Consultation response: PAS 110 recommendations
The Anaerobic Digestion and Biogas Association (“ADBA”) is the trade association that represents the range of
interests and matters related to the anaerobic digestion of organic materials (“AD”) across the UK, including the
collection of waste for use as feedstock. ADBA understands the complex range of skills required by developers
of new AD plants, from feedstock management through technology to energy production, markets and resource
to land.
The organisation has over 300 members from across the AD industry, including farmers, local authorities, waste
management companies, supermarkets, food processors, plant operators, energy and water companies,
equipment manufacturers and suppliers, consultants, financiers and supporting service companies. Anaerobic
Digestion can make a significant contribution to renewable energy, climate change, and critical resource
preservation targets, subject to the right policies being in place.
RBP Test
1. Do you agree that the limit should be reduced to 0.2 l biogas g-1 VS, and that the test duration should be
reduced to 10 days?
Although ADBA recognises the rationale for wanting to reduce the test duration to 10 days given the significant
criticism that has been levelled at the lengthy duration of the test currently, evidence shows that a 0.2 limit at 10
days would not significantly reduce commercial risk that failure would bring. We believe the limit should be raised
to 0.35 L/g VS, with the possibility of a rolling-average limit of 0.30 L/g VS.
The Southampton University report shows that 6 out of 24 samples failed the RBP test (ie 25%), which we
believe is a significant proportion and warrants further examination of the rationale for the 0.25 limit.
The original work on which the RBP test and limit were based was published by WRAP in 2010 in a report titled
‘Development and evaluation of a method of testing the RBP of digestates’. The limit on RBP proposed in that report of
0.25 L/g VS, which was subsequently adopted for PAS 110, was based on a very limited regime of testing, as well as
RBP values for other materials applied to agricultural land, including slurry and sewage sludge.
WRAP however published a report in September 2011 titled ‘Biofertiliser management: best practice for agronomic
benefit and odour-control’, which showed that RBP values for farm slurries could be far higher than 0.25, with 6 out of
10 samples falling above 0.25, and indeed nearly reaching 0.9 in one example.
2. Do you agree that the maximum period in which the net biogas production can remain negative is reduced
from the current 5 days to 4 days?
ADBA would be keen to see more evidence on this issue.
3. Do you agree that the RBP test should be applied to whole digestates only, rather than to separated
fractions? This would require differentiation within the PAS, since all other tests would apply to the digestate as
supplied to the end user, whereas the RBP test would be associated solely with demonstration of process
performance.
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ADBA believes that there is a risk that applying the RBP test to whole digestate only, rather than to separated
fractions, could mean that further treatment could take place prior to the digestate being spread to land which
this approach would not consider.
4a. Do you agree that the sampling point should be specified as the outlet of the final tank from which biogas is
collected for processing rather than simply vented?
No – we believe the primary focus of the RBP test should be the quality of the end product going to land.
5. The current (post-validation) testing frequency is 2 per 12 months and not within 3 months of each other, or
sooner if and when significant change occurs. Do you think that this should be changed? If so, please state how
and provide data to support your suggestion(s).
ADBA understands from members’ feedback that the cost of the current RBP test (£500) can be prohibitive, and
therefore believes there should be caution in extending sampling/testing requirements. Reducing the test
duration to 10 days therefore initially seems attractive, but data suggests that the 0.2 limit would not significantly
reduce the risk of failure and hence a far greater financial cost and commercial risk.
The raising of the limit to 0.35 L/g VS with the same testing frequency seems to be the most sensible option; with
the possibility of a rolling average limit of 0.30 L/g VS (although the increased costs of this mechanism would
have to be examined closely).
6. At present, PAS110 includes limits for PTEs, physical contaminants and bacterial indicators. Do you consider
that PAS110 should include further ‘environmental outcome’ tests? If so, please state what these should be and
provide data to support your suggestion(s).
We do not believe that the inclusion of further ‘environmental outcome’ tests should fall within the remit of PAS
110; this should be a matter for other environmental regulations.
7. There are no grounds for using VFA concentration as a product stability criterion. Do you agree with this
assertion? If not, please state why and provide data to support your case.
No – VFA concentration, along with other indicators such as alkalinity and pH level are relevant for product
stability and should therefore be considered.
9. The current RBP test protocol does not contain any instructions on outlier results due to lab or equipment
failures, e.g. leakage. Do you agree that such an amendment is appropriate?
