Wastewater Treatment

Aim: remove waste impurities and recycle

fresh water

Primary treatmentSewage 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.