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Influence of pipe material and surfaces on sulfide relatedodor and corrosion in sewers

Asbjørn Haaning Nielsen*, Jes Vollertsen, Henriette Stokbro Jensen,Tove Wium-Andersen, Thorkild Hvitved-Jacobsen

Section of Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark

a r t i c l e i n f o

Article history:

Received 13 May 2008

Received in revised form

4 July 2008

Accepted 7 July 2008

Available online 17 July 2008

Keywords:

Concrete corrosion

Hydrogen sulfide

Odor

Sewers

Sulfide oxidizing bacteria

* Corresponding author. Section of EnviroEngineering, Aalborg University, Sohngaards

E-mail address: ahn@bio.aau.dk (A.H. Nie0043-1354/$ – see front matter ª 2008 Elsevidoi:10.1016/j.watres.2008.07.013

a b s t r a c t

Hydrogen sulfide oxidation on sewer pipe surfaces was investigated in a pilot scale exper-

imental setup. The experiments were aimed at replicating conditions in a gravity sewer

located immediately downstream of a force main where sulfide related concrete corrosion

and odor is often observed. During the experiments, hydrogen sulfide gas was injected

intermittently into the headspace of partially filled concrete and plastic (PVC and HDPE)

sewer pipes in concentrations of approximately 1000 ppmv. Between each injection, the

hydrogen sulfide concentration was monitored while it decreased because of adsorption

and subsequent oxidation on the pipe surfaces. The experiments showed that the rate of

hydrogen sulfide oxidation was approximately two orders of magnitude faster on the

concrete pipe surfaces than on the plastic pipe surfaces. Removal of the layer of reaction

(corrosion) products from the concrete pipes was found to reduce the rate of hydrogen

sulfide oxidation significantly. However, the rate of sulfide oxidation was restored to its

background level within 10–20 days. A similar treatment had no observable effect on

hydrogen sulfide removal in the plastic pipe reactors. The experimental results were

used to model hydrogen sulfide oxidation under field conditions. This showed that the

gas-phase hydrogen sulfide concentration in concrete sewers would typically amount to

a few percent of the equilibrium concentration calculated from Henry’s law. In the plastic

pipe sewers, significantly higher concentrations were predicted because of the slower

adsorption and oxidation kinetics on such surfaces.

ª 2008 Elsevier Ltd. All rights reserved.

1. Introduction filled (gravity) sewers, the hydrogen sulfide gas is emitted

Some of the most challenging problems faced by sewerage

authorities are those related to the build-up of hydrogen

sulfide gas (H2S(g)) in sewers. Hydrogen sulfide is primarily

produced by microbial breakdown of organic matter with

sulfate (SO42�) as terminal electron acceptor. This strictly

anaerobic process mainly takes place in the sediments

and biofilms covering the wetted pipe surfaces. In partly

nmental Engineering,holmsvej 57, 9000 Aalbolsen).er Ltd. All rights reserved

from the wastewater stream into the sewer atmosphere.

The rate of air–water mass transfer of hydrogen sulfide is

controlled by several factors, the most important being

temperature, wastewater pH and the turbulence level

(Yongsiri et al., 2005). Hydrogen sulfide is an extremely

toxic gas with a characteristic smell of rotten egg. Every

year sewer workers are injured or killed due to exposure

to toxic/lethal levels of hydrogen sulfide. The threshold

Department of Biotechnology, Chemistry and Environmentalrg, Denmark. Tel.: þ45 96 358 468; fax: þ45 96 352 555.

.

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 4 4207

odor limit of hydrogen sulfide is very low and it is known

to be a major constituent of sewer odor (Thistlethwayte

and Goleb, 1972). In addition, hydrogen sulfide is the cause

of microbial induced concrete corrosion, which is

a problem of significant economic impact (Zhang et al.,

2008). The corrosion process results from adsorption of

hydrogen sulfide onto the concrete surfaces exposed to

the sewer atmosphere and subsequent oxidation of the

hydrogen sulfide to sulfuric acid (H2SO4). The oxidation

mechanism has been described to include chemical oxida-

tion of hydrogen sulfide to elemental sulfur, which is then

oxidized microbiologically to sulfuric acid by thiobacilli

(e.g., Vincke et al., 2000). The sulfuric acid reacts readily

with the alkaline components of the concrete. In severe

cases, corrosion rates exceeding 5 mm/year have been

observed (Mori et al., 1991).

