corrosion of concrete sewers—the kinetics of hydrogen sulfide oxidation

9
Corrosion of concrete sewersThe kinetics of hydrogen sulfide oxidation Jes Vollertsen , Asbjørn Haaning Nielsen, Henriette Stokbro Jensen, Tove Wium-Andersen, Thorkild Hvitved-Jacobsen Section of Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark ARTICLE INFO ABSTRACT Article history: Received 15 October 2007 Received in revised form 3 January 2008 Accepted 8 January 2008 Available online 20 February 2008 Hydrogen sulfide absorption and oxidation by corroding concrete surfaces was quantified in a test rig consisting of 6 concrete pipes operated under sewer conditions. The test rig was placed in an underground sewer monitoring station with access to fresh wastewater. Hydrogen sulfide gas was injected into the pipe every 2nd hour to peak concentrations around 1000 ppm. After some months of operation, the hydrogen sulfide became rapidly oxidized by the corroding concrete surfaces. At hydrogen sulfide concentrations of 1000 ppm, oxidation rates as high as 1 mg S m - 2 s - 1 were observed. The oxidation process followed simple nth order kinetics with a process order of 0.450.75. Extrapolating the results to gravity sewer systems showed that hydrogen sulfide oxidation by corroding concrete is a fast process compared to the release of hydrogen sulfide from the bulk water, resulting in low gas concentrations compared with equilibrium. Balancing hydrogen sulfide release with hydrogen sulfide oxidation at steady state conditions demonstrated that significant corrosion ratesseveral millimeters of concrete per yearcan potentially occur at hydrogen sulfide gas phase concentrations well below 510 ppm. The results obtained in the study advances the knowledge on prediction of sewer concrete corrosion and the extent of odor problems. © 2008 Elsevier B.V. All rights reserved. Keywords: Concrete corrosion Hydrogen sulfide Sewers Process kinetics 1. Introduction In Los Angeles, USA, back in 1895, Olmsted and Hamlin (1900) conducted one of the earliest studies on concrete sewer cor- rosion. They concluded that sulfuric acid was the cause of the corrosion; however, they did not link the occurrence of sulfuric acid to the occurrence of hydrogen sulfide. In the first decades of the 20th century, hydrogen sulfide was identified as the cause of the sulfuric acid and consequently of the concrete corrosion (James, 1917; Bowlus and Banta, 1932). However, not before the 1940s, a conceptual understanding of concrete corrosion was established through systematic research in the USA and Australia (Parker, 1945a,b, 1947; Pomeroy and Bowlus, 1946). A bacterium named Thiobacillus concretivorous was identified from strongly acidic corrosion products and believed the main re- sponsible for the corrosion process (Parker, 1945a,b, 1947). The bacterium has been renamed to Thiobacillus thiooxidans and subsequently to Acidithiobacillus thiooxidans (Kelly and Wood, 2000). Later on, other bacteria as well as fungi were reported to take part in the concrete corrosion process (e.g. Parker and Prisk, 1953; Cho and Mori, 1995; Nica et al., 2000; Hernandez et al., 2002; Okabe et al., 2007). Concrete sewer corrosion is initiated through a series of steps. First the pH of the alkaline and moist concrete surface is chemically lowered towards a more neutral pH by dissociation of hydrogen sulfide and by carbonation. Hereafter, different neutrophilic sulfide oxidizing bacteria and fungi colonize the concrete surface and contribute to a successive oxidation of SCIENCE OF THE TOTAL ENVIRONMENT 394 (2008) 162 170 Corresponding author. Tel.: +45 9940 8504; fax: +45 9635 0558. E-mail address: [email protected] (J. Vollertsen). 0048-9697/$ see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.01.028 available at www.sciencedirect.com www.elsevier.com/locate/scitotenv

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Page 1: Corrosion of concrete sewers—The kinetics of hydrogen sulfide oxidation

S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 4 ( 2 0 0 8 ) 1 6 2 – 1 7 0

ava i l ab l e a t www.sc i enced i r ec t . com

www.e l sev i e r. com/ loca te / sc i to tenv

Corrosion of concrete sewers—The kinetics of hydrogensulfide oxidation

Jes Vollertsen⁎, Asbjørn Haaning Nielsen, Henriette Stokbro Jensen,Tove Wium-Andersen, Thorkild Hvitved-JacobsenSection of Environmental Engineering, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark

A R T I C L E I N F O

⁎ Corresponding author. Tel.: +45 9940 8504; fE-mail address: [email protected] (J. Vollertsen

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.01.028

A B S T R A C T

Article history:Received 15 October 2007Received in revised form3 January 2008Accepted 8 January 2008Available online 20 February 2008

