CONTROLLING NUISANCE GAS EMISSIONS FROM WWTPS
OF CONCERN: odor compounds
may also need to worry about toxic volatile organic compound (VOC) emission (such
as chlorinated solvents; for example, trichloroethene TCE)
Note that there are numerical standards for allowable concentrations of hazardous air
pollutants, while there are no such limits for odor compounds. In many cases, analytic
methods to detect odor-causing compounds may not be as sensitive as the human nose to low
concentrations. In addition, odor compounds are generally not hazardous to human health, but
are simply a nuisance. Therefore, odor control requirements are largely a function of public
acceptability. If a wastewater treatment plant is located far away from most residents, odor
gases at the site are not a problem. However, if neighbors living nearby complain of odors,
then mitigation methods are needed. Therefore, in the long run it may be more cost effective
for a wastewater utility to purchase surrounding land to serve as a buffer zone rather than let
residential areas crowd the property boundary.
Specific chemicals which have objectionable odors:
Inorganics such as hydrogen sulfide (found in the highest concentrations at WWTPs)
and ammonia are the most problematic
Organic compounds containing sulfur or nitrogen are also a problem: amines,
mercaptans, indole, skatole, organic acids, organic sulfides
A summary of some of the major odor causing chemicals is provided in the table below. The
Henry’s constant is a measurement of the relative affinity of the compound for the vapor
phase versus liquid (at equilibrium, the concentration in air divided by the concentration in
water). The concentrations listed are in parts per million (by volume) where half the
population can detect the specific smell. Note how LOW these concentrations are for many of
the compounds!
Compound
Chemical
Henrys Characteristic Odor Minimum concentration
formula
const @
that 50% of an odor
25°C,
panel could smell, ppm
L/L
Ammonia
NH3
0.013
sharp, pungent
0.037
Amyl Mercaptan
unpleasant, putrid
0.0003
crotyl mercaptan
skunk-like
0.000029
Dimethylamine
C2H7N
0.006
putrid, fishy
0.047
Dimethyl sulfide
(CH3)2S
0.074
decaying vegetables
0.001
Hydrogen sulfide
H2S
0.385
rotten eggs
0.00047
Indole
C8H7N
0.0057
fecal, nauseating
ND
Methyl mercaptan
CH3S
0.132
decaying cabbage
0.0011
Skatole
C9H9N
0.0547
fecal, nauseating
0.0012
Thiocresol
C7H7OS
skunk, rancid
0.0001
Sources of odors:
Raw wastewater that goes “anaerobic” in the sewer lines (consumes all available oxygen)
contains anaerobic bacteria that will transform organic sulfur compounds into hydrogen
sulfide (H2S). This H2S will tend to be dissolved in the water until the sewage liquid is
exposed to turbulence in the inlet junction box or lift pumps and then the H2S will
volatilize into the air. Typical sewer off-gas contains 0.1-15 ppm H2S, and 0.01-0.05 ppm
organic odor compounds.
Anaerobic biological treatment processes may also result in the formation of H2S
Scum floating on the surface of primary settling tanks
Biosolids drying or dissolved air flotation thickening
Aerobic thermophilic sludge digestion, largely ammonia
Options:
1. place covers on unit processes such as primary & secondary clarifiers, trickling filters,...
these can be structural fabric, aluminum, etc.
