Hi Iman Jafari ,

Are you talking specifically about municipal water treatment systems? If so the likelihood is that the systems you are referring to are designed with more than one reactor in parallel so that one can be shut down for maintenance whilst at least one other identical/similar reactor remains working. This ensures that the required throughput of water is always achievable through the entire water treatment process during maintenance periods.

I hope this answers your question. If not I hope someone else can answer it better than I have been able to.

Iman Jafari, You need to be more specific about what reactors you mean and what their role is. It is often the case that one reactor is running and the other is not, due to regeneration of the catalytic bed which has been deactivated.. The same is the case with adsorbers. There must always be two, one of which is working and the other is where the adsorbent is regenerated.

Dear John Hoslett & Mirosław Grzesik ,

Many thanks for your valuable answers!

Likely for maintenance and reliability reasons as indicated, but another possibility is that when dealing with reactions which feature a declining rate over time, you can increase throughput by maximizing volume used for the high rate reactions (shorter RT) and not spending space on the low reaction rates that tend to dominate in the trailing period. In many cases, you can achieve greater throughput in parallel operation than in series. Of course, all quality specs need to be met, but in the economic world, capacity tends to trump efficiency or completeness.

Alternatively, parallel operation can be a sign of expansion. Additional units were added as capacity needs increased. Twinning (tripling, etc) is the easiest way to provide capacity expansion without excessive downtime that can be involved when modifying a circuit.