Beverage wastewater to single cell protein

1. Abstract

Nowadays, there is an increasing need to treat wastewater using microalgae from agriculture, industrial, and municipal areas. There are many conventional techniques such as the application of anaerobic digestion, biofilms, and membrane separation with varying pore dimensions that have already been widely applied and commercialized. This paper analyzes the traditional use of microalgae in wastewater treatment processes, identifying their drawbacks and current limitations while exploring potential solutions. We focus on three microbial protein production processes, each involving different bacteria: Microalgae (MA), Purple Phototrophic Bacteria (PPB), and Aerobic Heterotrophic Bacteria (AHB). The design of reactor systems for these processes is critically evaluated and optimized. In addition, the potential system boundaries and scenarios are designed for treating the beverage wastewater, with the aim of converting the beverage wastewater into the fish feed through the three processes. The carbon emissions from each stage of these processes are assessed using the life cycle analysis (LCA) techniques. The results are compared with each other to highlight differences in environmental impact across various aspects. The LCA examines factors such as feed and effluent flow rates, the use of fish feed alternatives, electricity, and other energy inputs. All findings are benchmarked against official data, adhering to the standards set in Singapore.

2. Background

In 1965, Singapore was partially self-sufficient in food supply with farmlands occupying approximately 25% of land. However, by 2014, farmlands occupied less than 1% of the land in Singapore. Hence, Singapore is reliant on the 160 countries which it imports food from (Ludher, 2016). Although the overall increase of food consumption was minimal, a breakdown of the food consumption across different categories showed that consumption of vegetables, fruits, and proteins (chicken, pork, and eggs) increased significantly while that of rice reduced drastically. This shows that there is a significant demand for protein among people (Boland, 2013). However, traditional methods of obtaining protein, primarily through animal and dairy production, are ultimately unsustainable. Therefore, it is crucial to explore alternative sources to meet the protein needs of both humans and animals.

One promising solution is the production of microbial protein supplements for animal feed from wastewater streams. In modern society, there is an increasing need for efficient treatment of wastewater from industrial, agricultural, and municipal sources. Singapore, in particular, achieves a 100% wastewater treatment rate (Liao, 2021). The procedure not only effectively treats organic wastewater but also converts organic matter into high-value protein products, reducing environmental burdens while providing a cost-effective and sustainable alternative protein source for agriculture and aquaculture. Meanwhile, the novel wastewater treatment method can avoid the drawbacks of traditional ones, such as anaerobic digestion and membrane separation technologies, are often associated with long start-up period, high energy consumption, operational costs, and environmental drawbacks, limiting their potential for large-scale commercialization (Liu, 2004).

3. Introduction and literature review

Currently, microbial protein has huge potential and prospects in wastewater treatment processes and reflects the implementation of renewable waste energy sources well. However, there still exist challenges in wastewater treatment by utilizing microalgae, and lots of applications are not commercially favored.  There are enormous techniques related to treating wastewater effluents using different types of algae and the results show high efficiency in pollutant removal by manipulating the COD values. For instance, the nanofiltration system developed recently for treating the wastewater from the yeast industry can increase COD retention and promote color and electro-conductivity (EC) removal by showing a decrease in permeate flux after long-term operation (Rahimpour, 2014). In the agricultural and livestock domains, it has already been shown that microalgae can promote the treatment of wastewater from food production.

The research of spiller emphasizes the treatment of organic wastewater to single cell protein by using the comprehensive LCA to evaluate the environmental effects of the microbial protein from potato wastewater which is treated as one of the food effluents as the substitution of the conventional soybean meal as one of the examples. In this case, the high-strength potato wastewater is treated under three different microbial conditions which are Aerobic Heterotrophic Bacteria (AHB), Microalgae together with AHB (MaB), and Purple Non-Sulfur Bacteria (PSNB) (Spiller, 2020). Since microalgae can decrease the amount of nutrients in wastewater by lowering the BOD and COD value significantly, it can be applied as a remediation agent although the chemicals and energy costs are relatively high (Udaiyappan, 2017). The entire removal of microalgae after the completion of treatment is necessary for the complete organic content assimilation.

