Sewage sludge spreading on arable land – long-term effects on soil and crops, as well as the presence of antibiotic-resistant bacteria
In 1981, field trials were started to investigate the short- and long-term effects of the spread of municipal sewage sludge on arable land. Initially, there were five test fields, but for a long time the project has been concentrated on two of them: Igelösa outside Lund, which receives sludge from the Källby Wastewater Treatment Plant (WWTP), and Petersborg outside Malmö, which receives sludge from the Sjölunda WWTP. The project has so far had nine stages of four years each. The period 2018-2021 will be the tenth stage of the project.
Municipal sewage sludge contains many desired plant nutrients, especially phosphorus. Sludge spreading is also positive for the soil’s organic content. But just like other organic fertilisers, sludge also contains heavy metals and other contaminants. The sludge quality has improved a lot over the years when it comes to heavy metals, not least through the possibility of certifying WWTPs through Revaq, but the debate and the question of whether sludge as a phosphorus source should be spread on arable land continues.
The project Sewage sludge spreading on arable land is unique in the world. It has contributed to a material that is sought after by the research world, both nationally and internationally, and the results have contributed to knowledge in several different issues. After more than 30 years of study in the two field trials, it is possible to say with great certainty about certain results, while other results are more uncertain and require further studies. Since 2010, an extended analysis program of both crop and soil has been carried out, which means that all 36 test windows have been analysed individually for plant nutrients and metals. This in turn has made it possible to perform statistical analyses of harvested products and soil in a better and safer way.
The experiments have to a large extent contributed to the state of knowledge we have today about sludge spreading on arable land.
From the report covering the years 1981-2014 (Andersson, 2015), it can be read, among other things, that sludge supply has the following effects on harvested product:
- In the experimental phase with sludge, the supply of sludge has on average given a 7% increase in yield, regardless of the supply of mineral fertilisers.
- Recalculated in the 2015 price level, the harvest-increasing effect of sludge supply has been 500-600 SEK per hectare and year.
- Cadmium has been given great interest during the experiments, but in addition 14 other metals have been analysed in the crops. None of these metals have increased in uptake into the plant with sludge supply, not even with triple sludge application. The following conclusion after 33 years of experiments is therefore very certain: Under the conditions prevailing at the experimental sites, sludge supply to arable land has no effect on the uptake of heavy metals by studied crops.
Impact of sludge supply on soil plant nutrient content:
- A large part of the phosphorus that is added becomes available to the plant. Phosphorus levels rise in the field.
- The soil organic content is higher in experimental stages that have received sludge compared with in experimental stages that have not received any sludge.
- Soil fertility increases.
Impact of sludge supply on the metal content of the soil:
- The sludge used has historically contained a lot of copper and mercury, which has meant that the content of these metals has increased in the soil.
- The zinc content has also increased, and there is a weak, not unambiguous, tendency for lead levels to rise
- In the years when tin has reached above the detection limit, the levels have increased.
- Cadmium levels tend to be slightly higher in Petersburg with sludge supply. However, the results are not unequivocal.
- For other metals there are no differences.
The water and sewerage industry has been waiting since the Swedish Environmental Protection Agency’s government assignment was published (Swedish Environmental Protection Agency, 2013) for new limit values for spreading sludge on arable land. Meanwhile, new contaminants have begun to come into focus: organic micro-contaminants, including microplastics, and the possible presence and survival of antibiotic-resistant bacteria.
There is concern that sludge spread increases the risk of developing and spreading antibiotic-resistant bacteria (Burch et al, 2014; Chen et al, 2016; Xie et al, 2016). There are antibiotic-resistant bacteria in all environments, but they are much more common in the intestinal flora in humans than in most external environments, e.g. soils and watercourses. As the bacterial communities that reach our municipal WWTPs primarily consist of faecal bacteria, the treatment plants are also environments where resistant bacteria are common, so also in Sweden (Bengtsson-Palme et al, 2016).
The project will
- continue to investigate the long-term effect on both soil and crop when spreading sludge on arable land in terms of nutrients, metals, micro-trace elements and organic substances.
- better understand and, in the long run, assess the risks of sludge spreading on arable land with regard to the presence of antibiotic-resistant bacteria and antibiotic-resistance genes.