Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed as water towers, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs are entirely underground, especially in more hilly or mountainous country. In the United Kingdom, Thames Water has many underground reservoirs built in the 1800s, most of which are lined with brick. A good example is the Honor Oak Reservoir, constructed between 1901 and 1909. When it was completed it was the largest brick built underground reservoir in the world and is still one of the largest in Europe. The reservoir now forms part of the Southern extension of the Thames Water Ring Main. The top of the reservoir has been grassed over and is now the Aquarias Golf Club.
Service reservoirs perform several functions including ensuring sufficient head of water in the water distribution system and providing hydraulic capacitance in the system to even out peak demand from consumers enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed so that energy costs in pumping are reduced by concentrating refilling activity at times of day when power costs are low.
