Water treatment process and operations Flashcards
Major water pollutants
suspended solids
organic matter
nutrients
pathogens
heavy metals
refractory organics
BOD
Biochemical oxygen demand is the amount of oxygen required by microorganisms to break down organic waste at a certain temperature over a particular period of time
high BOD = large quantity of organic material present (large number of bacteria present to dissolve the waste)
low BOD = small quantity of organic material present
what are the three types of wastewater treatment?
Physical treatment, chemical treatment, biological treatment
What is physical wastewater treatment? Give examples
Removal of pollutants (suspended solids) by physical forces
e.g. sedimentation, screening, filtration
What is chemical treatment? Give examples
Conversion or destruction of contaminants through chemical reactions
e.g. coagulation, flocculation, disinfection
What is biological treatment? Give examples
Conversion or destruction of contaminants using microorganisms
e.g. aerobic, anaerobic, anoxic treatment
Main unit operations in water treatment
Preliminary treatment
Primary treatment
Secondary treatment
Tertiary treatment
What are the three stages of preliminary treatment and what are the purposes of each?
Screens: remove debris and big chunks of raw material
Comminutors and grinders: chop or shred debris
Grit removal units/chambers: filter out sand and dust from water to avoid accumulation
What are the stages of primary treatment and what are the purposes of each?
sedimentation tanks/clarifiers/settling tanks: remove floating material
Rectangular v Circular tanks in primary treatment
Comes down to designer preference as lifecycle cost is in similar range
BUT
rectangular tanks use less land, longer path and are simpler
What are the stages of secondary treatment and what are the purposes of each?
trickling filter: bed of crushed stones/pebbles covered with slime which is an aerobic treatment system that utilizes microorganisms to remove organic matter from wastewater
activated sludge: aeration tank +secondary clarifier, inject compressed air through diffusers to provide oxygen and rapid mixing action
oxidation ponds: treat wastewater through the interaction of sunlight, bacteria, and algae. Algae grows using energy from the sun and carbon dioxide and inorganic compounds released by bacteria in water
What are the options of tertiary treatment and pros and cons for each?
chlorination
- taste and odour effects
- leaves a residual chloroform
ozone gas
- needs to be produced on site
- expensive
ultraviolet radiation
- no residual
- no taste/odour problems
- very expensive
What is filtration?
After the coagulation and sedimentation steps - removes further impurities by percolating it downward through a layer/bed of porous, granular material such as sand, pebbles
What are the types of filtration and the purposes of each?
Membrane filtration: removes particles, microbes and dissolved inorganic materials
Aeration: removes dissolved Fe, Mn and involved in taste/odour control
Water softening: removes dissolved Ca and Mg salts via chemical addition and ion exchange
Carbon adsorption: removes dissolved organic substances and involved in taste/odour control
What is sludge and what are the two types?
Residual material (biosolids) produced as a by products of WWP
Two types are primary sludge (generated from sedimentation and other primary processes), secondary sludge (activated waste biomass resulting from biological treatments)