We agree that such an amendment would be appropriate; however its scope should go beyond lab/equipment
failures in considering outlier results.
Pasteurisation
12b. Do you think that specific feedstocks should be exempted from the pasteurisation requirement? If so,
please state what they are and provide data to support your assertion that they should be exempted.
ADBA does not believe that forage presents any risk to the spread of plant pathogens, and therefore believes it
should be exempt from the pasteurisation requirement – in the UK forage that is fed to the cattle herd is not
pasteurised and is subjected to one day’s mesophilic digestion in three stomachs with a high risk of bypass.
We also believe that fusarium should be excluded from the pasteurisation requirement. Fusarium is not
considered to be anything but a naturally occurring soil borne fungus that should be routinely controlled whether
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there has been recycling on the particular soil or not. Fusarium will typically be digested in a mesophilic digester
in two days.
There is also a strong case for exempting brassicas from the pasteurisation requirement. Brassicas or brassica
roots are rarely used as a feedstock in AD, although there is the possibility of club root being used which is
sourced from brassicas.
However, the existence of club root in soils can be controlled by crop rotation, and is present in soil where
recycling has not taken place anyway. Pasteurisation at 70 °C would not eliminate club root anyway, meaning
the temperature would need to be raised if this is considered a genuine issue.
ADBA also believes that the current requirement to pasteurise non wastes when mixed with pasteurised wastes
represents an unnecessary regulatory burden for operators, and we believe that the real environmental risks of
such a change need to be considered in more detail.
15. If the current PAS110 pasteurisation regime were to remain in place, the concept of a standardised
approach for validating alternatives is extremely appealing. Do you support the proposal to use Plasmodiophora
brassicae and tomato seeds as indicator species for such validation? If not, please state why and provide data
to support your case
We do not agree with the proposal to use tomato seeds as an indicator species for validation. Tomato seeds are
easily controllable in farming systems, and we believe that if seeds should be used as a validation mechanism,
blackgrass, sterile brome, wild oats, cleavers, docks, thistles and other options would be more suitable given
their adverse impact on farming systems.
In addition, the “bag” method of tracing the tomato seed through the AD process is not applicable to wet AD.
The bag will be destroyed by pumps and macerators during the process, and if it did survive by chance, it would
be challenging to recover it from the process.
Potentially Toxic Element (PTE) limits
17. Do you agree with the conclusion that it would be more appropriate for PTE limit concentrations in digestates
to be set on a fresh weight basis? If not, please state why and provide data to support your case
Yes – this will properly reflect what those with a low dry matter digestate (around 3-4%) apply to land.
18a. Do you agree that it is appropriate to have separate limits for liquid and fibre digestates? If not, please
provide data to support your case.
Given the likelihood of PTEs appearing in the digestate fibre, we recognise the need for separate limits for liquid
and fibre digestates.
Our members have however indicated that they would be interested in seeing further information on why the
precautionary limit values for the liquid digestate are much lower than the theoretical values when compared to
the
fibre
samples.
18b. Do you agree that the distinction between liquid and fibre digestates should be set at 15% dry matter? If
not, please provide data to support your case.
This 15% dry matter distinction seems reasonable in principle, however it is possible in some cases that crude
separation may result in a dry matter of around 20%. It may therefore be worth consulting further with AD
operators who already separate to establish their views.
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18c. Do you agree with the proposed PTE limit values? If not, please provide data to support your case.
ADBA’s members would be interested in understanding the evidence base used in determining the PTE addition
rates. This is particularly important given the ‘Sludge (Use in Agriculture) Regulations’ and the ‘Code of Practice
for Agricultural Use of Sewage Sludge’ prescribe far higher addition rates, and it is not clear on what basis (i.e.
what environmental risk) these PAS 110 limits have been calculated on.
There also needs to be greater clarity on which samples include farm slurries co-digested with food waste, as
some slurries will contain higher limits of certain contaminants such as Cu and Zn.
20a. Do you agree that levels of physical contaminants should be set for digestates on an ‘as received’ basis? If
so, please state why and provide data to support your case. If not, please state why and provide data to support
your case.
Yes. Operators producing a digestate with a total low solids content (typically between 3-4%) are left with little
solid matter when separated after being put through a 2mm screen. Due to the limited content of solid in the
material, any piece of plastic or cornstarch will make up a significant percentage of the total dry matter content
and will therefore exceed the limit set out in PAS 110 even though these contaminants would represent a small
percentage of the total ‘as received’ sample
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