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 monitoring report (DMR) data and EPCRA Section 313 Toxic Release 

Inventory (TRI) data. 

ERG’s review of DMR data found that pulp and paper mills, petroleum refineries, textile mills, 

and oil and gas recovery facilities were discharging sulfide at levels significantly above the 

National Water Quality Criteria (NWQC) standard of 2 ug/L. Table 1 shows the sulfide 

concentrations measured from each of these industrial sectors along with the average facilitylevel flows and sulfide loadings to receiving streams. The data in Table 1 shows that each of the 

industrial sectors discharge average 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 Effluent Loadings From Four Industrial Sectors2

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, Number 274, pages 68599 -68615, December 28, 2009 2

 USEPA Discharge Monitoring Report Data, 2009 
Memorandum 

May 31, 2011 

Page 2 

Sulfide Chemistry

Sulfide is an anion of sulfur in its lowest oxidation state of minus 2 (S2−

). The dianion S2− exists 

only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H2S or 

alkali metal salts such as Li2S, Na2S, and K2S in the presence of excess hydroxide ions. The ion 

S2− is exceptionally basic with a pKa > 14. It does not exist in appreciable 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 hydrogen sulfide ion, hydrosulfide ion, 

sulfhydryl ion, or bisulfide ion. At still lower pH's (<7), HS−

 converts to H2S, hydrogen sulfide, 

as shown by the equation below.4

 

H2S  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 H2S.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.jsp 
Memorandum 

May 31, 2011 

Page 3 

solutions of transition metals cations react with sulfides to precipitate solid metal sulfide salts. 

The metal sulfide salts typically have very low solubility in water. 

Sulfide Treatment Options

The following discusses various treatment options for sulfide in industrial wastewaters. Although 

these treatment options presented below can remove sulfide from wastewater, 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 dissolved gasses such as hydrogen sulfide.6

Aeration processes are, generally, used in two types of water applications: air stripping - the 

process in which gas is removed from water, and 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 equilibrium. The Henry's Law constant is the ratio of the equilibrium 

concentration of a particular contaminant in air to its concentration in water. Thus, a higher 

Henry's Law constant indicates a greater tendency 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 20oC8, 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 thickness of film at air-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 classified 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 types of waterfall aerators 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 sulfide, and carbon dioxide. Packed towers can 

also be sized for flows ranging from a few thousand gallons per day to millions of gallons per 

day. The packed tower air strippers 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 countercurrent through the bottom of 

the tower. The large surface area provided by the packing material allows more liquid-gas 

transfer compared with other air 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 rectangular concrete tank in which perforated 

pipes, porous diffuser tubes or plates are installed. Compressed air is forced through these pipes 

 

6 USEPA Drinking Water Treatability Database, 

https://iaspub.epa.gov/tdb/pages/treatment/treatmentOverview.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.pdf
Memorandum 

May 31, 2011 

Page 4 

to produce air bubbles that rise through water, producing turbulence resulting in effective waterair mixing. Bubble aerators are effective in removing carbon dioxide, VOCs, gasoline 

components, hydrogen sulfide, methane, and radon from contaminated groundwater. They are 

mostly used by small water systems with low flow 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, water column depth, and air venting conditions. 

(3) Mechanical Aeration: Mechanical aeration employs motor driven impellers alone or in 

combination with air injection devices. They are also 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 through biological degradation. The mechanical 

aerators can be installed as surface aerators (on the float) or submerged (below the water level) 

aerators. 

(4) Pressure Aeration: Pressure aerators are typically used for oxidizing iron and manganese. 

There are two basic types of pressure 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 sulfide depends upon the aeration method selected, 

the pH of the water which dictates the applicable Henry's law constant, 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 that hydrogen sulfide is not destroyed, but is 

instead discharged as an air emission. 

Hydrogen Peroxide Oxidation

Hydrogen peroxide (H2O2) controls sulfide by oxidation to either elemental sulfur or sulfate ion 

depending on the pH of the wastewater. Hydrogen peroxide oxidation of sulfide has been 

demonstrated at both industrial facilities9 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 H2O2 is underdosed) or a white colloid (with complete 

oxidation). Colloidal elemental sulfur can be removed from the wastewater following H2O2

oxidation by flocculation with an anionic polymer and filtration. The reaction below shows the 

oxidation of hydrogen sulfide to elemental sulfur under neutral or slightly acid conditions. 

H2S + H2O2 ------> S0 + H2O 

The stoichiometry calls for 1.0 lb H2O2per lb H2S, and it is not unusual for efficiencies to 

approach 100%, particularly when the concentrations of other oxidizable 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 conversion 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 H2O2 and Ferric Iron Catalyst 

Reaction 

Time 

(minutes) 0 mg/L Fe3+ 1 mg/L Fe3+ 3 mg/L Fe3+ 5 mg/L Fe3+ 10 mg/L Fe3+ 25 mg/L Fe3+

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 

H2O2 dosed at 1.2:1 wt.% based on sulfide, pH 7.0, and 20oC in demineralized water. 

Under alkaline conditions (pH > 9.2), sulfide is converted to sulfate by H2O2 as shown by the 

reaction below. 

S2- + 4H2O2 -----> SO4

2- + 4H2O 

The stoichiometry calls for 4.25 lbs H2O2 per lb S2- , and again it is not unusual for reaction 

efficiencies to approach 100 percent, provided that the H2O2 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 H2O2 at alkaline pH. Consequently, as the pH increases above 9-10, 

there is generally little benefit to catalyzing the reaction. 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 H2O2 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 <1 
Memorandum 

May 31, 2011 

Page 6 

 H2O2 dosed at 5.0:1 wt.% based on sulfide and 20oC in demineralized water. 

Hydrogen peroxide oxidation of sulfide 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 302oF to 617oF. Gauge pressures of 300 to 3,000 lbs/in2 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 batch 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 demand 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 concentrations 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 production 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 pressure = 1,900 lb/in2, waste liquor flow = 8 gal/min 

Although the wet oxidation process has been demonstrated for treatment of sulfide in industrial 

wastewater, the system flows are far less than those provided in Table 1. Published data indicates 

systems are typically operated up to flows of approximately 70 gpm; 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 removal 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. Strong-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 sulfide concentrations were reduced to less than 0.2 

mg/L. Pilot plant trials were conducted in Pasco County, Florida to investigate 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, resins 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 Table 1 may be prohibitive. 

Conclusions 

The technologies presented in this memo will remove sulfide, however some are more applicable 

to the target industries than others. Wet oxidation and ion exchange are expensive and typically 

used on much smaller flows. Air stripping using either a tank-based system or packed tower is 

effective, however the hydrogen sulfide is discharged to the atmosphere rather than destroyed. 

Hydrogen peroxide oxidation removes 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.