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How about bio gas?

by Jim Heath

Copyright. All rights reserved.
Not to be mirrored on other websites.
First published: Isle of Man Weekly Times.
Published on this website: 2 Jan 2004.


A DOZEN streetlights in Douglas were once lit with gas taken from the sewers. The last of these lights, on Crellin's Hill, was dismantled in 1961. There was nothing wrong with it, nor with any of the others. Progress underground snuffed them out. The sewers were changed to a fully ventilated system, instead of accumulating pockets of gas that needed to be burned off.

At first it looks like we could be missing an opportunity. It's annoying to know that inflammable gas is being vented away -- wasted -- at the same time that great quantities of expensive gas are being imported. But the annoyance melts away as soon as you do some arithmetic. Sludge treatment plants have found that every person "on the line" adds about one cubic foot of gas production per day. That doesn't sound like much -- and isn't. It would satisfy only one-sixtieth of an average person's household energy demand (heating, cooking, the lot).

Even the paltry one cubic foot per day is a maximum figure. At a sludge treatment plant, the "batch" is maintained at the optimum temperature and kept in the tank until all the gas has bubbled off. In a town sewer, the sludge isn't normally at the best gas-producing temperature. Nor does it stay in the sewer long enough to allow anything like the full gas yield. The whole Douglas system probably generates about 1000-2000 cubic feet of gas per day -- hardly enough for ten families (assuming they would use it).

So if straight sewer gas is no help, what about setting up a proper sludge treatment plant? Unfortunately, in the cold British Isles, it takes all the gas produced just to run such a plant and to keep the waste at the right temperature. The real product is black, humus-like, sanitary fertiliser, not gas. So if those two long pipes heading out into Douglas Bay symbolise anything, it's lost crop food, not lost energy.

And if that's granted, and possibly lamented, it doesn't change the fact that there would be troubles with building a sludge-to-fertiliser conversion plant in Douglas. The sewage system is gravity-fed and there's no reasonable site at the natural collection point -- Douglas Beach -- for a sprawling plant. (Imagine the uproar if anyone suggested it.) So the sludge would have to be pumped to some remote and acceptable location, just for starters. And then the ratepayers would have to be convinced about the whole venture. It doesn't seem on.

But on a smaller scale, and using farm wastes, combined gas and fertiliser production can work. The fundamentals are simple. In order to produce bio-gas (methane) with any efficiency, the following things are essential: no air should be present during the digestion; the temperature of the batch should be kept at a constant 95oF; and the ratio of carbon to nitrogen in the material should be about 30 to 1.

All this caters for methane bacteria. These ancient, energetic organisms break down waste material in the absence of air -- under water, for example. Methane bacteria were busy in swamps 500 million years ago and they are busy right now in sewer mains, septic tanks, marshes and ponds all over the world.


Pig manure

The first private effort to produce bio-gas in a controlled way, and in quantity, was by a South African pig farmer (L. John Fry, R.A.F. Ret'd). He was driven to it by desperation: pigs produce a river of excreta (fifteen times as much as humans of the same weight); it is notoriously smelly, driving neighbours past endurance and leading to visits by the police; finally, flies love the nourishing stuff and breed in it.

As a way out, Mr Fry built the first "continuous-feed displacement digester" -- a closed concrete tank, fifty feet long, eleven feet wide and five feet deep. A few sealed hatches were left for access points, and a pipe led to a huge gas storage tank.

The digester was given an initial fill. Every day after that, raw manure from 500 pigs was mixed with water, poured into a loading pit, and pumped into the digester. The glass-tiled loading pit was hosed clean in order to stop fly breeding. Each daily load of manure progressed through the digester, pushed one stage further every 24 hours by the injection of fresh material at the input end. On average, the waste was in the digester about 30 days.

