Building a balcony compost system

Swiss households throw 52.4 kilos of organic waste per person per year into the taxed rubbish bag, of which 22.9 kilos are vegetable and fruit peelings. That figure comes from hand-sorting 500 kilos of rubbish across 33 municipalities, an exercise the Federal Office for the Environment runs every ten years. A properly sized balcony composter deals with the peelings, which is the single heaviest slice of that stream. It will not deal with cooked leftovers, dairy or meat, and those together weigh more.

That is the first thing worth knowing before buying a bin: balcony composting solves part of the problem, not all of it.

What is actually in your rubbish bag

The Swiss waste composition survey has run every decade since 1982. In 2022 the surveyors sorted bag contents into 32 waste types grouped into 14 fractions. Extrapolated nationally, that gives just under 1.3 million tonnes of household rubbish, or 148.2 kilos per person per year.

The organic fraction weighs 52.4 kilos per person, 35.4 % of the bag by weight. It is the heaviest of the fourteen, ahead of composite items (26.6 kg), plastics (19.9 kg) and paper (17.6 kg). The breakdown inside that fraction is where it gets interesting:

Waste typekg per person per yearShare of the bag
Peelings22.915.5 %
Other food, cooked and processed16.511.1 %
Uncooked fruit and vegetables5.73.9 %
Garden waste, meat and fish, dairy, liquidsroughly 7.3 combinedthe remainder

Peelings alone are the heaviest single waste type out of all 32 categories sorted. If you are vegetarian and cook a lot of fresh vegetables, your own share is probably higher than the national average.

The federal office puts the untapped material recovery potential still sitting in the bag at 31.3 kilos per person, of which 18.9 kilos are organic waste. Scaled up, that is 167,112 tonnes a year going to incineration instead of composting or anaerobic digestion.

The methane argument does not apply in Switzerland

Most composting guides explain that organic waste buried in landfill produces methane, a far more potent greenhouse gas than CO2, and that composting avoids those emissions. The reasoning is sound in a country that landfills its rubbish. It does not describe Switzerland.

Landfilling combustible waste has been banned since 1 January 2000. Swiss household rubbish goes to incineration plants, where it burns and feeds district heating and electricity networks. An apple core dropped into a taxed bag in Lausanne does not ferment in a landfill; it burns, carrying a high water content that drags down the plant’s energy yield.

So the real benefit of composting in Switzerland is a different one, and it holds up well. The Waste Ordinance requires under article 14 that suitable organic waste be collected separately and either materially recovered or digested. Composting at home closes that loop at the shortest possible scale: organic matter returns to a growing medium with no transport, no industrial processing and no burning of a water-logged material.

There is a caveat the Swiss technical literature states plainly. The report commissioned by the federal office on national composting data notes that emission factors for home composting are so heterogeneous that the uncertainty applied in the Swiss inventory is 100 %. The stated reason is blunt: there is no professional standard for how private individuals run the process, therefore no definable state of the art. A poorly aerated compost emits methane and nitrous oxide. A well-run one emits little. The difference lies entirely in how you manage it, which is what the rest of this article is about.

The same report points out that home composting does not even count as waste under Swiss law, because no change of ownership occurs. Nobody weighs what happens on balconies. Published estimates range from 14 to 63 kilos per inhabitant per year depending on the source, a fourfold spread that says plenty about how poorly measured this is.

Choose between the three systems by constraint, not by benefit

Comparisons usually present the three systems through their strengths. That is the wrong way in. Each has a constraint that rules it out in certain situations, and the constraint is what should decide.

The classic aerobic bin

A ventilated container alternating nitrogen-rich and carbon-rich material, with bacteria and fungi breaking it down in the presence of oxygen. Its constraint is volume: you need enough mass for the process to establish itself and enough room to turn it. Below roughly 40 usable litres, running it on a balcony becomes awkward.

Its strength is that it accepts woody material, dead leaves, stems and cardboard, and produces an amendment you can use directly when repotting.

