Natalie Lamb and the introduction to drinking water treatment

This post will provide an introduction to drinking water tretament, using Bristol Water, a water only utility in the UK as a case study.

An overview of Bristol Water

The two largest water treatment works at Bristol Water are Purton and Barrow.

Purton is located in the North of the region and is canal fed, while Barrow is in the South and reservoir fed. Purton Water Treatment Works has a maximum output of 165 million litres per day, providing 1/3 of Bristol Water’s drinking water supply.

The water quality in Barrow is easier to anticipate, for instance, it is known that challenges such as algal blooms are a likelihood in the summer. Purton, meanwhile, is more volatile so is more prone to weather quality shifts rather than seasonal shifts e.g., heavy rainfall and pesticide use leading to high pesticide concentrations. This makes the canal system a more risky water source, resulting in more chemicals and more energy to treat it, as well as relying on the local water quality scientists to provide a quick response to those changes in quality. One example of a response scientists could take could be to blend water with other sources of water or to stop the abstraction of water from the canal for a short period.

Some examples of challenges from the source water can include:

• Nitrate
• Microorganisms such as E. coli and Cryptosporidium
• Algae (although not as much of a risk at Purton than at Barrow)
• Geosmin (pronounced gee-oz-min), which can cause an earthy odour
• Pesticides such as glyphosate and metaldehyde
• Zebra mussels, which accumulate in biomass to narrow pipes and channels

Future treatment challenges for Bristol Water include ever tightening water quality standards, as they will never relax, they will only get tighter and lead, as around 50% of the pipes in Bristol city centre are made from lead (they would be uncomfortable having phosphate dosing for lead control off for 5 days).

 

What have you seen on site and what was it?

Treatment steps, using Purton as a case study site

1. Abstraction, including the pumping in of any raw water if required.

2. Removal. Large materials can also be removed such as shopping trolleys!

3. Blending of other water sources to help mitigate any challenges in source water, such as nitrate.

4. pH correction. This particular site uses a strong (96% sulphuric acid) but small dose. The source water is alkaline (pH 8) so an acid is added to decrease the pH to around pH 7 to ensure the coagulant is able to work efficiently. In hardwater areas (i.e. lots of calcium and minerals), such as this one, more acid is required to get the pH into the desired range. Due to this, the price of treatment of this water source for Bristol Water fluctuates seasonally (even though customer bills do not) as alkalinity shifts throughout the year because of the amount of acid required (high alkalinity = more acid needed).

5. Coagulation. Coagulants such as PAC (polyaluminium chloride) are a bit like a positively charged glue. If you add this to the water in the right conditions (such as the correct pH), particles will be attracted to it and stick together in a fluffy clump which becomes heavier and then can settle out. The clumps can also sometimes look blue, which you can see a little better in the coagulation photos. You might see on site a very fine trickle of the coagulant, this is done to encourage good mixing.  

6. Clarification by settlement. There are various different clarifiers. They are a process to settle out the heavy coagulant particles. These solids are pumped out to sludge. Around 90% of the solids removal happens here.

7. Filtration. Rapid Gravity Filters (RGF) are a bit like giant sand pits filled with anthracite media (like black sand). Water filters through the sand, removing the remaining solids. Depending on head loss (how much they are getting clogged), water is pumped up the wrong way, removing solids stuck in the filter media. This process is known as backwashing.

8. Ozonation. Ozone is a strong oxidant that is good at breaking down things such as metals, taste, odours etc.

If, like at this site, there is bromide in the raw water, it can combine with the ozone to form bromate. As such ozone levels have to be adjusted to ensure bromate levels are within the drinking water regulatory standards of 10 µg/L (this site normally sits at ~4 µg/L).

9. GAC (Granular Activated Carbon) Filters. These filters are essentially like giant Brita filters. They are also a great way to absorb things, like smell, colour, taste and pesticides.

10. UV. Ultraviolet light is used for disinfection. It was first adopted by Bristol Water in 2017 but it is now found at all of their sites. They mostly use UV for the microorganism Cryptosporidium because it is resistant to chlorine.

UVA is what causes wrinkles, UVB is what causes sun burn, UVC is the reason why you have to wear a mask while welding. UVC is the biocidal range of the UV spectrum. Even if there is a one second contact time, that is sufficient to deactivate microbes in the water.

However, you can’t get a sunburn from UVB if you are covered up because the light has to penetrate your skin- and the same is true for water. The water has to be clear, have high transmissibility, for UV to be able to act as a disinfectant. For instance, coke has a transmissivity of 0% while ultrapure water has a transmissivity of 100%. Bristol Water often has 98%.The higher this value, the less turbid the water and the more effective the disinfection.

11. Chlorination. While UV acts as the main method of disinfection (primary disinfection), chlorine is also added to provide a residual (secondary disinfection). This means that there will be some leftover chlorine presence at the customer tap to prevent microbial regrowth during distribution in the drinking water pipe network. On average, it takes ~24 hours for the water from the treatment works to reach the >400,000 people supplied in Bristol. When leaving the plant, chlorine concentrations are around 1 mg/l, while at the tap this is around 0.5 mg/l, since chlorine decays within the pipe network. In the summer these concentrations decrease quicker because heat speeds up chemical reactions, including those reactions which cause chlorine to decay.

Interestingly, this site produces their own chlorine, known as on-site electro-chlorination (OSEC). As such, liquid chlorine (sodium hypochlorite, chlorine gas dissolved in sodium hydroxide) is used rather than often the norm, chlorine gas, for health and safety reasons. They also intend to roll this out to smaller remote sites in future.