You tear the liner off the roll and it clings to itself — the static charge of polymer film. You thumb it open, press it into the caddy, scrape last night's dinner off the plate. The liner is warm where your fingers pinched it. Tomorrow it will hold coffee grounds, potato peelings, the fat trimmed off chicken thighs. By collection day it will be wet through, heavy, the contents shifting as you carry it to the kerbside bin. You will do this fifty-two times this year.
The material touching your hands is a petroleum-derived copolyester. The box it came from says compostable. The Seedling logo says the same. EN 13432 — the standard behind that logo — tests at 58°C. Your council's anaerobic digestion plant operates at 35-40°C. The gap is 18-23 degrees. It is also the gap between what the certification promises and what happens to that liner after you put it in the food waste caddy.
Both paper and bioplastic caddy liners are screened out and incinerated at AD plants before food waste is processed. The certification that distinguishes them in the shop is irrelevant to the processing that handles them after collection. This is documented. It is not disputed. And it raises a question that no council guidance, no Which? buying guide, and no Ethical Consumer review has asked: if the certification doesn't determine what happens to the liner, what does?
The material does.
The Standard
EN 13432:2000 is the European standard for industrial compostability — the standard behind the Seedling logo on the roll in your kitchen drawer. It requires 90% biodegradation within six months at 58 ± 2°C, and disintegration into fragments smaller than 2mm within twelve weeks.1 These are industrial composting conditions — large-scale facilities maintaining thermophilic temperatures through controlled aeration.
The standard measures whether material disappears. It does not measure what the material becomes. It does not test what metabolites accumulate in soil during degradation. It does not test ecotoxicological effects of degradation products on soil organisms.2 If you have been composting certified liners in your garden for a year, or two, or three — the standard behind that certification has nothing to say about what has been building up in your soil during that time.
The EU now acknowledges this. Regulation 2025/40 on packaging and packaging waste states that EN 13432 "needs to be revised and replaced" and that updated standards should reflect "actual conditions in bio-waste treatment facilities, including anaerobic digestion processes" and include verification that decomposition results in "carbon dioxide or methane and mineral salts, biomass and water" — an endpoint the current standard does not verify. Until revision, EN 13432 no longer serves as the basis for presumption of conformity.3
The Seedling logo — the mark consumers look for on caddy liners — is a trademark owned by European Bioplastics e.V., a trade association representing over 80 bioplastic companies. European Bioplastics declared lobbying expenditure of €100,000-€199,999 on the EU Transparency Register for 2023.4 In 2023, the organisation updated the Seedling logo to read "industrially compostable" — the previous version said only "compostable."5 The update is an implicit admission: consumers were reading "compostable" as a general property. It is not. It is a temperature-specific, facility-specific certification. And in the UK, the facilities it specifies are not the facilities that process your food waste.
The Screening
UK food waste from kerbside collection goes predominantly to anaerobic digestion plants, not industrial composting facilities. These plants operate at mesophilic temperatures — 35-40°C — approximately 18-23°C below the conditions EN 13432 certifies for.6
At these plants, all caddy liners — paper and bioplastic — are removed at the first stage of processing, before food waste enters the digester. This is not a failure; it is standard operating procedure. Stoke-on-Trent Council: "The plastic liners are removed from the recycling process as the first stage when the food waste arrives at the AD Plant."7 Lincolnshire County Council: "The anaerobic digestion facility removes all liners at the front end of the process whatever they are made of."8 The Environment Agency's Standard Rules for AD facilities specify "screening to remove plastic residues" as a required treatment step.9
Removed liners — paper and bioplastic alike — are incinerated for energy recovery or sent to landfill. If you have been using the council caddy since collection began, every liner you placed in it followed this route. The certification on the box described a different destination.
Paper liners exist. They survive a 3-7 day caddy cycle with wet food waste. They cost roughly the same. And they offer one thing bioplastic liners do not: if you choose not to put them in the food waste caddy, you can put them in your garden compost bin, where they decompose within weeks. This is not a certification claim. It is a material property of cellulose.