The wormery

Epigeic worms, usually Eisenia fetida, work through organic matter in a stack of trays. The constraint is thermal and biological: these are live animals, sensitive to frost and overheating alike, and they need feeding regularly, including while you are away.

Ethiopian work published in Scientific Reports gives a sense of what worm digestion actually changes. Over sixty days of treatment with Eisenia fetida, the carbon to nitrogen ratio fell from 21.39 to 8.63, total nitrogen rose from 1.38 to 2.19 %, available phosphorus from 11.17 to 20.85 ppm and exchangeable potassium from 2.26 to 4.43 mg/kg. The worms concentrate nutrients while the carbon is respired away. That, rather than any supposed speed advantage, is the real point of the method.

A study on rabbit manure and kitchen waste blends in Scientific Reports adds something useful for an urban balcony: treatments heavy on plant waste showed lower pH, lower electrical conductivity and lower bulk density, but higher organic matter, porosity and water retention. A wormcast made only from peelings is therefore a structural amendment more than a concentrated fertiliser, which suits containers well.

Bokashi

Fermentation in a sealed bucket with inoculated bran, without oxygen. Its constraint is that it does not produce compost. It produces an acidic fermented mass that then has to be buried in soil or added to a composter to finish breaking down. In a flat with no access to a container of earth at all, it is a dead end.

A participatory pilot study in FEMS Microbes followed six urban bokashi practitioners with 16S sequencing on 34 samples and shotgun metagenomics on 11. Microbial communities were dominated by lactic acid bacteria of the genera Lentilactobacillus and Lacticaseibacillus, metabolic pathways for organic acid production were present, and potentially pathogenic organisms appeared only in extremely low abundance. The authors conclude that urban bokashi is unlikely to carry increased health risks. They also note, and this matters for calibrating expectations, that applying the ferment or its leachate to soil supplies nutrients and organic acids that promote plant growth without substantially changing the soil’s microbial community composition.

So a bokashi bucket does not seed your potting mix with microbial life. It turns your waste into a pre-digested material that still needs burying.

Size the bin from your own numbers

Sizing is a calculation, not a guess. Start from the 22.9 kilos of peelings per person per year in the federal survey, adjust for how you cook, and convert to volume.

Fresh fruit and vegetable peelings are mostly water and have a loose bulk density of roughly 0.5 to 0.6 kg per litre. For one person, 22.9 kilos a year works out at about 40 litres of fresh material annually, or 750 millilitres to a litre a week. A vegetarian who cooks a lot of fresh produce can easily double that.

To that volume of nitrogen-rich material you need to add roughly the same volume of dry carbon-rich material: torn brown cardboard, dead leaves, untreated wood shavings. So budget two to four litres of weekly input for a two-person household.

Material loses around 75 % of its volume as it breaks down. For a couple, a bin of 60 to 80 usable litres is ample, with a residence time of several months. A 200-litre bin sold for gardens is useless on a balcony: it will never fill fast enough for the process to establish properly, and it takes up space you will resent losing.

The carbon to nitrogen ratio is the only setting that really matters

Every problem with home composting comes down to two variables: the ratio of carbon to nitrogen, and moisture. Everything else follows from those.

Peelings are what gardeners call greens: nitrogen-rich, water-rich, structurally weak. On their own they compact, drive out oxygen, and decomposition tips into anaerobic fermentation. That is where the sour smell, the fruit flies and the dark liquid at the bottom of the bin come from.

Browns supply carbon and structure: unprinted packaging cardboard, egg boxes, paper tubes, dead leaves, untreated wood shavings, dry stems. They resist compaction and keep air channels open.

The vermicomposting blend study cited above identified a carbon to nitrogen ratio of 21 as optimal for enzyme activity and microbial diversity. For a hand-managed balcony bin, one rule is enough: with every addition of peelings, cover with at least an equal volume of brown material torn into pieces a few centimetres across. When in doubt, add more brown. Too much carbon makes a slow compost; too much nitrogen makes a foul one.

For moisture the test is manual. Take a handful and squeeze: the material should hold together without a drop appearing between your fingers. If it drips, add dry cardboard and turn. If it crumbles apart, water lightly.

