1.      Explain why anaerobic treatment is expected to be popular in tropical developing countries like India. (2)



        Low power requirement, which is attractive for energy-poor developing countries.

        Minimal operation and maintenance skills and cost are required.

        Sludge production is low, hence sludge handling is easy

        Production of methane, which can be used as fuel

        High temperatures prevalent in tropical regions ensure that kinetics of anaerobic degradation is favorable, leading to faster treatment and smaller reactor sizes.




2.      Explain why attached growth anaerobic reactors are designed based on organic loading rate (OLR) and hydraulic retention time (HRT). (2)



The performance of an attached growth anaerobic reactor having a fixed amount of biomass per unit volume depends on how much substrate is available to the microorganisms, and how much time is available for consumption/degradation of this substrate. Organic loading rate (OLR) is a measure of how much substrate is available to the microorganisms. Hydraulic retention time (HRT) is a measure of the amount of time available to the microorganisms for the consumption/degradation of the substrate.




3.      Explain why a fluidized bed reactor is expected to be more efficient, (i.e., operate at higher OLR for a particular HRT and give comparable removal efficiency) as compared to a down-flow or up-flow anaerobic reactor. (2)



Other things (OLR and HRT) remaining same, the performance of an anaerobic reactor depends on the amount of biomass that is retained per unit volume of the reactor. In a fluidized bed reactor, the reactor configuration and mode of operation ensures that a larger amount of biomass can be maintained per unit volume of the reactor due to the larger area provided for attachment of biomass. Hence the improved performance.




4.      Explain why treatment of domestic wastewater by anaerobic methods is practical using only UASB reactors. (2)



Obtaining low effluent COD concentration is a requirement of any domestic water treatment method. Obtaining low COD concentration is possible only is the specific substrate utilization rate in the reactor is low, while the overall substrate utilization rate is high. This is only possible if the biomass concentration in the reactor (X) is high. Among anaerobic reaction configurations, UASB process is the only one that enables the maintenance of high enough biomess (X) concentration to make domestic wastewater treatment practical.



5.      Draw a neat sketch and explain the utility and working principle of a gas-liquid-solid separator (GLSS) in a UASB reactor. (2)


Gas Domes



















Biomass/sludge particles often escape from the sludge blanket, especially when they get buoyant due to the attachment of gas bubbles. The idea behind providing GLSS is to ensure that such buoyant sludge particles impact the GLSS surface. The impact of this collision is sufficient to ensure that the gas bubbles and sludge particles are separated. The sludge particles then come less buoyant and tend to settle back to the sludge blanket, while the gas bubbles escape and are collected in the gas domes. The settling of sludge particles are further assisted by the provision of a quiescent sludge settling zone as shown in the figure above. All these measures ensure that sludge retention in an UASB reactor is very high.