## 6 Specific Applications of Wastewater Bioprocessing with Simultaneous Biofuel Production
### Food Processing Plant Waste and DHA Production  
In addition to biofuels, microalgae also produce polyunsaturated fatty acids (PUFAs) that are of significant nutritional value. One of these is docosahexaenoicenoic acid (DHA), whose main production is based on fish oils (salmon, tuna, sardines). The main disadvantages of fish oil production are sustainability issues due to seasonal and climatic variations, overfishing, and depletion of fish populations. In addition, however, heavy metals (mercury) and other dangerous pollutants (PCBs and dioxins) have been tracked at elevated levels in some fish species. In addition, in relation to the production of fatty acids, there is a significant difference between those produced from fish oil and those produced from microalgae. In the first instance, extra separation and purification procedures are needed during DHA production in order to comply with industry standards. However, due to their lower trophic levels than fish and their status as primary producers, microalgae have far simpler fatty acid profiles, which facilitates separation [48]. In their 2018 study, [PERSON] et al. examined the economic feasibility of producing DHA by cultivating the strain _Aurantiochytrium_ sp. KRS101 on an orange peel waste (OPW) nutrient substrate that was acquired from a Korean orange juice factory. Orange is the most-used fruit in the fruit juice industry; 51.1 million tons of orange juice are produced annually worldwide. The biomass of the fruit, which is made up of solids like pulps and peels, makes up about half of its byproducts when juice is produced. As a result, millions of tons of orange peels could be made available for use as a microalgae growth substrate, although substrates such as anaerobic digestion effluent (ADE) are found to be effective, especially in carbohydrate level augmentation [49]. Although there is more potential for other uses, the primary use is as inexpensive feed. Orange peel extract (OPE) was optimized in the study using NaNO\({}_{3}\) (1.2 g/L) as a nitrogen source, which produced higher yields of DHA (0.63 g/L) than urea and NH\({}_{4}\)Cl. The yield of 0.63 \(\pm\) 0.02 g/L is 2.5 times that of the modified standard synthetic substrate and was recorded at 72 h of cultivation. At the same time, there was a high performance in FAME (1.97 \(\pm\) 0.07 g/L).  
To investigate economic viability, the authors calculated production costs comparatively between the typical synthetic substrate and the OPE with the addition of sodium nitrate. The cost of the synthetic substrate is 25.75 USD/1000 L, while that of the OPE is 3.52 USD/1000 L. Therefore, using _Aurantiochytrium_ sp. KRS101 on OPE substrate and 1.2 g/L NaNO\({}_{3}\) to produce 1 kg of DHA is 18 times less expensive than using modified basal medium with 20 g/L glucose and 2 g/L yeast extract. Given that OPE produced 0.63 g/L of DHA and the modified synthetic medium produced 0.25 g/L, the raw material costs per kg of DHA for OPE and the synthetic medium, respectively, were USD 5.6 and USD 103. According to the authors, the findings of the study support the effective exploitation of waste from food processing plants for microalgae crops that produce value-added products, such as DHA [48].