Advanced Design and Technologies for In-situ Reprovisioning of Sha Tin Water Treatment Works South Works Abstract: The Sha Tin Water Treatment Works is the largest water treatment plant in the Hong Kong Special Administrative Region, People s Republic of China. The South Works, using conventional water treatment process, was commissioned in 1964 while the subsequent expansions in 1973, 1976 and 1983 formed the North Works. The 48 years old plant and equipment in the South Works are approaching the end of their service life and require major renovation and replacement. The challenge is to complete the in-situ reprovisioning of the South Works and major common facilities for both the South Works and North Works within a site, which is already fully utilised, whilst maintaining the continuous operation, maintenance and security of the North Works. The key to success is to replace the conventional low-rate sedimentation process with high-rate robust process to free up the much-needed space. High rate lamella sedimentation basins, followed by ozonation and deep bed biological filtration, and UV disinfection at the downstream are proposed for the in-situ reprovisioning of the South Works. The paper presents the design approach and evaluation methodology of the water treatment processes and the formulation of the subsequent Master Layout Plan for the new South Works. Keywords: In-situ Reprovisioning, Water Treatment Works, High Rate Lamella Sedimentation, Deep Bed Biological Filtration, UV Disinfection 1
Introduction The Sha Tin Water Treatment Works (STWTW) is the largest water treatment plant of the Hong Kong Special Administrative Region (HKSAR) Government, People s Republic of China. It was commissioned in 1964 with an initial treatment capacity of 364,000 m³/day, which formed the current South Works. Three subsequent expansions in 1973, 1976 and 1983 formed the North Works, bringing the total nominal treatment capacity to 1,227,000 m³/day. The 48 years old plant and equipment in the South Works are approaching the end of their service life and require major renovation and replacement. The challenge is to complete the in-situ reprovisioning of the South Works with increased capacity from 364,000m 3 /day to 550,000m 3 /day, including major common facilities for both the South Works and North Works within the site, which is already fully utilised. It is also necessary to maintain continuous operation, maintenance and security of the North Works during the South Works reprovisioning. Water Supply Sources Currently STWTW receives water from 5 different sources: Dongjiang water is the main source, supplying water 11 months in a year. Plover Cove Reservoir (main storage of Dongjiang water) during Dongjiang shutdown. Shing Mun Reservoirs (yield from local catchment) by gravity. High Island Reservoir (yield from local catchment) with nominal supply only, and cosupply with Plover Cove Reservoir during Dongjiang shutdown. Direct catchment yields with high turbidity during rainy seasons. Figure 1 Water Sources for STWTW 2
These five water sources are different in character. Whilst the current Dongjiang water is excellent in quality, there have been prolonged periods in the past with high ammonia and manganese that require special attention. For the reservoir water, it is generally low in turbidity. High Island Reservoir is excellent in quality. Organics in the Plover Cove Reservoir tend to be high due to eutrophication, with occasional taste and odour problems due to algal bloom or slightly high MIB level in reservoir. Volume of catchwater yield during downpour can be very high and sufficient for normal demand without supplement from other sources. The challenge is the high turbidity from the runoff, jumping from the annual average between 10-15 NTU to the average maximum between 100-200 NTU during wet seasons (six months yearly), and a historical high of 400 NTU. Conventional Water Treatment Process With the constant changes of water sources during the day and throughout the year, the treatment process must be robust to meet the challenges from a continuous fluctuation in raw water quality. The treatment plant uses the conventional two-stage treatment process which is a typical design of the 1960-1980. It comprises: addition of lime for ph adjustment and alum for coagulation and flocculation; clarification to settle out the large particles as sludge for further treatment and disposal; chlorine to remove manganese and ammonia that may be present in the water; rapid gravity filtration to remove finely divided particles in clarified water; and disinfection with chlorine, corrosion control by ph adjustment with hydrated lime and fluoridation for dental protection. Figure 2 Conventional Treatment Process Adopted in STWTW 3
