Unit Treatment Processes in Water Engineering

Field Trip Report

Ballymore Eustace Water Treatment Plant

03/02/2010

Dara Crosbie

06465099

Group 5

Introduction:

On 3rd February 2010, I attended a field trip with my UTP in Water Engineering class to Ballymore Eustace Water Treatment Plant in Ballymore Eustace, Kildare. Ballymore Eustace Water Treatment Plant is responsible for providing most of Dublin City with a clean water supply and has been in operation for over 70 years. It is one of four treatment plants that supply the Dublin region and provides approximately 55-70% of the total supply to the area. The purpose of the treatment plant, as will other treatment plants of the same design, is to remove contaminants and unwanted substances from raw water and to make the water suitable for human consumption. The plant is being upgraded at the moment to increase its capacity from 250 Ml per day to 318 Ml per day with a peak capacity of 400 Ml per day. This represents an investment of 30 to 50 million Euro and involves the construction of new sedimentation tanks, filters, reservoir, administration and laboratory facilities. When upgrades are completed in 2011, Ballymore Eustace will be the largest electrochlorination plant in the UK and Ireland.

The raw water for the plant is sourced from the nearby Blessington lakes, mainly from the Poulaphouca impoundment, which was created in the 1940’s by damming the River Liffey. The dam was originally used to generate electricity, however nowadays it is only used to generate small amounts of electricity at peak times. The raw water is transported to the plant through a gravitational flow system some 1.5 km long.

Overview:

On arrival at Ballymore Eustace, we began our tour at the manifold building, where the raw water enters and is distributed to the plant. From there, we followed the path of the water as it progressed through the various stages of treatment – starting at the manifold building, on to the sedimentation tanks, then through the filters, and finally to the disinfection and fluoridation stage, after which it is stored for distribution. We also visited the on-site sludge treatment plant which treats the sludge produced by the various treatment processes. These processes are described in detail later.

Water treatment plants generally have on-site laboratories and resident chemists, and Ballymore Eustace is no exception. The resident chemist will check the alum dose

and pump settings daily and carries out jar tests in the lab on samples of water from various stages in the treatment process to measure colour, turbidity, and alkalinity. From the results of these tests the chemist can determine if the plant is working effectively. Changes in colour, turbidity or alkalinity can indicate that something is not working properly, that a filter needs to be backwashed or that a sedimentation tank needs to be cleaned. If the water is not up to standard they can adjust the alum dose or pump settings accordingly.

Manifold Building:

All raw water entering the treatment plant comes through the manifold building via three massive 1.6 meter diameter green pipes (the intake pipes from the reservoir).

Flow into the plant is controlled by valves, either electrically or mechanically, so that a steady flow of raw water is available at all times. These valves can be opened to allow more water flow to the plant during peak demand, closed partially to restrict flow to a minimum, or even closed fully to allow for maintenance. There are air vents built into the pipes to allow for venting of the pipelines, maintaining the negative air pressure needed by the gravitational flow system that transports the raw water from Poulaphouca Reservoir.

Aluminium sulphate, a flocculating agent which arrives on site as a concentrated liquid, is drawn from its delivery tank by dosing pumps and injected into the inflow pipes via small purple pipes (called the purple line) in the manifold building. The concentration of this aluminium sulphate varies from 3-5 mg/l and more is added during the winter than the summer. This is due to the difference in mean temperature between the two seasons. The dosing pumps can be regulated from the pump house or the control room so that the dose can be changed if required. From the manifold building, the water is distributed to the sedimentation tanks.

Sedimentation Tanks:

On the walk from the manifold building to the sedimentation tanks, we passed by the old sedimentation tanks, which are no longer in use. These tanks were horizontal flow sedimentation tanks that had to be manually de-sludged. The new upflow sedimentation tanks were built in the 1970’s and are still in use today.

The tanks themselves are sludge blanket clarifier type tanks, approximately 4 meters in depth. Each tank features vertical hexagonal tubes that increase the efficiency of the tank by providing a large surface area, meaning more floc can come into contact and settle.

Water coming from the manifold building first enters a chamber with splitter weirs feeding each of the sedimentation tanks. A secondary coagulant or polyelectrolyte is added as the water goes over the weir (this aids with mixing). A polyelectrolyte is a synthetic long chain polymer with plus and minus ions that floc particles can attach to.

It comes as a powder and is prepared by mixing with water, a task that is quite hard to carry out due to its high viscosity. Polyelectrolyte is added to the water in tiny concentrations, about 0.1 mg/l, yet has a dramatic effect, making the floc much heavier and easier to settle.

The water, having been dosed with polyelectrolyte, enters at the bottom of the sedimentation tank through pipework that is arranged to distribute it evenly over the entire plan area. As the water moves upwards, a floc forms and gets trapped by the sludge blanket. The water at the top of the tank, now free from that floc, flows over v-notch weirs into a decanting channel which takes it on to the next stage of treatment, filtration.

If the sedimentation tank is working properly, an observer should be able to see down through the water to the sludge blanket. If the water is cloudy and the sludge blanket cannot be seen, the tank is not working as expected. On our trip, we could see straight down to the sludge blanket, indicating that the sedimentation tanks were working properly. Sludge is removed periodically via a sludge hopper. The removal rate is based on the formation rate of the sludge and is typically about once every 15 minutes.

