It may not be pretty, but waste sludge can be more useful than you think. Irish researchers hope to turn these nutrient-rich leftovers into useful fertilisers. Large volumes of sludge arise from sewage and dairy waste, but the muck can be converted to something useful using simple techniques such as aeration or worm-composting.
It's a cradle-to-cradle approach that returns food-waste to the land, says Dr John Bartlett, director of the centre for sustainability at the Institute of Technology, Sligo. He heads research on the nutrient-rich bacterial sludge that is left over when urban wastewater is treated to break down sewage.
In Ireland, sewage sludge is commonly treated using the complex process of anaerobic digestion, where bacteria break down the organic matter in the absence of oxygen. It's a perfectly fine technology, says Bartlett, but his group is now taking a more straightforward approach - they simply mix air into the sludge.
Called autothermal thermophilic aerobic digestion (Atad), the process favours the growth of heat-generating aerobic microbes that raise the temperature of the sludge to around 55 degrees centigrade. This kills off disease-causing organisms that may be lurking in the mix, such as Salmonella.
"The microbes raise the temperature themselves," explains Dr Bartlett. "You do not heat the stuff up at any stage, start, in between or finish. You get that heat out of it and you get the pasteurised product," he says.
The treatment process also reduces the level of volatile solids in the sludge to levels where it won't attract pests like flies or rats. The resulting "biosolids" can then be spread on agricultural land and are comparable in nutrient value to the biosolids produced by anaerobic methods, according to Bartlett.
The group currently uses the aeration system to treat sludge from a wastewater plant in Killarney that supports a population equivalent to 45,000 people. And they are now looking to improve on the process.
They are using DNA technology to profile the populations of bacteria that grow in the sludge under these aerobic conditions, and are compiling a shortlist of desirable species in conjunction with Professor Tony Pembroke and his team at the University of Limerick.
"We are now getting at smaller subsets of the ecosystem, looking at sub-communities that we want to advantage and make life easier for them," says Bartlett. The group also plans to look at ways of harnessing the heat produced by the bacteria as an energy source.
Meanwhile, other research at IT Sligo looks to earthworms to reduce the burden of waste from the dairy processing industry. Each year the dairy industry in Ireland produces in excess of 100,000 tonnes of waste sludge, which is currently landspread. Percy Foster has been using worms to "vermicompost" the cheesecake-like sludge and has found the process can change the availability of nutrients to plants.
He diluted untreated and vermicomposted sludge with peat to create a range of plant fertilisers and trialled them on grass, barley, radish and marigold. When compared to "virgin" sludge, the vermicomposted product improved radish and barley yield, and resulted in more regular growth of grass.
On the other hand, the untreated dairy sludge supported better yield of the grass and marigolds. Either version of the sludge performed better as a fertiliser than using just peat.
"The overall picture from the trials was that the vermiculture and the dairy sludge had better growth than the peat alone," says Foster, who worked with Dr Billy Fitzgerald and Dr Don Cotton from IT Sligo. Glanbia were industrial partners on the project, which was funded by the Department of Agriculture.
Dr Claire O'Connell