Jellyfish Compliance with SQIDEP · MCU15/0122

                                                                                                                                            1
                                                                                                               Technical Papers




A STATISTICAL APPROACH TO
ASSESS STORMWATER TREATMENT
DEVICE PERFORMANCE DATA
A demonstration of how statistical methods can be
used to gauge whether or not presented data sets
are sufficient to draw reliable conclusions
C Kelly, A Bardak


ABSTRACT                                       There is a range of engineered                   analysis to be conducted, such as the
                                               stormwater treatment systems to                  paired t-test, and calculations involving
It has long been established that the pre-
                                               consider. Assessing the performance              confidence intervals, and the number of
treatment of stormwater runoff prior to
                                               of hard engineered systems requires              paired samples required to achieve a given
reaching its final destination is essential
                                               quantification of inputs and outputs to          confidence level. Table 1 displays a data set
for river and catchment health. There is a
                                               receiving environments under a diverse           of 10 qualifying events (Kelly and Bardak,
plethora of stormwater runoff treatment
                                               range of event durations, flow rates and         2015), which is used to demonstrate this
devices either available or about to
                                               loads (Herr and Sansalone, 2015).                statistical approach for three pollutants:
enter the market. Stormwater Australia
                                                                                                Total Suspended Solids (TSS), Total
has released a consultation draft of its          Stormwater Australia has released
                                                                                                Phosphorus (TP) and Total Nitrogen (TN).
Stormwater Quality Improvement Device          a consultation draft of its Stormwater
Evaluation Protocol (SQIDEP), which            Quality Improvement Device Evaluation               The Event Mean Concentration (EMC) is
includes a requirement to undertake            Protocol, or SQIDEP (2014), which                a flow-weighted concentration parameter
statistical assessment as part of the          includes a requirement for statistical           derived for each storm. It can be calculated
validation of performance data.                assessment to validate the performance           as follows (Geosyntec Consultants &
                                               data. There remains an important step            Wright Water Engineers, 2009):
  This paper uses data from a field-           in the further development of SQIDEP,
testing case study of an engineered            which is to engage with academia and
treatment technology to demonstrate            regulators to reach a consensus on the
how statistical methods can be used to         robustness of a statistical approach.
                                                                                                where:
determine whether or not the data sets
being presented are actually sufficient          Field-testing data are used in this
                                               paper to illustrate a statistical assessment
to be able to draw reliable conclusions.
                                               process to determine whether or not the
The assessment is conducted for Total
                                               data sets being presented are actually             The Concentration Removal Efficiency
Suspended Solids, Total Phosphorus and
                                               sufficient for statistically valid conclusions   (CRE) method was used to calculate the
Total Nitrogen data, and shows that the
                                               to be drawn. The paper presents a                per cent removal in Kelly and Bardak
required number of paired samples is
                                               worked example of the proposed analysis          (2015). It is also used in this paper to
different for each of these parameters
                                               technique using a data set reported in           demonstrate the statistical methodology.
and depends on the variability of the
                                               Kelly and Bardak (2015) for a proprietory        CRE% was calculated for each event as
data sets. It is necessary to engage
                                               stormwater treatment device (the                 follows (Geosyntec Consultants & Wright
with academia and regulators to reach
                                               Jellyfish Filter). Details on experimental       Water Engineers, 2009):
a consensus on the robustness of a             set-up, sampling methods used and
statistical approach, and this paper           definitions of qualifying events are
provides an example.                           reported in Kelly and Bardak (2015).
INTRODUCTION                                   METHOD DETAIL                                       It should be noted that there are
Stormwater runoff transports a range           AND RESULTS                                      several recommended methods that can
of pollutants and is a significant non-        Initially, potential outliers are identified,    be used to calculate the contaminant
point source of urban water pollution.         examined and removed (if necessary).             removal efficacy, such as CRE, Efficiency
It has long been established that the          The data is then tested for normality to         Ratio, Mass Reduction Efficiency, etc.
pre-treatment of stormwater runoff             identify which statistical methods can           Selection of a relevant percentage
prior to reaching its final destination is     be used on that set. For cases where             removal calculation method is discussed
essential for river and catchment health       the raw data is not normally distributed,        in SQIDEP (Stormwater Australia, 2014)
(Herr and Sansalone, 2015). Effective          but appears to be positively skewed,             and Geosyntec Consultants and Wright
treatment requires technologies that are       a lognormal transformation may be                Water Engineers (2009). The following
appropriate to site constraints, as well as    applicable (Singh et al., 1997). Normally        statistical assessment approach can be
rainfall and pollutant load characteristics.   distributed data allows standard statistical     applied to all of these methods.



                                                                                                                       MAY 2015 WATER