Water Chemistry

Neil Grant Kennet Water

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What is Water ?

• The combination of 2 Hydrogen Ions and 1 Oxygen Ion

• H2O

• Water is a transparent, tasteless, odourless and nearly

colorless chemical substance, which is the main constituent

of the Earth’s streams, lakes and oceans and the fluids of

most living organisms. Ref Wikipedia

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Definitions - Physical

• Physical Units – Minerals/Chemicals

Weight Kg g mg ug ng

Volume Litre ml ul nl

ppm ppb ppt

0.2mg/l equivalent to 0.2ul or 0.2ppm

Eg 0.2 mg/l equivalent to 200ug/l or 200,000ng/l (or

0.0002g/l)

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Definitions - Special

• Special Units – Minerals/Chemicals

Suspended solids g or mg/l

Turbidity (particles) NTU or FTU

Colour True/Apparent °Hazen or mg/l PtCo (how brown is it)

Conductivity uS/cm or m

pH units (none as it’s a ratio)

Hardness mg/l , ppm, °F, DH, Grains, ……..

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Regulations

“The Private Water Regulations 2016 amended 2018”

“The Water Supply (Water Quality) Regulations 2016”

Regulation 31- Municipal water components

Wras Fixtures and fittings attached to mains water

supply

BS EN National standards (chemicals)

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Turbidity

• Turbidity is the amount of particles in the water. Measured optically

(NTU/FTU). Light scattered at 90°. Could be 1 big particle or lots of

small particles

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Colour

• Colour of water is defined as Apparent or true colour

Apparent colour is the amount of light absorbed “raw” (at 400nm)

True colour is also called dissolved colour and is the amount of light

absorbed after the sample has been filtered through a 0.45um filter.

Brown scale not a blue scale (°Hazen or PtCo mg./l) .

• Upland waters have a high true colour reading due to dissolved

organics (humic acids) – rotting vegetation.

• Oxidised Iron rich waters have a high turbidity and apparent colour

due to iron particles. Labs often report just true colour .

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Total Dissolved Solids

• TDS (NOT SS)

• Amount of mater dissolved in water

• Inorganics (minerals) and organics (oils)

• Measured by weight of solid (mg/l)

• Heat sample until all water evaporated and weigh

the residue – the TDS

• Typical tap water is about 400 mg/l (0.4 g of

minerals in a litre of water)

• Seawater is about 35,000 mg/l (mainly NaCl)

• Peak Waste water softener >100,000 mg/l NaCl

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(Electrical) Conductivity EC

• Surrogate measurement for TDS

• Quick, easy and cheap

• Measurement of electrical current flow

• Use two electrodes at a given distance apart and measure EC the

water sample .

• The higher the conductivity the higher the number of electrons

present so by implication the higher the concentration of minerals

• Measured by Siemens or Micro Siemens (uS/cm)

• TDS to EC ratio about 0.7 ie 700mg/l will give an EC of 1000uS/cm

• No indication of which minerals are present – Hardness/Salinity

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pH

• pH is the ratio of the acid to alkali content

• Scale 1 to 14 with 7 as neutral (1 is acid H+, 14 alkali OH-)

• Quick, easy and cheap

• Soft Water tends to be slightly acidic

• Hard water tends to be slightly alkaline

• Acidic waters attack metal pipes, Alkaline water lay down scale –

Langelier Index

• Sometimes need a specific pH for a media to work

Eg iron needs a pH over 7 (alkaline) so iron is converted to iron hydroxide

Fe(OH)3

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Water hardness

• Total Hardness is a combination of temporary and permanent

hardness

• Total Hardness is the total of the Calcium and Magnesium minerals

• Temporary hardness is hardness which when heated changes form.

Ca(HCO3)2 Heat CO2 (gas) + H2O(liquid) + CaCO3 (solid - scale)

• Permanent hardness are calcium/magnesium minerals which don’t

change on heating eg calcium sulphate

• A water softener takes out all Ca/Mg ions so removes total hardness

(regenerated with salt)

• A dealkalisation resin can just take out the temporary (scale forming)

hardness but needs to be regenerated with concentrated acid

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Water hardness

• 300 mg/l total hardness is 0.3 grams of scale per litre of water

• 0.3 Kg of scale for every 1 m3 of water (1000 litres) that passes

through the pipes.

• 4 person house hold using 560 l/day is 200 m3/year or 60 Kg of

scale.

• In East Anglia where TH=450 mg/l that is 90Kg of scale per year

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Ground Water vs Surface Water

• Ground Water

• Water held underground in the soil or in pores and crevices in rock

• Abstracted from a Borehole, or Well

• Surface Water

• Water that collects on the surface of the ground

• Abstracted from a Lake, Burn, Stream, (puddle)

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Ground Water

• Ground Water Properties

• Highly affected by local Geology

• Mineral Rich

• Higher TDS/Conductivity

• Neutral to alkali pH

• Low in bacteria !!

• Low in surface water contamination !!

• “Often” relatively consistent (good or bad)

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Surface Water

• Surface Water Properties

• Highly affected by weather

• Low in minerals so low TDS

• Acid to Neutral (no hardness minerals)

• High in bacteria

• High in surface water contamination !!

• “Often” flashy (hard to treat)

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Ground Water Chemistry /Quality

Typical PWS Treatment 1 house + stable : Iron, turbidity, bacteria

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Potable Water

• Meets either Municipal or PW regulations – water quality

virtually the same

• Conductivity below 2500uS, pH 6.5 to 9.5, limits for many minerals

• Limit for metals and specific organics and total pesticides

• No limit (upper or lower) for total hardness

• Typical potable water soft water area EC of below 200uS, pH 7,

metals below 50ug/l (20% of range), no Ca/Mg, organics low

• Typical potable water hard water area EC of 600uS, pH 7.5, metals

below 100ug/l (40% of range), raised Ca/Mg, organics low

• If Municipal all waters will contain chlorine typ 0.05 to 0.5 mg/l

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Common Treatments of Potable Water

• Total Hardness (Softener, water conditioner)

• Taste and odour (smell)- typically Carbon filters CT=5 mins.

Chlorine is often the part which smells

• Particles (from pipes) – sediment filters 5 micron

• Organics, hormones, pesticides- carbon filters or under sink RO (if

using carbon to reduce organics CT at least 20 minutes). RO CT is

time taken for water to get through the membrane)

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Pure Water

• Good quality water with minerals and organics reduced

• Distillation Simple but slow and expensive

• Demineralisation moderate cost but uses dangerous chemicals

• Reverse Osmosis High cost but safe

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Pure Water

• Good quality water with minerals and organics reduced

• Distillation Simple but slow and expensive

• Demineralisation moderate cost but uses dangerous chemicals

• Reverse Osmosis High cost but safe

Demineralisation

• Simple two stage demineralisation plant

Water flows through cation resin first where positively charged atoms are held

(eg metals). This is regenerated with hydrochloric acid (HCl). All + atoms are

exchanged for H+

Water then flows through anion resin where negatively charged atoms are held

(eg chlorides, nitrates ). This is regenerated with sodium hydroxide (NaOH). All

- atoms are exchanged for OH-

H+ combines with OH- to give HOH or H2O

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Reverse Osmosis

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Reverse Osmosis

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Conclusions

Water chemistry quite straight forward and often

depends on source water

Like an MOT (only valid for that time an day)

Units can be confusing

Always get an analysis (if possible)

Write notes and compare how chemistry changes