Cover costs generally range from $12-$80 per ft2 surface area depending on type
fixed roof cover for primary clarifiers $12/ft2 in Orange County, CA
fixed cover costs generally 12-26/ft2
rigid floating covers $42-$56/ft2
flexible floating cover $3-$7/ft2
air supported structure $54-65/m3 to install; operating costs about 250000 kWH/yr
POTW process
Aer grit chamber
Mech grit chamber
Primary Clarifier
Activated sludge diff air
Activated sludge mech mix
Activated sludge pure oxygen
trickling filters
secondary clarifiers
tertiary filters
Cl contact chamber
aerobic pond
removal efficiency
operational
problems
fixed
cover
Y
Y
Y
Y
rigid
floating
N
Y
Y
N
flexible
floating
N
N
N
N
Air supp
structure
Y
POSS
Y
Y
Floating
spheres
N
N
N
N
Y
N
N
Y
N
Y
N
N
Y
NN
Y
Y
Y
Y
Y
POSS
N
POSS
N
N
N
N
N
POSS
N
Y
Y
Y
Y
Y
N
N
POSS
POSS
N
FIXED COVER
80-95%
corrosion of
concrete walls,
RIGID FLOATING
85-95%
difficulty of
maintenance,
AIR SUPP STR
41-95%
corrosion of equip
inside cover, inc liq
experience at
POTWs
difficulty w/
maintenance;
degradation fr
weathering & offgas
used at several
POTWs
complexity
not complex
cross media impacts
cost
degradation of seam
& cover due to
VOCs in WW
proc concrete walls;
degradation of cover
fr ww off-gas
used for digester
tank covers; used in
other ind. for yrs on
O/W separators
relatively simple
used in industry, but
not yet at POTWs
req more knowledge
than other covers
due to inflation sys
possibility of change possibility of change negligible
in sludge
in sludge
composition
composition
$13-26 / ft2
$42-100 / sq ft
$54-$65 /m3; high
energy costs
2. “deodorizing misters”
By misting the surface of open tanks, the volatilized odor compounds may dissolve in
the mist of air and return to the liquid. The misters may also contain “odor masking”
compounds. Many companies have worked to develop chemical mixtures that are specifically
designed to neutralize and/or mask odors from wastewater treatment plants.
3. pre-oxidation of odor compounds
One way to minimize odors through the wastewater plant is to add chemicals that will react
with the odor causing compounds. For example, chlorine can be added at the head of the
plant to pre-oxidize -> but this may result in THMs & other volatile organic chlorinated
compounds that are a health hazard risk volatilizing out later in the plant (such as chloroform).
Alternatively, the addition of FeCl3 (a common coagulant that may be present in sludge from
drinking water treatment plants) may also serve to reduce odor emissions through the plant.
Treatment of odor gases:
1. dry activated carbon adsorption
non-impregnated carbon
caustic-impregnated carbon (~5% by weight KOH, NaOH, Na2CO3)
non-impregnated carbon with ammonia addition
- general bed depths 1-6 ft, 250 cfm air/ft2
convey contaminated gases to the bed: due to corrosive nature use plastic or fiberglass
pipes
regenerate carbon on-site or send for off-site thermal regeneration
humidity in gases reduces sorption capacity
cost estimates (73 GAC suppliers in US, about 6 sell complete systems)
10,000 scfm flow and 100 ppmv inlet conc, capital cost $475K, O&M
$250K/yr
250-cfm flow capital $20K and O&M $148K
removal efficiency
operational problems
experience at POTWs
complexity
cross media impacts
carbon adsorption
can be high for low concs if low
velocity flow
humidity >80% results in lost
sorption capacity;
for odor control; industry for 50yrs
low, unless monitor breakthru &
regenerate on-site
high disposal or regeneration cost
2. wet scrubbers with oxidants (hydrogen peroxide, chlorine, ozone, potassium
permanganate)
may use spray chambers (usu. 50-75’ tall, 200 cfm air/ft2)
or packed towers (17’ packing, 30’ total height, ~300 cfm air/ft2)
high chemical cost, difficult maintenance & frequent operation attention needed,
safety hazards of chemicals
cost about $0.70 / 10,000 cfm
3. thermal
flares - low-cost, use for gases containing combustible components such as methane,
hydrogen, CO; need enough heating value in the gas for self-sustaining burn otherwise need to
add supplemental fuel (as would be the case in most WWTP off-gases)
catalytic oxidation - waste gas contacts catalyst bed to allow rapid reactions at 700-900°F.
