Il controlavaggio e la decalcificazione sono diversi.

Because the subject is made up of two things joined by e (and):

• Il controlavaggio
• la decalcificazione

Together they form a compound subject, so Italian uses the plural verb:

• sono = are

If there were only one subject, you would use the singular:

• Il controlavaggio è diverso.
• La decalcificazione è diversa.

The adjective diversi agrees with the two nouns together.

Here the two nouns are:

• controlavaggio — masculine singular
• decalcificazione — feminine singular

When Italian has a group with mixed gender (at least one masculine noun and one feminine noun), the adjective normally goes in the masculine plural.

So:

• masculine + feminine = diversi

If both nouns were feminine, you would use diverse.

Because the two nouns have different grammatical genders:

• il controlavaggio → masculine
• la decalcificazione → feminine

In Italian, nouns have grammatical gender, and the article must match the noun:

• il for many masculine singular nouns
• la for many feminine singular nouns

This does not necessarily mean the things themselves are male or female. It is just a grammatical feature of the noun.

In Italian, when you have two separate nouns joined by e, it is very normal to repeat the article before each one:

• Il controlavaggio e la decalcificazione

You generally cannot use just one article here, because the nouns have different genders anyway:

• il controlavaggio
• la decalcificazione

Repeating the article makes the structure clear and natural.

Here diversi is an adjective meaning different.

It comes after the verb sono and describes the subject:

• Il controlavaggio e la decalcificazione sono diversi.

So the structure is:

• subject + verb + adjective

This is similar to English:

• X and Y are different.

Important: diverso/diversi can sometimes also mean several/various in other contexts, but not here. In this sentence it clearly means different.

Yes, controlavaggio is written as one word.

It is a compound noun built from:

• contro- = counter-, back-, reverse-, against
• lavaggio = washing

In technical contexts, it often refers to backwashing or a reverse washing/cleaning process.

Italian often forms technical nouns by combining elements like this into a single word.

Each noun is singular by itself:

• il controlavaggio = one process
• la decalcificazione = one process

But because they are linked by e, together they create a plural subject. So:

• the nouns stay singular
• the verb becomes plural
• the adjective becomes plural

That is why you get:

• Il controlavaggio e la decalcificazione sono diversi.

Usually, no—not in normal standard Italian.

Italian uses articles much more often than English, especially with general nouns and abstract or technical processes. So:

• Il controlavaggio e la decalcificazione... = natural
• Controlavaggio e decalcificazione... = less standard here, unless used in a special style such as headings, labels, or notes

So in a normal sentence, keeping il and la is the safest choice.

E is the normal Italian word for and.

Sometimes you may also see ed, but that is mainly used before a word beginning with e to avoid the repetition of the same sound:

• pane ed erba

In your sentence, the next word is la, not a word beginning with e, so e is correct:

• Il controlavaggio e la decalcificazione

It is a noun related to removing calcium or lime deposits. Its structure is roughly:

• de- = removal/reversal
• calcific- = related to calcium/limescale
• -azione / -zione = noun ending meaning process/action

So decalcificazione is the process of decalcifying / descaling.

This ending is very common in Italian technical and abstract nouns:

• organizzazione
• purificazione
• sterilizzazione

The pattern is:

• Article + noun + e + article + noun + plural verb + plural adjective

So:

• Il controlavaggio
• e
• la decalcificazione
• sono
• diversi

This is a very useful pattern for Italian:

• Il tè e il caffè sono diversi.
• La lavatrice e l’asciugatrice sono diverse.
• Il problema e la soluzione sono diversi. (mixed gender, so masculine plural adjective)

Yes, that is the most natural reading here.

The sentence means that the backwashing process and the decalcification process are not the same thing. In other words, they are distinct or different from one another.

So this is not just saying they are both generally unusual or varied; it is specifically contrasting the two processes.