FUNDAMENTALS OF ELECTRICITY

Unit of charge:

Coulomb is used as the unit of charge.

Charge of an electron is,

e = -1.60217733 x 10-19 C

Electric current:

Directed flow of free electrons(charge)is called electric current.

Actual direction of current is from negative terminal to positive terminal.

Current flowing from positive terminal to negative terminal is called conventional current.

Types of electric current:

Electric current is classified into the following: -

1. Steady current
2. Varying current
3. Alternating current

Steady Current: When the magnitude of current does not change with time, it is called as steady current.

Varying current: When the magnitude of current changes with time, it is called as varying current.

Alternating current: When the magnitude of current changes continuously with time and direction changes periodically, it is called as alternating current.

Electric Potential:

The capacity of a charged body to do work is called as electric potential.
Electric potential, V = Work-done/Charge = W/Q
Unit of Electrical potential, V = Joule/Coulomb = Volt

Potential Difference:

The difference in the potentials of two charged bodies is called potential difference.

It is also called as voltage.

The current will flow in a circuit if a potential difference exists in the circuit.

Resistance:

The opposition of the flow of electric current is called its resistance.

Unit of resistance is Ohm and is denoted by Ω.

Factors upon which resistance depends:

The resistance of a conductor,

• Is directly proportional to its length(l).
• Is inversely proportional to its cross sectional area(a).
• Depends upon the nature of the material.
• Changes with temperature.

i.e.,

Rα l/a

R=pl/a

Where,

P=Resistivity

Resistivity or specific resistance:

It is defined as the amount of resistance offered by a material of length ‘l’ and a cross sectional area ‘a’.

It is denoted by p.

Effect of temperature on resistance:

1. The resistance of pure metal (copper, aluminum), conductors increases with increase in temperature. It has a positive temperature coefficient of resistance.
2. Resistance of electrolytes, insulators, semiconductors (silicon, germanium) decreases with increase in temperature. It has a negative temperature coefficient of resistance.
3. Resistance of alloys increases with increase in temperature but this increase is very small and irregular.

Ohm’s law:

It states that the ratio of potential difference (v) across the end of a conductor to the current (I) flowing between them is constant, provided the physical condition (temp) does not change.

V/I = Constant = R

Electric Power: Rate at which work is done in an electrical circuit is called electric power.

Electric power = Work done in electric circuit/Time

P = VIt/t = VI = I²R = V²/R

Unit of power is Joule/Sec or watt.

1KW = 1000 watts

1MW = 10⁶ watts = 10³ KW

Electrical Energy:

The total work done in an electrical circuit is called electrical energy.

Electrical energy = Electric power x time

= VIt

=I²Rt

=V²t/R

Its unit is Kwh (Kilowatt- hour)

1Kwh of electrical energy is called Board of time (BOT) unit.

Power Rating:

Ability of a resistor to dissipate heat without destructive temperature built up

MARINE SEWAGE TREATMENT PLANT

Principle of sewage treatment

• The breakdown of raw sewage in water is effected by aerobic bacteria if there is a relatively ample presence of oxygen, but by anaerobic bacteria if oxygen has been depleted.
• Aerobic bacteria require free oxygen to survive.They breakdown the organic matter to produce safe products such as water, carbon dioxide. Final discharge has a clean and clear appearance.
• Anaerobic bacteria can only multiply in the absence of free oxygen.They breakdown the organic matter into water, carbon dioxide,methane, hydrogen sulphide and ammonia which are noxious and toxic.

Pic: Sewage treatment plant

Components/Chambers/Tanks

• Wire mesh
• Primary tank
• Aeration chamber
• Settlement tank
• Chlorination and collection tank
• Blowers
• Chlorinator
• Low level, high level alarm

Operation

• Sewage water is introduced into a primary vessel through a wire mesh.
• It is then passed through a wire mesh which grinds the sewage and breakdown into smaller particles.
• A plate is provided which makes the sewage to rise and flow into the aeration chamber.
• Air is forced through the diffuser into the air chamber which helps the aerobic bacteria to grow and attack the sewage.
• Constant pressure of around 0.3-0.4 bar is kept for proper agitation and formation of bubbles.
• The sewage is decomposed into carbon dioxide, water and inorganic sewage.
• The mixture of liquid and sludge is passed to the settling tank from the aeration chamber.
• The sludge settle at the bottom and  clear liquid on the top, sludge is not kept in settling tank as it may produce anaerobic bacteria and foul gases are produced.
• The sludge formed is recycled by sending it back to the aeration chamber.
• The clear liquid produced from the settling tank is overflown and is disinfected with the help of chlorine.
• This is done due to the presence of E-coli present in liquid which has to be eliminated.
• The liquid during chlorination is kept for a period of at least 60 min.
• The collected liquid is discharged overboard when the high level alarm raises and the vessel is in proper geological location. 

Effluent quality standards

• The bio-chemical oxygen demand (BOD) is determined by incubating at 200 degree Celsius, a sample of sewage effluent which has been well-oxygenated. The amount of oxygen absorbed over a five-day period is then measured. The test measures the total amount of oxygen used for complete breakdown of organic matter. This indicates the strength of the sewage. IMO recommends BOD of less than 50 mg/L after treatment through sewage treatment plant.
• Suspended solids - this can give rise to silting problems. Suspended solids are measured by filtering a sample through a pre-weighed pad which is then dried and re-weighed. IMO recommends 50 mg/ litre after treatment.
•  Coliform count - The e-coliform is a family of bacteria which live in the human intestine. The result of this test is called the e-coli count and is expressed per 100ml. Presence of these organisms in water is an indication of pathogen (disease causing bacteria responsible for cholera, dysentery, typhoid). IMO recommends fecal coliform count of less than 250/100 ml. of affluent after treatment.

The rules which has to be followed for the discharge of sewage is given in annex IV as below.

Marpol Annex IV: -

• Vessel should be equipped with a certified sewage treatment system or holding tank.
• Within 3 miles of nearest land, sewage discharges are to be treated by a certified marine sanitation that is a sewage treatment plant device prior discharge.
• Between 3 miles and 12 miles from shore, sewage discharges must be treated by no less than maceration or chlorination.
• Sewage discharge beyond 12 miles from shore is unrestricted.
• The discharge of sewage should be such that the ship is en route and is proceeding not less than 4 nautical miles.