Biological waste from livestock has long been used to help make farm soil more fertile. However, while human waste has similar potential benefits, its use presents more challenges. Sewage contains high concentrations of heavy metals, and current treatment methods are unable to remove enough of these toxic metals to make biosolids safe for use on farms as fertilizer, so it often ends up being landfilled instead.
Now, however, researchers from the Masdar Institute in the United Arab Emirates have developed an energy-efficient, low-cost method for removing heavy metals from biosolids. The novel process 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. This removal rate is significantly higher than any previously reported removal rates; in the case of zinc, the removal was well below the tolerable concentration levels set by regulatory agencies.
Shadi Wajih Hasan, assistant professor of chemical and environmental engineering at Masdar Institute, developed the novel three-step treatment process that combines chemical conditioning, electrokinetic remediation and a post-treatment washing. He was lead author on a paper describing the process, which was published in August 2016 in the Nature-affiliated journal Scientific Reports. Co-authors included Ph.D. student Adewale Giwa, former master’s student Amna Al Housani, and postdoctoral researcher Iftikhar Ahmed.
“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,” says Hasan. “I think this is an opportunity that the UAE should explore in greater depth.”
In his paper, Hasan briefly explains the economic benefit of converting biosolids to fertilizer. He concluded 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 over the last 10 years. In 2014, the country’s fertilizer producers manufactured 5 million tons of products, earning $653 million in revenues. Currently ranked as the third-largest fertilizer producer in the Gulf Cooperation Council, the UAE’s fertilizer industry would greatly benefit from any technological advancement that would reduce production costs while improving the product quality of its fertilizer.
In addition to being a potential boon for the country’s revenue stream, the conversion of biosolids to fertilizer would also result in significant environmental savings.
Sewage sludge contains high concentrations of heavy metals that are not degraded through conventional physical, biological and chemical treatment processes. In many countries, including the UAE, the current approach to handling municipal sludge is to landfill it, which can contaminate the environment around it.
This treatment method is being scaled to commercial levels. Hasan ran a pilot-scale experiment of the system to determine its economic viability. The pilot-scale experiment showed excellent reproducibility of the technology, indicating that it could be scaled up for industrial process design.
This scalability factor is important if the new treatment method is going to be able to reproduce such high heavy metal removal levels on a large scale. One reason sewage sludge is landfilled is because of the high cost of treating it. Attempts to treat and remove the heavy metals from sewage sludge are usually expensive and energy-intensive, yielding only a slight reduction in the overall metal content. Thus, any potential solution to reduce the presence of heavy metals in biosolids must be low-cost and energy-efficient if it is to be applied on a mass scale.
Hasan’s three-stage treatment process not only meets the requirements of being potentially cost-effective and low-energy, but also results in a high-value product in the form of a nutrient-rich fertilizer.
The new treatment process uses a low electric field treatment, making it significantly more energy-efficient than conventional electrokinetic remediation processes. The kinetics of this low-strength electrokinetic treatment were 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,” says Hasan. “More importantly for this study, the nitrous acid can disrupt the organically bounded zinc and copper trapped in EPS.”
Electrokinetic 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. While the two steps of chemical conditioning and electrokinetics are quite effective, Hasan introduced a third step that led to even higher metal removal rates.
A biogenic chelating agent was used to post-treat the sludge after the low-strength, acidified electrokinetic process. This agent is a cation-capturing organic extract from citrus fruit peel waste known as pectin. Pectin from citrus peels is physically stable, exhibits a particularly high degree of metal uptake, and is also suitable for coagulating residual ions in sludge after electrokinetic treatment.
Hasan discovered that washing the biosolids in pectin removes more of the free ions that were not pulled to the electrodes during the electrokinetic treatment and settled to the bottom of the sludge.
“We are very excited by our discovery of the use of pectin as a post-treatment option,” says Hasan. “Pectin is also sourced from waste, which means we have developed an innovative method that leverages two separate waste streams to create a value-added product.”
This innovative three-step treatment strategy has successfully eliminated the presence of zinc to levels below the prescribed standard for unrestricted use of biosolids locally and globally, achieving a removal rate of 94 percent. Copper levels were also brought much closer to their unrestricted standard, with a 64 percent removal rate achieved.
Compared to landfilling and incineration, utilization of sludge for agricultural use is considered the best alternative for sludge disposal. Now, thanks to this research, an innovative, potentially low-cost method for removing heavy metals from sludge has been developed.
Source: Masdar Institute of Science & Technology