|03- Sewerage Tunnels|
|DOWN TO THE SEA IN SHIPS
Every year, maintenance engineers and staff of the Metropolitan Water Sewerage and Drainage Board drift down to Bondi in canoes through the main Bondi sewer. In the words of the old saying, they are literally up the creek without a paddle. One year, their annual inspection tour coincided with the City to Surf Foot Race. The Water Board boats beat most of the field home by sailing under College, Liverpool, and Oxford Streets, across Double Bay Valley to Bondi rather than sweating on foot over the top of it.
According to the maintenance engineers, drifting down a sewer is not too bad once you get used to it. The main qualification is that you need to be able to swim. Surprisingly, raw sewage does not smell very much at all. If anything, it has a strange, rather sickly sweet odour. The worst smell, according to the engineers, is something called "dunder" which is produced by CSR in its rum production process, and even this probably does not smell any worse than the CSR factory where it is produced. The brown colour of the water at Bondi also comes from CSR -this time from masonite production. About sixty per cent of Bondi's effluent is industrial and commercial.
The original Northern Bondi Ocean Outfall is fairly old. It was built in 1889. Originally it was intended to carry stormwater drainage as well as sewage and, as a result, it has managed to cope with the huge increase in population that has taken place.
At the outfall at Ben Buckler a large chamber, ten metres long by eight metres wide, and ten metres high, was cut out of the sandstone rock. From it a vertical shaft about four metres high topped by an eighteen-metre high ventilating stack was constructed. This was demolished in 1910 to make way for a thirty-metre reinforced concrete shaft.
Surveyors on pedestals (left and right) transferring alignment of the Bondi sewer down on access shaft.
How they built Sydney's first sewer in the 1850s. Note the Supervisor in the cutaway coat and top hat.
In 1930, the south-eastern discharge channel was lowered to a depth of five metres below the lowest spring tides so that wave action would not force gas back up the tunnel. Today, the Bondi system drains 3600 hectares from Balmain to the coast. There are thirty-four sewage pumping stations discharging into the system. At North Bondi, there is primary treatment of the sewage. It is screened and grit is removed. There-is also sedimentation and sludge , . digestion before the sewage is emptied into the sea under water. It is possible to treat sewage so that you can drink the purified water at the end of the process. However, the, cost .of this treatment is extremely high and the benefits are not correspondingly great. The sea dilutes the discharged sewage to such an extent that there has been no recorded case of catching a disease from the matter. The major cause of public complaint is grease which is sometimes visible along surf beaches near ocean outfalls. It is more of an aesthetic problem than a health one, and the new tunnels to be built out under the sea should solve it.
There are seven ocean outfalls. Besides Bondi, Malabar, North Head, Cronulla and Warriewood are the main ones. There are also thirteen inland water pollution control plants. Each has its own network of tunnels and sewers which have to be inspected and maintained, but the Bondi one is the oldest and, for this reason, perhaps the most interesting.
UNDER NORTH HEAD
An elevator takes visitors to North Head right through the headland to water level -a distance of about seventy metres. Emerging from the lift you come out into a huge underground cavern which has been carved out of the sandstone. Below your feet, sewage runs out into the sea. Thanks to fans blowing down onto the sewage there is practically no smell. The only unpleasant odour comes from large drums which are used to screen solid waste from the sewage. None of this is human waste. Most of it is cotton which comes from the family washing machine as it batters the fibres of clothes. When the fibres come together in the sewer the threads start to recombine and would look messy washed up on a beach. At the moment, this waste is collected in bundles for burying but eventually it will be removed automatically and sent by a conveyor belt to the surface for burning in an incinerator.
The construction work at North Head has been going on since the mid-1960s. It has been accomplished at some human cost. Two men lost their lives in a freak accident during the blasting of rock in one of the tunnels. During the blasting a huge piece of rock was disturbed in such a way that it stayed in place looking as if it was part of the sandstone roof. In fact it remained there for twenty hours after the.initia1 blast and three men were working under it when it eventually came down, crushing two of them to death.
When it is completed, the North Head works will give primary treatment to all sewage passing out to sea. Huge pumps housed in the underground chamber will pump the sewage to the surface for treatment. It will then descend once more to flow out to sea under gravity.
An ambitious tunnelling scheme is on the drawing boards to take the treated sewage three kilometres out to sea. A number of undersea sewage pipes have been laid in the United States and various underwater tunnels like those in Hong Kong and Japan have been constructed. But the board will be quite literally breaking new ground with its under-
sea tunnels from North Head, Malabar, and Bondi. The tunnel faces will have to be probed with some care before each stage of drilling. What would happen in the event of a
collapse during the construction of these undersea tunnels just does not bear thinking about.
