8) Pergamon

PH: S0273-1223(98)OOOO2-X

War. Sci Tech: Vol. 37, No. I, pp. 77-85,1998­Q 1998 IAWQ. Published by ElsevierScienceLid

Printedin GlUt Britain.0273-1223/98SI9'OO + 0-00

STUDY ON HYDROGEN SULFIDEGENERATION RATE IN PRESSUREMAINS

Mitsuo Kitagawa* Takatoshi Ochi** and Syuji Tanaka***

• Fukui Construction Office. Japan Sewage Works Agency. 2F-Tsutae·b iru,1905 Wada 2·chome. Fukui-city, Fukui 910. Japan.. Ductile Cast Iron R&D Department. Kubota Corporation. 26 Ohamacha 2-chomeAmagasaki-city, Hyogo 660. Japan••• Wastewater System Division. Public Works Research Institute ofMinistry ofConstruction. Asahi I. Tsukuba-city, Ibaraki 305. Japan

ABSTRACf

Experimental facilities comprising 1.8 km of pipeline, 100 mm in diameter and pumping equipment, wereinstalled in a wastewater treatment plant and operated continuously for more than one year to clarify themain factors governing hydrogen sulfide generation in pressure mains. The effects of temperature . organicmatter, and sulfate on sulfide generation rate were investigated based on observed values. The sulfidegeneration rate depended significantly on wastewater temperature . It was confirmed not empirically butexperimentally that the effect of temperature (T) was expressed by (1.065)T-20. In respect of organic matter ,it is considered that there is a lillie effect of organic matter concentration on sulfide generation rate when thefluctuation of soluble organic matter concentration is slight. However. based on observed values, it wasfound that sulfide generation rate clearly depended on sulfate concentration when the biofilm was ratherthick like these experiments . Also. partial penetration of sulfate into biofilm was confirmed using a biofilmmodel. Furthermore. biofilm model as a sound method for predicting sulfide generation rate was discussed .© 1998IAWQ. Published by Elsevier Science Ltd

KEYWORDS

Sewer systems; hydrogen sulfide; generation rate; temperature; organic matter; sulfate; biofilm.

INTRODUCfION

Hydrogen sulfide generation in pressure mains is caused by the microbial reduction of sulfate to sulfideunder anaerobic conditions. Generated hydrogen sulfide induces serious maintenance problems such as odorand corrosion of concrete structures. Rational counter measures for these problems should be based on anappropriate method of predicting sulfide generation. Numerous studies have been conducted, and severalempirical equations have been proposed (USEPA 1985; Boon and Lister 1975; Thistlethwayte, 1972). Inthese equations , it is considered that the sulfate in a pressured pipe is reduced by microorganisms in thebiofilm on the pressured pipe inner surface and by microorganisms suspended in pressured flowingwastewater. with little deposition. The following equation was proposed for sulfide generation rate inpressured pipe.

77

78 M. KITAGAWAetal.

dC dC 4 (T.20)Sulfidegenerationrate=- = V.-= { Flux(20). - + Rs(20)} . a

dt dX D

Flux (20) = sulfide surface flux at 20°C (g-S/m2.h)

C =sulfideconcentration(mg-Szl)t =time (h)V = constant flow velocity (rnIh)X =distance (m)D =pipe diameter (m)T =wastewatertemperature(0C)Rs(20)=sulfide generationrate in flowing wastewaterat 20°C (mg-sn.h)a=constant value

(1)

Proposed predictive equations for sulfide generation rate in pressured pipe are summarized in Table 1. Allequations include the effect of temperature. However, the value of a, the effect of velocity and sulfateconcentration, sulfate reduction rate in flowing wastewater, and influence of organic matter differ. As aresult, the sulfide generation rates predicted with these equations are not always the same. Therefore, it isverydifficult to design new sewer systemsrationally, using these equations.