To estimate the overall carbon emission during the single cell protein production process, the LCA mainly focuses on the environmental impact analysis of each stage or defined functional unit, and the schematic approach is designed to evaluate the entire process from raw material extraction, production, distribution, usage and disposal. The target of LCA is to identify the opportunities to reduce the negative environmental impacts during the fish feed production, transportation and anaerobic treatment processes, which can not only contribute to the design of more sustainable fish feed products and provide the comprehensive view of environmental impact of the whole process, but also inform. the regulation and standards for sustainability to provide a sound basis for decisions. For the system boundaries proposed for AHB, PPB and MA digestion processes, all of them are designed for cradle-to-cradle LCA and cradle-to-grave LCA is applied for embranchment of some functional units in which extra landfill process is required since some of the byproducts are not suitable for fish feed or SCP production. There are two commonly used methodological LCA approaches which are consequential LCA and attributional LCA. The latter one focuses on the environmental exchanges and product system description by analysing the data retrospectively ^[19], which means that it analyses environmental impacts of products that has already exists while the consequential LCA emphasize on the changes in the design of functional units that results in actions or decisions and broader systems effects. The attributional LCA is applied for this project since the system boundaries has already been fixed and the goal is to figure out the direct impact of associated with specific fish products. The system boundary in this design covers the supply chain from beverage wastewater influent to fish feed production and the function unit is defined as 1 tonne of wastewater that treated to obtain fish feed. And the downstream life cycle steps including consumption and waste management are not considered because it is not relevant for decision making. The inventory data can be categorized into energy and wastewater resources input together with byproduct wastes and main fish feed products output.

The aerobic heterotrophic bacteria (AHB) are defined as the bacteria that can utilize oxygen to maintain their metabolism and it can also gain carbon from different organic compounds. The AHB can be mostly found in soil environment since they can form. spores to survive in adverse conditions. The key feather of AHB is that oxygen is the compulsory requirement for their survive and thrive. And organic matter is consumed for energy and carbon since they are not capable of producing their own food. The AHB plays an important role in decomposing the organic material by breaking down the complicated substances into simple compounds such as monomers and oligomers. What’s more, AHB contribute tremendously to the nutrient cycle within ecosystem by assisting the bioremediation of contaminated environments and indirectly improve the soil fertility and contribute the plants health support. Different from AHB, purple phototrophic bacteria (PPB) are the groups of photosynthetic microorganisms that rely on the light as their primary energy source. They can be discovered in low oxygen level conditions includes lakes, ponds, and other aquatic environments. However, the oxygen is not produced as the byproduct through their unique way of metabolism. The bacteriochlorophylls and carotenoids are the two major pigments contained in PPB which can provide the color variance that ranges from brown, red and green etc ^[20]. And purple is the predominant color that is shown in most of the species. Since PPB do not produce oxygen when capturing light energy, they utilize sulfur, hydrogen sulfide and other organic matters to perform. anoxygenic photosynthesis and such process results in the production of sulfur compounds as byproducts, which is required to be further treated by extra stages. The PPB can be classified into two groups that are purple sulfur bacteria and purple non-sulfur bacteria. The former one applies sulfur as the main electron donor in their metabolism and the latter can utilize a wide range of organic substances so that it can be discovered in less-sulfur rich environments. Besides contributing to the nutrient cycle, the PPB plays a crucial role in sulfur recycling and organic compounds in anoxic conditions. Both PPB and AHB are researched in bioremediation and wastewater treatment by studying their adaptability, photosynthesis process and potential applications.

Microalgae (MA) are a diverse group of photosynthetic microorganisms that encompass various species.  Based on eukaryotic and prokaryotic division, microalgae are further divided into two different groups. Microalgae primarily inhabit aquatic ecosystems but are also found in terrestrial environments and extreme conditions such as saltwater, soil, sand, and snow (Mata et al., 2010). They are essentially unicellular, though some multicellular microalgae also exist, either as free-living individuals, colonies, or in symbiosis with other microorganisms (Radmer, 1996). Depending on the species, their average size ranges from 2 µm to 2 mm, and they have a simple morphology, typically either motile or non-motile. As photosynthetic organisms, microalgae are among the largest contributors to oxygen production, capable of synthesizing half of the oxygen in the atmosphere, which will contribute to the reduction of carbon emission (Metting F.B., 1996). Based on characteristics such as structural features, pigments, composition, and product storage, microalgae are classified into several groups. Currently, between 200,000 to 1 million species of microalgae have been recorded (Trevor et al., 1996)). Among the numerous species, green algae, cyanobacteria, Spirulina, diatoms, and golden algae are the most widely utilized due to their unique characteristics and the high-value compounds they produce. These microalga types are applied extensively in food, biofuel, health, and environmental sectors, driven by their rapid growth rates, high productivity, and adaptability to diverse cultivation conditions. To manufacture single-cell protein (SCP) applications in animal feed industries, green algae are widely used for their high protein content and balanced amino acid profiles. Additionally, green algae can be cultivated in wastewater environments, thus contribute to the waste 0water treatment. During their cultivation, they will absorb excess nitrogen and phosphorus, which reduces water pollution and contributes to bioremediation efforts (Becker, 2007).