The certification on the bioplastic liner certifies conditions your AD plant doesn't provide. The paper liner doesn't need a certification. It just composts. The distinction matters — not at the AD plant, where both are incinerated, but in the one scenario the consumer controls: the garden. And that scenario is the one already running in your garden if you compost.
The Material
The bioplastic caddy liner most widely distributed in the UK — and most likely the one you have been tearing off the roll — is made from Mater-Bi, manufactured by Novamont. On its consumer-facing product page, Novamont describes Mater-Bi as the "result of a series of proprietary and pioneering technologies in the field of starches, vegetable oils, and their combinations."10 The words "petroleum," "fossil," and "PBAT" do not appear.
The primary polymer in Mater-Bi is PBAT — polybutylene adipate terephthalate — a petroleum-derived copolyester. It typically constitutes 60-70% of the Mater-Bi blend by weight, with thermoplastic starch making up the remainder. Exact ratios are proprietary. A 2024 review in Macromolecular Materials and Engineering found that standard PBAT currently has approximately 37% bio-based content — meaning roughly 63% is fossil-derived.11 Novamont produces some bio-based feedstock at its MATER-BIOTECH plant in Bottrighe di Adria (30,000 tonnes per year), but the bulk of PBAT feedstock remains fossil in origin. The majority of a Mater-Bi caddy liner, by weight, is a petroleum-derived copolyester. The static charge when you tear it off the roll is the charge of polymer film. The warmth where your fingers pinch it open is the warmth of a plastic.
What that copolyester becomes in soil — the metabolites it releases as it degrades — is the subject of the next section. If you have been putting these liners in your garden compost, the transformation described below is already underway.
Novamont is wholly owned by Versalis S.p.A., the chemical subsidiary of Eni — one of the seven global oil and gas supermajors. Versalis completed acquisition of 100% of Novamont's share capital on 18 October 2023, having previously held 36%.12 Versalis was formerly known as Polimeri Europa, a conventional petrochemical producer of olefins, aromatics, and polyethylene, before rebranding in 2012.13
The consumer-facing marketing describes "vegetable oils and thistle starch." The corporate ownership structure shows Eni. Both descriptions are accurate. The question is whether a consumer encounters both.
The downstream marketing follows the same pattern. Retailers of Mater-Bi caddy liners typically describe them as "compostable" or "home compostable" while referencing EN 13432 — an industrial standard — as the certification. Vegware, one of the UK's most visible compostable packaging brands, illustrates the gap. Its product page describes its Mater-Bi caddy liner as "Certified home compostable" and "Made responsibly in Europe from Mater-Bi, a plant material made from vegetable oils and thistle starch."14 The certifications listed on Vegware's materials page are EN 13432 and TÜV Austria OK Compost — both industrial composting standards, requiring 58°C conditions. OK Compost HOME — the TÜV Austria certification for ambient-temperature garden composting — is not displayed.15 This analysis applies to any retailer using the same certification-to-marketing pattern.
"Certified home compostable" on the product page. Industrial composting certification on the materials page. This is the information gap — and it is not unique to any single retailer.
The Degradation
What happens when PBAT enters soil — through garden composting, digestate contamination, or litter?
It does not disappear. It transforms. And what it transforms into may be worse than the material itself.
PBAT is built from three monomers: adipic acid, 1,4-butanediol, and terephthalic acid. As the polymer degrades, these monomers are released back into the surrounding soil. Two of them are cause for concern.
