A amônia vira nitrito no biofiltro.

Because amônia is a feminine singular noun in Portuguese, so it can take the definite article a.

In Portuguese, general statements often use the definite article more than English does. So:

• A amônia vira nitrito no biofiltro.
• literally: The ammonia turns into nitrite in the biofilter

English often drops the article in scientific statements, but Portuguese frequently keeps it.

Here, vira means becomes, turns into, or changes into.

It comes from the verb virar, which can mean different things depending on context:

• turn
• become
• turn into

In this sentence, virar means to become / to turn into:

• A amônia vira nitrito = Ammonia becomes nitrite

This is a very natural way to describe transformation in everyday Brazilian Portuguese.

With virar meaning to become / turn into, Portuguese normally does not need em.

So the natural pattern is:

• algo vira outra coisa
  something becomes something else

Examples:

• A água vira gelo.
• A lagarta vira borboleta.

If you want a more formal or technical alternative, you could say:

• A amônia se transforma em nitrito.
• A amônia se converte em nitrito.

Those verbs do use em.

Because after virar, the result noun often appears without an article when you are naming what something becomes.

So:

• A amônia vira nitrito is more natural than
• A amônia vira o nitrito

Using o nitrito would sound more specific, as if you meant a particular nitrite already known in the context.

This is similar to how Portuguese often omits the article when a noun functions like a category or result.

No biofiltro means in the biofilter.

No is a contraction of:

• em = in
• o = the

So:

• em + o = no

Examples:

• no aquário = in the aquarium
• no filtro = in the filter
• no biofiltro = in the biofilter

In normal Portuguese, no. You should use the contraction no.

So:

• correct: no biofiltro
• unnatural/incorrect in standard usage: em o biofiltro

Portuguese regularly contracts em with definite articles:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Because the subject is singular: a amônia.

The verb has to agree with the subject:

• A amônia vira nitrito.
• As substâncias viram nitrito.

So:

• a amônia = singular → vira
• as amônias or another plural subject → viram

Yes. Vira is in the present tense, and here it expresses a general process or scientific fact.

Portuguese uses the simple present this way all the time:

• A água ferve a 100 graus.
• O gelo derrete com calor.
• A amônia vira nitrito no biofiltro.

So even though it describes a process, the present tense is completely normal.

Yes. That is also correct.

Compare the tone:

• vira → more everyday, direct, very common
• se transforma em → a bit more formal or explanatory
• se converte em → more technical/scientific

So all of these can work, depending on style:

• A amônia vira nitrito no biofiltro.
• A amônia se transforma em nitrito no biofiltro.
• A amônia se converte em nitrito no biofiltro.

Yes, somewhat. The original order is the most neutral:

• A amônia vira nitrito no biofiltro.

But you can move the location for emphasis:

• No biofiltro, a amônia vira nitrito.

That puts more focus on where the transformation happens.

Still, for a simple textbook-style sentence, the original version is the most straightforward.

A simple approximate pronunciation is:

• a a-MOH-nya VEE-ra nee-TREE-too no byoh-FEEL-troo

A few notes:

• amônia has stress on mô
• vira sounds like VEE-ra
• nitrito has stress on tri
• biofiltro usually has stress on fil

Also, in natural speech, a amônia may flow together smoothly, since the two vowels meet.

Yes. Biofiltro is masculine, which is why it takes o in the singular:

• o biofiltro
• no biofiltro

This matches the ending and the base noun filtro, which is also masculine:

• o filtro
• o biofiltro

Yes. Virar is a very common verb with several meanings, which can confuse learners.

Some common uses are:

• turn
  Vire à esquerda. = Turn left.
• flip over / turn over
  O barco virou. = The boat flipped over.
• become / turn into
  A amônia vira nitrito.

So in this sentence, the meaning is specifically to become / to turn into.