Warmer samples for a cooler world

By storing wastewater samples at higher temperatures, laboratories can substantially decrease the amount of energy they use, and thus reduce their carbon emissions, without significantly compromising sample integrity.

In many situations, samples taken for chemical and biological analysis need to be stored before they can be processed. These samples can range from tissue, faeces, soil and even sewage. Samples can be stored for hours to years, in the case of long-term biobanks, acting as windows into the past; but if samples are not stored correctly, that window can quickly become clouded.

There are various sample storage options, including room temperature, refrigeration and freezing, with storage at -80°C often seen as the gold standard. However, the ultralow temperature freezers used to reach these temperatures require large amounts of energy.

In a study recently published in the Journal of Virological Methods, sewage sample storage techniques were scrutinized, for the purpose of wastewater monitoring. Wastewater monitoring can tell you information about a community that produces that wastewater, including the prevalence of infectious diseases, known as wastewater-based epidemiology (WBE), thus contributing to pandemic preparedness.

Measurements from samples stored for up to a year at 4°C, -20°C and -80°C were compared. These measurements included important chemical markers like pH, conductivity, ammonia, orthophosphate and turbidity, as well as biological markers targeting the RNA virus (SARS-CoV-2) that causes COVID-19, and a naturally occurring DNA faecal virus.

Surprisingly, in most cases, storing samples long-term at -80°C was found to be unnecessary, with samples frozen in a standard -20°C freezer remaining just as stable. All chemical markers were also stable after a year of just being refrigerated (4°C), so long as they were kept in smaller tubes. The RNA virus showed a similar trend, with samples stored at 4°C, for up to seven months, even showing a better detection rate than in samples that were frozen. This is because as soon as a sample is frozen, ice crystal formation damages the RNA.

In addition, taking samples in and out of the freezer multiple times throughout the year did not have a large effect on many of the markers, except for turbidity. This offers flexibility when working with samples in biobanks. Being able to effectively analyse samples in biobanks is valuable, especially in the context of WBE, as it can give you historic information on the early stages of emerging diseases.

The paper, ‘Wastewater sample storage for physicochemical and microbiological analysis’, is published in the Journal of Virological Methods.