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Sulfur species are typically treated within a process and managed through reaction engineering. But what happens when small concentrations of sulfur species make their way into wastewater? For many substances, the result is the

creation of hydrogen sulfide (H2S), a dangerous, odorous substance. This article looks at various ways to handle sulfur species, particularly H2S, and offers specific treatment methods, along with the benefits and drawbacks of each method. Although sulfur treatment can be a significant challenge, a cost effective solution is available through the creative use of oxygen based chemistry.

What is H2S?H2S is a flammable, colourless gas that smells like rotten eggs. It occurs both naturally and from manmade processes. H2S can be released from volcanoes, sulfur springs, undersea vents, swamps, stagnant bodies of water and, most commonly, in areas with crude petroleum and natural gas production and refining. Other industries that have to manage the creation and treatment of sulfur are water treatment, landfill gas processing, manure handling, and pulp and paper. In all of these areas, sulfur species continue to be difficult to isolate and manage. In wastewater treatment, sulfur and H2S concentrations tend to be relatively low, yet high enough to cause issues with safety, corrosion and odour complaints. This article will review some techniques for treating sulfur species in wastewater and help identify the most effective way to manage this odorous gas when it occurs in the process.

Timothy Lebrecht, Air Products, USA, and Neil Hannay, Air Products, UK, offer several ways to treat H2S in wastewater and discuss the benefits and drawbacks of each.

§ Something water related

Reprinted from February 2015HYDROCARBON ENGINEERING

Reprinted from February 2015 HYDROCARBON ENGINEERING

Issues with H2SH2S is a chemical that comes with severe dangers. It is a strong acid when dissolved, extremely flammable and highly toxic. Since this article focuses on wastewater, the flammability hazard is not part of the discussion. However, the toxicity and odour of the substance cause this material to be one of the most challenging to handle. For example, elevated H2S levels can cause headaches and nausea at just 5x the odour detection threshold, assuming a detection threshold of 8 ppb (toxicity issue threshold would then be 40 ppb). H2S has been lethal to humans at acute concentrations generally exceeding 500 ppm.

H2S removal versus control In general, companies are very aware of processes that can generate or accumulate sulfur and H2S. The dangers associated with H2S, as well as the extreme odour, have companies doing what they can to make sure they remove as much of the material as possible within the process. Treating in process is the best way to address H2S.

An example from the refining industry is the Claus reaction, which transforms H2S to elemental sulfur. Sour water can be encountered and will need to be treated, but the vast majority of H2S is handled outside of the wastewater process. Treatment through specific reactions is most commonly done when volumes are large and there can be another use for the sulfur. The challenges become greater when H2S exists in small quantities. Developing a way to cost effectively manage the substance is challenging, but possible. In general, sulfur is an element that is necessary to sustain life. When sulfur is within an aerobic digestion wastewater system, it is readily converted to an odourless sulfate. When sulfurs are present in an anaerobic process, such as anaerobic digestion, however, H2S, mercaptans or thiols can be formed. The odours associated with sulfides can range from the smell of garlic to rotten eggs and worse. The wastewater team at a given facility needs a strategy to actively treat H2S in the wastewater stream, rather than wait for it to become an issue that can create problems at the facility or in the community. There are several good ways to treat H2S, each with its own advantages and disadvantages. These strategies may

Table 1. Methods of sulfur control

Method of control

Methods Action Main sulfur compounds produced

Further treatment required

Process application area

Typical requirement, lb/lb H2S

Comments

pH control acid

Acid dose for stripping pH<5

Removal by pushing equilibrium to H2S (sol)

H2S dissolved Gas stream H2S removal

Effluent flow pH and stripping control can be difficult to maintain at equilibrium, plus H2S is highly soluble.

pH control alkali

Base dosing for maintaining in solution pH>9

Control by pushing equilibrium to HS

HS Neutralisation for discharge

In lagoon pH too high for biological activity.

Redox/ORP control

Oxygen (as air or pure oxygen) Nitrate

Prevention, removal and control

SO SO4

Maintenance of high redox/ORP

Effluent flow and in lagoon

Must be maintained to prevent reduction of S and SO4

Oxidation H2O2 Permanganate Chlorine Chlorine dioxide Hypochlorite Ozone

Removal and control

S SO SO4

Removal of solid and/or prevention of reconversion to H2S

Effluent flow and in lagoon

Expensive; undesirable organic reactions; toxic chemical handling considerations.