All this is in the future. At present, the sewage is discharged into the sea at North Head and is only screened to remove solid waste like the cotton mentioned previously. Unlike
the sewage at Bondi and Malabar, it does not even receive further treatment at North Head yet. It is what the media like to call "raw sewage". In fact, as the board complains,
sewage in the media these days is rarely ever simply sewage. 1t;is invariably "raw sewage" -a double-barrelled term that seems to carry that bit of extra emotive sting. Presumably "raw sewage" is much more dangerous and repulsive than ordinary sewage. In fact, there is no difference. The "raw" is superfluous. It's either sewage or it isn't.
But with or without the prefix, exactly what is sewage? Put simply, sewage is.99.9 per cent water with solids (including human wastes) being only a very small part. Imagine one tonne of ordinary domestic sewage -that is, one kilolitre or 220, gallons. The total quantity of solids in it would be only about half a kilogram, or a bit over one pound. Only about half the solids would be visible, as the other half would have dissolved.
Besides human wastes, the solids consist of grease, food scraps, soap, detergents, and all the miscellany of bits and pieces which go down the plughole from kitchen, laundry and bathroom.
It is through these plugholes that most of the water enters the sewers. Some sewers carry a certain amount of liquid trade wastes. But household sullage is by far the biggest component of what is broadly called sewage. Thus sewage is far from being the nauseating slurry of human waste that one associates with the backyard pan toilet.
After treatment, sewage basically becomes two separate things -sewage sludge (the extracted solids, which are rendered harmless by digestion) and sewage effluent (the liquid left after primary, secondary and/or tertiary treatment.
There is a vast difference between sewage, sludge and effluent.
Construction of an access shaft in Paddington. The area lo the right was then a Chinese market garden.
Similarly, reports of "sewage pollution" of Sydney beaches often amount to cases of mistaken identity. Sometimes what is reported is a natural form of marine pollution. For example, rough foaming seas mash up a considerable amount of seaweed and other marine growth with the result that "rafts" of this material -together with a yellowish brown stain -are left along the high tide line. This "pollution" is evident from time to time on many beaches far removed from any sewerage outfalls. These days, it is mainly grease which, from time to time, affects the beaches.
Yet the treatment processes at the outfall plants, and the Board's strict trade waste control policies, have dramatically reduced the amount of grease which intermittently gets onto the beaches. That is not to say the Board is content with the present situation. If it was, why would it have spent, and be still spending, so many millions of dollars on upgrading its major coastal treatment plants -and planning to build deepwater outfall lines to carry treated effluent and digested sludge away from the coastline? It's a matter of looking at the problem factually, logically and sensibly. .
UNDER THE SUBURBS
"Danger. Laser" is the sign that greets you outside the Water Board tunnel at Deep Creek, Narrabeen. The notice goes on to warn workers of the dangers of looking directly at the laser beam which is used to keep a machine called a mole on track as it chews its way through the sandstone hill towards Frenchs Forest. With the help of the laser, the mole can tunnel its way through a hill and emerge on the other side with an accuracy of a couple of millimetres.
The yellow mole has already dug its way under Elanora Heights. The resulting tunnel is over a kilometre long. Lit by electric light at present, it has a railway to carry workmen backward and forward as they carry out the finishing touches such as concreting certain areas. The line will eventually be removed and the tunnel will carry sewage through to the Warriewood treatment plant.
The kilometre-long tunnel is an impressive piece of engineering especially when G9u. consider that the longest railway tunnel in Australia is the 1189-metre Woy Woy tunnel LR' the Sydney-to-Newcastle line. However to the engineers of the Water Board it is just one more in a network of over 300 kilometres of sewerage tunnel. The longest system runs from Blacktown to North Head and the deepest runs 100 metres below East Turramurra. The yellow mole responsible for the work these days costs over a million dollars, but it can be driven by one man-and it digs so much faster than the old method of blasting and hand digging that it actually saves the Board money.
Sydney sandstone is quite good to tunnel through, according to the Board's engineers. The only problem is that it is very abrasive and bits often have to be removed from the mole and sharpened after every seven-hour shift.
Construction of the Bondi sewer in what is now Blair Street.
Bit used by Water Board "'Mole" for drilling sewerage tunnel through sandstone,
|This section is based on the excellent book by Brian and Barbara Kennedy. (Subterranean Sydney (The Real Underworld of Sydney Town), Reed, Sydney, 1982. ISBN 0 589 50312 X). Copyright Brian and Barbara Kennedy and Reed Publishing.|