Table I. Predictiveequationsof sulfidegenerationin pressuredpipe

Equationnumber(1-1)(1-2)(1-3)

Flux(20)1.0x10-3.BOD

0.228x10-3.CODcr0.5xI0-3.V.BODO.S.S040.4

Rs(20)l.57x10-3.BOD

0.365xI0-3.CODcr

a1.071.071.139

SourceUS.EPA(1985)Boon (1975)Thistlethwayte (1972)

To improve the method of predicting the sulfide generation rate in pressure mains, several studiesintroducing the biofilm model have been reported (Nielsen 1987, 1988, Holder et al., 1985, 1989). Thereduction of sulfate in pressured pipe occurred mainly in biofilm adhering to the inner surface of the pipe.Sulfate reduction rate in pressuredpipe dependson the substrate transferrate from bulk water to the biofilmsurface and the substrate uptake rate in biofilm.Substrate transfer rate from bulk water to biofilm surface isdescribed by Fick's first law of diffusion. Equation 2 shows the sulfate transfer rate for example. Thesubstrate uptake rate in biofilm is described by Eq. 3, which is obtained by combining Michaelis-Mentonkineticsand substrate transport rate in biofilm using Fick's second law of diffusion.The integrationof Eq. 3yields the substrate transfer rate at biofilmsurface (Eq. 4). At the biofilmsurface, substrate flux ofEqs 2 and4 is the same; by numerical solution of Eqs 2 and 4, substrate flux for a given substrate concentration inwastewater can be calculated. Furthermore, reduced sulfate concentration at any point in a pressuredpipeline can be predicted by a combination of Eqs 2, 4 and 5 when the sulfate concentrationat the pipelineinlet is given. By introducing a biofilm model, it is considered that the sulfide generation rate can berationally predicted, but there are few studies in which the sulfate reduction rate is analyzed and confirmedusing a biofilm model, based on observedvalues in a sewer system.

With this as background,experimental facilities comprisingeplOOmm, 1.8kmpressure pipeline and pumpingequipment were built to clarify the main factors governing the sulfide generation rate. The experimentalfacilities were operated continuously for one year, and concentrations of organic matter, sulfate and sulfide,temperature, biofilm thickness, etc. were measured. Based on the observed values, sulfide generation ratewas calculated,and the effect of temperature and organic matteron sulfide generationrate was investigated.Also, the effect of sulfate was analyzedand discussedusing the biofilmmodel.

Ree- Db . (Sb - Ss )

Zb(2)

Hydrogen sulfide generation rate 79

Re =sulfate flux (g-S / m2.h)Ss =sulfate concentration in the bulk water (mg-SII)Sb =sulfate concentration at the biofilm surface (mg-SII)Db =diffusion coefficient in the wastewater (m2/h )Zb =diffusion layer thickness (m)

d2S Kof. SDf.-= ---

dZ2 Km +S(3)

Of =diffusion coefficient in the biofilm (m2/h)S =sulfate concentration in the biofilm (mg-SII)Z =distance from the biofilm surface (m)Kof =zero order volume constant (g-S/m3.h)Km =half order constant (mg-S/I)

dS I \/2 Km + Sb 1/2Re = - Of .- = (2 .Kof. Of) . { (Sb - Se) - Km. In ( ) }

dZ Z=O Km+Se(4)

Se =sulfate concentration at the pipe walI surface (mg-SII)

dSs dSs 4-= V.-= -Re(Ss) .-

dt dX 0(5)

Sulfate Ss

Pipeall

Ze

Partial~ Fullpenetration penetration

Turbulentflow

LimitingSulfate

[Bulkwater] [Diffusion layer] [Biofilm]Db OfLaminarflowZb

Sulfate

concentratio:::n=-~L~~~;;;;:::~-V

Figure \. Sulfate transfer and uptake in biofilm .

EXPERIMENTAL FACILITIES AND METHOD

Several measurements and experiments were conducted using the experimental facilities illustrated in Fig. 2.These facilities were completed in November 1994 at Kasumigaura Wastewater Treatment Plant. This WTPis located approximately 70km north of Tokyo, Japan, and domestic wastewater is mainly treated. Theamount of treated wastewater, and its treatment area are about 55,OOOm3/day, and 28.5km2, respectively .Continuous operation at a constant flow velocity (O.6m1s) commenced in December. From that time, waterquality analysis was conducted regularly. A gradual increase in generated sulfide concentration wasobserved. In February 1995, steady state condition was confirmed by observing little increase of generatedsulfide concentration. The wastewater for this experimental facility was drawn from a surge tank locatedbetween the grid chamber and the primary sedimentation tank of the WTP. The detention time of storage