A 2025 study by researchers at Nanjing Normal University and Nanjing University, published in Eco-Environment & Health, quantified PBAT microplastic and metabolite accumulation in soil over 150 days. They found 1,4-butanediol — the first of the two concerning metabolites — accumulating to 1.6 mg/kg in soil within 30 days from a single experimental loading. To put that concentration in context: the material producing it has a half-life of approximately 453 days in alkaline soil, meaning each addition builds on the last without meaningful clearance. One caddy liner per week, fifty-two weeks per year, in soil that clears the material over years, not weeks. A press release accompanying the study described the 30-day concentration as "high enough to temporarily stress soil organisms."16
The second metabolite — terephthalic acid (TPA) — is structurally a phthalate ester analogue. A 2022 in vitro study at the University of Messina exposed murine preadipocyte cells to TPA and found it triggered dose-dependent increases in adipogenic markers and activated estrogen receptor pathways — effects that were reversed by an estrogen receptor antagonist, confirming the endocrine-mediating mechanism [in vitro, murine cell study].17 A 2023 review in Environmental Science & Technology stated directly: "terephthalic acid, a monomer of poly(butylene adipate-co-terephthalate), is an endocrine disruptor that can harm the endocrine system of organisms" [review].18
A separate 2022 study on Arabidopsis — a model plant — found that PBAT microplastics caused greater disruption to the photosynthetic system than conventional LDPE microplastics. The researchers concluded that "degradation products from PBAT comprising adipic acid, terephthalic acid, and butanediol were more toxic than PBAT-MPs" — the intact microplastic [in vivo plant study].19
This is the toxicity inversion.
The very property marketed as the benefit — biodegradation — is the mechanism producing the harm.
The certification measures whether the material breaks down. It does not measure what it breaks down into.
A necessary caveat — and then a reframe. These studies were conducted at laboratory temperatures, likely 25-28°C. UK garden soil averages 8-12°C. At lower temperatures, PBAT degrades more slowly — which means metabolites accumulate more slowly too. One caddy liner per week in a 200-400kg compost bin produces concentrations far below the study conditions. The metabolite toxicity of PBAT at household scale and UK soil temperatures has not been studied.
But consider what "not studied" means for someone already in the system. If you have been composting bioplastic liners for three years, you have added roughly 156 liners to your compost heap. PBAT's half-life of 453+ days in alkaline soil — in conditions warmer than a British garden — means less than half the material from your first year has degraded. The second year's liners are still largely intact. The third year's are barely started. You are not at zero. You are at an unknown point on a rising accumulation curve, and nobody has measured where any individual garden compost heap sits on that curve. The experiment did not begin when you read this report. It began when you composted the first liner. The absence of a measured dose is not the absence of a dose. It is the absence of knowledge of the dose you already have.
And the soil is not the endpoint. It is the medium.
If you garden-compost — and this is the scenario where material choice matters — your hands are in that soil. Turning it. Spreading it on beds. Working it into the ground where you grow food. Dermal contact with soil is a recognised exposure pathway for organic contaminants; the Environment Agency's Contaminated Land Exposure Assessment framework includes dermal absorption from soil as a standard human health pathway.37 The metabolites that accumulate in PBAT-amended compost are in the same material you handle with bare hands every time you turn the heap.
And if you grow food in that compost — tomatoes, courgettes, salad leaves, the crops a kitchen garden produces — the root zone of those plants sits in soil containing TPA and 1,4-butanediol. Plant uptake of organic contaminants from soil into edible tissue is documented across multiple compound classes; a 2014 review in Environmental Science & Technology found that organic chemicals with moderate hydrophobicity — which includes low-molecular-weight phthalate esters and their analogues — can be taken up through root systems and translocated to shoots and fruit [review].38 TPA is a low-molecular-weight aromatic acid. The pathway from compost to root zone to edible tissue is physically open.
The metabolites do not stay in the dirt. They are in the compost you spread with bare hands. They are in the soil where you grow lettuce. The chain does not terminate at soil organisms. It reaches your kitchen table.
This has not been measured at household scale. The specific concentration of TPA in a tomato grown in home compost containing fifty-two degraded caddy liners has not been quantified. But the exposure pathways — dermal and dietary — are not speculative. They are the standard routes by which soil contaminants reach the human body. What has not been studied is the dose. What has not been studied is not the route.
