odor control with ProSweet*

Transcription

1 Water Technologies & Solutions capability profile odor control with ProSweet* introduction Driven by changes in the Clean Air Act Amendment (CAAA), the requirement for improved odor control may exist in many industrial facilities. Under this act, regulatory bodies will be forcing industrial plants such as paper mills, refineries and food processors to improve the quality of air they are emitting. This legislation is aimed at reducing toxic chemical emissions. Odor control has come into focus. Odors emitted from manufacturing facilities can impact an operation in many ways: Public complaints about odor can negatively affect an industry s image in the marketplace and its ability to carry out an effective public relations policy with the surrounding community. Safety concerns can arise from people working in areas where odors can overcome them. Productivity impact can be seen in areas of a facility where objectionable odors exist due to employee avoidance or neglect. Production can be affected through the presence of odors and odor producing conditions, which taint product quality. Equipment integrity can be threatened by the presence of many odors, which are corrosive in nature. It should be noted that not all manufacturing facilities produce odors or experience these problems, nor are all odors noxious or toxic in nature. However, one very large segment of the odor control market, which does fit this description and dictates immediate and complete attention, is the control of hydrogen sulfide (H 2 S) generation and evolution. Although the CAAA does not explicitly regulate hydrogen sulfide, it does call for the elimination of offensive and toxic odors. With this in mind, SUEZ has developed a treatment program which offers a solution to this industrial need in an easy to apply and economical manner. It is this technology, which affords SUEZ a definite competitive advantage in this area. odor control in the industrial market place Potential for odor control technologies exists in many industries: Pulp & Paper Mills Oil Refineries/Hydrocarbon Processing Industries (HPI) Foundries/Steel Mills/Automotive Plants Food Processing Facilities Textile Mills Other Industrial Manufacturing Markets The use of chemicals as a method of odor control is regarded as an acceptable treatment option because of the minimal amount of capital investment required. Other acceptable technologies, including combustion, oxidation, and stripping are also very efficient but require considerable capital equipment investment. In many of the industrial applications mentioned above such as paper making, oil refining and steel, odor control methods such as incineration, carbon adsorption, wet scrubbing, electrostatic precipitation, source modification, and odor masking may be found. In hospitals, office buildings and schools, other methods such as filtration and absorption are used. Many industrial facilities, however, find it is more cost effective to implement and operate a chemical based odor control program than to incur up front equipment purchases. Find a contact near you by visiting and clicking on Contact Us. *Trademark of SUEZ; may be registered in one or more countries SUEZ. All rights reserved. cp81.docx Aug-11

2 A chemical program is a direct benefit to those who have to put in place an odor control program now, with a low cost impact on their bottom line. With odor control programs involving equipment purchases, the time lapse between purchase and start-up may involve months, consuming valuable time and becoming costly in fines and neighbor complaints. background H 2 S is the most commonly known and prevalent odorous gas associated with wastewater treatment systems. It has a characteristic rotten egg odor, is extremely toxic, and is corrosive to metals. Process and waste streams, in which the direct introduction of sulfides occurs, are likely candidates for odor problems. Figure 1a: Effect of ph on hydrogen sulfide equilibrium In addition to direct process sources of reduced sulfur compounds, the other primary contributor of hydrogen sulfide odors from wastewater streams is the biochemical reduction of inorganic sulfur compounds. Under anaerobic conditions, sulfate-reducing bacteria use sulfate as an oxygen source to metabolize organics in the waste stream: SO 4 = + 2C + H 2 O 2 HCO H 2 S Most sulfate reduction occurs within a biological slime layer, which protects the sulfate reducers from oxygen present within the bulk waste stream itself. The rate at which hydrogen sulfide is generated is dependent upon the concentration of sulfate and organics in the waste stream, the level of dissolved oxygen, ph, temperature, and the velocity of the water. The conditions leading to H 2 S formation generally favor the production of other malodorous organic compounds such as mercaptans, thiophenol, and thiocresol. Investigations of the conditions favoring H 2 S formation can also help to quantify the potential for odor generation from other compounds. Thus, solving H 2 S odor problems can often solve other odor problems as well. figure 1b: Proportions of H 2 S and HS - in dissolved sulfide. H 2 S dissolves in water and dissociates according to the following reactions: H 2 S HS - + H + HS - S = + H + Figures 1a and 1b show the distribution of sulfide species as a function of ph. The relative H 2 S concentration increases with decreasing ph. At a ph of 7.0, H 2 S represents 50% of the dissolved sulfides present, while at a ph of 6.0, over 90 percent of the dissolved sulfides is in the form of H 2 S. If part of the dissolved H 2 S escapes to the atmosphere, the remaining dissolved sulfide will be divided between H 2 S and HS - in the same proportion as before because the equilibrium re-establishes itself almost instantly. Page 2 cp81.docx