A few key features of the earlier design include: Hydrated lime and alum addition at the inlet work is by dripping over a channel, followed by hydraulic mixing. This is an energy efficient design as no external energy input is required. The circular sludge blanket/reactor type clarifier is robust and capable of accommodating a wide turbidity range. However, shock loading from sudden changes in flow and quality must be avoided. The rapid gravity filter design uses dual media comprising fine sand and anthracite with a total thickness from 0.6m to 0.8m with supporting gravel layers (for filters constructed in different years). Treated water quality from this design serves the public well, and is generally able to meet the latest World Health Organisation (WHO) Guidelines for Drinking-water Quality. The Need for Reprovisioning STWTW used to supply treated water to approximately 40% of the population in Hong Kong in the 1980s and 1990s. With de-centralization of the population, more treatment plants were built in the 1990s. Commissioning of these treatment plants also serve to reduce reliance on production from STWTW and to improve reliability in security of water supply. Today, it still serves about 30% (two million people) of the population. With over 48 years of service, the aging plant and equipment are approaching the end of their service life. Increasing fault occurrences and unavailability of the spare parts caused significant maintenance problems. Furthermore, the treated water quality requirements have been raised progressively over the last 48 years with anticipated higher requirements in the future. Being an international city with high standards of living, the Water Supplies Department (WSD) of the HKSAR reviewed the international treatment plant design criteria in more advanced developed countries like US and the UK in formulating the appropriate design specifications for the reprovisioned STWTW. In addition to the compliance of treated water quality with the latest WHO s Guidelines for Drinking-water Quality, additional parameters have been incorporated in the design specification to supplement the WHO Guidelines in safeguarding drinking water quality. The key additional parameters include reduction of cryptosporidium and giardia (C&G) and virus by 4 log (ie, 99.99% reduction), as well as the treatment capability to cope with high iron, as well as removal of ammoniacal nitrogen (3.0 mg/l) and soluble manganese (1.5 mg/l) in raw water to take account of historical occurrence of these parameters. 4
In addition to the above treated water quality standards, WSD also required that in the new process design, the use of chlorine should be reduced as far as practicable to minimise the hazards associated with chlorine delivery and storage and halogenated disinfection byproducts. Traditionally, chlorine is used for oxidation of manganese and removal of ammonia, as well as final disinfection. Alternative treatment has to be used. Due to the above changes, the logical and practical solution is to reconstruct the plant to meet the current standards. Constraints in the Reprovisioning An investigation study was conducted in 2003, with the requirements that the treatment plant output be maintained at 900,000m 3 per day during the reprovisioning plant, and only two clarifiers could be taken out of service at any one time. New common facilities including chemical building, administration building, laboratory, power supply and pumping stations had to be reprovisioned as well. The first step taken was a review of the site utilization. Figure 3 shows the layout of the existing treatment plant, and the approximate area occupied by the clarifiers and filters. With the main treatment area occupying an area of approximately 80,000m 2, the clarifiers and filters take up respectively 46% and 20% of the site. A review of the existing treatment processes shows that the effective surface loading rate (SLR) of the clarifiers is about 2-2.5m/hr, whereas the SLR of the filters is about 9m/hr. A logical solution is to use high rate processes to reduce the footprint and free up more areas for other facilities for the reprovisioning of South Works with upgraded capacity from 364,000m 3 /day to 550,000m 3 /day. 5
Figure 3 Site Utilization Plan Whilst modern clarification and filtration processes are capable of offering higher SLR, the severe constraints mean that only relatively fast processes like Actiflo for clarification or low pressure membrane could be used. Apart from the operation and maintenance issues with these processes, the final layout of the treatment plant is considered unsatisfactory due to the small areas available for concurrent demolition and reconstruction. As a result of the findings from this study, WSD has carried out the following advance works to provide more area available for concurrent demolition and reconstruction: Obtained consent from the district council and residents association to remove part of the hill on the west of the site to construct a new chemical building. There are two main advantages: freeing up space early for reconstruction and offering a more central location for chemical dosing for the new treatment plant; Made modifications to the South Works filters to allow about half of the South Works filters to be demolished on commencement of construction; and Re-configured the water supply network and reduced reliance of supply from STWTW. These advance works allow other process alternatives to be considered. 6
