RFI - Attachment F - Engineering Report

nutrients. While the concentrations of nutrients are generally lower in gross pollutants
they contribute a significant proportion to the overall transport sediment load.

There is much debate on the definition of suspended solids. Some agencies consider the
definition of suspended Solids to be < 63 micron (North Shore City Council, New
Zealand), while others have suggested 500 micron (City of Portland, USA). Some
agencies have adopted a finer definition of suspended solids as the finer fraction of
particles often have a higher concentration of contaminants which are also more bio-
available in the receiving environment. However, in this study sediment has been
obtained before it has entered the reticulation system and research (Pratt & Adams,
Hannah) has shown that particle size distribution of particles leaving the road surface are
often coarser than those sampled further down the reticulation system.

For the purpose of this study we will adopt the following definitions for suspended solids,
bed load and gross pollutants. These were obtained through correspondence with the
CRC for catchment hydrology.

   •   Suspended Solids (TSS) <1mm (T. Wong, CRC Catchment Hydrology, pers.com)
   •   Gross Pollutants (GP) >5mm (Allison, CRC 1998)
   •   Bed Load (BL) 1mm<x<5mm

The Enviropod Filter.

The Enviropod Filter is a gully pit insert comprising of a structural frame work, an
overflow mechanism and removal filter bag. The Enviropod filter was retrofitted into 80
gully pits at the head of the Blackbutt Creek Catchment. By installing the Enviropods in
the gully pits, an extremely large screening area and storage volume is obtained within
the catchment. For the Blackbutt Creek catchment the 80 Enviropods provided
approximately 8.5 m3 of storage and 90m2 of screening area.

The performance of the Enviropod Filter (200 micron bag) has been independently
evaluated buy Auckland City Council (ACC) in the laboratory (Tonkin & Taylor). This
was conducted using a range of synthetic sediment samples and sediment collected from
a road in Auckland. The Enviropod filter essentially removed all the sediments greater
than 100 micron in size. Removal of sediments less than 100 micron was approximately
20%. On road sediment the Enviropod Filter was found to remove approximately 97%.

Figure 1 shows the relative efficiencies for an Enviropod Filter fitted with a 200 micron
filter bag, at varying particle size ranges.




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