80 M. KlTAGAWAttal.

tank was 2.5 hours when the sewage flow velocity in pressured pipe was O.6m1s, and ORP and DO of drawnwastewater were maintained under -lOOmV, and O.lmgll, respectively . Controlled wastewater was thenpumped to cast iron pipes of diameter IOOmm and total length J.8km. Parallel or series circuit operationmode can be selected by controlling of valves (see Fig. 2). The experimental facility pipe was laid flat inmost positions. To parallel operation mode, the sampling points were three points (S I, S4, S2), with anotherpipeline used for sulfide control experiments. In series circuit mode, the sampling points were SI-S7.Several measurements were conducted at two fixed flow velocities, O.6m1s and O.3m1s; in each case, theflow condition was turbulent. During only dry weather flow condition, concentrations of total and solublesulfide, sulfate. and organic substance (BOD, CODcr, DOC), water temperature, DO, ORP, biofilmthickness, and flow rate were measured regularly during one year. At the end of each run, unit portions ofpipeline were removed at three points. (SI-S3) to measure biofilm thickness. For measurement of biofilmthickness, only the wet biofilm adhering to the inner pipe surface was scraped and collected carefully, andthe volume of the wet biofilm was measured . Biofilm thickness was calculated from the measured volume.When the pipe portions were detached, no deposits on the pipe bottom were found.

(parallel circuit)

54(l/4L)

56

(Series circuit)

Pipeline(lOOmmDiax 1800mLon~)

51(inlet) \

5S(3/4L)~..__...----...-...;....-.;..,

out 53(outlet) 57

N.B.) 51-S7: measurement points

Pipeline( IOOmmDia)

Hydrogen sulfidegeneration pipe

Pumping

r'---r~~~....J..."'"1--.--+-------....--...

larySedimentationtank

Figure 2. Schematic diagram of experimental facility system.

RESULTS AND DISCUSSION

Sulfide ~eneration rate in bjofilm and flowjn~ wastewater

In series circuit operation mode, flowing wastewater was sampled at S I, S2 and S3, and sulfide generationrate in flowing wastewater (Rs) was measured in winter and summer. Rs, generated sulfide concentration perlitre of wastewater-hour, was O.12mg-SII.h in winter (T=16°C) and O.14mg-SII.h in summer (T=27°C).These values agreed well with proposed empirical equations I-I and 1-2. The sulfide generation rate inbiofilm was 4.96mg-SII.h in winter, and 1O.52mg-SII.h in summer. Since the ratio of Rs to generation rate inbiofilm was relatively small in this experiment, Rs is hereafter neglected in this study.

Effect of temperature

Table 2 shows the water quality results obtained over a year's measurement from February 1995 to February1996, and classified into 9 cases by combinations of temperature (by 1°C) and flow velocity (O.3m1s andO.6m1s). Water quality and sulfide generation rates given in Table 2 are averages for each run. The sulfidegeneration rate was calculated using Eq. 6.

dC toC toC - Cin + Cout-=- = V.--= ----dt tot ex H

(6)

Hydrogen sulfide generation rate 81

zxH =detention time (h) =--­

V(7)

Run o.

T-20

Flux (T) =0 . 172 x r. 065

Run No. : Velocityo O.6m/so 0.3m/s'!l 0.25

<,U)I~ 0 .2

S 0 .15)(;,

L;: O. 1ill

"t:l

;;: 0 .05

~C was calculated by sulfide concentration, measured at S I (Cin) and S2 (Cout). In addition, another ~Cwas calculated by sulfide concentration, measured at S2 (Cin) and S3 (Cout ), in the case of series circuits.Figure 3 indicates the relation between water temperature and the average, maximum and minimum value ofFlux (T) for each run. Remarkably, Flux (T) depended on temperature, increasing as the wastewatertemperature rose. e was calculated by the least squares method, using average Flux (T) and temperature foreach run; its value was 1.065. This value was almost same as in empirical equations I-I and 1-2.

0.3 ,...------ - - - - - ----;::-::;""'1

16 18 20 22 24 26Was t ewa t e r temperature rOC]

28

Figure 3. Observed sulfide Flux (T) versus wastewater temperature .

•• • ••

orrccted Flux(20)

" ·111=Flux(T ) 11065

100 200 300 400 500BOD [rng/l]

:;: 0.3 r---- - --,-----,M

.§til

OIl0°·2M

'X':>

ti:II)

;g 0 . 1"3VI

-.::lII)

UII)

~ a "'-- - - - - - - - - - - -'u a

Figure 4. Corrected sulfide Flux (20) versus BOD.

