MSc Aquatic Ecosystems Management

Small sewage treatment works: West Linton and Broughton 2003

Both works serve small communities in sensitive locations, on tributaries of an important salmon river, the Tweed
West Linton works serves a population of approximately 1400
It is about 50 years old
Raw sewage is pumped to the head of the works, and travels along channels where heavier particles of grit settle out
Larger floatable objects ("rags") are removed by a screen and squeezed and transported by auger to be dumped in a bag
Auger and bag
The sewage flows into the centre of the primary settlement tank and as the velocity decreases the solids settle out. The sludge can be piped out by opening a valve at the bottom
The settled effluent spills over the v-notch weir which surrounds the tank
This device skims off any surace scum or fats which accumulate
The sewage then flows through a constant-velocity channel with a weir set to 6 times dry weather flow, which diverts storm water.
It then enters a siphon chamber: water builds up until the bell is submerged, which starts the siphon. The full chamber of sewage then flows into the distributors of the trickling filter beds (bacteria beds)
The arms rotate automatically as the effluent emerges from the nozzles
Effluent has its BOD reduced by the action of aerobic bacteria and other organisms
Nozzles are spaced differentially along the arms to equalise the distribution of liquid across the surface
The bacteria bed consists of coarse irregular gravel which presents a large surface for colonisation by bacteria, protists and higher animals, as well as allowing good aeration.
The drain holes around the base of the bed allow the treated effluent to drain out, as well as air to enter the bed. Some worms which have been washed from the bed can be seen in the outflow.
Schematic diagram of a percolating filter bed
Effluent then flows into a secondary settlement tank, similar to the primary.
This also has a v-notch weir. The final effluent is discharged into the river with a final BOD of not more than 40mg/l
The primary and secondary sludges pass into this sludge balancing tank where they are allowed to settle some more: the clearer surface water is drawn off with this floating boom.
Sludge is finally pumped into the sludge digestion tanks where it is stored until it can be tankered away to Seafield for final treatment and disposal.
Broughton Sewage Treatment Works

Broughton has a population of approx. 520 and is served by a Biodisc system, also known as a rotating biological contactor.This is a self-contained commercially produced unit housed in a fibreglass enclosure, and has been in use for about 25 years.

The sewage is pumped into a small primary settlement tank. Sludge is collected and periodically tankered away for treatment.
Effluent flows into the larger section of the unit, a large trough with rotating discs, which are thus periodically immersed in the sewage and then exposed to the air. A biological film builds up on the discs and breaks down the BOD.