MEMORANDUM June 17, 2011 Page 1



### DRAFT DELIBERATIVE MEMORANDUM

TO: CWA Section 304m Project Record

FROM: Elizabeth Sabol and Mark Briggs, ERG

DATE: July 15, 2011

SUBJECT: Sulfide Treatment Technologies

Under Section 304m of the Clean Water Act (CWA), EPA is required to identify categories of sources discharging toxic or non-conventional pollutants for which EPA has not published effluent limitations guidelines under section 304( b)(2) or New Source Performance Standards. 1 As part of the analysis to identify categories of sources, EPA has tasked ERG to review wastewater discharge monito ring report (DMR) data and EP CRA Section 313 Toxic Release Inventory (TRI) data.

ERG’s review of DMR data found that pulp and pa per mills, petroleum refineries, textile mills, and oil and gas recovery facilitie s were discharging sulfide at levels significantly above the National Water Quality Criteria (NWQC) standa rd of 2 ug/L. Table 1 shows the sulfide concentrations measured from each of these indus trial sectors along with the average facilitylevel flows and sulfide loadings to receiving stream s. The data in Table 1 shows that each of the industrial sectors discharge aver age sulfide concentrations above the NWQC standard, however pulp and paper mills and oil and gas facilities discharge more than ten times the sulfide when compared to petroleum refineries and textile mills.

Table 1. Sulfide Concentrations and Efflue nt Loadings From Four Industrial Sectors 2

Industrial Sector Effluent Sulfide Concentration Range (mg/L) Average Effluent Sulfide Concentration (mg/L) Average Facility

Flow (MGD) Average Facility Flow (gpm) Average Facility Sulfide Discharge (lbs/day) Pulp and Paper 0.048 – 2.1 1.08 8.81 6,110 79 Petroleum Refining 0.0004 – 0.47 0.06 3.4 2,360 2 Textiles 0.005 – 6.4 1.02 0.34 240 3 Oil and Gas 0.008 – 16.5 5.6 0.68 470 31 MGD: million gallons per day

1 United States Federal Register, Volume 74, Nu mber 274, pages 68599 -68615, December 28, 2009
2 USEPA Discharge Monitoring Report Data, 2009Memorandum May 31, 2011 Page 2

Sulfide Chemistry

Sulfide is an anion of sulfur in its lowest oxidation st ate of minus 2 (S 2

− ). The dianion S 2

− exists only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H 2 S or alkali metal salts such as Li 2 S, Na 2 S, and K 2 S in the presence of exce ss hydroxide ions. The ion S 2

− is exceptionally basic with a pK a > 14. It does not exist in apprec iable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH ) 3 . Instead, sulfide combines with protons to form HS − , which is variously called hydr ogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. At still lower pH's (<7), HS − converts to H 2 S, hydrogen sulfide, as shown by the equation below. 4

H 2 S  HS - + H + and HS -  S - + H + [CITE]

Figure 1 shows the relationship between hydrogen sulfide and sulfide at various pH’s. As indicated, at pH’s below 5.0, nearly 100 percent of sulfide is present as H 2 S. 5 CITE

Sulfides are moderately strong reducing agents. They react with oxygen in the air in elevated temperatures to form higher-valence sulfur salts, such as sulfates and sulfur dioxide. Aqueous

3 Meyer B, Ward K, Koshlap K, Peter L "Second dissociation constant of hydrogen sulfide" Inorganic Chemistry 1983, volume 22, pp. 2345.
4 Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the elements (2nd Edn.), Oxford:ButterworthHeinemann.
5 GE Power and Water, Handbook of Industrial Water Treatment, Chapter 04, Aeration. http://www.gewater.com/handbook/index.jspMemorandum May 31, 2011 Page 3

solutions of transition metals cations react with su lfides to precipitate solid metal sulfide salts. The metal sulfide salts typically have very low solubility in water.

Sulfide Treatment Options

The following discusses various tr eatment options for sulfide in industrial wastewaters. Although these treatment options presented below can remove sulfide from wa stewater, the actual effluent concentrations attainable are a function of treatment system design, which is beyond the scope of this memorandum.