4. Aerobic Heterotrophic Bacteria (AHB)

4.1 Functional units and system boundary design

Figure 1. Schematic diagram of system boundary design of the AHB system.

The overall processes of treatment by AHB can be divided into six steps. For the first step all the beverage wastewater streams will be mixed and adjusted into suitable pH by addition of buffers and other substance rich in nitrogen and phosphorus so that its chemical stability is ensured. The mixing process can be supported by continuous stirred tank reactor (CSTR) in series so that the conversion of the unnecessary compound can be higher than single CSTR and the temperature, pressure and retention time of each reactor can be optimized easily. And then design of anaerobic treatment system is the most important step because this is the step where the complex substrate with high molecule weights will be decomposed into monomers or oligomers so that it becomes easier for anaerobic microorganisms to digest and break down in order to produce biogas and other digestates. The key point in the anaerobic treatment is that the oxygen is entirely excluded so that the final yield of desired products is ensured. In this step the high-rate anaerobic treatment facility is selected to enhance the efficiency of anaerobic beverage wastewater treatment process. The advanced technology selected in this system is up-flow anaerobic sludge blanket (UASB) reactor, which increases the contact between beverage wastewater and biomasses promoting the effective treatment process. For UASB reactor the water pass through the system upwards and degraded by anaerobic organisms and combined with the gravity suspends’ settling actions and with the addition of fluctuant and the biogas which is considered as the byproduct that can be used as power sources of electricity generation is collected at the top of system ^[10]. The sludge is formed as granules in the surface that aggregated by different bacteria. As the flow conditions vary, the selective environment is provided for those microorganisms to aggregate with each other and proliferate to form. dense biofilms. And the heat produced through electricity generation can also be reused to heat the digestion tank. The AHB photobioreactor was chosen in this case and it utilizes sunlight as primary energy sources to support the photosynthesis processes of microorganisms. And extra solar panels are required to be added for the collection of solar energy. Compared with other reactors, the open system photobioreactor has the advantage of simplicity and low maintenance cost. However, it must be exposed to outside environments together with the pond and raceway design. As a result, it is selected to apply the tubular closed system design since the light, temperature and contamination is easier to control. Additionally, the hollow fibre membrane is selected for the nanofiltration of the digestates because it has the advantage of providing large surface area for filtration in compact form. of tubular structures. And it is capable of filtering out the microorganisms and small particles efficiently. What’s more, it can be arranged in buddle so that it can be applied in large scale. However, the fouling and aging problems cannot be ignored for maintaining of sufficient separation performance. The reason why applying nanofiltration is that the general dimension of aerobic heterotrophic bacteria is around 0.5-5 μm and the pore size for general nanofiltration is around 0.5-2 nm and feeding pressure is approximately 10-25 bar ^[11]. The spray drying that atomizing the feed biomass into fine droplets and then rapidly drying those droplets by using hot air can well-preserved the quality of biomasses and the morphology and size of final digestate powder can be easily controlled during the drying process. But the inaccurate control of temperature will lead to the formation of byproducts and further separation processes are required. And the uniformity of biomass particle sizes can also be one of the challenges to handle. The quality of final dried AHB is heterogenous and some of the undesired products must be handled by direct landfill. The trucks are used for the transportation of dried digestate residue to landfill and fish feed production and the carbon footprint produced is significant. All the functional units in the schematic diagram are compared with the industrial standard to obtain the avoided wastewater treatment load, avoided fish feed and avoided transportation emissions. The goal of this design is to lower the environmental effect and investment that is associated with treating wastewater and make it into the more sustainable approach to water resource management.

