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A Summary of Information Regarding the

Presence, Fate, and Concentrations of

Reduced Sulfur in Pulp and Paper Mill Treated Effluents

NCASI 2015

Prepared in Support of EPA Effluent Limitations Guidelines Evaluations

BACKGROUND

Beginning with the 2012 reporting year, industrial releases of hydrogen sulfide (H2S) were required to be

estimated and reported under Section 313 of the Superfund Amendments and Reauthorization Act (SARA

313). EPA uses the data reported under SARA 313 for a variety of purposes. Among these uses, EPA’s

Office of Water considers reported SARA 313 releases to water as part of its activity to review and plan

for updating effluent limitations guidelines (ELGs) for industrial categories. Like other industrial

categories, the pulp and paper industry reported H2S releases beginning in 2012. In response to an EPA

request related to its recurring ELG review activities, NCASI has prepared this summary of information

related to the presence, fate, and concentrations of reduced sulfur, including H2S, in treated mill

wastewaters.

INTRODUCTION

Hydrogen sulfide (H2S) is one of several forms of reduced sulfur. It can occur in pulp and paper mill

wastewater from two sources. At some chemical pulp mills, use and recovery of pulping liquors can

produce H2S and some H2S precursors, and certain wastewaters from the liquor recovery cycle can

contribute H2S to mill wastewater. H2S may also be formed in wastewater collection or treatment systems

by way of biologically mediated reduction of sulfate (SO4=) or other oxidized sulfur species.

H2S is both volatile and readily converted to chemical forms that are rapidly oxidized in the presence of

oxygen in wastewater. Because most wastewater treatment systems used in the forest products industry

employ aerobic biological treatment or include aerobic zones prior to the release of treated wastewaters,

H2S does not occur at high concentrations in properly treated wastewaters. It is possible, however, for

trace levels of H2S to be present in some treated effluents.

ANALYZING FOR HYDROGEN SULFIDE

Analytical techniques for measuring H2S at low concentrations in aqueous samples are not

straightforward, owing to both the reactivity of H2S and the manner in which wastewater properties affect

the speciation of reduced inorganic sulfur. In particular, it is important to distinguish H2S from total

sulfide (TS) measurements. Sulfide determinations are, by necessity, method defined because sulfide

assumes various forms depending on sample pH (Figure 1), temperature, dissolved oxygen, ionic

strength, and the biological constituents present in the matrix. TS is defined herein as dissolved H2S plus

hydrosulfide ion (HS-) plus acid soluble metallic sulfide and sulfide weakly associated with organics in

the sample. Metallic and organic sulfides may be present in pulp mill wastewaters and may (but do not

always) serve as sources of H2S. The majority of metallic sulfides would not be significant sources of

H2S under normal wastewater treatment facility operating conditions, as they are mostly insoluble in

water and require a strong acid to increase the solubility (Moeller et al. 1980). Sulfide (S2-) would also be

considered a component of TS, but is present in negligible amounts unless sample pH is extremely basic.
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Figure 1. Dependence of Sulfide Dissociation on Liquid Phase pH

Analytical determination of H2S is challenging due to its absorptive, adsorptive, photo reactive, volatile,

biologically active, and oxidative properties, which can lead to imprecise measurements.

As noted, H2S concentrations in treated mill effluents will be less than TS concentrations, in part due to

the likely presence of metal and organic sulfide complexes disassociated during the analytical procedure.

NCASI has recently developed a sampling system that allows for measurement of “dissolved” sulfides in

aqueous samples (defined as sulfide forms passing a 0.7 μm filter) (NCASI 2015). H2S in an effluent

would be in the dissolved form, as would certain other reduced sulfur substances. Measurements of

dissolved sulfides, therefore, would represent a lesser overestimate of H2S concentrations. During

method development, only one treated final effluent was sampled; dissolved sulfides were less than the

analytical detection limit of 0.01 mg S/L for that sample. A comparison of total and dissolved sulfides in

various untreated or partially treated wastewater samples collected at five mills indicated that dissolved

concentration ranged from 0 to 82% of the TS concentration.

HYDROGEN SULFIDE IN WASTEWATER TREATMENT

As illustrated in Figure 2, several processes in a pulp and paper mill treatment system can impact the

concentration of H2S in final effluent. For example, aerobic biological activity can convert H2S to

oxidized species (e.g., SO4

2-

), while anaerobic activity can generate H2S from oxidized species.

Because sulfides can be both generated and destroyed in wastewater treatment systems, it is not possible

to make generic statements about the “treatability” of H2S across treatment systems of various types.