And the timeline matters. PBAT's half-life of approximately 453 days in alkaline soil — in best-case conditions warmer than a British garden — means each weekly liner addition builds on the previous one without meaningful clearance. At 52 liners per year, the cumulative PBAT load in a home compost system increases year on year. A 2025 study in the Journal of Hazardous Materials documented the degradation timeline: PBAT microplastic formation follows three phases — initial release (0-30 days), peak microplastic generation (60-120 days), and degradation (150-180 days).20 For four to six months after entering soil, PBAT is actively generating more microplastics — fragmenting into smaller particles before those particles begin to degrade. In UK conditions, this timeline stretches further.
Paper in the same compost bin: cellulose. Weeks. Gone.
TIMELINE OF PERSISTENCE
- Time in caddy: 3-7 days
- Time in AD plant: screened out, incinerated (same day)
- Paper in garden compost: weeks
- PBAT in garden compost: half-life 453+ days (lab conditions); years at UK temperatures
- PBAT metabolites in soil: unstudied at household scale
- PBAT metabolites in garden-grown food: unstudied
- Your cumulative PBAT load after 3 years of composting: unknown
The Displacement
If paper liners work, cost the same, and are genuinely home-compostable — why aren't they what you're using?
Because they're not what arrived in the post.
When your council rolled out food waste collection under the Simpler Recycling mandate — Environment Act 2021, Section 57, requiring all English councils to provide weekly food waste collection by 31 March 2026 — it sent you a caddy. And with that caddy: a starter roll of liners. Staffordshire delivered "a small kitchen caddy, a larger outdoor caddy, and a starter roll of liners" directly to homes.21 Waltham Forest provided "a roll of compostable liners to line the caddy with" and a "top-up delivery in 6 months."22 Derbyshire Dales supplies 52 free liners per year per household, conforming to EN 13432 with the Seedling logo.23 Scottish Borders distributed approximately 936,000 free liners annually for ten years before discontinuing in January 2026.24
Those free liners are bioplastic. Every council identified in this investigation distributed bioplastic liners. None distributed paper.
The first caddy liner a consumer uses is chosen for them. And it was free.
It is a procurement architecture. The ESPO Framework 860, Lot 9 — the purchasing framework most councils use for waste collection products — specifies liners that "conform to EN13432, are 100% biodegradable and compostable, are produced from starch-based renewable sources, and have a minimum 12-month shelf life."25 This language describes bioplastic. Paper caddy liners — which are not "starch-based renewable sources" and which may not seek EN 13432 certification because paper composting is a material property, not a credentialled achievement — are not included.
Paper liner manufacturers sell through Amazon and specialist retailers. They are not on the procurement frameworks. They do not arrive in council envelopes. They are not free.
The pattern has a name: The Certification Displacement Pattern — the mechanism by which a working traditional solution is displaced from a market by a certified modern alternative, not because the modern alternative performs better, but because certification creates legitimacy independent of outcome. The traditional solution, which predates the certification system, cannot cross the legitimacy moat that certification creates. Paper doesn't need a logo to compost. But in a market where the logo is the legitimacy signal, "doesn't need a logo" reads as "isn't verified."
The displacement operates through three interlocking mechanisms. First, pro-innovation bias — the documented cognitive tendency to prefer novel solutions over traditional ones, measured in mouse-tracking experiments that show unconscious pull toward innovative options even when the traditional option is functionally equivalent.26 Bioplastic feels like progress. Paper feels like going backwards. Second, the Green Halo effect — the documented tendency for eco-labels to cause consumers to infer quality, sustainability, and value beyond the label's actual scope. A 2021 study found that "providing accurate information did not avoid biased heuristic thinking" — the halo operates below the level of reading the fine print.27 Third, procurement lock-in — the ESPO framework, council free distribution, and EN 13432 specification language that structurally excludes paper without anyone deciding to exclude it.
This investigation continues below.
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The result: only 22% of UK adults say they know what happens to their waste once collected.28 The Seedling logo tells them enough to stop asking.
The Comparison
Modern paper caddy liners are not the fragile, disintegrating-on-contact objects the comparison assumes.
