In a move towards a more circular economy, South East Water has partnered with engineers from RMIT University and Manipal University, India, to develop a cost-effective and environmentally friendly way to remove heavy metals from biosolids, multiplying the number of ways biosolids can be reused and recycled.

As the world’s population grows, so too does our production of waste. According to the Australian and New Zealand Biosolids Partnership (ANZBP) Australia produces in excess of 349,000 tonnes of dry biosolids – or treated sewage sludge – each year. When treated and managed properly, biosolids can be used for a number of applications and can be beneficial to many industries.

The humble toilet flush is the beginning of a surprisingly long and complex journey for human waste. While wastewater is generally composed of 99.9 per cent water, complex treatment must occur to ensure that the wastewater is safe to be reused and recycled back into the environment.

Wastewater undergoes a variety of filtration and sedimentation treatment processes, which results in two substances: treated water and sewage sludge. The purity and composition of each substance determines how it is used. Treated water is most frequently released back into the ocean or used for irrigation as recycled water. The sewage sludge is treated further, turning it into biosolids and, potentially, biochar. Biosolids are then used as fertiliser, or in other agricultural applications, depending on their contaminant grade and treatment grade.

Biosolids can be a valuable resource. However, one of the biggest hurdles to recycling biosolids safely is contamination from heavy metals. Heavy metals contamination can render biosolids unable to be used for agricultural applications, as the contaminants can spread and have consequences for flora and fauna.

While heavy metals are not desired in biosolids, or their derived biochar, for land application, they can be a valuable resource in many applications if recovered efficiently. For example, copper and zinc, some of the major heavy metals in biosolids, have widespread applications in electrical materials, power lines and coating materials. Furthermore, they can also be used to produce catalysts for hydrogen or bio-oil production from a number of waste streams.

Innovative engineering

Engineers in Melbourne – led by RMIT University in collaboration with South East Water and Manipal University in India – have developed an innovative process to separate heavy metals from biosolids.

RMIT lead senior researcher and Deputy Director (Academic) of the ARC-funded Training Centre for the Transfor, `Kalpit Shah, said, “Our innovation helps ensure that the resulting biosolids do not leach heavy metals into the environment and retain the nutrients that can be used for land applications.

With further processing, the biosolids can be turned into high-grade biochar, which is a renewable energy resource and has a range of applications, including as a fertiliser.” The overall metal-removal process occurs over three stages: extraction, purification and recovery.

The PhD researcher on the project, Ibrahim Hakeem – who is now a postdoctoral research assistant at RMIT – explained, ‘’The first stage of the removal process involves the extraction of the metals from biosolids into the liquid stream (leachate).

This step employs a mild acid solution at ambient conditions to leach out the metals in biosolids into the solution by contacting the solids with liquid under agitation”. The leachate stream is then concentrated down through recycling processes.

Essentially, the separated solution, which contains low concentrations of metals, is further used as a solvent to extract more metals from biosolids by continuously recycling it until the metals reach saturation concentration. From there, the concentrated stream is treated with an alkali solution to precipitate the metals out of the solution, which are recovered in a stagewise approach.

“Overall, this innovative process at optimum conditions can remove approximately 80 per cent of metals present in biosolids and can recover more than 95 per cent of zinc, cadmium, and chromium, and more than 65 per cent nitrogen, copper, lead and arsenic from the solution. The removal efficiency can vary depending on the process conditions, source of biosolids and the chemical form of these metal ions within the biosolids matrix,’’ Dr Hakeem said.

Building the circular economy

The process is said to be a closed loop solution that causes the least harm to the environment. Professor Shah explained that, “The developed process uses a dilute concentration of common acid (no more than three per cent v/v) under mild conditions of temperature and pressure (ambient temperature and atmospheric pressure).

“The recycling of the leachate stream for further metal extraction advances upon traditional methods to reduce the total volume of water and acid solution required for the process. Lastly, the resulting acidic leachate stream is neutralised in the metal recovery stage, and the final effluent contains no acid or toxic chemicals.”

For South East Water, this new process opens many doors to further develop renewable, sustainable products and solutions. South East Water Research, Development and Innovation Manager, Dr David Bergmann, said, “At the end of a sewage treatment plant, you’re basically producing two materials. You’re producing recycled water of various grades that can be used for irrigation, for agriculture, all sorts of things.

Then there’s the solids component, the biosolids component, which contains everything that’s not in the water. “Traditionally, we’ve worked with farmers to use the biosolids nutrients in agriculture. It has to be done very carefully to get the level of nutrients right and make sure that the contaminants that might be there are managed as well so as not to create contamination issues into agriculture.

“But that in fact costs us substantial money to manage biosolids that actually if processed correctly, is a material of value. If we recover metals by this process we can produce cleaner products like biochar and cleaner metals with greater value rather than a complex mixture that is seen as a contaminated waste.

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