Running it week by week

Here is the method that works on a balcony, in six points.

Lay a drainage base of five to ten centimetres of torn cardboard and thin twigs before the first addition. That layer stops compaction at the bottom and keeps liquid from pooling.

Chop peelings into pieces of two to three centimetres. Decomposition speed depends on the surface area exposed to micro-organisms. Whole squash peel will sit there for months; the same peel diced will be unrecognisable in a few weeks.

Add and cover immediately. Fresh material left exposed is what attracts flies. Covering is not an optional refinement, it is the central habit of running an urban compost.

Turn every two to three weeks with a small fork or a corkscrew aerator. Oxygen is the limiting factor in a small-volume bin. On a balcony, regular turning makes far more difference than any shop-bought activator.

Shelter it from driving rain and direct sun. A waterlogged bin goes anaerobic; a bin cooking in full summer sun dries out and the process stops. Partial shade, raised a few centimetres with a saucer underneath to catch any leachate, suits most situations.

Let it mature before use. This is the step most often skipped, and the next section explains why it counts.

Compost maturity is an overlooked safety point

Immature compost is phytotoxic. It is still consuming nitrogen and oxygen as it decomposes, and in contact with roots it competes with the plant instead of feeding it. The symptoms are yellowing, stalled growth, sometimes dead seedlings.

A study of microbial indicators in municipal solid waste compost in the Journal of Environmental Health Science and Engineering measured germination index and microbiological quality across compost samples and found a high fraction of immature samples, with faecal enterococci detected at relatively high concentrations in every sample and surviving better than other indicators after land application. That was municipal waste compost, not domestic plant-peel compost, and the risk does not transfer directly. The principle does: maturity is something you verify, not something you assume.

The signs of mature compost on a balcony are tangible. The material is dark, crumbly and uniform, with no recognisable original inputs. It smells of woodland floor, never sour or ammoniacal. Its temperature has returned to ambient. Depending on season, volume and how finely you chopped, expect three to eight months between the last addition and that state.

A simple test before use: sow ten cress or radish seeds in a pot of the compost in question, and ten more in fresh potting mix. If germination in the compost is clearly lower or the seedlings yellow, it is not ready. Give it another month and repeat.

Using compost without burning your plants

Home compost is not potting mix. Its nutrient concentration and water-holding capacity make it an amendment to be blended, not a growing medium to use neat.

When repotting, mix one part mature compost to four or five parts potting mix, so 20 to 25 % of the blend. For hungry plants such as balcony tomatoes you can push to 30 %. Beyond that, water retention becomes excessive and root aeration suffers.

As a top dressing, spread one to two centimetres around established plants without mixing it in, then water. Nutrients will work their way down gradually. This is the safest method for plants already in place, since it avoids direct contact between roots and fresh compost.

A review in the International Journal of Food Science on organic fertilisers derived from food waste sums up the state of knowledge well: the potential to improve soil organic matter, water retention and micronutrient supply is solid, but outcomes remain highly variable depending on feedstock, processing and application practice, and unresolved risks persist around heavy metals, microplastics and pathogen survival. On a balcony fed with peelings from shop-bought vegetables those risks are low, but they depend on what you put in.

The liquid collecting under a wormery is not a miracle liquid feed. It is leachate, to be diluted heavily, on the order of one part to ten parts water, and used promptly. It has none of the stability of mature compost.

What actually attracts pests

Fear of rats and flies is the first objection to an urban composter, and it deserves a mechanical answer rather than a reassuring one.

Flies, mostly fruit flies and fungus gnats, lay on exposed sugary material. They have no interest in a surface covered with dry cardboard. If you see them, fresh additions are sitting uncovered or the bin is too wet. The fix is immediate: turn, add browns, cover.

Rodents are after fats, proteins and warmth. A closed bin receiving no meat, fish, dairy or greasy cooked leftovers holds little appeal. On an upper-floor balcony the question rarely arises. At ground level or in a courtyard, a lockable lid and a solid base are the minimum.

