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What methods could an underground society with no access to the surface help to speed the decomposition rate of bodies?
Ideally, is there a method they could speed decomposition and still use the bodies as compost? They want the useful materials to feed the crops, but don’t really want Grandma sitting around in their corn.
There are a number of insects which feed on various parts of a corpse: flesh, keratin and even bones.
Dermestids are also called skin or hide beetles. Their larvae have the unusual ability to digest keratin. Dermestid beetles arrive late in the decomposition process, after other organisms have devoured the soft tissues of the cadaver and all that remains is the dry skin and hair. Dermestid larvae are one of the most common insects collected by forensic entomologists from human corpses.
The family Cleridae is probably better known by its other common name, the checkered beetles. Most are predaceous on the larvae of other insects. A small subset of this group, however, prefers to feed on flesh. Entomologists sometimes refer to these Clerids as bone beetles or ham beetles. One species in particular, or the red-legged ham beetle, can be a problem pest of stored meats. Bone beetles are sometimes collected from corpses in the later stages of decay.
Carrion beetle larvae devour vertebrate carcasses. Adults feed on maggots, a clever way of eliminating their competition on the carrion. Some members of this family are also called burying beetles for their remarkable ability to interr small carcasses. It's fairly easy to find carrion beetles if you don't mind examining roadkill. Carrion beetles will colonize a corpse during any stage of decomposition.
Hide or skin beetles from the family Trogidae can be easily missed, even when they've colonized a corpse or carcass. These small beetles are dark in color and roughly textured, a combination that acts as camouflage against the background of rotting or muddied flesh. Though only 50 or so species are found in North America, forensic entomologists have collected as many as 8 different species from a single carcass.
and many more.
Just feed the corpse of the deceased to the larvae of these insects, then process them into an organic mixture that you disperse in the soil.
This has the advantage of not having to directly process the remains of the loved ones. Many cultures in the past had giving back the corpse to nature as part of their funeral rites.
I'm not entirely sure where smoke enters into this, but the fastest way to dispose of a body under these circumstances would likely be to cut it into small pieces and bury the pieces in the soil they're using to grow food in, probably with a chunk next to each plant. Better yet, grind it into a fine paste and spread it over the garden.
Whether that's more or less distasteful than having their dearly departed lie around in the crops is up to you.
By using lye (sodium or, i.c., potassium hydroxide), you can put your bodies (make sure they're dead) through a process called alkaline hydrolysis (just look at that link, *wink* *wink*).
The process takes only 4-6 hours (if heated to a temperature of around 160 °C (320 °F)).
From the Wikipedia article (slightly abridged):
The result is a quantity of green-brown tinted liquid [..], and soft, porous white bone remains [..] easily crushed in the hand [..] to form a white-colored dust.
The "ash" can then be returned to the next of kin of the deceased [or used as fertilizer]. The liquid [can be] [..] use[d] in a garden or green space.
Reading on, we can see it is actually an energy-efficient and relatively low-pollutive way of dealing with your dead.
You do need access to the outside world as you require a lot of trees and rainwater.
The trees don't need to be alive, though (which is good news if your civilization decided to take root underground because the surface of the earth has been turned into a barren wasteland), as you will only require their ashes to produce lye.
Above or below ground, don't let grandma go to waste. Instead, just turn her in at the nearest processing plant. You'll get a big cash payment (based on weight).
She's gone, but we're sure she'd be happy to know her family will have the cash to buy a bunch of those super-tasty green snack crackers everyone's been raving about.
Bodies are going to decompose anyway without light. There's a reason many cultures bury bodies.
There's a few options that come to mind, with various levels of morbidity.
You need light. Bodies contain fat, so you'd render down your dead for fat and burn the fat for lighting. This would also presumably, done right, denature proteins for reuse of everything else in other ways
Fungi don't need light. Inoculating the bodies with fugal spores then storing them in an appropriate way would allow for conversion. With the right fungus mix, it would break down chemicals and allow for eventual reuse of the biomass. Consider a mix between this - where coffins are made of lab grown mycelium meant to consume the body within and the exodian funeral practices from the wayfarer book series - essentially a culture who lived on generation ships and would compost, and recycle their dead to grow plants to live on and provide oxygen in an almost quasi religious fashion.
