Environmental Engineering Test 2 Nddc Scholarship

Biological oxidation ponds (stabilization ponds) utilize a symbiotic relationship between aerobic bacteria and algae. The bacteria consume and oxidize organic matter (BOD) for energy and growth (A), while the algae provide the necessary oxygen via photosynthesis (B). This natural process effectively purifies wastewater using solar energy and microbial activity.

Silica gel is a highly porous form of silicon dioxide that acts as a physical adsorbent. It is effective at trapping nitrogen oxides (NOx) from gas streams due to its high surface area and affinity for polar molecules. While active carbon is also an adsorbent, silica gel is frequently preferred in specific gas-cleaning applications for NOx removal.

Methyl iso-cyanate (MIC) is one of the most toxic industrial chemicals known, as evidenced by the Bhopal disaster. Its TLV is extremely low, at approximately 0.02 ppm, reflecting its extreme reactivity and the severe damage it causes to human tissue at even trace concentrations. Phosgene and ozone have slightly higher TLVs (around 0.1 ppm).

The Dobson Unit (DU) measures the total amount of ozone in a column overhead. A value of 230-300 DU is considered the global average thickness (equivalent to a layer only 3mm thick at standard temperature and pressure). Anything below 220 DU is scientifically defined as an 'ozone hole'.

Polyvinyl chloride (PVC) is often avoided for direct contact with foodstuffs. This is because certain additives used to make PVC flexible, such as phthalates, or trace amounts of the unreacted vinyl chloride monomer, can leach into the food. These substances are associated with various health risks, leading to the use of safer plastics like PET or HDPE for food packaging.

Nitrogen is physiologically inert but acts as a simple asphyxiant by displacing oxygen. If the concentration of N2 rises to 84% or higher, the oxygen concentration in the air drops below 16% (normal is 21%). At this level, the partial pressure of oxygen is insufficient to support human life, leading to rapid unconsciousness and death.

While specific values can vary slightly by regulatory body (OSHA vs. ACGIH), 5 ppm is a common safety threshold for irritant gases like nitrogen dioxide. Ammonia often has a higher 8-hour limit (25 ppm), but in many safety contexts, 5 ppm is used as a conservative 'trigger' level for monitoring these specific industrial vapors.

Boiler corrosion is primarily caused by dissolved oxygen and acidic conditions. Deaeration (A) physically removes oxygen; alkali addition (B) prevents acidic attack; and chemical scavengers like sodium sulphite or hydrazine (C) chemically react with remaining oxygen. Implementing all these methods ensures the longevity and safety of the boiler system.

Slow sand filters operate at a much lower velocity than rapid sand filters to allow for biological purification. The typical filtration rate is 100 to 200 litres per square metre per hour. This slow pace allows a 'Schmutzdecke' (a biological film of algae and bacteria) to form on the surface, which effectively removes pathogens and organic matter without the need for chemical coagulants.

Waste stabilization ponds, or lagoons, are a comprehensive natural treatment solution. They are shallow artificial lakes (A) that rely on aerobic bacteria and algae (B) to break down waste. When arranged in a series, they are highly efficient, producing a high-quality effluent (C) with very low suspended solids and BOD, making them ideal for rural or decentralized wastewater management.