Nitrogen - Nitrite

It should be first noted that Nitrates and Nitrites, both elements of Nitrogen, are very natural in our environment and crucial for our sustainable ecosystem.  With that said...

Nitrates can be found in drinking water supplies. Their presence in groundwater is generally associated with septic systems, confined animal feeding operations, or fertilizer use. These sources of nitrate contamination are more associated with rural settings, and are often subjects of drinking water source protection programs.

Nitrates are also present in treated wastewater, and as such can be present in surface water, or in treated wastewater used in groundwater recharge projects. These sources can pose risks to urban drinking water supplies.

Nitrates are also used in industry, for example, in the production of fertilizers and explosives.  (ref here)

If your water supply is a domestic well, nitrites might be a sign of coliform bacteria or a compromised well.  On a municipal line, nitrites are not as likely to be present at a high risk level, but it can  happen.  About 3% of municipal sources do show higher levels of contamination.

The most common method would be a Reverse Osmosis system.  This can be a whole house system, or at point of disbursal.  What is generally most common is a RO system at the point where drinking water is consumed (kitchen sink, typically).

Prices can range from a couple hundred into the thousands.  Here is an Amazon link to a selection of RO systems.  (I do not get an affiliate commission, this is just providing information for you.)

Here are a handful of links to provide more information on Nitrates in drinking water.

Understanding Nitrite as a Water Contaminant

California Water Boards - Nitrites

Where Are Nitrates and Nitrites Found? - Agency for Toxic Substances and Disease Registry