Drainage Services Department Research & Development Forum, 20 Nov 2013
Disinfection Dosage Control for Harbour Area Treatment Scheme
J.H.W. Lee, K.W. Choi and S.N. Chan
Hong Kong University of Science and Technology
Outline
1. Overview of the Harbour Area Treatment Scheme (HATS). 2. Study on sewage effluent E.coli standard to protect beach water quality 3. Hydraulics of the Advance Disinfection System (ADF) in SCISTW 4. In-plant E.coli survey in SCISTW 5. Conclusions
1 1. Overview of the Harbour Area Treatment Scheme (HATS).
Harbour Area Treatment Scheme (HATS)
Stage 1：Commenced in December 2001 (Chemically Enhanced Primary Treatment)
23.6 km of deep tunnels; 1.4 M m3/d of sewage receives CEPT
Disinfection (chlorination): Started in March 2010
Stage 2A : Commenced in 2014
2 Deep-tunnel sewage conveyance system
Screening Stonecutters Island Plants/pumping Sewage Treatment Submarine stations Work (SCISTW) outfall
23.6 km deep tunnels (>100m below ground level)
Sewage in Kowloon and Hong Kong Island is conveyed through 23.6km deep tunnels to the Stonecutters Island STW for Chemically-Enhanced Primary Treatment (CEPT).
HATS Stage 1 Chemically-enhanced primary treatment at Stonecutters Island STW Stop 600 tonnes/yr of sludge from entering the harbour Pollutants removal rate: 70% organics (BOD) 80% suspended solids 60% heavy metals 25% total nitrogen 50% phosphorus E.coli: before disinfection: 50% after disinfection: 7 5 10 → 10 cnt/100mL
Tsuen Wan
Approach Hoi Mei Lido Casam Wan Ting Kau Tsuen Wan Angler’s Gemini Beach HATS Outfall Length: 20m Tsing Yi HATS Outfall (~10-15m) Ma Wan Channel (30 (15m) Slope = 7% Kap ~ 8km 淨化海港計劃排污口 Shui
Mun
(25 -35m HATS -30m ) ) Victoria Due to elevatedHarbour E.coli level, Tsuen Wan beaches were closed in 2002-2010
Ma Wan Tung Wan Beach Length: 140m TW beaches reopened in Slope = 1% 2011 as water quality improved due to disinfection
WQO Compliance of Tsuen Wan beaches after disinfection - A beach is in compliance with the Water Quality Objective (WQO) if the annual geometric mean E.coli is < 180 count/100mL
Measured Annual Geometric mean E.coli of Tsuen Wan beaches after disinfection Year 2010 2011 2012 Rainfall (mm/yr) 2372 mm 1477 mm 1925 mm Ma Wan Tung Wan 17 10 22 Angler's 134 28 69 Gemini 137 19 40 Hoi Mei Wan 87 24 51 Casam 102 20 50 Lido 87 19 32 Ting Kau 141 56 87 Approach 124 57 83 Annual Ranking Good <25 Annual ranking of a beach = geometric mean of all E.coli Fair 25-180 sampling in bathing season (1 Mar-31 Oct). Poor 181-610 Very Poor >610
4 Improvement in water Quality (E.coli), Victoria Harbour E.coli (counts/100mL) 1.E+6 Depth average EPD Data 1.E+5
1.E+4 Pre-HATS: 1986-2001
