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ENGR405 > Wastewater > Flashcards

Wastewater Flashcards

1
Q

What are 3 main harmful effects of industrial waste?

A

High content of C N and P which cause:
1. Oxygen depletion in water bodies as they are overloaded with biochemical oxygen demanding waste
2. Eutrophication of lakes and rivers - as there is algal bloom resulting from too much nutrient sources
And
3. High toxicity and carcinogenicity due to heavy metals

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2
Q

What is BOD?

A

Biochemical oxygen demand is a measure of the biodegradable organic matter. Defined as the amount of oxygen required for bacteria to decompose organic matter under aerobic conditions. It is widely used to determine the organic pollution strength of industrial and domestic wastes

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3
Q

What is COD?

A

Alternative way to measure organic content in water. It is defined as the mass of oxygen required to completely oxidise an organic compound to CO2. COD uses chemical oxidation to determine organic content

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4
Q

Calculate COD of methane?

A

CH4 + 2O2 –> CO2+2H2O. 2 mol/mol = 64 g/16 g = 4 kg COD/ kg methane

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5
Q

What unit processes are normally involved in primary treatment?

A

Primary treatment involves the physical removal of solids as well as some contaminant reduction. BOD, TSS, TN, TP are all reduced. There is initial screening and grit removal which is done by screens and grit settling technologies. The aim of this step is to prevent damage to downstream processes/equipment. Then the solids are settled out using primary sedimentation tanks or dissolved air flotation. The aim of this step is to reduce volumes to secondary treatment, reduce aeration required and reduce secondary sludge volumes.

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6
Q

What is normally involved in secondary treatment?

A

Involves the removal of colloidal and dissolved contaminants, further reducing BOD, TSS, TP, TN. This id done through biological processes and is an accelerated and controlled version of the purification that would occur in natural systems. This involves attached growth processes with biomass including rotating biological contactors and trickling filters, suspended growth processes with activated sludge including contact stabilisation, AB process, and SBR, as well as anaerobic systems.

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7
Q

Tertiary treatment?

A

This step involves the further reduction of TSS, inorganics, organics, and pathogens to required limits. Filtration methods such as rapid sand filters and disc filters can be used as well as disinfection methods with chlorination, ozone and UV light. The final discharge can then go to waterways or be used for irrigation.

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8
Q

Draw a diagram of the Nitrogen cycle?

A

Draw it

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9
Q

What is nitrification?

A

Nitrification is the biological conversion of ammonia nitrate to nitrate nitrogen by ammonia oxidising bacteria. It is an aerobic process thus requires oxygen. It also requires alkalinity

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10
Q

What is denitrification?

A

Denitrification is the biological conversion of nitrate nitrogen to nitrogen gas. It is anoxic, releasing oxygen and alkalinity. However it requires an organic substrate in the form of carbon.

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11
Q

List 3 factors that affect growth rate of Ammonia oxidising bacteria

A

Alkalinity or pH optimal is 7.5 or slightly higher
Temperature - optimal is 25-30 - after this the growth rate drops significantly.
Oxygen - DIssolved oxygen content. Typically operate at DO = 1.5-2 mg/L
Can measure using extended BOD test, past around 6-8 days
Or can see how it affects maximum specific growth rate using equations (write equations)

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12
Q

How do oxidation ponds work?

A

The influent wastewater enters one end of the pond. The suspended solids (particulate BOD) in the water will settle in the pond forming a bottom sludge (called the anaerobic zone). The sludge is decomposed by anaerobic microorganisms forming CH4 and CO2. The dissolved organic matter (soluble BOD) remains suspended near the top of the pond. This is called the aerobic zone where the organic matter is oxidized by aerobic respiration. Sunlight is used as the inactivation mechanism for pathogens via internal and external photooxidation

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13
Q

How to upgrade ponds?

A

Can increase detention time in pond so there is a longer time for the BOD to decompose/oxidise.
Shallow with a large surface area as the effective zone of light penetration is 30 cm
Want to maximise wind impact on pond
Aerator/mixer assistance to increase DO content for aerobic respiration, also bringing non motile algae into light penetration zone
Baffles to encourage mixing and flow patterns - eg longer detention times

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14
Q

Explain Henrys Law in relation to the effectiveness of biofilters

A

Mass transfer in biofilters is controlled by solubility which is best described by Henrys Law - x = y/H
High Henrys constant indicates low solubility and therefore low mass transfer to water/wet film. As trickle beds efficiency relies on the contaminant diffusing into the water flow, this makes biofilters more attractive due to their reduced water content

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15
Q

Why is clay to sand ratio of the bed important?

