BIOMASS insights

“Make the most out of waste” – that is our claim in the Deep Purple project. During the last months we have put attention to the topic of biomass and the enrichment with Purple Phototrophic Bacteria on our social media channels. Find a summary of the most significant content below:

In the DEEP PURPLE project, mixed urban waste streams, namely wastewater, sewage sludge and the organic fraction of municipal solid waste (OFMSW) is used for the recovery of valuable resources. Currently, the majority of urban biowaste in the EU is incinerated or landfilled.

Purple Phototrophic Bacteria
Purple Phototrophic Bacteria

By using Purple Phototrophic Bacteria (PPB) in wastewater treatment, the amount of landfilled organic waste can be reduced by at least 20% compared to disposal solution(s) currently implemented for the same types of waste streams, such as anaerobic methanation of OFMSW and activated sludge wastewater treatment plants

In addition, the enriched PPB biomass is transformed into valuable compounds for chemicals and materials like PHA (Polyhydroxyalkanoates), ectoine and cellulose in a versatile, integrated and flexible Multi-Platform Biorefinery. This approach increases productivity and cost-efficiency and reduces the carbon footprint compared to existing, conventional solutions.

PPB bacteria growth
PPB bacteria growth (Copyright: URJC)

Purple Phototrophic Bacteria live for resource recovery
PPB are essentially growth-driven bacteria. They do not oxidize or reduce the resources. Instead, they fully assimilate them for their growth.
In an aerobic treatment, around 60% of the total organics is converted into CO2. Therefore, only 40% of the liquid fraction can be effectively transformed into biomass. In contrast, close to 100% of the organics in wastewater are assimilated and/or accumulated by PPB.
These adaptable microorganisms allow to produce different end-products, depending on feedstock availability and features, environmental conditions, market demand, etc. The transition from a bioplastic-based refinery into a biomass (fertilizers)-based refinery is possible, as rapid conversion of the photobiorefinery operation is feasible to quickly meet market demands.

settled PPB biomass
settled PPB biomass (Copyright: Aqualia)

Biomass process & treatment technologies
The use of PPB for wastewater treatment is an emerging technology that enables the recovery of organics, nitrogen and phosphorous from wastewater streams in a single treatment step by biomass concentration.
Microbial technologies need pre-treatment of the waste feedstock to ease its conversion into valuable products, comparable to “cooking” the residues for them. Just like in any other wastewater treatment plant, other technologies such as sieving, settling or thermal hydrolysis are involved. After the thermal hydrolysis, the suspension is separated into a liquid and a solid fraction. The liquid fraction is then treated with the PPB, while the solid fraction is transferred into an anaerobic digester.

PPB biomass in photobioreactor
PPB biomass in photobioreactor (Copyright: Aqualia)

The main operational parameter in this context is the organic loading rate (OLR), which can be easily controlled by the amount of OFMSW that is being hydrolysed and fed into the photobioreactor. The OLR serves to tune up the COD/N/P relationship (Chemical Oxygen Demand / Nitrogen / Phosphorus) of the inlet wastewater, controlling the nutrients limitation of the biological process. When the process is submitted to nutrients limitation (mainly P), the PPB use the excess of organics to build up PHA that can be extracted as bioplastics. On the contrary, a COD/N/P ratio close to the physiological ratio of the PPB can maximize biomass growth, thereby increasing the potential production of biomass-based organic fertilizers production. Even high P concentrations and irradiation intensity can be tuned up for optimizing P accumulation as polyphosphate, thereby obtaining high-P organic fertilizers. Moreover, diverse valorisation possibilities are presented both by the biogas conversion into valuable products as well as the valorisation of the cellulose recovered from wastewater.

The DP project is still in its infancy, but many researchers are working on an optimization of the process, thereby revealing promising results.


Pre- and pilot photobioreactors
Pre- and pilot photobioreactors (Copyright: Aqualia)

In DEEP PURPLE, the very first treatment plant with the use of PPB in Europe and the biggest in the world will be built. The aim is to scale-up the PPB anaerobic photobioreactor from a state-of-the art treatment capacity of 30 m3 /d wastewater (TRL6) to up 600 m3 /d, which will be the largest PPB photobioreactor constructed so far.
In a conventional wastewater treatment plant, all the secondary sludge is further digested and converted into methane, with a net efficiency of 50-60%. By contrast, DEEP PURPLE attempts to convert almost the 100% of the PPB biomass into bioproducts, comprising both the PHA accumulated during the photoheterotrophic metabolism, as well as the rest of the biomass with high N, P and K composition that will be transformed into organic fertilizers.

DEEP PURPLE will set up two biorefinery platforms in two different locations to produce sustainable biomass feedstock for several industrial sectors.

Pre-pilot photobioreactor
Pre-pilot photobioreactor (Copyright: Aqualia)

The DEEP PURPLE demo sites will convert 438,000 m3 of domestic wastewater and 164 tons of OFMSW into raw matters: 260 tons of enriched biomass feedstock, 16 tons of cellulosic material and 5 tons of biogas yearly for further transformation into raw bio products.

Deep Purple wastewater treatmentBy treating wastewater from 10,000 inhabitants in a DEEP PURPLE photobiorefinery, up to 200 kg of biofertilizers could be produced and 2,500m3 of treated water delivered. Besides, 1,000 kg CO2/day emission would be avoided and the energy surplus could power 400 households.

Optimization of the reactor’s operation is one of the highest challenges in DP as the work includes mixed microbial cultures and competence between microbial communities is unavoidable.

For more information, check out our publications:
“Coupling thermal hydrolysis with anaerobic digestion and photo-fermentation”
“Alkalinity as key factor in domestic wastewater treatment by mixed cultures of purple phototrophic bacteria”