December 2017 Tamar Estuary
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- Projects Estimated Cost ($ million)
Tamar Estuary River Health Action Plan 41 1. Impacts of these individual projects on Tamar Estuary Zone 1 enterococci concentrations are shown in Figure 12.
The New Margaret Street storage and New combined rising main can both be expected to lead to very substantial decreases in Tamar Estuary Zone 1 enterococci concentrations (15 per cent to 16 per cent). The West Launceston diversion is also very cost effective, leading to an 8 per cent decrease in concentrations for less than 20 per cent of the cost of the New combined rising main. However, the expected increase in flows to Ti Tree Bend treatment plant required exploration to see how the plant’s performance would be effected. Ti Tree Bend treatment plant was primarily designed to remove Tamar Estuary River Health Action Plan 42 suspended solids and treat pathogens from the combined system and, while has some benefits for reducing nutrient levels in effluent, it is not its primary purpose. The following figure shows how Ti Tree Bend functions. Figure 13. Ti Tree Bend treatment process Essentially, flows of up to 200ML/day will receive screening going into the plant, which stops a lot of the large pathogen carrying matter overflowing straight into the Estuary in very large rainfalls. Up to 120ML/day of flow goes on to receive primary treatment (essentially chlorine dosing), while the balance is discharged to the Estuary after the initial screening. Around 60ML/day goes on to receive secondary treatment, with the remainder that has received primary treatment bypassing that stage and discharged to the Estuary. In dry weather conditions, flows at Ti Tree Bend are around 12.2 ML/day. While the plant performs soundly with respect to treatment of pathogens in dry and low rainfall conditions, TasWater’s available influent and effluent monitoring from the plant suggests that in times of high rainfall, where flows are large, there is decreased efficiency in primary and secondary plant performance in terms of treatment of nutrients. The available data is inconclusive on the degree of that efficiency loss, but it would seem that higher levels of flow have the impact of mobilising nutrients already in the plant, such that nutrient load exiting the plant is higher than the load entering the plant once flows reach around 30ML/day for nitrogen and 90ML/day for phosphorous. As can be seen in Figure 14, the vertical change in the blue line represents the amount of avoided nitrogen from reduced combined system overflows due to the proposed projects, while the vertical change in the red line represents the increased nitrogen load that would exit Ti Tree Bend for the same level of flow.
Tamar Estuary River Health Action Plan 43
Clearly, while the benefits of the proposed projects for pathogen reduction are expected to be significant in total and have additional benefits in terms of reducing nutrient loads from combined system overflows, the Taskforce was of the view that all this benefit would be undone if the ecological health of the Estuary was made worse by a net higher level of nutrients then entering the Estuary. TasWater had previously looked at the potential benefits of upgraded nutrient treatment at Ti Tree Bend, utilising analysis conducted by CH2M Australia Pty Ltd and this forms part of the Stage 2 LSIP planning that focuses on a series of improvements at the existing Ti Tree Bend plant which could total up to $100 million. Through this work TasWater looked at the costs and effectiveness of several potential nutrient upgrade options, but for the purpose of this analysis the Working Group, in discussion with TasWater, incorporated intermittently aerated bioreactor, aerobic bioreactor and sidestream deammonification components. The cost of these works was estimated at around $10 million. CH2M Australia estimated total nitrogen effluent loads would decrease by roughly 53 per cent and total phosphorous by 72 per cent as a result of this upgrade. These assumptions were added to the proposed combined system projects and Figure 15 shows the expected impacts on Tamar Estuary Zone 1 concentrations. This figure shows very substantial benefits of the treatment plant upgrade in terms of decreased nutrient concentrations. It is estimated that total phosphorous concentrations would be expected to decrease by 18 per cent and total nitrogen by 26 per cent. While the TasWater/CH2M work was largely desktop and further detailed design is needed to be confident about the costing of this project and the magnitude of these reductions, it would seem that an investment of $10 million in nutrient removal upgrade would offset the decline in performance expected at Ti Tree Bend with increased flows. Tamar Estuary River Health Action Plan 44
This investment option allows the benefits of reduced combined system overflows in terms of enterococci to be retained while substantially decreasing nutrient concentrations, avoiding the potential decline that could be expected without such an upgrade. 6.3
The table below outlines the projects and actions recommended that the Taskforce believe will yield the best value for money improvements to the Estuary. Projects Estimated Cost ($ million) Catchment Actions
Brumbys-Lake, Macquarie, Meander & Tamar - Dairy 1.10 North Esk - Grazing 1.33
Upper Tamar – Grazing 1.66
Brumbys-Lake, Meander and South Esk – Grazing 5.41
Launceston sewage stormwater intrusion 0.50
Combined system actions
Esplanade storage 6.7 Forster St storage 8.4 New Margaret St storage 10.0 South Launceston Diversion 18.1 West Launceston Diversion 4.6 Yüklə 1,91 Mb. Dostları ilə paylaş:
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