ANSWER SCHEME
MIDTERM 2 ERT319
Semester 1 2012/2013
Question 1
Activated sludge process is one of the most important processes in biological treatment.
(a)
Sketch a diagram showing a complete mix activated sludge process.
[10 Marks]
Answer:
(b)
Foaming may be formed if the activated sludge tank contains soap, detergents & other
surfactants.
Analyze the froth formation, problems and solutions to be implemented to overcome
such problems.
[20 Marks]
Answer:
- Foaming: when the aerated wastewater which contains soap, detergents & other surfactants
- Foaming action produces froth that contains sludge solids, grease, and wastewater bacteria.
- Problems: Wind may lift & blow the froth  contaminate whatever it touches, slippery, and
difficult to remove once it has dried.
- Solutions: remove froth by spraying clear water or screened effluent through nozzles or,
adding antifoaming chemical additives in spray water
Question 2
Figure 1
Figure 1 shows a diagram of a type of biological treatment unit operation. Propose a removal
mechanism for this type of treatment.
[20 Marks]
Answer:
- As the shaft rotates, the surfaces of rotating disks alternately come in contact with the
microorganisms and organic content of wastewater and atmosphere.
-
During the rotation, the microbes get attached to the disks, and O2 from atmosphere is
transferred to the wastewater to maintain aerobic condition.
-
The microbes attached to the disks surface grow in the form of biological film and
consume organic content in the wastes.
-
After some times, as the bio-films thickens, an anaerobic condition develops nearer to
the disks surface, then the slime layer gets washed out (sloughing) by incoming
wastewater flow (the layer of microbes enters endogenous respiration and weaken the
attachment to the films surface and easily washed-out by next wastewater flow).
 The sloughed bio-films is ultimately removed in the secondary clarifier before final
disposal of treated effluent.
Question 3
(a)
Stabilization process of sludge is a process to reduce volume of waste, produce usable
gas and improve the dewaterability of the sludge. Stabilization includes digestions and
composting.
Evaluate the advantages and disadvantages of cylindrical anaerobic digestion.
[20 Marks]
Answer:
Advantages
Disadvantages
(b)
Figure 2
Figure 2 shows a schematic diagram of a waste dumping system. Analyze the functions of X
and Y.
[20 Marks]
Answer:
X: Leachate collection system
- Consisting of a series of drainpipes or a drainage layer to collect leachate / avoid
leachate from penetrating/ absorbing in the soil and contaminate the soil/
groundwater.
- Easy to collect leachate for further treatment.
Y: Primary barrier
- Consisting of layer of clay, bentonite-enhanced soil or hydraulic asphalt.
- Act as high-permeability layer to protect the leachate and landfill gas from
contaminate the surrounding soil and groundwater.
- Act as a barrier from which to separate and filter the leachate produced from
contaminating the soil and groundwater.