Producing drinking water

Question 1

What are the water supply options?

Answer 1

Groundwater:

• good quality
• no freezing problems
• water cooler in summer period

Surface water:
- can be used if not enough groundwater or if it is low quality

Artificial groundwater:
- surface water is filtered to a natural groundwater deposit

Question 2

Surface water 3 yields:

Answer 2

Yield:
the part of the water flow that can be continuously taken to other use.

Evaluating yield:
based on basic hydrological parameters (precipitation, evaporation, run-off)

Increasing yield:
Water rationing
Water supply from another water course

Question 3

The capacity of water supply and treatment equipment and distribution network is determined by?

Answer 3

The use of population size and specific use of water (inhabitant/day)

Question 4

The storage space needed to offset the peaks of 24-hour consumption is approximately?

Answer 4

15 % of the average water consumption.

Question 5

Factors affecting the quality of natural waters?

Answer 5

• annual and seasonal weather conditions: the amount of precipitation, the length of the ice-covered period, the thickness of the snow cover and the amount of spring melting water, wind, lighting conditions and temperature.
• quality of the soil and bedrock of the catchment area,
  size, shape, depth ratios and openness of the lake basin
• biological activities of the lake: chemical and biological oxygen demand, pH, nutrients in reactive form
• waste and catchment waters

Question 6

Two typical risks of drinking water?

Answer 6

• Health risk from harmful microbes
• Risk of chemical impurities

Chemical impurities usually exist at low concentrations. Health risk is in long term effects.

Question 7

What happens to the water source if pollutants are detected?

Answer 7

The composition and quality of water shared by water utilities shall be regularly monitored and, if necessary, the water is purified from pollutants or the use of water is discontinued if it does not meet the quality requirements.

Question 8

What can be possible chemical substances in water?

Answer 8

Chemical substances may be:

• natural substances soluble in water from soil and bedrock (arsenic, uranium, radon, fluoride),
• substances entering water, e.g. substances entering water (nitrates)
• soil-contaminated substances that enter groundwater (e.g. chlorophenol, trichloroethylene, petrol).

Question 9

Domestic water is produced by water utilities and the necessary methods depend on the quality of the water.

Process may contain the following stages:

Answer 9

• Filtering/Sieving: the largest debris, fish and parts of plants are separated from raw water.
• pH adjustment: for example with lime adjusting the pH of the water to suit the precipitation.
• Precipitation: aluminium and - iron salts are used to precipitate organic substance, humus.
• Clarification: precipitate is separated either in big settling tanks or smaller flotation units.

Question 10

What must be done when cleaning water for domestic usage? First steps

Answer 10

Surface water must never be drunk without cleaning it.

• Surface water is first filtered out of all loose and larger debris by passing the water through, for example, a fine sand filter.
• After filtration, the chemical purification steps begin. Humus /organic substances and phosphorus are precipitated for example with iron(II)-sulphate.

Question 11

Second steps of cleaning water for domestic usage?

Answer 11

• Disinfection can be done with sodium hypochlorite and ammonium chloride. Ozone is possible as well.
• Finally, the alkalinity and hardness of the water are increased with carbon dioxide and lime solution, so that the water is not corrosive the water supply network.

After possible filtration through activated carbon, the water can still be disinfected with ultraviolet (UV) light.

Question 12

What is the purpose of water disinfection?

Answer 12

The purpose is the destruction of pathogenic organisms in water.

The destruction of all organisms is called sterilisation.

Normally water facilities must have equipment for disinfection available.

Water cooking (15-20 min) would be enough, but this is too expensive for normal conditions.

Question 13

What are mostly used to disinfect water?

Answer 13

Chemicals are most often used to desinfect water, but irradiation, oxidation and electrochemical methods have also been studied/used.

Most commonly used: chlorination (inexpensive and easy, good after-effect) and ozonation.

Question 14

Water disinfection with chlorination:

Answer 14

• It should be remembered that the risk of drinking chlorinated water is low compared to unchlorinated water.
• Chlorination has an after-effect, the disinfectant effect continues in the distribution network.
• For disinfection to be successful, water must be of good quality:

Suspended particles < 1 mg/l
Organic substance as low as possible
Turbidity upper limit 5-10 NTU

• Nitrite and ammonia in water reduces results
• Reduced iron and manganese compounds reduce chlorination results
• The success of chlorination is also affected by the chlorine state, supply volume, water temperature, pH (pH <7.2).

Question 15

Chlorination methods:

Answer 15

• Simple chlorination is done using sodium hypochlorite (NaOCl) or chlorine gas (Cl2)
• Hypochlorite (liquid, chlorine 10%) is dispensed with an input pump
• Chlorine gas is dissolved in a small amount of water and the solution is passed to the desired location.
• It is often difficult to achieve a consistent result.

Question 16

What is overchlorination?

Answer 16

Overchlorination refers to a high chlorine dose, in which case the chlorine residue has to be reduced by dechlorination.

The activated carbon filter has also been used for dechlorination

Question 17

Amount of chlorine usage:

Answer 17

• The supply volume of chlorine depends on the quality of the water, but must be large enough (0,2-40 mg/l). 15 mg/l is rarely exceeded.
• The free chlorine residue on the network should be between 0.2 and 0.5 mg/l.
• The half-life of chlorine is 6 months.
• Even chlorine distribution and residence time in the pool (10-30 minutes) is important.

