Q.BOD of waste water is estimated by measuring the amount of:

Step 1 — Definition of BOD

Biochemical Oxygen Demand (BOD) refers to the amount of oxygen that would be consumed if all the organic matter in a given volume of water were oxidised by bacteria (aerobic decomposers) present in that water, typically measured over 5 days at 20°C.

Step 2 — Why it reflects pollution level

Sewage-polluted water contains large amounts of biodegradable organic matter, so more oxygen is consumed by the decomposing bacteria as they break it down — the more polluted the water, the higher the BOD.

Step 3 — Ruling out the distractors …

1. What water potential means

Water potential ($\Psi_w$), measured in pascals (Pa) or bars, is the free energy of water per unit volume — in effect, a measure of how able water is to move. Water always flows spontaneously from a region of HIGHER water potential to a region of LOWER water potential; this movement down the $\Psi_w$ gradient is osmosis.

Like any potential, it needs a zero point, and biologists define that zero point as pure water at standard temperature and pressure. That is not a measured result — it is the chosen reference:

Since pure water has the maximum possible concentration of water molecules and hence the maximum free energy, zero is also the highest value on this scale.

2. The consequence — why every solution is negative

Water potential has two components:

where $\Psi_s$ is the solute (osmotic) potential and $\Psi_p$ the pressure potential.

Dissolving a solute in water lowers the free energy of the water, so $\Psi_s$ is ALWAYS negative. Therefore, for a solution at atmospheric pressure ($\Psi_p = 0$):

$\Psi_w = \Psi_s < 0$ …

The key concept here is Biochemical Oxygen Demand (BOD). BOD tells you how much dissolved oxygen is used up by microorganisms over a fixed time (usually 5 days at 20°C) as they decompose organic matter in water. The more organic waste present, the more oxygen the microbes need, and the higher the BOD reading. Clean water has a low BOD (under 1 mg/L), while heavily polluted water can have BOD values in the tens or even hundreds of mg/L.

So the question is straightforward: among the four given values, which one is the largest? That sample has the most organic pollution.

1. List the BOD values:

   (A) $0.16\ \text{mg/L}$

   (B) $0.6\ \text{mg/L}$

   (C) $0.06\ \text{mg/L}$

   (D) $6\ \text{mg/L}$

2. Compare them directly. $0.06$ is the smallest, then $0.16$, then $0.6$, and finally $6$ — which is ten times larger than the next highest. …

Step 1 — The greenhouse effect in one line.

Clouds and gases in the atmosphere reflect ~$75\%$ of incoming solar radiation; the rest is absorbed by the Earth's surface and re-radiated as infrared (long-wave) radiation. Greenhouse gases — $\mathrm{CO_2}$, $\mathrm{CH_4}$, CFCs and $\mathrm{N_2O}$ — absorb this outgoing infrared and re-emit it, warming the Earth. Without them the average surface temperature would be about $-18^\circ\text{C}$ rather than the present $\approx 15^\circ\text{C}$.

Step 2 — The relative contributions to total global warming.

Sanity check on the total:

Note the ordering — it is the easiest way to remember: $\mathrm{CO_2} > \mathrm{CH_4} > \mathrm{CFC} > \mathrm{N_2O}$. (Contribution $\ne$ potency: a single CFC or $\mathrm{N_2O}$ molecule traps far more heat than a $\mathrm{CO_2}$ molecule, but $\mathrm{CO_2}$ dominates the total because it is released in vastly greater quantity.)

Step 3 — Test each option against that ordering. …

The question gives you stroke volume — $70~\text{mL}$ per beat — but doesn't give the heart rate. That's the key: cardiac output is not just stroke volume; it's stroke volume multiplied by the number of beats per minute. For an adult human at rest, the normal heart rate is about 72 beats per minute. This is standard physiological knowledge for such problems.

Let's walk through it.

1. Cardiac output per minute Cardiac output ($CO$) is defined as:

With stroke volume $= 70~\text{mL/beat}$ and heart rate $= 72~\text{beats/min}$:

2. Convert to litres per minute Since $1~\text{L} = 1000~\text{mL}$:

3. Extend to one hour One hour has 60 minutes, so: $CO_{\text{hour}} = 5.04 \times 60 = 302.4~\text{L/hour}$ …

Step 1 — What biogas is

Biogas is a mixture of gases (predominantly methane, with CO$_2$ and H$_2$S) produced by the anaerobic breakdown of organic matter — cattle dung ('gobar'), sewage and plant waste — by methanogens such as Methanobacterium. These bacteria are also present in the rumen of cattle, which is why cattle dung is such a good starting slurry.

Step 2 — Who developed the technology in India

• KVIC — Khadi and Village Industries Commission: promoted and popularised the village-level biogas (gobar gas) plant as a rural, decentralised energy source.
• IARI — Indian Agricultural Research Institute: carried out the technical/agricultural research behind plant design and digestion of cattle dung. …

Step 1 — What BOD actually measures

Biochemical Oxygen Demand (BOD) is the amount of oxygen that bacteria would consume in oxidising all the organic matter in one litre of water. So BOD is a proxy for organic load: the dirtier the water, the higher its BOD. Discharging high-BOD effluent into a river would let river microbes strip the water of dissolved oxygen and kill the fish.

Step 2 — The stages of sewage treatment (NCERT, Microbes in Human Welfare)

Primary treatment — physical

• Sequential filtration removes floating debris; sedimentation removes grit and soil. The settled solids form the primary sludge; the supernatant is the primary effluent.

Secondary treatment — biological (the key step)

• The primary effluent is passed into large aeration tanks and constantly agitated mechanically while air is pumped in.
• This lets useful aerobic flocs (masses of bacteria with fungal filaments in mesh-like structures) grow rapidly. They consume the major part of the organic matter in the effluent.
• Result: the BOD of the effluent falls sharply (by about 90–95%). NCERT states that treatment continues until the BOD is reduced, and it is precisely this BOD drop that makes it safe to release the water into a natural water body.

Step 3 — Why the other options fail

• (A) Removes debris — that is primary treatment (physical filtration/sedimentation), not secondary. …

The question is about a specific legal directive from the Supreme Court of India regarding the implementation of Bharat Stage VI (BS-VI) emission norms. This is not a physics or mathematics problem — it is a factual recall question from current affairs or environmental law, often appearing in Indian competitive exams like UPSC, SSC, or state PCS.

The key concept here is that the Supreme Court, in a landmark ruling, accelerated the timeline for BS-VI norms to combat air pollution, especially in the National Capital Region. The original government plan was to introduce BS-VI in phases, but the Court ordered a single nationwide deadline.

Let’s break down the timeline:

1. Original government plan: The Ministry of Road Transport and Highways had initially proposed a phased rollout — BS-VI in the National Capital Region from April 2019, and nationwide from April 2020 for new vehicles. However, this was later revised to a uniform nationwide date of April 2020 for all vehicles.

2. Supreme Court intervention: In a case concerning air pollution (M.C. Mehta vs. Union of India), the Supreme Court examined the feasibility of an earlier implementation. The Court noted that the auto industry had already been given sufficient time to prepare, and that delaying BS-VI would worsen public health.

3. The ruling: On October 24, 2018, the Supreme Court passed an order directing that no BS-IV vehicle shall be sold or registered in India after March 31, 2020. This effectively meant that from April 1, 2020, all new vehicles sold in the country must comply with BS-VI norms. The Court rejected requests for extensions from automobile manufacturers. …