Sewage treatment


The flow is then passed to the 4 grit settlement tanks:		Flow through the tanks is slowed down by the baffles, allowing heavier particles (road grit, etc.) to settle out:

Paddles scrape the grit into the centre, from where it is lifted and transported to a washer.		The grit is collected, drained and sent to landfill or for land reclamation.

The sewage passes from the grit separators via a set of penstocks and flow measuring flumes to the circular primary settlement tanks.		The sewage (average BOD about 150, Total Suspended Solids about 140mg/l) is fed to the centre of the tank and flows towards the perimeter. As it does so its velocity decreases and the particulate material settles out as sludge, removing 35% of the BOD and 55% TSS.

The effluent flows under a baffle designed to remove fats, and over a V-notch weir.		The effluent is a grey, opalescent colour at this stage, and its BOD and TSS is still too high (around 100 mg/l BOD, 70 TSS) to discharge into the Forth estuary, according to the consents currently set by SEPA.

The sediment sludge is collected by airlift pumps from the bottom of the tank: these tanks are relatively shallow as the site is built on reclaimed land on the coast, and suffers from a high water-table, which means empty tanks might float.		The gantry carrying the airlift system sweeps around the tank, skimming fats at the surface also

From this point onwards there are two streams to follow: the effluent, which is eventually discharged into the Forth Estuary, and the sludge, which meets up with sludge from other processes for final treatment. When the works was first commissioned, only this primary treatment was deemed necessary: the effluent went straight out to the Forth via a long sea outfall pipe with diffusers, and the sludge was dumped at sea by a boat, poetically called the 'Gardyloo'. The sludge has its water content reduced in Picket Fence Thickeners, a form of strainer. The water from this re-joins the effluent stream.		During storm events flow is diverted via the penstocks and overflow weirs into the four storm tanks (100m x 30m and 3.4 metres deep). The tanks are seen full here: the rainfall during the previous night had been prolonged but not unusual: any excess at this point would have to be spilled directly into the Forth as there is no further storage capacity, however, the sewage would be very dilute as the major part of the volume would be rainwater, and the sewers would already have been well flushed of solids.

Once the excess input has stopped, pumps at the bottom of the tanks are switched on to resuspend the sediment. After a storm the sewage in the tanks is pumped into the main treatment stream.		The effluent from the primary settlement tanks undergoes a biological treatment by the Activated Sludge process (ASP). The effluent passes through a raceway system of deep (6.1 metre) tanks which have air blown in at the bottom, forcing oxygen into solution. Flocs of bacteria grow aerobically, using the carbonaceous material in the sewage as an energy source and thus reducing the Biochemical Oxygen Demand (BOD).

The diffusers at the bottom of the lanes are arranged to give tapered aeration, with more air delivered at the start of the process where it is most needed,		while the diffusers towards the end are more spaced out.

If the BOD was not reduced in the treatment process, oxygen would be used up in the receiving water, resulting in anoxic conditions, killing marine organisms, and causing the sediments to produce hydrogen sulphide. The dissolved oxygen and solids content in the activated sludge tanks is closely monitored. The grey box on the rail houses the oxygen monitoring equipment.		A certain proportion of the bacterial flocs (the sludge) is returned to the start of the process to maintain a viable number of the essential bacteria. The activated sludge mix at the end of the process is passed to setlement tanks similar to the primary settlement tanks for the separation of the sludge and effluent.

The design of these tanks is slightly different from the primary tanks in that it has a slightly funnel-shaped profile: this picture shows the rotating gantry which sweeps the bottom of the tank and removes the sludge via the well in the centre: any not required to re-join the ASP joins the primary sludge stream.		The secondary effluent is much clearer than the primary, and its BOD is sufficiently low to allow it to be discharged (BOD10, TSS15 mg/l): in fact it may be mixed with some primary effluent which has not received the full treatment.

During winter the effluent flows straight out through the outfall tunnel, but during the bathing season it is disinfected by being passed under a bank of UV lights.		The effluent passes through mesh screens first to remove residual rags which have escaped the earlier screening: these are being cleaned while the system is not in use.

This problem has meant that the untreated fraction of primary effluent has had to be diverted from the UV disinfestion process and is treated by dosing with peracetic acid in an unused primary tank.

These pictures show the last set of UV tubes which irradiate the 5 lanes of effluent (lamps not in use at the time).		This is one rack of the UV tubes, each in a quartz tube: these racks are lowered directly into the flow of effluent

Returning to the sludge stream, sludge may be mixed with sludge brought in from smaller sewage works, before being pumped into the digesters. Air from parts of the sludge processing stream are passed over calcified seaweed (maerl) to remove odours.		The digesters contain anaerobic bacteria which break down organic material, generating methane. This is stored in the white sphere at the end of the row of digesters, and is used as a source of fuel for heating the digesters, and as for generating electricity to partially power the whole works, via a CHP plant.

The digestion reduces the total solids content of the sludge, making it easier to dispose of. It is relatively innocuous material at this stage, and is collected in this tank before final processing.		The final processes involve removing the water: the sludge passes to a tank where it is mixed with polymer which encourages it to clump together, and then to a belt thickener to remove much of the water.

polymer

This is a large fabric belt which allows the liquid to pass through, while the solids are carried to the end of the belt and dropped into a hopper.		Views of the belt thickeners

		The thickened sludge is then passed to centrifuges where remaining water is removed.

The water from these processes is returned to the treatment stream and undergoes primary and secondary treatment.		The solids from the centrifugation were formerly discharged at this point and either taken to landfill or used in land reclamation or forestry, uses which avoided direct contact with forage crops.

Recently drying and pelletising equipment has been commissioned.		This further dries the sludge using heat and compresses it into pellets which are bagged and sold: it may be used as agricultural fertiliser as the process sterilises the sludge.