawing a sludge
will convert a material with a jelly-like consistency to a
granular material which drains immediately A sludge at
2 to 8 percent solids placed on a sand bed and
allowed to freeze can attain 25 percent solids as soon
as it is thawed
Figure 1 Self-cleaning scum trough at Hampton, New
Hampshire wastewater treatment plant

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The key to successful performance is to apply the
sludge to the bed in layers about 8 cm deep and to
allow each layer to freeze before the next is applied. It
is essential for the sludge to freeze completely for the
dewatering benefits to be realized. A very deep layer
might never freeze to the bottom in locations where
alternating freezing and thawing conditions occur.
Design details can be found in references 2 and 3. A
preliminary esbmate of the total depth of sludge which
could be frozen in sequential 8 cm layers can be
determined with this equation:
= 1.76 (Fr) — 101
Where: T = total depth (cm) of sludge that could
be frozen during cold weather if
applied in 8 cm layers.
F maximum depth (cm) of frost
penetration into the soil at the location
of concern (From local experience or
records).
Figure 2 illustrates this relationship for the United
States.
Figure 2. Potential total depth (cm) of sludge that could
be frozen if applied in 8 cm. layers.

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Conclusions
Cold weather can seriously affect the operation and
performance of wastewater treatment facilities. Many of
these problems can be eliminated or reduced to
tolerable levels by carefully trained operators, and can
be avoided in future systems by appropriate design
modifications. Therefore, the design of a system should
consider what the site conditions will be like during cold
weather months when high winds, snow and ice are
prevalent.
References
1. Reed, 5, et al, Prevention of Freezing and Other
Cold Weather Problems at Wastewater Treatment
Facilities, SR 85-11, USA CRREL, Hanover, NH,
1985
2 Reed, 5, J Bouzoun, W Medding, A Rational
Method for Sludge Qewatering Via Freezing,
presented at Oct. 1985 WPCF conference, USA
CRREL, Hanover, NH, 1985
3 US EPA Process Design Manual for Dewatenng
Municipal Wastewater Sludges, EPA 625/1-82-01 4a
(a new edition, scheduled for publication in October
1987, now in preparation) US EPA CERI, Cincinnati,
OH
4 Operations Forum, Water Pollution Control
Federation, Volume 2, Number 12, December 1985
For more information contact:
EPA-OMPC (wH-595) EPA-wERL (489)
401 M Street, SW 26 West St Ciair Street
Washington, DC 20460 Cincinnati, OH 45268
(202) 382-7368 /7369 (513) 569-7931
Text prepared by Sherwood C Reed, USACOE-CRREL,
Hanover, NH, under EPMAG No DW 969361
Prepared by Environmentai Resources Management, inc

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