The method applied was LCA as this is a standardized analysis tool for compilation and evaluation of the inputs, outputs, and potential environmental impacts of a product system throughout its life cycle. In other words, it covers the environmental impact of every stage of a product or a system, from the utilized raw materials and their extraction, the production processes of the products, the operational impact “use phase”, and the end of life “disposal” .
This study follows the recommendations of the ISO 14040 and 14,044 frameworks regarding goal and scope definition, inventory analysis, life cycle impact assessment (LCIA), and interpretation, as shown in Fig. 1. The tool used to carry out the LCA is openLCA version 1.10, an open-source software with a broad range of compatible databases and assessment methods that help to quantify the impact of different processes on the environment. The unit processes are selected from ecoinvent 3.5, which is used as the primary database. Ecoinvent provides real-life data in the form of “flows” that are interconnected with the environmental data of these flows .
Goal and Scope of the Study
The overall aim of the Interreg NWE funded interdisciplinary research project “ALG-AD-Creating value from waste nutrients by integrating algal and anaerobic digestion technology” bringing together a group of scientists and engineers from 11 different partners from academics and industry in four countries across the NWE was to develop a circular economy solution to create wealth from waste by the integration of the algae technology into the NRD stream produced by anaerobic digestion plants by extraction and reduction of excess nitrate and making use of the scarce phosphorus. The goal of this study is to assess the life cycle of the ALG-AD technologies for their environmental impacts and to have valuable insights concerning the combination of biogas production and microalgae cultivation.
The three pilot plants are located in the NWE region, specifically, the UK (Langage-SU), Belgium (Innolab-UG), and France (CNRS-UBO). This region is characterized by wet and windy weather with intermediate temperature variations. The pilot plants are designed in combination with or as an extension of three different biogas plants where an AD is used to produce biogas from crops, agricultural, and food waste as feedstock. After dilution and treatment, the liquid fraction from the NRD produced by the AD is applied as a nutrient source to microalgae. The cultivated microalgae are harvested, partially dried, and post-treated as animal feed. The LCA compares the combined algal AD technology with an existing animal feed production, considering that the algal-based feed product will substitute the traditional feed product. This work defines SM from Brazil as the reference product since it is commonly used as animal feed in the NWE region. Hence, overseas transportation and occupation of agricultural lands could be avoided by replacing SM with algae-based animal feed. Figure 2 shows the process outline of the ALG-AD.
Functional Unit and System Boundaries
The overall functional unit (reference product) of the ALG-AD technology is described as “1 kg dry mass of algae-based animal feed”. This functional unit shall consider and credit potential by-products and alternative functions (such as excess energy of the AD process, produced fertilizer, avoided emissions from NRD storage and spread) resulting from the overall system.
The general approach of the LCA is gate-to-gate. It, therefore, does not include production, collection, and delivery of the biomass feedstock for anaerobic digestion and excludes the use of algae-based feed additive in livestock production (end product). The system boundaries of the LCA are shown in Fig. 3. The upstream processes include the production, pre-treatment of the digestate, and microalgae cultivation. The downstream processes include the harvest and post-treatment to process the algal biomass to the feed product. The overall ALG-AD technology system (Fig. 3) consists of four subsystems (SS):
SS1: Anaerobic digestion (including the combined heat and power plant)
SS2: Digestate processing (separation of solids, nutrient separation, and preparation, i.e. dilution with water)
SS3: Microalgae cultivation (inoculation, cultivation, and harvest)
SS4: Algae biomass processing (incubation and spray drying)
Within the system boundaries, the following inputs and outputs are considered:
Material input and output flows and their production processes: biomass feedstock, freshwater, glucose feedstock, additional nutrients, chemicals for cleaning, ammonia emissions during biogas production, and NRD storage
Energy inputs, outputs, and internal usage: for digestate processing, pumping the NRD fertilizer and carbon source supply to the algae culture as nutrient and for pH control, heating and cooling the photo-bioreactor (PBR), and harvesting and processing the algal biomass
Construction and infrastructure: buildings, greenhouse, filters, storage tanks for NRD fertilizer, pipelines, pumps, microalgae cultivation reactors, centrifuge, and incubation tank
The temporal boundary reflects a 1-year baseline period of the technology. Therefore, all data (by considering lifespan and useful life of construction, appliances, or tools) is annualized accordingly.
