What happens to Bristol’s sewage?

From flush to end of pipe, Bristol sewage

Human consumption produces colossal amounts of waste. Our waste takes many forms; every piece of disposable plastic ever made is still out there and one third of all food grown for consumption is wasted. This got me thinking; what about our literal waste, our defecation? Like all animals nature calls for us too, but unlike our earthly comrades, we put ours into a porcelain bowl and flush it away with a slosh of fresh water. Job done! But what happens to Bristol’s sewage once it is out of sight?

The Sewage Treatment Process

The first step in treating sewage is screening out non-organics and unflushables, such as grit and muck washed into drains by rain and household items including wet wipes, cotton buds and nappies. These items can hinder and disrupt the more important stages of sewage treatment. More importantly, they pose a serious pollution risk to water sources. In the UK, about 700,000 panty liners, 2.5 million tampons and 1.4 million sanitary towels are flushed down the toilet every day, blocking up drains and polluting water sources. 

Groups such as Surfers against Sewage and City to Sea campaign to build awareness around unflushable pollution, which makes up 7% of plastic pollution on UK beaches. Check out Natalie Fee’s TED talk on plastic pollution from items flushed down the toilet, and how you can help stop it.

After plastic pollutants are removed from the sewage, the next step is to separate the organic solid matter or ‘sludge’ from water. This video from the National Geographic explains the process at Becton, the sewage treatment plant responsible for London’s sewage.

The solid-free water can then be filtered through sand as it is returned to the water source; these sources are quality checked by the Environment Agency (or Natural Resources Wales in Wales) to regulate the water purity. Allegedly, wastewater that has undergone sewage treatment can even increase the purity of the water source it returns to.

Recycling Sludge

While Becton treatment plant chooses to use the organic solid matter as fuel, its nutrient content means it can be used in more environmentally friendly ways. The Forestry Commission suggests that sewage sludge can enrich soils on brownfield sites to grow new woodland. Similarly, as the sludge contains beneficial nutrients, such as phosphorous and nitrogen, it can be applied as agricultural fertiliser

What happens with sludge in Bristol?

While Bristol Water deals with water supply and catchment, Wessex Water deals with wastewater. The good news is that 100% of the sludge they treat is used to fertilise farmland and other areas. It is made usable by either the addition of lime or anaerobic digestion.

The company currently anaerobically digest 64% of the sludge, but are hoping to increase this figure because anaerobic digestion has another useful by-product – biogas – generated as bacteria digest and process the sludge. As part of a plan to be waste-free by 2020 (Wessex Water is already 100% zero waste in their offices), unflushables from the first stage of sewage treatment are diverted from landfill to a composting site in Avonmouth where they can be treated and recycled.

As some of these unflushables may contain traces of pollutants such as plastic it’s not certain they’ll be fully recycled. Recycling sewerage from urban areas is already risky enough as pollutants are more prevalent in modern cities.

Wessex water claim to be 98% waste free in its sewerage operations, with the remaining 2% being contaminated soil and construction waste, for which there is little that can be done right now.

On a city-wide scale cutting out water use in dealing with sewage while maintaining a high level of sanitation is impossible, so reducing landfill waste and minimizing water usage are focal points for sustainable sewage treatment in the city. As a raw sewage producer, it is up to every toilet user to make sure that non-decomposable material is kept out of the sewage system, and that it doesn’t end up impacting marine wildlife.

Article by Tom Stevens