Controlling sulfide generation in force mains by air injection

~

Pergamon

Wal. Sci. Tech. Vol. 37.No. I. pp. 87-9.5.1998. iC>

PH:S0273-1223(98)00003-1

1998 IAWQ. Published byElsevier Science LId Printed inGreat Britain. 0273-1223/98 S19'00 + 0-00

CONTROLLING SULFIDE GENERATION IN FORCE MAINS BY AIR INJECTION Takatoshi Ochi*, Mitsuo Kitagawa** and Syuji Tanaka*** • Ductile Cast Iron R&D Dept. KubotaCorporation, 26 Ohamacho 2-chome Amagasaki-city, Hyogo660, Japan •• Fukui Construction Office. Japan Sewage Works Agency.2F-Tsutae-biru, 1905 Wada2-chome, Fukui-city, Fukui 910,Japan ••• Wastewater systemdivision, Public Works ResearchInstituteofMinistryof Construction, Asahi 1. Tsukuba-city, lbaraki305, Japan

ABSTRACT Air injection into force mains has been used to control sulfide generation. However.the design criteria have not been clearly established. In this study. the minimum concentration of dissolvedoxygen (DO) required to prevent sulfide generation. and the oxygen balance in the force mains were investigated using an experimentalfacility. Air injection completely eliminated sulfide presenceat the pipe outlet when DO at the pipe end was 0.2 mgll or higher. Rcaerationfrom gaseousphase to wastewater was affected by sewage flow velocity and oxygen concentration in the gaseousphase. Oxygenconsumption rate in bulk water (Rr) ranged widelyfrom 3 to 18 mg/I.h. Oxygenconsumption rate in biofilm(Re) was measuredusing a rotatingreactor. Re seemed to increase in proportion to the square root of DO. @ 1998IAWQ. Publishedby Elsevier Science Ltd

KEYWORDS Hydrogensulfide; sulfide; air injection;force main; oxygenconsumption; reaeration; sewage. INTRODUCTION In long distance force mains sanitary sewage becomes anaerobic and hydrogen sulfide is generated, causing odor and deteriorating concrete structures. In recent years several methods have been proposed to control hydrogen sulfide generation. Air injection into the force mains is one useful and effective method to prevent hydrogen sulfide generation by keeping the sewage in aerobic conditions, and several studies of the air injectionmethod have been reported. However, a rational theory for controllingair injection volume to keep the sewage in aerobic condition has not been established. To prevent the sulfide generation effectively, oxygenbalance in air-injected force mains should be clarified, and air injectionvolume should be controlled based on actual oxygen balance in the force mains. In this report, oxygen balance in air-injectedforce mains was investigated and several factors such as reaeration rate (RO exchange coefficient (KL) and oxygen consumption rate in bulk water and in biofilm (Rr) and (Re) that affect oxygen balance were examined. Also, the minimum concentration of dissolved oxygen to keep the sewage in aerobic condition at the outlet of force mains was investigated.

87

T. oeHI et al.

88

MODELING OF OXYGEN BALANCE IN FORCE MAIN In air-injected force mains laid flat, two flow phases, i.e. gasous and liquid phase, are recognized as illustrated in Fig. I, same as the gravity sewers (U.S.EPA 1985). Oxygen in the gaseous phase dissolves into flowing wastewater by reaeration. At the same time, dissolved oxygen in the wastewater is consumed by microorganisms suspended in flowing wastewater and in the biofilm at the pressured pipe inner surface. Thus, dissolved oxygen concentration in an air-injected force mains is determined by the balance between the reaeration rate and the oxygen consumption rate. The oxygen balance in air-injected force mains was expressed by Equation( I) just as for gravity sewers. dDO/dt

= = = = = =

=Rf - Rr - RelR - dDsldt

(I)

DO concentration of dissolved oxygen in wastewater (mgll) t =time (h) Rf reaeration rate (mgll.h) R hydraulic radius (m) cross-section of sewage/wetted perimeter Rr oxygen consumption rate in the bulk water (mgll.h) Re oxygen consumption rate in the biofilm (g/m 2.h) Ds concentration of dissolved oxygen consumed by oxidation of sulfide (mgll)

=

Surface length L

Biofi 1m '- (Oxygen consumpt i on in b i of jim : Re)

Figure I. Simplified oxygen balancein air-injected forcemain.

