El compostaje doméstico usa el cubo de orgánico.

Because compostaje is a masculine noun in Spanish.

A very useful pattern is that nouns ending in -aje are usually masculine, for example:

• el viaje
• el paisaje
• el reciclaje
• el compostaje

So the sentence correctly has el compostaje.

Because doméstico is describing compostaje, and compostaje is masculine singular.

In Spanish, adjectives usually agree with the noun they describe:

• el compostaje doméstico
• la basura doméstica

So here doméstico matches compostaje, not some other implied noun.

Because the subject is el compostaje doméstico, which is singular.

So the verb must also be singular:

• El compostaje doméstico usa... = singular
• Los sistemas de compostaje usan... = plural

Even though compostaje refers to a process or activity, grammatically it is still a singular noun.

In Spain, orgánico is often used as a shorthand noun meaning organic waste.

So el cubo de orgánico means something like:

• the organic-waste bin
• the bin for organic waste

Here, orgánico is not just an adjective. It is being used as a noun-like label for a type of rubbish/waste.

A fuller, less shortened version could be:

• el cubo de residuos orgánicos
• el cubo para la basura orgánica

But in everyday Spanish, shorter forms like el cubo de orgánico are common.

Because de shows what the bin is for or what it contains.

This is very common in Spanish:

• cubo de papel
• contenedor de vidrio
• bolsa de plástico

So cubo de orgánico literally works like bin of organic waste, but in natural English we would usually say organic bin or bin for organic waste.

If you said only el cubo orgánico, it would sound more like the organic bin in the sense that the bin itself is organic, which is usually not the intended meaning.

Because after de, Spanish often leaves out the article when talking about a material, category, or type in a general way.

Compare:

• un vaso de agua
• un cubo de basura
• un contenedor de vidrio
• un cubo de orgánico

Here orgánico is being treated as a category label, so no article is needed.

If you added an article, the meaning or tone could shift, and in this kind of recycling vocabulary, de orgánico is the most natural everyday phrasing.

Here it is functioning like a noun.

Normally orgánico is an adjective:

• residuos orgánicos
• materia orgánica

But Spanish often allows adjectives to be used on their own when the noun is understood from context. In this sentence, orgánico stands for something like:

• residuos orgánicos
• basura orgánica
• materia orgánica

So a learner can think of it as a shortened noun phrase.

It is understandable, but many speakers might prefer a slightly more natural version depending on context.

Possible alternatives:

• Para el compostaje doméstico se usa el cubo de orgánico.
• En el compostaje doméstico se usa el cubo de orgánico.
• El compostaje doméstico utiliza el cubo de orgánico.

Why? Because El compostaje doméstico usa... makes composting sound like the active subject doing the action. That is grammatically possible, but Spanish often prefers se usa or a phrase with para when giving general instructions or explanations.

So the original sentence is understandable, but in an explanatory text, Para el compostaje doméstico se usa... may sound more idiomatic.

Yes. Both are correct.

• usa = more common, simpler, everyday
• utiliza = slightly more formal or technical

So these both work:

• El compostaje doméstico usa el cubo de orgánico.
• El compostaje doméstico utiliza el cubo de orgánico.

In ordinary speech, usar is usually the more basic and common verb.

Yes, literally cubo often means bucket. But in Spain it can also refer to a household bin, especially a small indoor bin used for separating waste.

So in recycling contexts:

• cubo often suggests a smaller bin at home
• contenedor usually suggests a larger public/street container

That means:

• el cubo de orgánico = the organic-waste bin at home
• el contenedor de orgánico = the larger collection container outside

This is a useful Spain-specific vocabulary distinction.

They show where the stress goes:

• do-MÉS-ti-co
• or-GÁ-ni-co

Without the written accent marks, Spanish stress rules would suggest a different pronunciation, so the accents are necessary.

They also help you recognise that both words are esdrújulas (stress on the third-to-last syllable), and all such words in Spanish always carry a written accent.