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					    A Basic Guide
          to
Water Treatment Plants
Clean Water Treatment .................................................................................. 3
  Boreholes and Wells ............................................................................................................................. 3
  Springs................................................................................................................................................... 3
  River Water .......................................................................................................................................... 3
  Major water treatment processes ....................................................................................................... 4
    Screening ............................................................................................................................................ 4
    Ozonation ........................................................................................................................................... 4
    Coagulation or Flash Mixing ............................................................................................................. 4
    Clarification........................................................................................................................................ 4
    Filtration ............................................................................................................................................. 4
    PH correction ..................................................................................................................................... 4
    Phosphate dosing................................................................................................................................ 4
    Chlorination ....................................................................................................................................... 5
    Storage................................................................................................................................................ 5
  Examples of Water Treatment Works ............................................................................................... 5
    Spring Water Treatment ..................................................................................................................... 5
    River Water Treatment ....................................................................................................................... 6
  Filtration Plants .................................................................................................................................... 7
    Filter Cleaning or Backwashing ......................................................................................................... 7
Waste Water Treatment .................................................................................. 8
  Waste Water Treatment Processes ..................................................................................................... 8
    Screening ............................................................................................................................................ 8
    Detritors.............................................................................................................................................. 8
    Settlement........................................................................................................................................... 8
    Biological filtration ............................................................................................................................ 9
    Activated sludge ................................................................................................................................. 9
    BAFF .................................................................................................................................................. 9
    Final settlement .................................................................................................................................. 9
    UV Irradiation .................................................................................................................................... 9
    Peroxyacetic Acid Addition (PAA) ................................................................................................... 9
    Membrane Treatment (Micro filtration)............................................................................................. 9
    Sludge treatment, recycling and disposal ......................................................................................... 10
    Odour Control .................................................................................................................................. 10
  Example of a Waste Water Treatment Works ................................................................................ 11




Tony Chapman                                                          Page 2                                                              08/12/2011
Clean Water Treatment
A clean water treatment plant is commonly referred to as a Water Treatment Works (WTW)

Treatment is the term used to describe the series of processes to which raw water is subjected to make it
safe for human consumption. Water treatment purifies and disinfects the raw water by a variety of
physical and chemical processes to remove unwanted substances and organisms and converts the raw
water into water that is safe to drink.

Water treatment facilities share several fundamental processes that are almost universal in their
application and sequencing. The first stages are to remove gross solids and then to promote
sedimentation or coagulation for the removal of finer solids and colloidal material - particularly from
upland water sources. Following this the water undergoes a filtration process and a chemical dosing
process.

The level of treatment needed depends on the source of the water – river, reservoir or underground
aquifer – and the nature of the catchment. For example, in areas of intensive agriculture, some
pesticides will be present in raw water and will need to be removed. These pesticides are washed off
fields into rivers and reservoirs when it rains. Groundwater generally requires far less treatment –
perhaps just disinfection – as the rocks act as a natural filter.


Boreholes and Wells
Water taken from underground wells or boreholes is usually of high quality as it has been naturally
filtered during its journey through the soil and rocks. At most sites a carefully controlled dose of
chlorine gas is all that is required to guard against bacteria. At some sites we have to give the raw water
a heavier dose of chlorine to kill off any bacteria; afterwards we reduce the chlorine level before the
water goes into supply.

Springs
Water taken from springs often contains some small particles and therefore goes through a filtration
process at a Treatment Works to remove them.

River Water
River water by comparison is dirtier. It undergoes extensive treatment which is carefully monitored.
Chemicals are added which bind the small pieces of debris in the water together. These form a blanket
of sediment in the settlement tanks and as the water passes through the blanket so it removes further
particles. Any remaining particles are removed by a filtration process. Finally chlorine is added to
ensure a safe and wholesome water supply.




Tony Chapman                                    Page 3                                       08/12/2011
Major water treatment processes
(Listed in the order in which they are carried out)

Screening
Debris such as leaves and plant fragments are removed by passing the water through a series of coarse
meshes or screens.

Ozonation
Ozone is made by combining three oxygen atoms to form one ozone molecule. Oxygen gas is passed
through ceramic tubes inside a metal vessel and electricity is transferred across these tubes to change
the oxygen into ozone.

Ozone is a powerful oxidant that encourages the break down and separation of molecules that are
suspended in the water. The main function of ozone at this stage is to halt the algal activity, breaking
up the algae and making it easier to remove. Ozone gas enters the water through a system of pipework
and diffusers in the bottom of the tank Ozone, a chemically active form of oxygen, is passed through the
water to destroy micro-organisms. Ozone treatment is becoming more common in the UK, but is not
necessary in all areas.

