Pollutants and Drinking water Treatment

Question 1

list some conventional and some emerging pollutants

Answer 1

conventional- pathogens, organic matter, salts, toxic substances
emerging- nanomaterials, endocrine disrupting chemicals, plastics.
emerging pollutants are synthetic or uncommon natural chemicals, potentially damaging to the ecosystem and human health

Question 2

Define dissolved oxygen (DO) and biochemical oxygen demand BOD

Answer 2

the amount of molecular oxygen dissolved in water in liquid phase. Oxygen demand is the amount required to degrade pollutants. BOD gives a measure of the oxygen used by microorganisms during oxidation of organic matter. it indirectly measures concentration of organic matter.

Question 3

how would you conduct a standard BOD test?

Answer 3

conditions: conducted over 5 days, 20 degrees, dark, excess of nutrients, dilution.
1. 10ml of sample is taken into 300ml bottle then filled with dilution water
2. initial DO is calculated then again after 5 days (20 degrees)
3. difference in reading is multiplied by dilution factor gives BOD

Question 4

How do you calculate BOD from a test?

Answer 4

BOD= ((DOi - DOf) - (1 - p)(Bi - Bf))/p
P= dilution factor = Vs/Vt
B= blank dissolved O2 concentration

Question 5

How do you calculate BOD at a given time (t)?

Answer 5

BOD(t) = L0 (1 - e^-(kt))
L0= oxygen equivalent of organic matter at t=0
L0=BODu

Question 6

What is Chemical oxygen demand (COD)?

Answer 6

the equivalent amount of oxygen required to oxidise any organic matter in water by means of a strong oxidising agent. This is much faster and has better repeatability than BOD.

Question 7

How would you model a ‘sag’ curve?

Answer 7

1. determine the initial conditions of the river
2. determine the deoxygenation rate (BOD test)
3. determine the reaeration rate
4. calculate DO deficit as a function of time
5. calculate critical distance and time

Question 8

How do you calculate deoxygenation rate constant in a river?

Answer 8

Kd(20) = k + vη/H
Kd= Kd(20) * θ^(T-20)
This accounts for increase in microorganisms activity in warmer temperatures

Question 9

How do you calculate the reaeration rate constant in a river?

Answer 9

Kr(20) = (3.9 * v^0.5)/(H^1.5)

varies with temp in the same way as Kd

Question 10

list some important water quality parameters

Answer 10

turbidity(cloudy), pH, hardness, alkalinity, nutrients, microorganisms.

Question 11

Define total suspended solids

Answer 11

solids in water that are large enough to settle out of the solution, or that can be filtered.
TSS = (final mass of filter- initial mass of filter)/water volume

Question 12

How can pH of water cause issues?

Answer 12

pH<7 corrodes metal pipes, pH>8 decreases efficiency of chlorine disinfectant. acceptable range 6.5

Question 13

What is hardness?

Answer 13

the amount of dissolved calcium and magnesium in the water. Carbonate hardness (CH) is temporary (removed by hot water) but non-carbonate hardness (NCH) is permanent.

Question 14

list the stages of drinking water treatment

Answer 14

screening, coagulation, flocculation, flotation and sedimentation, softening, filtration, disinfection, storage, distribution.

Question 15

What happens during screening?

Answer 15

relatively simple process by mechanically removing large solids such as logs, branches and fish as to protect pumps and pipes.

Question 16

What happens during coagulation and flocculation?

Answer 16

grouping small suspended particles and colloids into larger flocs, which can then later be removed. with coagulation we neutralize the surface charge of the particles by adding coagulants allowing particles can group (fast mixing tank). flocculation aims to achieve the optimal conditions for floc growth (slow mixing tank).

Question 17

What’s the difference between discrete particles and flocculating particles?

Answer 17

discrete particles do not change shape, size or velocity gradient over time. flocculating particles do.

Question 18

list the important processes in the global water cycle

Answer 18

precipitation, evaporation, transpiration, surface run-off (overland flow, water network run off), infiltration, interflow (soil moisture, vadose zone), base flow, groundwater flow (aquifers)

Question 19

What is meant by water stress?

Answer 19

Water stress occurs when the demand for water exceeds the available amount during a certain period or when poor quality restricts water use.

Question 20

Describe the urban water cycle

Answer 20

Urban water sources are stream, lakes, reservoirs and aquifers. Water is transferred from these sources to storage and treatment facilities, then transferred to end users via distributions system. After use wastewater is collected then sent to treatment.

Question 21

Who regulates provision of water and wastewater services in the UK.

Answer 21

Environmental Agency (environmental impacts), Office for Water Regulation (economical regulator), Drinking Water Expectorate (safe drinking water)

Question 22

What is a point source and a non-point source of pollution?

