A case study with MUSIC (A Model for urban stormwater improvement conceptualisation)
It is generally accepted that using gully pit inserts as pre-treatment can reduce the needed maintenance frequency of a bioretention device. However, the benefits of pre-treatment are not limited to maintenance as effective pre-treatment can allow a smaller bioretention device in the treatment train to meet regulatory pollutant removal requirements.
To demonstrate this, a conceptual MUSIC model was developed to illustrate that the addition of an EnviroPod200™gully pit filter as a pre-treatment device to standard bioretention can reduce the required size (footprint) of a bioretention device by 33%.
(MUSIC is Australia’s leading tool for water sensitive urban design and can model a wide range of treatment devices to identify the best way to capture and reuse stormwater runoff, remove its contaminants, as well as reduce runoff frequency.)
The two source nodes include a 1000 m2 landscaping section with 100% pervious area and a 3200 m2 car park with 100% impervious area. The sizing and properties used for the land use. Both sources include MUSIC X’s standard stochastically generated pollutant levels for Urban–Mixed land use. Runoff from these source nodes will flow straight into a bioretention or be pre-treated by EnviroPod200™ filters installed in the car park’s gullypits and then flow to a Bioretention system. The Node diagram is shown in above.
The EnviroPod200™ was modelled with a high flow bypass of 20 L/s with pollutant removals. The EnviroPod200™’s is an effective sediment removal device preventing sediment accumulation in the bioretention system and the system is a moderate remover of nutrients. While effective at removing sediment and particulate material, it has no mechanism for removing dissolved factions of phosphorus and nitrogen. However, reducing the nutrient concentration through the EnviroPod200™ increases the treatment train’s removal.
Two bioretention designs were considered in the model. The first Bioretention system has a 12 m2 filter area (no pre-treatment) and another with an 8 m2 surface area (pretreatment). The 8 m2 bioretention system received EnviroPod200™ pre-treated flows. All other parameters were kept consistent with the two bioretention designs.
The Treatment Train Performance
MUSIC allows the evaluation of a treatment train. The concentration of the influent flows primarily influences the performance of a treatment device. MUSIC considers the concentration reduction of each step of the treatment train. This reduces the device’s efficiency as the influent concentration is reduced. However, the overall efficiency is increased by passing stormwater runoff through numerous steps in a treatment train.
Adding the EnviroPod200™ to the site’s gully pits reduces the sediment load to the bioretention system from 595.7kg/yr to 115.6kg/year. Capturing sediment in the gullypits extends the life of the bioretention system before corrective and costly remedial maintenance is required (media and plant replacement). The addition of pre-treatment also provides additional resilience to the system by creating an extra barrier for unforeseen events such as spills that may occur in the catchment. The EnviroPod200™ requires routine maintenance that can be undertaken by hand or vactor truck.
Stormwater professionals have long supported the treatment train concept to improve the effluent quality of stormwater discharges. However, the cost of additional steps in the treatment train has often resulted in a single stand-alone device being adopted.
The principle of a treatment train involves targeting different contaminants with different particle sizes and speciation with treatment devices with other pollutant removal mechanisms and stormwater professionals have long supported the treatment train concept to improve the effluent quality of stormwater discharges.
However, the cost of adding additional steps in the treatment train has often resulted in a single stand-alone device being adopted.
By modelling the EnviroPod200™, we have demonstrated that an effective sediment pre-treatment device such as the EnviroPod200™filter with a bioretention device can remove dissolved pollutants and deliver greater efficiency with lower capital and operational costs.
Water-sensitive design(WSD) has multiple benefits beyond water quality. The use of treatment with WSD elements will allow significant uptake of WSD and provide improved urban environments. However, WSD systems are often susceptible to high sediment and gross pollutant loads, which can cause clogging and increase the need for expensive rehabilitation work over time.