ENVIRONMENTAL TECHNOLOGIES

Without negative side effects, new organic scavenger technology controls hydrogen sulfide odor in a variety of applications

BY CARLTON E. HAGEN and ROBERT W. HARTUNG, P.E.

New Chemical Treatment Method Controls Wastewater System Odor

While the control of air pollutants from various pulping and papermaking processes consumes considerable capital and operating expense (Table 1), those pollutants responsible for odor generation receive the most attention due to public awareness within communities located near mills. And, though many offending odors can be reduced by operational changes, others require some type of additional mechanical or chemical treatment.

Odors emanating from kraft mill operations are primarily due to generation of volatile reduced sulfur compounds (TRS), which consist of hydrogen sulfide (H2S), methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. However, H2S is the most commonly known and prevalent odorous gas. It has a characteristic rotten egg odor, is extremely toxic, and is corrosive to metals.

To combat the effects of H2S, a variety of chemical treatments have been applied in the past. However, many of these treatments have negative side effects, such as safety and handling problems, difficult application techniques, and production of inorganic sludge. To avoid these problems, a new approach to sulfide control, known as organic scavenger technology, has been developed.

This technique involves the use of an organic-based molecule that selectively reacts with any reduced sulfur compounds that have acidic protons. As case studies indicate, organic scavenger technology has proven an effective and economical alternative that avoids damaging problems associated with other chemical treatments, such as increased pH levels, excessive sludge, and corrosion damage to equipment.

SOURCES OF ODOR IN PAPER MILLS. TRS compounds are generated as a direct result of the kraft pulping and chemical recovery process. In aqueous waste streams, the primary sources of TRS compounds are digester blow steam condensate, turpentine condenser underflow, and evaporator condensate. In mills where hydrosulfite brightening is practiced, sulfides may also be generated. Table 2 shows the typical reduced sulfur gas concentrations from various kraft mill sources.

In addition to direct process sources of TRS, the other primary contributor of hydrogen sulfide odors from paper mill 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:

SO4 = + 2 C + H2O 2 HCO3- + H2S

Most sulfate reduction occurs within a biological slime layer that protects the sulfate reducers from oxygen present within the bulk waste stream itself. The rate at which hydrogen sulfide is generated depends on the concentration of sulfate and organics in the waste stream, the level of dissolved oxygen, pH, temperature, and the velocity of the water.

FORMATION OF H2S. The conditions leading to H2S formation generally favor the production of other malodorous organic compounds such as mercaptans, thiophenol, and thiocresol. Investigations of the conditions favoring H2S formation can also help to quantify the potential for odor generation from other compounds. Thus, solving H2S odor problems can often solve other odor problems as well.

H2S dissolves in water and dissociates according to the following reactions:

H2S HS- + H+

HS- S= + H+

Figure 1 shows the distribution of H2S and HS- species as a function of pH. The relative H2S concentration increases with decreasing pH. At a pH of 7.0, H2S represents 50% of the dissolved sulfides present, while at a pH of 6.0, more than 90% of the dissolved sulfides is in the form of H2S. If part of the dissolved H2S escapes to the atmosphere, the remaining dissolved sulfide will be divided between H2S and HS- in the same proportion as before because the equilibrium re-establishes itself almost instantly.

The distinction between the types of sulfide species is significant, because only the H2S can escape from solution and create odor and corrosion problems. It is important, therefore, to quantify the total and dissolved sulfides present and the pH of the wastewater.

TOXICITY AND SAFE PRACTICES. H2S is an acutely toxic gas. It is heavier than air, colorless, and has a characteristic rotten egg smell at low concentrations. But as the levels of H2S increase, workers are 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 come suddenly, followed by death if there is not a prompt rescue.

Even very low concentrations of H2S can cause serious health hazards. Death has resulted from concentrations of 300 ppm by volume in air.1 Such concentrations can be obtained in an enclosed chamber with high turbulence from wastewater containing
2 mg/l of dissolved sulfide at a pH of 7.0. Based on Henry's Law, Figure 2 Fshows H2S levels in the atmosphere (closed vessel) in equilibrium with the given concentrations of H2S in the water at the respective wastewater temperatures.

Preliminary monitoring program. Normally, repeated odor complaints by workers or from a community 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 supplied by Chemetrics or Hach should be used for wastewater testing. Normally, odor problems will occur when dissolved sulfide levels are 1 to 1.5 mg/l or greater.

