Water pollution
Pollution of the aquatic environment means the introduction by humans, indirectly or directly, of substance or energy (heat) that result in negative effects to plant, animal or human health.
Briefly describe the main effects of water pollution
Effects can be summarised by this list. These do not occur in isolation
* Aesthetic: By the release of pollutants with high suspended solids. It looks unsightly, e.g., litter, discolouration and smells. The colour and turbidity (a measure of cloudiness) of the water.
* Temperature: usually, heat
* Deoxygenation: lack of oxygen in the water leading to damage to aquatic life
* Toxicity: acute or chronic toxicity, causing harm to aquatic or human life. This can be caused by the release of pathogens (in raw sewage, for example), heavy metals, radioactive materials
* Sublethal toxicity: such as endocrine disruption or changes in biodiversity
* Acidity/alkalinity: disturbance of the pH regime
* Eutrophication: nutrients or biodegradable organics giving rise to excessive growths of some organisms
cycle of water use
Raw water → Treatment →Use → Collection → Treatment → Disposal or recycle
There are effectively two strands to consider when we think about water and liquid pollution.
1.How do we ensure that water is safe for us to drink/use?
2.How do we treat any liquid effluent once it has left our homes/businesses?
1991 Water Industries Act
Sets out duties & powers of water providers regarding water supply, provision of sewerage services & consenting requirements for the receipt of trade effluent.
1991 Water Resources Act
Sets out duties & powers of the EA for water resource management, abstraction & impounding, the control of pollution of water resources & flood defence. Regulates water resources, water quality and pollution. Outlines the functions of the EA
Where can raw freshwater be extracted from and what are the main sources of contamination within each one?
Raw freshwater is abstracted from lakes, rivers or underground sources. The lakes and rivers can be classified as lowland or upland depending on their location.
Lowland water sources contain high levels of natural organic and inorganic matter from human activity and are more prone to pollution. Consequently, they receive the highest level of treatment.
Upland waters and reservoirs contain higher naturally occurring organic matter levels from decaying vegetation. This, in combination with iron, is why many upland rivers are brown. They can quite often contain contamination from field run-off.
Groundwater is usually low in organic matter and less vulnerable to microbiological and chemical contamination. However, they frequently contain high levels of inorganic substances from rocks through which the water percolates. In addition, the presence of nitrogen compounds indicates possible pollution.
DRINKING WATER STANDARDS
- Organoleptic : These are parameters that are sensed by humans (by eyes, nose and throat)
- Physicochemical:
- Temperature
- pH
- Conductivity
- Substances undesirable in excess
- Nitrate - linked to blue baby syndrome
- Ammonia
- Toxic substances
can be geological or from industrial pollution - Microbiological species
Usually due to human & animal excreta - Minimum for softened water
- Ca & Mg
- scales boilers
- hard water linked to reduced heart attacks
The treatment train will vary according to the requirements for the supply and the nature of the water source but will usually contain the following:
- Pre-treatment
o Screening
o Storage
o Equalisation
o Aeration - Primary treatment
o Coagulation and flocculation
o sedimentation - Secondary treatment
o Filtration - Disinfection
- Advanced treatment
o Activated carbon
o Membrane processes
o Fe and Mn removal
PRE-TREATMENT types
- SCREENS: To remove the larger contaminants
- STORAGE & EQUALISATION: Water storage is required to even out any supply variation. Water is generally stored in an open reservoir or tank. These vessels also act as a buffer if the water source becomes polluted. Storage allows solids to settle, so maintaining the storage and equalisation tanks is necessary to remove any silt build-up.
- AERATION:
This type of pre-treatment is common for groundwater. It is used to:
▪ Release excess H2S
▪ Release excess CO2
▪ Increase O2
Aeration can be a simple mechanical process, such as allowing the water to fall over a series of cascades.
PRIMARY TREATMENT types
SEDIMENTATION
COAGULATION & FLOCCULATION
what is SEDIMENTATION, what are the key parameters and typical values in the design?
is solid-liquid separation using gravity to remove suspended solids. Sedimentation usually takes place in large rectangular or circular vessels.
The key parameters and typical values in the design are:
- Surface overflow rate ~20-35m3/day/m2
- Detention times ~2-8h
- Weir overflow rate ~ 150-300m3/day/m2
COAGULATION & FLOCCULATION,
The most common coagulants are, and their benefits:
Process to remove suspended solids by addition of chemical agent
The most common coagulants are:
- Aluminium sulphate (alum)
- Ferrous sulphate (ferric)
- Ferric chloride
Benefits
- Reduction in suspended solids
- Reduction in BOD5
- Removal of floating matter
SECONDARY TREATMENT – FILTRATION, what is the filtration medium?
The filtration medium used in secondary treatment is usually sand but can be other media such as coal. Slow sand filtration improves water’s appearance by removing solids and pathogens.
There are many classifications of filtration systems, and they include:
- Gravity or pressure
- Rapid, slow or variable filtration rates
- Cake or depth filtration
DISINFECTION
Disinfection is required to kill or render harmless the remaining pathogens. Like coagulation and filtration, the other
treatments remove >90% of bacteria and viruses. However, to meet the standards set by the EU and WHO additional disinfection is required.
The requirements of a good disinfectant are:
- Toxic to microorganisms at concentrations well below the toxic threshold to humans & higher animals
- Fast rate of kill
- Persistent enough to prevent the regrowth of organisms in the distribution system
Most common industrial disinfectants:
Chlorination
Chlorine dioxide
Chloramines
Ozone
UV radiation
The purpose of advanced water treatment is to:
- Take water treated by standard process and improve it to an exceptional quality required by a particular industry
- To treat water for a specific chemical or microbiological contaminant
advanced water treatment methods include:
- Iron and manganese removal (water softening)
- Ion exchange and inorganic absorption
- Adsorption of organics
- Membrane processes including reverse osmosis
What are the main sources of wastewater?
- Domestic
- Industrial
- Infiltration / inflow
- Stormwater
To whom does industry need to apply for a wastewater discharge permit?
The Environment Agency
The waste-producing company will need to provide the Environment Agency and/or the water authority with information about the effluent discharge. The permit and consent document will commonly include:
- Location of discharge & design of the outlet
- Steps required to minimise the polluting effect of the discharge
- Facilities to enable sampling
- Nature, origin, composition, temperature, volume, rate of discharge, periods when discharge can take place
- Making of returns
- Recording & monitoring requirements
The properties of interest for industrial effluent are:
▪ Suspended solids (mg/l)
▪ BOD (mg/l)
▪ COD (mg/l)
▪ pH
▪ Temperature
▪ Flowrate (normal operation, emergency operation, shut down)
