17. (Activated sludge processes): Indicate whether the following statements are

Question

17. (Activated sludge processes): Indicate whether the following statements are true (T) or false ('F"). 17.1. EAAS systems use long SRTs (20-30 days) to partially stabilize the biomass produced. This requires large bioreactors with HRTs (hydraulic retention times) of 24 h or more, but results in excellent process stability and the production of a high-quality effluent. 17.2. In SFAS systems, biomass and oxygen-enriched gas flow co-currently through a staged, covered bioreactor. This allows use of high volumetric organic loading rates and short HRTs S (1-2 days) are also generally use

Explanation / Answer

17.1 The given statement is true:

Extended aereation activated sludge systems are often used on waste waters that have not been treated in a physical operation to remove suspended organic matter, in that case it gets trapped in the biofloc and undergoes some stabilization and oxidation and have a large SRT (solid retention time)

17.2 The given statement is false

The biomass in each reactor of a multiple reactor system has unique specific growth rate in response to the growth limiting nutrients in the bioreactor, so the SRT is not related to the specific growth like in the case of the bioreactor and it is still a variable in the design process

At a given SRT and system HRT, the MLSS concentration entering the settler from the last tank of an SFAS system with influent distributed evenly along the system is less than the concentration from a CAS system or from a single CSTR. System performance, however, is only slightly worse than that of a single CSTR.

An SFAS system with influent distributed evenly along the system exhibits more diurnal variability in its effluent organic substrate and ammonia-N concentrations than a CSTR operating at the same SRT, but experiences about the same variability in its oxygen requirement.

The MLSS in an SFAS system has a concentration gradient from the influent to the effluent end, with the influent end of the system having higher concentrations. As a consequence, soluble organic substrate and ammonia-N exhibit the opposite concentration gradient. Increasing the recycle ratio around an SFAS system decreases the concentration gradient through the system and gives better system performance.

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