Stripping Air CO2

Gas stripping for downstream gas phase treatment

H2S in carrier gas

Gas collection and scrubbing

Effluent flow Low pH maintenance required for effective full sulfide removal; CO2 carrier gas is therefore preferable to air.

Precipitation Ferrous sulfate Reaction to solid precipitate

Fe2S3 solid Solids removal and disposal

Effluent flow 4 - 15

Bactericidal Acid/alkali Chlorine Permanganate

Kill all bacteria to remove biological reduction of sulfur species

n/a Ongoing requirement to ensure no biological activity

Effluent flow Applicable for corrosion and odour control in pipelines.

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vary by treatment type and include equalisation ponds, anaerobic ponds and deep storage industrial ponds. In general, the goal for H2S control in an equalisation pond is to keep organic material in solution and move it along quickly so that anaerobic conditions do not develop. Anaerobic ponds are the largest challenge, as sulfide gas bubbles (most commonly H2S) can rise from the pond. Strategies to eliminate this include pH control, stripping with scrubbing, and oxidation reduction

potential (ORP) control/oxidation. Deep industrial ponds can encounter the same issues as anaerobic ponds, but on a more seasonal basis. In general, the concepts of pH control, stripping and scrubbing, and ORP control/oxidation are the best means to control sulfur in wastewater. The level of acidity in the treatment basin can be a key way to make sure H2S does not leave the basin. When wastewater has a pH level of >9, nearly all H2S will stay in solution as HS and

Table 2. Comparison of redox control methods

Method of redox control

Efficiency losses w/ea 1 mg/l DO rise above 0

Typical energy required for dissolution (power)

Typical method of application

Typical dose rates quantity required

Mechanism Side reactions

Effect on ecosystem

Benefits Issues

Pure oxygen

2% 0.0 - 0.3 kW/kg

Continuous dissolved oxygen; automatically controlled injection

8 - 10 g O2/m3

5 - 10 lb/lb H2S

Maintains high dissolved oxygen preventing reduction of sulfur species. Local oxidation of sulfur species by biological and chemical processes to H2SO4, SO4 and S.

Biological BOD removal. Promotes bacterial activity.

Improves biodiversity and aerobic treatment. High dissolved oxygen for final discharge.

High efficiency. Low agitation of basin. Small footprint. No chemical handling. Fully automated. Low power.

Good dissolution required to ensure economics.

Air 10% 0.8 kW/kg Continuous surface aeration

50 - 100 g O2/m3 (150 - 300 g air/m3)

Maintains dissolved oxygen levels > 0.5 mg/l preventing reduction of sulfur species.

Biological BOD removal. Stripping of dissolved H2S due to nitrogen waste gas.

Improves biodiversity and aerobic treatment.

Low technology investment.

Dissolution rates drop in warm weather, when biological activity increases. Waste nitrogen gas can cause over mixing and exacerbate the release of H2S from the system. Air will strip H2S at effluent inlet due to high volume waste nitrogen.

H2O2 N/A N/A Batch delivery and application

N/A reaction not DO rise 1 - 5 lb/lb @ 100% H2O2

Local chemical reaction to S. Stochiometric 1:1 but in process nearer 5:1.

Chemical oxidation of organic compounds and biological components. Decomposes to oxygen and water.

Effectively disinfects local injection region entirely stopping water treatment.

Emergency control of H2S levels.

Biological destruction stimulates increased activity and anaerobic processes once oxidation potential lost. Hazardous chemical handling. Usually very local injection required.

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will not exit the treatment area. This may sound like a simple approach, however, in biological wastewater, the micro organisms are the key to treatment. These organisms cannot live in such a high pH environment. The challenge is that for these organisms to be healthy, pH must be much closer to neutral. Doing so keeps the bacteria healthy, but it does not keep the H2S in solution. Another means of controlling H2S is stripping it from the stream with air or CO2 and then scrubbing the H2S. This tends to be a costly approach due to the large volume of water that would need to be stripped of H2S, as well as the operating cost of a scrubber. In most instances, this is not the chosen path of treatment. The most common approach in water treatment is ORP control/oxidation. Oxidation involves the reaction of H2S with oxygen (O2) to form sulfur (S), sulfate (SO4), sulfuric acid (H2SO4) or other soluble sulfur compounds. In this way, the challenge of H2S is changed to an alternate chemical that is more simply treated or controlled. The issue with oxidation is that it requires the oxygen molecule in the wastewater at a concentration high enough to react with the H2S without slipping into anaerobic conditions. Ponds or treatment basins try to maintain dissolved oxygen (DO) content at an acceptable level, but due to process variation and seasonality, DO levels will vary. A summary of the different types of control for sulfur species odour is shown in Table 1. As described above, there are multiple