Aeration and Air Stripping

Aeration is a common method for removal of di ssolved gasses such as hydrogen sulfide. 6 Aeration processes are, generally, used in two types of water applica tions: air stripping - the process in which gas is removed from water, an d aeration - the process in which air or oxygen is transferred to water. Henry's law describes the tendency of a constituent to transfer from the liquid to the gas phase at equili brium. The Henry's Law constant is the ratio of the equilibrium concentration of a particular contaminant in ai r to its concentration in water. Thus, a higher Henry's Law constant indicates a greater tende ncy of species to volatilize. Compounds having Henry’s Law constants above 10 atm/mole fraction are readily air strippable. 7 The Henry’s Law constant for sulfide ion is not published since it is very low, however the Henry’s Law constant for hydrogen sulfide is 468 at 20 o C 8 , which indicates that sulfide must first be converted to hydrogen sulfide by the addition of acid to pH 5 or below (see Figure 1) . High temperature and turbulence promotes gas transfer by reducing th ickness of film at ai r-water interface. The efficiency of aeration depends almost entirely on the amount of surface contact between the air and water. Method of aeration can be clas sified into four general categories.

(1) Waterfall Aeration: Waterfall aeration uses the desorption principle and accomplishes gas transfer by causing water to break into drops of thin films, increasing contact surface area between the air and water. The five common t ypes of waterfall aerator s are spray aerators, cascade aerators, multi-tray aerators, cone aerators, and packed tower air strippers. Packed tower air strippers are most common since they can be designed to remove specific amounts of solvents and volatile SOCs, VOCs, ammonia, hydrogen sulf ide, and carbon dioxide. Packed towers can also be sized for flows ranging from a few thousa nd gallons per day to millions of gallons per day. The packed tower air stripper s consist of a cylindrical tower, a packing material (usually plastic, steel or ceramic) and a centrifugal blower . The contaminated feed water is pumped into the top of the tower and blowers are used to introduce air coun tercurrent through the bottom of the tower. The large surface area provided by the packing material allows more liquid-gas transfer compared with other ai r stripping methods. The number of stages in the packed tower is determined by Henry’s Law constant and the tower loading rate.

(2) Bubble Aeration: Bubble aerators consist of a rectangul ar concrete tank in which perforated pipes, porous diffuser tubes or plates are insta lled. Compressed air is forced through these pipes

6 USEPA Drinking Water Treatability Database, https://iaspub.epa.gov/tdb/pages/treatment/treatm entOverview.do?treatmentProcessId=-346223903#
7 Freeman, H.M., Standard Handbook of Hazardous Waste Treatment and Disposal, 1989.
8 Center for Disease Control, ATSDR, Toxological Profile, http://www.atsdr.cdc.gov/toxprofiles/tp114-c4.pdfMemorandum May 31, 2011 Page 4

to produce air bubbles that rise through water, produci ng turbulence resulting in effective waterair mixing. Bubble aerators are effective in removing carbon dioxide, VOCs, gasoline components, hydrogen sulfide, methane, and ra don from contaminated groundwater. They are mostly used by small water systems with low fl ow rates. Air sparging is similar to bubble aeration except it does not involve the use of a diffusion mechanism. In air sparging, pressurized air is injected directly into the reservoir, well , or aquifer. The removal efficiency depends on the amount and pressure of air injected, wate r column depth, and ai r venting conditions.

(3) Mechanical Aeration: Mechanical aeration employs moto r driven impellers alone or in combination with air injection devices. They are al so installed at water reservoirs to control taste and odor resulting from hydrogen sulfide or in wastewater treatment ponds to increase dissolved oxygen levels promoting pollutant removals th rough biological degradation. The mechanical aerators can be installed as surf ace aerators (on the float) or submerged (below the water level)
aerators.

(4) Pressure Aeration: Pressure aerators are typically us ed for oxidizing iron and manganese. There are two basic types of pressu re aerators, where either compressed air is injected directly in pressurized pipeline or water is sprayed into the top of a closed tank while the tank is continuously supplied with compressed air.

