Generally, if you have a drilled well that has a secure cap, and is protected from surface water runoff or flooding, bacterial samples should be taken 3 times per year, if these are satisfactory, a permanent treatment device should not be required.
If you have a dug well (whose joints are usually not sealed) or a sandpoint well that typically draws water from shallow water table, then bacteria may be present especially after storms or during spring snow melt.
In these types of wells a permanent treatment device should be considered. Constant, repeated shock chlorination is not a solution.
If you draw from an unprotected source like a stream, river or lake, then the water supply is not safe and should not be used as a source of drinking water unless it is adequately treated first.
“Simple” treatment systems installed on these source supplies may not render the water safe because the quality of the water may vary drastically from day to day. Consider not only bacterial contamination but also viruses, and parasitic cysts like cryptosporidium and giardia.
Treatment Device for Bacteria Removal
Treatment Device for Chemical Removal