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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 ( H2 S ) 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 e ffluent limitations guidelines (ELGs) for industrial categories. Like other industrial categories, the pulp and paper industry reported H2 S 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 H2 S , in treated mill wastewaters. INTRODUCTION Hydrogen sulfide ( H2 S ) 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 recov ery of pulping liquors can produce H2 S and some H2 S precursors, and certain wastewaters from the liqu or recovery cycle can contribute H2 S to mill wastewater. H2 S may also be formed in wastewater collection or treatment systems by way of biologically mediated reduction of sulfate (SO4=) or other oxidized sulfur species. H2 S is both volatile and readily converted to chemical forms that are rapidly oxidized in the pr esence 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, H2 S does not occur at high concentrati ons in properly treated wastewaters. It is possible , however , for trace levels of H2 S to be present in some treated effluents. ANALYZING FOR HYDROGEN SULFIDE Analytical techniques for measuring H 2 S at low concentrations in aqueous samples are not straightforward, owing to both the reactivity of H 2 S and the manner in which wastewater properties affect the speciation of reduced inorganic sulfur. In particular, it is important to distinguish H 2 S from total sulfide (TS) measurements. Sulfide determin ations 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 H2 S p lus 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 H 2 S. The majority of metallic sulfides would not be significant source s of H2 S 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 (S 2

-) would also be considered a component of TS , but is present in negligible amounts unless sample pH is extremely basic.2 Figure 1 . Dependence of Sulfide Dissociation on Liquid Phase pH Analytical determination of H2 S is challenging due to its absorptive, adsorptive, photo reactive, volatile, biologically active, and oxidative properties, which can lead to imprecise measurements. As noted, H 2 S 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 sul fide forms passing a 0.7 μm filter) (NCASI 2015). H2 S 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 H 2 S concentrati ons. 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 c omparison 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 H 2 S in final effluent. For example, aerobic biological activity can convert H 2 S to oxidized species (e.g., SO 4 2

-), while anaerobic activity can generate H 2 S from oxidized species. Bec ause sulfides can be both generated and destroyed in wastewater treatment systems, it is not possible to make generic statements about the “treatability” of H2 S across treatment systems of various types. However, because H2 S 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 H2 S 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 wast ewater treatment system (NCASI 2008).3

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 ( μ g S/L) Outlet

Stream Total Sulfide

Concentration ( μ g 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 H 2 S in final effluents because H2 S 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 sulf ide 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 (H2 S, HS -) plus sulfide weakly associated with either dissolved organic matter o r 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 H2 S 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).4

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 Furnish a Process Type b WWTP c 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 multi ple sampling dates and analyses ND = not detected above method detection limit
Table 2 represent s 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 f or treated mi ll effluents sampled more than once. TS concentration s were below the detection limit of the method for 6 of the 25 mills sampled. In cases where detect and5

non- detect values were found in effluents sampled more than once, non

- detects were replaced with one half of the method detection limit for purposes of calculating an average concentration. Figure 3 is a plot of the data from Table 2

. TS concentration s 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 H 2 S 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 ( H2 S ) was reportable under SARA 313 beginning in 2012 and the substance can occur at some pulp and paper mills. H2 S is one of several forms of reduced sulfur that are collectively referred to as total sulfides (TS). The information presented here in focuses on the presence, fate, and treated final effluent concentrations o f H2 S and total sulfides in pulp and paper mill wastewaters. Key point s include:

 H2 S 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 H2 S ;

 measurements for TS made at 25 mills showed that biologically treated final effluent concentratio n s 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 H2 S from TS data.6

REFERENCES Moeller, T., Bailar, J., Kleinberg, J., Guss, C., Castellion, M., and Metz, C. 198 0. Chemistry with inorganic qualitative analysis

. New York: Academic Press Inc. National Council for Air and Stream Improvement, Inc. (NCASI). 201 5

. Development and evaluation of a method for measuring dissolved sulfide in pulp and paper mill wastewate rs . 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 com pounds 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. Developm ent 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.