4.2 LCA Calculation and discussion of AHB

Table 1. the input parameters and output yield for AHB treatment system

The important data of system boundary and functional units design of AHB is selected for LCA calculation path is listed in table 1. Define the avoided wastewater treatment unit as the 1L conversion of wastewater treatment and the detailed LCA calculation path for treating 1 tons of wastewater is shown as follows:

The COD contained in 1 ton of beverage wastewater (approximated the density of water as 1kg/m³) : 1000L × 1698mg/L =1.698*10⁶mg

The amount of COD in feed per day: 350L/day ×1698mg/L = 594300mg/day

Since the removal efficiency is shown as 96.72%, the treated amount of COD is calculated as: 96.72% × 594300mg/day = 574809.96mg/day

The carbon footprint of beverage wastewater treatment can be approximated as the 0.7 kgCO2 eq/m³ of wastewater which represents the average value of former literature studies ^[12].To deal with the sludge produced during the AHB treatment process, it is found that the specific energy consumption for sludge dewatering can be approximated as 0.026 for influent pumping, 0.032 for primary settling tank and 0.024 for secondary settling tank (CSTR in series), 0.007 for anaerobic treatments, 2.116 for sludge dewatering in the unit of kWh/m^{3 [13]} and these energy are all  reflected in the electricity consumption which increases the total power required. And all the final treated sludge are transported for the biological fish feed production. It is discovered from previous literature that the carbon footprint formed during the fish or shrimp feed production is 0.9883 kgCO2eq/kg ^[14]. Since all the dried biomasses are packed and transported to different industries, the carbon emissions of 16-tonne truck which is 0.05kgCO2eq/tonne·km ^[15] is taken as the factor to evaluate the environmental impact caused by transportation. Since the AHB is functioning in the photobioreactor in which only the solar energy is applied, the carbon emission can be taken as 0.05kgCO2eq/kWh which is the carbon emission factor of most of solar panels ^[16]. And the landfilling process can also produce the average carbon emission of 20gCO2eq/kg ^[15]. Applying all the parameters discovered, the carbon footprint caused by wastewater treatment per day is computed as:

0.7kgCO2eq/m³ × (350.00L/day – 314.10L/day) × 0.001m³/L = 0.0252kgCO2eq/day

Combine with the carbon footprint of electricity generation in Singapore which is 0.412kgCO2eq/kWh ^[17], the extra carbon footprint required for CSTR operation, sludge drying, anaerobic treatment and influent pumping is calculated as:

(0.026kWh/m³ + 0.032kWh/m³ + 0.024kWh/m³ + 0.007kWh/m³) × 350.00L/day × 0.001m³/L × 0.412kgCO2eq/kWh + 35.90L/day × 2.116kWh/m³ × 0.001m³/L × 0.412kgCO2eq/kWh = 0.044kgCO2eq/day

Assume the sludge is produced in slow settling process, the density can be approximated as 1.038kg/L ^[18]. The total carbon emission of AHB photobioreactor, fish feed production and lan filling process can be calculated as follows:

For photobioreactor: 2.40kW × × 0.05kgCO2eq/kWh = 80.223kgCO2eq

For fish feed production: 1.038kg/L × 35.90L/day × 0.9883kgCO2eq/kg = 36.828kgCO2eq/day

Above all, assume that the distance required for the transportation of fish feed is 100km, the total carbon footprint produced in the system boundary can be calculated as:

(0.0252kgCO2eq/day + 0.044kgCO2eq/day + 36.828kgCO2eq/day) × + 80.223kgCO2eq + 0.05kgCO2eq/tonne·km × 100km × × 35.90L/day × 1.038kg/L × 0.001tonne/kg = 1113.19kgCO2eq

