We arrived at the Tourmakeady Water Treatment Plant at around 1 p.m. on Monday 9 November 1998.
First of all we were shown the source of the water. As you know the water was pumped form Lough Mask. We were shown the main controls, which controlled the rate at which the water was pumped. In this case 2 million gallons of water was pumped daily. The water is pumped at night to avail of half rate electricity charges. The cost of water was 34p per 1,000 gallons of water.
After being given all the details we moved onto the treatment plant where the water was treated before distribution. We were shown the different stages involved in the treatment of water. These include:
After these stages are carried out lime is added and fluoride is then added.
Next we were shown the Filtration beds. These were big concrete tanks filled with the water, which was ready to be treated. At first the water was very dirty and contained many dirt particles and germs. After the water had been treated in the filtration bed it was clean.
In the office we were shown that the water had a pH of 7.6. This meant that the water was just over neutral and that it was slightly alkaline.
Reasons why Water must be Treated:
In the office it was explained t us that water must be treated for various reasons which include:
Bacteria is present in all water. It is important that it be killed before the public consumes the water in order to prevent the spread of infection, diseases etc.
Water can be contaminated and must be treated to prevent the spread of diseases.
Dirt is commonly present in water. Again it must be cleaned to prevent the spread of harmful bacteria.
Lime and Fluoride are added to the water before it is distributed to thousands of home in the area. Lime is added to raise the pH when necessary, while fluoride is added to reduce dental decay. Chlorine is added to kill any bacteria that may be present in the water.
The treatment process is designed to remove from the raw water undesirable impurities and render it suitable for potable use. The incoming raw water will contain four main types of impurities.
Suspended solids which are small particles of clay and silt which would, if the water were allowed to stand, settle out themselves. They are removed by settlement and filtration.
Colour which is caused by colloidal matter in the water. These materials can only be removed by changing them into a form which will settle in the clarifiers and any carryover will be trapped in the filter media. Chemicals are added to the water to cause these minute particles to come together to form larger particles and settle out.
Biological contamination is the presence of bacteria. Most of these are removed in the settlement and filtration stages and any that survive these processes are killed off by the addition of chlorine.
Dissolved chemicals are invariably present and may be natural or synthetic chemicals which are washed into the water by rain. These must be removed by adding chemicals which will change them to a form which will settle out or change their chemical composition to a form which is acceptable.
Raw water is pumped from Lough Mask via a rising main to a balance tank. The capacities of the four pumps are: - 425m3
Level probes in the balance tanks control automatic operation of the pumps. When the level reached a pre-set low point, the raw water pump will start and continue to run until the pre-set high level is reached.
The raw water is gravity fed to the contact tank. The raw water is controlled by an electropuematic controller, which alters the position of the control valve to give the required rate of flow. Before entering the control tank, the raw water is dosed with ferric sulphate. The mixing of the ferric dosed water is effected by a series of baffles in the contact tank. The flow through the contact tank allows for a minimum contact time of three minutes.
The raw water is divided in two in the splitter chambers. Polyelectrolyte is dosed in the water via a spurge pipe located under the two adjustable weirs. Each half of the treated raw water is fed to one of the two Sedimentation tanks via a transient flow generator which creates a pulsed flow effect. The raw water inflow is therefore uniformly distributed over the entire flow area of the clarifier cell. This inflow is directed vertically downwards at an average orifice speed calculated to inhibit solids depositing on the cell floor.
A sludge blanket is formed in the cells which entrains most of the coagulated particles. The upper level of the blanket is maintained by the provision of a sludge generator channel in the centre of each cell. This channel allows for intermittent draw-off of sludge through the sludge holding tank from which it is pumped to the Picket Fence Thickener. Both cells are provided with draindown facilities and this drainage is pumped to the filter washings tanks or can be diverted to the Picket Fence Thickener if it contains sludge.
An assembly of inclined plates are provided in each cell immersed above the sludge blanket zone. This assembly increases the effective settling area approximately six fold and is designed to allow for laminar flow throughout the entire range of through put. Adequate interspersing of the plates obviates the possibility of blocking of the system in the event of the formation of algae growth. Floc particles which may escape upwards from the blanket zone settle on the surface of the inclined plates where they form a thin sludge layer which eventually slides back down in the blanket zone. The clarified water exits from the upper surface of the lamella assembly and is collected uniformly in a series of notched decanting channels at the top water level.
