Something water related

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1 omething water related Timothy Lebrecht, Air Products, UA, and Neil Hannay, Air Products, UK, offer several ways to treat H2 in wastewater and discuss the benefits and drawbacks of each. ulfur 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 (H2), a dangerous, odorous substance. This article looks at various ways to handle sulfur species, particularly H2, 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 based chemistry. What is H2? H2 is a flammable, colourless gas that smells like rotten eggs. It occurs both naturally and from manmade processes. H2 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 H2 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. Reprinted Reprintedfrom fromfebruary February

2 Table 1. Methods of sulfur Method of Methods Action Main sulfur compounds produced Further treatment Gas stream removal Process area requirement, lb/lb Comments ph acid Acid dose for stripping ph<5 Removal by pushing equilibrium to (sol) ph and stripping can be difficult to maintain at equilibrium, plus is highly soluble. ph too high for activity. ph alkali Base dosing for maintaining in solution ph>9 Control by pushing equilibrium to H Prevention, removal and H Neutralisation for discharge In lagoon Redox/ORP Oxygen (as air or pure ) Nitrate O O 4 Maintenance of high redox/ ORP and in lagoon Must be maintained to prevent reduction of and O 4 Expensive; undesirable organic reactions; toxic handling considerations. Oxidation Permanganate dioxide Hypochlorite Ozone tripping Air C Removal and O O 4 Removal of solid and/or prevention of reconversion to and in lagoon Gas stripping for downstream gas phase treatment in carrier gas Gas collection and scrubbing Low ph maintenance for effective full sulfide removal; C carrier gas is therefore preferable to air. Precipitation Ferrous sulfate Reaction to solid precipitate Fe 2 3 solid olids removal and disposal 4-15 Bactericidal Acid/alkali Permanganate Kill all bacteria to remove sulfur species n/a Ongoing requirement to ensure no activity Applicable for corrosion and odour in pipelines. Issues with is a that comes with severe dangers. It is a strong acid when, extremely flammable and highly toxic. ince 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 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). has been lethal to humans at acute concentrations generally exceeding 500 ppm. removal versus In general, companies are very aware of processes that can generate or accumulate sulfur and. The dangers associated with, 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. An example from the refining industry is the Claus reaction, which transforms to elemental sulfur. our water can be encountered and will need to be treated, but the vast majority of 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 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,, 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 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, each with its own advantages and disadvantages. These strategies may

3 Table 2. Comparison of redox methods Method of redox Efficiency losses w/ ea 1 mg/l DO rise above 0 energy for dissolution (power) method of dose rates quantity Mechanism ide reactions Effect on ecosystem Benefits Issues Pure 2% kw/kg Continuous ; automatically led 8-10 g /m lb/ lb Maintains high preventing sulfur species. Local oxidation of sulfur species by and processes to O 4, O 4 and. Maintains levels > 0.5 mg/l preventing sulfur species. BOD removal. Promotes bacterial activity. Improves biodiversity and aerobic High for final discharge. High efficiency. Low agitation of basin. mall footprint. No handling. Fully automated. Low power. Good dissolution to ensure economics. Air 10% 0.8 kw/kg Continuous surface aeration g /m 3 ( g air/m 3 ) BOD removal. tripping of due to nitrogen waste gas. Improves biodiversity and aerobic Low technology investment. Dissolution rates drop in warm weather, when activity increases. Waste nitrogen gas can cause over mixing and exacerbate the release of from the system. Air will strip at effluent inlet due to high volume waste nitrogen. destruction stimulates increased activity and anaerobic processes once oxidation potential lost. Hazardous handling. Usually very local. N/A N/A Batch delivery and N/A reaction not DO rise % Local reaction to. tochiometric 1:1 but in process nearer 5:1. Chemical oxidation of organic compounds and components. Decomposes to and water. Effectively disinfects local region entirely stopping water Emergency of levels. vary by treatment type and include equalisation ponds, anaerobic ponds and deep storage industrial ponds. In general, the goal for 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 ) can rise from the pond. trategies to eliminate this include ph, stripping with scrubbing, and oxidation reduction potential (ORP) /oxidation. Deep industrial ponds can encounter the same issues as anaerobic ponds, but on a more seasonal basis. In general, the concepts of ph, stripping and scrubbing, and ORP /oxidation are the best means to sulfur in wastewater. The level of acidity in the treatment basin can be a key way to make sure does not leave the basin. When wastewater has a ph level of >9, nearly all will stay in solution as H and

