# Effects of Caprolactam Wastewater on Algal Growth and Nutrients Removal by _Arthrospira platensis_
## 1 Introduction  
Microalgae are photosynthetic organisms that convert C, N, P, and light energy into biomass and can be applied in biofuels, food, feed, and biofertilizer industries [1]. The main factors limiting the commercial mass culture of microalgae are the high production costs and water resource consumption [2]. To alleviate this issue, wastewater has been considered as a medium for microalgae cultivation [3]. Coupling wastewater treatment with microalgae cultivation is an advanced strategy for wastewater recycling that can reduce microalgae cultivation costs and remove N and P pollutants. Many studies have confirmed that microalgae can grow in different wastewaters, such as municipal wastewater [4], olive mill wastewater [5], soy sauce wastewater [6], and agro-industrial wastewater [7, 8]. Caprolactam wastewater (WCP) is generated during the production of caprolactam (CPL). It is estimated that approximately 4.2 t to 4.7 t of wastewater will be generated for each ton of CPL production [9]. WCP contains NO\({}_{3}^{-}\), PO\({}_{4}^{3-}\), and organic matter [9, 10, 11]. WCP discharge may lead to a risk of environmental pollution. Biological and chemical methods have been applied to treat WCP [12, 13, 14]. Chemical methodologies such as electrooxidation, ozonation [13] and biochemical techniques, suchas the activated sludge (AS) process, anoxic/oxic (A/O) process, anaerobic-anoxic-aerobic (A\({}_{2}\)O) process, and AS mixed with specific CPL-degrading bacteria (ASMSB), have been investigated [12; 14; 15]. Furthermore, microwave and ultrasound [16; 17; 18] are also used to pre-treat wastewater. However, owing to the low organic C source in WCP, it is difficult to remove N and P using these processes. These processes also produce large amounts of sludge, which is a secondary solid waste [19]. Organic C addition and the disposal of large amounts of sludge increase the cost of wastewater treatment. Therefore, the removal of pollutants (N and P) in WCP using the AS, ASMSB, A/O, and A\({}_{2}\)O methods is not eco-friendly or economical. Microalgae cultivation in wastewater not only removes nutrients (N and P), but also produces high-value products. Some microalgae species can adapt to various types of wastewater, such as _Parachlorella kessleri_ QWY28 [20], _Chlorella_ sp. [21], _Arthrospira platensis_[22], _Spirulina maxima_ (_Arthrospira maxima_) [6], and _Desmodesmus_ sp. [23]. Among these species, _Arthrospira_ is considered a candidate for wastewater treatment because its biomass can be easily harvested by filtration [24] without the application of any flocculants [25]. Moreover, the protein content of _Arthrospira_ can reach up to 70%, which is significantly higher than that of other species of microalgae. Such a high protein content suggests that _Arthrospira_ can absorb more N from wastewater and thus has a higher N removal capacity [26]. Protein-rich _Arthrospira_ can be used as a feed additive to replace some fish meal and has broad application prospects in food and feed [26]. _Arthrospira platensis_ contains a special protein called phycocyanin (PC), which is a high value-added microalgae biotechnology product. PC has antioxidant, anti-inflammatory, hepatotoxicity, and free radical scavenging properties, and is widely used in the food, feed, and pharmaceutical industries [27]. Therefore, _Arthrospira_ has great potential for WCP treatment, and studies on the use of _Arthrospira_ to treat wastewater have attracted much attention. Our previous study focused on the effects of organic components [e.g., cyclohexanone (CHN) and cyclohexanol (CHL)] in WCP on the growth of _Arthrospira_. The results showed that the growth of _A. platensis_ was not inhibited by CHN, CHL, or a mixture of CHN and CHL under safe exposure concentrations. Thus, it is possible to use _A. platensis_ for the removal of N from WCP after proper dilution [11]. However, the effects of real WCP on the growth of _A. platensis_ have not yet been investigated.