Hydrogen sulfide absorption and oxidation by corroding concrete surfaces was quantified ina test rig consisting of 6 concrete pipes operated under sewer conditions. The test rig wasplaced in an underground sewer monitoring station with access to fresh wastewater.Hydrogen sulfide gas was injected into the pipe every 2nd hour to peak concentrationsaround 1000 ppm. After some months of operation, the hydrogen sulfide became rapidlyoxidized by the corroding concrete surfaces. At hydrogen sulfide concentrations of1000 ppm, oxidation rates as high as 1 mg S m−2 s−1 were observed. The oxidation processfollowed simple nth order kinetics with a process order of 0.45–0.75. Extrapolating theresults to gravity sewer systems showed that hydrogen sulfide oxidation by corrodingconcrete is a fast process compared to the release of hydrogen sulfide from the bulk water,resulting in low gas concentrations compared with equilibrium. Balancing hydrogen sulfiderelease with hydrogen sulfide oxidation at steady state conditions demonstrated thatsignificant corrosion rates—several millimeters of concrete per year—can potentially occurat hydrogen sulfide gas phase concentrations well below 5–10 ppm. The results obtained inthe study advances the knowledge on prediction of sewer concrete corrosion and the extentof odor problems.

© 2008 Elsevier B.V. All rights reserved.

Keywords:Concrete corrosionHydrogen sulfideSewersProcess kinetics

1. Introduction

In Los Angeles, USA, back in 1895, Olmsted and Hamlin (1900)conducted one of the earliest studies on concrete sewer cor-rosion. They concluded that sulfuric acid was the cause of thecorrosion; however, they did not link the occurrence of sulfuricacid to the occurrenceof hydrogen sulfide. In the first decades ofthe 20th century, hydrogen sulfidewas identified as the cause ofthe sulfuric acid and consequently of the concrete corrosion(James, 1917; Bowlus and Banta, 1932). However, not before the1940s, a conceptual understanding of concrete corrosion wasestablished through systematic research in the USA andAustralia (Parker, 1945a,b, 1947; Pomeroy and Bowlus, 1946). Abacterium named Thiobacillus concretivorous was identified from

ax: +45 9635 0558.).

er B.V. All rights reserved

strongly acidic corrosion products and believed the main re-sponsible for the corrosion process (Parker, 1945a,b, 1947). Thebacterium has been renamed to Thiobacillus thiooxidansand subsequently to Acidithiobacillus thiooxidans (Kelly andWood, 2000). Later on, other bacteria as well as fungi werereported to take part in the concrete corrosion process (e.g.Parker and Prisk, 1953; Cho and Mori, 1995; Nica et al., 2000;Hernandez et al., 2002; Okabe et al., 2007).

Concrete sewer corrosion is initiated through a series ofsteps. First the pH of the alkaline andmoist concrete surface ischemically lowered towards amore neutral pH by dissociationof hydrogen sulfide and by carbonation. Hereafter, differentneutrophilic sulfide oxidizing bacteria and fungi colonize theconcrete surface and contribute to a successive oxidation of

.

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Fig. 1 –Three of the six pilot scale reactors for investigation ofconcrete corrosion.

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reduced sulfur compounds to dissociated sulfuric acid. Thelast step is characterized by the pH of the concrete surfacefalling below 2 as acidophilic organisms take over and A.thiooxidans becomes the dominating microorganism (Parker,1947, 1951; Islander et al., 1991; Ismail et al., 1993; Okabe et al.,2007).

Pure culture studies on the growth of microorganism par-ticipating in the corrosion process indicate that hydrogensulfide itself is not necessarily a good growth substrate (Nicaet al., 2000). Parker (1945a) reports that hydrogensulfide inhighconcentrations is toxic for A. thiooxidans. However, Parker andPrisk (1953) report that this bacterium and several othermicroorganisms can utilize hydrogen sulfide as growth sub-strate at lowhydrogen sulfide concentrations—concentrationscorresponding to those typically found in sewer systems. Anumber of the organisms foundon corroding concrete surfacescannot utilize hydrogen sulfide but do utilize elemental sulfuror thiosulfate, suggesting an initial chemical and/or biologicaloxidation step from hydrogen sulfide to these intermediates(Parker and Prisk, 1953). Furthermore, yellowdeposits are oftenseen on corroding concrete, suggesting some accumulation ofelemental sulfur (Parker, 1945b; Islander et al., 1991).

