Northumbria University graduate Danielle Coffey has designed a device that transforms oils and fats leftover from cooking into soap, to prevent them from polluting household water systems.

Sápu, which is Icelandic for "soap", is a kitchen product that aims to encourage homeowners to collect and repurpose their own fat, oil and grease (FOG) waste from cooking by turning it into natural soap.

Developed as a more environmentally friendly alternative to sink disposal methods, the project aims to address the pressing future issue of water pollution.

The Sápu device offers a solution to the "common issue" of not knowing what to do with cooking byproducts, which are typically washed down the sink and contribute to water pollution or infrastructure issues such as fatbergs.

Coffey designed the product in response to a brief set for the university by the Room Y innovation department at British department store John Lewis.

Students were tasked with developing a proposal that makes use of untapped or neglected resources in the urban environment as a way of addressing the challenges of climate change, population growth and dwindling resources.

During her research, Coffey found that FOG blockages are responsible for around 80 per cent of water-system issues, and approximately £100 million worth of damage to systems and the environment.

She also discovered that incorrect disposal of pollutants by households are the biggest contributor to blockages in the water system.

Therefore, with her Sápu project, she aims to improve future water quality by "instilling behavioural changes" that will encourage people to correctly dispose of or reuse FOGs.

After cooking, the waste FOGs are poured into a three-part injection-moulded polypropylene filter, which has been ultrasonically welded together – a technique that applies high-frequency ultrasonic acoustic vibrations to items under pressure.

The waste is filtered through non-toxic, biodegradable filter paper made from a polysaccharide base – from a carbohydrate such as cellulose or starch.

This then filters down into the main compartment where it is mixed with two spoonfuls of lye (a metal hydroxide) and six spoonfuls of water.

This produces a liquid mixture that the user can personalise by adding herbs, essential oils, rinds and seeds such as chia seeds or lavender, before distributing it into moulds. The bars are left to solidify and form a soap for everyday household use.

Once the moulds have been filled, the different components of the Sápu device can be easily separated for cleaning.

The lye is kept in an injection-moulded polypropylene container with a recycled cork topper, which features a child-proof twist cap to ensure there is no accidental contact with the substance.

The designer hopes to develop her waste repurposing system in the future to make edible "fat balls" for animals by adding peelings and seeds to the soap mixture, or for plant care by adding the oil directly to soil as fertiliser, or spraying it like a pesticide.

Coffey is not the only graduate designer to create soap in a bid for more sustainable future. Mi Zhou from Central Saint Martins made toiletry bottles from cast soap that melt away once they are no longer useful.

Zhou dyed vegetable oil-based soap using natural pigments before forming them in a mould, and lining the bottles with a thin layer of beeswax to make them waterproof and prevent the liquid contents from dissolving the bottles.