Wastewater Treatment Aim: remove waste impurities and recycle fresh water

Primary treatment ZSewage settles in large tanks called “primary clarifiers” where sludge can settle and floating material (oils) can be skimmed off.

ZThe 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. ZThe separated grease/oil can be used for saponification: making soap. ZThe seperation process is aided by a mixture of calcium hydroxide and aluminum sulfate: Al 2 (SO 4 ) 3 (aq) + 3Ca(OH) 2 (aq) --> 2Al(OH) 3 (s) + 3CaSO 4 (aq)

Secondary Treatment ZGoal: degrade the waste aerobically. ZBacteria 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).

ZSecondary treatment processes are classified as either: ZFixed-film: organic material is degraded by bacteria as wastewater trickles through a bed of stones (called a trickling filter system).

ZOR suspended-growth systems such as ZThe 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. ZEmerging 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 ZRemoves heavy metal ions, phosphates, and nitrates. ZHeavy metal ions and phosphates can be removed by precipitation. ZAluminum sulfate or calcium oxide can be used to precipitate phosphates: Al 3+ (aq) + PO 4 3- (aq) --> AlPO 4 (s) 3Ca 2+ (aq) + 3PO 4 3- (aq) --> Ca 3 (PO 4 ) 2 (s)

ZHeavy metal ions can be precipitated as insoluble hydroxides or basic salts by the addition of calcium hydroxide or sodium carbonate. For example: Cr 3+ (aq) + 3OH - (aq) --> Cr(OH) 3 (s)

ZAll nitrates are soluble, so they are precipitation cannot be used. ZAnaerobic 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.