Table 2. the sum of LCA results of AHB treatment system

5. Wastewater to Single Cell Protein through Microalgae

5.1 Introduction of Microalgae-based Method

Microalgal-based wastewater treatment has emerged as one of the most promising approaches for both wastewater purification and nutrient recovery (Cai, 2013; Whitton, 2015). This technology has garnered increasing attention due to its high potential value across industrial and commercial sectors. The core of this process is a symbiotic relationship between microalgae and bacteria, which plays a pivotal role in enhancing treatment efficiency. This symbiosis provides protective mechanisms for the microalgae against toxic compounds present in wastewater, allowing them to perform. photosynthesis efficiently. Through this process, microalgae utilize carbon dioxide to produce oxygen, which is subsequently consumed by bacteria within the wastewater. The bacteria, in absorbing this oxygen, initiate the breakdown of organic carbon, nitrogen, and phosphorus compounds in the wastewater. Concurrently, bacterial will release carbon dioxide, inorganic nitrogen, and phosphorus (Oswald, 1957). These byproducts, in turn, serve as essential nutrients for the microalgae, thereby supporting their growth and enabling a sustained photosynthetic cycle.

Through this process, microalgae can effectively reduce nutrient levels in wastewater to extremely low concentrations, as they absorb these nutrients to fuel their growth. This capability not only helps prevent the eutrophication of natural water bodies but also enables treated water to meet rigorous discharge standards, further highlighting the environmental benefits of this symbiotic system. Furthermore, once the microalgae have proliferated, they can be harvested and processed as a valuable biomass. Through bioprocessing or other methodologies, this biomass can be transformed into raw materials for various applications, including the production of single-cell protein, among other bio-products (Clarens, 2010). This dual-purpose approach, which combines wastewater remediation with the generation of commercially valuable bioproducts, underscores the substantial potential of microalgal technology in sustainable wastewater management and resource recovery.

5.2 Functional units and system boundary design

Figure 2. Schematic diagram of system boundary design of the MA system

The process of converting beverage wastewater into fish feed involves a series of carefully controlled stages, each designed to maximize resource recovery and minimize waste. The initial step is the pre-treatment of wastewater, which stabilizes the pH and nutrient levels to ensure optimal conditions for subsequent processing. In this stage, beverage wastewater is treated with buffering agents (NaHCO₃, KH₂PO₄, Na₂CO₃) and nutrient supplements rich in nitrogen and phosphorus (CO(NH₂)₂, NH₄Cl, Phosphate Salts), enhancing its chemical stability. A continuous stirred tank reactor (CSTR) setup in series is used for this purpose, allowing precise control over variables such as temperature, pressure, and retention time. This configuration facilitates more efficient mixing and reaction, converting unwanted compounds and optimizing the wastewater’s readiness for anaerobic digestion.

The anaerobic system is the next stage in the process, where complex organic compounds are decomposed by anaerobic microorganisms into simpler molecules. This breakdown generates biogas, primarily methane (CH₄) and carbon dioxide (CO₂), which are collected as valuable by-products. During the anaerobic digestion process, biogas is generated, primarily composed of methane (CH₄) and carbon dioxide (CO₂). This biogas can be collected and utilized as a renewable energy source, either for direct combustion to produce heat or through biogas engines to generate electricity. This generated energy can support subsequent steps in the process, reducing the reliance on external energy sources and enhancing the overall sustainability of the wastewater treatment system. The CO₂ produced in this stage can be used directed to the microalgae photobioreactor, providing a carbon source to support microalgal photosynthesis. The anaerobic system operates in an oxygen-free environment, utilizing an up-flow anaerobic sludge blanket (UASB) reactor to enhance wastewater and biomass contact. This reactor design promotes effective digestion, allowing wastewater to pass upward, interact with dense microbial biomass, and release biogas at the top.

In the microalgae photobioreactor, the anaerobically treated wastewater, along with CO₂ and air, is utilized to cultivate microalgae through photosynthesis. This closed tubular photobioreactor is chosen for its ability to maintain controlled conditions, such as light, temperature, and contamination levels, which optimize microalgal growth and productivity. The microalgae assimilate CO₂ and other nutrients from the treated wastewater, resulting in high-nutrient biomass that serves as the basis for fish feed production.

After cultivation, the biomass moves to a membrane system, where it is separated into a liquid effluent and a solid component. The effluent, which contains dissolved nutrients, can be recycled within the system. The solid fraction is nutrient-rich and can be further processed into fertilizer, providing an additional sustainable product from the process. A hollow fiber membrane is used in this stage, offering a large surface area for filtration and effectively separating microorganisms and particles from the liquid.

