The Importance of the Circular Economy – The linear economic model where we “take, make and dispose” of things is not sustainable. It relies on large quantities of cheap easily accessible materials and energy. A circular economy is one that keeps resources in use for as long as possible and then recovers and regenerates products and materials at the end of each service life.
In the Water Industry, we can play an important role in the emerging circular economy, we receive huge amounts of “waste” water that is full of potentially valuable material that can be recovered and regenerated, and then either used by us or sold to be made into value added products.
Severn Trent have advanced plans to be at the forefront of the emerging circular economy through recovering materials, energy and clean water from the wastewater we treat. Delivering on our circular economy ambition is becoming more and more important as the resources required to effectively treat wastewater such as energy and chemicals are in increasing demand. By reducing the amount of resources we need to effectively treat sewage we can keep customers bills low.
The Spernal STW Test bed
This summer we will be opening our multi-million pound demonstration test-bed site at Spernal STW in Redditch which will be used to validate technologies and flowsheets that are consistent with a transition to a circular economy approach.
The test-bed offers us the exciting opportunity to evaluate energy neutral wastewater treatment and to recover valuable materials contained in wastewater such as fertilisers, bio-plastics, cellulose and even protein (for example for livestock and aquaculture feed, but ultimately perhaps even beef burgers!).
Energy neutral sewage treatment
The initial focus of our trial programme will be low energy treatment consisting of enhanced primary treatment and mainstream anaerobic treatment. An enhanced primary treatment stage removes more suspended solids and more organic load than conventional primary settlement tanks. This directs more of the organic load to the energy generating anaerobic sludge digesters and less to the energy consuming wastewater secondary treatment process. A number of enhanced primary treatment technologies are being considered for trialling on the testbed, including dissolved air flotation and micro-screen filtration. This trial work is scheduled to run in the second half of 2019.
We’ve been developing the anaerobic technology over the last 10 years or so with one of our research partners, Cranfield University. Anaerobic treatment is already successfully used in warm countries like Brazil, but recent research breakthroughs with anaerobic membrane bioreactors (AnMBRs) have developed a process capable of efficient treatment in temperate climates like the UK’s. Next year we’ll be commissioning, at Spernal, the largest demonstration scale AnMBR system in Europe, capable of treating up to 500m3/d. An AnMBR treats wastewater by using the same types of anaerobic micro-organisms used extensively to treat wastewater sludge in anaerobic digesters. However, in sludge anaerobic digestion the process is heated to between 35 and 40oC, with a municipal wastewater AnMBR the process has to be designed to successfully treat wastewater down to temperatures as low as 8oC. The system utilises an upflow anaerobic sludge blanket reactor (UASB) twinned with an ultra-filtration (UF) membrane. The bacteria break down the organic pollutants into biogas before being separated from the treated wastewater, and returned to the UASB by the UF membrane. The treated effluent is passed to a membrane contactor where any dissolved biogas is extracted. The biogas can then be converted to renewable electricity and used to power the treatment process. The AnMBR process is expected to work best where the majority of suspended solids have been removed from the influent and hence the combination of enhanced primary treatment with AnMBR is likely to be very complimentary.
In most of our wastewater treatment plants we also remove nutrients – nitrogen and phosphorus – to protect the streams and rivers we discharge into. Nutrients, however, are an essential resource, they are a component of all living cells and without them we can’t grow crops, fruit or vegetables, and they are an essential ingredient in the feed that farmers give to livestock. Nitrogen fertilisers are manufactured by the Haber-Bosch process which chemically converts nitrogen from the air to ammonia – this is a very energy intensive process. Phosphorus is a non-renewable, finite resource that is mined and incorporated into fertilisers. The Earth’s phosphate rock reserves are becoming rapidly depleted. There is a compelling argument, therefore, to recover both nitrogen and phosphorus from sewage.
Mainstream anaerobic treatment won’t remove nitrogen and phosphorus from the sewage which in the context of nutrient recovery is yet a further advantage. The effluent from an AnMBR will be relatively rich in nitrogen and phosphorus but crucially it is free from solids and hence very amenable to adsorption and ion exchange based nutrient recovery technologies that we will also evaluate at Spernal. These technologies, developed by Cranfield University, use a media – mesolite for ammonia recovery and an iron nano-particle embedded ion exchange bead for phosphorus recovery. The nutrients are removed through adsorption onto the media bed, meaning there is no need to dose chemicals. The media bed can then be regenerated, allowing the nutrients to be recovered as useful minerals, for example calcium phosphate and ammonium sulphate. Previous trial work at our Packington sewage treatment works was an important step in developing the technology at pilot scale , however further work is required to validate the compatibility of the process with upstream anaerobic treatment and to optimise the regenerant clean-up and nutrient recovery aspects. This will be the focus of the trial work planned at Spernal next year.
European research programmes and other opportunities
Our work in developing resource recovery from sewage has been strongly supported by our involvement in a number of EU research projects. The EU’s Horizon 2020 Innovation Fund has provided approximately £450,000 worth of funding for the AnMBR demonstration plant through the “NextGen” project. This European consortium, consisting of over 30 world-leading partners from across the EU, is setting out a four-year programme to evaluate and champion circular economy solutions and systems in the water sector. The project is also helping to fund another nine circular economy case studies across Europe. The “SMART Plant” Horizon 2020 project has been running since 2016 and has been instrumental in developing the nutrient recovery technology described above. Our project partners are also developing technologies for the recovery of cellulose and bioplastics from wastewater. The “WOW!” project, an EU Interreg North West Europe funded project is focusing on developing the markets for the materials we are able to recover from sewage.
Severn Trent believe that by embracing circular economy principles we will deliver energy neutral, bio-refineries at our sewage treatment works, creating valuable products from what has traditionally been viewed as waste.