The reaeration rate in air-injected force mains is expressed by Equation (2) same as gravity sewers. The exchange coefficient of KL(20) in gravity sewers is shown by Equation (3) (Parkhurst 1972). However. KL(20) of gravity sewers can not apply to air-injected force mains, because the flow conditions of air and liquid phase or the degree of the turbulence in air-water interface in air-injected force mains are completely different from those of gravity sewers. There are few reports about KL(20) in air-injected force mains. Also, VIA. the ratio of liquid-gas surface area to sewage volume in air-injected force mains differs from that in the gravity sewers, because the pressured air volume in air-injected force mains differs from the volume of gravity sewers. In Japan, Kameda (1993) proposed a calculation method for the increased head loss in airinjected force mains theoretically and experimentally. To calculate increased head loss, the void fraction, the ratio of gaseous phase to the entire pipe cross-section, is critically important. and the void fraction is obtained by air injection volume, water velocity, diameter and slope of pipe, and so on. In the case of flat air injected pipe, NY can be calculated by void fraction. Saturated dissolved oxygen concentration in the force mains can be expressed by Equation(4). Rf = Kz' (DOsp - DO) .

e (T·ZO) = KL(20)' (A / V ) . (DOsp - DO) . e (T-ZO)

(2)

Controlling sulfide generation in force mains

89

K2 =reaerationcoefficient (1/h) KL(20) = exchangecoefficient at water temperature 20°C (m/h) A =liquid-gas surface area (m2) V =sewage volume (m3) DOsp =saturatedDO (mgll) e = temperature correctionfactor = 1.024 with referenceto Elmore (1961) T =water temperature (0C) KL(20) =0.96 . (l + 0.17 . F2) . (s . u)318

(3)

F = Froude number = u/(g . V I A) 1/2 s =energy grade line (m/m) u =flow velocity (m/s) g =gravitationalacceleration(m/s2) DOsp :saturatedDO in the force main (mgll) =DOsa' (P + I) ' Oxi21

(4)

DOsa =saturatedDO under atmospheric pressure (mgll) P =pressure in the force main (kgf/cm2) Ox =oxygen concentrationin gaseous phase (%) Several studies about Rr and Re have been reported. According to USEPA (1985), Rr in force mains should be determined through sampling and analysis, and Rr values depending on the age of wastewater were suggested as a general guide. Also, it was shown that Re was directly proportional to the oxygen concentration and it increasesin proportion to the square root of the slope of the sewer and water velocity. The decreased concentration of oxygen in the gaseous phase between point A and B can be calculated by Equation (5), and if Rf, Rr and Re can be predictedrationally, dissolved oxygen concentration at any given point of an air-injectedforce mains can be calculatedby combination of Equations(I) and (5). 22.4 ' 100 ' V

aOx

=OXA - OXB =------32 ' 103' Rf' Qair

(5)

Qair = air injection volume (m3/h) aOx =decreasedconcentration of oxygen in gaseousphase (%) OXA =oxygen concentration in gaseous phase at point A (%) OXB = oxygen concentration in gaseous phase at point B (%) V =sewage volume (m3) In this report, oxygen balance in air-injected force mains were examined and investigated based on these equationsusing observed data of experimental force mains. MATERIALS AND METHODS The two experimental force mains, 100mm in diameterand 900 m long, as shown in Fig. 2 were installedat Kasumigaura Sewage Treatment Plant. One was for air injection, the other was for no air injection (sulfide generation confirmed). The pipes were laid almost flat. Wastewaterused in the experiments was drawn from a surge tank that is located between the grid chamber and the primary sedimentation tank and stored in the detention tank for 1 - 2 hours to adjust DO and oxidation reduction potential, Then drawn wastewater was supplied into the force mains. Points marked A-C and 1-5 in Fig. 2 indicatesamplingpoints. The experiments were conducted from October to November 1995, and from August to September 1996. Sewage flow velocity was 0.6 mls or 0.3 mls in each run. Injectedair flow rates were 11 % to 42% of sewage

90

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flow rates. Concentration of generated sulfide in the non-injected pipe was also measured for comparison with observed concentration in the air-injected pipe.

