(Capcom's primary storyline)
This unnamed treatment plant was a facility owned and operated by Umbrella USA. This facility was located some eight miles northwest of Raccoon City, and largely consisted on the banks of an underground river within a limestone cavern. A surface level also existed, which was based at the top of a cliff overlooking the Umbrella Chemical Plant. The primary purpose of the treatment plant was the processing of wastewater from NEST, the Arklay Laboratory and the Umbrella Executive Training School, the latter two it was connected to via underground streams. The facility was destroyed in July 1998 as part of a biohazard containment plan.
When the treatment plant was constructed is uncertain, but it is known is handled waste water from the Arklay Laboratory and Executive Training School, both constructed in the 1960s. It was certainly operational by 1978, and was used by Dr. James Marcus for the covert disposal of children who were killed during his work on the t-Virus Project. Marcus' body would itself be dumped in the same channel after his assassination in 1988.
Little is known of the treatment plant in the years between Marcus' death and the events of 1998, though evidence suggests that the facility was not being properly maintained by Umbrella USA, and by 1998 there were serious problems with rust. In May 1998, industrial waste was being redirected to the treatment plant in compounds and quantities it was not designed to process. As occurred elsewhere in the mountains, a t-Virus strain soon began spreading amongst the staff, possibly caused by the Queen Leech test-subject. Although signs point to an emergency taking place with at least one fatality and possibly more, the plant's laboratories nonetheless continued to study Plague Crawlers they had discovered.
On the early hours of 24 July, the treatment plant manager received an order for the plant to be immediately closed. Soon after this order was made, the facility was attacked in full by Queen Leech, which had until then resided in the training school. All remaining workers were infected with the t-Virus and mutated into Zombies. Soon after this, S.T.A.R.S. officer Rebecca Chambers arrived, searching for Billy Coen, an ex-Marine who had fallen into the water. Retrieving him from a reservoir, they navigated the plant's deteriorating structure and eventually reached the microwave furnace room on level B9 where they confronted Queen Leech. After a struggle, it chased them up a cargo elevator to the warehouse on level B1. As their second fight took place, the self-destruct system was activated by NEST chief researcher Dr. William Birkin to contain the outbreak. Consequently, the entire treatment plant is believed to have been destroyed.
The treatment plant was built within a series of large, naturally-occurring limestone caverns in the Arklay Mountains, and processed waste water dumped into underground river channels by Umbrella facilities. The treatment plant was directly connected to NEST. A cargo elevator led up to a large warehouse built near the executive training school.
The complex itself was largely made up of sewage pools and water treatment tanks, but also contained staff dormitories as well as a bar and recreation lounge; however, by the time Rebecca and Billy entered the plant the dank and gloomy facility was mostly home to zombies, lurkers and other by-products of Marcus' experiments.
- The Treatment Plant's layout as depicted in The Umbrella Chronicles' Train Derailment 3 is somewhat abridged; the elevator Billy and Rebecca use after their encounter with the T-001 takes them directly to the generator room located next to the alley leading to the incinerator chamber, whilst in Resident Evil 0 the elevator opens out onto a small walkway leading to the main power supply control room. These changes were presumably made in order to cut down the length of the level for pacing reasons.
- Despite containing the largest number of Leeches in any area (in the incinerator room) in the game, only one Mimicry Marcus is located in the entire Plant. However, it is the fastest one in the game.
- The treatment plant in Resident Evil 0 features an unusual geographical quirk dubbed by fans as the "magic elevator", which allows the player to travel from the treatment plant to the Umbrella Chemical Plant, and then up further to the training school's train yard. This was intended entirely for gameplay convenience to let players retrieve any remaining belongings.
- Resident Evil: The Umbrella Chronicles official website - "History of Biohazard" timeline
- Resident Evil 0 (2002), file: "Treatment Plant Manager's Diary".
- Resident Evil 0 (2002), room: "Treatment plant dormitory", examine: "It looks like it's been a while since this one croaked."
- Resident Evil 0 (2002), room: "Treatment plant dormitory", examine: "Several places are stained black with dried blood."
- Resident Evil 0 (2002), room: "No.1 laboratory", examine: "The capsules contain the larva of some kind of life-form. It's unlike anything you've ever seen before.".