ALG-AD Pilot Plants and Case Studies Outline and Description
This section describes three different ALG-AD pilot plants with combined algae and biogas technology and their specific settings and configurations: Langage-SU, CNRS-UBO, and Innolab-UG. The description is based exactly on how the real experiments were conducted without any improvement or literature-based protocols. More details about the subsystems are discussed in the “Life Cycle Inventory (LCI)” section.
The company Langage is active along the entire value chain, from the herding of cows to the sale of the dairy product. They have designed a closed-loop system consisting of the Langage Dairy Farm with 250 cows, the Langage AD plant, and the dairy production unit. The company uses bio-fertilizer (digestate) from the AD plant to grow lush grass to feed its Jersey herd, which then produces the rich Jersey milk, which is turned into dairy products such as clotted cream and ice cream and yoghurt. Wastes from both the product outlets and the dairy products factory are brought back to the AD plant to produce electrical power, heat, and fertilizers required to run the Langage closed-loop system.
In this section, the base scenario of the ALG-AD pilot facility Langage-SU is described reflecting that of the actual pilot plant which is located within the AD plant Langage-SU in Plymouth and operated in association with Swansea University (SU), both located in the UK (Fig. 4). Being located within the AD plant complex, the digestate treatment, microalgae cultivation, and harvesting take place in a specially built greenhouse. The AD consists of three digesters fed with food waste and waste from dairy products. A combined heat and power plant (CHP) is affiliated to the plant, where the biogas is used to produce electricity and heat, for internal and external use. The digestate is treated on-site to separate the solid part from the liquid part. The solid part is then sun-dried and transported for use as a fertilizer, whereas the liquid part is further filtered and stored in a digestate storage tank to be used in the microalgae cultivation phase. Concerning the cultivation of microalgae, Chlorella vulgaris is the specie of choice. The culture is first cultivated under phototrophic conditions and then under mixotrophic conditions before the algal biomass is harvested. Through hydrolyzation, the biomass can be treated to increase the bioavailability of the ingredients, respectively, the protein content, and then dried to be eligible for use with the animal feed.
This part entails the base scenario of the ALG-AD CNRS-UBO’s case study, where the processes to be mentioned reflect those of the actual pilot plant located in Brest (France) within the University of Western Brittany (UBO) and being investigated in close cooperation with the French National Centre for Scientific Research (CNRS), both located in France (Fig. 5). UBO is closely interlinked with the company Cooperl which is an agricultural and agri-food cooperative of the Grand-Ouest region of France. Over the years, it has become the French leader in pork with a very strong capacity for innovation and a perfect mastery of all the links in the chain from breeding, genetics, animal nutrition, building equipment, slaughtering and processing, and salting to a network of butcher’s shops and delicatessens and even on-farm shops. Beyond that Cooperl operates a centre based on the circular economy which is dedicated to the recycling of effluents into organic fertilizers, energy (steam, biogas, and biofuels), and recycled water.
The pilot plant for the cultivation of microalgae is located within the AD plant complex, however, in an uninsulated industrial building. The biomass feedstock is solid manure from local farms. Biogas is produced and sold to the national grid, while the energy required for running the plants is bought from the grid. The digestate is centrifuged, and the solid part is then dried and the liquid part delivered to the evapo-concentrator, to be filtered and purified. A heterotrophic-based reactor is used for microalgae cultivation. The species of microalgae used in this pilot is Aurantiochytrium m., which is a genus of eukaryotes. The algal biomass is harvested and treated to be used as animal feed.
In this section, the base scenario of the Innolab-UG pilot facility at Innolab, located in Oostkamp, Belgium, and affiliated to the Ghent University, is described (Fig. 6). Innolab is a laboratory offering a wide range of services in connection with biogas production, purification of waste water, and analysis of biomass. They are a research and service laboratory with more than 10 years of experience in the entire analytical follow-up and technical guidance of biological purification and processing. The AD plant, which is located in Pittem, 20 km away from the pilot plant at Innolab, is using food waste as the main feedstock. A CHP converts the biogas into electricity and thermal energy for internal use and to be fed to the grid. The digestate is treated first at the AD plant through centrifugation. The solid part of the digestate is dried and used as a fertilizer, whereas part of the liquid part is taken to the microalgae pilot plant Innolab-UG and further filtered before added to the PBR. Microalgae species mixture of Chlorella and Scenedesmus is used. After inoculation in the lab, the culture grows in a multi-layer horizontal PBR, and the microalgae are continuously circulated in the PBR under the sun and flow through a dark tank triggering a mixotrophic phase.
