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Microbial Biotechnology > Wastewater Treatment > Flashcards

Wastewater Treatment Flashcards

1
Q

What is the aim of wastewater treatment?

A

To reduce or remove organic and inorganic materials in wastewater, such as toxic metals, nutrients such as phosphate or nitrate and pathogens.

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2
Q

What is monitored in wastewater?

A

Organic carbon- requires oxidation, measured as BOD or COD,
Inorganics- nutrients such as N, P, and S or toxic metals,
Special organics- volatiles such as petroleum hydrocarbons,
Microbiology- pathogens present in human waster and indicator organisms.

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3
Q

What is BOD and COD?

A

BOD is the biochemical oxygen demand, this is the amount of dissolved oxygen consumed by microorganisms for the biochemical oxidation of organic and inorganic matter.
BOD5 is the five day measurement of the amount of O2 required to oxidise organic matter in a sample.
COD is the chemical oxygen demand.

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4
Q

What is the rough equation for the breakdown of organic compounds by microorganisms?

A

Organic compounds + O2 –> NH4^+ + H2O + CO2 + biomass

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5
Q

How is BOD measured?

A

BOD5 measurement- aliquots of wastewater are placed in a bottle, diluted and seeded with a culture. The sample is then saturated with oxygen and left in the dark. After five days at incubation at 20 degrees the amount of O2 consumed is determined.

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6
Q

What is nitrogenous oxygen demand?

A

This is the amount of dissolved oxygen required for biological oxidation of nitrogen in wastewater. This is due to nitrifying bacteria that oxidise NH4^+ to NO3-. Monitoring of the nitrogenous oxygen demand occurs after the BOD5 measurement due to the slower growth of nitrifiers.

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7
Q

What is chemical oxygen demand?

A

This is the amount of O2 required to oxidise carbon completely to CO2, H2O and NH4^+. Organic compounds are oxidised to CO2 by adding strong oxidising agents such as potassium dichromate, under acidic conditions.
The BOD5/COD ratio is the fraction of easily degraded organic matter.

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8
Q

What is total organic carbon?

A

This represents the total amount of organic carbon in a given sample independent of the oxygen state. It is determined by oxidation of the heated organic matter and measurement of the CO2 liberated.

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9
Q

How can suspended solids be measured and why do they need to be removed from the wastewater?

A

This is measured by filtering the sample, drying the residue at 104 degrees and determining the mass.
Suspended solids need to be liberated from the wastewater because if released in the effluent they can foul the waterways.

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10
Q

What are mixed liquor suspended solids?

A

This is a measure of the suspended solids in an aeration tank during the activated sludge process. It indicates the concentration of biomass in a wastewater treatment plant.

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11
Q

What is the primary and preliminary treatment of wastewater?

A

The objective of this is to remove debris and coarse materials from the wastewater. Physical processes such as screening and sedimentation are used. BOD is reduced by 30-40%.

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12
Q

What is secondary treatment of wastewater?

A

The objective of this is to use biological and physical processes to speed up the biological degradation of solids in the wastewater. It reduces BOD by 80-90%.

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13
Q

What is tertiary treatment of wastewater?

What is sludge?

A

This is when the secondary effluent is treated further to obtain a higher effluent quality.
Sludge is the semi solid material left behind after the wastewater treatment process.

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14
Q

What are the different types of secondary wastewater treatment processes?

A 
Trickling filters, 
Rotating biological contractors,
Activated sludge treatment,
Anaerobic digestion,
Waste stabilisation ponds,
Soil aquifer treatment,
Artificial wetlands.
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15
Q

What is the trickling filter method?

A

Watertight basins are filled with rocks or a highly permeable plastic medium. The wastewater is trickled through the medium and air diffuses up from the bottom. Microbes attack to the medium and form a biofilm and degrade the organic matter.

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16
Q

What is the rotating biological contractor method?

A

Circular disks are submerged in the wastewater and rotated slowly through it. Biological growth forms a slime layer, the rotation maintains the biomass in an aerobic condition.

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17
Q

What is activated sludge treatment?

A

This is a type of aerobic suspended growth process (suspended biofilm). It is the most common form of wastewater treatment. It uses an aeration tank and a settling tank to to produce clear effluent. The aeration tank is where organic matter is degraded. Any sludge produced here then settles in the settling tank and is then treated or disposed of.

