RFI - Attachment F - Engineering Report

CONCLUSION


Generally, ER criteria for stormwater treatment in the Sydney Region is based on removing a
minimum 85% of the Suspended Solids, 65% of the Total Phosphorus and 45% of the Total
Nitrogen average annual loads. The Efficiency Ratios determined by Contech Engineered
Solutions for the StormFilter® installed within the Mitchell Community College carpark are
related to thirteen (13) storm events over a twenty month (20) period, which resulted in the
removal of 90.4% of Total Suspended Solids, 86.1% of Total Phosphorus and 55.9% of Total
Nitrogen.

The procedures employed in collecting and testing the samples appear to be consistent with
those outlined in NWQMS (2000) and as such should provide a high degree of confidence
that the EMCs determined are correct for each of the samples collected. The determination
of the ERs is dependent on the difference between the influent and effluent EMCs.
Consequently, it is our opinion, that the ERs determined in the report are those that were
actually achieved by the system under field conditions, based on the sampling and testing
protocols employed, and similar ERs could be expected if the same system were installed
within a similar catchment in Australia.

Although a Particle Size Distribution was not carried out as part of the initial study, the texture
of the Suspended Solids was described as Loamy Sand. A Loamy Sand typically consists of
approximately 80% Sand, 15% Silt and 5% Clay (refer to Figure 1). AS 1726-1993 classifies
Sand as between 2.36 mm (2360 µm) and 0.075 mm (75 µm) in dia with Silts and Clays <
0.075 mm (75 µm) (refer to Figure 2). However, colloidal Clays are generally < 0.002 mm (2
µm) in dia. Consequently a Loamy Sand could contain a significant proportion of < 125 µm
dia soil particles. This has been confirmed by the recent GeoTesting Express ASTM D1140
sieve analysis, of the sediment retained within the system, which determined that in excess of
80% of the sediment was < 75 µm (refer to Figure 3), and provides a separate body of
evidence for the determination of a Suspended Solid Concentration (SSC < 500 µm)
Efficiency Ratio of 93.4% for the system (Contech Engineered Solutions, 2012).

Regardless, of the fact that a PSD was not undertaken initially, and that an ER for the
removal of the < 125 µm Suspended Solids component of the loads was not determined, the
low concentrations of Suspended Solids in the influent (10.3 mg/L to 98.2 mg/L) suggests that
an ER of at least 90% is possible when even higher concentrations of Suspended Solid, such
as those reported within the Australian literature, are encountered. Further, retention of the
solids in a dry to semi-dry condition limits the opportunity for anaerobic decomposition and re-
suspension of nutrients to occur.


                                             With Compliments,




                                        Hunter Environmental Services




HES – StormFilter® System Peer Review                                                         Page 6