China Opts For Fine Screens
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THREE ROTATING DRUMS, made of S31600 stainless steel, near completion in Auckland, New Zealand prior to being shipped to a waste water treatment plant in Shenzen, China. |
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Corrosion-resistant stainless steel used for waste water treament plants designed to last 25
years. By Steve Dawson
Nickel magazine, February 2003 -- The eyes of the world will soon be
on Beijing in 2008 when it hosts the 29th Olympiad, and one aspect of daily life that
will not go unnoticed is how China manages its water resources.
The Chinese government is determined to improve the quality of water in many regions and, toward that
end, has financed projects designed to improve methods of cleaning effluent that
goes into local rivers, lakes and seas.
Many foreign companies are racing to offer various water treatment technologies to China, among them an
environmental engineering firm in Auckland, New Zealand -- Contra-Shear Separation Technologies Ltd.
The company recently received a NZ$1-million contract to supply fine screens for the Nan Shan waste
water treatment plant in the city of Shenzen, in the southern province of Guangdong. The plant will treat
sewage generated by a city of 4.5 million.
Fine screening of sewage below 6 millimetres is a relatively new concept in many countries. By removing most of the inorganic matter in the primary stage, the fine screening process reduces the amount of solids released downstream. Contra-Shear has been supplying such systems to waste water treatment plants worldwide for more than 15 years.
As a part of the contract, Contra-Shear will supply and commission four patented Suboscreens (R) made
entirely of
S31600 stainless steel, and an integrated solids handling system.
The Suboscreen is a semi-submerged rotating drum. The units for Nan Shan have a drum diameter of 1.75
metres with a slot opening of 4.5 millimetres. Each unit is designed for a peak flow rate of 1,900
litres per second. Total plant capacity will be about 500 million litres per day.
"Sewage applications for Suboscreens generally require S31600 stainless steel, as it is more resistant to corrosion," says John Phillips, regional manager for Contra-Shear. "Although we use S30400 in other applications, we prefer to use S31600 because of the fluctuating pH and the presence of hydrogen sulphide. Moreover the screens need to be robust, with a design life of about twenty-five years."
Sewage enters the drum and undergoes a 90-degree change in direction as it flows through the 4.5-millimetre-wide wire slots. Solids are removed from the influent by the rotation of the drum and by lifting staves to the apex of the drum. Two external washing sparges mounted above the screen use screened effluent to dislodge the solids and drop them on to the discharge flume. These solids then flow to the inlet of the drainer screw press where they are washed, dewatered and compressed prior to discharge.
The key factor in winning the contract for the Nan Shan plant was the fine-screening concept,
combined with superior separation efficiencies and life cycle cost savings as a result of using
S31600. Phillips believes that many operators just look at the initial costs and ignore solid recoveries and
problems with inorganic matter in their downstream processes.
"For many plant operators, initial cost is the only factor," he argues. "As a result, they
compromise on materials of construction and look at the short term. We believe the key to a well-run plant is
a holistic design. By designing the head works properly and optimizing the flow of material, the load on the
rest of the plant is lessened. Furthermore flow through a Suboscreen is far greater than that through other
competing products, lessening the number of screens required to handle a certain flow. These factors
and the use of corrosion-resistant materials such as S31600 lead to lower maintenance
costs over the life of the units."
The efficiency of Contra-Shear's design was instrumental in winning the tender. Says Phillips: "The finer the screen, the greater the amount of inorganic matter, such as plastics, wooden particles and fibres, that can be removed from the effluent. However, fine screens can usually handle only smaller flow rates compared to coarse screens. Contra-Shear has designed a screen able to maintain a balance between flow rates and solid removal with fine screens and that is a major advantage."
The equipment to be used for Nan Shan recently arrived in Shenzen, and the treatment plant
should be up and running later this year.
Nan Shan is Contra-Shear's first foray into China and will be the most modern plant in the region. At
the same time, there are other, older plants in China that are in need of upgrading and this presents
the company with a great growth opportunity. "Most of the water treatment plants in China are now old
and in need of up-dating," says Phillips.
In addition to municipal applications, Contra-Shear recently received an order to deliver water treatment equipment to a textile plant in China.
Steve Dawson is a Toronto-based freelance writer.
Photo: CONTRA-SHEAR SEPARATION TECHNOLOGIES LTD.
 John Phillips |
