El compostaje doméstico reduce el residuo orgánico.

In this sentence, el compostaje doméstico is the subject, so Spanish normally uses the definite article here.

Spanish often uses el / la / los / las more than English does, especially when talking about something in a general sense:

• El compostaje doméstico reduce... = Home composting reduces...
• Literally, it looks like The home composting reduces...

This is very natural in Spanish. English often drops the in general statements, but Spanish frequently keeps it.

Here doméstico means household or at home.

So compostaje doméstico means:

• home composting
• household composting

Although doméstico can sometimes translate as domestic, in this context household or home sounds more natural in English.

In Spanish, adjectives usually come after the noun.

So:

• compostaje doméstico = domestic/home composting
• residuo orgánico = organic waste

This is the normal word order in Spanish:

• noun + adjective

English usually does the opposite:

• adjective + noun

So this is a very common pattern learners need to get used to.

Reducir is the infinitive, meaning to reduce.

In the sentence, the verb is conjugated:

• reducir = to reduce
• reduce = it reduces

The subject is el compostaje doméstico, which is singular, so the verb must also be singular:

• El compostaje doméstico reduce...
• Home composting reduces...

This is the third person singular of reducir in the present tense.

Because the subject is he / she / it form in Spanish.

Spanish verb endings show who is doing the action:

• reduzco = I reduce
• reduces = you reduce
• reduce = he/she/it reduces

Here the subject is el compostaje doméstico, which is treated as it, so:

• reduce = reduces

Again, Spanish often uses the definite article where English may not.

So:

• reduce el residuo orgánico

literally looks like:

• reduces the organic waste

But in context, this can simply mean:

• reduces organic waste

Spanish often prefers the article with nouns used in a general or category sense. This is very common and natural.

Spanish often uses a singular noun where English may use an uncountable noun or a broader category word.

Here el residuo orgánico means:

• organic waste
• organic refuse
• organic waste material

Even though English does not usually say an organic waste in this kind of sentence, Spanish can use singular residuo to refer to the category as a whole.

You could also see plural forms in other contexts, but the singular here is perfectly normal.

Both can relate to waste or trash, but they are not always used in exactly the same way.

• residuo is often more technical, formal, or environmental
• basura is more everyday and often means trash or garbage

In a sentence about composting, recycling, or environmental policy, residuo orgánico sounds more precise and appropriate than basura orgánica.

So residuo is a very common word in environmental vocabulary.

Often yes, but the exact nuance depends on context.

In residuo orgánico, it usually refers to organic matter, meaning waste of biological origin, such as:

• food scraps
• plant material
• biodegradable material

So here orgánico is not mainly about organic food certification. It is more about the type of waste that can decompose naturally.

Because reducir usually takes a direct object in Spanish.

So:

• reducir algo = to reduce something

Examples:

• reduce el consumo = it reduces consumption
• reduce los costes = it reduces costs
• reduce el residuo orgánico = it reduces organic waste

No preposition is needed before the thing being reduced.

In Spain, compostaje is pronounced approximately:

kom-pos-TA-he

A few useful points:

• j sounds like a strong English h, but rougher: he
• the stress is on ta
• g before e in orgánico would be a hard g, but here in compostaje the j is the rough sound

A rough pronunciation guide:

• compostaje doméstico reduce el residuo orgánico
• kom-pos-TA-he do-MES-ti-ko re-DU-the el re-SI-du-o or-GA-ni-ko

In much of Spain, c before e/i and z sound like the th in thin, but that does not affect compostaje itself.

Yes, casero is possible in many contexts, and it can also mean home-made or home/home-based.

But there is a slight nuance:

• doméstico sounds a bit more neutral, formal, or technical
• casero can sound more everyday

In environmental or informational writing, compostaje doméstico is often the more standard choice. So the original sentence sounds very natural for Spain.