Activated sludge
Activated sludge is a process in sewage treatment in which air or oxygen is forced into sewage liquor to develop a biological floc which reduces the organic content of the sewage. In all activated sludge plants, once the sewage has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage entering the tank. This fraction of the floc is called R.A.S - Return Activated Sludge. The remaining sludge, also called W.A.S - Waste Activated Sludge, is further treated prior to disposal.
Purpose
In a sewage treatment plant, Activated Sludge process can be used for one or several of the following purpose:
- removing carbonaceous pollution
- removing nitrogen pollution
- removing phosphate
General Principles
Definitions
- Raw water: water entering the system
- Mixed liquor: the mix of raw water and activated sludge.
- Return activated sludge (R.A.S): activated sludge extracted from the system and mixed with raw water to form the mixed liquor.
- Waste activated sludge (W.A.S.): activated sludge in excess that is extracted from the system to be directed to sludge treatment.
- Sludge age: the average time biological sludge stay in the system. In simpler words, it can be defined as the average age of a bactery in the system.
Arrangement
The general arrangement of an aerated sludge process for removing carbonaceous pollution includes the following items:
- Aeration tank where air (or oxygen) is injected in the mix of raw water and returned activated sludge
- Settling tank (usually referred to as "final clarifyer" or "secondary settling tank") to allow the biological flocs to settle, thus separating the biological sludge from the clear treated water.
This is illustrated in the following diagram:
Types of plants
There are a variety of types of activated sludge plants. These include :
Diffused Aeration
Sewage liquor is run into deep tanks with diffuser blocks attached to the floor. These are like the diffuser blocks used in tropical fish tanks but on a much larger scale. Air is pumped through the blocks and the curtain of bubbles formed both oxygenates the liquor and also provide the necessary stirring action. Where capacity is limited or the sewage is unusually strong or difficult to treat, oxygen may be used instead of air.
Cones
Vertically mounted tubes of up to 1 metre diameter extend from just above the base of a deep concrete tank to just below the surface of the sewage liquor. A typical shaft might be 10Metres high. At the surface end the tube is formed into a cone with helical vanes attached to the inner surface. When the tube is rotated, the vanes spin liquor up and out of the cones drawing new sewage liquor from the base of the tank. In many works each cone is located in a separate cell that can be isolated from the remaining cells if required for maintenance. Some works may have two cones to a cell and some large works may have 4 cones per cell.
Package Plants
There are a wide range of other types of plants, often serving small communities or industrial plants that may use hybrid treatment processes often involving the use of aerobic sludge to treat the incoming sewage. In such plants the primary settlement stage of treatment may be omitted. In these plants, a biotic floc is created which provides the required substrate.
Oxidation Ditch
In some areas where more land is available sewage is treated in large round or oval ditches with one or more horizontal aerators typically called brush or disc aerators which drive the mixed liquor around the ditch and provide aeration. These are oxidation ditches, often referred to by manufacturer's trade names such as Pasveer, Orbal, or Carrousel. They have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (i.e at breakfast time and in the evening).
Deep Shaft
Where land is in short supply sewage may be treated by injection of oxygen into a pressured return sludge stream which is injected into the base of a deep columnar tank buried in the ground. Such shafts may be up to 20 metres deep and are filled with settled sewage liquor. As the sewage rises the oxygen forced into solution by the pressure at the base of the shaft breaks out as molecular oxygen providing a highly efficient source of oxygen for the activated sludge biota. The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewage and sludge. Mixed sludge and sewage is decanted at the surface and separated into supernatant and sludge components. The efficiency of deep shaft treatment can be high but both installation costs and running costs are higher than most other methods and maintenance problems at many plants have given this method a poor rating