I want to know if I have the best way to combine 5 sentences.1. This waste poses a threat to civilization.2. This waste poses a threat to the...

Once again, your sentence is "correct," and also make use
of formal grammar and sentence structure. However, you might want to "mix things up a
bit," so that your sentences don't sound too similar in structure, which
might occur if this is a pattern that you
repeat.

By changing the structure of your sentence—the
order in which you place its various elements—your writing can sound more interesting.
This is especially true if you are completing this for a class where the sentence is
going to be one of many read. Yours may end up sounding like all
the other sentences; however, as you become a stronger writer, you will want to make
your work stand out. This is something that will serve with college application letters
and essays, as well as the pursuit of employment
prospects.

A perfect example of more sophisticated writing
can be seen with the simplicity of nursery rhymes, such as "Mary had a little lamb," or
first grade readers that use "subject+verb" sentence structures, such as, "I see the
apple." When you see this kind of writing, you recognize that your
own writing is far more sophisticated. However, it
making it more interesting, you don't want your writing to be difficult to understand,
but simply more engaging.

I would first offer some context
here, and then experiment with the order of the sentence. (And don't be afraid to use
commas as necessary, as well as semicolons or
colons.)

Waste
is a necessary by-product of any civilization, but correct processing of it is an
extraordinarily serious concern; it is imperative to avoid a potential threat to the
environment—as seen with the long-lasting and lethal effects of
radiation.

One thing to try
to do when joining these related ideas is to avoid using a word repeatedly.
You have already done this by using "pose" only once, though it
appears in two sentences. You have done the same with "Its radition is so…" Repeating
these words or phrases would make the writing sound monotonous; at the same time, this
repetition in your basic sentences also points to the relationship between these items
and helps you to find elements that can be combined in your final sentence. I took out
"poses" and wrote about "avoiding a potential threat." The content is the same: the
information you have highlighted is included. My suggestion looks at the information in
a slightly different way without losing the central meaning of the
information.

Your sentence is absolutely correct: there is
no question—it is nicely done. At some point you might want to try to experiment with
switching the word order (syntax), and/or the sentence structure to find a different
"sound" for your writing.