3 100 ppm : IDLH (Immediately Dangerous to Life and Health) Very low concentrations of this gas can cause serious health hazards. Death has resulted from concentrations of 300 ppm by volume in air. Such concentrations can be obtained in an enclosed chamber with high turbulence, from wastewater containing 2 ppm of dissolved sulfide at a ph of 7.0. Based on Henry s Law, Figure 2 was developed to show H 2 S levels in the atmosphere (closed vessel) in equilibrium with the given concentrations of H 2 S in the water at the respective wastewater temperatures. preliminary monitoring program Figure 2: Equilibrium concentration of H 2 S in air The distinction between the types of sulfide compounds is significant because only the H 2 S can escape from solution and create odor, corrosion, and health problems. It is important, therefore, to quantify the total and dissolved sulfides present and the ph of the wastewater. health effects of H2S H 2 S is an acutely toxic gas. H 2 S is heavier than air, is colorless and has a characteristic rotten egg smell at low concentrations. But as the levels of H 2 S increase, we become generally unaware of its presence. A person s ability to sense dangerous concentrations by smell is quickly lost. If the concentration is high enough, unconsciousness will occur suddenly, followed by death if there is not a prompt rescue. The following outlines the current exposure limits for hydrogen sulfide as set by NIOSH (National Institute for Occupational Safety and Health) and ACGIH (American Conference of Government and Industrial Hygienists). 1 ppm : TLV/TWA (8 hour maximum average exposure) 5 ppm : STEL (Short Term Exposure Limit) 10 ppm : Ceiling Recommended Exposure Limit Normally, repeated odor complaints are the first indicators of potentially damaging sulfide generation within a system. In more extreme cases, the problems are manifested by deteriorated conditions in pipes and electrical equipment or by structural failures. Evidence of sulfide generation warrants the implementation of a preliminary program to assess the overall potential for sulfide generation. Such a preliminary program should include a thorough investigation of odor complaints, and a systematic investigation of the wastewater collection and treatment system to identify major potential contributors. preliminary sampling The survey of odor generation should begin at the waste treatment plant and proceed upstream throughout the facility. The preliminary survey should consist of a simplified field analysis of sulfide levels in the water and air to determine the actual trouble areas. Hand held monitors can be used for air sampling. Test kits such as those supplied by Chemetrics can be used for wastewater testing. Normally, odor problems will occur when dissolved sulfide levels are 1.0 to 1.5 ppm or greater. The following locations should be checked: 1. Lift stations - sample wet-well influents, pump discharges, and ends of pressure mains to determine sulfide balance. 2. Gravity sewers - turbulence may release H 2 S gas and cause corrosion of manhole chambers. cp81.docx Page 3

4 3. Sludge holding tanks - residual H 2 S and anaerobic activity will cause problems. 4. Areas of turbulence and long detention times - sample locations of turbulence where sulfide may be released. Measurement of total sulfides before and after the point of turbulence can give an indication of the quantity of H 2 S released to the atmosphere. Recycle streams from thickeners, digesters, and other sludge treatment processes should also be sampled, as these can contain high concentrations of H 2 S that can result in severe odor and/or corrosion problems at the point of side stream return. Air samples from enclosed spaces exposed to wastewater may also be tested to determine the severity of odors. wastewater characterization The wastewater should be analyzed for the following: 1. Sulfates (SO 4 =), ppm 2. ph 3. Total Sulfides (TS), ppm 4. Biochemical Oxygen Demand (BOD 5 ), ppm 5. Dissolved Oxygen (DO), ppm 6. Temperature, C/ F 7. Suspended Solids (TSS), ppm In addition to these water analyses, H 2 S levels in the ambient air should also be determined with an appropriate air monitor. chemical treatment Numerous chemicals have been employed for control of sulfides in wastewater collection systems. Chemical addition can control sulfides by: 1. ProSweet Solution 2. Chemical oxidation (Cl 2,H 2 O 2 ) 3. Precipitation (Metal salts) 4. ph control ProSweet Solution The current family of ProSweet products includes three distinct categories of technology: Organic Scavenger Biomodifier Counteractant (Odor neutralizer) Organic Scavenger - The organic scavengers are comprised of a unique proprietary technology (non-amine based) and the traditional cyclic amines. ProSweet organic scavengers will selectively react with any reduced sulfur compounds that have acidic protons. Therefore, many malodorous sulfur odors can be treated successfully with these products. Proper treatment levels for ProSweet scavengers depend on many factors such as stream flow rate, temperature, H 2 S concentration, desired H 2 S removal efficiency, and ph. Assessment of these factors will aid the SUEZ sales representative in recommending treatment rates, control procedures, and specific application points. Dosages are often controlled based on actual operating conditions and the degree of scavenging required. It may not be necessary to scavenge all malodorous sulfides. Treatment levels will be dictated by perception and/or satisfactory monitoring levels. The selection of single or multiple feed-points is site specific. Sulfide containing streams should be identified, in addition to locations, with high H 2 S concentrations in the air. The feed point should be located at the generation point or upstream of the affected areas. ProSweet products can be fed with a standard metering pump to various locations such as full flowing pipelines, open channels, sludge lines, or sludge holding tanks. The benefits of using a ProSweet non-amine based organic scavenger includes: A selective activity with reduced sulfur compounds No ph change Wide ph range of applicability No sludge generation. Easy to handle and feed No adverse effect on biological wastewater system Page 4 cp81.docx

