Microalgal culture to treat  anaerobically digested abattoir effluent

Hajar Shayesteh

The continuous surge in human population coupled together with the growing demand for animal sourced protein has significantly increased the number of abattoirs around the world and the volume of wastewater generated in these facilities. Abattoir wastewater is typically characterized by high concentration of inorganic and organic matters, toxicants and solids (suspended and dissolved) which can negatively impact the environment if directly released. Although anaerobic digestion (AD) is widely employed as a primary biological treatment for reducing the concentration of organics in abattoir wastewater, it is inefficient for the removal of inorganic nutrients (e.g., phosphorus and nitrogen) and still requires subsequent treatment procedures before discharge to the environment. Microalgal cultivation offer an elegant solution and cost-effective tertiary treatment option for various wastewater. Cultivation of microalgae in anaerobic digestate abattoir effluent (ADAE) requires minimal to no freshwater input which is highly valuable especially in regions with water scarcity. Microalgae can efficiently assimilate various forms of nutrient from waste streams for the production of valuable biomass, representing the conversion of waste-to-profit. Moreover, nutrient-depleted water effluent after microalgal cultivation can be safe for potential discharge or reuse within the abattoir’s operations while the generated biomass can be used for multiple purposes such as animal and aquaculture feed, and organic fertilizer. To achieve maximum biomass production and nutrient removal from ADAE, it is essential to optimize the growth and operating conditions of microalgae cultures in the effluent. The productivity of microalgal cultures is generally limited by a combination of physical factors such as pH and CO2, as well as operational factors such as culture depth. Initially, Scenedesmus sp. was isolated as the most efficient (long-term survival and highest productivity) microalga capable of growing in raw ADAE. Then, the first growth condition to be optimized as part of this work was culture pH in which Scenedesmus sp. was cultured at different pH points (5.5, 6.5, 7.5 and 8.5) controlled by addition of CO2 (pH-stat systems) under outdoor local climatic conditions during summer. The highest Scenedesmus biomass productivity was obtained for the culture grown at pH 6.5 (19.24 g m−2d−1) which was 2.1 times greater than the culture without any pH control (9.13 gm−2d−1). In general, the growth was pH 6.5> pH 7.5= pH 8.5> uncontrolled pH> pH 5.5. The percentage of ammoniacal nitrogen (NH3-N) fixed into biomass was also highest (86%) for the cultures at pH 6.5. The percentage of NH3-N assimilation trended pH 6.5> pH 7.5> pH 8.5> uncontrolled pH> pH 5.5. More than 83% of phosphorus was assimilated for biomass production by the cultures grown at pH 6.5, 7.5 and 8.5 with no significant difference recorded among them which was almost twice of that for the uncontrolled pH (43%) treatment. In the next study, the effects of culture depth on the growth, photo-physiology and nutrient removal rate of Scenedesmus sp. grown in paddle wheel-driven raceway micro-ponds at 14 cm, 17 cm, 20 cm, and 23 cm depth during Austral winter and summer was evaluated. Cultures were operated semi-continuously with total four harvesting regime being performed (four replicates) and the hydraulic residence times ranged between 3 days in summer and 7 to 9 days in winter. Culture pH was kept constant at pH 6.5 for all treatments throughout the experimental period by periodical dosage of CO2 using a pH-stat system. Algal cultures grown in summer showed 2.3- 2.7 times significantly higher biomass productivity when compared to the same cultures grown in winter. Irrespective of the season, the maximum volumetric productivity of this alga was achieved at 14 cm depth. However, areal biomass productivity for culture grown at 23 cm depth was 12% and 30% higher than that of the culture grown at 14 cm depth in winter and summer, respectively. In addition, nitrogen, phosphorus and COD areal removal rates were significantly higher in cultures operated at 23 cm than all other treatments in both seasons. The effective quantum yield (Fq'/Fm') in summer was 23 cm depth = 20 cm depth> 17 cm depth = 14 cm depth while it trended 14 cm depth⩾ 17 cm depth⩾ 20 cm depth⩾ 23 cm depth in winter, indicating significance of operational conditions on algal photosynthesis. The outcome of this study showed that, irrespective of the season, operating the culture in higher depths significantly increased areal biomass productivity as well as nutrient removal rates when treating ADAE. A major outcome of this project was the 13-month semi-continuous cultivation of Scenedesmus sp. ADAE in outdoor raceway ponds to evaluate reliability of the microalgae culture during different seasons throughout the year. Over the course of summer and early autumn, the average weekly biomass productivity of Scenedesmus sp. cultures was 12.5 ± 0.6 g m-2 d-1 which was 16% and 30% higher than the productivities recorded in spring and winter. All available ammoniacal nitrogen (NH3-N) was found to be exhausted during each growth period with an average 33.6% nitrogen assimilation. The average phosphorus and COD (chemical oxygen demand) removals were 85.2% ± 1.9 and 37.5% ± 2.9 throughout the cultivation period. No significant differences were found in carbohydrate, lipid and protein content of Scenedesmus sp. during different seasons of the year. A further 53% increase in biomass productivity is expected to be achieved if CO2 is supplemented to control culture pH at 6.5. The last part of this project looked into the application of ADAE grown microalgal biomass as an organic fertilizer to clearly establish the waste into profit objective. For this, a 56-day soil incubation experiment was conducted to compare the effect of wet and dried Scenedesmus sp. microalgae biomass on soil chemistry, microbial biomass carbon (MBC), CO2 respiration, and bacterial community diversity. The experiment also included control treatments consisting of glucose, glucose + ammonium nitrate, and no fertilizer addition. All treatments except unamended soil (without addition of fertilizer) were adjusted to deliver the same amount of carbon to the soil. Microalgae biomass and ammonium nitrate were added at specific quantities to the soil so that the amount of N equalled 200 kg N ha-1. Soil chemistry, soil microbial biomass, CO2 respiration as well as diversity profiling of the bacterial community (16S rRNA region) on the Illumina Mi-Seq platform followed by an in-silico analysis of functional genes associated with soil N and C cycling processes was performed for all treatments. The results indicate that heterotrophic nitrification may contribute to nitrate production for both microalgae amendments, as evidenced by low amoA gene abundance and a decrease in ammonium with an increase in nitrate concentration. Additionally, dissimilatory nitrate reduction to ammonium (DNRA) may contribute to ammonium production in the wet microalgae amendment, as indicated by an increase in nrfA gene and ammonium concentration. This is a significant finding because DNRA leads to N retention in agricultural soils instead of N loss via nitrification and denitrification. Thus, further processing the microalgae through drying may not be favourable for fertilizer production as the wet microalgae appeared to promote DNRA and N retention.

Microalgal culture to treat anaerobically digested abattoir effluent

Files and links (1)

Abstract

Details

UN Sustainable Development Goals (SDGs)

Metrics