http://www.science.gov/topicpages/m/minimising+excess+sludge.html

Check this link.

hope it will be useful.

https://books.google.co.in/books?id=YiZz_xDk7rkC&pg=PA204&lpg=PA204&dq=cryptic+and+chronic+growth+of+bacteria&source=bl&ots=LEHLFL0vUV&sig=X3WrSpNCCneZUufCBWNhfBsHatI&hl=en&sa=X&ved=0ahUKEwjh5cScufvJAhVKGo4KHd1nATMQ6AEILjAC#v=onepage&q=cryptic%20and%20chronic%20growth%20of%20bacteria&f=true

 Dear Riccardo Campo chech this link.

I hope this link can help you

http://microbiology.okstate.edu/faculty/demed2/Notes/Microbial%20growthdoc.html

thanks for the questions and answers

The term 'cryptic growth' stands for a hidden growth of some individual cells in a starving bacterial culture in a death phase. The prevailing majority of cells do not grow at all,  the total biomass measured as cfu's or OD in a starving culture decline but few bacteria are able to multiply. A common opinion is that these survival cell use products of lysis of dying cells. There are no a simple method to measure the intensity of cryptic growth, mostly it is inferred rather than experimentally measured.

The term 'chronic' is applied to slow developing infection or other microbial processes. 'Chronic starvation' stands for a deeply limited growth when nutrients are not completely depleted (as in a starving batch culture) but rather are continuously supplied with extremely low rate (e.g. in retentostat, dialysis culture or chemostat culture with low feeding rate).