To give some idea of the amount of the currency that represents - there were more than 90 billion banknotes in circulation in India last March.
Most central banks destroy soiled and mutilated banknotes on a regular basis and replace them with new, crisp ones.
The Reserve Bank of India, similarly, shreds such notes and makes briquettes of them. But they are not your usual briquettes.
Briquettes - usually made of farm waste in India - are used for cooking, lighting and heating. They are cheaper than coal, have lower ash content, are less polluting, and easier to store and pack. They are mostly used as fuel in factory boilers.
But briquettes made out of shredded cash are brittle and serve no such purpose, a senior central bank official told me.
So the bank's 27 shredding and briquetting machines in 19 offices across India will now snip the expired banknotes into the smallest of pieces and the resulting briquettes will be then dumped in India's vast landfills.
Sometimes the shredded currency is also recycled to make files, calendars and paper weights and ballpoint pen shells, tea coasters, cups and small trays as souvenirs for guests.
The practice is similar in the US: counterfeit banknotes are sent to the Secret Service, while unfit notes are shredded and sent to landfills or given away as souvenirs to the public touring the Federal Reserve Bank.
Central bank officials believe shredding 20 billion banknotes will not be a huge challenge. In 2015-16 the Reserve Bank of India destroyed more than 16 billion soiled notes. More than 14 billion were removed in 2012-2013 after nearly 500,000 fake notes were found in the system.
"Destroying so much cash is not a challenge because we have enough shredding and briquetting machines with very high capacities. These are automatic machines which shred the cash into the finest of pieces," says an official.  So, India's mountain of expired currency will soon become rubbish, literally.

It is the latest development in efforts to solve the mystery of the aircraft, which went missing in March 2014.
The plane, flying from Kuala Lumpur to Beijing, had 239 people on board when it vanished.
It is presumed to have crashed into the sea after veering off course.
Three ships are searching a 120,000 sq km area of the southern Indian Ocean but have so far found no trace of the plane.
Five pieces of debris have been confirmed as definitely or probably from the plane.
Each was found thousands of miles from the search zone, though within the area models of ocean currents have indicated debris could wash up.
1. A section of wing called a flaperon, found on Reunion Island in July 2015 - confirmed as debris in September 2015
2. Horizontal stabilizer from tail section, found in Mozambique in December 2015
3. Stabilizer panel with "No Step" stencil, found in Mozambique in February 2016
4. Engine cowling bearing Rolls-Royce logo, found in March 2016 in Mossel Bay, South Africa
5. Fragment of interior door panel found in Rodrigues Island, Mauritius in March 2016
All the debris is being examined in Australia by the Australian Transport Safety Bureau (ATSB) and other experts.
They use manufacturing marks on the pieces as well as samples of marine ecology like barnacles to help confirm whether they are likely to have come from the missing Boeing 777.
Speaking on Thursday, Malaysia's Transport Minister Liow Tiong Lai said the team had "confirmed that both pieces of debris from South Africa and Rodrigues Island are almost certainly from MH370".
The ATSB also said both sections were "almost certainly" from 9M-MRO, which is the plane's registration.
No other 777 has ever crashed in the southern hemisphere, and none has reported missing pieces.
The ocean search, involving Australia, Malaysia and China, has scoured more than 105,000 sq km of seafloor so far, much of it areas which have never been explored before.
But the countries have agreed that in the absence of "credible new information" the search will end by the middle of the year.

Fishlock, 29, had been a player and assistant coach to Jo Montemurro, who has taken up a role with the club's men's team.
Her first match in charge will be the Round 11 fixture at Sydney FC on Sunday, 8 January.
Sydney are top of the table with Fishlock's side fourth.
India's central bank will have to destroy, by one estimate, some 20 billion "expired" banknotes after it scrapped two high-value denominations - the 500 ($7.60) and 1,000 rupee notes - this month to crack down on "black money" or illegal cash holdings.
[NEXT_CONCEPT]
Two pieces of aircraft debris found on beaches in Mauritius and South Africa almost certainly came from Malaysia Airlines flight MH370, say Malaysian and Australian officials.
[NEXT_CONCEPT]
Wales midfielder Jess Fishlock has taken over as Melbourne City coach for the final weeks of Australia's W-League season.