General Assumptions for the Three ALG-AD Pilot Plants
The pilot facilities of the ALG-AD are assumed to operate in the same location without any renovation and under the same settings and policies for 11 months of the year. For 1 month, the plants are not in operation to allow for maintenance work and major cleaning. This period of inactivity is divided into 4 weeks, and it is assumed that this production stop will happen during 1 week every 3 months. However, regarding the ALG-AD pilot plant CNRS-UBO in France, the facility is assumed to be running for 12 months a year due to the simplicity of cleaning and maintaining the reactors.
Energy consumption during the inoculum preparation at the ALG-AD pilot plants Langage-SU and Innolab-UG was not considered, as it is done only at the beginning of the pilot plant operation and due to the negligible impact of this process in comparison to the whole system . The embodied energy during the production and transport of equipment was not considered due to the complexity of acquiring such information. This complexity is based on the lack of reliable communication with the manufacturers and/or confidentiality. Considering a lifetime of only 1 year, constructions and equipment used within this pilot facility are assumed to have a maintenance-free 1-year operation. Since the pilot facility Langage-SU in Plymouth is located on-site of the AD plant, no transportation impact is included. The microalgae cultivation is assumed to be stable with continuous production throughout 1 year and with no contamination or crush of cultures. All the processes are done on-site with no transportation requirements to subcontractors. There was no monitoring of emissions within the pilot facilities, and hence, data for gaseous emissions were not included.
Life Cycle Inventory (LCI)
Data acquisition is a very challenging yet sensitive step in LCA. It depends on the reliability of the source, from a feasibility point of view and a veracity point of view. It also depends on the compatibility of these data with the database that is being used in the assessment, because in most cases, it is very hard to obtain and create LCA elements without having the background data at hand. Be it personal communication or literature review, there are challenges with the ease and willingness of providing the data in a profound manner and challenges with the accuracy, punctuality, and availability of the data, especially for new technologies and processes. In this study, primary data from the project partners working in the pilot facilities are used; however, some literature-based data was essential to create elements that were not provided or found on the database. Information about the equipment was gathered from the manufacturers’ websites and/or by interviewing colleagues from the relevant technical departments. Background data was based on ecoinvent database v3.5 .
Table S1 shows the inputs and outputs of the three ALG-AD pilot plant base scenarios presenting two different cultivation technologies, which are compared during this assessment. Inputs and outputs of the LCA model including equipment, materials, and consumables data are found in Table 1. The negative numbers for the nutrients nitrogen, potassium, and phosphates displayed in Table 1 are given to acknowledge and model the reduced environmental impacts by the avoided nutrient excess from the digestate, which is up taken by microalgae instead of being wasted as shown in the cultivation section of Table 1. All equipment used at the microalgae facility is modelled according to available data, either from the partners, manufacture websites, or contacts in the industry. The details for the equipment mentioned as items in Table 1 will be found in Table S2 in the supplementary information.
Anaerobic Digestion and Biogas Plant (Subsystem 1)
In this section, details about the AD plant at each of the three ALG-AD pilot facilities are communicated. Since the AD plants at the three locations are not entirely part of the project, not enough data could have been provided by the operators for a detailed description of subsystem 1. Therefore, for this study, readily available AD plants, as well as CHP units taken from the ecoinvent database v3.5, were scaled accordingly to conform to a more realistic and consistent estimate.
The Langage-SU AD plant comprises three mesophilic digesters each of 1, 000 m3 in volume. The digester is fed with around 20, 000 t/year of feedstock from food waste and dairy. The AD plant produces 80, 000 t/year of digestate and 1.8E6 m3/year biogas. The total produced energy by the CHP unit is around 500 kWh/year, from which the pilot facility is powered. However, the data used in this study is based on a readily available process on ecoinvent v3.5  and is scaled up to the size of that of the AD plant in Langage. For the CHP, also a readily available process from ecoinvent is used, representing the one in Langage-SU.
The AD plant located in Lamballe, France, has a mesophilic digester volume of 15, 000 m3 and is fed with wastewater from the slaughterhouse, pig manure, and recycled water. The plant produces around 156, 000 t of raw digestate per year and 530 m3/h of bio-methane, purified and fed to the gas grid. For this study, the data used for the AD plant is based on the readily available AD process, which was also used for Langage, however, employing a higher scaling factor. As Systemic does not have a CHP unit on site, subsystem 1 for CNRS-UBO comprises only the AD plant.