Coagulation or Flash Mixing
Particles which are too small to be removed by the screens are made to bind together to form larger
particles by the addition of an approved chemical. The water is mixed with coagulants, either ferric
(iron) sulphate or aluminium sulphate, and put into large tanks called clarifiers. The water and the
coagulant enter the tank from the bottom and flow upwards.

Clarification
The coagulants attract dirt particles and, as the particles become heavier, they hang suspended in the
tank forming a blanket, or floc. This serves as a filter, trapping additional coagulant and dirt. Eventually
individual particles become too heavy to be supported by the upward flow and they sink to the bottom
of the tank to form a sludge which is then removed for disposal. As well as removing dirt, the
coagulants also remove the colour, bacteria and other micro-organisms which the water may contain.

Filtration
The clarified water then passes through a filter made from layers of sand and gravel, which removes any
remaining particles. Once water has been filtered it is clean and colourless. To keep the filters clean,
they are periodically back-washed – i.e. clean water is forced back through the filter to remove any
build-up of particles and the wash water is sent to settling tanks for disposal.

PH correction
Once the water has been filtered, the acidity or alkalinity of the water is checked and adjusted if
necessary by the addition of chemicals. The water should be slightly alkaline; if the water is too acid it
could attack the metal pipes and fittings in the distribution system. If it is too alkaline it could lead to
deposits forming in the pipes which would clog up the distribution system.

Phosphate dosing
In areas where the water is soft (i.e. contains little calcium) minute quantities of lead from old pipes can
dissolve into the water. Phosphate – a common chemical present in most foodstuffs – is added to
prevent this happening. It coats the lead pipes and prevents the lead pick-up.



Tony Chapman                                    Page 4                                        08/12/2011
Chlorination
Finally the water is given a dose of chlorine which destroys any remaining bacteria. The chlorine levels
are adjusted to leave a small amount in the water to keep it bacteria-free on its way through the
distribution system. Chlorine is essential for public health, as it kills the bacteria responsible for
diseases like cholera, typhoid and dysentery that affect those countries where water is not chlorinated.
(It was the addition of chlorine to tap water in the Victorian era which wiped out cholera in the UK.)

Storage
Before being pumped to customers, the treated water will usually be stored in a sealed storage reservoir.
Many of these storage reservoirs are underground.


Examples of Water Treatment Works
Spring Water Treatment




Stage 1
Spring water is collected, dosed with chlorine to begin the disinfection process and pumped to the
Water Treatment Works.
Stage 2
Partially treated water enters the Splitter Weir Chamber where, if necessary, alum is added to aid the
filtration process. Here the flow is split between Stream 1 and Stream 2.
Stage 3
The partially treated water passes to the Flash Mixer where the alum, a coagulation aid, starts to bind
together the small particles which cause turbidity.
Stage 4
The coagulated water is then divided equally between Rapid Gravity Filters in Stream 1 and Stream 2.
Here the water is filtered through a graded sand and gravel bed, approximately one metre deep, before
being discharged.
Stage 5
Following filtration, the chlorine dose in the 'final water' is adjusted to its set point and it flows into the
Contact Tank.
Stage 6
After a minimum of 10 minutes contact with the chlorine dose, a number of monitoring checks are
conducted before the water is transferred from the Works to underground service reservoirs nearby.

Tony Chapman                                      Page 5                                         08/12/2011
River Water Treatment




Stage 1
Water leaves the River through intakes, with surface debris removed by a floating boom. It then passes
through a mechanically raked bar-screen before entering the Low Lift Pumping Station, where it is
strained through fine mesh rotating screens before being pumped to the Reservoir.
Stage 2
Water from the Low Lift Pumping Station enters the first compartment of the reservoir where 90% of
the solids sink to the reservoir floor. It then passes to the larger second compartment where it remains
for three days. During this time many of the bacteria and viruses die off before the water gravitates to
the Treatment Works where a preliminary chlorine dose is added to ensure the start of the disinfection
process.
Stage 3
A coagulant Poly Aluminium Chloride (PAC) is added at the Flash Mixer to bind any small particles.
Sulphuric Acid is also added to control the pH (acidity/alkalinity) and a Polyelectrolyte to improve the
coagulation process. The 'dosed water' is now retained for a short period to enable the 'binding process'
to start before the water passes to the 'Clarification Stage'.
Stage 4
The water dosed with coagulant is discharged into the bottom of 'upward flow clarifiers' and as the
water flows upwards so the particles bound together by the PAC form a sludge blanket just below the
surface of the water. The sludge blanket traps more particles as the water flows through it to the outlet
channels which span the clarifier at the water surface. From time to time some of the sludge blanket is
'drawn off' and discharged to the sludge processing plant.
Stage 5
The 'clarified water' is then divided equally between a number of rapid gravity filters, each containing a
gravel base and a bed of granular activated carbon which removes any remaining fine particles. The
granular activated carbon is also extremely useful in removing organic compounds which can cause
taste problems in the supply.
Stage 6
Following filtration the 'filtered water' is further dosed with chlorine to ensure adequate disinfection. It
remains in contact with a high dose of chlorine for a minimum of six hours in a covered contact tank.
Stage 7
After leaving the contact tank, the final water is dosed with Sulphur Dioxide to reduce the chlorine
residual to its set point before being pumped for distribution to customers. At this stage a number of
monitoring checks are conducted to ensure that the supply meets the necessary legislative standards




