Study of the Septicity Problem throughout the

Tunnel Systems under the Harbour Area

Treatment Scheme (HATS)

Feng Jiang

Professor,

School of Chemistry and Environment

South China Normal UniversityEmail: jiang.feng@foxmail.com / jiangfeng@scnu.edu.cnMobile / Tel: +86-13760612488

2 Assess the sulfide generation in HATS SCS

3 Simulation of HATS SCS by SPMM

4 Summary

1 General introduction

Contents

As the water quality in Victoria Harbour affects many people in Hong Kong, the Government initiated the

Harbour Area Treatment Scheme(HATS). It improves the water quality of Victoria Harbour through the

collection, treatment and disposal of sewage from both sides of Harbour.

1. General introduction

Stage 2A: • commissioning in December 2015;

• upgrading 8 PTWs, construction of 21km-

long and -70 to -160 m deep tunnel system;

• treats the remaining 25% of sewage

Stage 1: • commissioned in December 2001;

• upgrading 7 PTWs, construction of 23km-long

and -70 to -140 m deep tunnel system;

• treats 75% of the sewage, the capacity to

move up to 1.7 million m3/d.

long-distance long-time of sewage conveyance obnoxious gas(H2S) and corrosion

H2S SO42-

H2S

SO42-

H2S

Sulfur Oxidizing Bacteria

Sulfate Reducing Bacteria

Sewer atmosphere

Bulk sewage

Biofilm

H2SO4

sulfide is generally bio-generated in sewer biofilm, due

to the growth of the SRB colonized in biofilm (Ito et al.,

2002).

Mechanism of H2S production in sewers

Sulfate reduction and sulfide production：

𝑆𝑂42− + 𝐶

𝑆𝑅𝐵𝑆2− + 𝐶𝑂2

Anoxic sulfide oxidation：𝑆2− + 𝑂2/𝑁𝑂3

− 𝑆0/𝑆𝑂42− + 𝑁2

Hydrogen sulfide dissociation：𝑆2− ↔ 𝐻𝑆− ↔ 𝐻2𝑆

Hydrogen sulfide emission:

𝐻2𝑆(aq) ↔ 𝐻2𝑆(𝑔)

H2S(g)

conc.(ppm)Harmful effects

0.13 Min concentration can be smelt

… …

200-300Continuous exposure after one hour, there was a noticeable

conjunctivitis, and respiratory irritation

500-700 Loss of consciousness, apnea, and even death

1000~Loses consciousness immediately, breathes stops and died in

several minutes

average

peakHATS

H2S

Co

ntr

ol

End-of-pipe treatment

Forced Ventilation

DeodourizationUnit

In-pipe treatment

Chemical Dosing

To transform dissolved sulfide

H2O2, O2, NO3-,

NO2-

To inhibit sulfide formation

FNA, Molybdate

To precipitate dissolved sulfide

Fe2+, Fe3+

To suppress H2Sg

emissionNaOH, Ca(OH)2

Hydraulic flushing

How to control H2S(g) in sewer ?

• High Cost

• Chemical pollution

• Risk to downstream WWTP and ecosystem

Over dosage

• Ineffective H2S control

• Infrastructure corrosion

• Human health risk

Insufficient dosage

A sewer processes math model is essential to cost-effective H2S control in HATS

0

5000

10000

15000

20000

25000

4/10/2015 9:00 5/10/2015 21:00 7/10/2015 9:00

Flo

wra

te (

m3/h

)

Time

CW

SKW

TKO

KT

TKW

KC

TY

• The Sewer Process Mathematical Model (SPMM)

– Developed by Dr. Feng Jiang (SCNU) and Prof. G H

Chen (HKUST) since 2007

– To simulate all the physical, chemical, and biological

processes related to sewage quality changing in sewers.