catalyst usually noble metal such as platinum or oxides of Cu, Cr, Mn, Ni, and Co. S
compounds deactivate certain types of catalyst materials; life of catalyst 2-5 yrs; not common
in WWTPs; capital cost for 250 cfm system of $55K and O&M of $20K/yr
thermal incinerators (aka thermal oxidizers) - use supplemental fuel (usually natural gas) to
burn contaminated gas, temps 800-1500°F, use some form of heat recovery, large space
required (300 x 300 ft of land), need scrubber to clean gas of NOx, SOx, and acid gases
4. biological treatment methods: biofilters, biotrickling filters, activated sludge tanks
Biofilters: packed bed of media on which the biomass grows
humidify inlet air or sprinkle the top of the bed to maintain moisture
packing media is soil, peat, compost, wood chips (provide nutrients); generally replace
media every 6 mo to 3 yrs
need to maintain pH, since biodegradation of H2S & ammonia produces acidic conditions
optimum temperature 30-37°C, range 10-42°C; biofilters generate heat from microbial
activity which can allow odor control even with below freezing air temperatures
commonly 3-5 ft deep, 3-10 cfm air/ft2, 60 sec gas residence time, 40-80% porosity
low energy costs compared to other methods; only enough power to overcome 2-3” head
loss
cost: 10,000 cfm H2S 20 ppm in & <1 ppm out; $97.3K capital, $7.9K/yr O&M (1990)
cost: $0.10 / 10,000 cfm ; capital for 250-cfm system $34K, $7200 /yr O&M
cost: capital $17-$69 / cfm for a 25K-cfm system, $10-$40 / cfm for a 75K-cfm system
Biotrickling filters:
plastic media support, upflow air, recirculate some liquid with nutrients
1.14 cfm air/ft2, 24’ deep
Activated sludge treatment
use the odorous air from the headworks building as the inlet air fed into activated sludge
tanks (such as in the Los Angeles Hyperion treatment plant)
used for over 30-yrs at WWTPs; used at over 25 plants in U.S.
used in Japan where there is an “Offensive Odor Control Law”
should use non-ferrous piping and diffusers (probably coarse bubble) to prevent corrosion
& fouling
perhaps should use stainless steel dry filter fittings & flowmeters; have observed corrosion
problems with aluminum & steel guide vanes on the blower
no indication that odor air into activated sludge tanks increased problems with filamentous
bacteria
general aeration rate of 0.25 to 0.5 m2/1000 m3/d;
aeration rate 3.5K cfm for a 5-MGD conventional plant; could treat air from the influent
channels, aerated grit chamber, primary effluent launders, gravity thickener, bet
presses, and sludge holding tanks
no increase in odor over the activated sludge tanks when using odorous gases
longest experience at the Hyperion WWTP in Los Angeles, which installed odor treatment
from headworks, primary clarifiers, DAF thickeners, and effluent pump station in their
activated sludge basins in 1959. Since then, they have cleaned the blower twice,
reported no corrosion of fine bubble diffusers, and estimate odor removal at 96-99%
lab studies found removal of odor compounds to below detection limits of 0.1 ppm in an
activated sludge reactor of 2 to 4.2 ft depth; nitrification occurred in the reactor
if need activated sludge tanks for ww tmt, no additional capital costs other than selection
of proper materials to resist potential corrosion
References:
Ando, S. 1980. Odor Control of Wastewater Treatment Plants. Journal WPCF. 52(5): 90613.
Bowker & Associates. 1996. Biological Odor Control by Diffusion into Activated Sludge
Basins. NEWEA Journal. 30(2): 137-146.
Environmental Technology. 1988. 8(5): 18-19. Morris & Lecky. Controlling Wastewater
Treatment Plant Odors in a Resort Community.
Environmental Protection. Feb. 1998.
Romain, M. 1996. “Biotreatment of Odor-Containing Gases from Municipal Wastewater
Treatment Plants.” Masters Thesis, University of Washington.
Bishop, Witherspoon, Card, Chang, Corsi. 1990. VOC Vapor Phase Control Technology.
WPCF Research Foundation.
Torres, Devinny, et al. Biofiltration: Controlling Air Emissions through Innovative
Technology. 1997. WERF.
vanLith, C., G. Leson, and R. Michelsen. 1997. Evaluating Design Options for Biofilters. J.
Air&Waste Mgmt. 47: 37-48.
Williams, T. & F. Miller. 1992. Odor Control Using Biofilters. Biocycle. 33 (Oct/Nov):72-7,
75-9.