gomi no genryou ga hituyou desu.

It is read:

ごみ の げんりょう が ひつよう です。

A smooth reading without the spaces is:

ごみの減量が必要です。

That is a very common question.

減量（げんりょう） literally means reduction in amount/quantity.
So yes, it is often used for losing body weight, but it can also mean reducing other things.

In this sentence, ごみの減量 means:

• reduction of garbage
• reducing the amount of trash

So the word is broader than just personal weight loss.

Here, の connects two nouns:

• ごみ = garbage / trash
• 減量 = reduction

So ごみの減量 means the reduction of garbage or garbage reduction.

You can think of の here as showing the relationship between the two nouns, similar to:

• A の B = B of A
• or sometimes A-B in English

So:

• ごみの減量 = garbage reduction

In this sentence, が marks the thing that is being described as necessary.

Structure:

• ごみの減量 = garbage reduction
• が = subject marker
• 必要です = is necessary

So the whole sentence is saying:

Garbage reduction is necessary.

You can think of it as:

• X が 必要です = X is necessary / There is a need for X

Here, X is ごみの減量.

Because 必要 is a na-adjective / adjectival noun meaning necessary.

Japanese often makes sentences like this without using a verb like need in the same way English does.

So:

• 必要です = is necessary

This is a very common pattern:

• お金が必要です。 = Money is necessary. / I need money.
• 時間が必要です。 = Time is necessary. / We need time.

So ごみの減量が必要です is a normal Japanese way to say that reducing garbage is necessary.

Yes, ごみの減量は必要です is also possible, but the nuance changes a little.

• ごみの減量が必要です
  focuses on garbage reduction as the thing that is necessary

• ごみの減量は必要です
  presents garbage reduction as the topic, often with a nuance like
  As for garbage reduction, it is necessary

In many contexts, both are natural.
The が version is often a straightforward statement of necessity.

です makes the sentence polite.

Compare:

• ごみの減量が必要です。 = polite
• ごみの減量が必要だ。 = plain / less polite

So this sentence is appropriate in:

• public announcements
• school materials
• formal writing
• polite speech

It is natural, especially in formal or public-context language such as:

• environmental campaigns
• government notices
• school or workplace documents

A more action-focused version would be:

• ごみを減らす必要があります。
  = It is necessary to reduce garbage.

Difference:

• ごみの減量が必要です sounds a bit more formal and noun-based
• ごみを減らす必要があります sounds more verbal and direct

Both are correct and natural.

Because ごみ is very often written in hiragana in everyday Japanese.

It can be written as ゴミ in katakana or sometimes 塵 in kanji, but:

• ごみ is common and easy to read
• hiragana often feels neutral and standard in public writing

So seeing ごみ in hiragana is completely normal.

Not exactly.

必要です means is necessary or is needed, which is softer and more descriptive than must.

So:

• ごみの減量が必要です = Garbage reduction is necessary
• not exactly Garbage must be reduced, although that may be a natural English translation depending on context

If Japanese wants to sound more like must, it may use stronger expressions such as:

• ごみを減らさなければならない
• ごみを減らすべきだ

So 必要です expresses necessity, but not always as strongly as must.