In severe cases, hydrogen sulfide initiated concrete corro-sion approaches rates around 5 mm of concrete surface peryear (Mori et al., 1991; Roberts et al., 2002). It is therefore anengineering objective to predict the life expectancy of corrod-

Fig. 2 –Schematics of the

ing concrete in conveyance systems and to decide the need forand type of countermeasures. For pragmatic reasons, theprediction of concrete corrosion rates has hitherto been basedsolely on bulk water sulfide concentrations, and not on thehydrogen sulfide concentration of the sewer gas (Melbourneand Metropolitan Board of Works, 1989). The hydrogen sulfidesurface reaction kinetics of corroding concrete has receivedlimited attention (Æsøy et al., 2002), and the span from under-standing the microbial population dynamics of corroding con-crete to predicting concrete corrosion rates for engineeringpurposes has not yet been bridged.

Consequently, there is an engineering need for improvedunderstanding of the surface reaction kinetics of hydrogensulfide absorption and oxidation by corroding concrete. It isthe objective of this study to quantify hydrogen sulfideoxidation by corroding concrete surfaces in sewers and torelate these processes to concrete corrosion rates. A test rigdesigned for intermittent releases of hydrogen sulfide to theatmosphere in concrete sewer pipes constitutes the experi-mental basis of the study. Such a hydrogen sulfide releasepattern is typical for force mains discharging into gravitysewers at turbulent conditions.

2. Methods

2.1. Experimental setup

Hydrogen sulfide induced concrete corrosion was studied in atest rig consisting of 6 identical and parallel operated pilotscale sewer reactors (Fig. 1), operated with a free water surfaceto mimic gravity sewer conditions. The test rig was placed in asewer research andmonitoring station in the town of Frejlev, afew kilometers west of Aalborg, Denmark. The Frejlev mon-itoring station is located below ground with easy access to acontinuous supply of fresh wastewater from a purely residen-tial catchment of approximately 2000 inhabitants.

Each pilot scale reactor contained 10 concrete pipe segmentsof 0.2 m inner diameter and 0.2 m length, resulting in a totalreactor length of 2 m (Fig. 2). The pipe segments were cut fromstandard concrete pipes, producedaccording toDanish nationalstandards. The alkalinity of the concrete was equivalent to0.181 g CaCO3 (g concrete)−1 (±0014), determined on 8 concretesamples according to Snell et al. (1966–74). The specific densitywas determined to 2,340 kgm−3. The segments were assembledwith rubber rings and PVC end plates with openings for gascirculation and wastewater circulation was placed at each end

pilot scale reactors.

Page 3: Corrosion of concrete sewers—The kinetics of hydrogen sulfide oxidation

Table 1 – Test dates and temperatures

Date Aug18–21,2006

Sep18–20,2006

Oct 9–11,2006

Nov7–9,2006

Dec19–21,2006

Jan23–25,2007

Temperature 20 °C(68 °F)

17 °C(63 °F)

16 °C(61 °F)

11 °C(52 °F)

7 °C(45 °F)

3 °C(37 °F)

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of the assembled pipe. The test reactors were labeled I throughVI and the concrete pipe segments were labeled A through J,following the direction of gas flow. The water depth in thereactorwas 0.050m, and the total gas volumeof test reactorwas0.0542 m3. The total internal surface area of the reactor was1.55 m2, with 23% being the water surface, 56% being theconcrete test pipe (0.863 m2) and 21% being the gas circulationsystem and the end plates of the reactor.

By means of a blower, sewer gas was circulated througha PVC pipe of 22 mm inner diameter. The resulting gas ve-locity inside the concrete test pipe was 0.053±0.002 m s−1,corresponding to a circulation time of approximately 40 s. Toavoid jet streams at the gas inlet, the circulating air wasinjected through a diffuser. A 1mmhole wasmade in the gascirculation pipe in order to allow pressure equalization inthe otherwise airtight setup. Fresh wastewater was pre-settled and pumped into the reactor every 2nd hour. At thesame time, surplus wastewater as well as part of the sewergas was pumped out of the reactor. Subsequently, pure hy-drogen sulfide gas was injected from a flask containingcompressed hydrogen sulfide gas, resulting in initial hydro-gen sulfide concentrations in the gas phase around 1000 ppm.Wastewater was circulated at a low flow rate (approximately0.1 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 program-mable controller.

Gas phase hydrogen sulfide concentrations as well as gasphase temperatures were measured and logged by means ofelectrochemical hydrogen sulfide gas sensors (Odalog™). Thesensors had ameasuring range of 0 to 1000 ppm, and a nominalaccuracy of ±2 ppm. According to the manufacturer, the re-sponse time (T90) of the sensor is less than 60 s, which wasconfirmed experimentally. After exposing a new sensor toaround 1000 ppm of hydrogen sulfide gas for a short period, thesignal dropped to 100 ppmwithin approximately 30 s. The dropin signal from 100 ppm to 10 ppm took around 60 s, as did thedrop from 10 ppm to zero. However, as a measuring cell grewolder, the response time increased, and especially the timerequired for the signal to fall all the way back to zero increasedsignificantly—i.e. the signal had a ‘tail’, where it slowly droppedthe last few ppm till it became zero. When analyzing the data,the signal-‘tails’ were handled as a systematic error and sub-tracted from the measured signal. The error was typically lessthan 1% of the sensor range. Themeasuring cells were replacedwhen needed and the sensors were frequently calibrated by a100 ppm test gas.

