Frequently Asked Questions (FAQ) on Waste water / Sewage Treatment Plants (STP)

With this reply,I am answering both the above queries/comments in this one reply.

1. A vast majority of Sewage Treatment Plants(STP) in India today are based on a process that nature uses to take care of/dispose of waste.This is a process that involves the action of naturally occurring microbes on earth and in the air in consuming the waste.The microbes that do this work live/feed on air to be able to do this.This is why it is called an Aerobic process because air containing oxygen is used by the microbes as 'food'.
2. There are also other types of microbes which clean waste in a process where there is no air at all.This is called an Anaerobic process.This is usually found in the case of waste water with a very high pollution load.These bacteria are called 'methanogens' because this process generates methane gas (also called marsh gas) which can be used to light burners and heat any thing or to cook food.This reduces the pollution load substantially in the waste and it is then subjected to the above mentioned aerobic process.The waste from this process is sludge which is seperated from the treated sewage and disposed of after drying,when it can be used as manure in gardens.The same sludge is formed in the aerobic process (point 1) above.
3. Sewage differs from ordinary water by virtue of its Oxygen demand.There are two types of such demand. Biochemical Oxygen demand and Chemical oxygen demand.These are BOD and COD.
4. The STPs that operate on this natural process are called Activated Sludge Process (ASP) based plants.The have been around for close to a century by now and are used by a vast majority of those who purify sewage.Over this period, space has become very expensive and designers/manufacturers have developed modifications to reduce the 'foot print' of ASP based plants so as to occupy less space.
5. A new kind of STP has been developed that now uses a membrane to purify sewage and such plants are called Membrane Bio Reactors (MBR) which are very compact and produce a  quality of treated water far superior to that produced by ASP based STPs.MBRs occupy far less space than ASP based STPs.However, these are far more expensive and need automation to funtion optimally.

Sewage is a valuable resource in a world where fresh water is increasingly becoming scarce because of unchecked pollution of fresh water by untreated sewage.There will be a time when there may be no fresh water available,and,instead, treated sewage will have to be used instead, for human uses like drinking,cooking, bathing,etc. Singapore is one country which is already recovering sewage to meet 30% of its drinking water requirement.The whole world will now have to follow,particularly India.