4 Figure 1. Air Products' Halia mixer aeration system, consisting of supply, either as liquid or onsite generation, combined with OxyMix technology jointly developed by Aqua-Aerobic ystems, Inc. and Air Products. Figure 2. OxyMix technology installed in a wastewater treatment tank. will not exit the treatment area. This may sound like a simple approach, however, in wastewater, the micro organisms are the key to 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 in solution. Another means of ling is stripping it from the stream with air or C and then scrubbing the. This tends to be a costly approach due to the large volume of water that would need to be stripped of, as well as the operating cost of a scrubber. In most instances, this is not the chosen path of The most common approach in water treatment is ORP /oxidation. Oxidation involves the reaction of with ( ) to form sulfur (), sulfate (O 4 ), sulfuric acid ( O 4 ) or other soluble sulfur compounds. In this way, the challenge of is changed to an alternate that is more simply treated or led. The issue with oxidation is that it requires the molecule in the wastewater at a concentration high enough to react with the without slipping into anaerobic conditions. Ponds or treatment basins try to maintain (DO) content at an acceptable level, but due to process variation and seasonality, DO levels will vary. A summary of the different types of 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 techniques, process demand variance or seasonality require a dynamic way to the input of. These variations can be led proactively or reactively. A proactive approach is to install the necessary wastewater equipment to make sure that the 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 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 The most common way of ling sulfur species in a lagoon or treatment pond is through oxidation or redox/orp. Achieving the necessary level of in the pond or treatment basin requires air, pure 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 is added to the treatment pond, air can actually cause more to leave the pond than it stops through reaction with the. ly, air is not a reliable means for due to the possibility of stripping and the seasonal variability of retention with temperature. Although 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 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 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 activity increases, requiring even more energy. For, 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 requires unique equipment to ensure appropriate mixing and distribution in the treatment ponds. Good dissolution is to ensure the economics make sense for. The safety of 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. everal methods involve adding the under water in ways that increase its ability to mix with the wastewater based on a sensor s measurement of. Lastly, hydrogen peroxide ( ) can offer a solution. ly, is used as a reactive approach to water Ongoing supply can be challenging to distribute appropriately, which lends itself to be more of a reactive means of. ince distribution and are challenges, the cost in terms of product, labour and yield are significant issues. Due to the substance s high degree of

5 reactivity, can react with Table 3. Choosing the right supply mode impurities other than, thereby reducing its ability to upply features Flow range (tpd) Liquid 0-50 Onsite generation Pipeline* 100+ effectively treat. One of Commitment Low High Medium these side reactions is destruction of organisms in the Time to implement (months) basin, which can stimulate a second activity in anaerobic Location limitations Application best fit Yes No Yes processes once oxidation Flow Low Medium/high High potential is lost. The safety of Use pattern Variable teady Variable/steady using, which requires hazardous handling, *Gas piped in from remote air seperation plant also increases the cost of this option. As illustrated in Table 2, oxidation through the use of pure flexible source of. Evaluating the optimal mode of supply requires the review of a host of factors, including: creates the safest and lowest cost option. n ize of the requirement (average and peak demand). Oxygen Pure can be injected into wastewater in a number of ways. For years, this approach has been used in activated sludge systems to boost the 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. In the treatment of, tends to be better because it does not lift impurities in the air through stripping, and most of the is injected into the water without significant agitation of the pond. ome typical ways of adding 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 is through basic diffusion. This can be done by adding 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 through the depth of the pond. Each way has definite limits on efficiency. ly, only 10-15% of the 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. Diffusion based forced water/ 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 lb/d of per unit to a treatment zone. Oxygen supply The supply of is also an area where great expertise is needed. Oxygen can be provided from a liquid tank or an onsite generator. The most common mode of supply for delivered is via on road liquid tankers from a central manufacturing facility. The 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 pipeline, supply via pipeline could be the most cost effective and n Expected use pattern (continuous, seasonal, erratic). n Presence of other nearby consuming s, such as ozone. n Power availability and cost. n Proximity of delivered 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 supply. Conclusion ulfur species are problematic for many industries. However, there is hope for a way to this substance even when treatment conditions are variable. Equipment designed to use pure with an accompanying DO probe and a programmable logic ler can maintain the necessary content to transition 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 is costly, creates safety challenges, and is not a permanent solution. The more practical solution is pure, which provides a continuous supply to a treatment pond without exposure to a hazardous for workers and without danger of hurting or causing an issue with the planned treatment of the wastewater. References 1. Hydrogen ulfide; MD No ; Air Products: Allentown, Pa., February 8, MOUAVI et al., The Removal of from Process Air by Diffusion into Activated ludge, Env Tech, Vol 28, pp , HJORTH et al., Redox Potential as a Means to Control the Treatment of lurry to Lower Emissions, ensors 2012, 12, pp epticity in ewers: Causes, Consequences and Containment, Boon 1995, Water ci tech, Vol 31, No 7, pp Metcalf and Eddy Wastewater Engineering 2003 Fourth Edition. 6. EPA: Process Design Manual for ulfide Control in anitary ewerage ystems Economical, Efficient and Effective Mixing: Three Approaches to Controlling Odor in Wastewater Treatment Ponds White Paper Medora Corp. 8. NIELEN at al., Aerobic and Anaerobic Transformations of ulphide in a ewer, WEFTEC 2006.