Knowing the secrets of compost

Once the realm of green fingered types - composting is now attracting corporations. But can we reliably monitor bioaerosol emissions from these decomposing eco- stalwarts? The NPL think they have the answer.

Since the EU Landfill Directive came into force in 1999, organisations have been diverting biodegradable waste away from landfill sites. To achieve this, one of the UK’s approaches is to transform green waste into compost. As a consequence, many open windrow compost sites have been set up in UK – in which local green waste is transformed naturally into compost by microorganisms.

To ensure the optimal composting process, it must be aerated by regular turning. This action releases plumes of gasses and particles, which contain fungi and bacteria. Most problematic of these is a fungus called Aspergillus fumigatus, which might cause lung infections and respiratory problems.

The Environment Agency mandates that a risk assessment must be completed for any proposed composting site. This requires an analysis of bioaerosol airborne particles within the composting facility and at 250m of the site boundaries. All such measurements must be based on clear scientific evidence and show that bioaerosols can and will be maintained at appropriate levels.

New sites need to be started, or old ones expanded, to respond to an increase in the amount and variety of biowaste being composted. Furthermore, new techniques are being developed to deal with biowaste, helping provide more efficient and environmentally friendly ways of dealing with waste. It is important for those developing the techniques to have quick results regarding emissions to speed their development and ensure they meet standards.

“During the polymerase chain reaction, an enzyme copies and multiplies a specific sequence of DNA exponentially. The method relies on thermal cycling, where repeated heating and cooling leads to DNA melting and enzymatic polymerisation of the DNA target sequence”

The current method for monitoring bioaerosol emissions is somewhat inconvenient. Typically, bioaerosols are collected from the air onto filters. Filters are then washed off with a solution to re-suspend the microorganisms. This suspension is spread on a Petri dish containing a culturing medium.

After few day of incubation in a temperature-controlled environment, microorganisms multiply in order to form colonies on the Petri dish. The operator in the laboratory counts all colony-forming units. This is extremely time consuming but also raises concerns over reliability, as the technique risks underestimating levels of Aspergillus fumigatus.

It has been recognised for some time that there must be a better way of monitoring bioaerosol emissions. After carrying out a project for Defra to look at this problem and assess different methods, the National Physical Laboratory (NPL) believes it has developed a solution.

NPL is the UK’s National Measurement Institute and provides a wide range of measurement and monitoring services to business and the public sector, employing the highest standards of measurement. It has developed a new method for monitoring the emissions of Aspergillus fumigatus from composting sites, which is much quicker than the culture-based methods.

The method uses a highly sensitive technique called quantitative polymerase chain reaction (qPCR) to detect airborne Aspergillus fumigatus spore levels at composting sites. During the polymerase chain reaction, an enzyme copies and multiplies a specific sequence of DNA exponentially. The method relies on thermal cycling, where repeated heating and cooling leads to DNA melting and enzymatic polymerisation of the DNA target sequence. During the polymerisation step, a specific fluorescent probe is degraded that leads to an increase of fluorescence signal. The time when the fluorescence intensity reaches the fluorescent threshold value could be correlated to an amount of Aspergillus fumigatus spores according to a standard curve. In short, a sample that contains a large amount of spores will reach the fluorescent threshold value faster than a sample containing a lower amount of spores.  The qPCR method is commonly used for applications which detect and quantify extremely small amounts of DNA target, for example, the US postal service uses it for detecting anthrax in envelopes.

NPL is currently working with Enigma Diagnostics Ltd to develop an instrument capable of automating all the processes, including air sample preparation, qPCR and data analysis. The whole process can be carried out from the back of a specially equipped van, and results can be produced within two hours.

Bioaerosol analysis using the air sampler at a composting site

The air sampling method uses a Bertin Technologies Coriolis Cyclone Sampler with subsequent PCR determination. This sampling method impinges in a liquid phase all particles present into air; in this case Aspergillus fumigatus spores would be impinged into liquid. The target DNA is extracted from samples prior to qPCR, at the end of the qPCR run, data are processed to give a spores count per cubic meter of air.

Baptiste Lamarre, who was involved in the project at NPL says: “There is currently nothing like this available. The existing process is slow, inefficient and possibly unreliable. We are in need of a new way of measuring bioaerosol emissions. Having spent the last two years evaluating the different options, we believe this is it.”

The key advantages of this monitoring service over traditional culture-based methods are that it only requires short sampling times and can provide rapid analysis. The technique has high sensitivity and broad detection range, and can detect specific species. It also detects total spore count (viable and non-viable), which overcomes any issue of emission underestimation as a result of damage to the spores during collection and may aid differentiation between background spore levels and site specific emissions.

Since the Defra project ended, further refinement of the PCR methodology has been done to enhance the quantification at very low spore counts and develop a mechanism to identify and account for the observed 'inhibition' seen in some field samples. This has been achieved by developing an internal control to check for inhibition, which has been successfully demonstrated in trials.

Baptiste believes this technique represents the future of bioaerosol measurements. He says: “We have proved the validity of the technique and shown it is a viable option for monitoring potentially harmful emissions from composting facilities. We still have some refinement to do before we can use this with the certainty we need for the technique to become reliable enough for widespread commercial use, but we are close. This technology is now starting to attract some serious commercial interest and we fully expect this to completely replace previous methods.”

Some further work is still needed to investigate background measurements at a range of different locations. This will allow the levels of spores seen in this study to be placed in a wider context. Once it is fully tested, NPL plan to launch a service for waste management companies, using this technique to provide them with quick, easy and accurate analysis of bioaerosols.

This will streamline the process of identifying suitable composting sites and help them to meet regulatory requirements. It will also provide a reliable way of assessing whether more environmentally friendly methods of disposing of biological waste meet standards and so can start being used. Ultimately it is hoped that the technique will lead to reduced emissions from bio waste, improved safety of compost sites, greater ease in meeting regulations, and better health for those who work with or near bio-waste sites.