However, because H2S is both volatile and easily oxidized, and because most wastewater treatment

systems used in the forest products industry employ aerobic biological treatment or include aerobic zones

prior to release of treated wastewaters, it is likely that most H2S entering forest products industry

wastewater treatment systems is oxidized or otherwise removed from the wastewater prior to discharge of

treated final effluent. Table 1 shows available data comparing inlets and outlets from biological treatment

at four mills using aerobic treatment. Average values are shown where multiple measurements were

made. For calculating averages, non-detect values were replaced with one-half of the detection limit.

Removal at these mills was >98%, mostly due to oxidation in the wastewater treatment system (NCASI

2008).
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Figure 2. Sulfur Reactions in an Aerated Stabilization Basin Wastewater Treatment Plant

Table 1. Removal of Total Sulfide across Wastewater Treatment Systems (NCASI 2008)

Facility

Inlet

Stream

Total Sulfide

Concentration

(μg S/L)

Outlet

Stream

Total Sulfide

Concentration

(μg S/L) Removal

Mill 1 ASB inlet 2,570 ASB outlet 48 98.1%

Mill 2 ASB inlet (94.6% of flow) 511 ASB outlet 24.8 99.6%

Condensates (5.4% of flow) 102,825

Mill 3 ASB inlet 19,340 ASB outlet 91.5 99.5%

Mill 4 AS inlet 6,846 AS outlet 19 99.7%

ASB = aerated stabilization basin treatment

AS = activated sludge treatment

TOTAL SULFIDE IN TREATED FINAL EFFLUENTS

NCASI has previously measured TS levels in biologically treated final effluents from 25 pulp and paper

mills in the US and Canada. As noted, these data are not equivalent to concentrations of H2S in final

effluents because H2S is only one component of TS. TS data presented in this section were obtained by

analyzing final treated effluent samples using NCASI Method RSC-02.02 (NCASI 2007). The method

utilizes direct aqueous injection gas chromatography coupled with a pulsed flame photometric detector

(GC/PFPD). The concentration of sulfide measured using this method represents the total amount of

sulfide in the sample that is volatile at pH 2.5. It is believed that this includes all freely dissolved sulfide

(H2S, HS-) plus sulfide weakly associated with either dissolved organic matter or certain transition metals.

Importantly, because biological treatment plants are typically operated at a neutral pH (much higher than

2.5), these numbers represent a high biased estimate of H2S levels in the samples assessed. NCASI

Method RSC-02.02 is calibrated from 20 to 1000 μg S/L. The nominal method detection limit for TS is

32 μg S/L (0.03 mg/L). TS concentrations were converted from μg S/L to mg/L by multiplying the

μg S/L concentration by 1.06 (mass of hydrogen sulfide/mass of sulfur) and dividing the result by 1000

(1000 μg/1 mg).
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The quality of the data generated was assured by method protocols including calibration checks, daily

blank measurements, sample duplicate analyses, and matrix spiked samples with each set of samples

analyzed on a given day. In addition, surrogate spike recoveries were determined within each matrix

tested. All aspects of the quality control and assurance program were met or exceeded for the data

reported herein. Table 2 provides information regarding mill furnish, process type, wastewater treatment

plant (WWTP) type, number of samplings, and treated effluent concentrations.

Table 2 Mill Wastewater Treatment Plants Sampled

and Final Effluent Total Sulfide Concentrations (NCASI 2012)

Mill

Code FurnishaProcess Typeb WWTPc

Sampling

(n =)d

Average Conc.

(mg/L)

