Attachment 4 - Stormwater Managment Plan

SQIDEP Evaluation                                                     SPEL Basin, Sippy Downs


Based on the testing regime and submitted results TSS, TP, TN and gross pollutants can be
evaluated in this process. It is however acknowledged that by association and by reference
to other scientific studies hydrocarbons will be removed though no credit given.




Figure 6 Final Agreed Pollutant Reduction Performance

Scalability and Hydraulic Loading Rate

The question of scalability of these results has been considered as part of this review. The
design treatment rate of 10L/s was tested in the field and at least 4 events out of the
qualifying 18 events approach or exceed this value. Of these larger events there is a spread
of CRE values, with some well below the claimed reductions and some above. Viewing
these results more critically it would seem that the antecedent conditions, and shape of the
hydrograph are just as important precursors to the CRE as the actual flow rate. Clearly more
field data may better define these correlations, however given the 90% confidence rate
already, the care taken to remove outliers and non-qualifying events and defined SQIDEP
protocol it is accepted that natural variations will occur and that a treatment rate of 10L/s is
an acceptable limit.

How this 10L/s plays out in installations is a further consideration. SPEL have stated that
their preference is to procure the application in 10L/s modules. If this is adhered to then this
should have a level of acceptable saleability. Given that these are modules are only
procured by one manufacturer applying this limit this should minimise the misuse of this limit
– the same cannot be said for many other nutrient removal applications in the market.
However, this should strongly be written into any technical guideline associated with the
modules, so that any prospective purchaser or asset owner is aware of this limitation.

We note that this study tested a SPEL Basin accepting runoff from an 850m2 catchment.
Using a City of Gold Coast template this produces an annual hydraulic load rate of about
215m/year. We would therefore consider that a hydraulic loading rate up to 250m/year
would be an acceptable load rate for the device. Should the load rate for example increase
to say 500m/year then the same results found in field study would not be repeatable.

Limitations of Acceptance

The limitations of the acceptance of these testing results include:

    1. The results are for a road based catchment. The results lie within acceptable inflow
       limits for this type of catchment and based on the analysis are found to be
       acceptable. This does not necessarily relate to other catchment types, though it is
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