Process Considerations Comparing the conventional treatment process used at STWTW and the new treated water quality standards including the reduction in chlorine usage, the following observations are made: The hydraulic mixing facility does not provide sufficient mixing energy needed for the modern design. Hence a new flash mixing and flocculation facility has to be provided. Whilst the use of potassium permanganate for manganese oxidation is practised elsewhere, it requires very precise control which is not easy to achieve in practice for WTW receiving raw water with rapid quality fluctuation. Hence, it is not the preferred solution in Hong Kong. The alternative is to use ozone (pre-ozone). Experience of this process in Ngau Tam Mei WTW, another advanced treatment plant in Hong Kong, is very favourable. The use of ozone at this stage of the treatment has the added benefit of reducing taste and odour problems and breaking down organics in the raw water to facilitate better process performance downstream. The alternative for ammonia removal is to use biological filter, with exhausted granular activated carbon as media. This process is to be preceded by intermediate ozonation which will oxidize the organics in the raw water as food source for the biological process in the biological activated carbon (BAC), and replenish dissolved oxygen to the water. Use high rate clarification technology such as high rate lamella sedimentation and dissolved air flotation to substantially reduce the footprint. Disinfection using chlorine remains the most practical solution. All other alternatives will involve the use of alternative form of chlorine products either as hypochlorite solution or chloramines (a chemical product of chlorine and ammonia). They require more space for its installation and storage, lead to more traffic, noise and emission impact for frequent deliveries, and require more energy and E&M plant for its production with the disadvantage of being less effective in disinfection. C&G and virus, which cannot be completely removed by conventional filters, can be reduced / inactivated by a number of alternatives, such as intermediate ozonation, ultraviolet (UV) radiation and low pressure membrane filtration. Each with different capital cost, operating cost, land requirement and point of application. Taking into account the above considerations, six treatment process options that can satisfy the process needs have been selected. All six options invariably require the use of ozone for manganese oxidation and biologically activated filter for ammonia removal for the reasons given above. These six treatment process options are: 7
Descriptions of Process Options Option 1A Option 1B Option 2 Option 3 Option 4A Option 4B Clarification Ozone BAC - UV High rate lamella sedimentation, followed by intermediate ozonation, single stage deep bed biological filtration (BAC) combining the biological ammonia removal function and particle filtration in one single filter, and UV disinfection downstream of the BAC Clarification Ozone BAC - Granular Media Filtration - UV This option is similar to Option 1A, but with two stages of filtration, ie, different filters for biological filtration and particle filtration Clarification Ozone BAC - Low Pressure Membrane Filtration This option is similar to Option 1B, but uses low pressure membrane filtration to perform the function of particle filtration and C&G removal Low Pressure Membrane Filtration Ozone - BAC Low pressure membrane filtration to replace clarification, particle filtration and C&G removal, followed by ozonation and subsequent biological filtration for ammonia removal Clarification Ozone - BAC This is the same as Option 1A, except that instead of using UV downstream of the BAC for C&G inactivation, the intermediate ozone system is designed to carry out this function Clarification - Ozone - BAC - Granular Media Filtration This is the same as Option 1B, except that instead of using UV downstream of the BAC for C&G inactivation, the intermediate ozone system is designed to carry out this function The designed SLR of the high rate lamella sedimentation process is 10m/hr, which is about 5 times faster than the current