1.E+3 Post-HATS: 2002-2009 610 1.E+2 Pre-HATS Post-disinfection: 2010-now Post-HATS 1.E+1 Post-Disinfection West East 1.E+0 Significant reduction in WM4 WM3 VM8 VM7 VM6 VM5 VM4 VM2 VM1 JM4 EM1 EM2 EM3 HATS Central Lei Yue Mun E.coli in the eastern side after HATS Rise in E.coli in western harbour after HATS, but reduction after disinfection
Contribution of E.coli loading of various sources (pre-disinfection)
17 Total E.coli load = HATS + Victoria Harbour + Tsuen Wan + TW West = 2.1 x 10 count/d Outfalls, pre-disinfection Tuen Mun 0 1 2 4 km E.coli loading (count/d), E.coli loading (count/d) % to total load New Territories <1e+14 <5e+14 TW West Tsuen 7.2e+13, Wan <1e+15 <0.1% WTA 5.0e+15, <5e+15 2.4% <1e+16 Kap Tsing Shui <5e+16 Mun Yi <1e+17
<1e+18 SCISTW Ma Wan Rambler Channel Beaches Channel Kowloon
HATS
1.4e+17, NPA CEA 66% WEA Loading Flow E.coli E.coli % to WCA from (m3/s) conc. loading total Outfall (count/ (count/d) loading 100mL) (sewage) HATS 15.8 1.E+7 1.4E+17 78.6% CEA 1.316 1.E+7 1.1E+16 6.5% SBA Hong Kong Island CPA WCA 0.399 1.E+7 3.5E+15 2.0% Victoria Harbour WFA WEA 1.419 7.1% 1.E+7 1.2E+16 ABA 6.5e+16, 31% NPA 1.163 1.E+7 1.0E+16 5.8%
East Lamma Channel
West Lamma Channel
5 Contribution of E.coli loading of various sources (ADF)
16 Total E.coli load = HATS + Victoria Harbour + Tsuen Wan + TW West = 7.3 x 10 count/d Outfalls, post-disinfection 0 1 2 4 km E.coli loading (count/d), Tuen Mun E.coli loading (count/d) % to total load New Territories <1e+14 <5e+14 TW West Tsuen Wan 7.2e+13, <1e+15 <0.1% WTA 5.0e+15, <5e+15 7% <1e+16 Kap Tsing Shui <5e+16 Mun Yi <1e+17
<1e+18 SCISTW Ma Wan Rambler Beaches Channel Channel Kowloon
HATS 2.8e+15, 4% NPA CEA Loading Flow E.coli E.coli % to WCAWEA from (m3/s) conc. loading total Outfall (count/ (count/d) loading 100mL) (sewage) HATS 15.8 2.E+5 1.4E+17 6.9% CEA 1.316 28.5% 1.E+7 1.1E+16 SBA Hong Kong Island WCA 0.399 8.6% CPA 1.E+7 3.5E+15 Victoria Harbour WEA 1.419 1.E+7 1.2E+16 30.8% WFA ABA 6.5e+16, 89% NPA 1.163 1.E+7 1.0E+16 25.2%
East Lamma Channel
West Lamma Channel
Current License Standard for HATS Effluent
• Before disinfection (1.4 x 106 m3/d) – E.coli = 107 counts/100mL – Loading = 1.4 x 1017 counts/d • Stage I, ADF (1.4 x 106 m3/d) – Monthly Gmean E.coli < 2 x 105 counts/100mL – Loading = 2.8 x 1015 counts/d • Stage 2A (1.8 x 106 m3/d) – Monthly Gmean E.coli < 2 x 104 counts/100mL – Loading = 3.6 x 1014 counts/d • E.coli loading from HATS has much less contribution (from 66% to 4% of the total load) to beach water quality after disinfection • These standards are based on the results of water quality modelling in a previous EIA study (Maunsell | AECOM, 2007), which only considered the typical dry and wet season conditions and have not accounted for the diurnal E.coli variation. • Realistic re-examination of the effluent discharge standards in relation to the beach WQ is required.
