More than 70% of the sludge from wastewater treatment plants is used in agriculture to provide the soil with organic matter and nutrients such as nitrogen and phosphorus.
SARS-CoV-2 viral particles have been detected in the faeces of some patients. Although the World Health Organization considers that there is currently no evidence that SARS-CoV-2 can survive in sewage, it has been shown that related coronaviruses can remain infectious in this medium for several days. Before the slurry spreading season begins, the Ministries of Agriculture and Ecological and Inclusive Transition therefore issued an urgent request to ANSES to assess any possible risks associated with the spreading of sludge from urban wastewater treatment plants during the COVID-19 epidemic.
Since transmission occurs through the respiratory tract, the concerns mainly relate to exposure to droplets and dust that may be emitted during slurry application.
An analysis based on the effectiveness of sludge treatments on other viruses
In wastewater treatment plants, sludge intended for agricultural spreading undergoes several treatments: it is thickened and dewatered, then very often stabilised to block fermentation and limit odours. Lastly, in some plants it is heat-treated, limed with quicklime, or treated by digestion or composting to reduce the presence of micro-organisms.
The agricultural use of sewage sludge is governed by the Order of 8 January 1998, which sets out precisely the requirements to be met for sludge to be regarded as disinfected.
As data on the inactivation of SARS-CoV-2 in sludge and effluent are still very incomplete, ANSES based its analysis on knowledge of other viruses, such as enteroviruses, phages and animal coronaviruses, to estimate its resistance in sludge and in relation to the treatments applied.
The Agency examined the main factors underlying the effectiveness of the disinfection processes: time (for all processes), heat processes or those leading to a rise in temperature (digestion or composting of organic matter by bacteria, liming with quicklime) and the change in pH for liming.
Robust disinfection treatments are needed to make agricultural spreading safer
With regard to sludge produced during the epidemic period and undergoing disinfection treatment, the Agency considers that the risk of SARS-CoV-2 contamination is low to negligible, given the effectiveness of all the treatments applied: composting, thermal drying, thermophilic anaerobic digestion and liming.
However, it recommends reinforcing controls to verify the proper implementation of treatment processes and compliance with the protective measures that must normally be adopted by wastewater treatment plant workers and professionals carrying out the spreading (appropriate collective and personal protective equipment, hand washing, showering on completion of the activity, etc.).
For sludge produced during the COVID-19 epidemic that has not undergone any disinfection treatment, there are not enough data available at the present time to be able to precisely define the level of contamination by SARS-CoV-2. Furthermore, there is currently insufficient knowledge of the persistence and development over time of the infectivity of coronaviruses to be able to define a storage period beyond which the virus may be inactivated. Consequently, the Agency recommends that such sewage sludge should not be applied without first being disinfected. On the other hand, non-disinfected sludge produced before the start of the epidemic can still be spread. ANSES will remain attentive to future information and studies that may lead it to modify this assessment.
Lastly, given the lack of data for accurately documenting viral contamination in sludge and throughout the wastewater treatment system, ANSES believes it necessary to carry out specific scientific studies. In particular, it recommends further work on the monitoring of bacteriophages infecting intestinal bacteria, which are proposed as indicators of faecal or viral pollution.