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18
Q

How is sludge processed.

A

Sludge treatment and disposal is costly. Sludge from primary and secondary processes contains 1-5% solids. Treatment involves thickening and dewatering via Centrifugation and filtration. This results in sludge with a solids content of 20-40%. Dewatering is followed by stabilisation and final disposal of sludge.

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19
Q

What is sludge stabilisation?

A

This occurs after sludge dewatering and the purpose of it is to reduce the mass of the sludge, reduce its odour and make it safer. Processing includes: anaerobic digestion,
Composting,
Sludge may be heat stabilised or lime stabilised,
Incineration,
Landfill.

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20
Q

What is anaerobic digestion in terms of sludge digestion and processing?

A

This is a sludge stabilising process. A series of microbiological processes convert organic compounds to methane and CO2 and reduce volatile solids by 35-60%. It is carried out in the absence of oxygen by bacteria and archaea.
One stage digestion uses a single tank, two stage digestion uses two tanks, one for heating and mixing and the other for thickening and storage.

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21
Q

What bacteria carry out the initial and later stages of sludge anaerobic digestion?

A

Acidogenic bacteria carry out the initial stages, producing organic acids from the solids.
Methanogenic bacteria carry out the later stages, converting these acids or CO2 to methane.

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22
Q

Describe methods of composting.

A

Composting is a sludge stabilisation process. An aerated static pile can be used in which the sludge is mixed with a material such as woodchips and aerated for 20-30 days, this is then cured for 30 days, dried and then screened.

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23
Q

What is tertiary treatment of wastewater in terms of disinfecting the effluent?

A

This is a process used on the secondary effluent to produce a higher quality effluent. One method used is disinfecting, with chlorination and UV commonly being used.

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24
Q

What are the two main goals of the activated sludge process?

A

Oxidation of biodegradeable organic matter in the aeration tank. Soluble organic matter is converted into new cell biomass.

Flocculation- separation of the newly formed biomass from treated effluent.

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25
Q

What do activated sludge flocs contain?

A

They contain bacteria, Protozoa, viruses and fungi. Flocs also contain aerobic, anaerobic and anoxic zones and appear as an amorphous cell mass if viewed via microscope.

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26
Q

Describe bacteria in the activated sludge floc.

A

Bacteria make up the bulk of the biomass (approx 95%). Bacterial extracellular polymeric substances and filamentous bacteria play important roles in floc structure. Archaea are present but play minor roles. Beta proteobacteria are dominant in flocs.

27
Q

Describe Protozoa in the activated sludge flocs.

A

These are important in sludge ecology. They graze on bacteria and play important roles in maintaining floc size and density. The levels of certain Protozoa give indications for the status of the watewater, if a high number of ciliates are present then the water has a high BOD. If a high number of flagellates are present then there is a high concentration of organic solutes remaining.

28
Q

In terms of the activated sludge process what are mixed liquor suspended solids?
How can it be calculated?

A

The content of the aeration tank is known as the mixed liquor. Mixed liquor suspended solids is the concentration of the total amount of organic and mineral suspended solids including the microbial biomass in the mixed liquor.
Can be calculated by filtering the solid, drying at 105 degrees then measuring the mass of the solid and calculating the concentration.

29
Q

What is sludge age?

A

This is also known as the solid retention time (SRT), sludge age is the mean residence time of microorganisms in the system. It is the reciprocal of the microbial growth rate.
The equation is: MLSS x V/ (SSe x Qe) + (SSw x Qw)
Where is MLSS is the mixed liquor suspended solids, V is the volume f the aeration tank, SS is the suspended solids and Q is the quantity of wastewater.

30
Q

What is the hydraulic retention time?

A

HRT is the average time spent by the influential liquid in the aeration tank. Suitable contact time of wastewater with microbes is required for adequate degradation, typical HRT is 4-12 hours.
The equation is: HRT = V/Q where V is the volume and Q is the flow rate or influential wastewater.

31
Q

What is cell yield?

A

Cell yield is the amount of biomass formed per unit of substrate removed. Cell yield depends on growth rates, with faster growth rates generally meaning a higher cell yield. The equation for this is:
Y = X-Xo/So-S. Where Y is the cell yield, X is the final microbial concentration, Xo is the initial microbial concentration, So is the initial substrate concentration and S is the final substrate concentration.