6 2. Re-examination of effluent E.coli standard
Hong Kong’s beach water quality objective (WQO)
Minor illnesses E. coli * Water Quality Beach rate ** (counts Objective Grading water quality (cases per 1000 /100 mL) Compliance/ 泳灘水質 swimmers) 大腸桿菌 Exceedance 發病率 1 Good ≤ 24 Undetectable Compliance 2 Fair 25 - 180 ≤ 10 3 Poor 181 - 610 11 - 15 Exceedance 4 Very poor > 610 > 15
*Weekly Beach Grading: Geometric Mean E. coli level of the 5 most recent samplings (ClnEC5) Annual Beach Ranking: Geometric Mean E. coli level of all bathing season (Mar-Oct) samplings
** Skin and Gastrointestinal illnesses (Cheung et al. 1990)
7 Project WATERMAN 3D Hydrodynamic Model of Hong Kong waters (Water Research, Chan et al. 2013) Submarine outfalls
Salinity (ppt) 20.0 25.0 30.0 0.0 Dynamic coupling 2.0 4.0 (DESA) 6.0
8.0 Depth (m) Neap 10.0 Spring
12.0
14.0
16.0 Near-field model Far-field JETLAG Hydrodynamic Model
Bacterial Loading Current, Turbulent Mixing 潮汐流、紊流混合 Bacterial decay modeling
E. coli concentration (大腸桿菌含量)
Real-time forecast of beach water quality for Tsuen Wan beaches using 3D hydrodynamic model
8 Validation: 2006 (Wet Year, Pre-disinfection)
100000 2006 100000 E.coli (cnt/100mL) corr = 0.641 Wan Chai 2006 10000 HATS 10000 Lei Yu Mun 1000 1000 Meas. 100 Pred. West East
100 10
WM4 WM3 VM8 VM7 VM6 VM5 VM4 VM2 VM1 Pred. Depth Pred. Avg.E.coli (cnt/100mL) Pred. vs. Meas. (DA) Annual Mean, Depth Avg. 10 10 100 1000 10000 100000 Meas. Depth Avg. E.coli (cnt/100mL) Vertical structure, annual mean Meas. S Pred.
M
B
VM4 WM4 WM3 VM7 VM6 VM2
100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 West (Ma Wan) East (Lei Yu Mun)
Validation: 2010 (Average-Wet Year, Post-disinfection)
100000 E.coli (cnt/100mL) 2010 100000 corr = 0.712 Wan Chai 2010 HATS 10000 10000 Lei Yu Mun 1000 Meas. 1000 100 Pred West East 10
100 WM4 WM3 VM8 VM7 VM6 VM5 VM4 VM2 VM1 Pred. Depth Avg. E.coli (cnt/100mL) Avg. Depth E.coli Pred. Pred. vs. Meas. (DA) Annual Mean, Depth Avg. 10 10 100 1000 10000 100000 Meas. Depth Avg. E.coli (cnt/100mL) Vertical structure, annual mean
Meas. S Pred.
M
B
VM7 VM6 VM4 VM2 WM4 WM3
10 100 1000 10000 10 100 1000 10000 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 100 1000 10000 100000 West (Ma Wan) East (Lei Yu Mun)
9 Validation - Diurnal Beach E.coli Variation Lido beach 10000 E.coli Tide (m) 3.0 10000 E.coli Tide (m) 3.0 (#/100mL) LIDO (#/100mL) LIDO 2.5 Model 2.5 1000 610 1000 Field data 2.0 610 Tide 2.0 180 180 100 1.5 100 1.5
24 1.0 24 1.0 10 Model 10 Field data 0.5 0.5 Tide 1 0.0 1 0.0 0:00 6:00 12:00 18:00 0:00 0:00 6:00 12:00 18:00 0:00 18-Aug-2012, sunny day, semi-diurnal 27-Aug-2012, sunny day, diurnal Gemini beach
10000 E.coli Tide (m) 3.0 10000 E.coli Tide (m) 3.0 (#/100mL) GEM (#/100mL) GEM 2.5 Hindcast 2.5 1000 1000 Field data 610 610 2.0 Tide 2.0 180 180 100 1.5 100 1.5
24 1.0 24 1.0 10 Model 10 Field data 0.5 0.5 Tide 1 0.0 1 0.0 0:00 6:00 12:00 18:00 0:00 0:00 6:00 12:00 18:00 0:00 18-Aug-2012, sunny day, semi-diurnal 27-Aug-2012, sunny day, diurnal
Diurnal Beach E.coli Variation
• Beach WQ depends on tidal and solar radiation • During diurnal tides beach WQ is relatively better than during semi-diurnal tides • Longer travel time (from HATS to TW beaches) during diurnal tide, allows for more bacterial decay by solar radiation (Chan et al, 2013).