Sewer Process Mathematical Model

Biofilm/sediment

Water phase

Gas phase H2S(g)

SO42-

S2-//HS-/H2S(ag)

NO3-

S2-

Diffusion

emission

Attachment /Detachment

Biomass / particulate

biofilm-process

Schematic diagram of the main reaction related to sulfide in sewer

CorrosionH2SO4

Substance

concentrations

in biofilm

Changes in

biofiom

composition

(1) Agreement No. DEMP09/06 - Sewer Biofilm Modeling for sulfide Formation in Sewers

• Tung Chung pressured main sewer (TCS) and Tuen Mun gravity sewer (TMS)

(2) HKIA Sewer Network Study

• The sewer networks in the Hong Kong International Airport (HKIA)

SPMM can be used to simulate the biochemical process of biofilm and

wastewater, and also can predict the sulfide production and H2S releases,

e.g. the previous Applications of this SPMM

sulfide H2S(g)

Sampling Locations: SCISTW and 7 PTWs

Analyzing parameter: 20 parameters include

TS, DS, VSS, H2S(g), flowrate etc.

Sampling time: Every 2 or 12h for 7 days

• Data (2015.10.4 9:00 to 2015.10.7 9:00) for model

calibration

• Date (2015.10.7 9:00 to 2015.10.11 9:00) for model

verification

Sampling Locations: SCISTW and 8 PTWs

Analyzing parameter: 20 parameters include

TS, DS, VSS, H2S(g), flowrate etc.

Sampling time: Every 2 or 12h for 2 days

• (2015.12.7 14:00 to 2015.12.9 14:00)

HATS 1

HATS 2A

2. Assess the sulfide generation in HATS SCS

KC

PTW

TY

PTW

TKW

PTW

KT

PS

KT

PTW

TKO

PTW

CW

PTW

SKW

PTW

TUNNEL A

TUNNEL B

TUNNEL C

TUNNEL D

TUNNEL E

TUNNEL F

TUNNEL G

SCI

STW

Inverted Syphon

Pressured Main

Both (KT Riser Shaft)

HATS Stage 1

0 500 1000 1500 2000

SCISTW

output

PTWs input

Net sulfide input or output (kgS/d)

CW input KC input KT input SKW input

TKO input TKW input TY input SCISTW output

228

kgS2-/d

~70% DS were generated in the HATS 1

48

kgS2-/d

27

kgS2-/d

191

kgS2-/d

108

kgS2-/d

43

kgS2-/d

18

kgS2-/d

2089

kgS2-/d

Field experiment and data analysis

Liquid sampling

H2Sg monitoringat SCISTW MPS No.1

H2S(g) Conc. at MPS No.1

Average: 219 ppm

Maximum: 720 ppm

Wet well

H2S(g) conc. in HATS 1 outlet

0

5000

10000

15000

20000

25000

4/10/2015 9:00 5/10/2015 9:00 6/10/2015 9:00 7/10/2015 9:00

Flo

wra

te (

m3/h

)

Time

CW

SKW

TKO

KT

TKW

KC

TY

SCISTW output (10/4-10/7)

Based on the different sewer with different water quality and hydraulic conditions (e.g. pH,

flowrate, and dissolved sulfide), model should be used to predict and effective control H2S(g).

7 PTWs input (10/4-10/7)

Flowrat

e

DS

H2S(g)

pH

0

0.5

1

1.5

2

2.5

3

3.5

4/10/2015 9:00 5/10/2015 9:00 6/10/2015 9:00 7/10/2015 9:00

Dis

solv

ed s

ulf

ide

(mg

S/L

)

Time

CW

SKW

TKO

KT

TKW

KC

TY

DS

Flowrate

Strong fluctuation

Average sulfide Concentration:

H2S(g) measured: 219 ppm

simulated: 221 ppm

DS measured: 1.82 mgS/L

simulated: 1.94 mgS/L

Model Calibration of HATS stage 1 (10/4-10/7)

