Pink - Three Waters

Question 1

Attached growth reactors - trickling filters

Answer 1

Encourage the growth of microorganisms on the attached media. Units are circular to allow easy water distribution (<60m diameter).

Question 2

Attached growth reactors - rotating biological contactor (rbc)

Answer 2

Disco rotated through resivoiurs containing WW so about half of the disc is always immersed. Disc provides media for biofilm rotation provides airation. Typically better than trickling filters. Remove ~85% of BOD.

Question 3

Describe secondary wastewater treatment

Answer 3

Core of the WW treatment process.
- Complex organic matter broken down using biomass.
- Secondary clarifier for removal of biomass.
- 95% of the suspended solids are removed by bacteria.
- Big reduction of organic carbon.
- Biological process to remove organic pollutants (most important).
- Physical treatment to remove excess microorganisms from treated water.

Question 4

Describe the primary WW treatment process

Answer 4

Solids are separated, similar to sedimentation basis. Focuses on organics, removes grease and oil.
- solids settle floating material rises to the surface.
- Produces a homologous (uniform) liquid for later biological treatment.
- Primary sludge is pumped for sludge treatment and disposal.

Question 5

Equalisation basin + advantages

Answer 5

Ensures that the WW is pumped at a uniform rate-helps deal with the non-uniform supply.
- Flow equalisation and stability
- Biological treatment enhancement
- Improved secondary clarifier performance
- pH dampening
- Toxic substrate dilution (don’t want to kill the microbes in the biological stage)
- Partial primary treatment.

Question 6

Types of preliminary treatment + purpose

Answer 6

1) trash racks - remove trash (rags, branches, plastics)
2) screens (Bar screens) - remove smaller materials
3) shredders -grind larger solids into smaller solids
4) grit chamber - remove inorganic particles (sand, gravel, cinders) and fats, oils and grease - protect moving equipment (reduce abrasion)

Question 7

What are the 5 stages of WW treatment?

Answer 7

Preliminary, primary, secondary, tertiary, sludge treatment (biosolids)

Question 8

Wastewater network aim

Answer 8

WW is collected and transported via a network of pipes and pump stations to a WWTP water moves to the pump station primarily by gravity flow jump stations pump water from low lying areas over hills to WWTP, gets water to dat quickly, prevents solids from settling out >0.6 m/s velocity, minimise power consumption

Question 9

3 aims of WW treatment

Answer 9

• Produce clear waste stream
• Produce stabilised solid waste (sludge)
• Resource recovery

Question 10

5 effects of water quality on disinfection

Answer 10

• Particulates
• dissolved organic
• inorganic compounds and ions
• pH
• reactor design, (mixing and hydraulic conditions)

Question 11

Disinfection

Answer 11

• Last barrier between us and pathogens,
• most effective when the water is clean, (don’t want to create toxic substances),
• provides continuous supply of disinfection even when water is travelling through pipes to taps

Question 12

Chlorine advantages

Answer 12

Cheap, remains in water, protects you, easy to handle

Question 13

Log reduction

Answer 13

Relates to the % of microorganisms physically removed or inactivated by a given process.
1 log reduction - 90%
2 log reduction - 99%
3 log reduction - 99.9% (count the of 9s)

Question 14

Log removal

Answer 14

Log removal = log (cin / cout)
Smaller cout means that the removal is really good
Cin is the influent pathogen concentration
Cout effluent

Question 15

Flocculation

Answer 15

Is the agglomeration of destabilised particles into large size particles known as flocs by slow mixing which can be effectively removed by sedimentation or flotation. The floc aids are known as flocculate e.g. Organic polymers (captures neutralised articles and forms a branched polymer).

Question 16

Coagulation

Answer 16

Is the destabilisation of colloids by addition of chemicals that neutralise the negative charges; adding and rapid mixing of chemicals to remove particles from the water (flash mixing). Colloids are negatively charged therefore need to add counter charge - repel and stay in suspension- neutralise the charge to allow potential mixing

Question 17

Why do we reed coagulation/ flocculation?

Answer 17

Bigger particles have a faster settling velocity, therefore by making the smaller particles combine together we can ensure they settle last enough and therefore remove the contaminants from the water during this part of the treatment process. Gravity settling is not practical, possible for small particles,

Question 18

Describe slow sand filters (granular media filtration type)

Answer 18

• Slow water flow rate (2.5-10 m/day) allows biological growth to develop in the upper surface of the sand and is primarily responsible for particle and microbe removal.
• Schmutzdecke layer (1 -2 Cm slime).
• Cheap, easy to construct.
• Effective without pretreatment of the water by coagulations flocculation,

Question 19

Describe rapid sand filters (granular media filtration type)

Answer 19

• Stratified with usually 3 different layers ( anthracite, sand, gravel).
• Needs pretreatment of the water by coagulation flocculation in order to work well as this process also removes the suspended solids.
• High flow rate 60-240m/day, efficient.
• Most commonly used.

Question 20

Distinguishing feature between slow and rapid sand filters.

Answer 20

Rapid sand filters rely on the chemical process rather than the biological process in slow sand filters.

Question 21

Indicator organisms + how they should behave

Answer 21

Used to indicate the possible presence of disease causing constituents, to be a good indicator organism, needs to exhibit all of these characteristics.
- Applicable in water
- present when pathogens are
- no after growth in water
- absent when pathogens are
- constant characteristics
- persist longer than pathogens
- harmless to humans
- correlate qualitatively with pathogens
- present in greater numbers than pathogens
- be easily, accurately, and quickly detected.

Question 22

Absorbers - water softening and microbe reductions

Answer 22

Hardness ions are removed by adding line and Soda ash to precipitate them as carbonates, hydroxides, and oxides. Additional carbonates combine with Ca and mg to make the water soft. Pathogens aren’t taken care of until the pH>11. Basically a type of coagulation, flocculation process.

Question 23

Describe membrane filters

Answer 23

Microfilters, nano filters, ultra filters. Membrane and membrane seal integrity critical to effective performance. Ability to remove microplastics and forever chemicals. Still have to deal with the contaminants / brine left on the membrane.
Reverse Osmosis:
- >99.99%. Removal of cellular microbes
- > 99.99% removal viruses in ultra, nano and ro,
- <99% removal viruses by micro

Question 24

Describe roughing filters

Answer 24

Used in developing countries/ places where there is no centralised WTP, cheap, low maintenance, local materials. Removes large solids. Removes microbes, 1-2 log bacterial reduction, 90% turbidity reduction. Water flows horizontally

Question 25

Absorbers - describe filter- absorbers

Answer 25

Sand + granular activated carbon,
- reduces particles and organic,
- biologically active,
- microbial retention is possible.

Question 26

Absorbers - describe granular activated carbon adsorption.

Answer 26

• Removes dissolved organic,
• poor retention of pathogens especially viruses,
• biologically active, develops a biofilm,
• can shed microbes into the water,
• removes hydrophobic contaminants,
• improve performance of the filtration device.

Question 27

Factors effecting microbial reductions

Answer 27

• Detention time
• temperature
• microbial activity
• water quality: particles, salinity, dissolved solids
• sunlight → inactivation of bacteria, leaves, and faeces can get into water supply -sedimentation
• land use
• precipitation
• runoff or infiltration