Despite the significance of the hydrogen sulfide related

problems, several aspects concerning hydrogen sulfide

build-up in sewers are still not well understood. The rela-

tionship between hydrogen sulfide gas concentrations and

corrosion rates has recently been identified as one of the

main research gaps (Apgar and Witherspoon, 2007).

Hydrogen sulfide levels in the sewer atmosphere are rela-

tively easy to monitor using electrochemical sensors. It

would therefore be valuable if such measurements could

be used for predicting corrosion rates in actual sewers.

One aspect that is likely to play an important role for the

extent of the sulfide related problems is the characteristics

of the pipe surfaces. The corroding pipe surfaces may

frequently be disturbed because of variations of the water

level in the sewer. Such events will possibly remove the

matrix of biofilm embedded in loosely bound corrosion prod-

ucts that cover the concrete pipe surface. This is particularly

relevant for combined sewer systems, which are designed to

operate at full-flow capacity with a certain return period.

The uses of corrosion resistant pipe materials or liners will

possibly also influence the fate of the hydrogen sulfide gas.

Thermoplastics such as high-density polyethylene (HDPE)

and polyvinylchloride (PVC) are among the most widely

used corrosion resistant materials for sewer pipes (Apgar

and Witherspoon, 2007; Stewart, 2005).

Field studies have shown that adsorption and oxidation

of hydrogen sulfide onto concrete sewer pipes exposed to

the sewer atmosphere is a fast process that effectively

reduces the gas-phase concentration to a few percent of

the equilibrium concentration (e.g., Matos and Aires, 1995;

Nielsen et al., 2008). However, it has to the authors’ knowl-

edge not been documented to which extent sulfide adsorp-

tion and subsequent oxidation takes place on plastic pipe

surfaces. On concrete surfaces, sulfuric acid will react with

the alkaline components of the concrete, thereby neutral-

izing the acid (Sand, 1997). The plastic surfaces are inert

and will not neutralize the acid. It is therefore likely that

the pH will eventually become inhibitory for the sulfide

oxidizing bacteria, thereby reducing the rate of sulfide oxida-

tion. This will inevitably lead to higher sulfide concentra-

tions on the pipe surface, which in turn will reduce the

adsorption rate and result in higher hydrogen sulfide

concentrations in the sewer atmosphere and an increased

odor potential. In addition, it can be expected that hydrogen

sulfide gas is transported a longer distance downstream in

the gravity sewer system.

There is a need for better understanding the surface reac-

tion kinetics of hydrogen sulfide adsorption and oxidation

for managing odor and corrosion problems in sanitary sewer

systems. The objective of the present study was to quantify

the kinetics of hydrogen sulfide adsorption and oxidation on

concrete and plastic pipe surfaces under in situ conditions

and to investigate the effect of disturbing the pipe surface.

For the investigations, a pilot plant was constructed and oper-

ated under in situ conditions with intermittent injection of

hydrogen sulfide into the headspace of partially filled sewer

pipe reactors. The intermittent injection of hydrogen sulfide

gas replicates conditions immediately downstream of a force

main where sulfide related concrete corrosion and odor is

often observed.

2. Materials and methods

2.1. Experimental setup

Adsorption and oxidation of hydrogen sulfide gas on concrete

and plastic sewer pipes was studied in a pilot plant comprising

eight individually operated pilot scale sewer reactors. The

reactors were designed with a free water surface to replicate

gravity sewer conditions. Six of the sewer reactors were con-

structed of concrete pipe segments, one reactor was con-

structed of PVC pipe segments and one reactor was

constructed of HDPE pipe segments. The setup was placed in

a sewer research and monitoring station in the town of Frejlev,

a few kilometers west of Aalborg, Denmark. The Frejlev sewer

monitoring station is located below the ground with easy

access to a continuous supply of wastewater from a purely

residential catchment of approximately 2000 inhabitants.