sludge blanket/reactor type clarifier. The SLR of the filters (either biological filter or particle filter) is also increased from 9m/hr to 16m/hr by using a deep bed (2.1m vs about 0.9m in conventional filters) coarse monomedia (GAC with effective size 1.1mm for biofilter and anthracite with effective size 0.95mm for particle filter, against fine sand with effective size 0.65mm and anthracite with effective size 1.4mm for conventional filter). These high rate processes allow some existing areas to be freed up for provision of other new facilities such as pre-ozone, intermediate ozone and biofilters needed for the new process. Essentially, the difference between Options 1 and 4 is in the use of UV against ozone for C&G inactivation, and the difference between Options A and B is the use of one-stage or two-stage filters. To assist the evaluation process, a decision model was constructed. Taking into account the treatment requirements, technical as well as operational needs, the selection criteria in the model included: treatment effectiveness, constructability, public acceptance/aesthetics, operational issues, sustainability and impacts on distribution system as shown in Appendix 1. 8
The weighting for each factors and the technical scores for the six options were agreed at a workshop with all stakeholders. Results of the technical evaluation are shown in Appendix 2. After the technical evaluation from the decision model, the technical scores and the estimated whole life cost for a 20 year period were compared. As there is no cost data for similar plants in Hong Kong, and that the design is only at conceptual level, typical unit costs from North America were used where appropriate. As the main purpose of this evaluation is to compare the relative order between options, absolute values are not necessary. The evaluation results are shown in Table 1 below. Primary Factor Total Benefits (as Measured by the Decision Model) Estimated 20 year Life Cycle Cost Benefit-to- Cost Ratio (with cost measured in Billion HK$) 1A: Preozone- Clarification- Ozone-BAC- UV 1B: Preozone- Clarification- Ozone-Two- Stage Filtration-UV 2: Preozone- Clarification- Ozone-BAC- Membranes Option Number 3: Preozone- Membranes- Ozone-BAC 4A: Preozone- Clarification- Ozone-BAC 4B: Preozone- Clarification- Ozone-Two- Stage Filtration 79.5 77.7 76.6 75.4 81.1 79.4 HK$2,599M HK$3,050M HK$3,490M HK$3,381M HK$2,562M HK$3,049M 30.6 25.5 21.9 22.3 31.7 26.04 Table 1 Preliminary Benefit-to-Cost ratio for all Process Alternatives Given that the cost estimate is only at conceptual level, the ranking is viewed as groups rather than absolute ranking. On this basis, the preliminary modelling indicates that Options 4A and 1A are the leading options, followed by Options 4B and 1B. The last two Options 2 and 3 are both membrane related. The following observations can be drawn from the above analysis: In terms of effectiveness in achieving the treatment goals and other objectives, all options are satisfactory without significant difference in scoring; The main difference between groups, apart from physical land intake, is in capital and O&M costs; The most favoured options are those with single stage filtration (Options 1A and 4A). Apart from savings in capital cost for one less set of filters, the lesser head loss means that long term pumping cost of treated water away from the treatment plant is also lower. 9
Options 1B and 4B scored generally better than Options 1A and 4A from a treatment effectiveness standpoint, but scored lower overall due to other issues such higher capital and O&M costs, larger footprint, and operational complexity. It is also noted that the model showed a comparatively small difference between Options 1A and 4A, and between Options 1B and 4B. These pairs of options differ only in the type of primary disinfection used, with those options using ozone scoring slightly higher than those using UV. However, the differences are small, and can be considered to be the same in practical terms. The most unfavourable options are those using membrane filtration because of the higher capital and O&M costs. Taking into account the increasingly more stringent treated water quality standards by WHO and other authorities such as United States Environmental Protection Agency (USEPA), and potential deterioration of raw water quality in future, the following factors were considered in the final decision process with an aim to further reduce potential risks on public health by multiple barriers: A two-stage filtration process will provide an additional barrier for particle removal; and UV downstream of filters will provide an additional barrier for disinfection. From functional needs, the selected treatment process is therefore Option 1B: Clarification Ozone BAC - Granular Media Filtration - UV Because the