Week 11

Question 1

Preliminary/Primary treatment:

Answer 1

generally physical processes to prepare stream for subsequent biological treatment; removal and/or homogenisation of solids

Main purpose: Removal of suspended solids, large and small
◼ pH adjustment. This can lead to removal of solutes, e.g., raising pH to >7 can precipitate metals
◼ Equalisation: mixing then settling of waste streams
◼ Some volatile organic compounds (VOCs, e.g., many organic solvents) need to be removed prior to secondary treatment.

Question 2

◼ Secondary treatment:

Answer 2

aerobic or anaerobic biological
processes to convert biodegradable materials to gas and settleable sludge.
Can be safely discharged after disinfection.
Main purpose: Removal of organic contaminants and suspended solids
◼ Microbes, mainly bacteria, convert the BOD to more cells, gases and water
◼ Need to ensure conditions right for microbial growth, ie., check:
 Temperature
 Gaseous atmosphere
 pH
 Nutrients: N, P, Mg, S, trace metals
◼ Can be aerobic or anaerobic biological treatment

Question 3

◼ Tertiary treatment/Advanced treatment:

Answer 3

removal of residual microbes, fine solids, solutes such as N, P, colour & odour compounds. For water reuse/recycling
“Advanced wastewater treatment”
◼ “Polishing” water prior to release to environment or recycling
◼ Main Purpose: Removal of
 fine suspended solids
 colours, odours,
 Metal ions
 nutrients such as N & P
 microbes from secondary process
◼ Important for water reuse and recycling

Question 4

Colour removal

Answer 4

Colloidal and other very large colour bodies may be removed by coagulation and flocculation, then sedimentation, but usually colour-causing compounds
are removed by:
◼ Adsorption by activated carbon
◼ Oxidation by chlorine, ozone, UV radiation
◼ Membrane filtration by nanofiltration

Question 5

Oxidation of Organics

Answer 5

◼ Organics may be oxidised by chlorine, ozone or photooxidation
◼ Oxidise all organic materials…including bacteria, algae etc., and so also disinfects
◼ Reduces levels of BOD, colours, odours, oxidised cyanide to innocuous products
◼ Chlorine leaves a residual with oxidative/disinfecting effect
◼ Ozone produced on site, little residual (although increases dissolved oxygen level)
◼ Large doses of UV radiation cause photooxidation by radical-mediated processes, enhanced by dissolved oxygen

Question 6

Reducing Ions/Salinity

Answer 6

◼ Ion exchange: anions and cations adsorbed to ion exchange resin. For softening water, removal of heavy metals esp. toxic metals.
◼ Reverse osmosis: membranes which allow only water molecules to permeate (theoretically). Used widely for desalination…need high pressure to drive process…energy!

Question 7

Removal of P

Answer 7

◼ Goal is usually 0.5 mg P/L after treatment
◼ Biological processes 50-80% removal
◼ Chemical precipitation…iron salts, lime and alum give insoluble phosphates, >95% removal.
Resultant chemical sludge is an issue

Question 8

Removal of N

Answer 8

◼ N contributes to oxygen demand (via nitrates etc)
◼ Some removal via secondary treatment step: activated sludge, ….N→ biomass
◼ Some specially developed biological
processes…e.g., nitrification-denitrification in tertiary treatment
◼ Ammonia stripping

Question 9

Final Step: Disinfection

Answer 9

◼ Purpose: to inactivate pathogens so that they are not infectious to humans and animals
◼ Achieved by altering or destroying structures or functions of essential components within the pathogens, i.e., Proteins, nucleic acids, lipids
◼ Common disinfectants: chlorine (Cl), ozone (O3), ultraviolet (UV) irradiation

Question 10

Preliminary/Primary Treatment Methods

Answer 10

Bar Screens –large solids
Grit Removal –heavy solids
Air Stripping – VOCs
Oxidation – toxic compounds
Neutralisation – pH outside 6-9
Sedimentation – suspended solids

Question 11

Secondary Treatment Methods

Answer 11

Activated sludge (i.e.,aerobic) treatment –BOD/biodegradable organics
Anaerobic treatment –high content food waste
Sedimentation –suspended solids

Question 12

Tertiary treatment method

Answer 12

Micro-screening/sand filtration/membrane–fine suspended solids
Coagulation/Bio-treatment – P
Bio-treatment – N
Air Stripping – ammonia N
Ion exchange/reverse osmosis - metal ions
Oxidation/activated carbon/membrane – colour/odour compounds
Disinfection (Cl, UV or O3) – microbes/pathogens

Question 13

Removal of Fine Suspended Solids
Secondary treated wastewater may contain lots of fine suspended solids which can be removed by:

Answer 13

◼ Microscreening (rotary drum covered in fine mesh and waste fed to inside, 70-90% SS removal);
◼ Granular media filtration through sand, anthracite, diatomaceous earth (up to 90% removal);
◼ Coagulation with lime, alum, polyelectrolytes may be used prior to granular media filtration
◼ Membrane filtration (almost complete removal)

Question 14

Activated Sludge Process

Answer 14

◼ Wastewater is aerated
◼ Bacteria encouraged to grow, by providing:
 Oxygen
 Food (BOD), N, P and trace elements must be available
 Appropriate temperature (e.g., 10-30 ºC)
 Appropriate pH (usually 6-9)
 Sufficient contact time (hours)
◼ Microbes (bacteria) consume BOD:
 grow and multiply
 metabolism
◼ Microbial flocs settle in secondary clarifier (CRITICAL STEP!!!),
removed as sludge
 Some sludge (called?) is returned to aeration tank,
 Remaining sludge is wasted (treated eg., dewatered dried and
disposed/used as fertiliser)
◼ Treated WW is usually disinfected and discharged, or sent to tertiary
treatment