Trees in close proximity to structures can be hazardous due to their root systems affecting the foundations of the structure. This will be much more prevalent if the ground strata is clay.
When looking at trees and their proximity to drainage or sewerage systems, extra care must also be taken. Tree roots can be very fibrous and can easily enter drains or sewers, which then lead to the subsequent problems of trying to remove the roots. The usual reason roots enter the system is in search of water and if they can find evidence of water in the drains or sewers, then they will soon find a better way into the pipe, usually at cracks or joints.
Of course many pipeline systems are originally laid on gravel beds (and sometimes in gravel surrounds) which themselves are very good sources of water transmission, so that roots will frequently fill up the gravel bed or surround.
With sewage treatment systems, the tree roots can have severe effects on the structure of the system which has to be considered in the design and siting of the units, but there are other side effects which may contribute to the treatment process.
With reed bed or other types of treatment systems, the shading of the ultraviolet rays by trees on the natural plant life could well result in a reduction of growth rate on those areas shaded.