To some extent, hydrogen sulfide was lost to other pro-cesses than absorption and oxidation by the concrete surfaces;i.e. absorption by the wastewater within the setup, absorptionand oxidation by PVC surfaces, and consumption by thehydrogen sulfide sensor. To estimate these losses, an addi-tional pilot scale reactor was constructed, where the concretepipe sections had been substituted by 2 PVC pipe sections of200 mm diameter and 1 m length. This setup was operated inparallel with the concrete pilot scale reactors and in a similarmode; however, hydrogen sulfide was injected less often dueto a low hydrogen sulfide removal rate in the PVC pilot scalereactor. The total internal surface area of the PVC pilot scalereactor was 1.47 m2, with 23% being the water surface, 55%

being the PVC test pipe and 22% being the gas circulationsystem and the end plates of the reactor.

The hydrogen sulfide oxidation rate of each reactor wasmeasured with intervals of approximately 1 month (Table 1). Ahydrogen sulfide sensor was mounted on the PVC pipe re-cir-culating the sewer gas (Fig. 2), and the bi-hourlywastewater andsewer gas exchange were stopped, as was the wastewatercirculation, leaving the sewage in the pipe stagnant. The laterwas done to minimize air–water mass transfer of hydrogensulfide. The circulation of sewer gas was kept unchanged. Hy-drogen sulfide gas was injected to a concentration of approx-imately 1000 ppm and the decrease in hydrogen sulfideconcentration was logged every 15 s. The hydrogen sulfideinjection was repeated every 2nd hour for approximately 1 day,resulting in a total of 11–13 injections of hydrogen sulfide gas foreach of the 6 test reactors.

The hydrogen sulfide oxidation rate of individual concretepipe segments was tested to assess whether the segments of atest reactor exhibited similar activity. The tests were per-formed by placing a concrete pipe segment between the twoPVC pipe segments of the PVC pilot scale reactor. Individualsegmentswere tested in theperiod fromFebruary toApril 2007.The rates obtained for the individual concrete pipe segmentswere temperature adjusted to the average temperature of themeasuring period (12 °C) using an Arrhenius type equation.

Measurement of the surface pH was done by pH-strips(Merck) that were wetted with deionized water and pressedagainst the moist concrete surface for about a minute.

2.2. Simulation of measurements

The hydrogen sulfide oxidation rate was determined as theslope of the measured hydrogen sulfide concentration versustime, subtracting systematicmeasurement errors as describedabove. The calculated hydrogen sulfide oxidation rates weresimulated by Eq. (1). Fig. 3 illustrates the measuring principleand the interpretation of the data obtained.

dpH2S

dt¼ kppnH2S ð1Þ

where pH2S is the hydrogen sulfide gas phase concentration[ppm], n the reaction order (−) and kp the process rate constant[ppm1−n s−1].

Applying the ideal gas law, the volume-specific hydrogensulfide oxidation rate was converted from the unit of ppm s−1

into a flux, i.e. a surface-specific hydrogen sulfide oxidationrate, FH2S [gS m

−2 s−1], Eq. (2).

FH2S ¼ d pH2Sð Þdt

0:101332

RgTabs

Vgas

Aconc¼ kp0:1013

32RgTabs

Vgas

Aconc

� �pnH2S ¼ kFpnH2S

ð2Þ

Page 4: Corrosion of concrete sewers—The kinetics of hydrogen sulfide oxidation

Fig. 3 –Example of 24 h of hydrogen sulfide gas phase (g)measurement (upper graph) and the interpretation of a single hydrogensulfide injection (lower graph).

165S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 4 ( 2 0 0 8 ) 1 6 2 – 1 7 0

where Rg is the universal gas constant [J K−1 mol−1]; Tabs is theabsolute temperature [K]; Vgas the total gas volume in thereactor [m3]; Aconc the concrete surface subject to sewer gas[m2] and kF is the surface-specific process rate constant [mg Sm−2 s−1 (ppm H2S)−n].

As n and kF are dependent parameters, comparison be-tween experiments could not be made on the basis of the rateconstants alone. As a supplement representing low, mediumand high hydrogen sulfide concentrations, the correspondingrates at 10 ppm, 100 ppm and 1000 ppm were calculated andlabeled r10, r100 and r1000, respectively.

