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High Efficiency Sediment basins could reduce stormwater quality treatment infrastructure costs by $5.4 billion in South East Queensland over the next 30 year
Steve Dudgeonsteve.dudgeon@o‐2.com.au
Offices in Toowoomba, Brisbane and Sunshine Coast
Introduction• Cost of bioretentions in SEQ for the next 30 years• TSS contributions from development for both construction
and operational phases• Preferred solution to improve outcomes at similar costs• Likelihood of success of adoption of preferred solution • Cost effective solution that provides environmental, spatial
and temporal equivalence
SEQ broad scale costs The total area of broadhectare land available in SEQ for residential development is 39,120 hectares with an expected dwelling yield of 450,000.
Using Water by Design (2014) cost figures the total cost for bioretention basins will be $5.5 billion, including a $1.6 billion cost to councils. Based on these calculations the cost per dwelling is $12,222.
Alternative cost effective stormwater quality solutions
South East Queensland (SEQ) councils are currently looking for alternative cost effective stormwater quality solutions that achieve environmental equivalence to the currently accepted stormwater quality treatment best management practices.
Such as utilising offsite offsets such as streambank erosion restoration and/or onsite offsets such as High Efficiency Sediment (HES) basins.
Abatement CostsWaterbyDesign (2014) recently produced TSS abatement cost curves. Example costs for abatement of sediment include ‐
Bioretention – average cost $4,583/tonne
Wetland – average cost $10,322/tonne
Riparian Revegetation – average cost $16,754/tonne
Stream bank erosion mitigation works $500/tonne (per comm. SEQ Catchments) and HES basins $100/tonne
BLIGH TANNER
Construction phase on a 10 ha site at 100t/ha/yr = 1000t
Operational phase over 30 years on a 10ha site at 1.3t/ha/yr = 390t
Construction phase produces 2.5 times more TSS in 1 year compared to 30 years of operational phase
Preferred solution to improve outcomes at similar costs
Strategy •Install High Efficiency Sediment basins during construction and bioretention during operational phase.•Correctly designed and managed traditional sediment basins installed during the construction phase of development only remove 30% of TSS. •HES basins are capable of treating a far greater volume of runoff than a traditional sediment basin, and remove over 90% of TSS
High‐Efficiency Sediment (HES) BasinsRunoff Dosed with Coagulant at Inflow to Basin
Sediment drops out along basin length
Treated runoff discharges from basin via decant system
WINNER ENVIRONMENTAL INNOVATION 2014
TSS contributions from development for both construction and operational phases
High Efficiency Sediment Basins (HES) during construction can achieve in 1 year the equivalent pollutant load reduction of 30 years operation of a bioretention basin.
Graph shows modelled load reduction for a 10 ha site for both construction and operational and construction
Additional treatment capacity of a HES basin
Additional Treatment Capacity of HES Basins (Hatched Yellow Area) and Offset Potential (5ha site)
Additional treatment capacity of a HES basin
Long Term Runoff Treatment Comparison
Recent results at Sunshine Coast
‐Treated 131mm‐Maximum outflow turbidity of 31 NTU ‐Average outflow turbidity of 9 NTU
Cost comparison of preferred strategy
Analysis is based on a 1 ML basin requiring 20 treatments per year
Preferred strategy outcomes
Based on a 10 ha in SEQ
Phase Load (t)
Reduction Load Captured (t)
Construction 1000t 90% 900t
Operational 390t 80% 312t
Total 1,212t
HES Basin + Bio
Phase Load (t)
Reduction Load Captured (t)
Construction 1000t 30% 300t
Operational 390t 80% 312t
Total 612t
Traditional Basin + Bio
Low likelihood uptakeCivil contractors don't dewater as it can incur the cost of buying water for dust suppression and its an expense to flocculate and discharge a sediment basin.
If a standard sediment basin is dewatered after 10 or 15 days instead of 5 as required then the best sediment load reduction possible is reduced from 30% to 14%, and 5% respectively.
Low likelihood uptake cont.
ESC and sediment basin dewatering is very poorly regulated by both Councils and State government and efforts have been decreasing over the last 10 years.
Water by Design (2013) undertook a benchmarking assessment of ESC on land development on 57 sites for 6 SEQ councils and found only 5% of sites were substantially compliant.
Example of preferred strategy
Stockland's committed to using HES basins at Caloundra South Master Planned Community (2,310ha) due to high value receiving environment and passionate local community support for higher levels of protection.
A pilot project demonstrated that a 150mm event over 2 days was treated with no overtops and event grab samples showed concentration reduction as:TSS ‐ 98%TP – 95%TN – 45%
Alternative cost effective strategy – Onsite offset
O2 have recently begun discussion with 2 SEQ Councils regarding an onsite offset pilot project. One initial comment I received from an industry leading SEQ WSUD expert was.‘I have to admit I was a little shocked and confronted by your proposal. However now that I’ve had some time to think about it I would really like to explore it further’
Strategy ‐ install HES basins during construction and do not install bioretention basins. This will achieve equivalent or better environmental, temporal and spatial outcome for 45 times less cost.
TSS suitable metric or currency for off‐site or onsite solutions
The Water by Design (2014) discussion paper contends that ‐TSS may be a suitable metric or currency for off‐site (or onsite) solutions. This is because:•There is better data on the efficacy of a range of management actions in abating TSS, and from diffuse sources compared to other options.•There is a reasonable correlation between TSS and TP, and a fair correlation with TN.•The abatement cost curves show that only a few management practices have quite low TN or TP removal rates relative to their TSS reduction.•Nearly all aquatic ecosystems are sensitive to TSS, while sensitivity to TN and TP tends to vary depending on whether environments are N or P limited.
Environmental equivalence and cost reductionIn SEQ over 30 years, 909,500 tonnes of TSS will be produced from the operational phase and 3.9 million tonnes of TSS from the construction phase.
Bioretention basins cost an average of $4,583/tonne (Water by Design 2014) whilst HES basins only cost $100/tonne. To mitigate 909,500 of TSS using bioretention would cost $4.2 billion compared to $91 million for HES basins (46 times more cost effective).
Graph of 10ha site showing load treatment and cost comparisons for constriction and operational phases
Environmental equivalence and cost reduction cont.This strategy will save the councils and developers $4 billion over 30 years with no ongoing maintenance or renewal cost for councils and would also reduce dwelling costs by approximately $9,000.
HowLoads captured should be measured. This will require the HES basin to have flow, turbidity probes and event sampler at the inflow and outflow (which is standard design already).
Councils can take Voluntary Water Quality Offset Payment (VWQOP) in lieu of installing a bioretention or reduce size of treatment.
The contribution can be refunded based on TSS captured and agreed rate of $/tonne of TSS. This would be in excess of what would have been captured by a traditional basin (30% of construction load).
Thank you BRISBANE І SUNSHINE COAST | TOOWOOMBA
w o‐2.com.aue info@o‐2.com.au