whole treatment plant is to be reprovisioned in stages, it is inevitable that the treatment process for the new South Works will be different from the existing North Works. This include the provision of a separate pre-ozonation, coagulation and flocculation facility to the new South Works, whilst the North Works will only be provided with a new inchannel static mixer for alum and lime dosing. Programme Constraints and Final Site Layout Although the four clarifiers and half of the filters in the South Works can be demolished on commencement of the construction works, other key programme constraints must also be considered. These include: Current access to the North Works is via the South Works. An uninterrupted access for fire services and chlorine and other chemical delivery must be provided to maintain normal operation of the North Works during reprovisioning. Therefore an additional section of road must first be constructed at the north-east corner of the site to form a ring road from the site entrance via the sludge treatment area to the North Works filter area; 10
A new chemical building must be re-provided before the existing chemical building at the south-most part of the site can be decommissioned. That means the hill west of the site must first be removed for construction of a new chemical building; The state-of-the-art administration building cum laboratory including visitor facilities will occupy the area of the existing filter backwash facility at the eastern part of the site, which currently handles the washwater from both the South and North Works filters. This area can only be vacated after a new washwater handling facility (i.e. residual management facility) is provided in the South Works; and last but not the least, The remaining half of the South Works filters, which is currently used to provide the shortfall in filtration capacity from the North Works, can only be demolished when the water supply from STWTW could be reduced to 550,000m 3 /day provided that Tai Po Water Treatment Works Stage 2 is tentatively commissioned in early 2017. The final site layout plan is shown in Figure 4, and the state-of-the-art administration building cum laboratory is shown in Figure 5. Conclusion The reprovisioned South Works of STWTW will provide treated water meeting the latest treated water quality standards. Innovation includes the introduction of high rate lamella sedimentation, deep bed monomedia biofiltration and UV disinfection. The administration building cum laboratory including visitor facilities blend in well with the environment, embroidering sustaining design features aiming at least a gold standard for green building design according to the Hong Kong Building Environmental Assessment Method (BEAM). The congested site together with the constraint to maintain uninterrupted operation of the North Works albeit at a reduced capacity poses significant challenge to the design team. The solution adopted takes account of all physical, operational, and other constraints, and has been subject to a comprehensive evaluation of planning, engineering, environmental, cost and maintenance considerations. The selected multi-barrier design is considered to be the most feasible and practicable. The proposed layout also offers the most efficient and balanced land use arrangements. 11
Figure 4 Layout Plan of Reprovisioned South Works Figure 5 State-of-the-art Administration Building cum Laboratory 12
Appendix 1 - Criteria for the Decision Model Level 1 Criteria Treatment Effectiveness Constructability Issues Public Acceptance/ Aesthetics Operational Issues Level 1 Weight 30% 10% Level 2 Criteria Level 2 Weight Level 3 Criteria Level 3 Weight Effectiveness for Taste & Odour 10% Control None 3.0% Effectiveness for Organics 10% Removal None 3.0% Enhanced Primary Disinfection 20% None 6.0% Effectiveness for Ammonia 20% Removal None 6.0% Effectiveness for Manganese 20% and Iron Removal None 6.0% Effectiveness for Particulate 20% Removal None 6.0% Overall Footprint 35% None 3.5% Integration with existing North Works and future reprovisioned 15% None 1.5% North Works Impacts to Operation and Access to the North Works 15% None 1.5% during Reprovisioning Construction Schedule 35% None 3.5% Net Impact on Total Score 10% None 10.0% 30% Sustainability 10% Impacts Distribution System on 10% Health and Safety 5% None 1.5% Operator Friendliness 20% None 6.0% Reliability 20% None 6.0% Familiarity to WSD 20% None 6.0% Proven Track Record 20% None 6.0% Reliance on Proprietary Systems 15% and/or Equipment None 4.5% Waste Generation 30% Liquid Waste 40% 1.2% Solid Waste 60% 1.8% Carbon Footprint 70% None 7.0% Potential Bacterial Regrowth 50% None 5.0% Disinfection By-Product 50% Formation Potential None 5.0% TOTAL 100% 13
Appendix 2 Technical Evaluation Results from the Decision Model 14