3. Simulation of HATS SCS by SPMM

Model Verification of HATS stage 1 (10/7-10/11)

Simulated well

Average sulfide Concentration:

H2S(g) measured: 207 ppm

simulated: 214 ppm

DS measured: 1.89 mgS/L

simulated: 1.79 mgS/LPredictedwell

H2S(g)

H2S(g)

DS

DS

Flowrate

Flowrate

1. Locate the position of sulfide generation

2. Case study Case 1: Ultimate Flow Simulation

Case 2: Temperature effect

Case 3: Water Flushing

3. H2S control by chemical dosing Case 4: Super-oxygenation system(TKO) ;

Case 5: Nitrate dosing(TKW) ;

Case 6: NaOH (TKW) ;

Case 7: Nitrate + NaOH (TKW) ;

Case 8: Nitrate + NaOH (TKW) + Forced Ventilation (MPS1)

Overview

Model Application of HAST Stage 1

TunnelAverage flowrate

(m3/h)

Hydraulic

retention time(h)Lengh (km)

Calculated DS-in

(average mgS/L)

Calculated DS-out

(average mgS/L)

Net sulfide

production

(kgS/d)

A CW 2446 1.1 2300 0.81 2.35 90

B CW SKW 5357 0.7 2500 1.29 2.02 94

C TKO 5812 2.6 5300 1.64 2.93 180

D CW SKW TKO KT 25754 0.8 3300 1.36 1.57 130

E CW SKW TKO KT TKW 35390 1.5 5500 1.27 2.04 654

F KC TY 12210 1.3 3600 0.18 1.70 445

G KC 9643 0.3 800 0.19 0.15 -8

The position of sulfide generation

Long HRT result in high

sulfide concentration

Short HRT and high DO result in very

low sulfide concentration

Largest

Due to 35,390m3/h wastewater

HATS Stage 1

• The ultimate flow is predicted to be 1.4 times of current flow (average);

• sulfide production in Tunnels decreased by 12%, DS concentration decreased

by 27% (drop from 1.94 mgS/L to 1.42 mgS/L).

Current flow Ultimate peak flow

TunnelFlowrate

(m3/h)AverageHRT(h)

Simulated DS-out(mgS/L)

Net sulfideproduction（kgS/d）

Flowrate(m3/h)

AverageHRT(h)

Simulated DS-out(average mgS/L)

Net sulfideproduction（kgS/d）

A 2,446 1.1 2.35 90 2,556 1.0 2.13 81

B 5,357 0.7 2.02 94 7,083 0.5 1.69 87

C 5,812 1.3 2.93 180 6,869 1.1 2.78 188

D 25,754 0.8 1.57 130 34,560 0.6 1.28 50

E 35,390 1.5 2.04 654 52,201 1.0 1.44 539

F 12,210 1.3 1.70 445 16,066 1.0 1.38 463

G 9,643 0.3 0.15 -8 12,789 0.2 0.14 -15

HATS 1 47,599 1.94 1,585 68,267 1.42 1,393

Ultimate Flow Simulation

HATS 1Case Study 1

SCISTW

DS (mgS/L)DS variation(%)

Net sulfide production

（kgS/d）

28.7° C-

simulated1.94 0% 1,585

32.0° C-

simulated2.16 11% 1,837

25.0° C-

simulated1.61 -17% 1,207

18.0° C-

simulated1.16 -40% 692

Winter: 18° C

Spring-Autumn: 25° C

Summer: 32° C

Field experiment: 28.7° C

Temperature Bioactivity sulfide generation

Temperature Effect

HATS 1Case Study 2

Case Study 3Water Flushing

Tunnel E Tunnel F

Length (km) 5.5 3.6

Flowrate (m3/h)(10/4-10/7)

35,390 12,210

HRT (hour) 1.5 1.3

Unsatisfactory control effects

- Enormous flow requirement: 10 to 20 times of the average

flow, and increase with tunnel size;

- Flushing frequency: every 3~4 days, lasting for 1h;

Negative impact

- Stress the downstream treatment capacity

- High energy consumption

Flushing

EF

H2S control by chemical dosing

Q1: Where ?

