Managing Total Facility Odour Emissions – What is your “Odour Pie” Image?

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sesion02 mcginley00 A sewer District in the Southwest of U.S.A. operates a 30 MGD design average flow Activated Sludge and Tertiary Treatment facility (Facility). In 2007 an exploratory odour study was conducted to concentrate on three problem areas (pre-treatment, digesters, and sludge dewatering) to reduce odour emissions in a cost-effective manner.

Michael A. McGinley, P.E. and Charles M. McGinley, P.E.1

1. St. Croix Sensory, Inc., 1150 Stillwater Blvd. N., Stillwater, MN 55082 USA

 

Competing interests: The author has declared that no competing interests exist.

Academic editor: Carlos N Díaz.

Content quality: This paper has been peer reviewed by at least two reviewers. See scientific committee here

Citation: Michael A. McGinley, P.E. and Charles M. McGinley, P.E, 2014, Total Facility Odour Emissions – What is your “Odour Pie” Image?, Ist International Seminar of Odours in the Environment, Santiago, Chile, www.olores.org

Copyright: 2014 olores.org. Open Content Creative Commons license., It is allowed to download, reuse, reprint, modify, distribute, and/or copy articles in olores.org website, as long as the original authors and source are cited. No permission is required from the authors or the publishers.

Keyword: odour sampling, olfactometry, field olfactometer, odour pie, odour maps

 

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Abstract

   A sewer District in the Southwest of U.S.A. operates a 30 MGD design average flow Activated Sludge and Tertiary Treatment facility (Facility). In 2007 an exploratory odour study was conducted to concentrate on three problem areas (pre-treatment, digesters, and sludge dewatering) to reduce odour emissions in a cost-effective manner.

   A paradigm shift for odour studies:

    1. Traditional wastewater process studies focus only on the quality of the water discharges and the biosolids produced with air discharges often overlooked or underestimated. By incorporating the concept that each wastewater treatment facility has three effluents (water, solids, and air) a comprehensive facility analysis addresses odour root causes.
    2. Combining odours from all air emissions of a facility in an odour inventory, illustrated as an “odour pie” image, addresses the individual slices representing odour emissions from each process or system area with a proportional guide.
    3. Odour dispersion modeling predicts odour transport in to the surrounding community whilst it falls short of describing and measuring the actual odours experienced by citizens and reported by inspectors.

   In 2007, an odour emission inventory was created, an “odour pie” was reported, and recommendations were implemented. The District commissioners and stakeholders understood the clarity of the “odour pie” illustration and immediately grasped the overall concept of a slice-by-slice focus for overall improvement.

   Since the original 2007 odour study, major changes have occurred related to the facility operations including an increase in service population as well as process changes with added odour control. The two questions asked by the District commissioners and stakeholders are: (1) has our “odour pie” decreased and by how much? and, (2) which slice(s) of the “odour pie” do we address this year? Understandable graphics – the “odour pie” – yield clear focus and mission clarity.

   In 2013 the District revisited the Facility’s “odour pie”, updating the odour emission inventory and adding ambient odour surveys to understand how far Facility odours transport off-site into the surrounding community.

Introduction

The District’s wastewater treatment plant, a 30.0 MGD Design Average Flow Activated Sludge and Tertiary Treatment Facility, consists of:

    • Bar Screens
    • Grit Tanks
    • Primary Clarifiers
    • Aeration Tanks
    • Secondary Clarifiers
    • Tertiary Filters
    • Chlorine Disinfection Tanks
    • De-chlorination Chambers
    • Anaerobic Digestion
    • Belt Press Dewatering
    • Sludge Drying & Storage

   The District, in cooperation with municipalities connected to its sewer system, have been reducing total sewage flow to the treatment facility by removing infiltration and storm water flows with the adverse affect of increasing odorous gases in the collection system and at the Facility. Collection system odours were significantly reduced with chemical additions, i.e. nitrates and hydrogen peroxide.

   As a “good neighbor” the District continues to address odour generation potential. The 2007 odour study focused on known problem areas at the time:

    • Pre-Treatment (Influent & Primaries)
    • Digester Gases
    • Sludge Dewatering

This paper will present the District’s 2013 odour study results that include the updated comprehensive odour emissions inventory and the District’s “odour pie”. Further, the District commissioners asked that odour data be collected in the surrounding community to understand how far the Facility odours transport off-site.

