Pink - Three Waters Flashcards
Attached growth reactors - trickling filters
Encourage the growth of microorganisms on the attached media. Units are circular to allow easy water distribution (<60m diameter).
Attached growth reactors - rotating biological contactor (rbc)
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.
Describe secondary wastewater treatment
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.
Describe the primary WW treatment process
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.
Equalisation basin + advantages
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.
Types of preliminary treatment + purpose
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)
What are the 5 stages of WW treatment?
Preliminary, primary, secondary, tertiary, sludge treatment (biosolids)
Wastewater network aim
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
3 aims of WW treatment
- Produce clear waste stream
- Produce stabilised solid waste (sludge)
- Resource recovery
5 effects of water quality on disinfection
- Particulates
- dissolved organic
- inorganic compounds and ions
- pH
- reactor design, (mixing and hydraulic conditions)
Disinfection
- 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
Chlorine advantages
Cheap, remains in water, protects you, easy to handle
Log reduction
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)
Log removal
Log removal = log (cin / cout)
Smaller cout means that the removal is really good
Cin is the influent pathogen concentration
Cout effluent
Flocculation
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).
Coagulation
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
Why do we reed coagulation/ flocculation?
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,
Describe slow sand filters (granular media filtration type)
- 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,
Describe rapid sand filters (granular media filtration type)
- 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.
Distinguishing feature between slow and rapid sand filters.
Rapid sand filters rely on the chemical process rather than the biological process in slow sand filters.
Indicator organisms + how they should behave
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.
Absorbers - water softening and microbe reductions
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.
Describe membrane filters
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
Describe roughing filters
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
Absorbers - describe filter- absorbers
Sand + granular activated carbon,
- reduces particles and organic,
- biologically active,
- microbial retention is possible.
Absorbers - describe granular activated carbon adsorption.
- 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.
Factors effecting microbial reductions
- 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