5 Biomodifiers - Nitrate has long been used in facultative and anaerobic lagoons to control odors. Facultative and obligate anaerobic bacteria, which are responsible for odor and sulfide production, prefer nitrate (over sulfate) as an oxygen source when available. When nitrate is present, these sulfide producing bacteria use this to the exclusion of sulfate. This results in the production of nitrogen gas and other nitrogenous compounds rather than sulfide. In some cases, it is more appropriate to inhibit the biological production of malodorous H 2 S from water and wastewater streams with biomodifiers. ProSweet biomodifiers are a blend of a metabolic inhibitor to prevent the biological generation of hydrogen sulfide and an organic scavenger; providing a dual functional product. Proper treatment levels for this ProSweet technology depend on many factors such as stream flow rate, temperature, ph and levels of biochemical oxygen demand (BOD 5 ), sulfate and dissolved oxygen. Assessment of these factors will aid the SUEZ sales representative in recommending treatment rates, control procedures and specific application points Counteractant - These are chemicals that interfere with the malodor. The counteractant does not chemically react with the malodor, but reduces the perceived odor level by eliminating the objectionable characteristics of the malodor. The elimination or neutralization of odor does not indicate in any way that these chemicals have the ability to neutralize the health and safety effects the odor causing agent may have. This technology offers an effective way of dealing with a wide variety of odor types, mostly nuisance odors in oxidized form. This can also be used to control residual odors from sulfide treatment or odors generated by sulfides only if the H 2 S air levels are consistently below the safe exposure limits established by ACGIH and NIOSH. In this type of application, the malodorous air streams are treated with atomized or vaporized ProSweet product. For solid waste applications, the product may be sprayed on piles or landfills. Chemical Oxidation Chlorine donating materials are rarely used because of their safety and handling problems and the probability of THM formation. Hydrogen peroxide chemically oxidizes H2S according to the following reactions: ph < 8.5: H 2 O 2 + H 2 S S + 2H 2 O ph > 8.5: 4H 2 O 2 + S = = SO 4 + 2H 2 O At ph < 8.5, the stoichiometric H 2 O 2 requirement is 1g H 2 O 2 /1g H 2 S. In practice, a greater weight ratio may be required because hydrogen peroxide cannot selectively oxidize sulfides. The actual dosage rate will be proportional to the concentration of oxidizable compounds in the wastewater. Metal Salts The salts of many metals will react with dissolved sulfide to form metallic sulfide precipitates, thus preventing H 2 S release to the atmosphere. For effective removal of dissolved sulfides, the metallic sulfide formed must be highly insoluble. Iron salts have been used for sulfide control. The ferrous ion reacts with sulfide as shown below. Fe ++ + HS - FeS + H + Pomeroy found that the reaction of a mixture of iron salts with a molecular ratio of one part ferrous to two parts ferric was superior for sulfide control compared to the reaction of either one alone. The reaction of the mixed iron salts was hypothesized to occur as follows: Strong Alkalies Fe Fe HS - Fe 3 S 4 + 4H + Increasing the system ph reduces the proportion of dissolved H 2 S in the H 2 S and HS - equilibrium. For example, at a ph of 7.0, equal concentrations of dissolved H2S and HS - exist at equilibrium, while at a ph of 8.0; only about 10% of the dissolved sulfide exists as H 2 S. Since dissolved H2S is the only form which can be released to the atmosphere, it follows that increasing the ph would reduce odors and corrosion by maintaining the dissolved sulfides in the HS - and S -2 forms. cp81.docx Page 5