The settled water from both sedimentation tanks collects in a common settled water channel. The flow from the channels is evenly divided between all four filters of inlet penstock weirs. The clarified water then passes through a filter inlet distribution channel, which is fitted with orifice outlets at 500mm ors. To ensure uniform distribution of the inflow along the length of the filter. The filter level controller controls the water level. If the water level in the filter deviates from the level controller set point the controller causes an alteration in the position of the filter outlet valve proportional to the deviation. The higher the water level above the set point the more outlet valve opens and visa versa.
The water in the filter percolates down through the media which consists of approximately 1.5 m depth of 1.0 mm diameter sand. Any dirt or floc particles become entrained in the spaces between the sand grains. The filtered water then enters the filter under drainage system. The under drainage system in these filters consists of a series of air laterals dipped to water laterals on aluminium supported channels. The twin pipe lateral system is surrounded by a 150 mm deep layer of 12 mm/25 mm gravel overlaid with 100 mm deep layer of 6 mm/12 mm gravel. A further 100 mm deep transition layer of 6 mm/12 mm gravel is laid to preserve gravel/media interface during the high air intensity backwash. A 1.5 m deep course bed of 1.0 mm silica sand is laid on top of the transition layer.
During normal back washing of the filters, the settled water continues to flow into the filters causing a 'cross-flush'. This cross-flush, provided by the settled water, together with the simultaneous application of air and water to the sand bed which flushed the solids from the bed, sweeps the solids across the filter cross section and over the weir of the wash out channel. The inclined plate profile of the weir crest is designed to inhibit media loss during the backwash cycle.
The filter washing gravitates to the wash water tank from which it is pumped to the balance tank and re-circulated.
The filtered water collects in the filter channel and exits via the filter outlet into the filtered water channel Total filtered water flow is measured by a thin plate rectangular weir located in the filtered water channel. The level of water flowing over the weir crest is related to the flow rate. Both chlorine for sterilisation and soda ash for ph correction are dosed into the filtered water prior to the measuring weir.
Ferric sulphate is used as a coagulating agent to collect the dirt in the water, which is trapped in a sludge blanket in the settling tanks.
This blanket is located about 0.5 m below top water level. The clear water flows over the decanting channels and on to the filters and the sludge is bled off at regular intervals from the blanket thus maintaining a stable blanket in the settling tanks.
Polyelectrolyte As Coalgulant Aids.
The floc produced by the primary colagulants maybe improved by the addition of a small amount of polyelectrolyte.
Sedimentation
Sedimentation, sometines called clarification, is the removal
by gravity of settleable solids in water. The solids removed include
sand, silt, grit, chemical precipitates, pollutants, floc and
other settleable solids. The sedimentation process takes place
in the sedimentation or settling tank. Sedimentation occurs after
coagulation/flocculation and before filtration in a conventional
water treatment plant.
Rapid Gravity Filtration
The purpose of a filter is to remove suspended matter form the water passing through it. The filter consists of a bed of silica sand supported by layers of graded silica gravel over the underdrainage system. The details of media size and depth are given in the equipment specification. The underdrainage system consists of a network of plastic pipework for collection of filtered water and distribution of washwater and air scour. Each filter is a self-contained unit with individual control valves for inlet, outlet, backwash, washout and air scour.
The filter becomes progressively dirtier as more suspended matter is entrained on the media bed. This causes a resistance to flow through the bed and a difference in water pressure between the upstream and downstream sides of the filter bed occurs. This is called LOSS OF HEAD which increases as the filter gets dirtier. This difference in pressure is measured by a loss of head gauge which is graduated in metres of water. The filter is washed prior to the loss of head reaching 2 meters. The filters are washed once every 3 days regardless of head loss. If the loss of head is consistently allowed to exceed 2 meters 'mud-balling' occurs. Mud balling is the formation of lumps of mud below the surface of the media, which will cause deterioration in the quality of filtered water. The washing process is normally an air scour agitation followed by backwash with clean water. A description of the washing process is given under filter cleaning.
Chlorination
Sterilisation with Chlorine
Raw water will contain various types of bacteria, viruses, microscopic plants, fungi and so on. These are termed 'biological contamination' and certain species of them cause sickness and disease. Most of them are removed by settlement and filtration. However, in order to kill any remaining organisms chlorine is added to completely sterilise the water.
Fluoride
It is now accepted that fluoridation of a water supply to a level of 1 mg/l is both safe and effective in substantially reducing dental decay. The greatest reduction of dental decay occurs if fluoridated water is drunk during childhood throughout the period of tooth formation.
