2. REVIEW OF CHEMICALS FOR CEPT AND POTENTIAL DISINFECTION OPTIONS

2.1 Introduction

2.1.1.1 Under normal circumstances, the collected raw sewage would be pumped from the Ha Tsuen Pumping Station to San Wai STW for treatment. The treated effluent would be discharged through the NWNT effluent tunnel and Urmston Road outfall. For the purpose of this EIA, four treatment options for the expanded and upgraded San Wai STW have been critical examined in terms of their associated water quality and ecological impacts on the aquatic environment with the findings presented in Sections 5 and 8 of this EIA Report. The four treatment options considered are as follow:

2.1.1.2 This section summarises the findings of studies and trials carried out under several recent projects in Hong Kong with regards to the selection of chemicals for CEPT process and the performance of a number of potential disinfection options. More details of the studies and trials are included in Appendix 2A of this report.

2.1.1.3 The studies and trials reviewed in this section include:

2.2 Review of Chemicals for CEPT

2.2.1 Treatment Performance

2.2.1.1 Removal requirements for biochemical oxygen demand (BOD) and suspended solids (SS) at San Wai STW are likely to be approximately 50% and 70% respectively. Even allowing for an increase in wastewater strength, such removals would produce typical effluent quality of <100 mg BOD/L and <55 mg SS/L. Previous studies have shown that various chemicals can readily achieve these objectives, based on laboratory tests and pilot trials on effluents from several different catchments. These include ferric chloride, ferric sulphate, alum, and cationic polymeric coagulants, although the latter were found to be relatively expensive.

2.2.1.2 Experience over several years has shown that the use of ferric chloride at Stonecutters Island STW in moderate doses (10 mg/L as FeCl3 (i.e. 3.5 mg/L as Me3+) together with 0.1 mg/L of anionic polymer) consistently produces higher removals than the expected requirements for San Wai STW. The required dose rate for alum to achieve equivalent BOD and SS removal is likely to be around 7.5 mg/L Al2O3 (i.e. 4.0 mg/L as Me3+).

2.2.2 Chemical Supply

2.2.2.1 Availability of supply of chemicals such as ferric chloride, ferric sulphate or alum in the quantities required for this project is not expected to be a constraint on options.

2.2.3 Sludge Treatment

2.2.3.1 Recent studies and trials demonstrated no major difficulties in sludge dewatering for both the ferric chloride (pilot and full-scale) and alum (pilot scale) options.

2.2.4 UV Transmittance

2.2.4.1 San Wai STW's catchment is almost entirely domestic, although there are small "industrial" zones, e.g. in Tung Tau (the Yuen Long Industrial Estate will continue to discharge to Yuen Long STW). Transmittances after CEPT might therefore be expected to be similar to those found for NW Kowloon wastewater. The results from jar tests were as follows:

2.2.4.2 It is noted that, however, Stonecutters Island STW produces a transmittance slightly better than the jar tests with ferric chloride - typically in the range 26 to 30%.

2.2.4.3 The enhanced UV transmittance when using alum suggests that, based on the dose rates described above, lower power costs will be incurred for this option if UV is selected for disinfection. However, most studies suggest that ferric chloride is cheaper than alum for CEPT at these dose rates. For the ferric chloride option it may therefore be possible to improve effluent transmittance by increasing the coagulant dose to produce a more cost-effective option overall.

2.3 Review of Potential Disinfection Options and Their Implications for San Wai STW

2.3.1 Options Considered

2.3.1.1 This review is undertaken with reference to the findings of the studies and trials carried out under several recent projects in Hong Kong as discussed in Section 2.1 above. More details of the studies and trials are included in Appendix 2A of this report. The options considered in this review include:

2.3.2 Chlorination

Gaseous Chlorination and Dechlorination

2.3.2.1 Although the San Wai STW site is relatively remote from sensitive receivers, rural residential areas such as Tseung Kong Wai exist within the 1km radius (PHI boundary) of the site. Moreover, the proposed Deep Bay Link will be adjacent to the site boundary. The risks associated with transportation, handling and storage of large quantities of gaseous chlorine are considered to be significant when compared to the far safer chlorination alternative (sodium hypochlorite). Furthermore, the development of a large-scale gaseous chlorine storage and handling facility is likely to receive strong objection from local residents and other concerned individuals and groups.

2.3.2.2 It is not recommended that Gaseous Chlorination and Dechlorination be retained for further consideration.

Hypochlorination

2.3.2.3 The results of the SSDS Stage 2 PPFS trials are important for this option. These suggested that a minimum contact time of 30 minutes would be required to ensure adequate disinfection at times of peak diurnal concentration. If the peak diurnal concentration occurs at the same time as the peak diurnal flow at San Wai STW, and if the peak diurnal flow is twice the average design flow, the minimum contact time required at average design flow would be one hour.

2.3.2.4 Since the CEPT process will remove most of the dissolved sulphide in the influent at San Wai STW, the influent sulphide concentration will have negligible effect on chlorine dosage. During maintenance of the NWNT effluent tunnel, disinfection by chlorination and dechlorination would still be available. Besides, it is possible to incorporate the ability to increase chlorine dose during tunnel maintenance into the design.