Effect of or~anic mailer

Figure 4 shows the relation between BOD and corrected Flux (20). Corrected Flux (T) is the observedFlux(T) divided by (1.065)T.20. Corrected Flux (20) ranged from 0.115 to 0.22g-S/m2.h. Slight correlationwas found with Eq (I-I) within BOD range from IOOmgll to 200mgll , but an adequate correlation for allrange of observed BOD has not been found . Also, there was not an adequate correlation between correctedFlux (20) and COD, S-BOD, S-COD and DOC. It is known that sulfate-reducing bacteria utilize dissolvedorganic matter such as propionate, lactate, etc. In each run, the fluctuation in concentration of soluble

82 M. KITAGAWAela/.

organic matter such as S-BOD, S-COD, and DOC was slight compared with total BOD and COp (see Fig.5). It was considered that when the fluctuation of soluble organic matter is slight, there is little effect oforganic matter on sulfide generation rate. In these experiments, obvious effect of velocity on corrected Flux(20) or Flux (T) was not found.

o 100 200 300 400 500unit : rng/l600 700

BOD I • • ..CODer • • ..S-BOD i .... - - - - ------- --

S-CODer* r=--. ....DOC I ...

* Us i ng 1. OJ./m-f i It e r • Ave rage

Figure 5. Fluctuation range of observed organic mailer concentration (total and soluble).

Table 2. Experimental condition and observed values

Run Time Velo- Points Temp Sulfate DOC BOD S-BOD CODcr Sulfide Flux(T) BiofilmNo. city * I *2 generation rate thickness

m/s "C mg-S/I mg/I mg/I mg/I mg/I mg-S/Lh g-S/m2h Jlm1995 0.6 in 16.6 116 50 341 59 508 o 127*' 390Feb. out 15.7 80 48 350 52 +8%*4 _4%*5

2 1995 06 in 226 12.0 34 133 44 279 6.94 o 173 1550May out 22.1 7.5 31 133 41 263 +12% -15%

3 1995 06 In 270 9.5 38 93 25 374 10.66 0.267 920Sep. out 26.7 3.7 37 109 39 328 +9% -15%

4 1995 0.3 In 23.9 10. I 37 162 48 383 855 0.214 1380Oct out 23.6 55 41 143 47 388 +9% -11%

5 1995 0.3 In 229 113 47 115 36 347 695 0174 920Nov out 226 7.3 52 107 36 325 +11% -9%

6 1995 0.3 In 22.1 10.7 40 124 45 306 776 0.194 920Nov out 214 7.2 42 108 38 292 +13% -10%

7 1995 0.3 in 19.2 40 754 o 188 920Nov out 18.3 39 +7%-15%

8 1996 0.3 In 16.2 13 7 44 204 62 430 544 o 136 1180Jan. out 14.9 10.1 44 188 59 379 +14% _13°;0

9 1996 0.3 In 17.1 139 38 258 59 505 593 0.148 1180Jan out 16.2 10.3 39 244 57 488 +9% -23%

A 1996 0.3 SI 210 12.3 40 164 48 347 500Apr. S3 193 49 45 169 53 393

B 1996 0.3 SI 212 12.5 39 140 45 373 500Apr S3 19.6 4.6 41 156 48 417

*1 Dissolved organic carbon using 0 45 urn-filter, *2 L sing I 0 urn-filter• 3 Average. *4 (Flux max - Flux ave),' Flux ave. • ~ (Flux min - Flux ave) / Flux ave

Effect of sulfate

Figure 6 shows the relation between corrected Flux (20) and sulfate concentration. Slight correlationbetween corrected Flux (20) and sulfate concentration could be recognized as compared to the organicmatter. Nielsen and Jacobsen (1988) reported that sulfate was limiting for sulfate reduction only at relativelylow sulfate concentration, but sulfate limitation may be expected with very thick biofilms, or high Kofvalues. In the present experiment, relatively thick biofilm was observed, and partial penetration of sulfate tobiofilm could be considered. To confirm the effect of sulfate concentration on sulfide generation rate,

Hydrogen sulfide generation rate 83

additional experiments were conducted. Several measurements were measured at points (5 I-57) in AM (runA) and PM (run B) on the same day. The results are listed in Table 2. Biofilm thickness was 500~m, andlittle difference between organic matter concentration at 5 I and 53 .

• V=0.6m1

- =O.3m/s I

~ -~

I:. ,-...>. .....

••..

5 10 15ulfate concentration [rng- II )

0 '-- - - - - - - - -----'o

:2 O. 25 .-------- - ------,r-i.§C1 0.2.=!g O.1 5x:l

E:: O. 1~

"t.::; 0.05VI

"3

~o

U

Figure 6. Corrected sulfide Flux (20) versus observed sulfate concentration.