Fig. 4 –Simulation examples of measured hydrogen sulfidegas phase (g) oxidation rates, representing a low (kp=0.019;n=0.58), a medium (kp=0.094; n=0.50) and a high oxidationrate (kp=0.177; n=0.52).

3. Results and discussion

3.1. Testing of the pilot scale setup

Comparison between the hydrogen sulfide uptake rate ofthe PVC pilot scale reactor and the concrete pilot scale re-actors served to estimate the loss of hydrogen sulfide toother processes than absorption and oxidation on concretesurfaces. A quadruple determination at 7 °C showed that thePVC pilot scale reactor absorbed 0.00089 ppm s−1 (± 15%) at10 ppm (r10), 0.0091 ppm s−1 (± 17%) at 100 ppm (r100), and0.093 ppm s−1 (± 18%) at 1000 ppm (r1000).

All parts of the setup contributed to the hydrogen sulfideabsorption; i.e. the free water surface, the gas circulationsystem, the PVC endplates, the gas censor, and the 200 mmPVC test pipe sections (Fig. 2). The only difference between thePVC pilot scale reactor and the concrete pilot scale reactors wasthe test pipe itself. As the surface of the PVC test pipe alonewas55% of the total PVC pilot scale reactor surface, the hydrogensulfide losses in the concrete pilot scale reactors have likelybeen less than half of the hydrogen sulfide absorption found forthe PVC pilot scale reactor. Comparing the relative importanceof the hydrogen sulfide loss with the overall rates measured inthe concrete test pipes (Fig. 4), the losswasdeemed insignificantand not taken into account when analyzing the data.

All hydrogen sulfide absorbed by the concrete surface mustultimately have been oxidized. However, it cannot be excludedthat the absorption rate was higher than the oxidation rate ashydrogen sulfide might temporarily accumulate in the corro-sion products. To verify whether this was the case or not, initialexperiments were conducted to try saturating the setup withhydrogen sulfide. Two corroded concrete pilot scale reactorswere continuously exposed to 100–400 ppm hydrogen sulfideconcentrations for 1 h and hydrogen sulfide absorption rateswere measured immediately before and after. The hydrogensulfide absorption rates before and after the saturation attemptwere identical, and it was concluded that the absorption ratesreflected the hydrogen sulfide oxidation rates.

3.2. Hydrogen sulfide oxidation kinetics

A large variability in the hydrogen sulfide oxidation rates ofthe 6 pilot scale reactors was observed, however, the oxidationof hydrogen sulfidewas in all cases accurately simulated by anexponential function of the hydrogen sulfide concentration

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Fig. 5 –Correlation between the reaction order (n) and thehydrogen sulfide gas phase (g) oxidation rate at 100 ppm(r100). Regression line and 95% confidence intervals are shown.

166 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 4 ( 2 0 0 8 ) 1 6 2 – 1 7 0

(Eq. (1)). Fig. 4 gives an example hereof for a low, amediumanda high hydrogen sulfide oxidation rate. Due to the responsetime of the hydrogen sulfide sensor as well as incompletemixing within the system, the data obtained at high hydrogensulfide concentrations and high oxidation rates had to bedisregarded, cf. Fig. 4.

The reaction order (n) correlated weakly with the hydrogensulfide oxidation rates. However, on the 95% confidence level,the order increased with increasing rates. Fig. 5 shows thecorrelation for the hydrogen sulfide oxidation rate at 100 ppm(r100), but also the oxidation rates at 10 ppm and at 1000 ppmshowed a statistically significant tendency towards increasein the reaction order with increasing oxidation rate.

The underlying cause for the correlation between reactionorder and hydrogen sulfide oxidation rate is believed to bean incomplete mixing within the test pipes. The sensor formeasurement of hydrogen sulfide concentration is placed onthe sewer gas circulation pipe (Fig. 2) and the data recorded areassumed to represent the hydrogen sulfide concentration atthe concrete surface. However, the circulation time withinthe system is around 40 s, which—together with the fact thatthe rates at high hydrogen sulfide concentrations were severalppm s−1 (Fig. 4)—leads to the conclusion that there weresignificant hydrogen sulfide gas phase gradients along thelongitudinal axis as well as the radial axis of the concrete testpipes.