Methodology

   Two methodologies were used in the odour study: source odour sampling and ambient odour surveys (inspections). Air emissions from six process areas were sampled using standardized air sampling methods (point sources, area sources, volume sources).

    1. Pre-Treatment
    2. Primaries
    3. Digesters
    4. Dewatering
    5. Secondary
    6. Sludge Storage

   Air samples were evaluated for determination of odour thresholds and other parameters. Odour evaluations, following CEN’s EN13725:2003 and ASTM E679-04, reported detection and recognition thresholds (CEN, 2003) (ASTM, 2011).

   Point source odour emission rates were calculated as the product of the odour concentration (ouE/m3) and the air emission rate (m 3/s). Area source odour emission rates were calculated as the product of the odour concentration (using an equilibrium/flux hood) and the source surface area (m2) and a “near calm wind speed” of 0.1 m/s. Table 1 presents the summary of odour emission rates from the six process areas as well as the percentage of the total estimated odour emission rate from the Facility (110,000 ouE per second).

Table 1. Odour Emission Rates in European odor units per second

CATEGORY

ouE /s

PERCENT

Influent Structures

47,000

40

Primaries

35,500

30

Digesters

1,200

1

Dewatering

23,500

20

Secondary

5,000

4

Sludge Storage Area

6,000

5

TOTAL

118,200

100

   The ambient odour inspections where planned within a 2.5-km radius of the Facility. Nineteen (19) odour inspection locations were selected in the survey area, as illustrated on a Google Map in Figure 1, Odour Inspection Locations (Odor Track’rTM Program). Location #100 is the Facility entrance.

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Figure 1. Odour Inspection Locations (Odor Track’rTM Program)

   The ambient odour inspections were conducted over a seven-month period, from April through October 2013. A field olfactometer, Nasal Ranger®, was used to measure the ambient odour using the Dilution-to-Threshold (D/T) method. Odour inspectors conducted 126 odour surveys to quantify and describe ambient odours as well as document weather conditions.

Discussion

   Figure 2, 2007 Odour Pie, presents the results of the odour emission inventory of 2007. In 2007 the total odour emissions were 187,000 ouE/s with the following process percentages:

    • Pre-Treatment (32%),
    • Primaries (19%),
    • Digesters (19%),
    • Dewatering (24%),
    • Secondary (2.5%), and

· Sludge Storage (3.5%).

   The 2007 odour study focused on Pre-Treatment (Influent & Primaries), Digester Gases, and Sludge Dewatering. The District work reduced the total odour emissions from 187,000 to 118,000 ouE/s, a 37% reducing in total odour emissions.

   Figure 3, 2013 Odour Pie, presents the results of the odour emission inventory of 2013. In 2013 the total odour emissions were 118,200 ouE /s.

   Note that the 2013 Odour Pie is 37% smaller than the 2007 Odour Pie.

   Currently the Pre-Treatment (40%) is the largest “odour pie” segment of the Facility’s overall odour emissions. Pre-Treatment includes the open influent channel, the open grit tanks, and the pump building with bar screens and wet wells. The pump building fans exhaust to the atmosphere without odour control.

   The Facility’s Primaries (30%) are the second largest odour pie segment of the Facility’s overall odour emissions. The Primaries include 6 circular open tanks with the associated open influent channels and open scum and grit channels.

   The third largest odour pie segment is the Dewatering process (20%) with belt presses and bio-solids load-out trucks inside the dewatering building. The belt presses are vented through a small odour control device, which exhausts less than 1% of the dewatering buildings total odour emissions. The dewatering building fans exhaust to the atmosphere without odour control.

   The remaining Facility odour emissions include:

    • Secondary processes, i.e. clarifiers and aeration tanks (4%);
    • Sludge Storage Area (5%); and
    • Digesters (<1%).

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Figure 2. 2007 Odour Pie (Total 187,000 ouE/s)

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Figure 3. 2013 Odour Pie (Total 118,000 ouE/s) (37% reduced from 2007)

   Figure 4, Odour Eclipse Map (Odor Track’rTM Program), presents the odour inspection results from the seven-month study period. The data is presented using eclipse symbols with RED centers eclipsing the YELLOW circles at each inspection location. The larger RED eclipse circles represent stronger odours. Referring to the Figure 3 Odour D/T (Dilution-to-Threshold) Criteria (Eclipse Key), a full YELLOW circle (no eclipse) represents “no odour”. The ¼ RED eclipse represents less than 2-D/T. The ½ RED eclipse represents 2-D/T up to 7-D/T, and the Full RED eclipse represents odour greater than 7-D/T. Note, that non-detect odour inspections are not part of the odour averaging. The highest odours measured center around the Facility (Location #100).