A

When there is too much clay in the bed it can harden when not supplied with enough water resulting in bed cracking and significantly decreasing bed removal efficiency

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16
Q

Why is bulking agent important?

A

Allows for optimal contact of air and biofilter. Also, the bed would collapse in on itself (too compact) not allowing for effective airflow.

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17
Q

What is in wastewater?

A

Particulates/Suspended solids, organic matter, inorganic matter, heavy metals, hazardous + persistent organics

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18
Q

What are the chemical characteristics of wastewater?

A

pH, Alkalinity, Organics, nitrogen, phosphorus, sulphur, metals, soluble gases

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19
Q

What is the BOD test?

A

Measures the amount of oxygen consumed by the bacteria in the biochemical oxidation of the organic matter present in WW. Requires measuring the DO concentration at the beginning/end of a period and determining oxygen depletion. BOD bottle is incubated for 5 days at 20 degrees

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20
Q

What is ultimate BOD?

A

Oxygen required to complete oxidation, synthesis and endogenous respiration reactions
Usually run for 28 days

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21
Q

Effect of nitrification on BOD?

A

Usually takes 6-10 days to have a measurable effect.

Can use chemical inhibition.

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22
Q

Limitation of BOD test? (5)

A
  • Time taken (5 days)
  • Need a fairly high concentration of active bacteria
  • Only biodegradable matter is measured not total organics
  • Pretreatment may be necessary
  • As only a small sample is added to bottles accuracy reduces for high BOD samples
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23
Q

COD test? Limitations?

A

Mixing a very strong oxidant such as acid potassium dichromate with WW and digesting at 150 degrees

  • Theoretical > measured
  • Measurement doesn’t provide any info on biodegradability
  • Need to fractionate COD
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24
Q

4 goals of sampling?

A
  • Reproducible
  • Representative
  • Defensible
  • Useful
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25
Q

Compare 3 screen types

A

Bar screens - Have poor capture and can suffer from wear but continuous spray cleaning is not needed
Band screens - high removal, essential for more complex plants, preferred type
Horizontal rotating drum - Have milliscreens (standard for industry) or suboscreens (for large municipal flows), good capture rates, need continuous high P spray cleaning and hot water blasting for fats

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26
Q

What is the purpose of washing and dewatering screenings?

A

Less organics to landfill
Lower odour potential
Smaller volumes for transportation

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27
Q

What are 3 types of grit removal?

A

Aerated spiral flow - Consistent removal for a wide range of flows, low organic content in grit, pre aeration reduces septicity, Dis: power required from blower, aeration can drive off smelly volatile substances
Detritor (Based on principle of constant velocity)- Adv: Low head loss. Dis: Often difficult to get uniform flow, high organic content of grit, loss of grit disturbed by rake
Vortex grit removal. Adv: Removes high % of fine grit, maintains removal efficiency for wide range of flows, no submerged equip requiring maintenance, low footprint, low headloss.
Dis: Paddles collect rags, oil and grease accumulate, grit sump clogging, deep excavation or high structure required.

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28
Q

What is the aim of primary sedimentation?

A

Reduce process volumes to secondary treatment
Reduce secondary sludge
Reduce secondary treatment aeration
Captures nuisance material which help downstream processes run smoothly

29
Q

What is dissolved air flotation?

A

A proportion of flow is recycled and saturated with compressed air. As the recycle is released into tank the air is released as fine bubbles, air bubbles attach to solids lifting them to surface, floating material is skimmed off and bottom sludge from settling is also removed.
If particulate material has low density it may be more suitable. Can also be combined with acid dosing to ‘crack’ and remove proteins.
Alot smaller surface area required for DAF is one of main advantages.

30
Q

What is rotating biological contactor?

A

Attached growth process
Series of media discs on a rotating shaft, partially submerged in WW
Adv: they don’t require an air supply, so less power req, loss headloss
Dis: Generally only suitable for small sites, can have uneven biomass loading which can lead to shaft failure.

31
Q

What is trickling filter?

A

Attached growth process, WW is pumped above media and trickles down
Adv: Simple and reliable, low power input (more than RBC), reasonable effluent quality, low skill requirement for operation.
Dis: Land area req, limited control (more than RBC), potential for clogging, odour nuisance
Req secondary clarifier

32
Q

What is moving bed biofilm reactor?

A

Media is in tank, free to move around reactor

Adv: Can control liquid environment quite readily and hence the processes that are going on

33
Q

What is MABR?

