Sewage gas power coming into its own at city plant

Press, 21 October 1980, Page 21

Sewage gas power coming into its own at city plant

By

STAN DARLING

It was 1884, and a man named W. J. Dibdin was looking for a better way to',treat London’s sewage. He: up-ended an old boil* er and used it for a sludge tank- Windows were placed in the old boiler so the sludge digestion could be observed.

Fermentation was lively lifter a week, Dibdin reported. Sludge overflowed the boiler and ran down the sides, with clear water appearing at the tank's bottom. Sewage plant staff were not too enamoured of all that scientific tinkering. It created a terrific stink.

Dibdin was not quite sure hdw his experiments might pky off. He left it to later experimenters to come up. with more sophisticated ways of turning sludge into something useful.

Results of later tests are becoming more obvious in Christchurch as the Drainage Board gets ready to power most of its Bromley treatment plant with methane gas, a sewage by-product. The only thing keeping the plant from revving up one of its new power station’s big engines on the gas is an industrial dispute with the engineers who will keep an eye on the system. . Power production from sewage gas is not. really new. Many overseas plants have been using the process for years. Rising power prices, however,

have made it more economic to use a recycled sewage product instead of electricity from the national grid. Although the Bromley plant will always need some outside power help from the national grid to control its system, cost

savings of some $150,000 a year — at present electricity prices — have been suggested if methane gas proves as useful as expected. Two sewer pumps at Bromley have been driven by methan e-powered engines since the early

19605, Those engines, using a small percentage of regular diesel fuel in addition to the gas, have not been able to consume all the methane produced by plant digesters. Mr Humphrey Archer, the board’s deputy operations engineer, says that about 30 per cent cf the gas produced is regularly flared off.

That will change quickly when the power station comes on line. Each English engine has an output of 450 kw; each is designed for continuous duty. They are the type of engine used on many ships for auxiliary power generating, and have been specially converted to handle the methane gas. About a 10 per cent injection of diesel fuel is required to ignite each engine’s charge. Only one engine will be

used at a time. In addition, the two old engines in the pump room will continue to directly run tl.eir pumps. The remaining pumps will be run from the new power station.

An average of 500 kw per day of energy is imported from the national grid, and that can go up to 600 kw a day. So one engine will take away the need to import all but 50kw to 150 kw from the grid.

Mr Archer says it does not seem that, the Christchurch plant will be able to export any of its met-hane-produced power back to the grid. It may be more ceonomic for the plant to produce ways to use the gas itself if there is ever a surplus to power needs.

The Auckland sewage treatment plant has been producing power since the early 1960 s and exporting some of it.

However, Mr Archer adds, Auckland is now renewing engines there and has higher plant energy demands, so there will probably be less surplus

power to channel into the national grid from that source.

The Drainage Board also hopes to convert its two older engines to spark ignition. That would remove the need to ignite them with diesel, and possibly save another $50,000 a year of those fuel costs.

With the new power station, the treatment plant should come “fairly close” to supplying its own energy needs, Mr Archer says. There are ways to get more gas from the digesters —the plant has three, and room for a fourth — but the extraction process gets more expensive the further you go.

One option might be to use some of the gas as vehicle fuel, and that option will be among several future sewage gas possibilities presented to the

board later this year.

Board members will be told about ways to “squeeze more gas out of the sludge,” Mr Archer says, “and the costs of getting that extra amount.”

The Bromley plant has some high-pressure gas storage capacity that could act as a buffer if gas production becomes sporadic, and help ensure a steady supply. Sewage actually starts to decompose on its way to the treatment plant. Between two and four hours after settled sludge enters the digester, it starts to become gas.

So much water is coming into the plant each day — about 130,000 cu m, resulting in some 7000 cu m of sludge — that there is a rapid flowthrough of raw materials for gas production.

• About half of the total gas production comes off the sludge in the first five to 10 hours.

“The trick will be putting enough in storage to get a balanced flow,” Mr Archer says. “But the nat-

ional grid takes over automatically, anyway, if something happens.” The quality of sewage for gas-making varies, just as the flow varies during a day. Peak periods for sewage coming into the plant are not necessarily the times when the plant

needs the most power. Since the Bromley plant’s trickling filter system started working, the works has had two sources of sludge for the digesters. Mr Archer points out that there is already a high degree of recycling in

the treatment plant. For one thing, heat is recovered from the methane engine exhaust and jacket water. That provides most of the heat needed for the sludge digesters. Aside from other gas use options, Mr Archer says there is always the possibility of growing crops on board land that could be fed into the di* gesters to create more gas. Digestion reduces the bulk of sewage sludge by from 25 to 30 per cent as it gives off the gas. Gas yields will differ from plant to plant, but studies have shown an average yield of about .03 cu m per head of population each day. In the early 1900 s, a septic tank at the Exeter sewage works produced gas to light the works. A Birmingham drainage engineer had seen small engines driven by sewage gas in Australia and India, and in 1921 he started an English digestion plant that drove a gas engine coupled to pumps. Six years later, a further step was taken — an alternator set was driven by a sludge gas engine. In 1935, a big works in Middlesex started using sewage gas to drive engines, heat buildings, and fuel works vehicles. Methane from that plant also became one of the ingredients of an incendiary bomb made during the Second World War.