Sewage Treatment MCQ Quiz - Objective Question with Answer for Sewage Treatment - Download Free PDF

Explanation:

Imhoff tank results in 60 - 65 % removal of solids and 30 - 40 % removal of BOD.

Imhoff tank:

• An Imhoff tank is an improvement over the septic tank in which the incoming sewage is not allowed to get mixed up with the sludge produced and the outgoing effluent is not allowed to carry with it a large amount of organic load, as in the case of the septic tank.
• Imhoff tanks combine the advantages of both the septic as well as sedimentation tanks.
• An Imhoff tank is two storey tank in which the upper chamber served as sedimentation (aerobic condition) and the lower chamber served as digestion chamber (anaerobic condition).
• They find use in case of small treatment plants requiring only primary treatment.
• They are quite economical and do not require skilled attention during operations.
• The result obtained is quite good, with 60 to 65 % removal of solids and 30 to 40 % removal of BOD.
• There is no problem of sludge disposal, as in the case of sedimentation tan

Wastewater Stabilization Ponds (WSPs)/Oxidation pond:

It is also called lagoons or stabilization ponds, are large, shallow ponds designed to treat wastewater through the interaction of sunlight bacteria, and algae. Algae grow using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water.

During the process of photosynthesis, the algae release oxygen needed by aerobic bacteria. Mechanical aerators are sometimes installed to supply yet more oxygen, thereby reducing the required size of the pond. Sludge deposits in the pond must eventually be removed by dredging.

Algae remaining in the pond effluent can be removed by filtration or by a combination of chemical treatment and settling.

The ponds can be used individually or linked in a series for improved treatment. There are three types of ponds, (1) anaerobic, (2) facultative, and (3) aerobic (maturation), each with different treatment and design characteristics.

Concept:

Trickling Filters:

• A trickling filter is an attached growth process i.e. process in which microorganisms responsible for treatment are attached to inert packing material.
• The packing material used in attached growth processes includes rock, gravel, slag, sand, redwood, and a wide range of plastic and other synthetic materials.

Figure: Trickling Filter

The process of trickling filter is as follows:

• The wastewater in the trickling filter is distributed over the top area of a vessel containing non-submerged packing material.
• Air circulation in the void space, by either natural draft or blowers, provides oxygen for the microorganisms growing as an attached biofilm.
• During operation, the organic material present in the wastewater is metabolized by the biomass attached to the medium. The biological slime grows in thickness as the organic matter abstracted from the flowing wastewater is synthesized into new cellular material.
• The thickness of the aerobic layer is limited by the depth of penetration of oxygen into the microbial layer.
• The micro-organisms near the medium face enter the endogenous phase as the substrate is metabolized before it can reach the micro-organisms near the medium face as a result of increased thickness of the slime layer and lose their ability to cling to the media surface.
• The liquid then washes the slime off the medium and a new slime layer starts to grow. This phenomenon of losing the slime layer is called sloughing.
• The sloughed-off film and treated wastewater are collected by an under drainage which also allows circulation of air through the filter.
• The collected liquid is passed to a settling tank used for solid-liquid separation.

Sedimentation with Coagulation

Sedimentation with coagulation is a water treatment process used to remove suspended solids from water by adding a coagulant to form flocs, which then settle out of the water.

Analyzing the Given Statements

1. Statement 1: "Coagulation is usually found necessary for water having turbidity greater than 30-50 ppm." (Correct)

   • Coagulation is typically required for water with higher turbidity levels to effectively remove suspended particles.

2. Statement 2: "Alum produces heavy floc compared with iron salt coagulants." (Incorrect)

   • Iron salt coagulants tend to produce heavier and more compact flocs compared to alum.

3. Statement 3: "It is difficult to dewater the sludge formed by alum coagulant." (Correct)

   • The sludge formed by alum coagulant has a gelatinous nature, making it more challenging to dewater compared to other coagulants.

Explanation: 

(i) To ensure that deposition of suspended solids does not take place, self-cleansing velocities using Shield's formula is considered in the design of sewers.

    \(V_{sc}=\sqrt{\frac{8k}{f}\left ( G-1 \right )gd}\)

Where K is a constant that depend upon the type of solids present in the sewage
f = Friction factor

(ii) The above formula indicates that the velocity required to transport material in sewers is mainly dependent on the particle size and specific gravity and slightly dependent on conduit shape and depth of flow.