EcoSack produces a double-ply 70gsm waterproofed kraft paper liner, manufactured by The Compost Bag Company Limited from Scandinavian softwood. The double-ply construction provides structural wet strength without reliance on chemical waterproofing agents. The product holds wet food waste for the 3-7 day caddy cycle.29
Alina produces an FSC-certified virgin kraft paper liner with starch-based waterproofing — the cleaner alternative to PAE (polyamidoamine-epichlorohydrin) resin, which accounts for approximately 90% of the wet-strength paper market and generates toxic halogenated byproducts during manufacture.30 Alina's starch-based approach avoids this chemistry. The glue and ink used in Alina's liners are stated as tested for safe composting.31
Paper liners breathe. This is a functional advantage: paper absorbs moisture and allows airflow, reducing odour and leachate. Bioplastic liners trap moisture.
Price: paper caddy liners cost approximately £0.17-0.30 per bag for 10L liners, depending on brand and pack size. Bioplastic liners cost approximately £0.10-0.16 per bag at comparable sizes. Paper is marginally more expensive at point of purchase — pennies per bag — but carries no certification cost premium for a test that never applies to its processing pathway.33
Home compostability — the one scenario the consumer controls — is where the materials diverge decisively. Paper liners decompose in garden compost within weeks. This is not a certification claim; it is what cellulose does.
Bioplastic liners do not. The Big Compost Experiment — a 2022 UCL citizen science study involving 9,701 UK participants — found that 60% of certified home compostable items did not fully disintegrate in home composting conditions.34 A separate 2015 Garden Organic experiment tested a certified home-compostable caddy bag specifically: at 12 months, it had fully degraded in only 26% of compost heaps. Cardboard, in the same heaps, achieved 95%.35
A note on paper's own chemistry: not all paper liners are chemically equivalent. The industry-standard wet-strength agent — PAE resin — generates halogenated byproducts (1,3-DCP and 3-MCPD) described as toxic in a 2023 review of polymeric wet-strength agents.30 Modern formulations have reduced these compounds, and starch-based alternatives (such as Alina's claimed approach) avoid them entirely. When choosing paper liners, the material specification that matters is the waterproofing method: starch-based or mechanical double-ply construction over PAE-treated paper.
A note on Alina: the company sells both paper and bioplastic caddy liners. The recommendation in this report applies to Alina's paper product range only.
The Counter-Position
Paper manufacturing is more polluting than plastic manufacturing. This is real and it matters.
The UNEP Life Cycle Initiative's meta-analysis of single-use bag lifecycle assessments found that paper bags need to be reused four to eight times to match the lifecycle impact of a single-use plastic bag.36 Paper production requires more energy (the kraft pulping process is inherently energy-intensive), generates more solid waste, and consumes substantially more water than plastic film production. These penalties are driven by the pulping process itself and apply at any product scale — a 5-8g caddy liner still requires pulping, pressing, drying, and waterproofing.
This counter-argument is strongest for consumers who do not home-compost. If your caddy liner goes into the food waste caddy, gets collected, arrives at an AD plant, gets screened out, and is incinerated — the end-of-life is identical for paper and bioplastic. Both are burned. For this consumer — and this is the majority — the manufacturing footprint is the only material distinction. And on manufacturing footprint, bioplastic is lighter.
The report acknowledges this honestly. The case for paper does not rest on manufacturing cleanliness. It rests on three things:
First, the information gap. Regardless of which liner a consumer uses, they cannot currently see that the Seedling logo certifies conditions their AD plant doesn't provide, that the material is majority fossil-derived, or that "vegetable oils and thistle starch" is a marketing description of a petroleum copolyester. This gap exists whether or not they garden-compost.
Second, the home-composting escape. For consumers who do compost — and the report is explicit that this is the scenario where material choice matters — paper degrades in weeks without generating persistent microplastics or releasing endocrine-active metabolites into soil. This is the one scenario the consumer controls, and it is the one scenario where paper is unambiguously superior.
Third, the manufacturing data compares shopping bags (40-80g per bag), not caddy liners (5-8g per bag). No lifecycle assessment of paper versus bioplastic caddy liners specifically exists in published literature. The manufacturing penalty is real but its magnitude at caddy-liner scale is unquantified. The report does not claim it is negligible; it flags that the comparison data doesn't match the product.
