namagomi ha mizuke ga ooi desu.

It is read:

なまごみ は みずけ が おおい です。

In normal Japanese writing, it would usually appear without spaces:

生ごみは水気が多いです。

Word readings:

• 生ごみ = なまごみ
• 水気 = みずけ
• 多い = おおい

Because 生 has multiple readings in Japanese, and in this word it uses the kun-reading なま.

So:

• 生 can be read in many ways, such as せい, しょう, なま, い-きる, う-まれる, etc.
• In 生ごみ, it means something like raw / fresh / uncooked, so the reading is なま.

This is just something you have to learn as a vocabulary item:

• 生ごみ = なまごみ

生ごみ usually means kitchen waste, food scraps, or organic household garbage.

It often refers to things like:

• vegetable peels
• leftover food
• fruit skins
• other wet, decomposing trash from cooking or eating

So it is more specific than just ごみ (garbage / trash).

水気 means moisture, wetness, or water content.

That is different from just 水, which usually means water itself.

So:

• 水 = water
• 水気 = moisture / dampness / water content

In this sentence, the point is not that there is literal water sitting there, but that 生ごみ contains a lot of moisture. That is why 水気 is a natural choice.

は marks the topic of the sentence.

So 生ごみは means something like:

• As for food waste, ...
• Speaking of food waste, ...

The sentence is making a statement about 生ごみ, so it becomes the topic.

A very literal breakdown is:

• 生ごみは = as for food waste
• 水気が多いです = moisture is abundant / there is a lot of moisture

This is a very common Japanese pattern.

• は marks the topic
• が marks the thing that has the property being described

Here, the topic is 生ごみ.
Within that topic, 水気 is the thing being described as 多い.

So the structure is roughly:

• 生ごみは = as for food waste
• 水気が多い = its moisture is a lot / it has a lot of moisture

A useful way to think about it is:

As for food waste, moisture is plentiful.

This is why both particles can appear together naturally.

多い is an i-adjective meaning many / much / a lot.

In this sentence, 多い directly describes 水気:

• 水気が多い = there is a lot of moisture / it has high moisture content

You may also hear たくさん, but it works differently. It is more noun-like or adverb-like, and it does not fit this sentence as neatly.

Natural:

• 水気が多いです。

Possible in other structures:

• 水気がたくさんあります。

Less natural here:

• 水気がたくさんです。

So 多い is the standard, straightforward choice.

Yes, 多い is already a complete adjective, but です is added to make the sentence polite.

So:

• 水気が多い。 = casual
• 水気が多いです。 = polite

Important point:

• です here adds politeness
• it does not change 多い into a noun or use a copula the same way English uses is

This is very common with i-adjectives:

• 暑いです
• 高いです
• 多いです

Yes, it is natural.

It sounds like something you might see or hear in:

• garbage disposal instructions
• recycling guides
• environmental explanations
• household advice

It has a practical, explanatory tone.

A similar sentence you might also see is:

• 生ごみは水分が多いです。

The difference is roughly:

• 水気 = moisture / wetness
• 水分 = water content, often a bit more technical or objective

Both can work, but 水気が多い is perfectly natural.

No. Japanese normally does not use spaces like this in ordinary writing.

The standard written sentence is:

生ごみは水気が多いです。

Spaces are often added in textbooks or beginner materials to make the structure easier to see.

So the spaced version is for learning, not normal native writing.

A very literal gloss would be:

As for food waste, moisture is much.

That sounds unnatural in English, but it helps show the Japanese structure:

• 生ごみは = as for food waste
• 水気が = moisture
• 多いです = is a lot / is abundant

More natural English translations would be:

• Food waste contains a lot of moisture.
• Kitchen waste has a high moisture content.
• Food scraps are very wet.
  (This last one is less literal, but close in meaning.)

The casual form is:

生ごみは水気が多い。

Only the final です is removed.

You can compare:

• polite: 生ごみは水気が多いです。
• casual: 生ごみは水気が多い。

The adjective 多い itself does not change here; only the politeness level changes.