The AD plant belonging to the ALG-AD system of Innolab-UG is not located at the PBR site but in Pittem. With a capacity of 180, 000 t of feedstock per year, the four digesters and a post-digester with a total volume of 20, 000 m3 are fed with organic bio-waste, manure, and other waste sources. Through the CHP unit, the plant can produce around 32, 500 MWh and 29, 100 MWh of heat and electricity per year, respectively. The readily available AD process  is scaled up to model the one in Pittem. Two readily available CHP processes are selected with the same load for the CHP unit.
Digestate treatment (Subsystem 2)
After disclosing the data used for the AD plant and the CHP unit, subsystem 2 (see Figs. 4, 5, 6) considers the treatment of the digestate before adding it to the cultivar. In addition to the on-site treatment of the digestate at the AD plant, where separation of the solid digestate is essential for fertilizing applications, the digestate is further treated at the pilot facility for sterilization and further separation of the concentrate. The technology used at each location and its details are described in the following subsections.
At Langage-SU, the digestate is treated at the microalgae facility through filter membranes. A 0.2-μm mesh size filter membrane is used along with a dirty pump for 2 h for every 150 L digestate treated. The solid part is separated and dried and added to the solid fertilizer. The liquid portion is stored in a 1, 000-L intermediate bulk container (IBC) tank at the facility to be ready for use during the cultivation process.
At Lamballe, France, the digestate is treated in two stages, the first is through a decanting centrifuge with the addition of polymer, and the second is through an evapo-concentrator. The treated digestate is then stored in a 1, 000-L IBC tank.
After treating the digestate at the AD plant in Pittem, some 80 L per week are transported to the microalgae facility in Oostkamp. The digestate is further filtered using the paper filtration technique to eliminate more solid chunks.
Microalgae Inoculation, Cultivation, and Harvesting (Subsystem 3)
This is considered to be the main subsystem of the ALG-AD technology. It includes inoculation, cultivation of the algal species, and harvesting the algal biomass. For the three facilities, the inoculation process differs mainly in the algal species inoculated and the frequency of inoculation. There are many differences between the facilities for the cultivation, as will be discussed in the following subsections. This is also true for the different harvesting processes and frequencies.
The inoculation process starts in three lab-scale glass flasks of 1 L each and then moved to carboy bottles of 5 L each before they are added to three 80-L polyethylene bags filled with water for final inoculation. The nutrients used during inoculation are in the form of F/2 media. After treatment, the growth medium is moved to the 7, 000-L vertical tubular PBR (V-PBR) and diluted with fresh tap water. The tap water is chemically treated with bleach and sodium thiosulphate and is set in continuous motion using pumps to prevent bacterial contamination. The cylinders of the V-PBR are made of polymethyl methacrylate sheets (PMME), whereas the pipework and valves are made of polyvinylchloride (PVC). Stainless steel frames are used to support the PBR structure.
During cultivation, filtered digestate from subsystem 2 is added at an average of 90 L/batch. Liquid CO2 (31 L/h), which comes in pressurized bottle packs, is continuously added to adjust the pH of the culture. A heat pump and a lighting system consisting of fluorescent lamps with an annual energy consumption of 34 MWh and 1.8 MWh are employed. For the circulation of the culture, a pump is used with an annual energy demand of 2 MWh. After 5 days of cultivation, around 15% of the cultivar is filtered in a 0.2-μm mesh size filter membrane to concentrate the culture preparing for the mixotrophic phase. The mixotrophic bioreactor (MBR) is a downsized version of the PBR, where only around 250 L of the cultivar can be grown before centrifugation. The algal biomass production increases from a concentration of 0.4 g/L in the PBR to 6 g/L after membrane filtration and finally to 12 g/L after 48-h growth period in the MBR. The mixotrophic phase requires an additional carbon input provided in the form of 3 kg dextrose per batch of 250 L. A suspended centrifuge runs for 6 h every batch for the harvesting process, yielding 270 g/L algal biomass paste.
