Introduction

With increasing shortage of freshwater resources caused by both ever-growing water demand and aggravating water pollution in human societies, wastewater reuse has been more and more considered of vital necessity for worldwide sustainable development (Greenway 2005). Among various approaches, secondary municipal wastewater reuse in water bodies is quite promising, offering ornamental, recreational, impounding, and humidifying functions, since suspended solids and organic pollutants in wastewater have already been substantially removed for turbidity and odor reduction (Ak and Gunduz 2014). Besides, generation of secondary municipal effluent is continuous and of large quantities, which guarantees sufficient freshwater supply for water reuse.

One major limitation of secondary municipal wastewater reuse in water bodies, however, is potential eutrophication caused by remaining high concentrations of nutrients in the effluents. Conventional treatment processes such as activated sludge and trickling filters can effectively remove biodegradable organic pollutants, yet removal of inorganic nutrients is usually inadequate (Arbib et al. 2014; Mohammad et al. 2013). Excessive concentrations of nutrients in water bodies, particularly nitrogen in the forms of NH4 +-N and NO3 −-N, and phosphorus in the form of PO4 3−-P, have been identified to induce microalgal blooms, causing oxygen reduction, turbidity increase, odor, and death of desirable flora and fauna, which may all severely impede the function of water bodies (Chan et al. 2014; Kosaric et al. 1974; National Research Counsil (U.S.) 2012). Removal of excessive nutrients from municipal secondary effluent, therefore, is required before water reuse, and such treatment is generally referred to as advanced municipal wastewater treatment, tertiary wastewater treatment, or effluent polishing (Craggs et al. 1996; Gao et al. 2016b; Schumacher and Sekoulov 2002). Besides realizing proper water reuse, removal of nutrients from secondary municipal wastewater is also an effective approach for phosphorus recovery, since possible depletion of phosphorus resources within the twenty-first century has raised increasing global concerns, and about 8% of the mined phosphate ends up in secondary municipal wastewater (Midorikawa et al. 2008; Rittmann et al. 2011).

In order to prevent possible eutrophication, upper limits of effluent nutrient concentrations when discharged into water bodies have been proposed in different countries. United States Environmental Protection Agency (2012) suggested residual total nitrogen (TN) of 5 mg/L and total phosphorus (TP) of 1 mg/L, European Commission Directive (1998) 98/15/EEC established TN of 10 mg/L and TP of 1 mg/L, Chinese State Environmental Protection Administration (2002) specifically forwarded regulations for wastewater reuse in scenic water bodies: TN of 15 mg/L and TP of 1 mg/L when reused in rivers and TP of 0.5 mg/L when reused in lakes and waterscapes. It has to be noted that, however, even when nutrient concentrations are far below the above upper limits, there could still be a good chance of eutrophication, since some species of water bloom microalgae were observed to display rapid growth even under TN of 0.28 mg/L and TP of 0.01 mg/L (Miller and Maloney 1971). Therefore, reduction of nutrients to as low as possible concentrations, and with relatively simple operation and modest cost, should be considered as the top focus for advanced municipal wastewater treatment before water reuse.

Various physical, chemical, and biological processes have been developed for advanced nutrient removal (Abe et al. 2014; Ak and Gunduz 2014; Chang et al. 2014; Ghosh and Gopal 2010; Healy et al. 2010; Hossein et al. 2013; Kalkan et al. 2011; Kney and Zhao 2004; Li et al. 2010; Mohammad et al. 2013; Wu et al. 2013b; Xie et al. 2007; Xu et al. 2015; Yang et al. 2011; Zhang et al. 2014), with detailed comparison displayed in Table 1. Among these processes, only very few of them can simultaneously remove both nitrogen and phosphorus from low concentrations in secondary municipal wastewater to even lower levels. Microalgae-based advanced wastewater treatment, on the other hand, has received growing attention in recent years owing to its outstanding advantages, including (1) simultaneous efficient N and P removal via microalgal photosynthetic assimilation; (2) cost effective and environment friendly as no additional chemicals are required, while oxygen generation, carbon dioxide mitigation, and metal ion reduction can be realized at the same time; and (3) potential utilization of the harvested microalgal biomass for production of food, feed, fuel, fertilizers, and fine chemicals (Arbib et al. 2014; Chan et al. 2014; Hoffmann 1998; Martinez et al. 2000; Schumacher and Sekoulov 2002). A schematic diagram of microalgae-based advanced municipal wastewater treatment for water body reuse is depicted in Fig. 1.

