Sewage sludge may not look pretty, but it could make for a sustainable source of plant nutrients.
After urban wastewater is pushed through a treatment plant, it comes out on the other end as sewage sludge. Scientists writing in the journal Frontiers in Nutrition say that this treated sewage may make for an excellent fertilizer option, as well as a sustainable means of recycling phosphorus.
Phosphorus is an essential plant nutrient, but one that is quickly becoming depleted. Without phosphorus, food production would cease. Consequently, high phosphorus content is turning sewage into a potentially precious resource.
Whether sewage sludge could rival commercial fertilizers for agriculture depends, in part, on how well plants are able to take up its phosphorus.
Researchers from Madagascar and France grew ryegrass and a flowering plant called fescue in soils with either no fertilizer, sewer sludge that had been treated with heat, or commercial triple superphosphate fertilizer. An isotopic phosphorus label was applied to trace the mineral’s uptake by the plants.
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After a few weeks, the researchers harvested the plants and weighed the shoots that had grown upward and the roots within the soil.
Application of the commercial fertilizer improved shoot growth and provided plants with a boost in phosphorus. This was not only because the fertilizer added phosphorous to the soil, but also because plants grown in the fertilizer tended to use the nutrients in the soil more efficiently.
Plants treated with sewage also acquired more phosphorous than the controls, but to a lesser extent than those given commercial fertilizer.
The reason for the discrepancy might be that the sewage stimulated the activity of microbes in the soil, which compete with plant roots for available phosphorus. Indeed, sewage-containing soil has higher microbial biomass, but as study lead author Andry Andriamananjara explains in a press release, the phosphorus captured by these microbes can still become available to the plants over time.
Further, he points out that “sewage sludge is a non-limited continuously available and sustainable fertilizer source," and therefore recommends its use as a fertilizer.
Meanwhile, the use of sewage sludge in agriculture — for food crops in particular — is widely disputed. Though it can increase phosphorous content of the soil, the sludge also leaves behind low levels of heavy metals and toxins. Some of these may bind to the soil, leach into groundwater, or get absorbed by plants, leading to concern over potential health risks to humans and animals.
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Editor's note (Aug.17): Paragraph 10 has been added.