Each sewer reactor comprised of a reaction chamber made

of sewer pipe segments, an air circulation system, a waste-

water circulation system and appurtenances for H2S gas injec-

tion and wastewater supply and removal (Fig. 1). In the

concrete reactors, the reaction chamber consisted of 10

concrete pipe segments of 0.2 m length each, resulting in

a total reactor length of 2 m. The pipe segments were cut

from standard concrete pipes produced from Portland cement

according to Danish National Standards. The reaction

chamber of the plastic pipe reactors consisted of two 1.0 m

long PVC or HDPE pipe segments (Uponor A/S, Hadsund, Den-

mark). The reactor segments were sealed with rubber rings

and a PVC plate with openings for gas and wastewater circula-

tion was mounted at each end of the reaction chamber. The

main characteristics of the pilot scale test reactors are

summarized in Table 1. The air circulation system consisted

of a 3.0 m PVC pipe with an inner diameter of 22 mm and

a centrifugal fan blower. The resulting gas velocity inside

the reaction chamber was 0.053� 0.007 m s�1, corresponding

to a circulation time of approximately 40 s. The gas velocity

is comparable to velocities measured in real sewers (e.g., Mad-

sen et al., 2006). To avoid jet streams at the gas inlet, the circu-

lating air was injected through an air diffuser. A 1 mm drilling

was made in the air circulation pipe in order to allow pressure

equalization of the otherwise airtight setup.

Fig. 1 – Schematic illustration of the pilot scale reactors, c.f. text.

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 44208

2.2. Run-in of experimental setup

The reactors were run-in for a period of approximately 18

months before the experiments were started. During the

entire run-in period, hydrogen sulfide gas was injected

from a flask containing compressed hydrogen sulfide gas

(Yara Praxair, Fredericia, Denmark) at fixed time intervals.

The hydrogen sulfide concentration in the gas phase was

around 1000 ppmv immediately after injection. The

frequency of hydrogen sulfide gas injections was adapted

to the rate of hydrogen sulfide adsorption and oxidation.

This ensured that unrealistic high hydrogen sulfide concen-

trations did not build up inside the reactor. The time interval

between hydrogen sulfide gas injections was set at 1 h for

the concrete pipe reactors and 12 h for the plastic pipe reac-

tors. Every 2 h, approximately 30% of the wastewater was

replaced by pumping fresh pre-settled wastewater into the

reactor. At the same time, surplus wastewater as well as

part of the sewer gas was pumped out of the reactor. The

wastewater was circulated at a low flow rate (approximately

0.1 L s�1) with the purpose of achieving mixing of the waste-

water without causing disturbance of the corrosion process.

The operation of the setup was automated by a PLC control

system. During the run-in period, the temperature of the

gas phase varied between 20 �C and 3 �C.

Table 1 – Main characteristics of the pilot scale testreactors

Concrete HDPE PVC

Inner diameter of reaction chamber (m) 0.200 0.184 0.188

Water depth (m) 0.050 0.041 0.041

Gas volume (m3) 0.051 0.044 0.047

Wastewater volume (m3) 0.012 0.009 0.009

Surface area of reaction chamber

exposed to the gas phase (m2)

0.838 0.794 0.816

Surface area of air circulation system (m2) 0.207 0.207 0.207

2.3. Hydrogen sulfide oxidation experiments

During experiments for determining the kinetics of hydrogen

sulfide adsorption and oxidation, the reactors were operated

with pulse injection of 1000 ppmv H2S(g) similar to the run-in

period. However, the wastewater circulation was turned off

in order to minimize the loss of hydrogen sulfide resulting

from adsorption by the wastewater. The circulation of sewer

gas was kept unchanged. In each experiment, hydrogen

sulfide removal from the gas phase was measured over

a 96 h period for the plastic pipe reactors and 24 h for the

concrete pipe reactors. This resulted in 8–24 injections of

hydrogen sulfide of which the first 8–10 injections were used

for determination of reaction kinetics.

The hydrogen sulfide gas concentration was measured

using Odalog� gas detectors (App-Tek International Pty Ltd,

Brendale, Australia) fitted to the air circulation pipe (Fig. 1).

The gas detectors were routinely calibrated using 100 ppmv

H2S or 250 ppmv H2S calibration gas balanced in air (Euro-

Gas Management Services Ltd, Plymouth, UK).

The total sulfide concentration of the wastewater was

measured by the methylene blue method (APHA et al., 1995).

