Chemical disinfectants are hazardous to lab staff, and if discharged into the wastewater, pose a health risk to wildlife, ecosystems and the general public. Paul Birchmore makes the case for thermal decontamination.
The work of research and bio-analytical labs can involve the handling of biologically active materials such as bacteria, viruses, cells, tissues, and other microorganisms. Therefore, it is imperative for the labs to implement rigorous safety protocols and effective decontamination procedures. Failure to adhere to these measures could lead to laboratory-acquired infections, unintended gene exchange amongst biological matter, or even worse, accidental release of a controlled biological agent – the greatest fear of any lab manager.
One of the most important aspects of biosecurity is ensuring that effluent wastewater is decontaminated properly and free from any harmful microorganisms, before it is discharged into the sewer. This requires a process of effluent decontamination designed to treat these wastes to ensure that any organisms that might be present are rendered safe.
Decontamination downsides
Pathogens can be killed by a variety of chemical disinfectants such as chlorine dioxide, sodium hypochlorite, hydrogen peroxide and a range of organic chemicals, but many of these are highly toxic, potentially carcinogenic, corrosive and possibly explosive. The chemicals require specialist storage, frequent replacement, and manual handling and present a hazard to lab staff who handle them. Additionally, once the wastewater has been sterilised, disinfectant neutralisers, bringing their own set of risks and prerequisites, must be added before the wastewater can be disposed.
Another issue is that chemical disinfectants are only effective against bacteria if they can reach the cell wall, so solid material in the effluent can shield bacteria from the disinfectant chemical, resulting in ineffective decontamination. To be certain that effluent is safe, it is necessary to ensure that there is excess disinfectant remaining in the wastewater after the requisite contact time, which is dependent on both the chemical being used and the organisms to be inactivated. And if excess disinfectant is not neutralised prior to discharge of the wastewater, it can be released into the environment, presenting a hazard to wildlife, ecosystems, and the general public.
Heat sterilisation, using temperatures of 121°C and above, kills all forms of life, including viruses, bacteria, fungi and spore forms, and provides an effective way to decontaminate effluent wastewater. All the effluent is heated uniformly, including any solids content, and this means there is no escape for the pathogenic organisms. Temperature, being a simple and reliable online measurement, is readily calibrated against a standard thermometer and is easily recorded for control purposes, making validation much easier than with chemical systems. And without the need for harmful chemicals, the process has much less environmental impact.
Harnessing biological indicators
Biological indicators and spore tests can be used to provide an additional level of confidence of complete sterilisation. A known solution of Geobacillus Stearothermophilus spores can be injected at the start of the sterilisation process and samples from the discharge can be tested. A simpler test, used on smaller EDS systems, is a biological indicator ampule. This is a glass ampule full of liquid and spores which is placed in the biowell – a tube that goes into the kill tank (see diagram). The ampule is recovered and examined at the end of the sterilisation cycle. The presence of ≈ will cause the liquid to change colour, giving staff a clear signal if the process was effective.
Heat sterilisation EDS systems are usually batch processes, using one or more jacketed pressure vessels/kill tanks, heated by high pressure steam. Liquid waste is pumped, or gravity fed into the pressure vessel until the level reaches a pre-set point which starts the heating/sterilisation cycle and, in a multi-tank system, the incoming flow is diverted to another tank until it too is at the correct level to begin the process.
Where effluent flows are intermittent, a single kill tank can be used in combination with a holding tank. This stores the effluent until the kill tank is available and then a pump simply transfers it to the kill tank.
The temperature of the kill tank contents is raised to 121°C or 135°C and maintained for a preset time to effect sterilisation. The sterilised effluent is then cooled to the allowable discharge temperature (normally 60°C) before the contents of the tank are discharged to sewer.
- Paul Birchmore is AstellBio Sales and Marketing Director