paths to consider, but the most common path, especially when anaerobic issues are the root cause, is oxidation. When employing oxidation or ORP control techniques, process demand variance or seasonality require a dynamic way to control the input of oxygen. These variations can be controlled proactively or reactively. A proactive approach is to install the necessary wastewater equipment to make sure that the dissolved oxygen level is relatively consistent. A reactive approach is to wait for the pond or treatment basin to be overcome with an impurity and then treat on an as needed basis. The reactive approach can be lower cost if H2S does not start to leave the treatment system. However, if the levels of the gas leave the water and create a toxic or odorous environment, the price can be quite high both in cost and company image.

Oxidation and ORP controlThe most common way of controlling sulfur species in a lagoon or treatment pond is through oxidation or redox/ORP control. Achieving the necessary level of dissolved oxygen in the pond or treatment basin requires air, pure oxygen or hydrogen peroxide. Each of these substances has pros and cons, which are compared in Table 2. Air typically has the lowest operating cost; however, due to the way oxygen is added to the treatment pond, air can actually cause more H2S to leave the pond than it stops through reaction with the oxygen. Typically, air is not a reliable means for control due to the possibility of stripping and the seasonal variability of O2 retention with temperature. Although oxygen in air is readily available, the challenge is getting it into the water solution. Large air based mixers and aerators can require large horsepower motors and significant capital to get enough oxygen into the water solution. One issue this may create is an increase in overall VOCs into the atmosphere by stripping the treatment pond of impurities and pushing them into the air. Additionally, over mixing the pond can exacerbate the release of H2S because of the high volume of waste nitrogen coupled with increased sediment disturbance in lagoon type basins. Also, dissolution efficiency rates tend to drop in warm weather when biological activity increases, requiring even more energy. For H2S, the air based approach needs to be handled carefully, as the solution may cause more issues than the initial problem or vary by season. Pure oxygen requires unique equipment to ensure appropriate mixing and distribution in the treatment ponds. Good dissolution is required to ensure the economics make sense for oxygen. The safety of oxygen also needs to be carefully understood, and special consideration of the handling and care for the equipment is necessary. Oxygen based equipment can add the product without stripping the VOCs. Several methods involve adding the O2 under water in ways that increase its ability to mix with the wastewater based on a sensor’s measurement of dissolved oxygen. Lastly, hydrogen peroxide (H2O2) can offer a solution. Typically, H2O2 is used as a reactive approach to water treatment. Ongoing supply can be challenging to distribute appropriately, which lends itself to be more of a reactive means of control. Since distribution and control are challenges, the cost in terms of product, labour and yield are significant issues. Due to the substance’s high degree of

Figure 1. Air Products' Halia® mixer aeration system, consisting of oxygen supply, either as liquid oxygen or onsite generation, combined with OxyMix® technology jointly developed by Aqua-Aerobic Systems, Inc. and Air Products.

Figure 2. OxyMix® technology installed in a wastewater treatment tank.

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reactivity, H2O2 can react with impurities other than H2S, thereby reducing its ability to effectively treat H2S. One of these side reactions is biological destruction of organisms in the basin, which can stimulate a second activity in anaerobic processes once oxidation potential is lost. The safety of using H2O2, which requires hazardous chemical handling, also increases the cost of this option. As illustrated in Table 2, oxidation through the use of pure oxygen creates the safest and lowest cost option.