The specific gravity of grit is usually in the range of 2.4 to 2.65. Gravity sewers shall be designed for the velocities as given in Table below:

Explanation:

Activated sludge process:

• The essential features of the activated sludge process are an aeration stage, solids-liquid separation following aeration, and a sludge recycle system.
• Wastewater after primary treatment enters an aeration tank where the organic matter is brought into intimate contact with the sludge from the secondary clarifier.
• It requires less space, does not produce obnoxious odor, and requires less time for wastewater treatment.
• It requires skilled supervision

Followings are the classification of secondary treatment units:

Explanation:

Detention time for different type of Treatment unit

Concept:

Sludge volume index is the volume occupied in mL by one gm of solids in the mixed liquor after settling for 30 minutes.

The recommended value of the Sludge Volume Index (SVI) for municipal sewage is approximately lying in the range of 80 to 150.

\({\rm{Sludge\;volume\;index}} = \frac{{{\rm{Volume\;of\;settled\;sludge\;in\;ml}}}}{{{\rm{MLSS\;present}}}}\)

Calculation:

Given,

Volume of settled sludge = 100 ml, Drying weight (MLSS) = 800 mg

\(\rm SVI = \frac{{100}}{{800 \ \times \ {{10}^{ - 3}}}}\ ml/gram\)

SVI = 125 ml/gram

The correct answer is septic.

Key Points

• A septic tank is concrete, fibreglass, or plastic subterranean chamber through which domestic wastewater (sewage) flows for basic treatment.
• Anaerobic action occurs within the septic tank. The term "septic" refers to the anaerobic bacterial environment that forms in the tank and decomposes the waste that is expelled.
• Domestic sewage, as well as animal and poultry wastes, are examples of N-rich materials that supply nutrients for anaerobic organism development and multiplication.

Important Points

• A pressure/vacuum vent is installed in pressure tanks to avoid venting loss due to boiling and breathing loss due to daily temperature or barometric pressure variations. 

Concept:

Sludge volume index is the volume occupied in mL by one gm of solids in the mixed liquor after settling for 30 minutes.

The recommended value of the Sludge Volume Index (SVI) for municipal sewage is approximately lies in the range of 80 to 150.

\({\rm{Sludge\;volume\;index}} = \frac{{{\rm{Volume\;of\;settled\;sludge\;in\;ml}}}}{{{\rm{MLSS\;present}}}}\)

Calculation:

Given,

Volume of settled sludge = 100 ml, Drying weight (MLSS) = 800 mg

\(\rm SVI = \frac{{100}}{{800 \ \times \ {{10}^{ - 3}}}}\ ml/gram\)

SVI = 125 ml/gram

Concept:

A septic tank is an underground chamber made of concrete, fiberglass or plastic through which domestic wastewater (sewage) flows for basic treatment. Action within the septic tank is anaerobic in nature.

The term "septic" refers to the anaerobic bacterial environment that develops in the tank which decomposes the waste discharged into the tank.

Followings are the classification of secondary treatment units

Explanation:

Explanation:

The four processes of wastewater treatment are:

1. Preliminary Treatment

• Screening, grit chambers, and skimming tanks.

Note:

In Preliminary Treatment:

• Oils and greases are removed by skimming tanks.
• Floating materials are removed by screening.
• Grit and sand are removed by grit chambers

2. Primary Treatment

• Sedimentation (basically involves the process of sedimentation or settling),
• Chemical-aided Sedimentation

3. Secondary or Biological Treatment

• Activated sludge process
• Aerated lagoons         
• Sequencing batch reactor
• Aerobic digestion
• Trickling filters
• Rotating biological contractors

4. Tertiary or Advanced Treatment

• Solids removal
• Biological nitrogen removal
• Biological phosphorus removal
• Disinfection.

Explanation:

Soak pit: 

• A soak pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground.
• A soak pit is a covered, porous-walled chamber that allows water to slowly soak into the ground and also known as a soakaway or leach pit, and the pre-settled sewage from the septic tank is release to the underground chamber from where it infiltrates into the neighboring soils.
• So soak pits are preferred where the soil is porous in nature.

Explanation:

A trickling filter is a wastewater treatment system that removes organic matter from wastewater. It is an Aerobic attached growth system. 

From the above table, we can see that the most suitable option will be option A ie 110 - 330 ML per hectare per day