Action 1: Addition of alum
Most of the large, heavy particles in the wastewater entering the plant are removed in the grit removal system. Much of what remains in suspension is extremely small in size and carries a negative electrostatic charge on its surface. The particles remain in continuous random movement (known as Brownian motion) because of the electrostatic charge.
The repulsion forces exerted by the charge are strong enough to overcome gravity and the attraction charges between particles (known as Van der Waal’s forces). Because of this, separating the solids from the liquids is difficult without the aid of a coagulant.
Aluminum sulphate, alum, is used in the EPT clarifiers as the primary coagulant. When added to the wastewater the alum dissolves into an aluminum ion and a sulphate radical. The positively charged aluminum ion neutralizes the negatively charged particles in the wastewater, allowing them to bind together and settle. This process is almost instantaneous and particle destabilization is practically complete in less than one second. The aluminum ion then hydrolyzes, forming a jelly-like aluminum hydroxide floc, which catches and gathers the destabilized particles.
Action 2: Addition of polymer
Aluminum hydroxide floc has a relatively weak physical structure which is prone to break up. A polymer, which acts as a “glue” of sorts, is added to the wastewater as the aluminum hydroxide floc is being formed. The polymer strengthens the floc and helps to bind adjacent flocs creating larger clumps that settle more easily due to their size and weight.
Action 3: Flocculation
The formation of larger floc clumps is improved by gentle mixing. Careful selection of the mixing energy is required to achieve sufficient collisions without causing the flocs to break up. Efficient floc formation will occur when mixing energy and chemical additions are optimized.
Action 4: Plate settling
At normal operating rates, up-flow clarifiers require very large surface areas to avoid suspended particulates being carried over in the effluent. The use of plate settlers removes the need for such large surface areas. Flocculated wastewater flows upward between the inclined plates of the plate settler.
As the plates are closely spaced, the particulates in suspension have only a short distance to fall before settling on the plate surface. Further clumping of the floc takes place as it gathers on the inclined plate surface and slides down to exit at the bottom of the plate module. This results in better solids-liquid separation and clarifier operation rates that are up to 10 times more efficient than clarifiers that are not equipped with plate settlers.
Action 5: Sludge collection
A slowly moving rake mechanism at the bottom of the tank collects and moves sludge settled underneath the plates across the entire tank and into a small concrete box known as a sludge sump. The rake consists of a chain and an attached series of planks, called a chain and Flite system. Sludge that has accumulated in the sump is pumped to the anaerobic digesters for further treatment prior to disposal.
Action 6: Scum collection
Some light materials in the wastewater, together with the foam formed through mixing, float to the surface. This material, called scum, is collected using a rake mechanism similar to that used for sludge removal. In places where the rakes cannot reach, a water spray is used to move the scum. Scum is then collected in a concrete box known as a scum pit, where it is mixed with some additional water in the pit. It is then pumped as slurry and mixed with sludge originating from other locations within the Gold Bar Plant before awaiting treatment.
Action 7: Air scour
During treatment, some of the floc may stick to the clarifier plates, preventing sliding down to the bottom of the tank where it is collected as sludge. Over time, accumulated floc on the plates can reduce the capacity and overall effectiveness of the plates, requiring the plates to be cleaned occasionally. To loosen this accumulated floc, large volumes of air are bubbled through the plates.
The loosened floc then slides off the plates, collecting at the bottom of the tank as sludge. This process, known as air scour, will ensure that the plates remain clean and effective.