At all rates of flow to the treatment works rotary distributors must be fed at rates sufficient to ensure that the jets of sewage will rotate the arms. If the flow to the distributors is too low they might stand still and dribble. If this happens sewage will pass through one spot on the filter and effluent quality may deteriorate. Rotation of rotary distributors at all rates of flow may be achieved by means of a dosing syphon. This is used to hold back the flow until there is adequate volume and head to ensure efficient and continuous operation of the distributor.
The device shown in the diagrams below consists of a dosing tank fitted with a syphon which, when primed by the rising level of sewage, discharges to the filter under sufficient head to rotate the distributor. When the flow into the chamber is less than the discharge capacity of the syphon, the water level falls in the chamber until eventually the syphonic action is broken. The chamber refills with sewage until the syphon is re-primed and this charges to the filter once more. This cyclical action is then continued until the flow through the works increases to match the capacity of the syphon.
Dosing syphons – how they work
This diagram shows adosing siphon after and before discharge. The main operations comprise:-
The dosing chamber fills, causing the air pressure in the dome to increase and sewage to be displaced from the U-tube.Sewage then siphons over into the siphon pipe and the surge fills up the U-tube again.
The level in the dosing chamber falls to unseal the air intake pipe, allowing air to enter the dome and break the siphon. The height of the U-tube, labelled B, controls the difference in sewage levels between the chamber and the dome, labelled A. If it is too short, the sewage will not rise high enough to cover the dome completely, a pocket of air will remain in the dome and the siphon will function erratically If the arm is too long, the sewage
will rise above either the siphon pipe or the overflow pipe outside the dome and dribble over to the distributor.
Construction and Maintenance
The main castings for each syphon are in cast iron, but the air pipes are usually manufactured from galvanised
mild steel. These are the weak link in the equipment as they can corrode and become blocked by grease and
fat. Inspection of dosing syphons should cover blockages in the air intake and U-tube and corrosion that might affect the lengths of these pipes. Wear, corrosion or damage to the lower edge of the dome may affect the breaking of the syphon. While there is no power or energy requirements for syphons it is essential that the air pipes are kept clean and airtight.
Failure of the dosing syphon is a common cause of noncompliance.
Filter bed performance is best if dosing occurs 8 to 12 times an hour. If possible syphons should be adjusted to achieve this dosing frequency by reducing the volume of the chamber ( bricks/concrete blocks). On small works it is sometimes difficult to achieve this dosing frequency; the frequency may be as low as once or twice per hour. If compliance is at risk recirculation should be considered.