The samples for total sulfide determination were preserved

by addition of a 10% w/v zinc acetate solution (25 mL L�1)

into the sampling bottles. The dissolved oxygen concentra-

tion, pH and temperature of the wastewater were measured

using WTW pH/Oxi 340i multi-parameter instruments (WTW

GmbH, Weilheim, Germany).

The concentration of elemental sulfur in the material

deposited on the plastic pipe surfaces was analyzed by ion

chromatography after conversion to thiosulfate. The depos-

ited material was sampled using a small stainless steel

spatula. Before analysis, the material was washed in deion-

ized water and dried to constant weight in a dessicator. The

elemental sulfur was transformed to thiosulfate by reaction

with sulfite (30% w/v) in alkaline solution (1 M NaOH) at

60 �C for 12 h. The thiosulfate concentration was subse-

quently measured by ion chromatography with suppressed

conductivity detection (Dionex, Sunnyvale, Ca, USA) using

potassium hydroxide (KOH) as eluent and an anion exchange

column (Dionex AS11). The same ion chromatography method

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 4 4209

was also used to measure sulfate, thiosulfate and sulfite in

water droplets collected from the sewer crown.

Total, volatile and fixed solids of the corrosion products

were analyzed according to Standard Methods (APHA et al.,

1995). The alkalinity of the concrete in CaCO3 equivalents

was analyzed according to Snell et al. (1966–1974). The method

was modified for larger samples of approximately 5 g

concrete. The surface pH of the sewer pipe segments was

measured by pressing pH-strips (Merck, Darmstadt, Germany)

that were wetted with deionized water against the moist

surface for about a minute.

2.4. Interpretation of experimental results

The hydrogen sulfide removal rate was determined from the

slope of the measured hydrogen sulfide concentration versus

time as illustrated in Fig. 2. The removal of hydrogen sulfide

was interpreted by considering two removal mechanisms;

i.e., adsorption and oxidation on the pipe wall exposed to

the sewer atmosphere (designated as oxidation in the

following) and absorption by the wastewater phase. The

kinetics of hydrogen sulfide oxidation was evaluated in terms

of n-order kinetics (Eq. (1)).

�dpH2S

dt¼ knpn

H2S (1)

where pH2S is the partial pressure of H2S gas (ppmv), t is time

(h), n is the reaction order (�) and kn is the oxidation rate

constant (ppmv1�n h�1).

The kinetics of hydrogen sulfide absorption by the waste-

water phase was calculated from the classical rate equation

for air–water mass transfer of gases derived from the two-

film theory (Eq. (2)):

dCSð�IIÞ

dt¼ KL

Aw

Vw

�CH2S;Eq � CH2S

�(2)

Fig. 2 – Example of 48 h of hydrogen sulfide gas-phase measure

corresponding hydrogen sulfide removal rates (lower graphs).

where CS(�II ) is the total sulfide concentration in the waste-

water (g S m�3), KL is the mass transfer coefficient (m h�1),

Aw is the surface area of the wastewater (m2), Vw is the waste-

water volume (m3), CH2S;Eq is the dissolved H2S concentration

of the wastewater at equilibrium with the gas phase

(g S m�3), and CH2S is the actual H2S concentration of the

wastewater at the specific pH (g S m�3).

Considering the ideal gas law and the geometry of the

experimental setup, the overall removal of hydrogen sulfide

gas can be described by Eq. (3):

�dpH2S

dt¼ KL

Aw

Vg

RTabs

MwP106�CH2S;Eq � CH2S

�þ knpn

H2S (3)

where Vg is the volume of gas in the reactor (m3), R is the ideal

gas constant (m3 atm K�1 mol�1), Tabs is the temperature (K),

Mw is the molar weight of sulfur (g mol�1), P is the pressure

(atm).

The water-phase concentration of H2S at equilibrium with

the gas phase can be calculated from Henry’’s law (Eq. (4)):

CH2S;Eq ¼ Mw10�3

�pH2S

HA

�(4)

where HA is the Henry’s law constant for hydrogen sulfide

(L atm mol�1).