This ALG-AD pilot CNRS-UBO differs from the other two ALG-AD facilities as here no prokaryotic microalgae but a unicellular eukaryotic heterotrophic fungus-like clade of Stramenopiles is applied.  Since they are considered as an increasingly important source of polyunsaturated fatty acids (PUFAs) for biotechnological industries, these organisms are subsequently referred to in some literature and marketing sources as being derived from “algae”, despite their non-photosynthetic source organism.  The species Aurantiochytrium m. applied in CNRS-UBO belongs to the family of Thraustochytrids, which are associated with two main features responsible for the increasing interest in this family of microorganisms. They display high growth performance, reaching very high cell concentrations in a few days, and exhibit the striking ability to produce and accumulate docosahexaenoic acid (DHA).  In contrast with most microorganisms that produce saturated fatty acids as energy storage lipids, Thraustochytrids synthesize long chain-polyunsaturated fatty acids (LC-PUFAs), like DHA, as energy storage.
Aurantiochytrium m. is cultivated under heterotrophic conditions in batch system production. This entails that the inoculation is freshly prepared for every batch. The inoculum is prepared in 1-mL cryovials stored at − 80 °C then left for 6 days with the addition of 20 g glucose, 2 g yeast, 2 g peptone, and 15 g sea salt for every litre. The inoculum is then moved into 24-L carboy bottles filled with water. The bioreactor used at CNRS-UBO comprises three PET columns of 800 L total volume and 500 L active volume each. As the cultivation type is heterotrophic, this eliminates the application of lighting systems and carbon dioxide. However, to ensure optimal growth conditions, the aforementioned additives are added with the following quantities: 2.5 t/year of 900 g/L corn syrup as a source of glucose, tap water, and sea salt for digestate dilution and a mixture of yeast and peptides of around 4 kg/year each, diluted in tap water. Three IBC tanks of 1 m3 each are used to store those three additives. Circulation pumps are used at the beginning of each batch during the addition and starting of the culture, whereas an air-blower running at one-fourth of its power rating is operating continuously. The reactors are cleaned after every batch with water. The cultivar is harvested twice a week through a tangential flow filtration system that concentrates the diluted algal biomass to an average of 90 g/L biomass paste to be further processed in subsystem 4.
At Innolab-UG, the inoculation is like that at Langage-SU, done only at the beginning of the cultivation period. Lab-scale inoculum preparation follows almost similar procedures like the one at Langage-SU with the only difference regarding the source of nutrients, whereas at Innolab-UG treated digestate in place of F/2 media is applied. A horizontal tubular PBR (H-PBR) with a volume of 800 L is used for the cultivation process. The lifetime of the H-PBR at Innolab-UG is estimated at around 10 years as it is made of glass, whereas that made of plastic would attain scratches from scrubbing and cleaning that would impact the light penetration through the tube walls, hence estimated to have around only 5 years lifetime. After filling the H-PBR with the inoculum and adding UV-treated tap water, the cultivation process runs for almost 3 weeks before being harvested. During this period, liquid carbon dioxide (12 L/h) is supplied through pressurized bottles to adjust the pH of the cultivar. The cultivar grows on sunlight during the daytime; however, few fluorescent lamps are utilized to keep the culture from crashing during dark hours. The treated digestate is supplied at a rate of 30 L/month. After reaching a biomass concentration of around 2.7 g/L, about 500 L of the cultivar are filtered in a 0.2-μm mesh size filter membrane reaching a concentration of 150 g/L.
Biomass Treatment (Subsystem 4)
Subsystem 4 covers the post-treatment of the algal biomass. The hydrogenation of the harvested algal biomass is carried out once for all three facilities simultaneously. Therefore, the data used for the LCA model is identical for all ALG-AD pilot facilities. However, each facility would be modelled to have the hydrolyzation equipment on-site. These are incubation tank with temperature and pH control, settling tank, and spray dryer. During the hydrogenation process, water for dilution is added depending on the amount of the harvested biomass with a 1:1 ratio and protease as the enzyme for hydrolyzation. Then, the hydrolysate is heated to 90 °C to stop the enzyme activity. After hydrolyzation, the hydrolysate is left to settle in an IBC tank as a conditioning step before going through the spray dryer. Modified starch is added to the biomass being dried as a protective agent to the algal biomass and ascorbyl palmitate, which acts as an antioxidant.
Soybean Meal Production (Reference Scenario)
Data used to model the soybean meal (SM) scenario is based on “Life cycle inventories of bioenergy”.  which considers materials energy consumption, Land transformation and emissions. In contrast, data for overseas transportation is acquired from  the Central West (CW) scenario in Brazil, where more deforestation and transport take place.