To the knowledge of the authors, no previous study exists onthe hydrogen sulfide oxidation kinetics of corroding concrete;

Fig. 6 –Left photo: A slightly corroded concrete pipe with a cleA heavily corroded concrete pipe. Right photo: The surface te

however, Æsøy et al. (2002) report a study on concrete corrosionkinetics. They studied hydrogen sulfide corrosion bymeasuringthe corrosion depth of concrete coupons subject to wastewaterand hydrogen sulfide gas at relatively constant gas phase con-centrations of 0 ppm, 2–4 ppm and 15–25 ppm. They concludedthat the corrosion followedaMonod typeexpressionwithahalf-saturation constant of 2 ppm and a maximum corrosion depthof 15mmyear−1 (25 °C). Assuming a linear relationship betweenconcrete corrosion rates and surface rates of hydrogen sulfideoxidation, the results can be compared. The saturation kineticsreported by Æsøy et al. (2002) are not in agreement with thefindingsof thepresent study,however, their conclusion isbasedon only 3 data points—one being the corrosion rate at 0 ppmhydrogen sulfide—and the statistic evidence on whetherthe corrosion actually follows a Monod type expression seemsrather weak.

3.3. Development of corrosion

Some months after startup of the test rig, commencement ofcorrosion was observed as the formation of a thin layer ofcorrosionproducts at the concretepipesegments. The corrosionproducts first became visible close to the water surface andformed a clearly visible and sharp corrosion front. With time,the corrosion front crept towards the sewer crown (Fig. 6, leftphoto) and pH measurements of the pipe surfaces confirmedthe visible signs of corrosion. The surface pHwas neutral at theun-corroded crownof thepipe anddecreased towards thewaterline. After approximately 4 months of operation, all pipes wereseverely corroded with surface pH values below 1–2 (Fig. 6,middle photo). The color of the corrosion products ranged fromgrey to yellow, indicating a rather high content of elementalsulfur, confirming the observation by Parker and Prisk (1953),who found significant amounts of elemental sulfur in corrosionproducts from concrete sewers.

The hydrogen sulfide oxidation rates of the pilot scalereactors increased from the first measurements in August tillSeptember (Fig. 7). In August, visual inspection had shown theconcrete surfaces to be only partly corroded (Fig. 6, left photo),whereas 1 month later, all concrete surfaces were attacked(Fig. 6, middle photo). After September, no systematic changein the hydrogen sulfide oxidation rates could be observed(Fig. 7), even though the temperature in the test pipes de-creased by 14 °C (Table 1). In some of the test pipes, the activityactually increased with decreasing temperature, and someof the highest oxidation rates were found at the lowest

arly visible corrosion front at the pipe soffit. Middle photo:xture of a heavily corroded concrete pipe.

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Fig. 7 –Development in hydrogen sulfide oxidation parameters. Oxidation rate constant (kF), reaction order (n), and oxidationrates at 10 ppmH2S (r10), 100 ppmH2S (r100), and 1,000 ppmH2S (r1000). The rates are not temperature corrected.

167S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 4 ( 2 0 0 8 ) 1 6 2 – 1 7 0

temperatures. After the test period, the concrete segmentswere washed and inspection showed that the heaviest cor-rosion had occurred close to the water line.

Comparing the individual segments of the test pipes, thehydrogen sulfide oxidation rate of every second segment wastested. For each section, between 5 and 14 hydrogen sulfideinjections were measured. Fig. 8 shows the oxidation rates at100 ppm (r100). No dependency was found of the oxidation rateon the location of the segment within the pipe, and in generalthe variability between pipe segments of the same test pipewas of the same magnitude as the variability between testpipes. Oxidation rates at 10 ppm and 1000 ppm are not shown,but gave similar results.

Based on the assumption that the most heavily corrodedpipes also had the highest hydrogen sulfide oxidation activity,the concrete test pipes and their individual segments wereinspected and characterized. Visual differences between the

Fig. 8 –Comparisonof hydrogensulfide oxidation rate of individuato 12 °C.

degree of corrosion as well as surface texture were observedbetween the 6 test pipes as well as between the individualconcrete pipe segments of each test pipe. Comparisonwith thehydrogen sulfide oxidation rates of the individual pipe sectionsrevealed a weak tendency towards the slower pipes having aslightly closer surface texture compared to the faster pipes,however, the visual examination was not conclusive on apossible correlation between the hydrogen sulfide oxidationrate and the surface texture.

The observed oxidation rates caused a rapid reduction in thegas phase hydrogen sulfide concentrations. For the pipesexhibiting the highest oxidation rates, 1000 ppm of hydrogensulfide was oxidized in just 10 min, resulting in initial fluxes ashigh as 1 mg H2S-S m2 s−1 (Fig. 7). Due to the yellowish color ofthe corrosion products, it seems reasonable to assume thata fraction of the hydrogen sulfide was oxidized to elementalsulfur and possibly also to other intermediate products, e.g.

l concrete pipe segments. The rates are temperature corrected

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168 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 3 9 4 ( 2 0 0 8 ) 1 6 2 – 1 7 0

thiosulfate, trithionate, tetrathionate (Parker, 1945b; Parker andPrisk, 1953; Islander et al., 1991). This hypothesis is supported byJensen et al. (submitted for pulication) who studied the hy-drogen sulfide oxidation kinetics of concrete corrosion productsin acidic suspension and found that elemental sulfur was theimmediateproductof theoxidation.They furthermoreobservedsignificant oxygen consumption in the absence of hydrogensulfide, indicating a delayed oxidation of intermediates.