No a i r- injected pipe (900m)

.,

2

__~-:---:.:..-+~.--. ..

~_--,,3

5

4

Compressor (Air injection)

Primary Sedimentation tank

Figure2. Schematic diagramof experimental pipes.

RESULTS AND DISCUSSION Minjmum djssolved

oxy~en

concentration to keep the wastewater in aerobic condition

Table I shows the concentration of dissolved sulfide and DO measured at the experimental force mains inlet (Point A), intermediate point (Point B or 3) and outlet (Point C or 5). In the non-injection, with a sewage flow velocity of 0.3 mls (Case 2-2), dissolved sulfide concentration was 0.4 mgSn at the inlet, and increased to 5.6 mgSn at the outlet. With a sewage flow velocity of 0.6 mis, dissolved sulfide concentration at the outlet was 3.8 mgSn in Case 2-1, and 6.0 rng-Szl in Case 4-1. In contrast, when air was injected, dissolved sulfide was not detected at the force mains outlet in any case. DO was 0 mgll throughout the non-injected force mains, but was 0.2 - 1.8 mgll at the air-injected force mains outlet. Thus, it could be considered that sulfide generation at the pipe outlet was completely prevented when the concentration of dissolved oxygen at the pipe outlet was 0.2 mgll or higher. Table I. Measurement of dissolved sulfide and DO in experimental force mains Sewage Air injection volume rate flow of flow (B) rate(A) m3/min m3/min (mls) 0.060 I-I 0.283 (0.60) 1-2 0.283 (0.60) 0.030 1-3 0.141 (0.30) 0.060 1-4 0.141 (0.30) 0.030

Case

B/A

% 21 11 42 21

2-1 0.283 (0.60) 2-2 0.141 (0.30) 3-1 0.283 (0.60) 3-2 0.283 (0.60) 4-1

0.283 (0.60)

0.090 0.045

32 16

Temp. Dissolved sulfide of Inlet Inter- Outlet sewage mediate mgSn mgSn mgSn °C 23.2-23.7 0.2 0.0 0.0 23.1-23.7 0.1 0.0 0.0 22.8-23.4 0.3 0.0 0.0 0.0 22.0-22.4 0.2 0.0

Inlet

rng/l 0.0 0.0 0.0 0.0

DO Inter- Outlet mediate rng/l mg/l 1.4 0.9 0.5 0.2 1.4 1.3 0.8 0.8

22.9-23.7 21.7-23.2

0.2 0.4

2.2 2.3

3.8 5.6

0.0 0.0

0.0 0.0

0.0 0.0

26.0-27.7 24.8-25.7

0.4 1.0

0.0 0.0

0.0 0.0

0.0 0.0

2.8 I.3

1.8 0.5

24.8-27.7 0.7

3.9

6.0

0.0

0.0

0.0

Control1ing sulfide generationin force mains

91

RatiQQf !iQuid-~as surface area tQ sewa~e vQlume(biQfiJm state in air-injected force main) During each run, two flow phases, gaseous and liquid, were clearly recognized in experimental force mains. There was nQ biofilm to the top of the pipe, while it fully covers all other parts. Table 2 shQWS investigation results concerning the state of biofilm adhesion to the pipe interior. The surface length (L) as shown in Fig. I, averaged about 50 mm. Film thickness measurements revealed that thick biofiJm (0.45 mm or more) adhered, except at one location. Table 2. State of biofiJm adhesion to the pipe interior Date

