RFI - Attachment F - Engineering Report

2.2 FIELD TRIAL RESULTS

Of the four CFS observed in the field trials, Auckland City determined that two CFS products met operational
performance requirements for their project, and these were selected for further laboratory testing.

The two selected CFS performed favourably on all six performance attributes listed in 2.1 above.
Some interesting issues encountered with some of the CFS in the field trials were:

e Unnecessary use of the CFS overflow mechanism, resulting in untreated flows.

© Mechanical overflow mechanisms such as flaps were prone to failure by blockage.

« Flows bypassing the filter bags due to gaps in the seal between the CFS and the catchpit wall. These
gaps were a result of poor installation of the CFS.

¢ A small sediment storage capacity, which would resuit in a high cleaning frequency or loss of captured
sediments during high flows.

e One of the CFS did not have an overflow mechanism to bypass the filter bag under high flows. This
resulted in localised ponding/flooding around the catchpit when flows exceeded the filter bags capacity.

e One filter bag was seen to invert causing the sediment that had been captured to wash down the
overflow mechanism and out of the outlet pipe.

* The filter bag from one of the CFS was sucked down the catchpit outlet pipe, creating a blockage and
decreasing the capacity of the outlet.

2.3 FIELD TRIAL OUTCOMES
The two main outcomes of the field trial were
1. A CFS ability to function well is very dependant on how well it is installed. Careful specification of

contract requirements and installation monitoring is required to ensure good quality control of CFS
installation.

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It is essential that CFS have an efficient and reliable overflow mechanism. Mechanical flaps observed
in the field trial generally were unsuccessful.

3 LABORATORY TESTING

The laboratory study was conducted by Auckland Uniservices, in the School of Engineering at the University of
Auckland.

The laboratory tests were used to quantify the effectiveness of the CFS under different flow rates and with
different sediment particle sizes and concentrations. This provided a simplified and controlled representation of
field conditions to test the relative performance of the CFS. The laboratory data also enables the performance
of these products to be predicted from a measured sediment particle distribution. Laboratory tests were also
carried out to determine the head loss across the two synthetic filter fabrics to establish the potential for limiting
catchpit hydraulic capacity and relative overflow potential between different CFS.