A

Membrane aerated biofilm reactor.
Media is a membrane
Adv: Always have o2 right on surface of biofilm and oxygen is depleted as it goes through to bulk liquid phase. Bulk liquid phase is often anoxic - good dentitrification

34
Q

What is a suspended growth process?

A

Typically have reaction tank followed by solids separation with recycled solids back into tank. Microorganisms are maintained in suspension - process air is provided to maintain biomass. Biomass is separated from WW prior to discharge.

35
Q

Complete mix vs plug flow activated sludge?

A

Complete mix tanks allow for short circuit paths :(
Complete mix can cope with higher strength in flows and can be used in series to achieve greater BOD reduction
Plug flow minimises risks of short circuit paths, only used for domestic WW, high strength WW fed into plug flow can cause low DO

36
Q

Three functions of activated sludge clarifier?

A
  • A thickener to produce a continuous underflow of thickened sludge for return to the biological reactor.
  • A clarifier to produce a clarified effluent
  • A storage tank to store sludge during peak flows
37
Q

What are the advantages (5) and disadvantages (5) of extended aeration?

A 
Adv:
- No PST
- Longer SRT - so low sludge production
- Stable sludge - won't stink
-Simultaneous denitrification due to areas with low DO concentrations
- Can be designed for nutrient removal
Dis:
- Often poor settling
- Longer SRT, higher O2 demand
-Sludge doesn't digest well
- Can be difficult to control extent of anoxic zones
- Large footprint
38
Q

What is contact stabilisation?

A

Generic AS system, main reactor is contact tank below is stabilisation tank where RAS is prepped as it enters contact tank.
Adv: Lower aeration, lower volume for given SRT
Dis: Nitrification is limited by retention is contact stage (not good at nutrient removal)

39
Q

What is AB process?

A

Absorption-Biooxidation - 2 AS plants
1st one is very high rate, enhanced primary
Adv: Good for higher strength influents
Dis: Extra space req, higher cap cost

40
Q

What is SBR?

A

Sequencing batch reactor - no clarifier
Fill stage -> React stage -> Settle stage -> Decant stage
Adv: Single tank
Dis: You only take WW in one stage of this cycle

41
Q

What are configurations for AS N removal?

A

LE - Anoxic -> Aerobic
MLE -> Anoxic -> Aerobic with internal recycle
4 stage Bardenpho - Anox - Aerob - Anox - Aerob
Have final aerobic stage to get rid of nitrogen gas so it doesn’t cause sludge to float in clarifier.
Oxidation ditch

42
Q

What are configurations for P removal?

A

UCT
Johannesburg (anoxic tank in RAS line)
5 -stage bardenpho ( Anaerobic stage 1st)

43
Q

What are the combined fixed + suspended growth processes?

A

Trickling filter solids contact
MBBR/AS
IFAS

44
Q

What is filtration?

A

Removal of particulate material

Reduction of BOD, TP, TN associated with organic solids

45
Q

What are the types of filters?

A 
Rapid gravity (water flows down through support media)
- typically used for treating waters from rivers and lakes 
Continuous flow sand - compact and smaller footprint with a higher loading rate compared to rapid gravity, full covered 
Disc filters - even smaller footprint, low headloss and low energy input, fully covered
46
Q

What are different disinfection types?

A

Physical - Heat, and UV light
Chemical - chlorine, ozone
Unit processes - already in plant, like screens and AS

47
Q

What factors influence the action of disinfectants? (5)

A 
Contact time
Concentration of disinfectants
Intensity and nature of physical agent or means 
Temperature
Types of organisms
48
Q

Advantages (3) and disadvantages (2) of UV disinfection

A 
Adv:
No disinfection by-products 
No chemicals - and associated delivery storage and hazards
Less operator intervention 
Dis:
Power use and replacement parts
49
Q

What is UV disinfection?

A

A physical process where energy is absorbed by DNA. This causes photochemical damage of DNA/RNA within the organism.

50
Q

What are extractive MBRS

A

Used to extract specific components from the WW eg priority pollutants

51
Q

What are MABRs

A

Uses gas permeable membranes to directly supply high purity O2 without bubble formation to biofilm. In conventional AS processes the process efficiency is controlled by availability of air, 90% of this is lost and vented to atm, MABR solves this.

52
Q

What are membrane separation reactors?

A

UF or MF replaces most tertiary treatment methods as has good effluent quality

53
Q

What are membrane bioreactors?

A

Used for enhancing solids separation in AS reactor. Consists of a bioreactor with suspended biomass and then solids separation by MF or UF membranes
Produces good quality effluent better than clarifiers
Can be either immersed or external (passed through membranes in series)

54
Q

What are the advantages (5) and disadvantages (5) of using membrane bioreactors?