Table 1 Comparison among processes for advanced nutrient removal from secondary wastewater
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Fig. 1
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Schematic diagram on microalgae-based advanced municipal wastewater treatment for water body reuse. Dashed arrows and braces indicate optional conditions and products

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One particular challenge of microalgal advanced municipal wastewater treatment compared to treatment of other types of wastewater is that concentrations of nutrients and N:P ratios in secondary municipal effluent are much lower and imbalanced than in other types of wastewater or the commonly used microalgal cultivation mediums (Table 2) (Abe et al. 2014; Li et al. 2011; Wu et al. 2014; Zhou et al. 2012). Therefore, comprehensive considerations targeting nutrient removal from this specific type of effluent should be paid. This review summarized the performance of microalgae-based advanced municipal wastewater treatment in previous studies, analyzed the mechanisms and influencing factors of advanced nutrient removal, and integrated a kinetic model to evaluate assimilation-related nutrient removal. Limitations and prospects of full-scale microalgae-based advanced municipal wastewater treatment were also suggested. The manuscript could offer much valuable information for future studies on microalgae-based advanced wastewater treatment and water reuse.

Table 2 Comparison of nutrient concentrations in various wastewater streams and commonly used microalgae cultivation medium
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Nutrient and organic substance removal and other environmental benefits in microalgae-based advanced municipal wastewater treatment

In microalgae-based advanced municipal wastewater treatment systems, nitrogen and phosphorous pollutants are effectively removed, and there are other additional environmental benefits such as increased dissolved oxygen, reduced bacteria, and removal of heavy metals, all of which improve the feasibility of treated municipal secondary effluent for water reuse.

N removal

Nitrogenous pollutants can be effectively removed via microalgae-based advanced wastewater treatment. Removal of nitrogen in most studies was expressed by removal efficiency (in percentages) (Gómez-Serrano et al. 2015; Ramos Tercero et al. 2014), and generally only data sets of total nitrogen (TN) rather than detailed major components (NH4 +-N, NO3 −-N) were available (Shi et al. 2014). Concerning the fact that these studies were neither carried out with wastewaters of similar compositions nor with identical microalgal strains under the same culturing conditions, comparison of N removal performances among various studies based on removal efficiency alone cannot tell the whole story. For better evaluation of a specific microalgal advanced municipal wastewater treatment system, not only should removal efficiencies of TN, NH4 +-N, and NO3 −-N be given, removal rates of the above nitrogenous pollutants (the amount removed per volume/area within per unit time) are also indispensable, since high removal efficiency may not guarantee high removal rate, while fast N removal is always among the top priorities for feasible large-scale application. Another parameter evaluating microalgal N removal capacity is specific removal rates, determined via dividing N removal rates by initial microalgal biomass concentration at the start-up of advanced treatment process. Comparison among different advanced treatment systems based on such parameter, however, is really scarce (Aslan and Kapdan 2006).

According to available results (summarized in Fig. 2), over half of the advanced municipal wastewater treatment systems could achieve TN removal efficiency of higher than 80%, while TN removal rates in most studies were below 7.0 mg/L/day, corresponding to an average hydraulic retention time (HRT) of longer than 3 days. For better removal performance evaluation, final concentrations of major nitrogenous pollutants should also be provided, as they are direct indexes determining whether the treated wastewater is ready for reuse in water bodies without causing possible eutrophication. Information on microalgal strains and major cultivating conditions (operating mode, pretreatment, lighting, mixing, HRT, pH, temperature, etc.) should be provided as well, as variance in these conditions may result in widely different removal performances (Boonchai and Seo 2015; Kosaric et al. 1974; Shi et al. 2007). Another aspect worthy of attention is the comparability of N removal between different culturing systems, as removal rates in suspended systems were generally expressed in milligrams per liter per day, whereas for attached systems such parameters were commonly expressed in grams per square meter per day (Craggs et al. 1996); reliable inter-conversion of removal results is therefore expected for comprehensive evaluation among various treatment systems.