The effectiveness of aeration for removing su lfide depends upon the aeration method selected, the pH of the water which dictates the applicable Henry's law cons tant, design factors such as air to water ratio, flow and loading rate, available area of mass transfer, temperature, and algae production. The major drawback to aeration is th at hydrogen sulfide is not destroyed, but is instead discharged as an air emission.

Hydrogen Peroxide Oxidation

Hydrogen peroxide (H 2 O 2 ) controls sulfide by oxidation to eith er elemental sulfur or sulfate ion depending on the pH of the wastewater. Hydroge n peroxide oxidation of sulfide has been demonstrated at both industrial facilities 9 and municipal wastewater treatment plants. At neutral or slightly acid pH conditions, the product of sulfide oxidation is predominately elemental sulfur which appears as a yellow colloid (if H 2 O 2 is underdosed) or a white colloid (with complete oxidation). Colloidal elemental sulfur can be removed from the wastewater following H 2 O 2 oxidation by flocculation with an anionic polym er and filtration. The reaction below shows the oxidation of hydrogen sulfide to elemental sulfur under neutral or slightly acid conditions.

H 2 S + H 2 O 2 ------> S 0 + H 2 O

The stoichiometry calls for 1.0 lb H 2 O 2 per lb H2S, and it is not unusual for efficiencies to approach 100%, particularly when the concen trations of other oxidi zable substances (e.g., thiosulfate) are low, and when the reaction is accelerated by a catalyst such as ferric iron. 10 The effect of the ferric iron catalyst on the rate of hydrogen sulfide c onversion to elemental sulfur is

9 U.S. Peroxide LLC, Re-Refinery Used Oil – Sour Water Treated with Hydrogen Peroxide, Case Study. 2011.
10 Water Environment Federation Proceedings, Anaerobic Digester Hydrogen Sulfide Removal, 2007.Memorandum May 31, 2011 Page 5

illustrated in Table 2. Table 2 shows that 252 mg /L of hydrogen sulfide can be reduced to less than 1 mg/L in 15 minutes at a ferric iron dose of 5 mg/L.

Table 2. Hydrogen Sulfide Oxidation by H 2 O 2 and Ferric Iron Catalyst

Reaction

Time (minutes) 0 mg/L Fe 3+ 1 mg/L Fe 3+ 3 mg/L Fe 3+ 5 mg/L Fe 3+ 10 mg/L Fe 3+ 25 mg/L Fe 3+

0 252 252 252 252 252 252

2 218 192 154 108 56 24

5 152 126 92 52 14 <1

10 104 54 30 5 <1

15 74 22 3 <1

30 28 9 <1

Hydrogen sulfide concentrations in mg/L H 2 O 2 dosed at 1.2:1 wt.% based on sulfide, pH 7.0, and 20 o C in demineralized water.

Under alkaline conditions (pH > 9.2), su lfide is converted to sulfate by H 2 O 2 as shown by the reaction below.

S 2- + 4H 2 O 2 -----> SO 4 2- + 4H 2 O
The stoichiometry calls for 4.25 lbs H 2 O 2 per lb S 2- , and again it is not unusual for reaction efficiencies to approach 100 percent, provided that the H 2 O 2 is added in a controlled fashion and the reaction medium is thoroughly mixed. This is due to the much faster reaction brought about by the increased reactivity of H 2 O 2 at alkaline pH. Consequently, as the pH increases above 9-10, there is generally little benef it to catalyzing the re action. Table 3 shows 258 mg/L of sulfide can be oxidized to sulfate at pH 9 in 15 minutes. Since sulfate is very soluble in water, no additional wastewater processing (e.g., filtration) is required following peroxide oxidation.

Table 3. Sulfide Oxidation to Sulfate with H 2 O 2 at Increasing pH

Reaction Time (minutes) pH 9 pH 10 pH 11 pH 12

0 258 258 258 258

2 162 102 36 22

5 76 26 2 <1

10 12 <1

15 <1Memorandum May 31, 2011 Page 6

H 2 O 2 dosed at 5.0:1 wt.% based on sulfide and 20 o C in demineralized water.

Hydrogen peroxide oxidation of su lfide at pH’s above 9.2 requires a peroxide dosing system and a reaction tank with a mixer sized to provide approximately 15 minutes of hydraulic residence time.