A SW kraft ASB 1 0.24

B SW, deink GW, TMP, recycle AS/ASB 1 ND

C SW, OCC kraft ASB 3 0.09

D SW kraft, dissolving kraft ASB 3 0.10

E SW, OCC kraft, recycle ASB 1 ND

F SW/HW, OCC kraft, recycle ASB 1 0.16

G SW, HW kraft ASB 2 0.13

H SW, HW, deink kraft, recycle ASB/AS 2 0.18

I SW kraft ASB 2 0.04

J HW, SW, OCC kraft, recycle, NSSC ASB 2 0.05

K HW, SW kraft ASB 1 ND

L SW, deink TMP, recycle AS 3 0.10

M HW, OCC NSSC, recycle ASB/AS 2 0.24

N HW/SW kraft AS 1 ND

O SW kraft ASB 2 0.04

P SW kraft ASB 8 0.28

Q SW/HW, OCC kraft, recycle ASB 1 0.23

R SW/HW kraft ASB 1 ND

S SW/HW kraft ASB 2 0.07

T SW/HW kraft ASB 1 0.03

U SW/HW kraft ASB 2 0.29

V SW/HW kraft AS/ASB 2 ND

W SW kraft, GW ASB 1 0.10

X SW, HW, not deinked kraft, NSSC, recycle ASB 1 0.06

Y SW kraft ASB 1 0.04

Mean Concentration 0.10

Range of Concentrations 0.02-0.29

a HW = hardwood; SW = softwood; OCC = old corrugated containers

b GW = groundwood; TMP = thermo-mechanical pulping; NSSC = neutral sulfite semi-chemical

c AS = activated sludge; ASB = aeration stabilization basin

d

n=1 provides concentration for a single analysis and sample date, n>1 provides mean of multiple sampling

dates and analyses

ND = not detected above method detection limit

Table 2 represents data from mills that were investigating odor-related issues in which sulfide may have

been a factor. It is, therefore, more representative of mills with higher levels of sulfide in wastewater and

effluent. An average is presented for treated mill effluents sampled more than once. TS concentrations

were below the detection limit of the method for 6 of the 25 mills sampled. In cases where detect and
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non-detect values were found in effluents sampled more than once, non-detects were replaced with onehalf of the method detection limit for purposes of calculating an average concentration.

Figure 3 is a plot of the data from Table 2. TS concentrations in biologically treated final effluents ranged

from non-detect to 0.29 mg/L with an average concentration of 0.10 mg/L and a median concentration of

0.07 mg/L. Non-detect values are plotted at 0.01 mg/L, approximately one-half of the detection limit.

For reasons noted earlier, concentrations of H2S in these effluents would have been less than or equal to

the TS concentrations shown.

Figure 3. Final Effluent Total Sulfide Concentrations at 25 Pulp and Paper Mills (NCASI 2012);

[average concentration of 0.10 mg/L (solid line); median concentration of 0.07 mg/L (dashed line);

Non-detect values plotted at 0.01 mg/L (dotted) line]

SUMMARY

Hydrogen sulfide (H2S) was reportable under SARA 313 beginning in 2012 and the substance can occur

at some pulp and paper mills. H2S is one of several forms of reduced sulfur that are collectively referred

to as total sulfides (TS). The information presented herein focuses on the presence, fate, and treated final

effluent concentrations of H2S and total sulfides in pulp and paper mill wastewaters. Key points include:

 H2S is not directly measureable in aqueous matrices because of its variable presence depending on

pH, temperature, ionic strength, and organic and inorganic complexes;

 TS measurements are defined by the measurement method used and yield a high biased estimate of

H2S;

 measurements for TS made at 25 mills showed that biologically treated final effluent concentrations

ranged from non-detect to 0.29 mg/L (average 0.10 and median 0.07 mg/L), and that 6 of the 25

samples were non-detect at a nominal detection limit of 0.03 mg/L;

 removal of total sulfide across aerobic biological treatment at 4 mills was greater than 98%; and

 a new sampling system developed by NCASI may allow measurement of dissolved sulfides, which

would lessen the high bias in estimates of H2S from TS data.
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REFERENCES

Moeller, T., Bailar, J., Kleinberg, J., Guss, C., Castellion, M., and Metz, C. 1980. Chemistry with

inorganic qualitative analysis. New York: Academic Press Inc.

National Council for Air and Stream Improvement, Inc. (NCASI). 2015. Development and evaluation of

a method for measuring dissolved sulfide in pulp and paper mill wastewaters. Technical Bulletin (in

press). Research Triangle Park, NC: National Council for Air and Stream Improvement, Inc.

National Council for Air and Stream Improvement, Inc. (NCASI). 2012. The presence, fate, and

ecological significance of hydrogen sulfide in pulp and paper mill effluents. Technical Bulletin

No. 997. Research Triangle Park, NC: National Council for Air and Stream Improvement, Inc.

National Council for Air and Stream Improvement, Inc. (NCASI). 2008. Emissions of reduced sulfur

compounds and methane from kraft mill wastewater treatment plants. Technical Bulletin No. 956.

Research Triangle Park, NC: National Council for Air and Stream Improvement, Inc.

National Council for Air and Stream Improvement, Inc. (NCASI). 2007. Development and application

of a method for measuring reduced sulfur compounds in pulp and paper mill wastewaters. Technical

Bulletin No. 933. Research Triangle Park, NC: National Council for Air and Stream Improvement,

Inc.