Fig. 2
figure 2

Nitrogen removal in microalgae-based advanced municipal wastewater treatment systems (Arbib et al. 2013; Boonchai and Seo 2015; Chan et al. 2014; Gómez et al. 2013; Gao et al. 2016b; Gao et al. 2014; He and Xue 2010; Li et al. 2010; Martı́nez et al. 2000; Kosaric et al. 1974; Ruiz-Marin et al. 2010; Ruiz et al. 2013a; Shi et al. 2007; Shi et al. 2014; Cho et al. 2011; Wang et al. 2010; Wu et al. 2013b; Xu et al. 2015; Yang et al. 2011; Yang et al. 2016; Zhang and Hong 2014; Zhang et al. 2014)

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P removal

Although as the other nutrient that causes eutrophication, studies with data on both removal efficiency and removal rate of phosphorus are not as many as for nitrogen. As shown in Fig. 3, TP removal rates with wide variances of up to over 200-fold exist among different studies. This may relate to other major factors including microalgal strain, cultivation mode, operating mode, pretreatment method, lighting, stirring/mixing, HRT, pH, temperature, etc. As it is the case with N removal, final TP and PO4 3−-P concentrations should also be provided for better P removal performance evaluation, as they are direct criteria for qualifying the reuse of treated secondary municipal wastewater in water bodies. Another issue similar to microalgae-based N removal is comparability of P removal performances between suspended and attached systems (expressed respectively by mg/L/d and g/m2/d) needs to be improved (Craggs et al. 1996; Guzzon et al. 2008; Sukačová et al. 2015).

Fig. 3
figure 3

Phosphorus removal in microalgae-based advanced municipal wastewater treatment systems (Arbib et al. 2013; Boonchai and Seo 2015; Chan et al. 2014; Gómez et al. 2013; Gao et al. 2014; Kosaric et al. 1974; Li et al. 2010; Martı́nez et al. 2000; Ruiz-Marin et al. 2010; Ruiz et al. 2013a; Shi et al. 2014; Cho et al. 2011; Wu et al. 2013b; Xu et al. 2015; Yang et al. 2011; Yang et al. 2016; Zhang and Hong 2014; Zhang et al. 2014)

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BOD removal

Organic substances in secondary municipal effluent are mostly inert after traditional biological treatment; thus, it is difficult for microalgae and possibly existing bacteria to utilize them (Drexler et al. 2014; Gao et al. 2016b), leading to very limited observations of microalgae-based advanced BOD removal (El Hamouri 2012; Gómez et al. 2013). A more frequently reported phenomenon, on the other hand, is the elevated SS and BOD concentrations in treated effluent, since microalgal cells may not be effectively removed after advanced treatment (Almasi et al. 2014). It has to be noted that, however, even if satisfactory water-algae separation is achieved, treated effluent may still exhibit elevated BOD concentrations due to excretion of soluble algal products (SAPs, consisting of carbohydrates, amino acids, amino sugars, proteins, lipids, and organic acids) during growth-related nutrient removal (Arbib et al. 2013; Gao et al. 2016b; Nguyen et al. 2005; Silva et al. 2015; Wang et al. 2010; Zhang et al. 2014; Zhuang et al. 2016), which may induce bacterial and/or fungal contamination, nutrient removal reduction, odor pollution, and carcinogenic by-product generation when traditional chlorination is followed after advanced municipal wastewater treatment (Cho et al. 2011; Hulatt and Thomas 2010; Nguyen et al. 2005; Pivokonsky et al. 2006; Yu et al. 2015). Shortening of HRT through membrane filtration via constantly replacing secondary municipal wastewater for microalgal growth could effectively overcome such BOD increase (Gao et al. 2016b). Membrane fouling, however, is an issue worthy of concern, especially with coexistence of bacteria, which is often the case for affordable large-scale advanced municipal wastewater treatment systems, since sterilization is not an option (Xu et al. 2016).