Surface length L : biofilm not adhered (rom) Film thickness (rom) Point I Point 3 Point 5 Point I Point 3 Point 5 1995 Nov. 70 32 0.46 0.50 1996 Aug. 30 65 40 0.71 0.23 0.48 1996 Sep. 38 48 40 0.83 1.44 0.61 Liquid-gas surface length (L) was obtained from void fraction calculated by Kameda's model, and that length ranged from 52 to 66mm. These calculated results agreed well with observed surface length. As AIV, liquid-gas surface area to sewage volume, in Bquatiomz) can easily calculated by surface length(L), it is considered that AIV can be well estimated by Kameda's model. ReaeratiQn rate Rf Oxygen concentrations in the gaseous phase at each sampling point of experimental force mains were measured, and Rf was obtained by Equation(5) using the difference of oxygen concentration of each point. Observed Rf values in the experimental force mains were shown in Fig.3. Rf values with a sewage velocity of 0.6m/s were obviously larger than those of O.3m/s.There was positive correlation between Rf and oxygen concentration in the gaseous phase.

40 .---\\---------:-----, Symbols Case Sewage flow velocity (mls)

•.... ,•

'bo 20

.5

6::

10

o

o

~

14 16 18 20 22 Oxygen concentration in gaseous phase [%]

*

3-1 1-1 3-2 1-2 1-3 1-4

0.60 0.60 0.60 0.60 0.30 0.30

Air injection volume rate of flow (m:'min) 0.090 0.060 0.045 0.030 0.060 0.030

Figure 3. Relationshipbetween Rf and oxygenconcentrationin gaseousphase.

KL(20) values calculated by Equation (2) using obtained Rf values and AIV values by Kameda's model are shown in Table 3. KL(20) ranged from 0.27 to 0.42 mIh with a sewage flow velocity of 0.6m/s, and from 0.17 tQ 0.20 mIh with a sewage flow velocity of 0.3m/s. The average values of KL(20) were 0.32m1h and 0.19 mIh with sewage flow velocities of 0.6m1s and 0.3 m/s, respectively. This means that KL(20) values were greatly affected by sewage flow velocities. In contrast, there was no clear correlation between KL(20) and air injection volume rate. KL(20) values in the experimental force mains was compared with those in the gravity sewer pipe calculated by Equation(3). KL(20) calculated by Equation (3) are 0.15 mIh and 0.1Omlh with sewage flow velocities of 0.6m1s and O.3m1s, respectively, which are about half the KL(20) values shown in Table 3. As the gaseous velocity is higher than the liquid velocity in the horizontal profile pipelines in general. turbulent flow is

T. OCHI et al.

92

caused by the difference between these two velocities (Japan SME, 1989). It is considered that turbulent flow results in an increase in KL(20) in the force mains. Table 3. KL(20) in the force main Case 3-1

I-I 3-2 1-2 1-3 1-4

Sewage flow velocity (m/s) 0.60 0.60 0.60 0.60 0.30 0.30

KL(20) (m/h) Point 1-3 Point 3-5 0.29 0.34 0.27 0.28 0.31 0.35 0.28 0.42 0.17 0.20 0.18 0.20

Air injection volume rate of flow (m3/min) 0.090 0.060 0.045 0.030 0.060 0.030

Ollnen consumption in bulk water Rr Figures 4 and 5 show the measurement results of Rr and the relation between dissolved CODer (S-CODcr) and VSS, respectively. Rr ranged widely from 3 to 18 mg/I.h. Positive correlation was found between Rr and S-CODcr, while there was no clear correlation between Rr and VSS.

..

20.----------,

:;: IS

I.

.:0-.. -" ••

..

.w- • • •,

~IO

r1

•

:-"

~.1Ilo

~ 5

••

Ol---

o

.......J

SO

100

150

S-CODcr [mgll) Figure4. Relationship Rr and S-CODcr.

20

:;: IS

••

~10 ~

5

•

~

• • .,\\ • • , , .a: •

........

...'"

