Biofilter lets nature take its course at Woodward Avenue WWTP
By Dan Chauvin, C.E.T., Jim Joyce, P.E., Joe Uglevich, P.E.
and Frank Burford, P.Eng.
Figure 1-1
Woodward Avenue WWTP: raw wastewater pumping station and headworks
wastewater and airflow schematic.
The Woodward Avenue Wastewater
Treatment Plant (WWTP)
is owned by the City of
Hamilton, and treats wastewater
from the Ontario communities of
Hamilton, Stoney Creek, Glanbrook
and Ancaster, with a combined population
of approximately 380,000.
Hamilton has quite an extensive and
complex wastewater system including
2300 km of sanitary sewers and 600
km of combined sewers, which can
convey up to four times the average
day flow to the Woodward WWTP during
wet weather conditions. Treatment
is by a conventional activated sludge
treatment system with an average day
design flow rate of 409 ML/d and peak
design of 614 ML/d.
The plant, operated and maintained
under contract by American Water
Services Canada Corp., has undergone
a number of upgrades that include the
installation of a fine bubble aeration
system, construction of a waste activated
sludge (WAS) system, construction
of additional primary clarifiers
and a retrofit of the existing secondary
clarifiers.
Most recently, the City retained
AWS to undertake upgrades to the
WWTP headworks facility in an effort
to address operational and hydraulic
restrictions. Upon reviewing commitments
the City had made to address the
Hamilton Harbour Remedial Action
Plan (HHRAP), specifically the reduction
of combined sewer overflows
(CSO), it was determined that the final
design capacity of the headworks facility
would need to match the 1350
ML/d capacity of the main pumphouse
to treat wet weather flows historically
bypassed from the WWTP.
The Headworks Replacement
Project incorporated new equipment
for screening and grit removal, provided
redundancy in case of equipment
breakdown and matched the rated flow
capacity for the main pumphouse.
An important design requirement
for the new facility was the implementation
of an odour control system, with
the added challenge of undertaking
this significant construction activity
with minimal impact on treatment
processes.
Originally, it was envisioned that a
single odour control system would be
used to treat the air from the headworks
facility. However, field investigations
and odour sampling indicated
that the main pumphouse and the associated
influent conveyance channels,
located upstream of the existing headworks
facility, contributed a significant
volume of odorous air to the headworks
facility at the Woodward WWTP.
The main pumphouse and headworks
facilities are hydraulically and
pneumatically connected because there
is a common air space between them
through the enclosed conveyence
channels. Therefore, as wastewater is
conveyed downstream, so are the
odours and air from the pumphouse
wet well. The odours are then released
into the headworks facility (Figure 1-
1). Without intervention, this phenomenon
was expected also to occur following
the commissioning of the new
headworks facility. Any odour control employed at the new headworks building
would have limited overall impact
as the more significant odour source
was found to be upstream.
One solution was to block the air
from the pumphouse wet well from
entering the influent conveyence channel.
However, this would have resulted
in moving the odour problem
upstream. So, an odour control system
would have been required at the main
pumphouse to treat air emissions from
the wet well plus a second at the headworks
facility to control odours generated
in the influent channel and the
headworks.
An evaluation of the existing natural
airflow and ventilation dynamics
within the influent pumping station
and wastewater conveyance channels
was conducted in order to understand
the magnitude of the situation.
It was determined that it was possible
to treat the air from the main
pumphouse, wastewater conveyance
channels and the new headworks facility
influent channels in a centralized
odour control system by extracting air
from the influent conveyence channels
into a biofilter. Additionally, the operating
level (building) air from the
headworks facility would be treated in
a second odour control system (carbon
scrubber). This approach would
accomplish the following objectives:
Treat multiple existing odour
sources (raw wastewater pumphouse,
connecting conduits and headworks
influent channels) in a single new
odour control system constructed as
part of the headworks replacement
project;
Minimize odour transmission from
the wastewater channels to the operating
(building) levels;
Separate the airstreams in the headworks
facility to provide the most cost
effective and efficient treatment; and,
Minimize air requiring treatment
from the headworks building proper,
by containing odours in the channels.
