Knowledge partner GWI
Tuesday, 28 March 2023
Management of sewage sludge is a significant and rising cost for utilities. GWI looks at the potential of process engineering for maximum resource recovery to create a biorefinery and capitalise on the value of waste.
A growing market for sustainable opportunities
GWI estimates that total global expenditure on sludge management will
reach $15.5 billion in 2022, with this figure set
to rise as populations grow and disposal routes dwindle. This is particularly felt in mature
markets like Europe where
regulations regarding disposal are becoming stricter.
Germany, for example, has put in place legislation that will ban land application of sludge as a
disposal route from
2032 – following similar bans in place in the Netherlands and Switzerland. Emerging innovations
in resource recovery
technology, however, may open the door to offset sludge disposal costs by introducing a plethora
of value-generating
streams.
The biorefinery concept looks at the potential of redesigning sludge treatment plants to
capitalise on such revenue
streams. Instead of focusing on decreasing waste volumes, these plants could be meticulously
engineered to maximise
recovery of resources at every treatment stage. Taking inspiration from oil and petrochemical
refineries, where
processes are designed for maximum efficiency and the production of profitable end-products,
biorefineries can use
sewage as a feedstock to produce valuable products.
By bringing this resource recovery process into the treatment of sewage sludge, biorefineries
can also support a move
towards a circular economy, as recovered products can reduce the need for raw material
production. While fully maximised
resource recovery from sludge may still be a futuristic dream – captivating the minds of
bioresource process engineers
rather than the plans of the average utility – innovative technologies are already bringing many
aspects of the
biorefinery into reality.
What resources can be recovered in a biorefinery?
Increasing emphasis on recycling and reusing waste materials is resulting in a greater appreciation of the resources they contain. Sewage sludge is full of potential for utilities, with many already extracting energy, phosphorus and nitrogen via anaerobic digestion and nutrient recovery processes, respectively. Tentative steps are being made into recovering biopolymers and producing hydrogen, and the recovery of volatile fatty acids and cellulose could become more viable in the future. Hence, the biorefinery concept could transform sludge management from a necessary cost to a valuable opportunity and offer a sustainable potential business model.
Source: GWI
As can be seen in the above graphic, there are many possible avenues for resource recovery from
the treatment stream of
sewage sludge. The level of market readiness of the technologies involved, however, varies
significantly. Biogas capture
from anaerobic digestion, for example, is widely commercialised as an energy generation process
for both onsite use and
export to national grids. One emerging market, with limited applications thus far but showing
great potential, is
ammonia recovery.
Reducing emissions and creating value with ammonia recovery
By recovering and reusing ammonia from sludge treatment, utilities can
meet regulatory standards on nutrient pollution
while generating an additional product. Furthermore, when ammonia is left in sludge, it releases
the highly potent
greenhouse gas of nitrous oxide (N2O), with ammonia recovery being an emerging strategy to
mitigate these emissions. GWI
models estimate that N2O released from sludge management accounts for 5.2 million tonnes of
carbon-equivalent emissions
globally each year; this represents 32 per cent of greenhouse gas emissions from sludge
management.
Liquor and condensates from sludge treatment have high ammonia levels, with UK-based engineering
company Atkins
reporting that 2-3 per cent of the UK’s total ammonia production could be offset using recovery
from sludge. The key
technologies to achieve this recovery are chemical and thermal ammonia stripping, both of which
can be applied in liquor
treatment. Companies offering these solutions include Anaergia, Organics Group and Nijhuis Saur.
Once ammonia is recovered, the most established end-use is for fertiliser production, in place
of ammonia produced
through the dominant yet fossil-fuel dependent Haber-Bosch process. This highly
ammonia-demanding industry presents the
opportunity for two-fold emissions reductions: replacing a carbon-intensive new material with an
emissions-reducing
recycled material.