A 
Adv:
-Higher volumetric loading rates and thus shorter reactor HRT
-Longer SRT ie less sludge production
-Can operate at low DO concs with potential for simultaneous nitrification/denitrification in long SRT designs 
-High-quality effluent
-Less space required
Dis:
- High capital costs 
- Limited data on membrane life 
- Potential high costs of membrane replacement
- Higher energy costs
- Need to control membrane fouling
55
Q

What are the advantages (6) and disadvantages (5) of anaerobic treatment?

A

Adv:
- Reduction of sludge production up to 90%
- Reduction of space required when using high rate systems
- Reduction in energy requirement from aeration
- Production of CH4 energy (biogas)
-Sludge can be stored unfed for long periods
-Granular sludge can have market value
Dis:
-Effluent often requires further treatment
-Nutrient removal is limited to biomass growth
-High temps are often required to get necessary reaction rates
-Odour potential
-H2S yields a poor quality of gas and reduced COD removal

56
Q

What are the different anaerobic systems?

A 
Contact
BVF
CIGAR
AN-MBR
UASB
EGSB
IC
ECSB
57
Q

What to consider when choosing an anaerobic reactor? (6)

A 
Suspended solids
Toxic components
Strength and composition of biodegradable COD
Nutrients
Temp
Presence of alternate electron acceptors
58
Q

What is anaerobic digestion?

A

Biological degradation of organic solids in the absence of free oxygen

59
Q

What are the different types of sludge digestion

A

MAD - mesophilic

TAD - Thermophilic

60
Q

Advantages (4) and disadvantages of TAD?

A

Adv:
Increased reduction of pathogens
Increased solids destruction and biogas production
Less solids for further treatment
Less retention time required
Dis: Odorous sludge and biogas, additional HXs

61
Q

Advantages of TPAD (4)?

A

Thermophilic followed by mesophilic

  • Less odour than thermo alone
  • Less foam than meso alone
  • Stable operation
  • Improved dewaterability of digested solids
62
Q

What are the steps in applying treated effluent to land?

A
  1. Identify potential land parcels considering elevation, contours, price, neighbours and waterways
  2. Investigate soil properties
  3. Assess crop conditions (Ryegrass of forests)
  4. Water and nutrient balance
    Know when to irrigate
    Limit for applying N to land + uptake of nitrogen from crops
  5. Irrigation methods
    Centre Pivot
    Solid Set
    Rootzone dripline
63
Q

What are different irrigation methods?

A

Centre Pivot - low cost, bad for contours, large setback
Solid Set - higher capital cost and maintenance, good for contours, large setback
Rootzone dripline - can be above or below ground, $$, small setback, can operate on contours but difficult to install.

64
Q

What are the practical problems of secondary clarifiers?

A

Poor flocculation = poor settling
Scum accumulation on surface
(these problems affect final effluent quality increasing TSS and BOD)

65
Q

What upgrades have been made to secondary clarifiers?

A

Flocculation zones - upper deflector preventing the accumulation of scum, lower deflector preventing disturbances in separation and thickening zones
Scum baffles - which prevent floating biomass from escaping to the effluent
Stamford baffle to direct sludge flow back to separation zone

66
Q

What are the solids management options?

A

Preferable to resuse solids where financially viable to conserve the nutrients and avoid consuming landfill space. However there are constraints on discharges to land under RMA
Options are to spread on land or combust for energy recovery
Ash from combustion can be made into bricks or landfilled
Costs of solids handling can be 30-50% of the total WWT cost

67
Q

How to prep solids for reuse?

A

Dewatering
Pathogen reduction
Vector attraction reduction

68
Q

Why would you use constructed wetlands?

A

Have a visual appeal and are favoured for cultural reasons

69
Q

Compare 3 high rate anaerobic systems

A

Aerobic contact processes are characterised by their relatively simple design consisting of a completely mixed digestor (reactor) following by a settling tank or clarifier. The settled solids are recycled back into the process allowing ACP to obtain high solids retention time with a high concentration of solids in the reactor.
In UASB systems the wastewater moves up through the reactor. A GLSS device is used to separate and remove the biogas, enhance sludge settling by reducing liquid turbulence in the settler, and prevent expansion of the sludge bed in the digester.
EGSB systems are similar to that of UASB. They are characterised by the slight expansion of the sludge bed as a result of high superficial liquid velocities and the lifting action of the gas in the bed. This results in high contact between the sludge and wastewater, and allows for significantly higher loading rates compared to UASB systems

ENGR405 (3 decks)

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