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Figure 4. Odour Eclipse Map (Odor Track’rTM Program)

   The ambient odour study covered a 6-month time period (Spring through Fall seasons) with 120 ambient odour surveys conducted. During that time period, 6 trained odour investigators made 2100 odour inspections. Over half the inspections (1200) encountered ambient odours to measure and describe.

   The top 5 odour descriptors observed and record by the odour investigators are listed in Table 2, Odour Descriptors.

Table 2. Odour Descriptors

Descriptors

# of Observations

Sewer

240

Septic

110

Musty

100

Grassy

110

Floral

105

   Sewer odour was the first or second top odour descriptor in the near vicinity of the facility. Figure 5, Odour Descriptor Map (Odor Track’rTM Program), presents the odour descriptor summary at the 7 survey locations near the Facility, plus the Facility entrance location (#100). Note the Sewer RED color, the Septic PURPLE, and the Musty ORANGE color, three odour descriptors associated with the Facility’s top three on-site odour sources (Influent Structures, Primaries, and Dewatering).

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Figure 5. Odour Descriptor Map (Odor Track’rTM Program)

 

Conclusions

   Because traditional wastewater process studies focus on the quality of the water discharges and the biosolids produced, air discharges are often overlooked or underestimated. By incorporating the concept that each wastewater treatment facility has three effluents (water, solids, and air) a comprehensive odour management analysis can be completed to achieve effective water treatment. The total odours from a facility can be illustrated as an “odour pie” image, with individual slices representing odour emissions from each process or system area.

   The Facility’s current “odour pie” image spotlights two odorous process areas, Pre-Treatment and Primaries, that account for the majority of the odours generated (70%). These two process areas are currently “uncontrolled”, except for hydrogen peroxide addition to the influent (i.e. preventative odour generation). Odour management strategies will now focus on the cost-effectiveness of the hydrogen peroxide addition, alternatives and/or additional chemical additions, i.e. iron salts, and Pump Building exhaust odour control.

   The effectiveness of odour management at Pre-Treatment will be evaluated for downstream odour reduction effectiveness at the Primaries. The Primaries will be studied in greater detail to identify the odour emissions from process components, i.e. open channels (turbulence), scum accumulation/removal, and weir drop distances (turbulence).

   The odour generated at the Facility and transported off-site to the surrounding community impacts citizens and the community’s quality of life. The 120 comprehensive ambient odour surveys, conducted over six spring-summer-fall months, identified Facility odours of sewer, septic, and musty reaching off-site. Of the total 240 sewer odour observations, 215 (90%) were reported in the immediate vicinity of the Facility. Likewise, of the total 110 septic odour observations, 105 (95%) were reported in the immediate vicinity of the Facility. Of the total 100 musty odour observations, 63 (63%) were reported in the immediate vicinity of the Facility. These three odour descriptors are closely linked to the three top odour source types identified in the Facility’s “odour pie” image, Pre-Treatment (sewer & septic), Primaries (sewer, septic), and Dewatering (musty). Note that odour dispersion modeling predicts odour transport based on meteorological data bases, whilst it falls short of describing and measuring the actual odours experienced by citizens and reported by inspectors.

   Two smaller odour pie segments, Sludge Storage and Digesters, require continued vigilant observations and considerations. The Sludge (bio-solids) Storage area becomes “actively odorous” during intermittent removal of the bio-solids. A special odour study is prescribed for activities in the Storage area. Further, the digesters are well known to be an infrequent but high level odour source when digester gas is released from pressure relief valves.

   The District now has an updated “odour pie” image and community odour eclipse and descriptor maps for Facility odour management strategic planning.

References

ASTM International. 2011. E679-04(11): Standard Practice for Determination of Odor and Taste Threshold by a Forced-Choice Ascending Concentration Series Method of Limits. Philadelphia, PA. USA.

Committee for European Normalization (CEN). 2003. EN13725: Air Quality – Determination of Odour Concentration by Dynamic Olfactometry. Brussels, Belgium.

Odor Track’rTM Program. 2013. www.OdorTrackr.com. St. Croix Sensory, Inc. Stillwater, MN. USA.

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