Filters:

After passing through the sedimentation process, the water still contains aluminium floc that needs to be removed by filtration. The filter beds at Ballymore Eustace are rapid-gravity sand filters consisting of sand overlying gravel media. Water enters the filters at the top and flows down through the sand bed, which traps the floc in-between the pores and removes it. Beneath the tank there is an underdrainage system consisting of a series of pipes. These pipes remove water uniformly into an open channel that flow onto the next stage. There is usually 1-1.5 m of water above the sand bed at any time.

Beneath the filter a series of pipes and pumps are found, along with a venturi meter, which measures the pressure differential causing the flow through the filter.

There is a valve controller at the outlet of the filter which can be opened or closed in response to variations in flow. Just after a filter has been cleaned, the valve can be partially closed, providing a resistance to flow. As the filter is used, the floc captured by the sand particles builds up and resists the flow of the water through the bed, causing a loss of head through the filter. The valve is then gradually opened to compensate for this loss of head.

When a filter clogs up with material it had filtered out of the water, it needs to be cleaned, a process called backwashing. Backwashing of a filter is done on the basis of a specific time interval or a pre-determined head loss through the filter bed. While on our field trip to Ballymore Eustace, we got to see this process first hand. The filter is taken out of service and air is forced upwards through the bed, dislodging and removing trapped floc particles. After this air scour, the wash water is removed by forcing it over the weir at the top of the filter. This is done by pumping clean water up through the bed. The rate of water push up is so that no sand is washed out of the filter and only the floc particles and trapped material are removed with the washout. When the backwashing of the filter is complete, the outlet valve removing washwater through the brown pipe is closed and the water returns to its original level. The inlet valve is then re-opened and filtration starts again.

The clean water used in backwashing is stored on site in a water tower, which also provides clean water for use in the lab.

Disinfection and Fluoridation:

The final step in the treatment process before the water can be distributed or stored for drinking water supply is disinfection and fluoridation. First, the water is dosed with lime to correct the pH. Then, chlorine is added as a disinfectant to kill any micro-organisms that weren’t removed during previous treatment steps or that could possibly get into the water by the time it reaches the consumer (e.g. during storage in an open reservoir). Finally fluoride is added as required by law. Now the treated water can be distributed for general consumption by the public.

Sludge Treatment:

The sludge that results from the sedimentation stage of the treatment is a waste that needs to be treated before it can be disposed of. The sludge sent to the sludge recycling plant is 98% water and the aim of treating the sludge is to remove as much of this water as possible to make the sludge easier to handle.

The first step in the sludge treatment process is thickening. A polyelectrolyte is added to the sludge and it is placed in a sedimentation tank. A rotating scraper scrapes off and removes the sludge that settles for further treatment. Solids content increases from 2 to 4 percent during this step.

The second step in the process removes the bulk of the water. The sludge removed from the first step is sent to the sludge de-watering plant where water is squeezed out under high pressure (about 5 bars) using a plate press pressure filter. This is a batch process and involves filling the pressure filter with sludge, pressing in a horizontal direction, and removing the sludge cake by opening the press. Solids content increases to about 23% during this step and the end product after this process is a cake that can be removed to landfill.

Conclusions:

I found this field trip very interesting and informative. It provided me with a chance to see the theory, processes and equipment I have learned about in lectures up close and in physical form. I especially relished the opportunity to have seen the process of backwashing a sand filter in person. I could also see first hand how well the water treatment process works, turning murky, turbid raw water into the crystal clear potable water that we are used to drinking from our taps. I was amazed to see how clear the water was after emerging from the filtration stage.

While on this field trip, I could see the upgrade works being carried out on the plant and could appreciate the importance of Ballymore Eustace and indeed all water treatment plants in supplying the population with clean, healthy drinking water. I also realised the gravity of the problems facing us as civil engineers in designing for an increasing demand in high quality potable water in the future and solving the problems and challenges that will emerge as a result of this demand.

Even with all the upgrades, as well as water leakage reduction, Ballymore Eustace and the other water treatment plants supplying the Dublin region will not be able to meet the rising demand for potable water in the future. It is projected that between 2006 and 2016 demand will increase by 100 ML per day. Beyond 2016, alternative sources of raw water will need to be looked at. Options that Dublin City council are currently investigating are:

a) Abstracting from the River Shannon at Lough Ree and transporting to Dublin for treatment.

b) Desalination from the Irish Sea.c) Using the River Liffey and River Barrow conjunctively.

References:

1. Upgrading works, Pierse Contractors - http://www.pierse.ie/project_details.php?id=35

2. Upgrading works, BAM Civil - http://www.asconrohcon.com/live/bam_civil/aproject_details.asp?section1=1&sub_section=4

3. Dublin Region Water Supply Project – http://www.watersupplyproject-dublinregion.ie/index.php?page=presentation

4. Dublin City Council - http://www.dublincity.ie/WATERWASTEENVIRONMENT/WATERPROJECTS/Pages/WaterProjects.aspx

5. Siemens Electrochlorination plant - http://www.edie.net/products/view_entry.asp?id=3505

6. Ballymore Eustace handout given on trip.