The idea being tested is that in a batch reactor there is no feed or withdrawal during the reaction. You load the reactor with the reactants, seal it, and let the reaction proceed (often with stirring) for a set time. During this stage the volume stays fixed and there is no flow in or out. After the reaction is complete, you remove the product. This contrasts with continuous-flow reactors like complete-mix (CSTR), plug flow, and packed bed, where reactants are constantly entering and products are leaving as the reaction proceeds. So, the batch reactor is the one with no flow in/out during the reaction.

The main idea here is how coagulants help remove suspended particles by destabilizing them and forming larger aggregates. Coagulants neutralize or compress the electrical charges on particle surfaces, reducing repulsion so particles can come together. When the repulsion is lowered, mixing or natural motion lets particles collide more easily. Those collisions lead to the growth of larger aggregates, or flocs, which can then settle out or be filtered. Among the statements, reducing repulsion is a true benefit because it directly enables particle aggregation. Allowing particles to collide is also a benefit, since collisions are the mechanism that starts the flocculation process. Creating flocs is the end result of successful collisions and destabilization, making removal via sedimentation or filtration possible. Increasing color stability, however, is not a benefit of using coagulants. Coagulation is used to remove color-causing substances by forming flocs that are then removed, which typically reduces color, not stabilizes it.

Rapid mixing is the quick, thorough distribution of added coagulants in the water to destabilize suspended particles and kick off floc formation. The goal is to achieve uniform contact between the coagulant and colloids so their charges are neutralized and small particles begin to clump together. This requires high-energy mixing for a short time, after which gentler stirring allows larger flocs to form and then be removed by sedimentation or filtration. That’s why dispersing chemicals quickly is the best description. The other options don’t fit: dissolving chlorine is about disinfection, not the rapid distribution of coagulants; increasing temperature isn’t the aim of mixing; and filtration is a later step, not the mixing phase.

Attached microbial growth is described by biofilm. A biofilm is a community of microorganisms that adhere to a surface and produce a sticky extracellular matrix that helps them stay attached and stick together. This matrix creates a protected, structured environment with channels for nutrients and waste, which often makes the community more resistant to disinfectants and physical removal. You commonly find biofilms on pipes, medical devices, and teeth, where cells remain anchored and can act in a coordinated way. Suspened growth, by contrast, describes microbes floating freely in liquid rather than sticking to a surface. Aerobic and anaerobic refer to oxygen requirements for metabolism, not whether the organisms are attached.