The concentrated microalgae biomass is then subjected to spray drying, a process that removes moisture from the biomass and converts it into a stable dried microalgae (MA) powder. This powder is rich in protein and can be used as a high-quality fish feed. Spray drying involves atomizing the biomass into fine droplets, which are rapidly dried by hot air, preserving the nutritional content of the microalgae. This drying method allows for control over particle size and morphology, ensuring uniformity and ease of handling in downstream applications.

Finally, the transportation stage involves packaging and transporting the dried microalgae powder to fish farms, where it is used as a nutrient-rich feed source. The inclusion of dried microalgae in fish diets supports aquaculture by providing a sustainable, protein-rich alternative to traditional feed ingredients. This entire process exemplifies a closed-loop system, where wastewater is transformed into valuable products such as fish feed and fertilizer, reducing environmental impact and supporting resource-efficient water management.

5.3 Life Cycle Inventory

Pre-treatment Stage: pump1

Inputs: Beverage wastewater, buffering agents (sodium bicarbonate, potassium dihydrogen phosphate), nitrogen and phosphorus supplements (ammonium chloride).

Outputs: Chemically stabilized wastewater, with optimized pH and nutrient levels for microbial treatment.

Anaerobic System: pump2

Inputs: Pre-treated wastewater, electricity (for mixing and temperature control).

Outputs: Methane (CH₄), carbon dioxide (CO₂), dewatered sludge (partially recycled for fertilizer production), and a small amount of discharge water.

Microalgae Photobioreactor: pump3

Inputs: Anaerobically treated wastewater, CO₂, light, air.

Outputs: Nutrient-rich microalgae biomass.

Membrane Filtration System:

Inputs: Microalgae culture, electricity (for pumps and maintaining pressure).

Outputs: Solid and liquid fractions.

Solid: Nutrient-rich residue used as fertilizer.

Liquid: Effluent for discharge or recirculation within the system to reduce water consumption.

Spray Drying:

Inputs: Concentrated microalgae biomass, electricity and heat

Outputs: Dried microalgae powder, water vapor.

Transportation:

Inputs: Fuel.

Outputs: Transportation emissions (CO₂).

5.4 Goal and Scope

Chlorella is rich in protein and essential amino acids, making them ideal candidates for single-cell protein (SCP) production used in food and feed industries. In this part a Life Cycle Assessment (LCA) of using Chlorella sorokiniana will be conducted for single-cell protein (SCP) production from beverage wastewater, focusing on eutrophication potential (EP), global warming potential (GWP), and overall energy consumption. The primary goal of this LCA is to analyze the environmental impact of converting beverage wastewater into fish feed. The functional Unit is defined as the production of 1 kilogram of dried microalgae (MA) powder as fish feed. The stages covered are Wastewater pre-treatment, Anaerobic digestion (UASB reactor), Microalgae cultivation in photobioreactors, Membrane filtration, Spray drying, and Transportation to fish farms.

5.5 GWP Calculation

The Global Warming Potential (GWP) was assessed to quantify the total greenhouse gas emissions resulting from the treatment of 1 liter of beverage wastewater. The GWP calculation includes methane emissions from the anaerobic digestion stage, CO₂ emissions associated with energy use, and CO₂ emissions from transportation.

According to Alexandre’s report, the main parameters of the anaerobic digesters are show in the following table.

In the anaerobic digestion stage, methane (CH₄) is produced as a by-product. Given that the system consists of two reactors with a combined volume of 1252 m³ and an organic loading rate of 3.23 kg VSS/m³/day, the methane production per day can be calculated as follows:

Daily Methane Production= 0.62 m3·kg−1 VSS×3.23 kg VSS·m−3·day−1×626 m3=1253.62 m3 CH₄·day−1. The daily flow rate is 62.6 m3·day−1=62,000 L·day−1, Methane production per liter of wastewater=1253.62 m3/62,000 L·day−1=0.02 m3 ·L−1, The density of methane at 35°C is 0.66 kg·m−3, so the Methane production per liter of wastewater can be converted to 0.02 m3 ·L−1×0.66 kg·m−3=0.0133 kg CH₄·L1.The global warming potential factor of CH₄ is 25, so the CO₂ equivalent per liter=0.0133 kg CH₄·L1×25=0.33kg CO₂-eq/L

