Transcript AD-waterwaste_treatment

Wastewater Treatment
Aim: remove waste
impurities and recycle
fresh water
Primary treatment
Sewage settles in large tanks
called “primary clarifiers”
where sludge can settle and
floating material (oils) can be
skimmed off.
The goal of these tanks is to produce a
fairly homogenous liquid that can be
biologically treated and a settled sludge
that can be separately treated.
The separated grease/oil can be used for
saponification: making soap.
The seperation process is aided by a
mixture of calcium hydroxide and
aluminum sulfate:
Al2(SO4)3(aq) + 3Ca(OH)2(aq) --> 2Al(OH)3(s) + 3CaSO4(aq)
Secondary Treatment
Goal: degrade the waste
aerobically.
Bacteria and protozoa (which are
kept alive by a supply of oxygen
and a substrate) consume the
biodegradable soluble organic
contaminants and bind the less
soluble materials into floc (flakes).
Secondary treatment processes are
classified as either:
Fixed-film: organic material is
degraded by bacteria as
wastewater trickles through a bed of
stones (called a trickling filter
system).
OR suspended-growth systems such
as
The activated sludge process: it is
more effective. Sewage is aerated
with pure oxygen in a sedimentation
tank. Sludge that settles out contains
micro-organisms that digest organic
waste. Some of it is recycled.
Emerging water will be disinfected
with chlorine or ozone, rendering it
suitable for drinking.
HOWEVER
Fixed-film systems are more able
to cope with drastic changes in
the amount of biological material
and can provide higher removal
rates for organic material and
suspended solids than suspended
growth systems.
Tertiary Treatment
Removes heavy metal ions,
phosphates, and nitrates.
Heavy metal ions and phosphates
can be removed by precipitation.
Aluminum sulfate or calcium oxide
can be used to precipitate
phosphates:
Al3+(aq) + PO43-(aq) --> AlPO4(s)
3Ca2+(aq) + 3PO43-(aq) --> Ca3(PO4)2(s)
Heavy metal ions can be
precipitated as insoluble hydroxides
or basic salts by the addition of
calcium hydroxide or sodium
carbonate. For example:
Cr3+(aq) + 3OH-(aq) --> Cr(OH)3(s)
All nitrates are soluble, so they are
precipitation cannot be used.
Anaerobic denitrifying bacteria can
reduce the nitrates to nitrogen (gas),
or the nitrate-infused water can be
passed through algal ponds where
algae will utilize the nitrates as
nutrients.