10 Predicted daily beach grading distribution – bathing season 5 • ADF, C0 = 2 x 10 count/100mL, current license standard N = 245 4 • 2A, C0 = 2 x 10 count/100mL, current license standard 5 • 2A, C0 = 2 x 10 count/100mL, relaxed standard TWM - Daily geometric mean GEM - Daily geometric mean 100% 100% 89% 90% HATS ADF License 87% HATS ADF License 80% HATS2A License 80% HATS2A License 64% HATS2A Relaxed HATS2A Relaxed 58% 60% 54% 60% 45% 40% 40% 36% 40%
20% 20% 11% 5% 6% 7% 5% 2% 0% 1% 0% 0% 0% 2% 0% 0% 0% 0% 0% Good (<24) Fair (25-180) Poor (181-610) V. Poor (>610) Good (<24) Fair (25-180) Poor (181-610) V. Poor (>610)
LIDO - Daily geometric mean APP - Daily geometric mean 100% 100% 89% 86%84% HATS ADF License HATS ADF License 75% 80% HATS2A License 80% 71% HATS2A License 67% HATS2A Relaxed HATS2A Relaxed 60% 60%
40% 40% 29% 27% 21% 20% 14% 20% 8% 7% 6% 3% 4% 1% 0% 0% 0% 3% 2% 1% 0% 0% 0% 0% Good (<24) Fair (25-180) Poor (181-610) V. Poor (>610) Good (<24) Fair (25-180) Poor (181-610) V. Poor (>610)
Recommendation on effluent E.coli standard • Under HATS-2A (Q = 1.8 x 106 m3/d), HATS effluent E.coli standard in bathing season can be relaxed from the current license level of 2 × 104 count/100mL to 2 × 105 count/100mL. • A less restrictive effluent E.coli standard (e.g. 8x105 count/100mL) can be adopted for diurnal tides without violating the WQO. • For the non-bathing season, effluent E.coli standard can be further relaxed to 7x105 count/100mL for the compliance of WQOs of other sensitive receivers and for the protection of winter beach users • More understanding on the disinfection system is required for optimizing disinfection dosage : – Hydraulics and mixing of the ADF – Relation between chlorine dosing and effluent E.coli concentration
11 3. Hydraulics of the Advance Disinfection Facility
Stonecutters Island Sewage Treatment Work Plan view General Layout of ADF Main Pumping Station
Flow distribution Chamber Sedimentation Tanks Chlorine Dosing
Chamber 15 Dechlorination Effluent Box Culverts Dosing 2 x 2.5 m x 2.5m
12 25m Influent HATS Pumping Station Effluent Flow Distribution Chamber Chlorine Emergency overflow dosing unit
3.5m x 3.5m twin box culvert Effluent Channel Chamber 9 Flocculation Tanks NaOCl Rapid Mixing Tanks Storage
Flocculation Tanks
Sedimentation Tanks
Northwest Kowloon Chlorine Pumping Station Contact Channel, 950m , 2.5 x 2.5m twin box culvert
Longitudinal section from the Flow Distribution Chamber to Chamber 15 Dechlorination chemical 25m storage Dropshaft emergency Flow Distribution to outfall overflow Chamber Chamber No.9 Ground Level Chamber 15 ~5-6 mPD Chlorine contact culvert 950m Chamber 15 Sedimentation Tank +0.85 mPD High Tide 2.3 mPD Chlorine +0.743 mPD dosing unit
Low tide Tide 0.1 mPD -0.5 mPD 90m Dechlorination dosing 3.5m x 3.5m 25m twin box culvert 1.8m wide x 4m high twin box culvert 950m Diffuser and risers 2.5 m x 2.5m twin box culvert
-14.5 mPD ......