The white filamentous mould that appears on cardboard consists of normal, desirable decomposer fungi. It signals that the process is working. There is no need to remove it.

One note of caution belongs here, even though it mainly concerns professionals. A study of bioaerosol exposure in biodegradable waste facilities published in Frontiers in Public Health measured, across six composting facilities, Aspergillus fumigatus concentrations reaching 2.4 × 10⁵ copies per cubic metre and endotoxins frequently exceeding the occupational exposure value of 90 EU/m³. Those levels come from industrial halls handling tonnes of material, not a 60-litre bin on an open balcony. The precautionary principle still holds for anyone immunocompromised, asthmatic or allergic to moulds: turn the heap in open air, preferably while the material is damp so fewer spores disperse, and keep your face away from the bin as you turn it.

Volatile compounds are a siting question

Compost releases volatile organic compounds, and the quantity depends heavily on what goes in. A study in Atmosphere measured VOCs from yard waste compost and from mixed food and yard waste. Methanol and ethanol were more than a thousand times more concentrated in the mixtures containing food waste than in yard waste alone, with ethanol reaching 27,000 ppm in the hottest piles, while terpenes were slightly higher in yard waste alone. The authors note that methanol was observed above permissible indoor levels.

Those measurements come from professional windrows of several cubic metres, where the temperature rise is nothing like that of a small bin. They still carry a direct practical consequence: a composter fed with kitchen waste belongs outdoors, on an open balcony or terrace, not in a closed utility room or unventilated storeroom. Wormeries and bokashi buckets, which stay at ambient temperature, are the only systems reasonably suited to an indoor space, and even then only a ventilated one.

What you can compost, and what causes trouble

Fruit and vegetable peelings, cores, leafy tops, spent houseplant foliage, coffee grounds with their paper filter and unprinted brown cardboard form the core of the stream. They break down well and cause no particular difficulty.

A few materials call for nuance rather than prohibition. Citrus acidifies the mix and slows microbial activity when it dominates; in moderate proportion within a varied blend it passes without trouble. Onion and garlic have the same reputation, for the same reasons and with the same answer. Eggshells supply calcium but break down extremely slowly: crush them finely or you will still be finding them whole a year later.

Some materials have no place in a balcony bin. Meat, fish, dairy and greasy cooked leftovers attract pests and smell strongly in a small, poorly aerated volume. Yet they account for 16.5 kilos per person per year in the federal statistics, which is exactly why a home composter never replaces municipal organic collection where it exists: industrial digestion plants do handle them.

Diseased plants leave the domestic loop, because a balcony bin never reaches the thermophilic temperatures that sanitise professional windrows. Bags and cutlery labelled compostable or biodegradable are designed for industrial temperature and humidity conditions that a 60-litre bin does not reproduce: they come out intact or fragmented. Treated wood, coal ash and vacuum cleaner bag contents do not belong there either.

Five mistakes that come up every time

Composting without browns. This causes the large majority of failures. Keep a bag of torn cardboard permanently beside the bin. Without a reserve within reach, you will not cover your additions.

Choosing a bin that is too big. An oversized volume fills too slowly, the material dries on top and compacts below, and the process never properly starts.

Using compost too early. Impatience is understandable after months of waiting, but immature compost damages the plants it was meant to feed. The germination test costs ten seeds and a fortnight.

Leaving the bin in full sun. On a south-facing balcony, summer dries the material out and decomposition stops. Partial shade, or simply a piece of cardboard laid over the lid, is enough.

Confusing bokashi with compost. A bokashi bucket produces acidic fermented material, not a finished amendment. Without a pot of soil or a composter to receive it afterwards, the chain stops halfway.

Adapting the system to your situation

Studio with no balcony and no access to soil. None of the three systems works end to end. Look instead for a municipal organic waste collection point or a neighbourhood composter; many Swiss towns have installed them.

Balcony of two to four square metres, one or two people. An indoor wormery or a small 60-litre aerobic bin. The wormery if space is tight and you worry about odours, the aerobic bin if you also generate plant waste.