32
Q

Are low growth rates desirable?

A

It would lead to less sludge to deal with however it also means slow and large treatment facilities.

33
Q

What is the food microorganisms ratio?

A

This indicates the organic load into the activated sludge system. It is controlled by the rate of sludge wasting. A low F/M ratio generally leads to a more efficient wastewater treatment. A typical activated sludge system has a F/M ratio of 0.2-0.5kg BOD5/day/kg/MLSS.
Equation is: F/M = Q x BOD5/ MLSS x V

34
Q

What happens in the clarifier of a activated sludge system?
What does settling require?
What must growth encourage in an activated sludge system?

A

In the clarifier the solids settle and separate from the treated wastewater.
Settling requires the microbes to have flocculated and formed aggregates (flocs).
Growth in an activated sludge system must encourage floc forming organisms.

35
Q

Settling is best observed in a clarifier when?

A

Microbes are in the endogenous stage of growth.
Sludge age is at least 3-4 days.
There is a low F/M ratio.

36
Q

Other than the floc forming microorganisms, what else is required to form flocs?

A

Extracellular polymeric substances. These consist of sugars, uronic acids and amino acids important in floc formation.

37
Q

What is the sludge volume index?

A

It is the ability of the sludge to settle and is monitored by wastewater treatment plants by measuring the sludge volume index (SVI).
SVI = Vs/ Vo x MLSS x 0.001
Where SVI is the sludge volume index, Vs is the volume of settled sludge after 30 min, Vo is the volume prior to settling, MLSS is the mixed liquor suspended solids, V is the volume of the tank.

38
Q

Why is bulking and foaming major problems in wastewater treatment plants?

A

They prevent effective settling in the clarifier and thus cause increased amounts if solids in the effluent.

39
Q

Why is dispersed growth a problem with activated sludge systems?

A

Because when this occurs microorganisms don’t form flocs and therefore are dispersed. This results in no zone settling or sludge.

40
Q

Why is slime and viscous bulking a problem?

A

Because the microorganisms are present in large amounts of extracellular slime. This results in reduced settling and virtually no solid separation.

41
Q

What is the problem with pin flocs?

A

These are small compact and roughly spherical flocs that results in a low SVI. Can occur when there are no filamentous bacteria.

42
Q

What is the problem with bulking?

A

This is when filamentous organisms extend from flocs into the bulk solution and interfere with compaction and settling of activated sludge, high SVI.

43
Q

What is the problem caused from foaming and scum formation?

A

This can be caused by a build up of non-degradable surfactants. The foams float sludge to the surface of treatment units. Foam accumulates and purifies. Solids may overflow into secondary effluent or onto walkways.

44
Q

What factors influence filamentous bulking?

A

Wastewater composition- high carbohydrate concentration in wastewater increases filamentous growth.
F/M ratios- low F/M ratio most common cause of filamentous bulking.
Sludge loading and sludge age- generally a high sludge age and low sludge loading results in an increase in filamentous organisms.
pH- optimum pH for aeration tank is 7-7.5, a pH of less than 6 favours fungi which can cause bulking.
Sulphide concentration- high sulphide concentration favour sulphur filamentous bacteria.

45
Q

In what year was the activated sludge process developed and who by?

A

It was developed in 1914 by Arden and Lockett.

46
Q

Why is it important that the activated sludge process can be operated to achieve biological removal of nitrogen?

A

It is important because both Phosphorus and Nitrogen can cause eutrophication, additionally ammonia is toxic to fish. Additionally nitrate entering the water can cause blue babies syndrome. This is when nitrate converting bacteria in the GI tract convert nitrate to nitrite which competes with oxygen for binding to haemoglobin.

47
Q

Organic nitrogen is converted to what en route to the wastewater treatment plant?

A

It is converted to ammonium

48
Q

What is the enzyme that catalyses the reaction between amino acids and water to form ammonium and carbon dioxide?
Write the equation to this.

A

Urease.

H2NCONH2 + H2O –> CO2 + 2NH4^+

49
Q

How does liming and air stripping remove nitrogen?

A

Liming reduces increases the pH to 10-11, resulting in more OH^- ions. These react with ammonium to form NH4OH which is then converted to ammonia and water. The ammonia is then removed via air stripping.

50
Q

How can breakpoint chlorination be used to remove nitrogen?

