Farmers may soon be using sewage sludge to help make farm soil more fertile.
Researchers from Masdar Institute in the United Arab Emirates have developed an energy-efficient, low-cost method for removing heavy metals from biosolids, a challenge in the past.
The research team has demonstrated the ability to remove over 90 percent of zinc and over 60 percent of copper from sewage sludge collected from the Masdar City wastewater treatment plant using the new system. This rate is significantly higher than any previously reported removal rates, and for zinc the removal was well below the tolerable concentration levels set by regulatory agencies.
The three-step treatment process combines chemical conditioning, electrokinetic remediation and a post-treatment washing.
“If the UAE could convert the 26,000 tons of biosolids that are generated yearly from Abu Dhabi’s urban wastewater treatment plants into fertilizer, the environmental impact of keeping this waste out of the landfill and the economic benefits of creating a valuable fertilizer product that could be sold would be significant,” Shadi Wajih Hasan, Ph.D., assistant professor of Chemical and Environmental Engineering at Masdar Institute, said in an statement. “I think this is an opportunity that the UAE should explore in greater depth.”
The process uses a low electric field treatment, making it significantly more energy-efficient than conventional electrokinetic remediation processes. The process wa improved through the use of aqua regia acid.
“Aqua regia contains nitrite, which gives rise to free nitrous acid that can increase biodegradability, disrupt extracellular polymeric substances (EPS) and microbial cells, and reduce sludge particle size by breaking down EPS,” Hasan said. “More importantly for this study, the nitrous acid can disrupt the organically bounded zinc and copper trapped in EPS.”
The treatment involves running an electric current through the sludge between two electrodes to break the bonds of the metal ions. While electrokinetic remediation can be very effective at removing metal ions, its widespread application has been hindered because it is energy-intensive and expensive.
Hasan’s team found a way around these limitations by conditioning the biosolids first with the aqua regia.
Hasan explains that the use of sludge as land fertilizer would potentially yield over $2 million per year as revenue for Abu Dhabi, boosting the country’s fertilizer industry, which has grown by more than 50 percent in the last 10 years.
The conversion of biosolids to fertilizer would also result in significant environmental savings because sewage sludge contains high concentrations of heavy metals that are not degraded through conventional physical, biological and chemical treatment processes. In many countries, municipal sludge is landfilled, which can contaminate the environment around it.
Hasan is scaling the treatment method to commercial levels, which can determine if the method is going to be able to reproduce the high removal rates on a large scale. Attempts to treat and remove the heavy metals from sewage sludge are generally expensive and energy-intensive, while yielding only a slight reduction in the overall metal content.
Any potential solution to reduce the presence of heavy metals in biosolids must be low-cost and energy efficient to be applied on a mass-scale.