This allows for a lower air exchange
rate to be utilized at the operating
(building) level, thereby lowering the
costs (operating and capital) and size
of dedicated odour control and the
heated supply/makeup air system.
Accordingly, the airstreams were
segregated so that the high concentration,
high humidity airstream would be
treated by a 5,000 cfm inground biofilter.
The air from the operating (building)
level, residuals containers, and
covered screw conveyors in the headworks
facility would be treated in a
15,300 cfm single bed carbon adsorber.
Because the air at the operating
level is less odorous and better conditioned
(by the makeup/supply air system), carbon fouling potential and
maintenance frequency were significantly
reduced.
The biofilter was centrally located
between the two influent conveyence
channels, occupying an area of the
plant site which had limited use for any
other type of process.
Biofilters, sometimes called Soil
Bed Filters or Compost Filters, are an
innovative alternative to physical/
chemical foul air treatment that has
become increasingly popular in recent
years. When odorous air passes
through a biofilter, gases are adsorbed
onto media particles and absorbed into
the moist, biologically active water/-
media boundary layer surrounding
media particles. Once this occurs, biological
degradation of the absorbed
odour compounds occurs and only
clean, odour-free air is released from
the biofilter. Essentially, the biofilter
allows nature to do the “dirty work”.
Biofilters act as efficient biological
reactors, where a thriving microbial
population exists. In order to support
microorganisms effectively, an optimal
choice of media and moisture content
is critical. The odorous air provides
the food source for the bacteria in the
media. As such, the foul air is provided
continuously to assure biological
growth and maintenance. Interruptions
in the air/food source would cause
stress and possible death of the
microorganisms. Because the WWTP
operates continuously, there will never
be a lack of “food” for the biofilter.
The Woodward WWTP biofilter
installation includes a pipe network of
perforated pipe through which the
odorous air is distributed (Figure 2-1).
The perforated pipe is below the media
and surrounded by granular material.
The granular material helps to distribute
the odorous air throughout the area
of the biofilter. Above the piping and
granular material are the filter media.
A membrane is typically used to separate
the media and the underlying
granular material to prevent the migration
of finer media into the granular
material.
Extensive research has been performed
to determine an optimum
biofilter media. Several media types
have been successfully utilized, including:
compost, peat, soil, wood chips,
and various mixtures of these materials.
Compost “overs”, a product generated
from screening compost and
generally discarded, are an excellent
source of biofilter media. These
“overs” were utilized in the construction
of the biofilter at the Woodward Avenue WWTP. Generally, the local
climate conditions, local availability,
and economy dictate the media mixture.
The biofilter was provided with an
underdrain system to remove rainwater
and excess irrigation water. Many
installations include a mist or spray
system to humidify the inlet air, as well
as an external or internal irrigation
system to improve media moisture
content, promote biological activity,
prevent media drying and cracking,
and flush the decomposed odour residuals
from the media. External irrigation
consists of pop-up type spray irrigation
systems, permanent arcsprayers,
or irrigation hoses. Internal
irrigation typically consists of soaker
hoses embedded within the biofilter
media. The biofilter at the WWTP
utilizes treated plant effluent water for
irrigation, providing both moisture and
nutrients for the microrganisms.
The biofilter is virtually maintenance
free. During dry summer
months, the irrigation system must be
activated to maintain the moisture
level in the media. Over the years, the
media break down and must be periodically
removed and replaced with new
material. The spent media are non
hazardous and can be used as shrub
mulch or re-composted.
The odour control systems at the
WWTP Headworks Replacement were
commissioned in the spring of 2003.
Microrganisms are now busy treating
the foul odours from the front end of
the plant and doing it for free.
Dan Chauvin is the Manager, Water
Quality with the City of Hamilton. Jim
Joyce is the President of Odor and
Corrosion Technology Consultants
(OCTC) and the principal author of
the Manual of Practice No. 22, Odor
Control in Wastewater Treatment
Plants, jointly published by WEF and
ASCE. Joe Uglevich is a Project
Manager with OCTC, responsible for
the odour control aspects of the
Woodward Avenue WWTP Headworks
Project. Frank Burford is a Senior
Project Manager with AWS Engineers
& Planners Corp.
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