Previous investigations have shown that hydrogen sulfide

absorption by the wastewater is negligible compared to

surface oxidation in the concrete reactors (Vollertsen et al.,

2008). This process was therefore omitted when analyzing

data from the concrete pipe reactors. For determining the

value of KL in the plastic pipe reactors, the reactors were

cleaned and the wastewater was replaced with a 1% w/v

zinc acetate solution in six independent experiments. Zinc

acetate effectively precipitates the H2S absorbed by the water

phase, thereby maintaining a CH2S of 0 g S m�3. Eq. (2) was

adapted to the experimental conditions and the value of KL

was determined from the measured hydrogen sulfide gas

ment in the HDPE pipe reactor (upper graphs) and the

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 44210

concentration and the increase in the water-phase concentra-

tion of zinc sulfide over a period of 48 h (Eq. (5)):

dCSð�IIÞ

dt¼ KL

Aw

VwMw10�3

�pH2S

HA

�(5)

The hydrogen sulfide oxidation rate expressed in units of

ppmv per hour is specific to the geometry of the experimental

setup. For comparison with other systems, the volume-

specific hydrogen sulfide oxidation rate was converted into

a surface-specific hydrogen sulfide oxidation rate (Eq. (6)).

r ¼ knpnH2S

Vg

Ap

PRTabs

10�3Mw (6)

where r is the surface-specific hydrogen sulfide oxidation rate

(mg S m�2 h�1) and Ap is the pipe wall area exposed to the gas

phase (m2).

Because the rate constant and the reaction order vary

between experiments, a comparison of these parameters alone

is not sufficient to characterize the kinetics of hydrogen sulfide

oxidation. Instead, sulfide oxidation rates were calculated for

low (10 ppmv), medium (100 ppmv) and high (1000 ppmv)

hydrogen sulfide concentrations, and labeled r10, r100 and

r1000, respectively. Unless otherwise stated, all rates have

been corrected to 15 �C according to an Arrhenius-type equa-

tion with a temperature coefficient of 1.024, which is typical

for diffusion limited processes (Hvitved-Jacobsen, 2002). While

analyzing the data from the plastic pipe reactors, the area-

specific hydrogen sulfide oxidation rate was assumed identical

for the air circulation system and the reaction chamber. For the

concrete pipe reactors, the contribution to the hydrogen sulfide

removal from the surface of the air circulation system was

neglected according to Vollertsen et al. (2008).

3. Results and discussion

3.1. Run-in of the pilot scale reactors

During the first couple of months of the run-in period, the

removal rate of hydrogen sulfide gas increased significantly

in the concrete pipe reactors. This was most likely due to

Fig. 3 – Pictures of the interior of one of the concrete pipe reactor

8 months of operation.

a colonization of the concrete surface by sulfide oxidizing

bacteria (Islander et al., 1991). On the interior surface of the

concrete pipes, a porous layer of corrosion products developed.

This layer increased the surface area of the pipe exposed to the

sewer atmosphere significantly and thereby probably also

influenced the removal rate. A similar development of the

sulfide removal rates was not observed for the plastic pipe

reactors which remained slow throughout the run-in period.

After approximately 4 months of operation, all concrete

pipes were severely corroded and the surface pH had dropped

below 1–2 (Fig. 3). The porous layer of corrosion products

covering the concrete pipe surfaces exposed to the sewer

atmosphere consisted of loosely bound material with little

mechanical strength. The material was most likely gypsum

(CaSO4$2H2O) and bassanite (CaSO4$0.5H2O) resulting from

the reaction between sulfuric acid and the alkaline compo-

nents of the concrete (Davis et al., 1998). The alkalinity of

the concrete was determined at 0.181 g CaCO3 (g concrete)�1

(�0.014) (Vollertsen et al., 2008). This value is typical for stan-

dard concrete pipes (U.S. EPA, 1974). The dry matter content of

the loosely bound material was 65.0% (� 2.4%) and the weight

loss from ignition at 550 �C constituted 2.7% (� 0.5%) of the dry

matter; i.e., the material was mainly inorganic.

During the run-in period, the interior surface of the plastic

pipes became covered by a thin layer of yellow material

(Fig. 3). Water droplets were retained at the sewer crown

whereas the remaining part of the pipe perimeter remained

fairly dry. In addition, the yellow layer was not evenly distrib-

uted along the perimeter of the pipe. The layer was thickest

near the water line as depicted in Fig. 3. It was therefore not

possible to obtain representative samples for characterization

of the water content. Material sampled near the water line

was analyzed for elemental sulfur, which was found to

comprise 88% w/w (�2.4%) of the dry matter. This demon-

strated that elemental sulfur is a major product of hydrogen

sulfide oxidation on the plastic pipe surfaces. The relatively

low weight loss from ignition of the porous layer of corroded

concrete indicates that elemental sulfur was not a major

part of the corroded material, as it would have evaporated.

