A water treatment plant draws water from a river. The first chamber contains rotating screens. What is the specific purpose of this screening stage, and why must it come before all other treatment steps?

To kill bacteria using a fine mesh that filters out microorganisms To remove dissolved chemicals that cause taste and odour problems To remove large debris (leaves, plastic, fish) that would block or damage downstream equipment such as pumps and filter membranes To adjust the pH of the incoming water before coagulant is added

Alum (aluminium sulphate) is added to river water containing fine clay particles. The clay particles carry a negative surface charge and repel each other, staying suspended indefinitely. How does alum solve this problem?

Alum coats the clay particles with a hydrophobic layer, causing them to float to the surface for skimming Alum provides positively charged aluminium ions that neutralise the negative surface charge on clay particles, allowing them to collide and stick together into settleable flocs Alum reacts with clay to form a soluble compound that can be removed by activated carbon Alum raises the pH above 10, causing clay to precipitate directly

A wastewater treatment plant's primary sedimentation tanks remove approximately 55% of suspended solids and 35% of BOD. Why does significant BOD remain in the water after primary treatment?

The sedimentation tanks are too small and need to be enlarged to remove more BOD Much of the BOD is dissolved organic matter or fine colloids that are too small to settle by gravity alone and require biological degradation in secondary treatment BOD is caused by heavy metals that are not affected by sedimentation Primary treatment is designed only to remove pathogens, not BOD

An activated sludge reactor in a sewage treatment works maintains millions of micro-organisms in an aerated tank. Operators measure BOD₅ at the inlet as 220 mg/L and at the outlet as 15 mg/L. Approximately what percentage of BOD has secondary treatment removed?

68% 79% 93% 99%

After secondary treatment, an effluent still contains 18 mg/L of total nitrogen and 4 mg/L of phosphorus. These nutrients cause algal blooms in the receiving river. What treatment stage is required to remove them, and what are the two main approaches for nitrogen removal?

Primary treatment; nitrogen is removed by adding alum to precipitate ammonium salts Secondary treatment; nitrogen is consumed by activated sludge bacteria as a nutrient for growth Tertiary treatment; nitrogen is removed biologically by nitrification (ammonia → nitrate) followed by denitrification (nitrate → nitrogen gas) Disinfection stage; UV radiation breaks down nitrogen compounds directly

A river ecologist reports that fish are dying in a section of river below a food-processing factory's outfall. Water samples show dissolved oxygen at 1.2 mg/L (normal is >6 mg/L). The factory discharge has a BOD₅ of 800 mg/L. What is the causal chain linking the factory discharge to the fish deaths?

The factory chemicals directly poison the fish through toxic reactions The warm discharge water raises river temperature, which reduces oxygen solubility High BOD organic waste stimulates bacterial growth, which consumes dissolved oxygen faster than it can be replenished, creating anoxic conditions that suffocate fish The discharge raises river pH above 9, which is toxic to freshwater fish

A town is built on a floodplain. Historical records show the local river has a flow of 450 m³/s exceeded once in 100 years. Engineers designing a hospital use the 1:1,000-year flood (650 m³/s) as the design standard. Why is a more severe flood standard applied to the hospital than to nearby houses?

Hospitals are larger buildings and physically require more structural protection from floodwater The 1:1,000 standard is the legal minimum for all buildings near rivers Hospitals are critical infrastructure — they must remain operational during a disaster when they are needed most, so a lower probability (more severe) flood standard is justified by the consequence of failure Insurance companies require the 1:1,000 standard before they will insure any healthcare facility

The Thames Barrier in London has 10 rotating steel gates, each spanning 61 metres, that rise from the riverbed to block tidal surges. In what way does this design differ fundamentally from a fixed floodwall, and what operational advantage does it provide?

The barrier generates tidal power electricity when gates are raised, making it economically self-funding The gates can be lowered flat on the riverbed to allow normal river traffic and commercial navigation, then raised only when a surge is forecast, preserving the economic use of the river The rotating design allows gates to spin continuously, generating turbulence that dissipates wave energy before it reaches London The barrier can be partially closed to reduce river flow for water extraction during droughts

A developer proposes a large car park in a city centre. The planning engineer notes that the car park will convert 2 hectares of soil to impermeable tarmac. What will this change do to the stormwater runoff regime, and which Sustainable Urban Drainage (SuDS) feature best mitigates it while also adding ecological value?

Runoff will decrease because the smooth tarmac allows water to flow quickly to drains without pooling Runoff volume and peak flow rate will increase sharply; a bioswale (vegetated drainage channel) both slows and infiltrates runoff while providing habitat Runoff will be unchanged because stormwater drains are designed to handle any surface type Runoff volume increases, but a standard concrete detention basin mitigates this without any need for vegetation

A landfill is lined with compacted clay and a high-density polyethylene (HDPE) membrane. A leachate collection system runs beneath the waste. Why are both a clay layer AND a synthetic liner used together rather than relying on one alone?

The clay layer provides structural support for the HDPE, which would collapse under the weight of waste without it Clay provides chemical adsorption and a low-permeability barrier; HDPE provides a near-impermeable physical barrier — together they compensate for each other's failure modes to protect groundwater The HDPE is required by law and the clay is added only for cost reasons because HDPE is expensive Both layers perform identical functions, providing redundancy in case one layer is accidentally omitted during construction

An LCA (Life Cycle Assessment) study compares a reusable ceramic coffee mug against a single-use paper cup. The study finds the ceramic mug must be used 50 times before its environmental impact per use is lower than the paper cup. A customer uses the ceramic mug 200 times. What does this result demonstrate about LCA methodology?

LCA studies are unreliable because they depend on assumptions about user behaviour Manufacturing phase impacts are significant — an apparently 'eco-friendly' product can have a higher initial environmental burden that is only recovered through sufficient reuse Paper cups are always more sustainable than ceramic mugs regardless of use frequency The mug should be discarded after 50 uses because further use provides no additional environmental benefit

A former petrol station has contaminated soil with benzene (a volatile organic compound). Four remediation options are proposed: pump-and-treat groundwater, in-situ bioremediation, soil vapour extraction, and excavation and landfill. Which option is most directly suited to removing volatile contaminants without excavating the soil?

Pump-and-treat — groundwater is extracted, treated at the surface, and reinjected In-situ bioremediation — nutrients are injected to stimulate bacteria to metabolise benzene Soil vapour extraction — a vacuum is applied to draw volatile contaminants out of the soil as vapour for above-ground treatment Excavation and landfill — contaminated soil is dug out and transported to a licensed hazardous waste facility