Other environmental benefits

Other environmental benefits of microalgae-based advanced municipal wastewater treatment include dissolved oxygen concentration increase, bacterial reduction (pathogen removal), and metal ion removal (Gómez et al. 2013; Singh and Dhar 2010). When nutrients in secondary municipal wastewater are removed via photosynthetic microalgal growth, a large amount of oxygen is released into the culturing medium, raising dissolved oxygen (DO) concentrations in treated wastewater to 100–300% of saturation values in open systems (Craggs et al. 1996) and over 400% of saturation values in closed photobioreactors (Molina et al. 2001). Increased DO concentration is advantageous for treated effluent reuse in water bodies, since it can effectively remove the offensive smells from secondary municipal effluent and prevent water from turning black or odorous (Ummalyma and Sukumaran 2014). Such complementary DO elevation in microalgae-based systems, however, is generally only achievable with energy-intensive aeration in traditional tertiary wastewater treatment processes (Jiang et al. 2010).

In addition to nutrient removal and DO increase, microalgae-based advanced municipal wastewater treatment may also facilitate bacterial reduction. Such phenomenon, however, is mostly observed only in attached treatment systems, where bacteria are removed from the treated wastewater via interrelated effects of algal biofilm adsorption, photo-oxidation, and pH increase (Schumacher and Sekoulov 2002). For suspended systems, only a few strains of microalgae have been reported to inhibit the growth of some specific bacterial strains (Yu et al. 2015), whereas more frequently observed are the negative impacts of bacteria on microalgal growth and growth-related nutrient removal. Detailed interactions of microalgae with other microorganisms are discussed in section “Intrinsic factors.”

Traces of metal ions including Cu2+, Zn2+, Fe3+, Al3+, Mn2+, Ca2+, Mg2+, K+, Na+, etc. have also been widely reported to be effectively removed by microalgae-based advanced municipal wastewater treatment systems (Gómez et al. 2013; Gao et al. 2016b; Wang et al. 2010). Mechanisms of the aforementioned removal are mainly (1) absorption (microalgal growth required), (2) detoxification by metallothioneins, (3) complexion by chelatins and polysaccharides, (4) alkaline precipitation, and (5) adsorption. Adsorption is considered as most contributive to overall metal ion removal, which can be realized by both live and dead algal cells (Kumar et al. 2015; Singh and Dhar 2010). The removed metal ions can be recovered through microalgal biomass harvesting, then recycled for use in other industrial sectors, offering additional benefits for microalgae-based advanced municipal wastewater treatment (Mallick 2002).

Mechanisms of nutrient removal in microalgae-based advanced municipal wastewater treatment

Since nutrient removal is the biggest contribution to microalgae-based advanced municipal wastewater treatment, mechanisms of nitrogen and phosphorus removal should be well understood and taken full advantage of, so as to optimize advanced treatment performances.

Mechanisms of N removal

Nitrogenous pollutants in secondary municipal wastewater streams are observed in the forms of ammonium (NH4 +-N), nitrate (NO3 −-N), nitrite (NO2 −-N), and organic N, among which NH4 +-N and NO3 −-N are dominating components that can be utilized by microalgae, although traces of dissolved hydrophilic organic N components have also been reported to be assimilated via microalgal growth (Liu et al. 2012). Since redox reaction is not involved during assimilation thus less energy is required (Cai et al. 2013), NH4 +-N is regarded as the form of N source most readily taken up by most microalgal species, and NH4 +-N removal by microalgae from secondary municipal wastewater has been reported to be much faster than NO3 −-N removal in many studies (Gao et al. 2016b; Ruiz-Marin et al. 2010). Besides being utilized by microalgal cells and assimilated into various substances such as glutamine, proteins, enzymes, peptides, chlorophylls, genetic materials (RNA, DNA), and energy transfer molecules (ADP, ATP) (Ruiz et al. 2013b), NH4 +-N in secondary municipal wastewater may also be removed through N2 loss (due to bacterial nitrification-denitrification) and NH3 volatilization (affected by pH, temperature, and mixing conditions) (Cai et al. 2013), as outlined in Fig. 4. With co-existence of bacteria in microalgae-based advanced municipal wastewater treatment systems, NO3 −-N could also be removed via bacterial denitrification other than microalgal uptake (Craggs et al. 1996). Such tributary N removal does not impede advanced municipal wastewater treatment performances as long as nitrogenous pollutant removal is the only target. However, N loss via gaseous N2 or NH3 should be kept at a minimum level if recovery of N from the harvested microalgal biomass is also expected, not to mention volumes of NH3 volatilization may lead to possible air pollution (Wang et al. 2015).