During the run-in period, the surface pH of the plastic pipes

rapidly dropped below 1 where it remained throughout the

s (left) and the HDPE pipe reactor (right) after approximately

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 4 4211

experimental period. This indicates that sulfuric acid was also

produced from the oxidation of hydrogen sulfide. This was

confirmed by analysis of condensate water collected from

the air circulation system, which was found to contain

7.4% w/v (�0.1%) sulfuric acid. The pH value of this solution

should theoretically be 0.1 (Nordstrom et al., 2000). However,

pH measurements on the plastic pipe surfaces using indicator

strips gave slightly higher values between 0.2 and 0.4.

The investigation of hydrogen sulfide loss from absorption

by the wastewater in the plastic pipe reactors showed that this

process was only of minor importance for the overall removal

of hydrogen sulfide gas. When the wastewater was replaced

by a zinc acetate solution, less than 10% of the injected

amount of hydrogen sulfide gas was removed by air–water

mass transfer. The KL value was accordingly determined at

0.045 m d�1 (�0.005 m d�1) for the HDPE reactor and

0.053 m d�1 (�0.003 m d�1) for the PVC reactor. A similar inves-

tigation was omitted for the concrete pipe reactors because

the absorption process was negligible compared to oxidation

on the surface of these reactors (Vollertsen et al., 2008).

3.2. Kinetics of hydrogen sulfide oxidation on plasticand concrete pipes

After the run-in period, the rate of hydrogen sulfide oxidation

had reached a constant level in each reactor and experiments

were conducted to compare the rate of hydrogen sulfide

oxidation on the different pipe surfaces. The experiments

showed that the kinetics of hydrogen sulfide oxidation was

highly dependent on the pipe material. Overall, the reaction

was approximately two orders of magnitude slower on the

plastic pipe surfaces compared to the concrete pipe surfaces

(Fig. 4). The hydrogen sulfide oxidation rates on the PVC and

HDPE pipe surfaces were not significantly different.

The observed hydrogen sulfide oxidation rates for the

concrete pipe reactors are comparable to those reported for

submerged sewer biofilms exposed to sulfide concentrations

up to 5 g S m�3 (Nielsen et al., 2005). To the authors’ knowl-

edge, no previous studies have investigated the kinetics of

adsorption and oxidation of hydrogen sulfide gas on plastic

sewer pipe surfaces. The lower hydrogen sulfide oxidation

rates of the plastic pipes support the explanation that the

pH can become inhibitory for the sulfide oxidizing bacteria,

thereby reducing the rate of sulfide oxidation. The minimum

pH at which sulfide oxidizing bacteria isolated from corroding

Fig. 4 – Box-plot of the hydrogen sulfide oxidation rates of

the concrete, PVC and HDPE pipe reactors after 16 months

of operation. Area-specific oxidation rates at 10 ppmv H2S(g)

(r10), 100 ppmv H2S(g) (r100) and 1000 ppmv H2S(g) (r1000) are

shown. All data are temperature corrected to 15 8C.

concrete sewers are able to grow is around 0.5 (Islander et al.,

1991). This is consistent with pH values measured on the

plastic pipe surfaces. Another important aspect may be

the difference of the surface area of the plastic pipes and the

corroded concrete pipes.

The reaction order with respect to hydrogen sulfide gas

concentration (n) was of the same magnitude for both the

concrete and the plastic pipe reactors. The median values

(�standard deviation) of n were determined at 0.54 (�0.10)

and 0.61 (�0.13) for the concrete and plastic pipe reactors,

respectively. The reaction order does not indicate whether

the process is chemical or biological; e.g., for oxidation of

dissolved sulfide in both active and sterilized cell suspensions,

the reaction order with respect to sulfide is reported in the

range of 0.6–0.8 (Buisman et al., 1990; Nielsen et al., 2003).

The rate constant (kn) was determined at 226.4

(�186.4) ppm1�0.54 h�1 for the concrete pipe reactors and 9.6

(�10.2) ppm1�0.61 h�1 for the plastic pipe reactors. Thus, the

standard deviation was of the same magnitude as the median

value for both reactor types.

