In this article we will discuss about:- 1. Operations and Units of an Activated Sludge Plant 2. Operational Difficulties in Activated Sludge Plant.
Operations and Units of an Activated Sludge Plant:
Fig. 13.2 shows a typical flow diagram for an activated sludge plant. The activated sludge process is usually preceded by preliminary and primary treatments of sewage for which screening chamber, grit chamber and primary settling tank are provided. The activated sludge process involves three basic operations viz., mixing of activated sludge, aeration of mixed liquor, and settling in secondary tank.
Thus an activated sludge plant essentially consists of the following:
(i) Aeration tank containing micro-organisms in suspension in which the reaction takes place;
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(ii) Aeration systems to transfer oxygen;
(iii) Secondary settling tank to separate and thicken activated sludge;
(iv) Activated sludge recirculation system; and
(v) Excess sludge wasting and disposal facilities.
Mixing Regime:
The mixing regime employed in the aeration tank may be plug flow or completely mixed flow. Plug flow implies that the incoming mixture of sewage and returned activated sludge moves down progressively along the aeration tank essentially unmixed with the rest of the tank contents.
Completely mixed flow involves rapid dispersal of the incoming mixture of sewage and returned activated sludge throughout the tank. In the plug flow system the oxygen demand and the F/M ratio will be highest at the inlet end of the aeration tank and will then progressively decrease. In the complete mix system, the oxygen demand and the F/M ratio will be uniform throughout the tank.
Flow Scheme:
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The flow scheme involves the pattern of addition of the mixture of sewage and returned activated sludge to the aeration tank and that of obtaining the sludge to be recycled and also the pattern of aeration. The addition of the mixture of sewage and returned activated sludge may be at a single point at the inlet end of the aeration tank or it may be at several points along the length of the aeration tank.
The sludge to be recycled may be obtained directly from the settling tank or through a sludge reaeration tank (see Fig. 13.2). Further the aeration may be at a uniform rate or it may be varied from the head of the aeration tank to its end.
Operational Difficulties in Activated Sludge Plant:
There are two most common difficulties encountered in the operation of activated sludge plant:
1. Rising sludge; and
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2. Bulking sludge.
1. Rising Sludge:
Occasionally sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The cause of this phenomenon is denitrification, in which the nitrites and nitrates in the sewage are converted to nitrogen gas. As nitrogen gas is formed in the sludge layers, much of it is trapped in the sludge mass.
If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids.
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The problem of rising sludge can be overcome by:
(i) Increasing the rate of return activated sludge pumping from the activated sludge settling tank;
(ii) Decreasing the rate of flow of aeration liquor into the offending tank if the sludge depth cannot be reduced by increasing the return activated sludge withdrawal rate;
(iii) Where possible, increasing the speed of the sludge collecting mechanism in the settling tank and;
(iv) Decreasing the mean cell residence time by increasing the sludge wasting rate.
2. Bulking Sludge:
A bulked sludge is the one that has poor settling characteristics and poor compactability.
Sludge bulking can be caused by
(i) The growth of filamentous organisms (primarily Sphaerotilas) that will not settle, or
(ii) The growth of micro-organisms that incorporate large volumes of water into their cell structure, making their density near that of water and thus causing them not to settle.
The factors which may lead to the development of sludge bulking are:
(a) Inadequate air supply (i.e., under aeration);
(b) Low pH value;
(c) Prolonged detention of sludge in the settling tanks;
(d) Accumulation of sludge at the bottom of the aeration tank;
(e) Sudden change in the character of sewage due to discharge of septic solids from the sewage system;
(f) Introduction of certain industrial sewages containing elements which favour growth of filamentous organisms.
When sludge bulking occurs, the sludge does not readily settle down, but remains in suspension in the secondary settling tank and it is carried with the effluent from the secondary settling tank which will result in the effluent of poor quality.
Further where extreme bulking of sludge exists, a portion of the suspended solids from the aeration tank will be discharged in the effluent which will result in the loss of MLSS from the aeration tank. Another problem with sludge bulking is that large volume of sludge will have to be handled.
The various remedial measures which may be adopted for controlling the bulking of sludge are enumerated below:
(i) Reducing the sewage flow to the aeration tank for a short period.
(ii) Reducing the suspended solids in the sewage.
(iii) Prolonged aeration.
(iv) Reaeration of returned activated sludge.
(v) Chlorination of sewage or return activated sludge, so as to control the growth of filamentous organisms. A reasonable range of chlorine requirement is between 0.2 and 1.0 percent by weight.
(vi) Raising of pH value of sewage to about 8 or more by adding lime.
(vii) Addition of hydrogen peroxide to control the growth of filamentous organisms.