Figure 7 shows observed sulfate and sulfide concentration at each point. Decreased sulfate concentrationwas almost the same as increased sulfide concentration, so sulfide generation rate could be calculated usingEqs. 2, 3, 4 and 5. Clearly, the sulfate concentration did not decrease linearly, and sulfide concentrationincreased non-linearly. It was found that the sulfide generation rate depended on sulfate concentration.

m3.h I

• ulfide (Run-A)

• ulfide (Run-B)

OJ ulfate (Run-A)

ulfate (Run-B)

::o

• Measuring point----

5 -

0::::: IS .------- - - -,A.:::....--=== 7=;f-- -Vl ------CoE

<5~ 0 ~_-'- --J

~ 0 20 40 60 80 100Vl Detention time in pressured pipe [minute]

eo 10

.~

ilvc8

Figure 7. Observed concentration of sulfide and sulfate versus detention time.

IlisCussjon ysjn~ bjofilm model

To confirm whether the sulfate had already partly penetrated biofilm at 5 I, the sulfate flux, i.e., sulfide fluxwas calculated by numerical solution of Eq. 3. In the biofilm model, the Kof value is very important and canbe obtained using a drum reactor. In this study, the Kof value was not confirmed. The Kof value was firstcalculated using the difference between sulfide concentrations at 5 I and 54, in the case of full penetration ofsulfate to biofilm. In this case, the sulfide flux was directly proportional to the product of Kof value andbiofilm thickness. The calculated Kof value was 340g-S/m3.h. (see Fig. 7) Using this Kof value, the sulfateflux, i.e., sulfide flux was calculated by Eq. 3. For the Km and Df values in calculating of Eq. 3, the samevalue as in the report of Nielsen et al. (1988) was used. The result is shown in Fig. 8.MT 37,1.0

84 M. KITAGAWA et al.

Ob scrv ed sulfatecon cnrrauon range... : ~: ..:-

0_ - - - - - - - ---0o III 15 ~o ~ ;

. lufatc onccntrauon Il1Ig-. III

Ko f · ~~Ug. /1113 hBrofilm thicknc: s (Zc) 0 5111111or-e77 " 10-1' 111 2 /11

. . KIII OIIII!-SII

.. KIII " "~ ·1 IIlll-SI)

• Kill O '~(lIllI!- S/J

...

"

_ 0 25 ,...------------,..<::

E 02Vii!l 0,15z~ 0, 1

;g 0 05:;

'"

Figure 8, Predict ed sulfide Flux versus sulfate concentration with various values of Kill ,

Sulfide concentration in the pipeline was then calculated by numerical solution of Eqs 2, 4 and 5, for severalKof values, Calculation was conducted with and without diffusion layer considered. In these calculations,the effect of the Km value was rather small. The diffusion layer thickness was calculated using the equationdeveloped by Linton and Sherwood (1950). This equation was used in the report (Holder et at.. 1989) inwhich observed data in laboratory experiments were compared with predicted value s using the biofilmmodel , to confirm the model 's appropriateness. Figure 9 show s the result when Of , Db and observed Ze were2.77xlO,6m2/h (80 % of Db ), 3.46xI0-6m 2/h , 0.5mm respectively and Km value was neglected. The mostexpected Kof value for observed sulfide concentration was determined by the least squares method. Thisvalue was 7IOg-S/m3.h with diffusion layer considered. and 450g-S/m3.h without. The effect of diffusionlayer could not be neglected, because the flow velocity of each run was rather low. The actual Kof valuewould exceed 450g-S/m3.h.

Kof = -l 5 Ug-S/I1I ~ h

Ko f 3·lOg- SII1l~ h

Zb> 0 111111

Kof = 71Ug- 111I3.h

R

R2 6E;:t 4e8'" .,~ -~ 0"-- - - - - - - - - ---'

o 20 40 60 80 100Detention lime 111 pressured pipe Inunut e I

Kof 2 ~ 5UI! - . 111I ~ hKof = 3~u g-S/11I3 h

l bz 0 UK3 111111

O:v.· '----- - -------'

o :!O 40 60 0 100

Detennon time 111 pres ured pipe [minute J

~ I O ~ I O

Vi R .AIA n Kof' > 710g. 111I3 h :n:OIl ~) 8E R I ·B(PM) =

Figure 9, Obser ved and calculated sulfide concentration versus detent ion lime (V=() 3m/s. T=20°C).

