RFI - Attachment F - Engineering Report

stormwater treatment                                                                                               refereed paper



                                                between 71 mg/L (forested catchments)
                                                and 232 mg/L (urban roads). Fletcher et
                                                al. (2004) recommend using a value of
                                                ca. 120 mg/L for roads and ca. 100 mg/L
                                                for most other land uses. Both sources
                                                propose a median value of ca. 40 mg/L
                                                for forested catchments. The influent
                                                concentration of Suspended Solids at
                                                Streets Creek is within the typical range of
                                                average annual EMCs proposed within the
                                                literature; however, no data was collected
                                                during large wet-season storm events.
                                                Consequently, the median influent EMC
                                                reported herein should not be regarded
                                                as indicative of an annual median value.

                                                   Effluent EMCs recorded for SS <500
                                                micron were in the range of 8 to 62 mg/L.
                                                The median effluent EMC was 20 mg/L.
                                                Mean removal efficiency for SS<500
Figure 4. SS <500micron data (JCU + SW360).     micron, calculated by aggregate load           Figure 6. Total Nitrogen (SW360 and
aggregate load reduction of 99%. SSC
                                                reduction, was 78%. It is evident from         JCU combined).
(ie, SSC is defined as the sum of SS            Figure 4 that the Stormwater360 and JCU           Particle size distribution (PSD) by laser
<500 micron and SS >500 micron) is              data sets are in relatively good agreement     diffraction was performed for the SS <500
‘suspended’ in the sense that all these         with each other, with the exception of         micron fraction for three storms during
particles were sufficiently suspended           the JCU outlier, which represents the          the Stormwater360 evaluation. Inspection
to reach the system. However, SS                first storm from JCU’s research project.       of the three cases analysed consists of
<500 micron represents what is more             This storm was deemed an outlier for all       particles between ca. 10 microns and 200
commonly understood by the term                 water quality parameters due to possible       microns in diameter. There is substantial
‘suspended solids’. It excludes coarse          sampling errors and has been removed           variation between the three events.
settleable sediment, which, while being         from the analyses. The box plot in Figure      • Storm 2 influent PSD centred at ca.
a management issue, does not represent          4 shows that the combined dataset is             20 microns for a removal efficiency
such an acute threat to water quality.          also clustered around an influent EMC            of approximately 65%;
  Figure 4 shows influent and effluent          of ca.100 mg/L and an effluent EMC of
                                                                                               • Storm 3 influent PSD centred at ca.
data (Stormwater360) for SS <500                ca.20 mg/L. In practical terms, 10 mg/L
                                                                                                 100 microns for a removal efficiency
micron, together with the results               approximates the system’s irreducible
                                                                                                 of approximately 85%;
published by JCU. In the scatter plot,          EMC for under-500 micron suspended
the filled-in circles represent data from       solids. The box plot in Figure 4 indicates     • Storm 6 influent PSD centred at ca.
the trial reported herein, and open circles     that, over the course of two trials, the         35 microns for a removal efficiency
represent data from the previous JCU’s          effluent EMCs from the system, were              of approximately 75%.
research project. The exception is the          typically within the range of 10 to 40 mg/L.     Generally, the higher removal efficiency
JCU outlier represented as an open                                                             would be expected for the coarser
square, which has not been included in                                                         samples, and this was the case for all
this evaluation. The line of best fit shown                                                    three storms sampled.
as a solid straight line was calculated by a
least squares linear regression for all data                                                   Total Nutrients
points except the JCU outlier (intended
to be informational only). Its relative slope                                                  The system achieved an aggregate load
provides an appreciation of the trend of                                                       reduction for total phosphorus (TP) of
the removal efficiency for the treatment                                                       47% (note, JCU recorded a load reduction
train. The dotted curves represent                                                             of 70%), the median influent and effluent
the 95% confidence limits for these                                                            EMCs for TP were 0.123 mg/L and 0.055
                                                                                               mg/L respectively (refer to Table 2).
same data points. The true statistical
                                                                                               Duncan (1999) and Fletcher et al. (2004)
significance of the regression lines is
                                                                                               recorded EMCs within a similar range and
open to interpretation and requires further
                                                                                               Fletcher (2004) recommends mean TP
investigation, due to the limited number
                                                                                               concentrations of between 0.25 and 0.50
of data points available for this analysis.
                                                                                               mg/L for most land uses. Similarly, BMP
  Over the six storms analysed by                                                              Database (2010) suggests that a typical
Stormwater360, the influent EMC for SS                                                         range for TP concentrations in stormwater
<500 micron was in the range of 48 to                                                          is from 0.11 to 0.38 mg/L, across a range
180 mg/L with a median influent EMC of                                                         of land uses. In this context it is apparent
105 mg/L. Duncan (1999) literature review                                                      that the influent TP concentration at the
determined that the median concentration        Figure 5. Total Phosphorus (SW360 and JCU      Kuranda site is towards the very low end
for most land uses (roofs excepted) lies        combined).                                     of published data. Consequently, the 47%

4 SEPTEMBER 2011 water                                                                             technical features