(written from a Production point of view)
The Donald C. Tillman Water Reclamation Plant is located in Van Nuys, Los Angeles, Southern California, on the ground of the Woodley Park, which is also known for its Japanese Garden. Its address is 6100 Woodley Avenue, between Victory Boulevard and Burbank Boulevard. The 6.5-acre Japanese Garden, designed by Dr. Koichi Kawana, was built between 1980 and 1983 and celebrated its opening in 1984.
Named after city engineer Donald C. Tillman, the plant began operating in 1985. Because of its futuristic look, the plant and garden are well known filming locations, mostly for their various Star Trek appearances mixed with matte paintings.
The location can also be seen in episodes of Knight Rider (1986), Murder, She Wrote (1993), Mighty Morphin' Power Rangers (1995), L.A. Heat (1997), Charmed (1999), and Numb3rs (2009) and was featured in the action comedy Dead Heat (1988, starring Joe Piscopo), the crime drama Rising Sun (1993), the science fiction film CyberTracker (1994), the action film Red Sun Rising (1994), the comedy Bio-Dome (1996), the comedy Austin Powers: International Man of Mystery (1997), the thriller Most Wanted (1997), the science fiction thriller Terminal Error (2002, with Marina Sirtis and Michael Nouri), and the science fiction film Sci-Fighter (2004). 
The first appearance of the location was seen in the first season episode "Justice" in 1987 when the plant and the garden portrayed the surface of the planet Rubicun III. The scenes were filmed on 11 September 1987.
Extended with a matte painting by Illusion Arts, Inc. and Syd Dutton it made its second appearance in the fifth season episode "The First Duty" in 1992. This time it was used as the grounds of the Starfleet Academy on Earth. The cast and crew filmed the exterior scenes on Monday 27 January 1992. This was the first time location manager Lisa White coordinated the filming with the cooperation of the Tillman Water Reclamation Plant associates Bob Krivak and Gene Green. According to the call sheet for this day, sunrise was at 6:56 a.m. and sunset at 5:20 p.m. The lunch break was between 12:00 and 12:30 p.m. and the catering service Cynthia's Distinctive Catering provided 110 hot lunches. Beside actors Patrick Stewart, Wil Wheaton, Ray Walston, and Richard Rothenberg, eighteen background talents and three stand-ins worked on location. It was also the first time DGA Trainee David G. Trotti worked on The Next Generation. The set decoration required two statues, fake rocks, flags and a flag pole, and a tool bin.
The location shot extended with a matte painting from "The First Duty" was later re-used for the fifth season cliffhanger "Time's Arrow".
The TNG Season 4 DVD special feature "New Life and New Civilizations" featured several shots of this location.
Outdoor scenes for the first season episode "Time and Again" were filmed at the Tillman Water Reclamation Plant in 1994. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 136) The plant's Japanese garden, normally seen in many other Star Trek episodes shot at the plant, is not seen this time, however, but rather the industrial-looking front part of the main building is shown portraying the building on the unnamed planet.
The Tillman Water Reclamation Plant was also added to a matte painting for several episodes of Enterprise again as location for Starfleet Headquarters. The matte painting first appeared in the first season episode "Shadows of P'Jem" and later in the episodes "Shockwave, Part II", "Regeneration", "First Flight", and "Home". A long shot was used in "Demons".