2.3.2.5 Regardless of the performance of hypochlorination in disinfection, there have been growing worldwide concerns on the toxic effects and carcinogenic potential of chlorinated by-products (CBPs) to both human and aquatic life. Such concerns were also expressed by several speakers and public comments on the SSDS/HATS project. There are still uncertainties on the carcinogenic potential of a number of CBPs. Moreover, CBPs consist of a whole range of halogenated organic compounds, many of which are yet to be identified, with unknown health effects and fate in the marine environment.

2.3.2.6 Discharges of CBPs are also being controlled in wastewater discharges in other overseas countries. In the UK, the use of chlorination is not approved for "long-term" disinfection of wastewater effluent. Several treatment works remain where the use of hypochlorite is allowed until such time as the effluent consents will be updated, but these are the minority.

2.3.2.7 The effluent standard required for San Wai STW is not the most stringent, and options other than chlorination are feasible. In view of the large quantity of effluent and the discharge close to the marine reserve, options such as UV are likely to lead to less concern from the community than chlorination.

2.3.3 UV Irradiation

2.3.3.1 The evidence gathered from several previous studies shows that UV disinfection would be feasible to disinfect CEPT effluent to the required E.coli standards. The effluent to be disinfected in this project will be a mix of CEPT effluent from the San Wai STW catchment and biological effluent from Yuen Long STW. The influent to the disinfection system will therefore have a relatively low suspended solids concentration. The mean UV transmittance is expected to be 15% or better if ferric chloride is used for CEPT at San Wai STW, and 25% or better using alum as the CEPT coagulant.

2.3.3.2 During maintenance of the NWNT effluent tunnel, UV disinfection would still be available. The UV dose and/or coagulant dose could also be increased to enhance disinfection during tunnel maintenance if necessary.

2.3.3.3 A key advantage of UV disinfection is the lack of by-products in the effluent. This has strongly influenced disinfection policy elsewhere. For example, in the UK the Environmental Agency policy remains as defined by the NRA in the early 1990s. This provides that UV is the only approved method for disinfection in the long-term (other methods are allowed as interim measures in particular circumstances). Chemical disinfectants are not viewed with much favour, due to concerns over THMs from chlorination and aldehyde residues from peroxides and ozone.

2.3.4 Ozonation

2.3.4.1 As noted in the previous studies, the effectiveness of ozonation varies greatly with effluent quality. Given that the San Wai wastewater is quite weak and that the San Wai CEPT effluent will be mixed with biological effluent from Yuen Long STW, the required ozone design dose may be less than the 30 mg/L (and 40 mg/L short-term peak) derived for the SSDS Stage 2 options.

2.3.4.2 During maintenance of the NWNT effluent tunnel, disinfection by ozonation would still be available. Besides, it is possible to incorporate the ability to increase ozone dose during tunnel maintenance into the design.

2.3.4.3 Nevertheless, recent studies have concurred that ozonation is likely to be much more expensive in both capital and operating cost terms than UV or hypochlorination / dechlorination. Land requirements would also be greater for ozonation.

2.3.5 Microfiltration

2.3.5.1 Microfiltration will achieve excellent removal of E-coli, but is would require a very large site area for disinfection of primary effluent. Microfiltration would also require additional sludge dewatering and disposal facilities. This process is not cost effective as a disinfection technology. It would only be viable if further improvements in other parameters (e.g. effluent suspended solids) are required or if biological treatment is needed, in which case the technology could be considered as part of the biological treatment stage (as membrane bioreactors). During maintenance of the NWNT effluent tunnel, disinfection by microfiltration would still be available.

2.3.6 STW Layout and Land Requirement for Different Options

2.3.6.1 Other than the disinfection facilities, other parts of the proposed expanded San Wai STW would not be affected by the selection of disinfection options. However, the land requirement for different disinfection options would be quite different. In general, the land requirement would be the highest for the microfiltration option, followed by the ozonation and chlorination options. The land requirement for the UV disinfection option would be the lowest among the reviewed options.

2.3.7 Summary

2.3.7.1 A summary of the disinfection options reviewed in this section is presented in Table 2.1. Among the reviewed disinfection options, UV irradiation is considered as a suitable option with less impacts on environment as well as other related aspects.

Table 2.1  Summary of Disinfection Options Review

2.4 Conclusions

2.4.1 Chemicals for CEPT

2.4.1.1 The review concluded that various chemicals namely ferric chloride, ferric sulphate, alum, and cationic polymeric coagulants can readily achieve the required effluent quality of <100mg BOD/L and <55 mg SS/L for the proposed expanded San Wai STW.

2.4.1.2 Sludge dewatering has been demonstrated to be without major technical difficulties for both the ferric chloride and alum. Nevertheless, there appears difference in the UV transmittance by using difference chemicals and hence the performance of UV disinfection. The final choice of chemical is likely to depend on the results of UV transmittance tests and review of chemical costs to be undertaken during the detailed design stage of the project.

2.4.2 Potential Disinfection Options

2.4.2.1 A number of potential disinfection options namely chlorination, UV irradiation, ozonation, and microfiltration have been reviewed for the proposed expanded San Wai STW. Among the reviewed disinfection options, UV irradiation is considered as a technically feasible option suitable for the proposed expanded San Wai STW. This option imposes minimal environmental impacts and lack of by-products in the effluent, and also require the lowest landtake among the reviewed options.