Sporadic odors in a collection system are almost impossible to completely treat and are usually from an isolated event.  Although odors may seem random at first, with monitoring, trends can be found and quite often related back to the diurnal flow patterns.  Diurnal flows, which are basically the trend between collection system flows and human activity, can change throughout the day. 
 
Adapting your approach or modeling your odor treatment to account for diurnal flow is not as difficult as it may sound. You may already have access to many of the resources required, which will allow for a greater understanding of the odor issues and how to proceed with treatment options.  These options should be optimized to match the diurnal flow pattern, which will ultimately reduce treatment cost and increase treatment levels.  Understanding this influence and correlating it with system retention times may also explain phantom odors.

Odor and Corrosion Generation 

Odor generation has a lot of contributing factors or building blocks.  Having a stable base of organic material and BOD is usually enough to cause a large odor issue to develop.  Hydrogen sulfide is the most common source of odor issues, with a pungent rotten egg smell that is noticeable in concentrations as low as 0.1 ppm or 100 ppb.  Hydrogen sulfide is thus the design driving factor for monitoring and quantification of treatment at most treatment plants or collection system emission points.

Figure 1 lists additional predominant substances that cause odors from a wastewater perspective.  These substances are the main compounds emitted from sewers but, by far, aren’t the only ones.  If a collection system has a large industrial influence, the list of odor compounds can grow exponentially, based on industrial effluent as well as on the mixing or breakdown of that effluent in the collection system. 

Figure 1 : Common Wastewater Odor Compounds


As no two sewers are exactly alike, they won’t have exactly the same odor generation or emission rate.  Most sewers have a similar basis for odor generation and thus, modeling and estimating can be done. 

In order to understand how to control odor, and specifically hydrogen sulfide, it’s necessary to look at odor’s production at the biological level in the wastewater system.  Simply stated, the occurrence of sulfides in wastewater results from water-borne facultative bacteria’s need to find an oxygen source to maintain their existence.  Facultative bacteria prefer free or “dissolved” oxygen, which is very scarce in wastewater streams and, as a result, is consumed rapidly.  Sulfate, another sulfur compound found in most wastewater systems, contains oxygen molecules.  The facultative bacteria attach to the sulfate compound and strip away the oxygen, thereby creating sulfides.  Through various biological processes, the sulfides become the source material for the formation of hydrogen sulfide and sulfuric acid compounds.  These compounds, respectively, cause the severe odor and corrosion problems in a typical wastewater collection system as seen in Figure 2. 

As noted above, the presence of hydrogen sulfide can also lead to corrosion concerns.  When hydrogen sulfide is present in a moisture-laden atmosphere, sulfuric acid is created.  These droplets will oxidize almost everything they come in contact with.  As the hydrogen sulfide levels increase, so does the generation of sulfuric acid which increases corrosion or oxidation of everything in that area.  In some instances, gaining corrosion control is the key objective in wastewater treatment for hydrogen sulfide – even more so than odor control.

In any system, there are three significant contributors of hydrogen sulfide to the control point.
1)  Existing dissolved hydrogen sulfide in the contributing flow;
2)  Production of hydrogen sulfide by bacteria in the bulk liquid; and
3)  Production of hydrogen sulfide from the slime layer or sediment within the collection system. 

A control point is a location on the sewer where odors are being emitted.  An individual sewer can have multiple control points.  As the sewage moves along the sewer, it emits odors at all atmospheric release points. 

When considering odor or hydrogen sulfide generation, it’s important to consider what sends the dissolved sulfide into the air.  There’s always the natural instability of dissolved sulfide in water as well as the saturation levels that the liquid media can hold.  The liquid media’s pH is also a contributing factor to the stability of the dissolved sulfide.  As seen in Figure 3, the dissolved sulfide is driven to the hydrogen sulfide compound under increasingly acidic conditions.  This aspect should be taken into consideration, especially if there’s an industrial input on the collection system that discharges varying pH material. 

In addition, turbulence is a physical characteristic of sewers that can drive dissolved sulfide into the atmosphere.  Any sewage drops, turns, or thrashing will release hydrogen sulfide if it has been generated in the liquid media.  Changing small flow characteristics in the collection system can sometimes reduce the release of hydrogen sulfide.  Any changes to reduce turbulence would help keep the dissolved sulfide in suspension.  Simple changes can help, such as reducing the wastewater flow speed by increasing pump run times but decreasing the pump rate; implementing a VFD pump arrangement versus an on/off pump cycle arrangement; or removing obstructions or large elevation changes at a control point.

Odor Testing and Monitoring

Once control points have been identified, atmospheric and liquid testing should be done to quantify the raw levels.  For liquid testing, special sample bottles must be used to preserve the dissolved sulfide.  In atmospheric testing, ‘bag sampling’ is commonly used for complete spectrum analysis which will also quantify the hydrogen sulfide levels.  This type of analysis is useful, especially early on, as it provides accurate quantification of all compounds and can eliminate or raise awareness of treatable compounds.  Testing will also aid in gauging successful treatment, if required later on.  There are numerous methods for testing, depending on the level of detail required.  Most third-party labs will be able to handle both liquid and gas samples for analysis.  Treatment suppliers and consultants may also be helpful sources when engaging in testing and monitoring, as they may have the resources readily available to do all of the initial testing. 