Tony Chapman                                    Page 6                                        08/12/2011
Filtration Plants
The filtration stage is performed by a variety of designs and plant. A common method in the EC is the
rapid gravity filter, but others such as slow sand filters are used. More recently, ultra-filtration methods
using membrane technology are being increasingly deployed, especially when there is a perceived
increased risk of water borne pathogens such as Cryptosporidium being present in the source water.
With both rapid gravity and slow sand filters, water is percolated through a sand or anthracite medium.
The particulate matter is entrapped within them medium and water passes through a series of perforated
tubes at the base of the filter for collection and removal to the next stage.

After a few days, filters start to become blocked by the solid particles they remove from the water.
They are cleaned by a backwash process, which can be started on a timed basis or by measuring the
reduction in hydraulic throughput due to solids build up. This backwash is carried out with air and
water and in effect fluidises the medium to release particulate matter. Then the filter is slowly returned
to service.


Filter Cleaning or Backwashing
The filter cleaning process and description for a Rapid Gravity Filter are shown below




                         .


   1. Compressed air is bubbled upwards through the filter bed for approximately 5 minutes.
   2. Wash water is passed upwards via the filtered water channel, through the filter bed for
      approximately 3 minutes.
   3. The wash water drains away through an outlet valve flushing the surface of the filter as it flows
      out.
   4. The filter inlet valve is then opened, the filter refills and is returned to service.




Tony Chapman                                    Page 7                                        08/12/2011
Waste Water Treatment
A waste water treatment plant is commonly referred to as a Waste water Treatment Works
(WwTW) or a Sewerage Treatment Works (STW)

Wastewater or sewage is collected from properties and businesses, then taken through the
sewerage system to treatment works, where it goes through a number of processes designed to
separate water from solid material. Human waste products form a relatively small proportion
of the total volume of sewage.

The treated water (effluent) is returned to rivers and seas while the solid matter (sewage
sludge or „biosolids‟) is disposed of in a number of different ways, including incineration,
landfill, and application to certain types of agricultural land.

Sewage treatment is strictly controlled, to comply with UK codes of practice and European
directives.

Waste Water Treatment Processes
The treatment of sewage can be broken down into four stages
   1. Preliminary treatment – Removal of large solids and grit.
          o Screening
          o Detritors
   2. Primary treatment – Removal of suspended solids and sludge.
          o Settlement
   3. Secondary treatment – Using bacteria to break down the biodegradable matter.
          o Either biological filtration, activated sludge or BAFF
   4. Tertiary treatment – Disinfection and nutrient removal.
          o UV irradiation
          o Peroxyacetic Acid Addition
          o Membrane Treatment (Microfiltration)

Screening
Wastewater is pumped to the screening plant where paper, sticks, cloth and other large items
are taken out and chopped up for landfill or burnt in incinerators.

Detritors
The wastewater passes slowly through detritors, where grit from the roads settles and is taken
out for landfill.

Settlement
In this first major stage of treatment, the sewage is pumped into large settling tanks where the
heavier solid material settles to the bottom of the tank. This reduces the biochemical oxygen
demand (BOD) by 20 per cent, while total suspended solids are reduced to at least 50 per cent
before the sewage can be discharged. Solids settle to the bottom as „sludge‟ which is
automatically scraped away at regular intervals. The sludge is treated and used as fertiliser or
may be pumped to a central works for further processing.




Tony Chapman                              Page 8                               08/12/2011
Biological filtration
The waste water is spread from a moving distributor arm over a deep granular filter bed called
a percolating filter. Bacteria, fungi and other organisms feed on the organic matter to leave a
virtually clear liquid that then flows from the base of the filter bed.

