Storm in a brickworks

| July 29, 2022

An ABC News headline proclaimed on 27th July that a Melbourne development could pollute the Yarra River. The development had been promoted as ‘the world’s most sustainable shopping centre’. The ABC story reported that “a 20-hectare Burwood Brickworks Development (BBW) has been approved to allow untreated stormwater run-off to flow into a local creek, sparking concerns about increased pollution in the Yarra River.”

However, nearly all urban stormwater flows from hard surfaces into natural waterways without treatment. Water tanks are popular in the suburbs and these are filled with stormwater from the roofs of residential buildings and shops.

The quality of stormwater from rooftops at the BBW would be the same as that in surrounding suburbs and could be used for many purposes other than drinking. Reserving high quality stormwater for environmental use is often neglected, which seems to be the case at the BBW.

A key principle in wastewater treatment is to avoid mixing higher quality (waste) water streams with water of lower quality. This is really just common sense but it appears that the BBW site drainage system does not separate stormwater flows according to quality.

Assuming that dwellings and shops cover 60% of the site, an average of approximately 80 million litres of relatively clean roof runoff could be released from the site annually. This might have been achievable by keeping roof and road drainage separate with a partitioned retarding basin.

Clean roof runoff would tend to improve water quality in the receiving environment, especially during the drier months. It may also have reduced the need for flow attenuation and more advanced treatment of a smaller volume of polluted stormwater might have been a practical option.

It is often claimed that constructed wetlands will ‘purify’ water. It is rarely acknowledged that the plant species usually selected are dormant in winter when their effect on water quality is minimal. With optimum depth, surface area and water retention time, significant water quality improvement can be expected in constructed wetlands at low to average flows in summer.

However, pollution loads are greatest during storm events when very fine non-settleable (colloidal) material carrying pollutants pass through the wetlands, sometimes picking up previously deposited nutrient-rich organic matter.

The wetland substrate (e.g., gravel beds) in which reeds and other species are planted become saturated with phosphorus and have to be replaced. Sustainability of wetland water treatment systems depends on skilled management, including biomass harvesting and monitoring. Algae blooms, including toxic cyanobacteria, are to be expected in retarding basins containing water enriched with nutrients. There is also a significant risk of nuisance aquatic weed growth, odour  and adverse health effects on humans, birds and animals.

It has evidently been estimated that the proposed stormwater treatment system at BBW would not meet the design nitrogen and phosphorus load reduction criteria set by the EPA.

However, the lower Yarra River is essentially estuarine and the exchange of nutrients between the water and sediment is complex and not completely understood. Apart from nitrogen and phosphorus, salinity, pH, temperature, and dissolved oxygen levels are some of the water quality indicators to consider.

It would be possible for fish kills to occur even if the nitrogen load limit had been achieved or even surpassed. This can happen when conditions are such that a high proportion of the nitrogenous content is in the form of toxic ammonia, especially when the level of dissolved oxygen in the water is low.

The designers of the BBW development stormwater strategy seem to have been preoccupied with the volumetric and drainage engineering aspects to the exclusion of water quality considerations. The result is that an opportunity to demonstrate innovative and environmentally beneficial stormwater management has been lost.

Melbourne Water has reportedly deemed the proposed BBW stormwater scheme ‘unsafe’. They have obviously grasped the potential for severe reaction from the wider community to inequitable maintenance costs and adverse impacts on health, amenity and the environment.

The developers will pay into a fund for stormwater offsets elsewhere. That’s like paying an airline to plant trees to compensate for carbon emitted by aircraft during flight. The environmental benefit of offset measures such as litter traps and relatively ineffective small wetlands elsewhere in the Yarra catchment is questionable.

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