For determining the kinetics of hydrogen sulfide oxidation,

the concentration of dissolved H2S in the wastewater (Eq. (3))

was assumed to be zero. This is a fair approximation as the

wastewater pH was generally above 8.0 where more than

90% of the dissolved sulfide is present as HS� ion, which

cannot be transported across the air–water interface. In addi-

tion, dissolved sulfide was constantly removed by oxidation in

the water phase, which is a fast process compared with the

air–water mass transfer of hydrogen sulfide (Nielsen et al.,

2006). This assumption was supported by regular measure-

ments of the total sulfide concentration of the wastewater,

which was always below 1 g S m�3.

The results clearly illustrate that the pipe material has

a marked effect on how much hydrogen sulfide can poten-

tially be removed by adsorption and subsequent oxidation

on sewer surfaces. This will affect the potential build-up of

hydrogen sulfide gas in the sewer atmosphere as well as the

downstream transport of hydrogen sulfide gas.

3.3. Effect of pipe surface characteristics

For investigating the effect of disturbing the pipe surface, the

concrete pipes were washed using a hose and a brush. This

removed the loosely bound corrosion products, thereby

exposing the aggregates of the concrete (Fig. 5). In the plastic

pipes, washing removed the yellow layer covering the

surfaces exposed to the sewer atmosphere.

After washing the pipes, the hydrogen sulfide oxidation

rate was monitored for 5–6 weeks. In all concrete pipe reac-

tors, the rate was significantly lowered immediately after

the washing (Fig. 6). However, the rate increased again during

the experimental period, particularly during the first 10–12

days of the experiments. After 30–40 days, hydrogen sulfide

had recovered to a level similar to that before the pipes were

washed (Fig. 4). The observed (exponential) increase of the

hydrogen sulfide oxidation rates was most likely the result

of growth of sulfide oxidizing bacteria colonizing the concrete

surfaces. In one reactor (VI), the washing was repeated twice

within 5 months with comparable results. Reactor V was acci-

dentally flooded at 35 days into the experiment due to a failure

Fig. 5 – Pictures of a concrete pipe segment before and after washing.

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 44212

of the wastewater discharge pump. This incident resulted in

a significant lowering of the hydrogen sulfide oxidation rate

although the layer of corrosion products was not removed.

The fast re-establishment of the surface-specific oxidation

rates is consistent with the findings of Islander et al. (1991). In

their study, the surface pH of concrete samples exposed to

sewer gas in an experimental chamber was monitored

following a flushing with wastewater. The surface pH rose

immediately after flushing, but returned quickly to a very

low background value. Based on these observations, they

concluded that heavy and frequent flushing was necessary

to significantly reduce corrosion rates. Occasional flushing of

the sewer pipes which removes the layer of corrosion prod-

ucts is therefore unlikely to have a lasting effect on the

Fig. 6 – Development of the hydrogen sulfide oxidation rates af

oxidation rates at 10 ppmv H2S(g) (r10), 100 ppmv H2S(g) (r100) and

corrected to 15 8C. Reactor VI was washed twice within 5 mont

confidence interval calculated from 10 injections of hydrogen s

corrosion process. The return period for such incidents must

be a few days for this to be the case. However, flushing inci-

dents might be important for supplying nutrients and trace

minerals to the biomass colonizing the pipe surface exposed

to the sewer atmosphere.

Although the concrete pipe reactors were all identically

produced and operated under identical conditions, the

kinetics of hydrogen sulfide oxidation exhibited significant

variability between the reactors. The slowest reactors (I and

IV) were approximately one order of magnitude slower than

the other reactors throughout the experiments. The reason

for this variability is not known.

Contrary to the concrete pipe reactors, washing the plastic

pipes did not significantly change the hydrogen sulfide

ter washing the six concrete pipe reactors. Area-specific

1000 ppmv H2S(g) (r1000) are shown. All data are temperature

hs. Each dot and error bar represents the median and 95%

ulfide.

Fig. 7 – Development of the hydrogen sulfide oxidation rates after washing the PVC and HDPE pipe reactors. Area-specific

oxidation rates at 10 ppmv H2S(g) (r10), 100 ppmv H2S(g) (r100) and 1000 ppmv H2S(g) (r1000) are shown. All data are temperature

corrected to 15 8C. Each dot and error bar represents the median and 95% confidence interval calculated from 8–10 injections

of hydrogen sulfide.

