In biological treatment, what acts as food for the bacteria?

The organic matter (BOD) of the sewage

What is the difference between oxidation and synthesis in aerobic treatment?

Oxidation breaks organics to CO2, water and energy; synthesis builds new cells

Oxidation makes new cells; synthesis gives CO2 and energy

Oxidation occurs without oxygen

Which comparison between grit chamber and sedimentation tank is correct?

Grit chamber: about 0.2-0.3 m/s and 30-60 s; sedimentation tank: very low velocity and 2-3 h

Grit chamber: 2-3 h detention; sedimentation tank: 30-60 s

Both have a flow velocity of 0.2-0.3 m/s

Grit chamber is placed after the sedimentation tank

How does the sludge differ between the two units?

Grit is dry inert material; sedimentation sludge is wet, putrescible and needs digestion

Grit is putrescible; sedimentation sludge is inert sand

2079 Chaitra · 4+4 marks

Biological treatment principles; grit vs sedimentation

Short answer

Biological treatment uses microorganisms (mainly bacteria) to oxidize organic matter (BOD) and produce new cells under aerobic conditions, controlled by F\/ M, sludge age and oxygen. Grit chambers remove heavy inorganic grit (specific gravity2.65, size >0.15 mm) with high flow velocity (0.2–0.3 m\/ s) and short detention (30–60 s), whereas sedimentation tanks settle fine organic solids (gravity ~1.2) at very low velocity (~0.005 m\/ s) over2–3 hours.

Explain briefly the principles of biological wastewater treatment. Enlist the differences between grit chamber and sedimentation tank.

Principles of biological wastewater treatment

Biological treatment removes dissolved and colloidal organic matter by the metabolic action of micro-organisms, mainly bacteria.

Food and organisms: the organic matter (BOD) of sewage is the food for the bacteria. They need oxygen (aerobic process), nutrients (N and P), pH of 6.5-8.5 and temperature of 20-35°C.
Oxidation (energy): $\text{organics} + O_2 \rightarrow CO_2 + H_2O + \text{energy}$
Synthesis (growth): $\text{organics} + O_2 + N \rightarrow \text{new cells}$
Endogenous respiration: when food runs short the cells oxidise their own mass, $\text{cells} + O_2 \rightarrow CO_2 + H_2O + NH_3$ .
The new cells are removed as sludge by settling, leaving a clear effluent with low BOD.
Anaerobic treatment (no oxygen) gives methane and CO₂ and is used for sludge digestion and strong wastes.
The process is operated as suspended growth (activated sludge) or attached growth (trickling filter, RBC), controlled by the $F/M$ ratio, sludge age, oxygen supply and temperature.

Differences between grit chamber and sedimentation tank

Point	Grit chamber	Sedimentation tank
Purpose	Removes heavy inorganic grit (sand, gravel, cinders)	Removes settleable organic and fine suspended solids
Particle size / $G$	About 0.15 mm and above, $G\approx 2.65$	Fine particles, $G\approx 1.2$ or less
Position	Before the sedimentation tank (after screens)	After the grit chamber
Flow velocity	About 0.2-0.3 m/s, kept constant	Very low, about 0.3 m/min (about 0.005 m/s)
Detention time	30-60 seconds	2-3 hours (primary)
Size	Small, narrow, long channel	Large rectangular or circular tank
Velocity control	Needed (proportional weir or Parshall flume)	Not critical
Sludge	Dry, inert grit; removed by hand or scraper	Wet putrescible sludge (about 95% water) needing digestion
Surface loading	High, 800-1200 m³/m²/day	Low, 25-40 m³/m²/day

✓ Answer checked by Claude Opus 5.5 (high effort)

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