Chlorine is a very toxic chemical so remember the safety regulations.

Question 18

Water disinfection with ozonation:

Answer 18

• Ozone is a very effective oxidizing and disinfecting gas.
• Ozone is the three-molecular form of oxygen, which is the strongest oxidizer of the chemicals used in water treatment.
• Ozonation of water removes many impurities in water, such as bacteria, iron, manganese, hydrogen sulfide, radon and organic substances.
• Also removes odor and flavor.
• If the ozonation effect is too low, ketones and aldehydes that affect taste are formed.
• Disinfection dose is usually 0.5-0.9 mg/l, i.e. the feed volume 2-3 mg/l.

Question 19

Negative effects of ozonation:

Answer 19

• Ozone is an effective disinfectant, but its tendency to decompose in water hampers its use.
• Ozone breaks down water humus-based substances into a smaller molecular form-
• The disadvantage is that the half-life of ozone is short (165 min, 20 C), and thus contact with pathogens remains short.
• At high concentrations (15%) ozone may explode.
• Relatively expensive equipment and high operating costs.

In some cases, ozonation can replace chlorination, but can not be the only treatment (does not form a residual concentration).

Question 20

Water disinfection with UV-treatment:

Answer 20

• Ultraviolet light is radiation that has shorter wavelength than visible light. Its wavelength is in the range of 315 to 100 nm.
• UV radiation power is based on the penetration of intense light through the cell wall of the virus or bacterium causing a disturbance in DNA function. The pathogen can no longer reproduce and dies.
• UV radiation use should be taken into account: water turbidity <1 NTU, humus, iron content, water layer thickness.

No harmful end products, not very effective against bacteria, but is effective against viruses.

Question 21

How is humus usually removed?

Answer 21

The removal of organic matter contained in humus-rich waters is traditionally carried out by a coagulation-flocculation process.

Partial coagulation-flocculation (10-30%)
Complete coagulation-flocculation (40-60%)
Sometimes additionally active carbon treatment (70%)

Question 22

Removing Iron and Manganese:

Answer 22

Can be done with ion-exchangers

• But at the same time calcium and magnesium are removed (so hardness must be adjusted to prevent corrosion)
• Iron and manganese attach to the ion exchange mass (needs maintenance).
• The iron and manganese in the humus are not removed well.
• Iron can also be removed in soil (before the water enters the well).

Question 23

Removing iron through coagulation:

Answer 23

The principle in general is to convert iron compounds into a ferric form.

• Suitable methods vary according to iron content and iron form (aeration, neutralisation, coagulation, filtration).
• Several oxidizers can be used to oxidize iron into ferrous form: oxygen, chlorine, potassium permanganate.
• Manganese removal usually harder than iron.
• High pH (=10) is required. Therefore, the pH of the water must be restored afterwards. Calcium permanganate usually functions as an oxidizer.

Question 24

Ion exchangers are used in:

Answer 24

Water purification industry.

• In domestic water supply, used in water softening.
• Ion exchangers can be used to remove impurities from the water: calcium, magnesium, radium, nitrate, arsenic, etc. Ion form substances.
• Ion exchange capacity must be considered, i.e. the flow must be dimensioned correctly

Question 25

Ion exchangers are divided into two main groups:

Answer 25

Downstream exchangers:
in which both water and regenerated material flow in the same direction.

Countercurrent exchangers:
with opposite flow directions.

Question 26

Based on the ion exchange mass, the exchangers are divided into:

Answer 26

Cation exchangers: copper, iron, lead, zinc, calcium, magnesium.

Anion exchangers: phosphate, arsenate, sulphate, nitrate, chloride, bicarbonate, radium.

Question 27

Removal of possible radon:

Answer 27

If there is radon in raw water, it must be removed from all water that is passed to households.
Either activated carbon filtration or aeration may be used (aeration is good in the case of high Radon concentrations.)

Question 28

Removal of possible fluoride:

Answer 28

Either a reverse osmosis device or an activated alumina device to clean the water used for cooking and drinking.

Question 29

Removal of possible arsenic:

Answer 29

Can be removed fairly effectively with a reverse osmosis device (RO) (membrane filtration) and a highly effectively with activated alumina filtration (AA) or iron masses (FE) (precipitation techniques).

Question 30

Domestic water preparation: groundwater

Answer 30

At groundwater based plants a simpler treatment is sufficient:
it is often enough to control acidity and prepare for disinfection, for example by UV treatment.

If groundwater contains iron, it can often be removed by aeration and filtration (sometimes coagulation is required). It’s harder to remove fluoride.

Precipitation with aluminium salts, adsorption to alumina or reverse osmosis are some possibilities.

Radon can be removed by aeration or activated carbon filtration.

Question 31

What is reverse osmosis?

Answer 31

Reverse osmosis is a process by which pressure forces the solution (usually clean water) to move through a semi-permeable membrane from a more concentrated solution towards a dilute solution.

Question 32

Example of water cleaning proccess:

Answer 32

1. Groundwater is pumped from sieve tube wells to aeration, where water oxides and carbon dioxide is removed.
2. Limescale is added to oxidized water to make iron and manganese precipitate in the mixed tanks. In the clarification tanks precipitates descend to the bottom.
3. pH of water going to filters is adjusted by sulphuric acid to 8,3 - 8,6.
4. Overflow goes to sand filters where final precipitation residues are removed.
5. Disinfection if needed with sodium hypochlorite (NaClO)