septic system

Reader's Digest 12月号のSamoaについての記事からです。
They replaced the wiring, plumbing and septic sytem, added new metal roofs and painted the fales white with blue trim.
(Note: "fale": The classic Samoan dwelling is a round bungalow called a fale.)
"septic sytem" はインフラの一種と推測できますが、"sewage" とは別物の様です。
辞書を見ます。　OneLookで検索すると "septic sytem" を載せているのは辞典だけで、辞書はリストには出てきません。
・Glossary of Water Resource Terms: an on-site system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank that receives waste from a residence or business and a system of drainage lines or a pit for disposal of the liquid effluent that remains after decomposition of the solids by bacteria in the tank.
Wikipediaもリストにあったので見てみると、"septic tank" に転送され次ぎの説明がありました。
A septic tank is a key component of the septic system, a small scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations.
この後も説明は続きますが、長くなるので "septic" を説明している個所だけを引用します。
The term "septic" refers to the anaerobic bacterial environment that develops in the tank and which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other on-site wastewater treatment units such as biofilters or aerobic systems involving artificial forced aeration.
"septic sytem" の主要な部分が "septic tank" である事が分かります。
また、多くの辞書にも "septic tank" の説明はありました。
American Heritage Dictionaryの説明を下に引用します。
A sewage-disposal tank in which a continuous flow of waste material is decomposed by anaerobic bacteria.