The following locations should be checked for sulfide content:

1. Lift stations. Sample wet-well influents, pump discharges, and ends of pressure mains to determine sulfide balance.

2. Gravity sewers. Check sewers, since turbulence may release H2S gas and cause corrosion of manhole chambers.

3. Sludge holding tanks. Residual H2S and anaerobic activity will cause problems in holding tanks.

4. Areas of turbulence and long detention times. Sample locations of turbulence where sulfide may be released. Measurement of dissolved H2S before and after the point of turbulence can indicate the quantity of H2S released to the atmosphere.

Recycle streams such as supernatants from thickeners, digesters, and other sludge treatment processes should also be sampled, as these can contain high concentrations of H2S that can result in severe odor and/or corrosion problems at the point of sidestream return. Air samples from enclosed spaces exposed to wastewater may also be collected to determine the severity of odors.

Analyses. The wastewater should be analyzed for the following:

1. Sulfates (SO4=), mg/l

2. pH

3. Total sulfides (TS), mg/l

4. Biochemical oxygen demand (BOD), mg/l

5. Dissolved oxygen (DO), mg/l

6. Temperature, C

7. Suspended solids (TSS), mg/l.

CHEMICAL CONTROL METHODS. Historically, there have been a number of commonly used chemical techniques employed for control of sulfides in wastewater systems. These include the use of oxidizing agents, metal salts, biomodifiers, organic scavengers, strong alkalis, and masking agents. Table 3 summarizes the benefits and limitations of some of the chemical options discussed in this section.

Oxidizing agents. A number of different oxidizing agents have been used for the destruction of H2S odors. While in some cases these approaches have been successful, all oxidizers must overcome the demand in the waste stream. In the presence of organics-such as those occurring in wastewater streams-the high demand can result in unfavorable economics for the use of oxidizers.

Chlorine can be added to wastewater either as hypochlorite or chlorine gas. If excess chlorine is added to wastewater containing sulfide, the sulfide is oxidized to sulfate according to the following:

H2S + 4Cl2 + 4H2O 8 HCl + H2SO4

Note that the resulting acids will decrease the pH of the wastewater. Also, the sulfide is oxidized to sulfate which can be re-used by sulfate--reducing bacteria, resulting in more H2S production. Chlorine donating materials are rarely used because of their safety and handling problems and the probability of THM formation.

Chlorine dioxide, like chlorine, will react quickly and completely with H2S.

8ClO2 + 5H2S + 4H2O 5SO4= + 8Cl- + 18H+

Note the similar results to chlorine addition.

Hydrogen peroxide chemically oxidizes H2S according to the following reactions:

pH < 8.5: H2O2 + H2S S + 2H2O

pH > 8.5: 4H2O2 + S= SO4= + 2H2O

At pH < 8.5, the stoichiometric H2O2 requirement is 1g H2O2/1g H2S. 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 H2S release to the atmosphere. For effective removal of dissolved sulfides, the metallic sulfide formed must be highly insoluble.

Iron salts are the most commonly used salt for sulfide control. The ferrous ion reacts with sulfide as shown in the following:

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 with the reaction of either one alone.2 The reaction of the mixed iron salts was hypothesized to occur as follows:

Fe++ + 2Fe+++ + 4HS- Fe3S4 + 4H+

The primary disadvantage to this approach is the production of an inorganic sludge.

Biomodifiers. The use of nitrate to control odors and hydrogen sulfide in wastewater collection systems has gained in popularity. 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 as an oxygen source over sulfate when available. When nitrate is present, these sulfide-producing bacteria use it to the exclusion of sulfate. This results in the production of nitrogen gas and other nitrogenous compounds rather than sulfide. However, raw wastewater does not normally contain any nitrate, so it must be artificially added.

Organic scavenger technology. A new approach to sulfide control involves the use of an organic-based molecule that will selectively react with any reduced sulfur compounds that have acidic protons. There is no demand due to the presence of other contaminants, so that the application dosage is proportional to the level of sulfide present. Therefore, many malodorous sulfur compounds can be economically treated with this approach even though other contaminants are present.

This organic scavenger forms a soluble complex with the sulfide present. Therefore, no sludge is produced that could add to the dewatering load and disposal cost to the mill. In addition, both the scavenger and its reaction product are pH neutral and therefore have no major effect on the waste stream pH, so post pH adjustment is not required. Also, implementation of the program is easy to accomplish.

