Mitigating excessive ammonia nitrogen in chicken farm flushing wastewater by mixing strategy for nutrient removal and lipid accumulation in the green alga Chlorella sorokiniana

Highlights

• Excessive ammonia nitrogen in CFFW was a major factor limiting the algal growth.

• Mixed wastewaters reduced ammonia nitrogen content and balanced nutrient profile.

• Algal biomass yield on mixed wastewaters had been improved significantly.

• Lipid content and nutrient removal on mixed wastewaters had been promoted notably.

Abstract

This study aimed to evaluate algal growth, lipid production, and nutrient removal in chicken farm flushing wastewater (CFFW). The excessive ammonia nitrogen (EAN) content in the CFFW wastewater represented a major factor limiting the algal growth. A strategy of mixing CFFW with municipal wastewater (MW) that contained less ammonia nitrogen was adopted. The results showed that the mixed wastewaters reduced ammonia nitrogen content, balanced nutrient profile, and promoted biomass production. The residual nutrients in mixed wastewaters were significantly reduced due to the algal absorption. Furthermore, alga grown on mixed wastewaters accumulated a higher level of total lipids and monounsaturated fatty acids that can be used for biodiesel production. The key issue of low biomass yield of algal grown on CFFW due to the inhibition of EAN was efficiently resolved by mitigating limiting factor to algal growth basing on mixing strategy, and accordingly the nutrients in the wastewater were significantly removed.

Introduction

Livestock and poultry wastewaters contain a high amount of heavy metals, nitrogen, phosphorus, and antibiotics (Milbrandt et al., 2018); if disposed without sufficient treatment, these wastewaters can seriously pollute the environments and adversely affect the human life (Milbrandt et al., 2018). The fundamental aims of wastewater treatment are to remove the high levels of nitrogen, phosphorus, chemical oxygen demand, and other organic matters (Nzayisenga et al., 2018). The conventional methods for wastewater treatments have been used to purify the manure and wastewater from chicken farm, including composting process (Wan et al., 2020), fermentation (Baltrėnas and Kolodynskij, 2019), and anaerobic digestion (Zahan and Othman, 2019, Zahan et al., 2018). However, these conventional methods for wastewater treatments are costly and inefficient; as such, there is an urgent need to develop innovative wastewater treatment technologies that are sustainable, cheaper, and more efficient.

One promising treatment has been to use wastewaters for microalgae cultivation (De Bhowmick et al., 2019, Ge and Champagne, 2016). This is a twofold strategy. On one hand, the nutrient-rich wastewaters can support algal growth, thus contributing to nutrient removal while also reducing the nutritional costs for biomass production (Lu et al., 2016, Tan et al., 2016). On the other hand, algal biomass contains abundant energy-rich components that can be converted into various biofuels and bioproducts (Ho et al., 2013). This microalgal cultivation-based treatment is environmentally amenable and sustainable because it does not generate additional pollutants, and provides an opportunity for nutrients recycling (Tan et al., 2016). Microalgal cultivation has been successfully tested in various wastewaters, including industrial (Wang et al., 2016), municipal (Ge and Champagne, 2017, Reyimu and Özçimen, 2017), agricultural (Khalid et al., 2018), food processing (Gao et al., 2018, Gupta and Pawar, 2018, Yang et al., 2015), and livestock (Mark et al., 2017). Recently, some studies have focused on the combined use of microalgal biotechnology to purify the manure and wastewater from chicken farm, for instance, Li et al. (2018) used Chlorella sp. to cultivate in the anaerobic digestion liquid digestate of chicken manure to produce biodiesel. Wang et al. (2014) did a series of pretreatment of poultry manure wastewater to approach the cultivation of Chlorella vulgaris. However, this microalgae culture system has not been used effectively in the treatment of chicken farm flushing wastewater (CFFW).

Usually, the livestock wastewater contains high levels of chromaticity and turbidity that lower light intensity for the photosynthesis of microalgae, and thus requires physicochemical treatments prior to microalgae cultivation (Kim et al., 2014). Livestock wastewater also contains excessive ammonia nitrogen (EAN) that may inhibit microalgae growth. For example, ammonia nitrogen content in piggery wastewater ranges 500–1600 mg/L in anaerobic digestate and 2400–3600 mg/L in raw wastewater (Boursier et al., 2005). Although multiple dissolved inorganic nitrogen forms such as NH₄⁺, NO₂⁻, and NO₃⁻ can serve as nutrient sources for algae growth (Sniffen et al., 2018, Wang et al., 2018), NH₄⁺ is first assimilated because it does not need to be reduced for amino acid synthesis. However, a combination of high NH₄⁺ concentration (1000–2000 mg/L) and basic pH (>8.0) will shifts the chemical equilibrium from NH₄⁺ to NH₃ which is toxic to microalgae (Källqvist and Svenson, 2003) and the inhibition by certain ammonia nitrogen concentration levels for algae is species-specific (He et al., 2013, Tan et al., 2016, Wang et al., 2018). Therefore, we hypothesized that it is the EAN that leads to the inhibition of algal growth on original livestock wastewater. Various strategies have been developed to reduce the toxic effects of ammonia nitrogen on algal growth, including dilution of ammonia nitrogen concentration, selection of tolerant algae strains, deployment of microalgae-bacteria/fungi/macroalgae symbioses, and other effective pretreatment methods.

Dilution is an effective method for reducing growth inhibition caused by EAN. Dilution of raw piggery wastewater and anaerobic digestate of piggery effluent (ADPE) with fresh water has been used for microalgae cultivation (Jia et al., 2016, Wang et al., 2012). However, dilution using fresh water is not a viable option because of the limitation of fresh water supply and potential problems with the disposal of a larger volume of wastewater into environments (Ayre et al., 2017). A better alternative would be to mix different wastewaters with contrasting profiles; this strategy could improve the nutrient profiles suitable for algae cultivation. For instance, mixing dairy wastewater of low ammonia nitrogen with slaughterhouse wastewater with high ammonia nitrogen improved the algae biomass yield (1.32–2.66 g/L) (Lu et al., 2016). Similar results were also obtained by mixing dairy final effluent with pulp and paper influent (Gentili, 2014). However, the studies on application of mixing strategy in CFFFW have not been reported.

The main aim of this study was to evaluate the capability of algal growth, lipid production, and nutrient removal in chicken farm flushing wastewater (CFFW). The specific experiments included: (1) analyzing the nutrient and metal element profiles of CFFW, (2) isolation and optimization of growth conditions of algal strains, (3) measuring algal growth and nutrient removal in original CFFW, (4) determining the factors that limited algal growth on CFFW, (5) mitigating the limiting factor by dilution with fresh water, (6) mixing CFFW with municipal wastewater (MW) to improve the nutrient profile and algal growth, (7) measuring nutrient removal rate for total nitrogen (TN), ammonia nitrogen (TAN), phosphorus (TP), and chemical oxygen demand (COD), and (8) analyzing nutrient composition of algae (lipid and fatty acids accumulation) grown on mixed wastewaters.