RFI - Attachment F - Engineering Report

3rd Party Modelling Comparison
The following information was supplied from Hughes Trueman. Hughes Trueman
conducted contaminant modelling using the Music model loading on various catchments
for KMC.

The following table indicates the average loading from the seven catchments used in the
modelling exercise. These values were simulated in a wet year, dry year and average year
to determine the variation based on rainfall.

               Year                 Rainfall         TSS     TP         TN       GP kg/ha
                                     mm             kg/ha   kg/ha      kg/ha
           Dry (10%ile)                868          203      0.4        3.4          45
              Average                 1383          539      1.2        9.5          73
           Wet (90%ile)               1972          786       2        16.2         115
     Blackbutt Creak Enviropod
          Removal Rates               1127          294      0.36       1.4         68
               Table 11 Average unit export rates (Middle Harbour Catchments)


The comparison of loading rates with the removal loading rates obtained in the Blackbutt
creek catchment show that significant removal was being obtained.

The CRC assume (CRC 2003) GPTs have a removal efficiency of 30% for Phosphorous
and 13% for Nitrogen. Comparing the loading rates form Hughes Truman, it is estimated
that the Enviropod is removing 65% TSS, 40% total phosphorous, 21% total nitrogen and
nearly 100% gross pollutants.

Applying the results of the ACC laboratory testing (Tokin & Taylor) and the CRC
adopted PSD for the Music model; the Enviropods should have obtained a removal of
48% for TSS. Observations on the site suggest that the particle size distribution in the
runoff from system is much coarser than those adopted for Melbourne and Brisbane.

The higher removal rate obtained by the Enviropod filter than a standard GPT device is
attributed to the following:
     1. The Enviropod filter fitted with a 200 micron filter bag has the ability to remove
        fine particulates. Auckland city council laboratory testing has shown 20%
        removal efficiency for particles less than 100 micron in size and 98% efficiency
        for particles 100 micron to 1000 micron in size. (Tokin & Taylor).
     2. The hypothesis that soil and rock particles entrained in the runoff are eroding and
        breaking down as they are travelling overland and in the reticulation system.
        Capturing of the particulates at the source before they enter the reticulation
        system limits this mechanic break down of the particles




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