The selection of single or multiple feed-points is site specific. Sulfide-containing streams should be identified, in addition to locations with high H2S concentrations in the air. The feed-point should be located upstream from the affected areas. Scavengers can be applied to various locations, such as full flowing pipe lines, open channels, sludge lines, or sludge holding tanks.

Strong alkalis. Increasing the pH reduces the proportion of dissolved H2S in the H2S-HS- equilibrium. For example, at a pH of 7.0, equal concentration of dissolved H2S and HS- exist at equilibrium, while at a pH of 8.0, only about 10% of the dissolved sulfide exists as H2S. Since dissolved H2S is the only form that 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- form.

Masking agents. The first step in controlling odor always should be the removal of the source. If the removal of the source is not possible (as in a kraft mill), chemical control of the sulfide species in the water phase is acceptable and quite common. However, use of masking agents to attack an H2S problem is ill-advised because it does not mitigate the adverse health effects of the gas.

TECHNOLOGY APPLICATION. As the following case studies indicate, organic scavenger technology is effective for a variety of applications.

Sludge plant. At a northern recycled newsprint mill, an odor problem from the sludge plant impacted both the employees and the surrounding community. It was determined that hydrogen sulfide was the offending source of the odor.

Hydrogen sulfide levels in the air were measured between 70 and 100 ppm at the sludge press. Mill operators also found 300 ppm to 500 ppm of H2S in their holding tank, and significant odors throughout the entire mill and the neighborhood.

Initially, ferric chloride was used to control the odor, but was difficult to handle and resulted in corrosion of chemical feed equipment. Results were not effective, and use of ferric chloride was excessive. Based on jar test data, the organic scavenger chemistry was selected and feed was initiated. Feed was injected at the Moyno pump just before the material mixes in the sludge. Odor reduction occurred, and H2S levels dropped very quickly. H2S levels dropped to almost undetectable levels at the sludge press and were slightly detected in the holding tank. No odor complaints were received around the mill at all.

Waste treatment facility. A Canadian newsprint mill was receiving complaints from employees and the neighboring community due to H2S generation at their waste treatment facility. In the waste treatment system layout, waste plant influent is split between two parallel primary clarifiers, and 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.

Air and water samples determined that sludge was the source of odor generation. Odors originated both from the primary clarifier itself and the sludge dewatering equipment. Ambient H2S levels between 50 and 150 ppm (mg/l) were measured at the primary clarifier outlet and the overflow lift station. Around the sludge presses, H2S 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 provided little odor generation relief. It was decided not to inject iron salts into the primary clarifier inlet because of the risk of iron carryover into the aeration basin. Also, 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 organic scavenger was then applied to the influent of the primary clarifiers. In less than 24 hours, odors were under control as shown in Table 4. Over time, not only were the odors eliminated, but also other benefits to system operation were realized. The growth of filamentous bacteria in the secondary clarifier was reduced, creating a faster-settling sludge. Also, it was assumed 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 addition of chlorine and defoamer to the clarifier to combat the excessive growth of filamentous bacteria was eliminated. Furthermore, because of better quality solids, polymer requirement for the belt press dewatering declined by 20%.

Liquor spills and dumps. At a northwestern kraft mill, the organic scavenger technology is used to reduce H2S odors that are generated during liquor spills and liquor dumps. When liquor is spilled or dumped, it goes to a single containment sump where it is then pumped over to the primary clarifier. The pH of the wastewater increases enough so that an automatic acid feed is required. When the acid is added, there is a localized pH drop that results in the evolution of H2S gas.

In addition to a pH probe, there is also a conductivity probe. A pH and conductivity increase is an indication that a liquor spill or dump has occurred. Operators manually operate the scavenger pump, injecting the chemical into the wastewater. The H2S stays in solution and is not evolved as a gas. n

REFERENCES

1. Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment Plants, U.S. EPA Technology Transfer Series, 1985.

2. Pomeroy, R.D., and Bowlus, F.D., Progress Report on Sulfide Control Research, Sewage Works Journal, 1946, Vol. 18, No. 4, pp. 597-640.

CarlTON E. Hagen is marketing manager-U.S. pulp and paper industry and Robert W. Hartung, P.E., is product manager-water/wastewater treatment for BetzDearborn Water Management Group, Horsham, Pa.