Tunnel -90 mPD
13 Stonecutters Island Main CEPT Tanks Pumping Station (SCIMPS)
Flocculation tanks Flow distribution (4 in series x 2) Chamber (NaOCl dosing) Air injection
Rapid mixing tanks (8) Flocculant dosing
Sedimentation Main influent tanks (19x2) distribution channel
NWKPS
Daily sewage inflow pattern
The daily flow pattern of SCIMPS+NWKPS SCIMPS is similar to that of an Monthly avg. individual screening plant (e.g. NWKPS) - Min flow at 4am - Max flow at 10am and 11pm Avg. flow = 16m3/s
Monthly averaged flow 2 2 1.8 May 2011 1.8 May 2011 Aug 2011 Aug 2011 1.6 1.6 Dec 2011 Dec 2011 1.4 1.4 Mar 2012 Mar 2012 1.2 1.2 1 1 0.8 0.8 Avg. flow Avg. flow 0.6 0.6 3 = 4m3/s = 12m /s 0.4 0.4
0.2 NWKPS 0.2 SCIMPS 0 0 0 3 6 9 12 15 18 21 24 0 3 6 9 12 15 18 21 24
14 Chlorine Disinfection of CEPT Effluent
Chlorination: • Sodium hypochlorite • Design dosage: 10-20 mg/L (20 tonnes /day)
Dechlorination: • Sodium bisulphate • Design dosage: 2-4 mg/L (4.2 tonnes /day)
Aimed E. coli reduction: 3 order (107 → 104 counts/100mL)
Beach water quality standard: 180 counts/100mL
The dosing control system
Inflow meas. at MPS and NWKPS
15 Hydraulics of the ADF system
1. Flow distribution chamber – 3D flow modelling 2. Contact box culvert – 1D modelling (FDC to Chamber 15) 3. Internal hydraulics of HATS outfall (Chamber 15 to outfall), saline intrusion
Flow Distribution Chamber (Chlorine dosing unit)
From sedimentation Weir tanks 2 3.5m 3.5m outlet culvert to Chamber 9 and 15 Chlorine dosing unit
Weir 3D view Plan view 2 3.5m 3.5m To Chamber 9 and 15 inlet culvert from sedimentation tanks Dosing Unit
Submerged flow
Jets 1.7 m opening
Free surface flow
Vertical section view Dosing unit (front view) x 2
16 Simulated flow in Flow Distribution Chamber with 20 m3/s inflow (Surcharged)
(downstream looking)
Contact time in the two culverts and spatial variation of TRC concentrations at Chamber 15 • The higher flow rate in the right-side culvert leads to shorter contact time and higher residual Cl concentration. • Consistent with the finding that the TRC in right-side culvert is 1.2-1.8 times of that in the left culvert (Shang et al. 2011).
Total flow (m3/s) 8 16 26 Flow condition Free surface Surcharged Surcharged
Contact time in left-side culvert (TL, min) 18.3 14.4 8.9
Contact time in right-side culvert (TR, min) 15.6 10.9 6.7
TL / TR 1.2 1.3 1.3
Test me: 10 am, Jan 13, 2011 2.75 2.85 2.95 -- -- 2.75 CR/CL for -- -- 2.10 1st order decay 2 min. 2.40 right rate (min-1) difference in 2.10 1.55 2.05 1.75 culvert contact time ------0.074 1.16 1.50 1.20 1.40 1.45 1.20 left 1.45 1.40 0.149 1.35 1.60 1.30 1.55 1.45 culvert 1.45 1.35 0.80 Intake of 1.35 0.287 1.78 Feedback 1.40 Return flow 1.30 Control Based on DSD CEPT Effluent Chlorination Study of Feedback 1.35 Meas. TRC st 1.35 Control (2003), the 1 order decay rate for TRC is estimated (Shang et al. 2011) to be 0.074 – 0.287 min-1.
17 Near field jets from the chlorine dosing unit
NaOCl = 1.21; sewage = 1.02 Average dosing jet 1.60 L/s
All the jets will sink as the chemical is heavier than the sewage.
Only some of the jets from the upper series of dosing holes may pass through the opening above the weir directly, the jets are more likely to hit the weir structure across the flow distribution chamber.
1D hydraulic modelling of the ADF system FDC – Chamber 15
The 1D unsteady finite difference model using Preissmann four-point implicit scheme with “Preissmann slot” for surcharged conditions is employed. The model is capable of modeling subcritical/supercritical transition and free surface/surcharged conditions
50 nodes, Δx = 5.0 - 36.0 m, Δt = 10.0 s, Manning’s n = 0.016
U/S BC: D/S BC: measured sewage Measured water level at flow rate (MPS) Chamber 15
18 Computed water surface profile from the FDC to Chamber 15 during night time
The flow in the ADF system is fully surcharged under peak flow before midnight and becomes free surface flow during low flow.
Sewage inflow, predicted and measured levels at FDC, chamber 9 and 15
The predicted water level variations at FDC and Chamber 9 agree well with the observed one.
