State point Analysis – Clarifier design rules
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The State point analysis is a mathematical model which helps to evaluate clarifier operation and performance. For the explanation it is important to understand the two main factors for the proper functioning of clarifiers:
1. Solids concentration or solids loading rate (SLR) in g/l.
2. Solid flux rate which describes the number of solids passing through the clarifier in a certain time (detention time). To be able to compare different flux rates its set into relation with the available clarifier surface area, therefore the flux rate equals solid amount in kg/m2*hr.
For further explanation the two factors are set in a diagram relation: The solid concentration is on the x-axis and the solid flux rate is on the y-axis. The diagram includes three curves.
1) The settling curve shows how fast the solids can be settled down. Example: At a usual MLSS of 3,500mg/l we have 6kg/m2/hr settled solids.
If the curve is lower, we have less settled solids per hour therefore the settling is slower.
If the curve is higher, we have more settled solids per hour therefore the settling is faster. The settling curve is determined by settling tests based on the SVI.
2) The overflow line represents the surface overflow rate (SOR) of the clarifier. The SOR is calculated by dividing the influent flow by the clarifier surface area and can therefore be adjusted by changing the influent flow rate. For example: Decreasing the flow rate to the clarifier will flatten the curve whereas increasing the flow rate will steepen the curve.
3) The underflow line shows the solids loading rate in relation to the expected underflow concentration. The underflow concentration is the water-sludge concentration on the bottom of the clarifier. By increasing or decreasing the RAS (return activated sludge) rate the curve can be moved up or down. More solids from RAS result in a higher MLSS and less solids from RAS result in a lower MLSS.
Next all three curves are shown in the diagram at the same time. The most important point is the intersection between the overflow line and the underflow line called the state point which shows the current operation of the clarifier. The state point must fulfill two requirements for a proper functioning of the clarifier:
1) The state point must be below the settling curve.
If the state point is above the settling curve the solids do not have enough time to settle down and will be carried out over the effluent launders.
2) The complete underflow curve must stay underneath the settling curve. If not, the sludge does settle but it doesn’t thicken fast enough so that the sludge cannot be removed, and the sludge blanket builds up in the clarifier tank.
Wastewater treatment plant operators have different measures to adjust the operating conditions of the clarifier such as changing the water flow rate, the RAS rate and the MLSS by adjusting the SVI. Again, changing the flow rate results in a more flat or more steep overflow line, changing the RAS rate or MLSS will move the underflow curve up or down. Finally, the gap between the underflow and settling curve shouldn’t be too big to save power costs of RAS pumps.
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