It is nearly impossible to eliminate waste completely, but it is essential
to remove or degrade harmful material before it is discharged or disposed
of. On many sites ground water must also be collected and processed,
as it may contain traces of the chemicals made and used on the site.
In the event of accidental spillage this collection is essential.
It is common for all the waste to be treated together in one plant,
rather than treating waste from each process separately. This "end
of pipe" treatment can be achieved by physical, chemical or biological
||Materials are separated
using a range of methods including filters, distillation, centrifugation,
membrane techniques and "steam stripping" (volatile substances
removed using steam).
||The type of reaction depends on the
nature of the waste, but may involve neutralisation of acids/alkalis,
oxidation of toxic substances, or electrochemical reduction of metal
ions. In some cases the treatment results in a useful product.
many materials. The process may be aerobic (with oxygen) or anaerobic.
Aerobic treatment results in the production of carbon dioxide from
respiration, but anaerobic treatment results in both carbon dioxide
and methane (which can be collected and used as a fuel).
As treatment is generally more economic in larger quantities, some chemical
manufacturers will choose to have their solid and liquid waste collected
and treated off-site by a specialist company. This may be by a company
that specialises in recovering energy by the blending of solvent wastes
for use as a fuel - more details can be found in the Energy
Some materials are particularly difficult to break down. This is often
the case with waste from pharmaceutical manufacturing. Research into the
use of ultrasound (high frequency sound) is giving promising results.
In liquids the sound waves can produce very high temperatures and pressures
in short-lived bubbles - several thousand °C and several hundred atmospheres
- while the bulk of the liquid remains cool. These conditions are sufficient
to break down contaminants into degradable products.
have also been used to remove contaminants from soil. Researchers have
used supercritical carbon dioxide to remove organic waste, including
the pesticide DDT, from soil.