Abstract
Microalgal heterotrophic bioreactors are a potential technological development that can convert organic matter, nitrogen and phosphorus of wastewaters into a biomass suitable for energy production. The aim of this work was to evaluate the performance of microalgal heterotrophic bioreactors in the secondary treatment of cattle-slaughterhouse wastewater and the reuse of microalgal sludge for biodiesel production. The experiments were performed in a bubble column bioreactor using the microalgae Phormidium sp. Heterotrophic microalgal bioreactors removed 90 % of the chemical oxygen demand, 57 % of total nitrogen and 52 % of total phosphorus. Substantial microalgal sludge is produced in the process (substrate yield coefficient of 0.43 mg sludge mg chemical oxygen demand−¹), resulting in a biomass with high potential for producing biodiesel (ester content of more than 99 %, cetane number of 55, iodine value of 73.5 g iodine 100 g−¹, unsaturation degree of ~75 % and a cold filter plugging point of 5 ºC).
Highlights
The high water consumption by slaughterhouses results in large volumes of wastewaters characterized by high organic load, due to the presence of blood, manure, fat, undigested stomach contents, and intestinal contents
The cattle-slaughterhouse wastewater composition, that takes into consideration one year of sampling, indicated a high concentration of organic matter, nitrogen, and phosphorous, characteristic of food processing wastewater, resulting in C/N (49.59 ± 9.4) and N/P (6.74 ± 3.8) ratios that are suitable for microalgae growth
The microalgal heterotrophic bioreactor applied to the treatment of cattle-slaughterhouse wastewater removed 2.7, 68.5, and 0.01 mg L−1 d−1 of organic matter (COD), total nitrogen (N-TKN) and total phosphorus (P-PO4−3), respectively, resulting simultaneous conversions in the order of 90 % of chemical oxygen demand (COD), ∼57 % of N-TKN, and ∼52 % of P-PO4−3 (Table 1)
Summary
The high water consumption by slaughterhouses results in large volumes of wastewaters characterized by high organic load, due to the presence of blood, manure, fat, undigested stomach contents, and intestinal contents. The wastewater of slaughterhouses has a high pollution load (Mittal, 2006) These agro-industrial wastes have a high concentration of organic matter and are a suitable environment that favors microalgae heterotrophic cultivation, where nitrogen (N) and phosphorus (P) are usually found in favorable carbon/nitrogen (C/N) and nitrogen/phosphorous (N/P) ratios that support microalgal growth. These wastewaters exhibit assimilated compounds and in most cases an absence of toxic compounds or growth inhibitors (Queiroz et al, 2013). This blue green algae shows considerable potential for being used as biocatalysts in environmental biotechnology processes because of its robustness and simple nutritional requirements (Cañizares-Villanueva et al, 1994; Al-Thukair et al, 2007; Guiry and Guiry, 2013)
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