The honest framing: paper has a higher manufacturing footprint. Bioplastic has a worse end-of-life profile. For non-composters, the practical difference is marginal. For composters, paper wins decisively. For everyone, the information gap is real.
What Would Change This Analysis
Two pieces of evidence would materially update this conclusion.
First: a UK-soil-specific PBAT metabolite study. All current degradation data comes from laboratory conditions at temperatures above typical British garden soil (8-12°C). A study measuring terephthalic acid and 1,4-butanediol concentrations in home compost bins at realistic UK temperatures, with household-scale loading (one liner per week), and ecotoxicological endpoints measured in compost-amended soil, would either confirm or weaken the metabolite toxicity dimension. If cumulative metabolite concentrations at UK temperatures prove negligible, the toxicity inversion becomes academic. If they prove cumulative over years of garden composting, it becomes urgent.
Second: a caddy-liner-specific lifecycle assessment. The manufacturing emissions counter-argument currently relies on shopping-bag data. A cradle-to-grave comparison of paper versus bioplastic 10L caddy liners — covering raw material extraction, manufacturing, transport, use, and end-of-life (including home composting as a disposal route) — would clarify whether the manufacturing penalty holds at this product scale and whether end-of-life advantages offset it.
Neither study currently exists in published literature. Their absence is itself the finding: the material consumers are asked to choose between has not been studied at the scale and conditions of actual use.
The Levers
The certification on your caddy liner certifies conditions your AD plant doesn't provide. Both paper and bioplastic liners are screened out and incinerated. The distinction that matters is what happens when the material enters soil — through garden composting, digestate contamination, or litter. Paper degrades into cellulose. PBAT degrades into metabolites that have not been studied at household scale.
What you can do now — no cost:
Line your caddy with newspaper. Several councils explicitly accept this. Lincolnshire: "you can use whatever works best for you."8 If you garden-compost, compost the paper liner directly — it degrades in weeks. If you are currently putting bioplastic liners in your garden compost: stop. They do not compost at garden temperatures. They fragment into microplastics over months and release degradation products into your soil over years.
What to buy when your current supply runs out:
Paper caddy liners. The material specification that matters:
- Waterproofing: Starch-based (cleanest) or mechanical double-ply construction (avoids chemical wet-strength agents). Avoid PAE-treated paper if disclosed.
- Paper source: FSC-certified or sustainably sourced kraft paper.
- Home compostable: By material property. No certification needed — cellulose composts. OK Compost HOME is a bonus, not a requirement.
- Price benchmark: £0.17-0.30 per bag for 10L.
Two products meeting this specification are currently available:
EcoSack — Double-ply 70gsm waterproofed kraft paper from Scandinavian softwood. EN 13432 certified. Available via Caddy Liners Direct, Amazon, and specialist retailers.29
Alina paper caddy liners (not their bioplastic range) — FSC-certified virgin kraft paper with starch-based waterproofing. EN 13432 certified. Available via Amazon.31
What would close this gap systemically:
Council guidance that distinguishes paper from bioplastic liners and explains the home-composting difference — rather than recommending "compostable liners" as a single category. Procurement frameworks (ESPO Framework 860) that include paper liner specifications alongside bioplastic. PAS 110 digestate testing that distinguishes biodegradable plastic fragments from conventional plastic and monitors below the 2mm sieve threshold. And the EN 13432 revision that EU Regulation 2025/40 already mandates — extended to include metabolite toxicity testing, not just disappearance.
The returning incumbent is not an alternative. It is a restoration. Paper wrapped kitchen waste for centuries before bioplastic existed. It didn't stop working. It was displaced by a certification system that created legitimacy independent of outcome — a system now correcting itself, one regulatory recital at a time. The consumer doesn't have to wait. Paper liners are on Amazon. They cost the same. They compost in your garden. The only thing missing was the information to choose them.
Now you have it.