I am doing forty days, one guy mention that organ start to shut down after 20 or 30 days. Is that true?

Thanks for answering.

No that is not true, Muslim people fast for 30 days every year and with the muslim population I'm pretty sure what he said is inaccurate. But I do not see why you are fassting.

Im 50# overweight and drop 11# on 5 days, mostly fecal matter. First time fasting and is not that bad. The black tea and the Zen meditation is helping somewhat. Also have issues with my knees because of the weight.

Read this article at WebMD about fasting: http://www.webmd.com/diet/fasting

I would not recommend fasting as a form of weight loss. You will not keep the weight off and will gain it all back almost immediately. This constant fluctuation of your body weight is not healthy. If you want to lose weight and keep it off, you should make some lifestyle changes. Dieting doesn't work either. You need to reform how you live.

I have lost a ton of weight and kept it off because not only did I stop eating unhealthy foods, I don't even allow them in my home anymore. If I do not purchase candy, chips, soda, etc, I cannot eat them.

You should also get some kind of moderate exercise each day. This does not mean you have to run 20 miles or something crazy.

No that is not true, Muslim people fast for 30 days every year
Muslims observe Ramadan by fasting during the daytime. They eat during the nighttime. "Fasting means abstaining from several things, including food and drink, from just before the starting of dawn (i.e. just before the start of Fajr) until sunset, with the intention of fasting." ~from muslimconverts.com

Becoming anorexic, even short term, won't help you, only harm you. Why not try proper diet and exercise?

It doesn't cause organ failure or else in the short term like a week or a month, but after 30 days, you might start to see definite muscle loss and slow heart beat. Heart is also built up of some amount of muscle, and you don't want to lose your heart muscles and have a heart failure, so keep in mind that it gets dangerous after 30-35 days.

Water fasting is extremely beneficial if you know how to prepare for it and how to break your fast.

I don't know much about water fasting and the like, but with juice fasting you can fast for up to 120 days no problem. I know people who have done it and run marathons, competed in triathalons, and all sorts of crazy things. Just remember you need to be getting proper nutrition and plenty of fluids. Low impact exercise like swimming is great for melting away the pounds too.