Oxygen injectionPure oxygen can be injected into wastewater in a number of ways. For years, this approach has been used in activated sludge systems to boost the dissolved oxygen content when other means have been exhausted. Oxygen tends to be a way to boost a treatment pond’s performance without increasing its size. These same methods are also good for the treatment of H2S. In the treatment of H2S, oxygen tends to be better because it does not lift impurities in the air through stripping, and most of the O2 is injected into the water without significant agitation of the pond. Some typical ways of adding oxygen are: diffusion into a pipeline, diffusion into a grid in the treatment pond, a floating mixer aerator unit or a floating diffuser based system. Each of these systems varies in yield and complexity. The simplest way to add the pure O2 is through basic diffusion. This can be done by adding O2 through a lance into a line of water that feeds the treatment basin. Another simple way is to have a static diffusion grid in the treatment pond bubbling oxygen through the depth of the pond. Each way has definite limits on efficiency. Typically, only 10 - 15% of the oxygen put into the pipe or diffusion grid is captured by the water, and the rest escapes into the atmosphere. There are several other approaches that use a more complex method for adding oxygen. Diffusion based forced water/oxygen systems can be operated. Industrial gas companies like Air Products offer this type of equipment. Oxygen efficiency can reach up to 90% under specific operating parameters. Air Products offers such items as the Halia® Mixer Aerator and the Halia® Venturi Aerator units for both deep and shallow treatment pond conditions. These units, pictured below, can add up to 10 000 lb/d of oxygen per unit to a treatment zone.

Oxygen supplyThe supply of oxygen is also an area where great expertise is needed. Oxygen can be provided from a liquid oxygen tank or an onsite generator.

The most common mode of supply for delivered oxygen is via on road liquid oxygen tankers from a central manufacturing facility. The oxygen is stored as a liquid at the site in an insulated tank and vaporised at the time of use. This is the most flexible mode of supply.

Oxygen can also be generated onsite using cryogenic or adsorption technologies. At locations in the vicinity of an oxygen pipeline, supply via pipeline could be the most cost effective and

flexible source of oxygen. Evaluating the optimal mode of supply requires the review of a host of factors, including:

n Size of the oxygen requirement (average and peak demand). n Expected use pattern (continuous, seasonal, erratic). n Presence of other nearby oxygen consuming applications, such

as ozone. n Power availability and cost. n Proximity of delivered oxygen source.

Table 3 provides rough guidance about the best mode of supply in the context of these parameters. From the early stages of the project, an industrial gas company like Air Products will work closely with a wastewater treatment plant to jointly determine the best mode of oxygen supply.

ConclusionSulfur species are problematic for many industries. However, there is hope for a way to control this substance even when treatment conditions are variable. Equipment designed to use pure oxygen with an accompanying DO probe and a programmable logic controller can maintain the necessary dissolved oxygen content required to transition H2S and other sulfur species into treatable sulfates. Other types of treatment can prove to be more costly or create more problems than they solve. For example, temporary fixes, such as hydrogen peroxide, do not solve the issue. Use of H2O2 is costly, creates safety challenges, and is not a permanent solution. The more practical solution is pure oxygen, which provides a continuous supply to a treatment pond without exposure to a hazardous chemical for workers and without danger of hurting or causing an issue with the planned biological treatment of the wastewater.

References1. Hydrogen Sulfide; MSDS No. 300000000081; Air Products: Allentown,

Pa., February 8, 2014. 2. MOUSSAVI et al., The Removal of H2S from Process Air by Diffusion

into Activated Sludge, Env Tech, Vol 28, pp. 987 - 993, 2007. 3. HJORTH et al., Redox Potential as a Means to Control the Treatment of

Slurry to Lower H2S Emissions, Sensors 2012, 12, pp. 5349 - 5362.

4. Septicity in Sewers: Causes, Consequences and Containment, Boon 1995, Water Sci tech, Vol 31, No 7, pp. 237 - 253.

5. Metcalf and Eddy Wastewater Engineering 2003 Fourth Edition.6. EPA: Process Design Manual for Sulfide Control in Sanitary Sewerage

Systems 1974. 7. Economical, Efficient and Effective Mixing: Three Approaches to

Controlling Odor in Wastewater Treatment Ponds – White Paper Medora Corp.

8. NIELSEN at al., Aerobic and Anaerobic Transformations of Sulphide in a Sewer, WEFTEC 2006.

Table 3. Choosing the right oxygen supply mode

Supply features Liquid oxygen Onsite generation Pipeline*

Flow range (tpd) 0 - 50 50 - 150+ 100+

Commitment Low High Medium

Time to implement (months) 1 - 2 10 - 18 6 - 8

Location limitations Yes No Yes

Application best fit

Flow Low Medium/high High

Use pattern Variable Steady Variable/steady

*Gas piped in from remote air seperation plant