Composting in an HDB Flat: Small-Space Methods That Work in Singapore's Climate

Inside an HDB flat, the only composting methods that work without outdoor space, a balcony, or neighbours complaining about smell are a bokashi bin or a small vermicomposting bin. Traditional aerobic composting needs a garden pile or a large tumbling bin. It demands volume, air circulation, and a buffer zone that a high-rise flat simply does not provide. Singapore's climate accelerates decomposition and odour. Any method that depends on open airflow and frequent turning will generate enough smell to reach the corridor. This guide compares bokashi and vermicomposting on the criteria that matter inside an HDB unit: odour risk, space footprint, the type of food scraps each handles, what the output is and what you do with it, and the ongoing cost for consumables. It also quantifies how much food waste a single household can divert from incineration, using NEA's 2023 data that shows 755,000 tonnes of food waste generated nationally, of which only 18% is recycled.

Aerobic composting requires a carbon-to-nitrogen ratio of roughly 30:1, a moisture content around 50%, and enough oxygen that the pile stays below 65°C. Inside an HDB unit, the only place to achieve that is a ventilated balcony. Even then, the bin must be large enough to hold at least a cubic metre of material to maintain thermophilic temperatures. Without that volume, the pile stays cool, attracts fruit flies, and produces a sour smell as anaerobic pockets form. Singapore's humidity pushes the moisture content above the ideal range within hours of adding wet food scraps. The only cure is adding dry browns: shredded paper, cardboard, dried leaves. Most flat residents do not have these in the quantities needed. The result is a bin that smells of rot within a week, generates leachate that stains the floor, and attracts cockroaches. No balcony? Aerobic composting is not viable. A balcony with a bin smaller than one metre tall and wide? It will fail.

A bokashi bin is a 15- to 20-litre bucket with a tight-sealing lid and a tap at the bottom. You add food scraps in layers, sprinkling each layer with inoculated bran. The process is anaerobic fermentation, not decomposition. The bran microbes pickle the food waste, suppressing putrefaction and producing a sweet-sour smell that does not carry beyond the bin. The tap drains a dark liquid. Collect it every few days and dilute it 1:100 with water to use as plant fertiliser or pour it down the sink.

What the Fermentation Bucket Handles

The bin accepts all kitchen food waste: cooked food, meat, fish, dairy, bones, citrus, onion skins, and mouldy leftovers. No other indoor method handles meat and dairy safely. It fits under a kitchen sink or on a small counter. A 15-litre bin processes roughly 1-2 kg of food scraps per week for a two-person household.

Odour and Pest Risk

If the lid is sealed, the bin produces no detectable odour. If the seal fails or you leave the lid off, the smell turns foul within hours. To prevent fruit flies, bury each new layer under the previous layer of pickled waste and keep the lid closed. A properly sealed bin does not attract ants or cockroaches because the fermented environment is acidic.

Output and What to Do With It

The fermented waste is not yet compost. After two weeks, you have pickled scraps that need to be buried in soil for another 2-4 weeks to mature into usable compost. Without access to soil, you have three options: bury it in a planter box on a common corridor (if your HDB block has one), give it to a community garden, or bag it and drop it at an NEA-designated food waste collection point. The pickled waste has a strong acidic smell when first opened. It dissipates within minutes.

Ongoing Cost

The inoculated bran costs roughly SGD 15-25 per kilogram. A 1 kg bag lasts a two-person household about 2-3 months. The bin itself is a one-time purchase of SGD 30-60. The ongoing consumable cost is the bran refill, roughly SGD 60-120 per year.

Criterion	Bokashi Bin	Vermicomposting Bin
Space required	15-20 litre bucket; fits under sink or on counter	20-30 litre bin; needs counter or shelf space
Odour risk	None when sealed; sour-sweet when opened	Minimal if managed; earthy, not rotten
Food waste types	All kitchen waste including meat, dairy, bones, citrus	Fruit and vegetable scraps only; no meat, dairy, oily food, citrus
Processing time	2 weeks fermentation + 2-4 weeks soil maturation	3-4 months for finished vermicompost
Output	Pickled waste (needs soil); leachate for plants	Castings (solid) and worm tea (liquid)
Consumable cost per year	SGD 60-120 (bokashi bran)	SGD 30-50 (worm bedding, new worms twice a year)
Suitable for composting without balcony Singapore	Yes, fully indoor	Yes, fully indoor
Suitable for odour-free composting HDB Singapore	Yes, when sealed	Yes, if managed correctly

A vermicomposting bin uses composting worms. The most common species for indoor use is Eisenia fetida, the red wiggler. They consume food scraps and produce worm castings, a nutrient-rich soil amendment. The bin is a shallow container (20-30 litres, 15-20 cm deep) with bedding made of shredded newspaper or coconut coir. Add food scraps in small quantities every few days. The worms eat them, leaving castings that are harvested every 3-4 months.

What Worms Eat and What They Reject

The worms cannot handle meat, dairy, oily food, citrus, onion, or garlic. They thrive on fruit and vegetable peelings, coffee grounds, tea bags, crushed eggshells, and bread. Adding the wrong waste causes the bin to smell and attracts pests. Keep the bin in a temperature range of 15-25°C. This is achievable inside an air-conditioned apartment. In a non-AC unit, Singapore's heat stresses the worms. Place the bin in the coolest part of the flat, away from direct sunlight.

Odour Control and Fruit Fly Prevention

A healthy worm bin smells like damp earth. A sour or ammoniac smell means you are overfeeding. Stop feeding for a week and add dry bedding. Fruit flies appear when exposed food scraps sit on the surface. Bury scraps under 5 cm of bedding and cover the bin with a sheet of cardboard or newspaper. A tight-fitting lid is essential. Odour-free worm composting depends on the bin being closed.

Output and What to Do With It

Worm castings are a finished fertiliser. Mix them directly into potting mix for houseplants or balcony plants. The liquid that drains from the bin, worm tea, is diluted 1:10 with water and used as a foliar feed. Both outputs require plants to use. If you have no plants, give the castings to a neighbour or drop them at a community garden.

Ongoing Cost

A starter culture of red wigglers costs roughly SGD 30-50. Bedding materials (coconut coir or shredded paper) are cheap or free. New worms breed in the bin, but in Singapore's climate the population needs replenishing roughly twice a year if the bin is kept indoors without AC. Total consumable cost is SGD 30-50 per year.

The failure case for both methods is the same. You start with enthusiasm, overfill the bin, fail to manage the output, and quit after the first fruit fly outbreak or sour smell. For the fermentation bucket, the mistake is leaving the lid loose. For the worm bin, it is adding too much food too fast. Start small. Get a 15-litre bucket or a 20-litre worm bin. Commit to emptying the fermented waste into soil or giving the worm castings to a neighbour within two months. If you have no access to soil and no neighbour who gardens, choose the fermentation method and use the NEA's food waste collection points. That step, finishing the output, is what turns a composting attempt into a real diversion. Without it, you just have a bin that fills up and gets thrown into the refuse chute.