In Fig . 8, predicted sulfide flux was almost constant (about 0.16g-S/m 2.h) when sulfate concentration rangedto more than about 15mg-S/1 which was called limiting sulfate concentration (see Fig . I), but was notconstant (less than 0.16g-S/m2.h) within observed sulfate concentration ranges in runs A and B. Because theamount of reduced sulfate in biofilm is in proport ion to the Kof value, the limiting sulfate concentrationincreases as the Kof value increases. If the Kof value was more than 340g-S/m3.h, the limit ing sulfateconcentration exceeded l5mg-S/1. Therefore, it could be concluded that the sulfate has partly penetrated thebiofilm throughout the pipeline in runs A and B. In Fig. 9 with and without diffu sion layer and with severalKof values, predicted sulfide concentration is different, so that Zb and Kof value s should be determined by

Hydrogensulfide generationrate 85

an appropriate method. Besides that, the flow condition (turbulent or laminar flow) and the status ofsubstrate penetration into biofilm (partial or full penetration) were critically important. The effect of flowvelocity on sulfide generation rate could be rationally explained by investigating the influences of velocityon several parameters used in the biofilm model, such as Zb and Zf.

CONCLUSION

To systematically clarify the main factors governing hydrogen sulfide generation in pressure mains.experimental facilities comprising 1.8 km of pipeline. 100 mm in diameter, and pumping equipment. wereinstalled in the WTP and operated continuously for more than one year. Using these facilities. the effect oftemperature. organic matter. and sulfate on hydrogen sulfide generation rate was examined and investigatedbased on values observed in experimental facilities .

The sulfide generation rate in pressure mains depended significantly on wastewater temperature. It wasfound that the effect of temperature m was expressed by (1.065)T-20based on observed values during oneyear. This value, 1.065, was almost the same as the proposed value empirically . An adequate correlationbetween organ ic matter concentration and corrected Flux (20) was not found. It is considered that there is alittle effect of organic matter on sulfide generation rate when the fluctuation of soluble organic matterconcentrations is slight.

Slight correlation was seen between corrected sulfide flux (20) and sulfate concentration when the distancebetween each measuring point was rather long, but in additional experiments using a narrowed distancebetween measuring points, it was clearly found that the generated sulfide concentration did not increaselinearly. When the biofilm is rather thick as in these experiments, sulfide generation rate clearly depends onsulfate concentration. Using the biofilm model, it was confirmed that sulfate partly penetrated throughoutthe experimental facility 100 mm pipeline when observed biofilm thickness was 0.5mm and flow velocitywas 0.3m/s. Also using biofilm model, the sulfide concentration in pressured pipeline was simulated underseveral conditions. Calculated results indicated that the Kof value of this experiment was between 450g­S/m3.h without a diffusion layer considered and 71Og-S/m3.h with it considered. To establish the rationalmethod of determining Kof, Zb, Zf value in several conditions is the subject for future study.

REFERENCES

Boon, A. G. and Lister, A. R. (1975). Fonnation of sulfide in rising main sewers and its prevention by injection of oxygen. Prog.Wat. Tuh., 7(2), 289-300.

Holder, G. A., Vaughan, G and Drew, W (\ 985). Kinetic studies of the microbiologicalconversion of sulfate to hydrogen sulfideand their relevance to sulfide generation withinsewers. WaL Sci. Tech., 17(2·3), 183·196.

Holder, G. A., van Oorschot, R. and Hauser, J. (\989) . Experimental and theoretical studies of sulfide generation in seweragesystems. Wal. Sci. Tech., 11(8·9), 757·768.

Linton. W. H. and Sherwood, T. K. (\950). Mass transfer from solid shapes to water in streamline and turbulent flow. Chem: Eng.Prog.45(5), 158-264.

Nielsen, P. H. (\987). Biofilm dynamics and kineticsduring high-rate sulfate reductionunder anaerobicconditions.Appl. Environ.Microbiol ., 53. 27-32.

Nielsen, P. H. and Hvitved-Jacobsen,T. (\ 988). Effect of sulfate and organic matter on the hydrogensulfide formation in biofilmsof filled sanitary sewers.J. WPCF., 60(5), 627·634.

Thistlethwayte,D. K.B. (1972). The Control o/Sulfides in Sewerage Systems. Butterworths,Sydney. p. 15USEPA(1985). Design Manual; Odorand Corrosion Control in SanitarySewerage Sstemsand Treatment Plants.pp. 2(}'22