Biofilm recovery (Regrowth)
Bacteria associated with biofilms are much more difficult to kill and remove from surfaces than planktonic organisms. According to Characklis (1990), numerous investigators and plant operators have observed "a rapid resumption of biofouling immediately following chlorine treatment." Incomplete removal of the biofilm will allow it to quickly return to its equilibrium state, causing a rebound in total plate counts following sanitization.

In figure below (Mittelman 1986), shows typical regrowth following sanitization. Initially, the bulk water bacteria count dropped to zero after sanitization, but this was followed by a gradual increase in numbers to levels at or below the pretreatment levels. In this example, regrowth started after 2 days and was back up to equilibrium levels after 20 days. This is similar to results seen in in-house sanitization testing at Edstrom Industries.

 
According to Characklis (1990), biofilm recovery may be due to one or all of the following.

1. The remaining biofilm contains enough viable organisms that there is no lag phase in regrowth. Thus, biofilm recovery after shock chlorination is faster than initial accumulation on a clean pipe.
2. The residual biofilm on the surface makes it rougher than clean pipe. The roughness of the deposit may provide a stickier surface which adsorbs more microbial cells and other compounds from the water.
3. The chlorine preferentially removes extracellular polymers and not biofilm cells, thus leaving biofilm cells more exposed to the nutrients when chlorination ceases.
4. Surviving organisms rapidly create more slime (extracellular polymers) as a protective response to irritation by chlorine.
5. There is selection for organisms less susceptible to the sanitizing chemical. This is usually the organisms that produce excessive amounts of slime like Pseudomonas.