Dosing Location : SCISTW or

KT, TKO, TY,…?

Q2: What ?

Dosing chemical: Ca(NO3)2,

NaOH, O2….

Q3: When and how?

Dosing strategy: dosage,

stable or dynamic?

Objective:• H2S(g) at SCISTW MPS 1 ≤ 20 ppm

• DS concentration at SCISTW MPS 1 ≤ 1.5 mg/L

The location selection for septicity control mainly considered:

The target H2S(g) control location - SCISTW

The net sulfide production contribution- tunnel E contribute 41% of the net sulfide production in HATS 1 system

The mixing and reaction time of dosage;

3050 kg/h dosing rate at six PTWs: TKW, KT, TKO, SKW, CW and TY, respectively.

610 kg/h dosing rate at TKW, KT, TKO, SKW and CW simultaneously

Best dosing location：TKW

Dosing Location Selection- Trial with Nitrate Dosing

HATS 1

Oxygen

o Only TKO can install this system;

o Long distance transportation consumed too much

oxygen dosage.

o Low control efficiency to SCISTW

Case 4: Super-Oxygenation System (TKO) HATS 1

Case 5: Nitrate Dosing (TKW)

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000

H2S

(p

pm

)

Dosage (kg/h)

Oxygen

Calcium nitrate

Sodium hydroxide

Case 6: NaOH Dosing (TKW)

Nitrate or

NaOH

• For DS control (<1.5 mg/L), it works

• Increasing nitrate dosage is inapplicable to

control H2S(g) at < 20 ppm

Add NaOH at a rate of 998 kg/h at TKW

Little change of DS concentration;

H2S(g) dropped from 221ppm to 18 ppm.

Forced Ventilation

Nitrate + NaOH

Case 8: Nitrate + NaOH (TKW) +

Forced Ventilation (SCISTW)

Case 7: Nitrate + NaOH dosing (TKW)

Nitrate + NaOH

HATS 1

To achieve the control target of :

H2S(g) ≤ 20 ppm

DS ≤ 1.5 mg/L

Ca(NO3)2 :1250 kg/h + NaOH : 848 kg/h.

√√

To achieve the control target:

H2S(g) ≤ 20 ppm

DS ≤ 1.5 mg/L

Ca(NO3)2 : 1250 kg/h + NaOH 498 kg/h + forced

ventilation: ACH of 5

√√

Case Note Recommendation

1Ultimate Flow Simulation

Flowrate sulfide H2S(g) -

2 Temperature effect T sulfide H2S(g) -

Objective:• H2S(g) at SCISTW MPS 1 ≤ 20 ppm

• DS concentration at SCISTW MPS 1 ≤ 1.5 mg/L

3 Water Flushing Unsatisfactory control effects X

4Super-oxygenation system(TKO)

Low efficiency X

5 Nitrate dosing(TKW) H2S(g) at < 20 ppm X

6 NaOH (TKW) H2S(g) at < 20 ppm, but little change of sulfide

X

7Nitrate + NaOH

(TKW) sulfide √ H2S(g) √ √

8Nitrate + NaOH

(TKW) + Forced

Ventilation (MPS1)sulfide √ H2S(g) √ √

4. Summary

The developed SPMM well predicted the sulfide production and H2S releases in

HATS stage 1 tunnels.

By model simulation, the recommended strategy for septic control in HATS stage

1 is combined dosing of calcium nitrate and sodium hydroxide at To Kwa Wan

PTW, with forced ventilation at MPS 1 of SCISTW

The SPMM is an efficient and helpful tool to identify the key factors to the

serious septic problem and reduce hydrogen sulfide pollution in the HATS

system