Leaves, stems, roots and microorganisms in the potting mix also act as a filtering system.
Besides, a filtering system is a temporary solution and is not appropriate for all kids.
It would be technically possible to use a filtering system, as we have done with email.
"Even if Google comes up with an airtight filtering system, that doesn't moot the charge that they've done something improper in the past, " says Boehm, who has been an outspoken critic of Google's intellectual property policies.
These Living Machines recycle wastewater by filtering it through a system of bacteria and plants.
For example, he said that digital technology offered vast opportunities for pupils to learn with others but these ideas were simply not filtering through to the education system where tests and examinations still focus on individual attainment.
The second was in failing to warn the new owners, the Michaels, that the fire alarm system had been subjected to filtering while a previous fault was fixed.
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The system will remember your choices and change your filtering preferences accordingly.
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"There's no guarantee that they are going to be able to get the offensive material out of the system quickly, " said Geoff Haggart, European head of filtering firm WebSense.
Since researchers, administrators and grant-awarding bodies all take note of which work has got through this filtering mechanism, the competition to publish in the best journals is intense, and the system becomes self-reinforcing, increasing the value of those journals still further.
Its no-contact drive system uses an advanced low-noise motor and geared belt to ensure control over speed while filtering out vibration.
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