MSc Aquatic Ecosystems Management

Small sewage treatment works: Penicik, Rosewell and Cardrona 2006

These works serve medium to small communities
Penicuik works serves a population of approximately 16,400

Original design incorporated primary treatment (settlement) but this has been discontinued with the tank being used for stormwater retention

Treatment is Sequencing Batch Reactor, with 3 tanks (compare Kinneil Kerse)

intake to works is 366 litres/sec DWF but SBR treatment is limited to 123 l/s

3-6xDWF is diverted to storm tanks. First is blind (tank must be pumped out after use). When this is full it overflows into the second, which is the disused primary tank.

>6xDWF is diverted at the intake and discharged directly into the river via a screen

Consents for discharge from the main output (not stormwater overflow) are according to the Controlled Activities Regulation (CAR), which is more stringent than the Control of Pollution Act values:

BOD mg/l
COD mg/l
Ammonia mg/l

SBR tanks 20m diameter, full capacity 1516m 3, decant capacity 556m 3,

Normal cycle is 4.5 hours:

1.5h fill & aerate
1.5h aerate when full
1h settle
30min decant

Decant is by decant arms made by Purac

Ammonia reduction depends on denitrification which is temperature dependent: it is inhibited at temps below 8-10°C; Scottish Water has a get-out clause for the consent for temperatures below 5°C.



Rosewell WWTW serves a population equivalent of 1350
Overview of the site. Raw sewage enters via screens to remove'rags' . A weir to divert flows greater than 6x Dry Weather Flow (DWF) lies alongside the screens. The diverted flow is discharged directlly into the Sheil burn.

Flow < 6xDWF passes through constant-velocity grit separator channels, with the flow rate monitored at flumes. After the grit channels another weir diverts flow >3xDWF.
The diverted flow is collected in the new tank on the middle left of the previous picture.

All remaining sewage goes into paired rectangular settlement tanks, which have a retention time of 2h at 3xDWF.
The overflow from the settled sewage passes through a mixing chamber then it then enters a siphon chamber lying between the tricking filter beds: water builds up until the bell is submerged, which starts the siphon. The full chamber of sewage then flows into the distributors of the beds (bacteria beds).
The arms rotate automatically as the effluent emerges from the nozzles. 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.
These beds are unusual in thet they are not built within a circular solid retaining wall, but are built in a 'landscaped' form retained by drystone walling. Effluent has its BOD reduced by the action of aerobic bacteria and other organisms.
Treatment by the beds is now insufficient in this plant, and the effluent is next passed to a more recent tricking filter bed of artificial media. This also assists with denitrification, lowering the ammonia levels in the final effluent.
The final stage is the removal of particulates, which is accomplished by the Dynasand tower, seen to the left of the nitrifying tower.
For comparison, see the page on the West Linton works. This also shows the Biodisc plant at Broughton
Cardrona Treatment Works
The Cardrona system has a design capacity of 1520 population equivalent. It serves the new village plus a 100-bed hotel, plus staff accomodation. With the seasonal variation in the Hotel occupancy, the total load fluctuates

The system started with 2 reed beds in operation and now has 4 (full capacity), with the newest being 1 year old (Nov. 2006). It has been enlarged as the number of houses has increased in recent years. The reed beds have a surface area of 1400m2.

Sewage is pumped into a balancing tank to accomodate variations in flow (time of day, etc). The input has a BOD of approx. 200. It is not screened, which may cause problems. From the balancing tank it is pumped (2 pumps) at 11m/s into 4 septic tanks, in pairs.

The outflow from the septic tanks has a BOD of 110-120, which is not regarded as being a very good level of preliminary treatment. It is gravity fed to the reed beds. The surface inflows are at 3 points along the outer long edges of the beds, in coarse gravel to diffuse the flow, and the effluent flows through finer gravel which has the reeds rooted in it, towards an outflow pipe in the centre of the bed.

The final effluent flows via a discharge pipe into the river Tweed.
Consents for BOD and SS are 30 mg/l; in practice it usually achieves 8-12

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