RMSE is around 0.17m – 0.19m
19 Chamber 15
+10 mPD
+2.0 mPD +1.83 mPD Low Tide -0.83 mPD 0 mPD Box culvert -0.5 mPD (2.5m high) 8.0 m -10 mPD -14.5 mPD ......
-20 mPD Dropshaft Diffuser and risers
4.5 m -80 mPD Tunnel
-90 mPD 5.0 m -90 mPD
-100 mPD
Internal hydraulics of HATS outfall
Chamber 15 ΔH = Water level in Chamber 15 – Tide level +10 mPD
+2.3H 15mPD ΔH High Tide +2.0 mPD +0.67H TmPD 0 mPD Box culvert -0.5 mPD (2.5m high)
8.07.3 m -10 mPD -14.5 mPD ......
-20 mPD Dropshaft Diffuser and risers
4.5 m -80 mPD Tunnel
-100-90 mPDmPD 5.0 m v -90 mPD
-100 mPD • Head-discharge relationship (ΔH-Q curve) • With known tide level, Q and head-discharge relationship at the downstream (Chamber 15), the upstream hydraulic profile (FDC-Chamber 15) can be determined by 1D model
Head-discharge ΔHf – Q relation (spring tide)
Q (m3/s) Q (m3/s) 30 30 9-Dec-2011 10-Jan-2012
25 25
20 20 Q 20.1 H f
15 15
10 10
5 5
dH-corrected (m) dH-corrected (m) 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5
Q (m3/s) Q (m3/s) 30 30 24-Dec-2011 9-Jan-2012
25 25
20 20
15 15 3 10 10 When Q < 15 m /s, higher head is required occasionally. 5 5
dH-corrected (m) dH-corrected (m) 0 0 0 0.5 1 1.5 2 2.5 0 0.5 1 1.5 2 2.5
20 Saline Intrusion in HATS Outfall – purging flow analysis 12 11 ……………….. 2 1 2nd riser
2 gh 2 V a 2 2 2 a h a K 2 K 2 2 K 1 2 R 8a f d A 31 j 2 2 x x nth riser (n = 3 – 12) 2 AVn + BVn + C = 0, Solve for Vn
2 2 1 a h a A K n K n K K n 2 R 8a f d 31,n 32,n A j n n a A B K K n n1 U 31,n 32,n A A n1 n n Assume the most downstream riser is 2 1 2 1 A 2 C U K K n1 U g(h z z ) intruded, the purging flow for this 2 2 2 31,n 32,n A n1 n n 2 n a riser can be determined using Yau n1 n1 2 Ui g hfi K32,i (1997)’s method. i2 i3 2 h = riser height = 14m Head loss: d = riser diameter = 0.85m, a = riser area
KR, = head loss of rosette = 1 D = tunnel diameter = 1.5-3.25m, A = tunnel area o K31, K32 = head loss of 90 diversion V = velocity in riser (K31 = 0.9-1.5, K32 = 0-0.3) U = velocity in tunnel Kf = friction loss coef. = 0.025 z = end level of riser h = friction head loss f Line of symmetry
Summary on internal hydraulic analysis of HATS outfall
• Empirical head-discharge (ΔHf - Q) relationship is derived from measured data of sewage discharge, tide level and water level in Chamber 15. • For Q > 15 m3/s, the relationship well describe the measured data. • For Q < 15 m3/s, higher head may be required to discharge the same flow, especially during spring tides, suggesting the change of internal hydraulic condition due to saline water intrusion into the outfall. • A purging flow analysis shows that Q = 15.9 m3/s is required to purge intruded saline water, consistent with the findings from H-Q relation.
21 4. Field Study of SCISTW effluent E.coli variation - (Feb and Mar 2013)
Motivation: • There is no in situ study on the effectiveness of the disinfection system; only lab study (jar tests) on chlorine dosing and E.coli kill have been conducted before its commencement. • Only one single grab sample is taken everyday for E.coli measurement in Chamber 15. Diurnal variation of effluent E.coli level is not well understood.