When testing is employed, trending is very important.  Quite often an odor monitoring plan is put in place early on so that records can be kept.  Just as with wastewater sampling, grab samples only give a snapshot of what is going on at a certain time.  The odor monitoring plan would then become part of the total odor management plan.  These plans can be designed for an individual issue or control point or can be applied to many problems in the region or town. 

There are a few options on the market for ongoing monitoring, specifically for the hydrogen sulfide parameter.  Based on the testing that’s done, daily generation rates can be estimated by taking the mg/L of dissolved sulfide and multiplying it by the flow and a conversion factor. 

Million US gals per day x 8.34 lbs per gal x ppm of  dissolved sulfide = lbs of sulfide produced per day
(Thousand meters3 per day x ppm of dissolved sulfide = kgs of sulfide produced per day)

For example, if a small system has 264,000 US gals (1,000 m3) per day of flow and at the control point test shows 5 ppm dissolved sulfide, then 11 lbs (5 kgs) of sulfide would be produced daily.

Most treatment options take into account the daily sulfide production rate for initial estimates on treatment cost and general approach to the issue.

Diurnal Flows

While there are many influences on odor generation, one key factor that many collection systems face is diurnal flow patterns.  This characteristic relates to the change in flow seen over the course of the day as it relates to human activity.  Flows are above average in early morning and early evening when most people are at home.  If a collection system has a large industrial influence, the diurnal flow characteristic may not be as evident but rarely can it be dismissed.   

Figure 4 lists the basic parameters of a small collection system.  Looking at the data, nothing seems irregular and a 2.5-hour retention time is not high enough to set off odor generation “alarm bells.”  



Despite the average retention time being 2.5 hours, when this data is input into a standard flow curve that breaks down the flow on an hourly basis, the maximum and minimum retention times are quite different (Figure 5).  Concern may be raised over a 4 - 5 hour retention time, especially if there’s an existing loading on the wastewater coming into the pumping station. 

If the wastewater chemistry remains the same in the collection system but the flow patterns change, odors may not be generated evenly over the course of a day.  Retention time changes drastically affect odor generation.  The longer the wastewater is in a captive system like a sewer, the more time the biomass has to break down food sources and generate byproducts that then lead to gases such as hydrogen sulfide.  A lot of factors play a role in odor generation, but the diurnal flow pattern is going to compound a negligible or non-existent issue into something that causes problems at certain times of the day. 

Understanding the effect diurnal flows may have on odor generation is key, as is applying it to actual systems and their individual retention times.  In Figure 6, an untreated municipal sewer system’s odor generation is being monitored in the form of hydrogen sulfide.  Looking at the timescale on the bottom, this initially appears to be completely the opposite of how diurnal flow affects odor generation.  Are the highest hydrogen sulfide results seen when you would expect to see the highest flows or the lowest retention times?

This collection system has roughly a 6-hour retention time, so the monitoring results taken from the downstream control point at the end of the collection system can have up to a 4 – 6 hour delay from when the actual generation occurred.  In Figure 7, the timescale has been altered to show when the wastewater that causes the high levels actually enters the collection system.  This view shows that the odor generation originates during the early morning low flows and the mid-day flows.  Wastewater that enters during these times has longer retention times and thus causes higher odor levels at the control point downstream.

Treatment Considerations

A lot of treatment options can be applied when an odor control issue is either present or expected to occur.  Treatment options can be split into two main areas: liquid-phase treatment options that treat the wastewater and vapor-phase options that treat the atmospheric conditions generated.  The main liquid-phase options are oxidizers, iron salts, or nitrate addition.  The main vapor-phase options are carbon adsorption, biofiltration/bioscrubbers, or wet/chemical scrubbers.  Just as there are many factors that contribute to odor generation, there are many factors to consider when choosing the treatment technology.  A few points that should be considered:

• Do you require treatment at one specific location or at multiple release points on a sewer?
• At the control point, what are the acceptable levels / targets for control?
• For odor control, the typical target is a daily average of 1 ppm H2S or lower.
• For corrosion control, the typical target is a daily average of 5 ppm H2S or lower. 
• What space is available at the control point or an upstream dosing location?
• Does the system generate odors at differing levels throughout the day? (i.e. diurnal flow generation)
• Can the treatment option be tailored or optimized for uneven odor generation?
• Does odor generation decrease in the cooler months due to a decrease in sewage temperature, or does the generation increase due to a decrease in average daily flows and increased retention time?

Conclusion

These are just some of the points to initially consider on an odor issue.  When engaging a treatment option, technical help from the supplier or consultants should be readily available.  As the odor field has been a market with increasing attention and expansion, consulting firms now specialize in this field as well as some well established suppliers who have a variety of proven technologies that can be applied to varying situations. 

Remember that no two sewers are exactly alike, so treatment options must be considered on a case-by-case scenario.  Applying one proven technology to every odor issue will most likely not achieve the best treatment or the most cost-effective solution.  That is not to say that if the odor issue has similar characteristics to a situation that has an effective treatment option already in place, the situation can’t be approached in a similar manner.  Just ensure that testing and monitoring of the situation is done so that a complete understanding of the issue can be achieved prior to applying a technology.  Also if possible, try to break down your daily flow rates to find the trends for diurnal flow influence and account for its treatment characteristics. 

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