Activated sludge
The waste water is fed into concrete tanks containing specially grown bacteria which feed on
the waste particles. The bacteria require a supply of oxygen to function correctly and the
process is accelerated by rotating paddles and jets of air.

BAFF
This process is currently very popular, as it is rapid and the land requirement minimal, even
compared to an activated sludge plant. Hence, Biologically Aerated Flooded Filters (BAFF)
plants are ideally suited for areas with restricted land availability such as cities, towns or
coastal areas. The process is completed in relatively small tanks equipped with aerators and
filled with small particles (such as polystyrene balls) which float in each of the cells and are
covered in millions of microscopic bacteria. The bacteria population is kept healthy by
blowing air into the wastewater and the bacteria then feast on any remaining solid particles in
the wastewater. The cleaned wastewater is then drawn off from the top of the tank.

Final settlement
In the final stage of the treatment process the waste water is then passed into settlement tanks,
known as humus tanks. These tanks allow any suspended matter produced in the previous
stage to settle out and the clean water is finally released to the river

UV Irradiation
Ultra-violet treatment disinfects waste water by inactivating viable micro-organisms likely to
cause subsequent infection of people. The UV irradiation bombards effluent with UV light
which breaks down the DNA structure of micro-organisms rendering them incapable of
reproduction and infection.

Peroxyacetic Acid Addition (PAA)
The PAA (Oxymaster) is a mixture containing acetic acid, hydrogen peroxide, peroxyacetic
acid and water. PAA can quickly be added, at low capital costs, to existing sewage treatment
works bu due to the high dose rate the operational costs are large, making this an expensive
disinfection system.

Membrane Treatment (Micro filtration)
Membrane treatment or micro filtration operates by forcing effluents to pass though a hollow
cylinder with multi-layered walls permeated with microscopic pores. Fluid may pass through
these walls made up of membranes. However, as the membranes pores are only 2 microns in
diameter, any bacteria, viruses or other particles greater than this size will be separated from
the liquid. These solids build up on the outside of the membrane walls as fluids pass. Regular
high pressure backwashing ensures the solids and algae are swept away from membrane
walls. The backwash effluent is fed back to the primary treatment plant.




Tony Chapman                              Page 9                                08/12/2011
Sludge treatment, recycling and disposal
The by-product of sewage treatment (sludge) may be treated in a variety of ways and is
viewed as a valuable agricultural resource.
    Thickening or dewatering – the water content of the wet sludge is reduced by
      gravitational or mechanical processes (centrifuges, presses and vacuum filtration).
    Digestion – bacteria and micro-organisms are used to break down the organic matter
      in the sludge to reduce the volume of solids.
    Composting – aerobic fermentation.
    Thermal drying – using high temperatures to reduce the sludge to a granular form (80
      per cent solids).
    Incineration or combustion – high temperature oxidation to produce an inert ash; the
      heat is usually recovered for other uses.

Odour Control
The control of foul smells can be achieved by containing the gases produced from the
treatment process then chemically “scrubbing” them before releasing to atmosphere.




Tony Chapman                          Page 10                             08/12/2011
Example of a Waste Water Treatment Works




1 Interceptor sewer
Wastewater and rainfall enter the works through an interceptor sewer.
2 Solids removal
A number of screens take out all solids larger than 6mm, such as paper, sticks, cloth and other
items. These are washed and compressed, and fall into skips on automatic turntables.
3 Grit removal
Sand and grit which has been washed down sewers is removed and is taken away for landfill.
4 Biological treatment
Honeycombed discs, called submerged biological contractors (SBCs), slowly turn in
the flow of wastewater. The SBCs provide a home for millions of bacteria which „eat‟ any
harmful micro-organisms and other pollutants in the wastewater.
5 Settlement
After biological treatment, the flow of wastewater is slowed down in 14 large settlement
tanks, where any solid particles fall to the bottom. These form a liquid sludge at the bottom of
the tanks, which is removed every two hours.
6 Sludge
The watery sludge is thickened in two gravity belt thickeners, and taken away by road tankers
for treatment at another site where it is pressed to a dry soil-like cake then used in landfill.
7 Odour control
This Treatment Works is fully covered and ventilated and all odours are removed from the air
using two triple stage scrubbers, before air is released into the outside atmosphere.
8 Return to sea
The final water is a clear liquid which is pumped out to sea through a buried outfall pipe, five
kilometres long. Out at sea, wave action, currents, sunlight, salt and marine micro-organisms
complete the natural purification process.




Tony Chapman                             Page 11                               08/12/2011

				
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