Over the past decade, water utilities around the world have been looking for technologies to recycle wastewater. As one of the major industry players, the French company Veolia was at the forefront of this pursuit. As early as 2011, its AnoxKaldnes has carried out pilot projects in three European places - Brussels North Wastewater Treatment Plant in Belgium, Eslöv in Sweden, and Leeuwarden in the Netherlands, by using industrial and municipal organic wastes as raw materials to recover degradable plastics - polyhydroxyalkanoates (PHA), which were mixed with microbial cultures from sewage.

PHA plastic extracted from sludge | Source: STOWAtencent

However, in 2016 Veolia abandoned the pilot project that was carried out in the Belgian sewage plant. Fortunately, the Dutch water committee took over, and the project has continued to this day. Why did the Dutch water committee still want to save the project that was abandoned by Veolia? Read on and find out.

Turning sludge into plastic

When it comes to the recycling of sewage resources, the first thing that people think of is the recycling of nitrogen and phosphorus. In fact, these are just the tip of the iceberg, as there are far more resources contained in sewage than that. E.g., phosphorus recovery is already considered a routine practice of many sewage plants. The Geestmerambacht sewage plant in the west of the Netherlands recycles cellulose into road construction materials, and the Zutphen sewage plant in the southeast successfully extracts alginate extracellular polymer (EPS) from sludge.

In 2015-2016, five Dutch water committees set up a project called PHARIO, targeting PHA extracted from sewage sludge.

PHARIO is the abbreviation of the Dutch "PHA uit RIOolwater", that is, PHA from sewage. As the name suggests, the principle is to use the activated sludge produced by sewage treatment as the raw material for the production of PHA. In fact, it is not easy to recycle such polymers. For example, Veolia began to explore the technology of recycling PHA from sewage more than a decade ago (Veolia has registered the trademark Cella™), but without success. Fortunately, the Dutch water committee had the confidence to continue the project, and this is how the PHARIO project was born.

PHARIO's pilot plant at the Bath Sewage Plant | Source: STOWA

What is a PHA?

PHA is the English abbreviation of Poly-Hydroxy-Alkanoate. It is an aliphatic copolyester with different structures, which can be synthesized by microorganisms through the fermentation of various carbon sources. The most common of these are poly-3-hydroxybutyrate (PHB), polyhydroxy valerate (PHV), and their copolymers (PHBV).

Chemical formulas of PHB and PHB | Source: STOWA

You may have heard of the biological phosphorus removal process. In fact, the biological production principle of PHA is to use phosphorus accumulating bacteria (PAOs) to absorb volatile fatty acids (VFAs) in water under an anaerobic environment to form PHA in cells. There are also many research groups in Chinese universities that are studying the recycling of PHA.

Multiple applications of PHA bioplastics | Source: ResearchGate

PHA has many excellent properties such as biodegradability and biocompatibility, and play an important role in biomedical materials, tissue engineering materials, sustained-release materials, electrical materials, and packaging materials. If the copolymer PHBV is synthesized, the weak point of PHB due to its high crystallinity can also be improved, thereby improving the mechanical properties, heat resistance, and water resistance. It has a wide range of uses, such as disposable tableware, non-woven fabrics, packaging materials, agricultural films, toys, wraps, glue, fibers, and other degradable products.

The PHARIO project cooperates with a number of companies to produce various products with PHA extracted from sludge, such as the card case below.

PHARIO chose the Bath sewage plant in Zeeland, the Netherlands as the test site, because the sewage plant adopts a biological denitrification process, including pre-anoxic denitrification and chemical phosphorus removal. These process conditions are conducive to screening and culturing phosphorus accumulating bacteria (which can accumulate PHA).

The test data showed that the PHA content obtained by the pilot plant in Brussels, Belgium, and the activated sludge secondary treatment system of the Bath sewage plant in the Netherlands was similar, up to 0.47 gPHA/gVSS. Generally speaking, 0.40 gPHA/gVSS is regarded as a pass line for industrial application, which shows that in terms of pure technology, PHARIO can integrate PHA production technology into the production line of urban/industrial sewage plants.

The Bath WWTP sends excess sludge to the pilot plant as the basic feed for the production of PHA. During the pilot test, the production of PHA was stable above the level of 0.40 gPHA/gVSS. They also introduce external feeds, such as VFA-rich liquids or primary sludges from adjacent confectionery factories; and mixtures of pure acetic and propionic acids are also used as feeds. The PHA obtained from the WWTP will be sent to AnoxKaldnes' pilot refinery in Lund, Sweden, for purification and recycling. Over a 10-month period, the pilot system established a capacity of 1kg PHA/week.

In October 2015, the Branbantse Delta Water Board, which is part of the wastewater treatment plant, handed over the first 1kg of PHA to Oerlemans, a local plastic packaging factory, to make shipping bags.

This pilot test initially demonstrated the feasibility of producing high-quality PHA from activated sludge, and the cost of PHA produced from activated sludge is competitive compared to market prices. The project team has estimated that the PHA produced based on activated sludge can be reused more than 5 times. Life cycle analysis (LCA) results also show that the cost is only one-fifth of the current biogas recovery from sewage, and the environmental impact is 30% of the current conventional PHA production method. However, after the project ended, the project team wasn’t happy with the final results. The Dutch water committee then launched new projects that included larger-scale tests, and increased production capacity to convince downstream manufacturers.

PHA2USE

The five Dutch water committees, together with STOWA, the sewage treatment company Paques, and the sludge incineration company HVC, created a new project called PHA2USE. They chose the sludge incineration plant in the city of Dordrecht near Rotterdam for a larger pilot test.

PHA2USE team | Source: STOWA

This new bioplastic pilot system consists of three major components, including a fatty acid storage tank, a bioreactor for bioplastics production, and a settler for mud-water separation. The budget for the project was nearly 2.5 million euros.

Pilot system of PHA2USE | Source: h2owaternetwerk

These PHBVs are stored in bacterial cells, so to obtain PHBV, they must be isolated from the cells first with the extraction technology. PHA2USE is targeting a production capacity of 25kg a day.

Equipment for extracting PHA | Source: h2owaternetwerk

Back to Basic Research

In fact, in 2020, PHA2USE started another EU project - Interreg NWE WOW! Moreover, the Wetsus Institute in the Netherlands and the Avans University of Applied Science had optimized the extraction process.

Why do they have to go back to the laboratory for research after they have achieved the scale of the pilot test? This is because there are some basic principles that neither the PHARIO nor the PHA2USE team have understood, so the reproducibility and stability of the process cannot be guaranteed. To achieve mass production, they are looking for answers to more fundamental questions, such as:

• Conditions for maximum PHA content and yield?
• Effects of different feed substrates on the bioprocess?
• Impact of reactor configuration and design on yield?

Wetsus Institute is a well-known European scientific research institution integrating the superior resources of colleges and universities, environmental protection enterprises and governments of various countries. It is mainly engaged in the research and development of sustainable water environment technology and is committed to solving global water environment problems. It conducted a lot of basic research on the resource utilization of sewage treatment, and these research experiences are also very useful for the extraction of PHA from sludge, and Wetsus and Paques also had a close cooperative relationship before, so it is a matter of course for the PHA2USE project to find Wetsus.

Apart from the bioconversion issues, the commercialization of sludge PHA also faces other challenges, such as whether the extracted bioplastics can be directly used in existing machinery and equipment in the plastics industry? Is there regulatory support for direct use in food packaging? Can consumers accept plastic products derived from sewage? The time will show the answers to some of these questions.