Shifting perspectives on agricultural spreading
Given the high nutrient value of sludge, it comes as no surprise that
ammonia recovery isn’t the sole contribution
sludge plants can make to the fertiliser industry. Agricultural spreading of sludge is a common
disposal method,
providing a profit-generating route while providing farmers with a fertiliser option that is
significantly less
expensive than synthetic fertilisers.
This disposal route is generally available to sludge that has been put through digestion for
stabilisation to reduce
pathogens and odours, creating spreadable digestate. In the UK, for example, over 90 per cent of
sewage sludge disposed
of is recycled to farmland, according to the regulator Ofwat. This practice is also dominant in
Spain, Australia and
China.
However, concerns over the high potential concentration of nutrients, heavy metals and emerging
contaminants (such as
PFAS, microplastics and pharmaceuticals) in digestate are impacting the availability of
agriculture spreading as a
disposal route. This can be seen in Switzerland and the Netherlands, where legislation currently
bans spreading
digestate to land.
Similarly, Germany’s 2017 Sewage Sludge Ordinance will come into force in 2032, banning land
application from plants
over 50,000 population equivalent. While standards ensuring sludge quality for spreading exist
in some countries – for
example, via the Biosolid Assurance Scheme in the UK and EPA biosolid standards in the USA –
uncertainty remains over
the future of digestate spreading.
Biochar: one to watch
One digestate alternative gaining traction is biochar. Biochar is a high-carbon solid produced
by the thermochemical
treatment of sludge, using processes of pyrolysis or gasification. This high-heat treatment has
been shown to remove
PFAS and other contaminants of concern from the resulting biochar.
Additionally, biochar fertilisers are not water-soluble, and nutrients are released slowly, thus
limiting eutrophication
concerns linked to agricultural nutrient leakage into surface waters. This high stability
further increases the appeal
for biochar fertilisers, as their efficacy is measured in centuries – compared to weeks of
efficacy for synthetically
produced fertilisers. Finally, biochar serves as a stable form of carbon sequestration, thereby
supporting the Net Zero
carbon emissions targets being set by a growing number of utilities.
Source: GWI
The biochar market is still emerging, however. A plethora of alternate uses for the product are
being researched, while
companies including Anaergia, Kore Infrastructure, Pyreg and Aqua Green lead the market for
biochar fertilisers.
Furthermore, regulatory barriers such as the EU Fertiliser Regulation, which was updated in July
2022 and excludes
sewage sludge as an acceptable feedstock for biochar fertilisers, currently limits the scope of
the market.
Some EU member states – including the Czech Republic, Denmark, and Sweden – have taken matters
into their own hands
recently by introducing country-level regulation which allows sewage sludge biochar to enter
fertiliser markets.
Additionally, the industry standard organisation European Biochar Certificate is conducting a
review which may see
sewage sludge included as an approved fertiliser feedstock; this move may prompt wider EU-level
change. If and when the
legislative landscape shifts to better support sewage sludge biochar, there may be significant
potential for growth.
A path towards a future circular economy
Along with legislative support, as highlighted, technological development is still needed for
many of the resources
within sludge to be recovered effectively. For example, volatile fatty acids are beginning to
raise excitement in the
industry, but recovery processes have yet to be demonstrated. Growing enthusiasm for circular
economy principles is
however encouraging this research and development into how such resources can be transformed
into value again. This can
be seen in funding opportunities like Ofwat’s Innovation Fund and the EU’s Horizon Europe fund
which are available to
support early-stage adoption of sustainable emerging technologies.
As technological development is achieved and the water sector implements such innovative
solutions, there is scope for
significant value-generation to occur in the coming decade. While change is likely to be
gradual, the biorefinery could
demonstrate the opportunity to fully incorporate circular economy principles into the wastewater
sector and close the
resource loop by minimising wasted materials.
This article has been produced by GWI, with the
full ‘Biorefinery market map (January 2023) available
here
References: Circular-economy-sludge-biorefineries