By the time it arrived at the far end of the tank, the sludge was practically odourless, entirely free of harmful bacteria, and much richer in nitrogen than manure composted in an open-air heap. The process would have been worth it just to get rid of the flies and to make peace with the neighbours. But there was also an energy bonanza: 3000 cubic feet a day of bio-gas, continuously, month after month. Part of the gas was used for heating and cooking; the rest powered a diesel generator that supplied electricity to the whole farm.

What works in South Africa (and now in lots of other places) will work on the Isle of Man. Even a dozen pigs can supply a small but practical digester. (The rule of thumb is that 46 pigs equals one gallon of petrol per day. More precisely: the daily excreta from 46 pigs will yield methane with an energy content equal to one gallon of petrol.) So the 4000 pigs on the Isle of Man add up to a petrol output of 87 gallons per day -- not much for the Island as a whole, but quite a lot for the pig farmers.

Other wastes

For cattle, the conversion figure is 5.5 cows per gallon of petrol. (Which assumes that all the manure can be collected.) If you want to use chickens, you'll need 810 of them. The figures are rough but they give some idea of the scope for different farms.

In the sort of continuous feed digester described above, only animal wastes are any good. Vegetable material is too fibrous. It accumulates into thick scum on top, slowing the process and finally stopping it. (There's a scum problem with animal manure as well, but the scum forms much more slowly and there are ways of dealing with it -- and with other small technicalities I haven't mentioned).

But if you want to extract methane from wastes like cauliflower leaves, you can do it. You have to use the "single-batch" method. Pack everything into an air-tight vat, keep it at the right temperature, and draw off methane for as long as it comes. Again, this can be done on any scale, for oil-drum size upwards. The only snags are that the gas output is variable as the batch ages and you have to clean out the whole digester after each batch is exhausted. Nevertheless, the procedure could be mechanised. The free gas and high-quality fertiliser make it inviting. (Don't forget: methane-digested compost is a lot richer in nitrogen than ordinary compost.)

Manx bio-gas?

A few Manx farmers mentioned to me that they have considered methane production, but haven't become serious about it. But if methane could be liquefied and used in tractors, it would be different. No one has come up with a reasonable way of doing this. It takes 5000 pounds per square inch to liquefy methane at ordinary temperatures, and that isn't safe for farm use. Massive steel containers aren't on. At the other extreme, nor is a huge low-pressure balloon stretched above the tractor.

Some farmers also expressed anxieties about general safety. If air gets into the digester -- or the first bit of gas produced isn't vented off so as to expel any air already there -- and anything should spark the mixture, the explosion could be memorable. Yet knowing the dangers, they can be avoided. To my knowledge, there's never been a methane digester explosion. But someone will get careless or unlucky.

I should warn about one hidden and rather strange hazard: no compost heaps should be near the digester, at least not if the compost contains animal remains. Rotting meat gives off the gas phosphine. And if phosphine blends with methane and oxygen, the mixture bursts into flame like a sudden demon. This goes on all the time in marshes. Rotting animals provide the phosphine, the marsh provides the methane, and lonely walkers are provided with will-o'-the-wisps.

The greatest promise for methane production may not lie with processed animal manure or with ordinary vegetable wastes. The highest methane yields are from primitive algae. Elsewhere, schemes are afoot to unite algal ponds, methane production, and waste heat from power stations.

Here's the proposed cycle: power station waste-heat would warm pools of algae, producing tons of the wispy green material; the algae would be fed into digesters, generating methane; and the methane would be piped back into the engines of the power station. Almost a perpetual-motion machine. Except that the power station wouldn't run solely on methane -- just get a boost. And not to be forgotten, the sun contributes too, quietly and hugely, as it radiates energy into the fast-growing algae.

The hub of this system is the interplay between two of the oldest organisms on the planet: methane bacteria and algae cells. Something so fundamental and simple has appeal. After all, dead and fossilised ooze -- oil -- powers the world right now. Why not living ooze to help power whatever type of world lies ahead?



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