Spotlight On: Sustainable Sewage Sludge Treatment
An essential part of water management is the treatment of sewage sludge, a byproduct of wastewater treatment that’s typically made up of human waste, microorganisms, food waste, trace chemicals and inorganic matter from products.
It can also come from industrial facilities, either entering public sewers to be treated alongside municipal sludge or being diverted to a treatment plant to be processed along with sewage sludge.
Once this sludge has been treated, it becomes what is known as biosolids, which are then sent to landfill, incinerated or applied to land.
Why is sludge treatment necessary?
Wastewater solids (sludge) need to be separated from sewage so that clean water can then be recovered. Once separated, sludge can be treated to reduce pathogens and other harmful microorganisms so as to reduce the risk of harm to environmental and public health.
Sludge treatment also has inherent value because biogas can be recovered through anaerobic digestion, while biosolids can be used as a nutrient-rich product that can be of benefit to soil.
The urgent need for new treatment models
According to the Chartered Institute of Water and Environmental Management (CIWEM), new more sustainable models for sewage sludge treatment are now urgently required to help address challenges such as microplastics and PFA chemicals in sludge that’s being recycled for use on agricultural land.
Some 3.6 million tonnes of sludge is recycled for this application each year – and there are many acknowledged concerns about this particular practice, with places like Switzerland and the Netherlands already banning sludge recycling because of contaminant presence.
In the UK, banning the agricultural use of sludge would mean that a significant supply of low-cost fertiliser would be lost, which in turn would drive up the use of chemicals, as well as the loss of valuable materials like ammonia and phosphorus. It would also mean that a new method of sludge disposal would be required.
Thermochemical alternatives
The good news is that research and development is now ongoing into how our wastewater industry processes can be transformed so that disposal is safer and more sustainable, and so that new revenue streams can be identified.
Research conducted by Cranfield University, alongside various water companies, has found that thermochemical technologies like hydrothermal oxidation and pyrolysis could be viable alternatives.
Another option is hydrothermal oxidation, where sludge is oxidised into a nutrient-rich liquid, containing ammonia and phosphorus. Here, although energy is required to create the appropriate pressured environment, the process then generates its own heat source and there’s also no liquid return and no need for drying treatments at the start of the process.
Pyrolysis involves treating sludge at high temperatures in the absence of oxygen, which breaks down organic matter and reduces pollutant concentrations, including pharmaceuticals, microplastics, antibiotics, hormones, PFAs and so on.
Outputs from this process include bio-oil and bio-char, the latter of which can (in principle) be used as an agricultural soil conditioner once regulatory approval is in place. Energy-rich gas is also produced, made up of carbon monoxide, hydrogen, ethylene and short-chain organic compounds.
There are drawbacks to this method, however, including the fact that the sludge has to be dried out first, which is energy intensive. Additionally, useful materials are also destroyed alongside the contaminants.
What about PFAs?
One of the biggest issues where water quality is concerned is the presence of forever chemicals like PFAs, which are toxic and associated with various negative health outcomes, including certain cancers, thyroid disease and fertility problems.
Further research from Cranfield University and water suppliers is now investigating the possibility of a whole-system approach to managing these chemicals in drinking water, ensuring effective removal and minimising operational costs.
Removal options include reverse osmosis, nanofiltration and other such membrane processes, or using sorbent materials like activated carbon – but it’s important to note that these all produce waste streams with high PFA concentrations, waste that is expensive and difficult to dispose of and which can potentially cause environmental pollution.
“There’s no single answer to producing cleaner cycles of water, wastewater and biosolids. Which is why this new phase of testing and demonstrations, assessing the role of each option in different circumstances, will be critical for making sure there is a platform of evidence for long-term, sustainable transformation in the water sector,” the CIWEM observed.
It will certainly be interesting to see how these projects develop and how they can be of benefit to water quality in the future… and we’ll make sure that we keep you updated every step of the way.