Answer 22

point source- localised released pollutants (sewer/ treatment plant)
non-point source- released over a large area (agriculture/ industrial estate)

Question 23

List two nutrients of major concern for a natural receiving water

Answer 23

nitrogen and phosphorous promote excessive growth of algae and cyan bacteria which die then block the light at the surface

Question 24

What’s the difference between CBOD and NBOD

Answer 24

CBOD- OD only from carbon containing compounds

NBOD- OD only from nitrogen containing compounds

Question 25

Why must coagulation and flocculation done in different speed tanks?

Answer 25

A high-energy, rapid-mix to properly disperse coagulant and promote particle collisions is needed to achieve good coagulation. Over-mixing does not affect coagulation, but insufficient mixing will leave this step incomplete. flocculation needs to be slow to ensure flocs that group together don't tear apart or shear as they are very difficult to stick back together again

Question 26

Describe the physical differences between Type I, Type II and Type III settling

Answer 26

I) particles settle independently, constant velocity and don't stick II) particles collide and stick, settling velocity is generally increasing III) grate concentration such that flocs settle as a mass and form the sludge zone

Question 27

Describe how a dissolved air flotation tank operates

Answer 27

DAF is a technique to promote flotation by dissolving air in water under pressure. When pressure is release, water becomes supersaturated with bubbles. Bubbles attach to particles causing them to float to the surface.

Question 28

Explain the process and the reasons for using lime soda for softening water

Answer 28

to promote a precipitation reaction. it is the most economical source. (quicklime)

Question 29

What is the critical settling velocity?

Answer 29

critical settling velocity when 100% of particles can be removed, %= 100 * Ut/Uc for tank to work Ut>=Uc

Question 30

How do you design a settlement tank?

Answer 30

-determine settling velocity (Ut) and set overflow rate (Uc) to equal or less than this. Uc=Q/A, A=L*W=Q/Ut, Uh=Uc=Q/WH.

Question 31

How do you determine the settling velocity for different flows?

Answer 31

laminar (Re<1)- stokes law applies, normal Ut equation | transitional (Re>1)/ turbulent flow (Re>10^4)- iteration method

Question 32

What are the aims of the softening process?

Answer 32

reducing hardness by removing minerals which limit effectiveness of detergents which cause 'scale'. Fe and Mg can leave rust coloured stains on clothing. also removal of dissolved gases which contribute to bad odour and taste.

Question 33

describe the process of filtration.

Answer 33

aims to remove suspended solids and colloidal particles too small for sedimentation removal. performance is measured by the effluent turbidity.

Question 34

describe a filter design for slow sand filtration

Answer 34

- loading rate Q/As = Va = 2.9 - 7.7 - influent placed on top of sand filter bed - pores get blocked in the top layer so performance degrades progressively. - top 75mm sand is periodically removed.

Question 35

describe a filter design for rapid sand filtration

Answer 35

- loading rate = Q/As = Va > 120 - coarse layered filters - similar to sedimentation process - filters are periodically backwashed, sand grains are pushed apart and filtrated particles removed

Question 36

What is the effective size?

Answer 36

10-percentile diameter, 10% by weight of filter material is less than this diameter. particle size distribution curve for <10% is defined as the effective size. Determined by sieve analysis.

Question 37

What is the uniformity coefficient?

Answer 37

``` UC = ratio of the 60th percentile to the effective size UC = d60 / d10 ```

Question 38

describe the process of backwashing

Answer 38

carried out by flushing water back up through the filter. backwash water goes back to the inlet. anthracite is lighter than sand and gravel the formation naturally reforms. system must be designed to handle flow with at least one filter out of service.

Question 39

Describe an ideal disinfectant

Answer 39

- toxic to pathogens and not humans - versatile against different micro-organisms - fast acting - effective in presence of interfering materials - affordable

Question 40

What are the two stages of disinfection?

Answer 40

Primary disinfection - inactivation of microbes in water | Secondary disinfection - maintaining a disinfectant residual in the treated water distribution (residual maintenance).

Question 41

list some examples of chemical and non-chemical disinfectants

Answer 41

chemical : chlorine, chlorine gas, sodium hypochlorite (bleach) non-chemical: ozone, irradiation (UV), sonication, gamma radiation

Question 42

what is chick's law?

Answer 42

disinfection rate of organisms = reaction rate * concentration (number/volume) dN/dt = -K * N

Question 43

what is Watson's law?

Answer 43

relative risistance of organisms = disinfection concentration ^(dilution factor) * time k = t * c^n

Question 44

What is the chick-watson law?

Answer 44

ln(Nt/N0) = -K * c^n * t | k=inactivation constant

Question 45

How do you calculate the percentage removal based on the log-removal?

Answer 45

LR = log(influent concentration / effluent concentration) | % removal = 100 - (100/(10^LR))