Assume the depth of the system is 4 meters, the surface area would be = 626 m³/4 m = 156.5m2. Using the specific power consumption of 10 W/m³ for mixing (from Mendoza et al., 2013), we would calculate the energy demand as: Mixing Energy= 10 W/m³×626 m3=6260W=6.26kW. The retention time is 20 days, so the annual operation days can assume to be 245 days. The total annual mixing energy is 6.26 kW×24 hours×245 days= 36808.8 kWh. The pumping system is assumed to be the same power consumption which is 72 kWh·day−1. So, the total annual pumping energy is 72 kWh·day−1×245 days=17640 kWh. With a daily flow of 62.6 m³ (62,600 liters) over 245 days, the total volume treated annually = 62,600 L·day−1×245 days=15,335,000L. Thus, the energy per liter (FU)= (36808.8kWh+17640 kWh)/15,335,000L=0.0036 kW·L−1. The Sludge Processing Energy is assumed to be 0.2 kWh/L (based on sludge processing requirements). Total energy per functional unit=0.0036+0.2=0.2036kWh·L−1, CO₂ emissions with a conversion factor of 0.300 kg CO₂/kWh=0.2036 kWh/L×0.3 CO₂/kWh=0.062 kg CO₂ /L.

Assume the photobioreactor is used for large scale microalgae process whose energy consumption is about 12kWh, the energy consumption for a photobioreactor is 12.0kW/h × 20h× 0.05kgCO2eq/kWh /500L/h =0.024kgCO2eq/L, and for fish feed production: 1.038kg/L × 35.90L/day × 0.9883kgCO2eq/kg /1000L/day = 0.0368kgCO2eq/L.

The distance of the transportation is assumed for about 20km and the emission factor for transport is about 0.1kg CO₂/ton·km. The CO₂ from Transport per Functional Unit= 1kg×20km×0.1kg CO₂/ton·km=0.002 kg CO₂ /L.The final GWP =0.33kg CO₂ eq/L+0.002 kg CO₂ /L+0.062 kg CO₂ /L+0.024kgCO2eq/day+0.0368kgCO2eq/L =0.4548 kg CO₂-eq /L

5.6 EP Calculation

According to Alexandre’s report the nutrient concentration in the effluent can be summarized as 40 mg/L Nitrogen, 8 mg/L Phosphorus. The emission factors of 1 kg Nitrogen is 0.42 kg PO₄³⁻-eq, the one of 1 kg of phosphorus = 3.06 kg PO₄³⁻-eq. The function unit is 1 liter of beverage wastewater. EP from Nitrogen=40 mg/L×0.42 kg PO₄³⁻-eq=0.0000168 kg PO₄³⁻-eq/L, Ep from Phosphorus=8 mg/L×3.06 kg PO₄³⁻-eq=0.00002448 kg PO₄³⁻-eq/L. Total EP= 0.0000168 kg PO₄³⁻-eq/L+0.00002448 kg PO₄³⁻-eq/L=0.00004128 kg PO₄³⁻-eq/L=41.28 mg PO₄³⁻-eq/L.

The EP of anaerobic digesters=72 kWh·day−1+194 kWh·day−1= 266 kWh·day−1. The EP factor of power produced in Singapore is 45.8 mg PO₄³⁻-eq/L. EP per liter= 266 kWh·day−1×45.8 mg PO₄³⁻-eq/L=12,182.8 mg PO₄³⁻-eq/day, the pumping system =216 kWh/day×45.8 mg PO₄³⁻-eq/kWh. The total EP per liter=0.3526 mg PO₄³⁻-eq/L.

The energy of the photobioreactor is 12kWh×20h×45.8 mg PO₄³⁻-eq/kWh /50 L·day=219.84 mg PO₄³⁻-eq/L. The Sludge Processing Energy is assumed to be 0.2 kWh/L×45.8 mg PO₄³⁻-eq/kWh=9.16 mg PO₄³⁻-eq/L.

The distance of transportation is assumed at 10 km, the EP= 2.4 mg PO₄³⁻-eq/ton-km ×10km=24 mg PO₄³⁻-eq/L.

The total EP=-41.28 mg PO₄³⁻-eq/L+0.3526 mg PO₄³⁻-eq/L+219.84 mg PO₄³⁻-eq/L+9.16 mg PO₄³⁻-eq/L+24 mg PO₄³⁻-eq/L=212.0726 mg PO₄³⁻-eq/L