Wet Oxidation
Wet oxidation is an aqueous-phase oxidation process brought about when an oxidizable inorganic such as sulfide in water is mixed thoroughly with a gaseous source of oxygen (usually air) at temperatures of 302 o F to 617 o F. Gauge pressures of 300 to 3,000 lbs/in 2 are maintained to promote reaction and control evaporation. 11 In the wet oxidation process, sulfide is converted to sulfate.
Wet oxidation can be performed on either a batc h or continuous operation. Elements of the system include a high-pressure liquid feed pump, oxygen source (air compressor or liquid oxygen vaporizer), reactor, heat exchangers, and process regulators. Systems for hydrogen sulfide treatment are designed on oxygen dema nd and typically range from 0.17 to 1.7 lbs oxygen per gallon of wastewater fed.
Wet oxidation systems have been demonstrated to reduce sulfide concentr ations in refinery scrubbing liquors by more than 99.9 percent. At a refinery in Illinois, wet oxidation treating spent caustic scrubbing liquors from an ethylene pr oduction facility produced an effluent with a sulfide concentration of 0.7 mg /L as shown in Table 4.
Table 4. Wet Oxidation Results for Spent Caustic Scrubbing Liquor at a Refinery

Component Waste Scrubbing Liquor Wet Oxidized Scrubbing Liquor
pH 1.3 8.1
COD (mg/L) 32,600 700
Sulfide (mg/L) 12,300 0.7

Treatment Conditions: temperature = 580oF, reactor gauge pr essure = 1,900 lb/in2, waste liquor flow = 8 gal/min

Although the wet oxidation process ha s been demonstrated for treatm ent of sulfide in industrial wastewater, the system flows are far less than t hose provided in Table 1. P ublished data indicates systems are typically operated up to flows of approximately 70 gp m; nearly three times less than the flows predicted for the textile industry.

Ion Exchange

11 Freeman, H.M., Standard Handbook of Hazardous Waste Treatment and Disposal, 1989.Memorandum May 31, 2011 Page 7

Ion exchange is a proven technology for remova l of hydrogen sulfide from drinking water. For sulfide removal, resins are generally strong-base anion exchangers with quaternary ammonium groups which function over a broad pH range. Str ong-base anion exchange resins are regenerated using strong bases such as sodium hydroxide.

Anion exchange has been used to remove sulfide from drinking water in Florida. 12 Tests using raw groundwater with sulfide concentrations ranging between 0.94 mg/L and 2.64 mg/L were pumped through a packed-bed anion exchange resin at a loading rate of 2 gpm per cubic foot of bed volume. The results showed effluent sulfid e concentrations were reduced to less than 0.2 mg/L. Pilot plant trials were conducted in Pasco County, Florida to invest igate the effectiveness of magnetic ion exchange (MIEX) resin in removing sulfides from the County ground water supplies. The trial showed that very effective sulfide removal could be achieved along with a 50- 60% reduction in the TOC level. 13

Although anion exchange will remove sulfide from water, the primary downside is that chemical wastes are produced during resin regeneration, re sins are prone to fouling, and packed bed systems can plug. In addition, both the capital and operating costs for systems large enough to handle the flows provided in Ta ble 1 may be prohibitive.

Conclusions

The technologies presented in this memo will rem ove sulfide, however some are more applicable to the target industries than ot hers. Wet oxidation and ion excha nge are expensive and typically used on much smaller flows. Air stripping using e ither a tank-based system or packed tower is effective, however the hydrogen sulfide is discha rged to the atmosphere rather than destroyed. Hydrogen peroxide oxidation remove s sulfide, but sulfur metal may be generated at neutral and slightly acid pHs. Hydrogen peroxide oxidation at pH’s above 9.2 produces sulfate, however the pH must be raised to achieve treatment, and, then lowered to near neutral prior to surface discharge.

12 Contrino, C.R., Removal of Hydrogen Sulfide from Groundwater Using Packed Bed Anion Exchange Technology, Florida Water Resources Journal, November 2007.
13 Bourke, M., Use of Continuous Ion Exchange Process to Remove TOC and Sulfides From Florida Water Supplies. Orica Water Treatment, Inc., 2000.