Cities are overflowing with valuable biomass if you look in the right places: namely, in their sewage systems. The organic materials in urban wastewater can provide cheap, local, and abundant raw materials for biobased and circular fuels, fertiliser, and materials.
Valorising this waste means setting up urban biorefineries that can extract plant and animal materials from sewage streams and process them into safe products.
But are urban biorefineries economically feasible? Or are they an ideal dreamt up by urban planning researchers?
Two European projects are proving the concept is a viable sustainable policy tool for municipal authorities and a profitable activity for waste management companies.
The urban biorefinery in theory
In recent years, the urban wastewater biorefinery concept has become a darling of urban planning researchers interested in designing sustainable cities. There is no shortage of papers dreaming up the raw materials, end products, and blueprints of future municipal biorefineries.
The appeal is obvious: these waste infrastructures can serve three functions that blend financial and ecological benefits. The most obvious function is that they would help purify sewage streams and help city water use efficiency.
Yet existing wastewater plants also do this. What sets the urban biorefinery apart is that at the same time as it purifies wastewater, it churns out high value raw materials from the waste it extracts.
This supports the third purpose of the urban biorefinery. The fact it squeezes high value goods out of waste means it can, in theory, lower the cost of municipal wastewater management.
Selling goods made from wastewater recoups some costs of its purification, the argument goes, meaning public and private entities are incentivised to prevent untreated sewage entering the environment.
The urban biorefinery in reality
The urban biorefinery is far from being an everyday piece of urban infrastructure. However, EU organisations are trying to tackle the technical, cultural, and economic challenges of realising this.
Between 2017 and 2022, the Bio Based Industries Joint Undertaking (BBI JU) funded the EU URBIOFIN project. URBIOFIN aimed at showing the technoeconomic feasibility of turning urban waste into fuel and materials, making municipal waste management more circular as a result.
To achieve this aim, URBIOFIN established lab, pilot, and semi-industrial demos of an urban biorefinery design in three spots across Spain. The first module that made up the URBIOFIN three-part biorefinery was located in PERSEO Biotechnology’s plant (L’Alcudia, Valencia). Modules two and three were sited in the Innovation Centre of Urbaser (Zaragoza) and the CLAMBER biorefinery (Puertollano, Ciudad Real) respectively.
This three-part system was designed to be modular, meaning anyone who is actually implementing it can pick and choose which segments they want to set up, depending on factors such as budget or which biobased products there are markets for.
Biofuels, bioplastics, biofertilisers
Each segment of the URBIOFIN biorefinery took in different inputs and yielded different biobased products. Some of the outputs created in the first or second modules can become feedstock for later stages in the production line.
In the first site, organic matter from urban waste is turned into bioethanol and bioethylene. The bioethanol byproducts from the first module are sent onto the second module, which makes natural polyesters from volatile fatty acids. These natural polyesters can be further processed into biopolymer-based consumer goods. The second module also produces biogas.
The third module features microalgae, which are used both as a biomanufacturing tool and as an ingredient for organic fertiliser outputs.
In module three, biogas from module two can be delivered to the microalgae, which convert the gas into either biomethane – a higher value product useful as vehicle fuel or for the power grid – or PHA biomass, feedstock for PHA plastic particularly suited to agricultural and horticultural applications.
The same algae can then be hydrolysed and concentrated, producing either dry granular fertiliser or liquid biobased stimulants for the agricultural market.
Who would buy from the biorefinery?
The project argued there were four major groups that could benefit economically from its biorefinery: the plastics industry, waste management companies, municipalities and the biobased fertiliser industry.
By creating high value goods from low value materials, waste companies can reduce the cost of waste treatment. Plastics companies looking to expand their biodegradable and biobased offerings could also be potential customers for an urban biorefinery of this kind.
The system can help both public authorities and private entities involved in waste management anticipate future sustainability laws that reward or mandate circularity.
One existing EU regulation that the project was geared towards was the Waste Framework Directive which lays down waste management principles that promote sustainability. Under the directive, preventing waste is the number one priority with preparation for re-use sitting just below it. Disposal is the very last resort. There are also the 2022 revisions to the EU Urban Wastewater Directive, which emphasises achieving circularity in wastewater management.