A

Breakpoint chlorination oxidises ammonium to N2.

NH4 + 2Cl + O2 –> N2 + 6HCl + 2H2O

51
Q

How can microbial assimilation be used to remove Nitrogen?

A

Microbes require N for growth. Nitrate, nitrite or ammonia can be used however they only remove a small amount of the total amount of nitrogen.

52
Q

What is nitrification?

What is denitrification?

A

This is a process which converts ammonia to nitrite. It is a aerobic process, carried out by nitrosomonas and Nitrobacter bacteria.
This is an anaerobic process in which nitrate is reduced to gaseous nitrogenous compounds.

53
Q

What would the set up of an activated sludge system be when nitrogen is being removed?
What are the equations for the processes going on in each type of tank?

A

The settled sewage would be fed into an aerobic tank before being fed into an anoxic tank and then another aerobic tank before finally entering the clarifier. Here sludge can either be returned to the first aerobic tank or the effluent leaves.
For the aerobic tank the equation is: BOD + O2 -> NO^3- + CO2
For the anoxic tank the equation is: BOD + NO^3- -> CO2 + N2

54
Q

Describe the order of tanks for the Bardenpho process.

A

The influential first enters a anoxic tank before entering an aerobic tank. After this it enters an anoxic tank before then entering another aerobic tank. Finally after this it enters a clarifier. Sludge can be returned to the first anoxic tank from this or can leave as effluent.

55
Q

Nitrification occurs in how many stages?

A

It occurs in two stages. Ammonia oxidation and nitrite oxidation.

56
Q

Describe some characteristics of a bacteria that is involved in ammonia oxidation.
How many steps is the process of ammonia oxidation?

A

Nitrosomonas is an example. It is a slow growing bacteria with the doubling time being 8 hours.
Two steps.

57
Q

What is the two step reaction equation for ammonia oxidation?
What are the enzymes that catalyse each step?
What is the intermediate between ammonia and nitrite?

A

NH3 + O2 + H2 –> NH2OH + H2O –> NO^2- + 5H^+
Ammonia monooxygenase,
Hydroxylamine oxidoreductase,
The intermediate is Hydroxylamine.

58
Q

Describe the characteristics of a nitrite oxidising bacteria.

A

An example is Nitrobacter. It is a chemoautotroph and it utilises energy from the nitrite oxidising process to fox CO2.
The reaction it conducts is:
NO^2- + O2 –> NO^3-
Enzyme that catalyses this is: nitrite oxidoreductase.

59
Q

Other than nitrification, what other reaction is important for removing nitrogen from activated sludge?
What does this process require?

A

Chemoheterotrophic denitrification.
The process requires organic carbon for energy. Denitrifying bacteria use nitrogen as the terminal electron acceptor instead of oxygen.
Pseudomonas is an example of a denitrifying genus of bacteria.

60
Q

What is the equation of denitrification?

What are all the equations in the reduction of nitrate?

A 
BOD + NO^3- --> N2 + OH- + H20 + CO2 + Biomass
NO^3- +2H+ + 2e- --> NO^2- + H2O
NO^2- + 2H+ + 2e- --> NO + H2O
2NO + 2H+ + 2e- --> N2O + H2O 
N2O + 2H+ + 2e- --> N2 + H2O
61
Q

In municipal wastewater phosphorus is usually in the range of what?
What is this phosphate in the form of?
What can phosphorus in wastewater lead to?

A

10-20mg/l in the form of orthophosphate PO4^3-.

Can lead to eutrophication.

62
Q

Give some methods to remove phosphorus.

A

Phosphorus assimilation by microbes.
Chemical precipitation, such as with Al^3+.
Physical methods such as membrane filtration.
Combined biological and physical processes.
Enhanced biological phosphorus removal.

63
Q

How does enhanced biological phosphorus removal work?

A

It works by accumulating phosphorus as polyphosphate by polyphosphate accumulating organisms. These are activated sludge bacteria and they accumulate Phosphate intracellularly as polyphosphate granules.

64
Q

What are the advantages of EBPR?

What are the disadvantages of EBPR?

A

Advantages: running costs are lower,
It is better environmentally.
Disadvantages: the systems intermittently fail,
The set-up costs are high,
There needs to be a back up in case of failure,
The details of the microscopy are not so well understood.

Microbial Biotechnology (6 decks)

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