0

0

SO

100 ISO VSS [mgll)

200

Figure5. Relationship Rr and YSS.

Ollnen consumption rate jn bjofilm Re To study the influence of DO on Re, experiments were conducted by using a biofilm reactor, as i11ustrated in Fig. 6. Aerated sewage in the detention lank was sent to the reactor at a constant flow rate. To keep the sewage in a completely mixed state, a vertical stirrer inside the reactor was rotated at a constant rate (60 rpm) and the diffusion layer on the biofilm surface can be neglected. Biofilm fully grew up before the experiments and the thickness was approximately 0.7mm. Concentration of dissolved oxygen at inlet and outlet of reactor and Rr of wastewater were measured. Then, Re was calculated by Equation (6).

ControJling sulfide generationin force mains

Re = (

ODin - DOout

H

- Rr

93

0 ) .--

(6)

4

=

ODin DO in the inlet (mg/l) DOout = DO in the outlet (= DO in reactor: mgll) H = detention period in reactor (h) D = reactor inner diameter (m) The relation between obtained Re by calculation of Equation (6) and observed DOout that is the same as DO in reactor is shown in Fig. 7. Water temperature during the experiment was 27°C. Fig. 7 shows that Re depends on DO in the reactor. On the hypothesis that the diffusion layer on the biofilm surface has no influence, and that the half saturation constant is extremely small, Re for partly penetrated biofilm can be expressed using Equation (7). (Jansen 1985) Re

=(2 'Of

=

Kof)I/2. 00 1/2 kl/2a ' 00 112

(7)

=

Of diffusion coefficient of DO in the biofilm (m 2/h ) Kof = zero order volume constant (g/m 3.h) k l12a half order surface rate (gll2/mll2.h)

=

The k l/2a value that was obtained by least square method was 0.537 g1/2/m II2.h. Calculated Re curve by using 0.537 gll2/ mll2.h is shown in Fig. 7. Re seemed to increase in proportion to the square root of DO.

Pump

St i r r er i. 10cm .1 Short Detention tank Reactor pipe Figure 6. Schematicdiagram of rotating reactor.

2 r-r=:====;-----,

~

',IMeasured I

using reactor

246 8 DO in reactor [mgll]

10

Figure 7. Relationship betweenRe and DO.

As mentioned above, DO balance in force mains can be expressed as in equation (1). Therefore already obtained Rf, Rr, Ds, DO. AIV, P and Qair by experimental data were applied to Equation (1), to obtain Re,

94

T. OCHI et al.

The results are shown in Fig. 8. Re ranged from 0.25 to 0.9 g/m 2.h. Re values calculated by using kl/2a that was obtained by reactor was shown in Fig.8. As measured Re values were agreed somewhat with the computed values using biofilm reactor, the data of biofilm reactor can be applied to Re in the pipe with observed DO range in this experiment, practically. Measured Re values were slightly less than values using biofilm reactor, because the diffusion layer on the biofilm surface can be neglected in the biofilm reactor, while the diffusion layer possibly affected Re in the experimental pipes. 1. 5 I2

'2

Re = 0.537 DO

~

<, OIl

•

·

• • •

O. 5 II>

• l

Measured 11\ experimental pipe

0:

0 0

3 2 DO [mg/I]

5

4

Figure M. Comparison between Re values computed by biofilm model and measured in experimental pipe.

Examination of

oxy~en

balance in the force main

Figure 9 shows the oxygen balance in the force mains, calculated using the above-obtained Rf, Rr and Re etc by experimental data. In Both Case 3-1 (air injection volume: 0.090 m 3/min.) and Case 3-2 (0.045 m 3/min.), oxygen in the pipe was mostly consumed by Rr and Re; only a small percentage was consumed for the oxidation of sulfide; a little DO remained in the sewage. Of total oxygen supplied, the percentage remaining in the gaseous phase at the outlet (Point 5) was 84% in Case 3-1 and 74% in Case 3-2. This means that oxygen dissolved in sewage accounted for 16% in Case 3-1 and 26% in Case 3-2, the rest of the oxygen being released while still mixed in the air. In these experiments, Rr and Re were set at 13.5 mg/l.h and 1.6 g/m 2.h, respectively, and air was injected in quantities 3 times (Case 3-1) and 1.5 times (Case 3-2) the quantity consumed by Rr and Re. If Rr and Re had been at setting values, 33% (Case 3-1) and 67% (Case 32) of oxygen would have been consumed in the force mains. While measured Re was markedly smaller than the intended value, oxygen was not consumed in the force mains to the extent intended. e 3-1(air-inlect. 009m3/mm)