Balcony over six square metres with plantings. An aerobic bin of 80 to 120 litres, fed also by stems and leaves from your plants. This is the configuration where the loop genuinely closes, since the compost produced returns to the pots that supplied part of the material.

Frequent absences. Avoid the wormery: worms will not survive three weeks without feeding in summer. An aerobic bin tolerates temporary neglect without harm.

Flat with an unheated balcony in a cold region. Vermicomposting stops below roughly 10 °C and the worms die in frost. Plan to bring the bin into a frost-free space over winter, or choose an aerobic system that will simply slow down.

What these data do not tell you

The 52.4 kilos of organic waste per person come from a sample of 33 municipalities and around 500 kilos of hand-sorted waste. The national extrapolation of total waste weight carries a 95 % confidence interval of 132 to 164 kilos per person per year, as the report itself states. National averages also mask substantial differences between urban and rural municipalities, and between charging systems.

Swiss home composting figures are estimates, not measurements. The official report applies 100 % uncertainty to the quantities, because nothing is weighed when no change of ownership takes place.

The vermicomposting work cited was conducted in Ethiopia and on blends including rabbit manure, under climatic conditions and with substrates unlike those of a Swiss balcony. The orders of magnitude for how the carbon to nitrogen ratio and nutrients shift remain instructive; the absolute values do not transfer.

The volatile compound and bioaerosol measurements come from professional facilities handling tonnages, not from domestic bins. They indicate a direction, not an exposure level on a balcony.

Finally, no available study quantifies the net carbon balance of a Swiss balcony composter against incineration with energy recovery. Both routes have balances that depend on local parameters: plant efficiency, collection distance, how well the compost is run. Anyone stating a precise figure on this point is claiming more than the data support.

Frequently asked questions

Does a balcony composter smell? A well-run aerobic compost smells of woodland floor. A sour, ammoniacal or putrid smell signals excess moisture, lack of oxygen or uncovered nitrogen-rich additions. All three are corrected by adding browns and turning.

How long before the compost is usable? Three to eight months depending on season, how finely you chopped and how regularly you turned. Vermicomposting is faster on small volumes; bokashi needs two to four weeks of fermentation followed by several weeks buried.

Can you compost through winter on a balcony? Yes, but decomposition slows sharply below 10 °C and effectively stops near freezing. Keep adding and covering: the material will restart in spring. For a wormery, move the bin somewhere frost-free.

Should you buy a compost activator? No. The necessary micro-organisms already live on the plant material. A handful of mature compost or garden soil does the same seeding job for nothing.

Does home compost replace shop-bought fertiliser? Not entirely. It supplies organic matter, improves substrate structure and water retention, and provides slow-release nutrients in variable amounts. For demanding container crops over a full season, a supplementary feed is often still needed.

Can compostable bags go in the bin? No, unless they explicitly state home composting. Most certifications target industrial conditions, at temperatures a balcony bin never reaches.

Can wormery leachate be used neat? No. It is a leachate to dilute at around one part to ten parts water. Used neat it can burn fine roots.

What should I do with surplus compost? Community gardens, urban allotments and gardening neighbours will happily take it. Some municipalities also accept surplus in their green waste collection.

Where to go next

Composting changes how you look at what you peel. Once the bin is running, a good deal of what used to go straight in ends up in a stock, a jar of pickles or a batch of peel crisps first, and leafy tops get turned into pesto rather than chopped for the bin. The same logic applies to growing your own: the compost you produce goes straight back into herbs grown on the balcony and into the substrate for micro-greens on a windowsill.

A balcony composter fits naturally into the wider approach set out in balcony permaculture basics, where waste, growing and harvesting form one system rather than three separate hobbies. It also supports a winter balcony garden of lettuce, kale and mizuna, which keeps producing material long after the summer crops are done.

On supply, a seed swap between neighbours complements a productive balcony well. For the kitchen side, keeping fresh herbs year round and local medicinal herbs worth cooking with both benefit from pots refreshed with your own compost, while forgotten vegetables such as Jerusalem artichoke and parsnip and local legumes generate very little waste for the nutrition they deliver.

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