Alison Margaret Brown reports on the benefits and risks of using biochar from waste
The UK government is committed to moving towards a circular economy, which not only means minimising waste but reusing it and promoting efficient use of all resources. However, when waste is transformed into a product, the product needs to meet regulatory requirements such as those linked to end-of-waste status. These regulatory hurdles protect us from hazardous materials re-entering circulation, but they create challenges for new waste-to-product transformations as they require demonstration of existing market and demand. Markets are difficult to develop without regulatory consent providing confidence to the buyer – the proverbial ‘chicken and egg’ paradox.
The pyrolysis process
Biochar made from waste is one such transformation. Biochar is the principal product formed from pyrolysis, the heating of biomass or biosolids to between 300-900°C in the absence of oxygen. The carbon in biochar is largely recalcitrant, so advantageous for carbon sequestration. The pyrolysis process generates energy and stabilises most of the feedstock carbon as non-degradable biochar, preventing its conversion to greenhouse gases. The current practice of storage and application to land of manures, slurry and sludge (83 million tonnes per annum) results in numerous environmental problems, so stabilisation of biosolids such as biochar has many advantages. Adding the right biochar to soil improves soil structure, water retention and nutrient delivery, reducing the potential for leachate to rivers.
Better for soil
A significant advantage of correctly run pyrolysis of waste feedstocks is that feedstock contaminants, such as pharmaceuticals, personal care products, antibiotic-resistant genes, mobile genetic elements, microplastics, synthetic organofluorine chemicals and other organic chemicals, found in waste streams (particularly sewage sludge) are destroyed or denatured. With these advantages, it is not surprising that biochar made from sewage sludge is now being added to land in Denmark, Sweden, the Czech Republic, Australia and Israel, and is becoming favoured over using sewage sludge directly, as still practised in the UK.
There are risks with using biochar that need to be guarded against. Biochar is essentially non-degradable which makes its addition to soil irreversible and any heavy metals that enter biochar from the feedstock are not destroyed, so we have to make certain contamination is not hidden in biochar by ensuring proper standards and certification. A revised UK-based Biochar Quality Mandate is being developed to support this and will be available for public consultation in 2023.
One barrier to taking advantage of biochar-linked opportunities is the regulatory framework. Animal litter and manures are not considered waste when applied directly to land, but using pyrolysis to treat them is considered a waste treatment process, with the end-product categorised as waste. Obtaining end-of-waste status is complex and time-consuming, and seemingly contradictory when the manures and litter can be used on land without treatment, despite risks linked to chemicals, pathogens and antibiotic-resistant genes. Changes to the regulatory framework to support pyrolysis of animal manures, backed by quality and sustainability certification, could be a way to overcome this ‘chicken and egg’ dilemma.
The benefits of biochar in farming
Farming has been in the spotlight for high greenhouse gas and ammonia emissions from slurry and intensive farming. The addition of biochar to slurry can significantly reduce ammonia and greenhouse gas emissions. Not only is this good for the climate, it also prevents loss of the fertilising value of the slurry and reduces leachate when the slurry is added to land. Ammonia, from intensive poultry farming, causes air pollution, damages biodiversity, affects poultry health and is extremely unpleasant for workers. If biochar is added into the litter, it can absorb ammonia, improving the air quality and preventing damage to animals’ feet. Poultry litter is an ideal feedstock for biochar and surplus energy produced by pyrolysis could be used to support barn heating and ventilation, further contributing to the circular economy.
Restoring ocean health
Scotland is the third biggest producer of farmed fish (214,000 tonnes per annum) after Norway and Chile. Much of the fish farm detritus (uneaten food, faeces and dead fish, together with chemicals, pharmaceuticals and microplastics) is released into the sea. This release of nutrients squanders a valuable resource and impacts marine ecosystems. However new closed, contained, recirculating aquaculture systems now exist. If these were used to prevent the detritus entering the ocean, it could be pyrolysed, producing a bio-sterile, contaminant-free, high-phosphorus, renewable, recycled nutrient source. Making this valuable fertiliser could reduce marine pollution, helping restore ocean health.
Alison Margaret Brown is SAGES-SEFARI intern at the University of Edinburgh
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