The hydrogen sulfide oxidation rate of corroding concrete isof the same magnitude as oxidation rates reported by Nielsenet al. (2005) for submerged sewer biofilms. They found thatbiofilm oxidation of hydrogen sulfide could be described by ahalf order reaction in dissolved oxygen and sulfide, with a rateconstant of around 0.1 (g S)0.5 (g O2)−0.5 m h−1 (biofilm grown athigh sulfide loads and 20 °C). At 20 °C, oxygen saturation and0.37 g S m−3 (i.e. corresponding to hydrogen sulfide saturationat a gas phase concentration of 100 ppm), the correspondingsurface-specific hydrogen sulfideoxidation rate is 0.05mgSm−2

s−1—i.e. well within the range of oxidation rates for corrodingconcrete at a gas phase concentration of 100 ppm H2S (Fig. 7).

The present study does not state whether or not the rapidoxidation of hydrogen sulfide is a biological process, a chem-ical process or a combination of biological and chemical pro-cesses. As reported by Parker and Prisk (1953), both hydrogensulfide and oxidized sulfur compounds—for example elemen-tal sulfur—can be utilized by the microorganisms that are themain responsible for the corrosion process at acidic condi-tions. It is furthermore known that chemical oxidation ofhydrogen sulfide to elemental sulfur takes place (Parker, 1947).Jensen et al. (submitted for pulication) found that biologicalhydrogen sulfide oxidation typically accounted for more than95% of the total oxidation rate and that chemical oxidationplayed aminor role. However, it stands to reason that in either

Fig. 9 –Hydrogen sulfide gas phase concentrations and hydrogengravity sewer. (g) indicates ‘gas phase’ and (w) indicates ‘waterslope is 0.5%, water temperature is 20 °C and pH is 7.0.

case the surface texture of the corroding concrete is an im-portant factor, as the surface area of corroding concrete ismuch larger than that of a smooth concrete surface (Fig. 6,right photo).

3.4. Corrosion rates

The observation that hydrogen sulfide was rapidly oxidizedand that elemental sulfur seemed to be present in the cor-rosion products, substantiated a hypothesis that the oxida-tion of hydrogen sulfide to dissociated sulfuric acid tookplace in steps: First the bulk of the hydrogen sulfide wasrapidly absorbed by the corrosion products and oxidizedto intermediate products—mainly elemental sulfur—uponwhich the intermediates slowly became oxidized to sulfuricacid. The hypothesis is further substantiated by the observa-tions of Jensen et al. (submitted for pulication), who docu-mented a similar hypothesis when investigating the sulfideoxidation in concrete corrosion product in acidic suspen-sions. Also the observations by Parker (1945b) that the im-mediate product of hydrogen sulfide oxidation primarily iselemental sulfur support this hypothesis.

The concrete surface of the experimental setup (Fig. 2) wassubjected to approximately 1.1 g S m−2 d−1, all of which ulti-mately could be oxidized to sulfuric acid. However, a loss ofcorrosion products containing intermediateswouldmeana lossof potential sulfuric acid and consequently reduced concretecorrosion. Another phenomenon of similar effect is loss ofsulfuric acid by droplets of condensate falling back into thesewage (Melbourne and Metropolitan Board of Works, 1989). Inthe experimental setup, little condensate was observed at theconcrete surfaces and loss of sulfuric acid by droplets was notobvious.

sulfide oxidation rates at steady state conditions in a concretephase’. When nothing else is stated, pipe diameter is 1.0 m,

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The alkalinity of the concrete pipeswas equivalent to 0.181 gCaCO3 (g concrete)−1 with a specific density of 2340 kg m−3. Atthis alkalinity and specific density, corrosion could cause aloss of concrete around 3.1 mm year−1—this is assuming thatall hydrogen sulfide ultimately was oxidized to sulfuric acidwhich then attacked the concrete. The actual amount of acidattacking the concrete has not yet been quantified. However,visual inspection of the test pipes corresponded well with thecalculated corrosion rate. The bulk of the oxidized hydrogensulfide must therefore ultimately have ended up attackingthe concrete, and only a smaller fraction would have droppedback into the sewage without reacting with the concrete ofthe pipes.

