Microbes in Human Welfare Set-1

Q1. A rural biogas plant produces $500\ \mathrm{m^3}$ of biogas per day with methane content $60\%$ by volume. Given the calorific value of methane as $35.8\ \mathrm{MJ\;m^{-3}}$ and $1\ \mathrm{kWh}=3.6\ \mathrm{MJ}$, the daily energy (in kWh) obtainable by complete combustion of the methane fraction is closest to (use $V_{CH_4}=0.6\times500$, $E_{\mathrm{MJ}}=V_{CH_4}\times35.8$, $E_{\mathrm{kWh}}=E_{\mathrm{MJ}}/3.6$):

Q2. A sewage treatment plant receives wastewater at $10{,}000\ \mathrm{m^3\,day^{-1}}$ with influent BOD $C_0=300\ \mathrm{mg\,L^{-1}}$. Primary treatment removes $30\%$ of BOD and the secondary biological stage removes $85\%$ of the BOD entering the secondary stage. Calculate the approximate BOD (in mg L$^{-1}$) of the final effluent and indicate whether it meets the discharge standard of $30\ \mathrm{mg\,L^{-1}}$ (use $C_{\text{after primary}}=C_0(1-0.30)$ and $C_{\text{final}}=C_{\text{after primary}}(1-0.85)$):

Q3. In a batch fermentation, Saccharomyces cerevisiae is grown on a medium with initial glucose $100\ \mathrm{g\;L^{-1}}$. After 48 h the broth contains $45\ \mathrm{g\;L^{-1}}$ ethanol. Assuming the theoretical maximum yield is $0.51\ \mathrm{g\ ethanol\;per\;g\ glucose}$, calculate the percentage of glucose converted to ethanol and state which inference is most reasonable about oxygen availability during the run (use $\%\ \text{conversion}=\dfrac{45}{100\times0.51}\times100$):

Q4. For composting, the optimal C:N ratio is $30:1$. Waste A has $45\%$ C and $2.0\%$ N (dry basis); Waste B has $35\%$ C and $0.5\%$ N (dry basis). If $x$ is the mass fraction of A in a dry mixture, solve $\dfrac{0.45x+0.35(1-x)}{0.02x+0.005(1-x)}=30$. What mixture (by mass percent) of A:B achieves the target C:N?

Q5. An activated sludge aeration tank has volume $V=5000\ \mathrm{m^3}$ and MLSS $X=3000\ \mathrm{mg\;L^{-1}}$. Influent flow $Q=2000\ \mathrm{m^3\;day^{-1}}$ with BOD$_5$ $C_0=250\ \mathrm{mg\;L^{-1}}$. Using $F/M=\dfrac{Q\,C_0}{V\,X}$ (convert mg L$^{-1}$ to g m$^{-3}$ as $1\ \mathrm{mg\;L^{-1}}=1\ \mathrm{g\;m^{-3}}$), compute $F/M$ in $\mathrm{kg\;BOD\;kg^{-1}\;MLSS\;day^{-1}}$ and choose the most likely operational problem at that $F/M$:

Q6. In anaerobic fermentation by yeast the overall stoichiometric reaction is $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$. If 180 g of glucose is completely fermented according to this reaction, the theoretical mass of ethanol produced is closest to:

Q7. A rural biogas digester fed on cow dung (C:N ≈ 25:1) produced gas with ~60% CH₄. The farmer mixed in a large amount of poultry manure and the measured methane fraction dropped to ~45% with accumulation of volatile fatty acids (VFAs). Which of the following is the most likely explanation for this observation?

Q8. A chemostat is used to produce a growth-associated metabolite. The microorganism has a maximum specific growth rate $\mu_{\max}=0.45\ \mathrm{h^{-1}}$. The reactor is initially run at dilution rate $D=0.20\ \mathrm{h^{-1}}$ and later the operator increases $D$ to $0.50\ \mathrm{h^{-1}}$. Which outcome is most likely?

Q9. Assertion (A): Alternating aerobic and anoxic zones in a wastewater treatment plant can achieve efficient nitrogen removal without external carbon dosing.
Reason (R): During the aerobic nitrification stage all easily biodegradable organic carbon is consumed, leaving virtually no electron donors for denitrifiers in the subsequent anoxic zone.
Which one of the following is correct?

Q10. Assertion (A): Wastewater treatment plants act as hotspots for horizontal gene transfer (HGT) of antibiotic resistance genes.
Reason (R): High microbial densities, continuous exposure to sub-inhibitory concentrations of antibiotics and co-selective agents (e.g., heavy metals), together with prevalence of mobile genetic elements (plasmids, transposons) under selective pressure, promote HGT.
Which one of the following is correct?