Objectives: • To examine the daily variation of E.coli concentration under constant chlorine dosage and diurnal flow variation; • To study the change in the E.coli concentrations at different locations of the ADF system; • To establish a relationship between the chlorine dosage and effluent E.coli concentration for dosage optimization;
22 First Survey (24-28 Feb, semi-diurnal tides) 2.5
2
1.5
1
0.5 Chlorine dosage 14 mg/L 12 mg/L 16 mg/L 10 mg/L 0 2/24/201324-Feb 2/24/2013 12:00 2/25/2013 25-Feb 2/25/201312:00 2/26/201326-Feb 2/26/201312:00 2/27/201327-Feb 2/27/201312:00 2/28/2013 28-Feb 2/28/2013 12:00 3/1/2013 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00
Second Survey (3-7 Mar, diurnal tides) 2.5
2
1.5
1 Chlorine dosage 0.5 14 mg/L 12 mg/L 16 mg/L 10 mg/L 0 3/3/20133-Mar 3/3/2013 12:00 3/4/2013 4-Mar 3/4/201312:00 3/5/2013 5-Mar 3/5/201312:00 3/6/2013 6-Mar 12:003/6/2013 73/7/2013-Mar 3/7/201312:00 3/8/2013 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00
Sampling locations
A. after settling tank before disinfection (pump sample); B. chamber 9 (grab sample at mid-depth); C. chamber 15 before dechorination (grab sample at mid-depth); D. chamber 15 after dechlorination (pump sample)
E.coli, NH3-N, TRC, pH, salinity, temperature are measured at 3 hr interval
emergency Flow Distribution overflow Chamber Chamber No.9 Ground Level Chamber 15 ~5-6 mPD Chlorine contact culvert 950m Sedimentation Tank +0.85 mPD High Tide 2.3 mPD B Chlorine +0.743 mPD A dosing unit
Low tide Tide 0.1 mPD C D -0.5 mPD 90m Dechlorination dosing 3.5m x 3.5m 25m twin box culvert 1.8m wide x 4m high twin box culvert 950m Diffuser and risers 2.5 m x 2.5m twin box culvert
-14.5 mPD ......
Tunnel -90 mPD
23 The disinfection is more effective during 5-6 Mar (< 104 count/100mL) than 26- 27 Feb (> 2x105 most of the time)
Temperature appears to play a significant role in disinfection efficiency (average of 24.4 vs 21.4 oC)
Measured E. coli before FDC, E. coli and ammonia levels at Chamber 15 before dechlorination together with the measured H2S gas concentrations at DOU 1 inlet during the in-plant surveys (Cl - chlorine dosage = 16 mg/L)
Chlorine dosage vs. the effluent E. coli level at Chamber 15 - Winter (Dec 2011 – Mar 2013)
• Cl dosage <= 8mg/L has practically no disinfection effect • Cl dosage > 16 mg/L would be likely to achieve at least 3 log kills of E. coli
24 Summary of in-plant survey
Two in-plant diurnal E. coli surveys are carried out in February and March 2013.
The disinfection is found to be more effective during the March survey than the February survey. The difference in efficiency can be attributed to two factors (i) the higher temperature (average of 24.1 vs 21.5 oC); and (ii) the higher sulphide levels of the February survey. These are consistent with the expectation that higher temperature can increase the sulphide formation, resulting in higher chloride demand and lower disinfection efficiency (Shang et al. 2011).
There are clear variation patterns for sewage flow, ammonia and H2S gas, while no consistent changing pattern is observed for E. coli either before or after the chlorine disinfection. Ammonia concentration peaks daily at noon time, while the sulphide level peaks twice daily at early morning and around 18:00 in the afternoon.
Conclusions: Proposed HATS Operation Strategy for Disinfection Dosage Control
1. Apply chlorine dosage of 10 mg/L during the non- bathing season to achieve an effluent E.coli level of 2x106 count/100mL 2. Apply chlorine dosage of 12-14 mg/L during the bathing season on days with diurnal tide to achieve an effluent E.coli level of 8x105 count/100mL 3. Apply chlorine dosage of 16-18 mg/L during the bathing season on days with semi-diurnal tide to achieve an effluent E.coli level of 2x105 count/100mL 4. Improve plant operation to reduce sulphide levels in sewage inflow to reduce the chloride demand
25