According to the EPA (Environmental Protection Agency) publication 'The Provision and Quality of Drinking Water in Ireland (2006-2007)': Drinking water must be clean and wholesome - Leaving Cert Chemistry - Question 7 - 2008

To ensure low levels of micro-organisms in drinking water, the following treatments are commonly used:

1. Chlorination: Chlorine is added to the water, acting as a disinfectant that kills harmful bacteria and viruses.
2. Ozonation: Ozone may also be introduced, which has strong oxidizing properties and can effectively kill micro-organisms without leaving harmful residues.
3. UV Treatment: Ultraviolet light is another method employed to inactivate bacteria, viruses, and other pathogens present in the water.

If the pH of public water supply falls outside the range of 6 – 8, several issues may arise:

1. Corrosion of Pipes: Water that is too acidic (pH < 6) can corrode plumbing and infrastructure, leading to the leaching of metals like lead and copper into the water supply.
2. Taste and Safety: A pH that is too high (pH > 8) can result in a soapy taste and may cause issues with finding correct balances in water treatment processes.
3. Effect on Aquatic Life: Such pH levels may be harmful to aquatic ecosystems, affecting respiration and overall health of aquatic organisms.

The concentration of Pb²⁺ must be kept below 10 µg/l because lead is a toxic substance that can cause serious health problems, including neurological damage and developmental disorders in children. To remove heavy metals such as Pb²⁺ from large quantities of water, the following methods are commonly applied:

1. Chemical Precipitation: Adding chemicals that cause lead to form insoluble compounds which can then be physically removed.
2. Ion Exchange: Utilizing resins or materials that can swap lead ions with less harmful ions in the water.
3. Filtration: Effective filtration systems that target heavy metals can also help in ensuring that levels remain compliant with regulations.

The nutrients referred to in the context of eutrophication are primarily:

1. Nitrates (Nitrogen Compounds): Often originating from fertilizers and sewage.
2. Phosphates (Phosphorous Compounds): Found in detergents and runoff. Their levels are typically lowered during the tertiary stage of sewage treatment, which is specifically designed to remove remaining nutrients from treated wastewater.

The term 'tertiary' in the context of sewage treatment refers to the final stage in the wastewater treatment process after primary and secondary treatments. It involves additional purification methods, such as filtration and chemical treatments, aimed at removing remaining contaminants and nutrients, particularly phosphorus and nitrogen compounds, to enhance water quality before discharge or reuse.

For the analysis of brewery effluent, the following conditions are essential:

1. Light Conditions: The analysis should be conducted in the dark to avoid photosynthetic reactions that could affect the results.
2. Temperature: The effluent should be analyzed at a controlled temperature of around 20 °C (293 K) to ensure consistency.
3. Duration: The duration of the test should be adequately maintained for 5 days, allowing for sufficient biological activity to be observed.

To calculate the BOD (Biochemical Oxygen Demand) of the brewery effluent, we use the following formula: BOD = rac{(D_1 - D_2) imes ext{dilution}}{ ext{volume of sample}} Where:

• $D_1 =$ dissolved oxygen concentration at the start (time = 0)
• $D_2 =$ dissolved oxygen concentration after incubation (at 5 days)

Based on given data:

• Initial DO ($D_1$): 9.8 mg/L at 0 h
• Final DO ($D_2$): 5.1 mg/L at 5 days
• Dilution: 500 mL
• Volume of sample: 100 mL

Plugging these values in: BOD = rac{(9.8 - 5.1) imes 500}{100} = rac{4.7 imes 500}{100} = 23.5 ext{ mg/L} Thus, the BOD of the brewery effluent is 23.5 mg/L.