The biobased fertiliser industry is another sector that would benefit directly from the biorefinery tested in the programme. The EU’s Products Regulation of 2019 is set to grow the European market for organic fertilisers. Demand is set to expand further thanks to the EU target of putting more land under organic farming.
Biorefineries close the city-rural nutrient loop
With its inclusion of agricultural products, the URBIOFIN project shows how another advantage of urban biorefineries is that they can connect urban and rural bioeconomies.
The chemical and material flows of urban and rural ecosystems are usually badly integrated, leading to waste on a huge scale that degrades crop soil quality, a common phenomenon in highly urbanised societies that use a linear food production system.
In general, when food crops are transported to consumers in cities, the nutrients that they once took up from rural soils become dissipated in urban sewage and waste management systems.
Unless food waste is deliberately concentrated, treated, and transported back to the farm, plant nutrients do not find their way back to the soil to support a new round of food production.
As a result, soils are degraded over time and agriculture becomes over-reliant on non-renewable, carbon-intensive and easily-over-applied synthetic fertilisers that damage habitats and biodiversity.
To fix the problem of rural-to-urban nutrient loss, east Asian societies systematically collected human sewage from cities and transported it to the countryside for use as manure. This was a traditional practice in both China and Japan until around the middle of the 20th century. Urban biorefineries that output organic fertilisers are an updated way to close the age-old problem of the city-rural nutrient loop.
Lighthouse cities lead context-specific biorefinery demos
URBIOFIN ended in 2022 but the HOOP project, backed by the EU’s CORDIA, has taken up its mission to mainstream the urban biorefinery.
While UROBIOFIN aimed to demonstrate technoeconomic feasibility, HOOP is focused on pathways to implementing bioeconomies in concrete contexts.
Rather than focusing narrowly on biorefinery design, HOOP is interested in how to adapt biorefinery and waste collection systems to specific urban ecosystems where they operate, local contexts marked by unique infrastructures, populations, and waste content.
To generate knowledge about context-specific biorefineries, the project brought on board eight cities that it calls lighthouse cities: Albano-Laziale (Italy), Almere (The Netherlands), Bergen (Norway), Kuopio (Finland), Münster (Germany), Murcia (Spain), Greater Porto (Portugal), and Western Macedonia (Greece).
The lighthouse cities all have one thing in common: they are home to either public or private entities keen on developing and implementing new urban wastewater valorisation projects.
HOOP offers these lighthouse cities assistance in analysing local waste flows to understand what kinds of biowaste valorisation processes would work in context.
It also offers peer reviews, training sessions, study visits, and online webinars and forums where project participants can share information on biorefinery implementation issues and insights with international counterparts
Beyond fuel, plastic, and fertiliser
HOOP is particularly interested in backing R&D into high-value products beyond compost, biogas, or biofuels – the standard triad of biobased products associated with biorefineries.
HOOP envisages more varied and emerging industries based on wastewater feedstock, including rearing economically useful bacteria or insects on feed using waste.
For example, BIR, a private Norwegian waste management company, joined HOOP to help support its R&D into turning commercial food waste into feed for insects and micro-algae. The company wants to process these organisms into feed for the salmon aquaculture industry.
Biomanufacturing based on fermentation processes features heavily in their plans too, planning to use recovered cooking oils and spent coffee grounds as sources of carbon to feed bacteria capable of producing P3HB, a chemical with cosmetic, medical, packaging, and 3D printing applications.
HOOP is also pushing to make human food ingredients from recovered animal byproducts and the fermentation of used cooking oils. This interest in novel products has an economic purpose: some niche compounds can fetch higher market prices than biofuels or compost, pushing up the economic viability of the plant.
Projects like HOOP make the urban biorefinery more likely to be adopted by urban authorities interested in sustainable planning. By helping actors on the ground realise context-specific projects and facilitating knowledge exchange on the bugs, barriers, and opportunities of implementation, it is building up a set of guidelines that other actors can then use to plan new projects more easily.
Along with URBIOFIN, HOOP offers practical demonstration of the technical, economic, and policy feasibility of these infrastructures, moving the urban biorefinery from research to reality and replication.