3.5. Extrapolation to field conditions

The concentration of hydrogen sulfide in the atmosphere of aconcrete gravity sewer is determined by the rate of hydrogensulfide release from the bulk water, the rate of hydrogen sulfideoxidation by the corroding concrete, as well as ventilation andgas exchanges with the urban atmosphere. Assuming steadystate conditions and disregarding ventilation and gas ex-changes, the hydrogen sulfide concentration in the seweratmosphere can be found from balancing the hydrogen sulfiderelease from the bulkwaterwith thehydrogen sulfide uptake bythe corroding concrete. Fig. 9 shows the result of suchcalculations at steady state conditions. The release of hydrogensulfide from the bulk water into the sewer atmosphere wasdetermined according toYongsiri et al. (2005). Theuptake rate ofhydrogen sulfide by the corroding concrete was determinedfrom Eq. (2) applying the median values from September toFebruary (n=0.569 and kF=0.00488 mg S m−2 s−1 (ppm H2S)−n).

Steady state conditions were calculated for bulk waterhydrogen sulfide concentrations up to 5 g S m−3. Within thisrange, the effect of pipe slope, wastewater pH, and pipe diam-eter were analyzed (Fig. 9). In general, the calculated gas phaseconcentrations were low compared to equilibrium conditionsaccording to Henry's law. For example, at pH 7, 20 °C, and a bulkwater hydrogen sulfide concentration of 2.5 g S m−3, thetheoretical gas phase concentration at equilibrium is around350 ppm. However, even at high slope and large pipe diameters,calculated steady state concentrations were less than 5–10% ofthe theoretical equilibrium concentration. These results arein good agreement with field observations e.g. Nielsen et al.(in press) and Melbourne and Metropolitan Board of Works(1989) who both report sewer gas phase concentrations ofhydrogen sulfide to be generally less than 10% of the theoreticalequilibrium concentration.

The simulations of hydrogen sulfide oxidation rates showthat significant uptake and consequently significant corrosioncan occur even though the gas phase concentration of hy-drogen sulfide is low (Fig. 9). For example for a pipe diameter of1.0 m, a slope of 0.3%, a pH of 7.0, and a water phase hydrogensulfide concentration of 2 g Sm−3, the gas phase concentrationat steady state is less than 4 ppm. However, the hydrogensulfide oxidation rate of the corroding concrete surface is about0.01mgSm−2 s−1. Assuming a concrete alkalinity of 0.2 gCaCO3

per g of concrete and that all absorbed hydrogen sulfide isoxidized to sulfuric acid, this uptake rate corresponds to acorrosion rate of 2 mm per year.

4. Conclusion

The test rig study on hydrogen sulfide corrosion of concretesewers revealed that hydrogen sulfide absorption and oxidationis a fast process compared to hydrogen sulfide release from thebulk water of a gravity sewer, and that corroding concrete iscapable of oxidizing hydrogen sulfide at rates similar to sub-merged sewer biofilms. The knowledge gained complementsthecentury-oldqualitativeunderstanding thathydrogensulfidecorrosion reduces the life expectancy of concrete sewers witha conceptual and quantifiable understanding of the hydrogensulfide absorption and oxidation processes.

The study focused on intermittent hydrogen sulfide loads,simulating gravity sewer conditions downstream of a forcemain. Corrosion of concrete surfaces was observed within afewmonths after upstart of the test rig, with surface pH valuesfalling below 1–2. The corrosion began close to the water line,but within 4 months, the corrosion front had crept all way tothe pipe crown. After the test period, inspection showed thatthe heaviest corrosion attack had occurred close to the waterline.

The observed oxidation rates increased with increasinghydrogen sulfide concentrations and showed no tendencytowards saturation, even at gas phase concentrations as highas 1000 ppm. The hydrogen sulfide oxidation kinetics followedsimple nth order kinetics,with a process order in the range from0.45 to 0.75 and an average rate constant of 0.005 mg S m−2 s−1

(ppm H2S)−n.Extrapolating the results from the test rig to gravity sewer

systems, the fast hydrogen sulfide oxidation rate balancedwith the slower hydrogen sulfide release rate at rather low gasphase concentrations. The immediate consequence hereof isthat it is the hydrogen sulfide release rate that controls thecorrosion rate and that high corrosion rates can occur eventhough hydrogen sulfide gas phase concentrations are low.

The knowledge gained on the kinetics of hydrogen sulfideoxidation by corroding concrete surfaces is crucial when pre-dicting sewer corrosion. Together with an understanding ofother in-sewer processes, the knowledge gained can be appliedto assess the extent and propagation of corrosion along a sewerpipe. Sewer regions in risk of severe attack can be identified andthe results applied to develop preventive maintenance strate-gies. Due to the malodorous probabilities of hydrogen sulfide,the knowledge on hydrogen sulfide oxidation kinetics further-more contributes to a more detailed prediction of the extent ofodor problems caused by sewer systems.

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