ase 3-2(aJr-mlect 004 m3/mm) '0

0

s;

e 100 OJ

' OJ

100

90

.EO

90

z;

E

.EO

.,:JI c

90

'0

...

:JI

c

E

80

1111

70 60

'"

C ., 0

.,

~

o..

A

I 2 3 4 5 amp lin g pomt

.,:JIc 9

:JI

60

'"

.,

~ e,

~

Oxygen consumption for oxidation of sulfide Oxygen consumption by Rr

80 70

C .,

I

~

0 '0

.,

oxygen .Dissolved In sewage

J

· .9x gen consumption_b~ R~ J Oxygen remaimng 11\ gaseo us phase

0 A

I

2

3

4

5

arnphng point

Figure 9. Percentage of oxygen consumption in force main.

Nevertheless, even with a large quantity of oxygen remaining in the air, DO in sewage in the second section of the force mains decreased, as shown in Table I; in Case 3-2, for example, DO was 0.5 mg/l at the outlet. In other words, if oxygen concentration in the gaseous phase declines further, Rf also decreases further, presumably making it impossible to keep sewage aerobic. As this suggests, when air injection volume is

Controlling sulfide generation in force mains

95

based on oxygen consumption, oxygen consumption can exceed oxygen dissolution , depending on gaseous and liquid flow conditions, even if oxygen is still present in the gaseous phase. Based on the above, it is important to determine the air injection volume by the study of oxygen balance. CONCLUSIONS I. Air injection completely eliminated sulfide presence at the pipe outlet, when DO at the pipe end was 0.2 mgll or higher.

2. NY, liquid-gas surface area to sewage volume, can be obtained from void fraction calculated by Kameda's model. 3. KL(20) values in the force mains were greatly affected by sewage flow velocity . In contrast, there was no clear correlation between KL(20) and air injection volume rate. 4. Rr ranged widely from 3 to 18 mg/I.h. Positive correlation was found between Rr and S-CODcr, while there was no clear correlation between Rr and VSS. 5. Re, measured using a rotating reactor, increased in proportion to the square root of DO. 6. Oxygen in the force mains was mostly consumed by Rr and Re. Only a small percentage was consumed for the oxidation of sulfide; a little dissolved oxygen remained in sewage. REFERENCES Elmore. H. L. and West. W. F. (1961 ). Effects of water temperature on stream reaerat ion . J. San it. Eng. Div.• 87. 59. Japan soc iety of mechanical engineers (1989) . Handbook ofgas-liquid two-phase flow technolog y. pp. 265. Jansen. J. L.Cour and Harremoes, P (1985) . Removal of soluble substrates in fixed films . Wat. Sci. Tech .• 17(213), 1-14. Kameda, Y. and Morita. T. (1993) . Two-phase flow study using 200mm diameter pipe. Japanese Journal of Multiphase Flow.• 7(3).250-261. Nielsen. P. H.• Raunkjer, K.• Norsker, N. H., Jansen, N.A. and Hvitved-Jacobsen, T. (1992) . Transformation of wastewater in sewer systems-a review. Wat. Sci. Tech .• 25(6). 17·31. Parkhurst. J. D. and Pomeroy. R. D. (1972) . Oxygen absorpt ion in streams. J. San it. Eng. Div., 98(SAI). 101. US.EPA (1985) . Design manual. Odor and corrosion control in sanitary sewerag e systems and treatment plants . 10-13.