6 ProSweet Program Applications In order to successfully apply the ProSweet odor control products, intimate contact between H2S and the products is necessary. During the system survey it is critical to identify an appropriate feed point for the treatment program. The survey will help to pinpoint areas where the treatment program can be set up for maximum effectiveness. In general, odor control products should be fed at the H 2 S generation area or ahead of the highest concentration of H 2 S odor points. For example, feed ahead of the clarifier if the odor is coming from the clarifier; or, feed ahead of the inlet to the sludge holding tank if the odor is highly concentrated in the holding tank. Figure 4: Paper Mill System Wastewater Treatment The H 2 S meter and the Hach sulfide kit were used to test for H 2 S in the air and water at selected sites. The level of H 2 S in the filtrate water from the twin belt filter presses was 10 ppm and, in the surrounding air at the sludge holding tank from the primary clarifier, it was 20 ppm. A feed point for ProSweet ahead of the sludge holding tank was the selected location to reduce the concentration of H2S in the system. Figure 3: Automotive Foundry Wastewater Treatment As an example (Figure 3), in a large Midwestern automobile foundry wastewater system, there was no H 2 S in the effluent of the clarifier. There was less than 1 ppm of H2S in the water after the sand filters effluent; 1 ppm after the carbon filters and 12 ppm of H 2 S in the Parshall Flume pit. The H 2 S meter was used for H 2 S determinations in the air at these locations and only at the Parshall Flume pit was there a significant accumulation of H 2 S in the air. The accumulation of H 2 S at this location was 20 ppm. At this level, H 2 S is quite unpleasant and unsafe; workers usually try to avoid the area until it is dissipated by opening doors, windows and operating exhaust fans. In this example, a feed point ahead of the Parshall Flume was selected as the desired location to eliminate the heavy concentrations of H2S. In another example, at a large Midwestern paper mill a survey of the physical equipment at the sludge dewatering building was made (Figure 4). Figure 5: General Layout of waste treatment system at newsprint mill A newsprint mill was receiving complaints from employees and the neighboring community because of H2S generation at their waste treatment facility. The general layout of the waste treatment system is shown in Figure 5. Waste plant influent is split between two parallel primary clarifiers. Overflow from the clarifiers enters an activated sludge system. Biological solids are then removed in the secondary clarifier prior to effluent discharge to the river. Secondary sludge is dewatered on a belt filter press, while mixed primary and secondary sludge is dewatered in a screw press. Page 6 cp81.docx

7 Air and water samples determined the source of odor generation to be the sludge. Odors originated both from the primary clarifier itself and the sludge dewatering equipment. Ambient H 2 S levels between 50 and 150 ppm were measured at the primary clarifier outlet and the overflow lift station. Around the sludge presses, H 2 S was measured between 25 to 50 ppm in the air. The mill evaluated ferric sulfate addition to the sludge ahead of the presses in an attempt to react with the sulfide present. This approach provided little relief on odor generation. It was decided not to inject iron salts into the primary clarifier inlet because of the risk of iron carryover into the aeration basin. It was feared that the resulting precipitation of phosphate would make nutrient control difficult and aggravate an existing filamentous bulking problem in the secondary clarifier. A proprietary non-amine based organic scavenger was then applied to the influent of the primary clarifiers. In less than 24 hours, odors were under control (Table 1). Over time, not only were the odors eliminated, but other impacts on system operation were realized. The growth of filamentous bacteria in the secondary clarifier was reduced, creating a faster settling sludge. It was surmised that the elimination of ionic sulfide from the water reduced the available sulfur supply to the Thiothrix ssp. in the aeration basin. As a result, the need for chlorine and defoamer addition to the clarifier because of the adverse effects of excessive growth of filamentous bacteria was eliminated. Because of better quality solids, polymer requirement for the belt press dewatering declined by 20%. Table 1: Location Ambient H 2 S Levels Before and After Organic Scavenger Application. Before Scavenger After Scavenger application monitoring Besides your sense of smell, there are a few essential testing items you should consider when scoping out for odor control treatment program or servicing an existing one. The following list should prepare you for the necessary testing. ph meter Chemets Sulfide Test Kit Drager Tube Kit H 2 S air monitor Dissolved Oxygen meter BioHealth Wastewater Test Kit The ph meter will allow you to determine the amount of H 2 S available in the water stream that can evolve. The Chemets kit is a convenient method to determine the total sulfide content in the water stream. This can be used to determine the initial ProSweet feed rate. When servicing an odor control program, use a continuous air monitor to measure the ambient H 2 S levels -- this method will indicate the ultimate effectiveness of the program. Use the Drager Tube Kit when running bench tests. This is a good way to show the effectiveness of a product and to determine the initial feed rate. Use the Dissolved Oxygen meter to determine if anaerobic conditions exist. Anaerobic conditions can lead to biological sulfate reduction and production of H 2 S. The BioHealth Test Kit is a technology based on measurement of Adenosine Triphosphate (ATP). This can be used to monitor anaerobic biological activity, especially in situations where you would not expect to have any aerobic biological activity. Primary Clarifier Primary Sludge Press Secondary Sludge Press 50 to 150 ppm 25 to 50 ppm 25 to 50 ppm 2 to 3 ppm 1 to 2 ppm < 1 ppm cp81.docx Page 7