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 4 4213

oxidation rate (Fig. 7). In addition, the rate did not increase

during the subsequent 45–53 days of experiments. During

the washing of the plastic pipes, most of the material covering

the pipe walls exposed to the gas phase was removed. Since

this did not significantly change the rate of sulfide oxidation,

it is likely that the hydrogen sulfide oxidation on the plastic

pipe surfaces was mainly a chemical process.

Fig. 8 – Simulated hydrogen sulfide gas-phase

concentrations and hydrogen sulfide oxidation rates at

steady state conditions in a concrete (left) and plastic (right)

gravity sewer, c.f. text.

3.4. Extrapolation to field conditions

In order to illustrate the implications of the findings, the

kinetic data were used to model steady state conditions in

an actual sewer. Model simulations were made for bulk water

concentrations of total sulfide up to 5 g S m�3 and pH values

between 6 and 8. The simulations were based on the median

parameter values for hydrogen sulfide oxidation on concrete

and plastic pipes. Air–water mass transfer was calculated

according to Yongsiri et al. (2005) assuming steady uniform

flow conditions. The pipe diameter was assumed 1.0 m, water

depth 0.25 m, slope 0.5%, water and air temperature 15 �C. The

simulations showed that the steady state hydrogen sulfide

concentration in the sewer atmosphere is significantly

affected by the pipe material. In the examples illustrated in

Fig. 8, the use of plastic pipes results in hydrogen sulfide gas

concentrations that are approximately 10 times higher than

in the concrete sewer. The steady state concentrations in

the concrete sewer are generally below 5% of the theoretical

equilibrium concentration calculated from Henry’s law,

whereas the hydrogen sulfide concentrations in the plastic

pipes are between 10 and 75% of the equilibrium value

(Fig. 8 middle). Despite the big difference in the steady state

hydrogen sulfide concentration, the oxidation rate on the

concrete pipes was in this example higher than that on the

plastic pipe only by a factor 3 (Fig. 8 bottom). This illustrates

that the air–water mass transfer is typically the limiting

process under normal flow conditions in sewers.

The highest sulfide oxidation rate of almost

200 mg S m�2 h�1 was estimated for the concrete sewer in

the example with pH 6 and a total sulfide concentration in

the wastewater of 5 g S m�3. In such cases with fast sulfuric

acid production, 30% of the acid can be expected to react

with the alkaline components of the concrete (U.S. EPA,

1974). Assuming that all the hydrogen sulfide is oxidized to

sulfuric acid, a surface-specific oxidation rate of

200 mg S m�2 h�1 will result in a corrosion rate of approxi-

mately 3.8 mm per year for concrete with an alkalinity of

0.181 g CaCO3/g and a density of 2340 kg m�3.

Considering potential odor and toxicity problems, it is

evident that the use of plastic pipe will significantly enhance

the risk of problems. Not only will the maximum hydrogen

sulfide gas concentrations be higher in plastic pipes, but also

the length of sewer affected will increase as sulfide is only

slowly removed by adsorption and oxidation on the pipe

surfaces.

4. Conclusions

A pilot scale investigation demonstrated that hydrogen sulfide

adsorption and oxidation on corroded concrete sewer pipe

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 2 0 6 – 4 2 1 44214

surfaces was approximately two orders of magnitude faster

than on plastic pipe surfaces. Model simulations based on

kinetic data from the experiments indicated that hydrogen

sulfide gas concentrations in concrete sewers – under steady

uniform flow conditions – would amount to a few percent of

the equilibrium concentration calculated from Henry’s law.

In plastic pipe sewers, the slower surface reaction results in

significantly higher hydrogen sulfide gas concentrations and

thus an increased odor potential.

Removal of the layer of reaction (corrosion) products

covering the pipe surfaces was found to reduce the rate of

hydrogen sulfide oxidation in the concrete pipes significantly,

but had no observable effect in the plastic (PVC and HDPE)

pipes. The high rate of sulfide oxidation in the concrete pipes

was restored within 10–20 days. Thus, such events must

happen frequently in order to have a permanent effect on

corrosion of concrete sewers.

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