Fig. 4
figure 4

Mechanisms of nitrogen removal in microalgae-based advanced municipal wastewater treatment

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Considering the detailed composition of total nitrogen, since by far most studies on microalgae-based advanced municipal wastewater treatment were conducted either under sterilized conditions or with microalgae as overwhelmingly dominant, removal of NO3 −-N can be attributed to microalgae alone. However, since pH was not often controlled at neutral and may increase up to over 9.0, NH3 volatilization could be one major cause of NH4 +-N removal, which has not been thoroughly discussed yet (Arbib et al. 2013; Gao et al. 2014; He and Xue 2010).

Mechanisms of P removal

Phosphorus in secondary municipal wastewater streams generally exists in the form of inorganics and organics, with soluble orthophosphate (PO4 3−-P) contributing to the largest share and often as the only assimilative component for microalgal growth (Dueñas et al. 2003). As one of the two major nutrients for microalgae, P is required to assimilate energy transfer molecules (ADP, ATP), nucleotides (DNA), nucleic acids (RNA), phospholipids, proteins, and intermediates used for carbohydrate metabolism (Cai et al. 2013). For some species (e.g., prokaryotic cyanobacteria and eukaryotic coccal green algae), PO4 3−-P can be accumulated as polyphosphate granules within microalgal cells (either with or without previous P starvation) (Powell et al. 2009; Ruiz et al. 2013b), enabling luxurious P uptake/removal from secondary municipal wastewater (Guzzon et al. 2008; Sukačová et al. 2015). Precipitation is another major approach for P removal besides microalgal assimilation and luxurious uptake (Wang et al. 2014b). Accompanied by cations such as Ca2+ and Mg2+ (commonly observed in secondary municipal wastewater), elevated pH (>8.5), and photosynthesis-generated dissolved oxygen, precipitations of Ca3(PO4)2 and Mg3(PO4)2 would contribute up to 50% of reduced P concentrations in treated effluent (El Hamouri 2012; Gao et al. 2014; He and Xue 2010; Sukačová et al. 2015). Mechanisms of P removal in microalgae-based advanced municipal wastewater treatment are illustrated in Fig. 5.

Fig. 5
figure 5

Mechanisms of phosphorous removal in microalgae-based advanced municipal wastewater treatment

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Influencing factors of nutrient removal in microalgae-based advanced municipal wastewater treatment

Factors influencing microalgae-based nutrient removal can be divided into three categories: intrinsic, environmental, and operational. Each category alone or combined can contribute greatly to the performance of microalgae-based advanced municipal wastewater treatment for water reuse.

Intrinsic factors

Intrinsic factors mainly refer to characteristics of the adopted microalgal strains. Up to now, the number of strains capable of growing in secondary municipal wastewater with simultaneous nutrient removal to low enough levels is quite limited (see Table 3). Considering a lot of the reported studies were conducted using sterilized effluent, under continuous illumination, with extra CO2 enrichment, and under good pH and temperature control (Sukačová et al. 2015), the number of microalgal strains suitable for advanced municipal wastewater treatment in large-scale outdoor non-sterilized environment-fluctuating conditions may be even smaller.

Table 3 Reported microalgal strains with satisfactory growth in real secondary wastewater
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Properties of ideal microalgal strains for advanced nutrient removal and reuse in water bodies could be categorized into three levels: necessary, basic, and additional. In terms of necessary properties, strains for advanced municipal wastewater treatment must have (1) fast growth rates, (2) high nutrient requirements (higher than average N and P contents in microalgal biomass) (Guzzon et al. 2008; Shilton et al. 2012), and (3) the ability to utilize low concentrations of nutrients and reduce them to even lower levels (Li et al. 2010; Wu et al. 2015). For basic properties, advantageous strains should be (1) easy to separate/harvest after effluent polishing (Attasat et al. 2012; Markou and Georgakakis 2011), (2) tolerant of environmental fluctuations (Chevalier et al. 2000; Mallick 2002; Morita et al. 2002; Suh and Lee 2003), and (3) resistant to bacterial/fungal contamination and zooplankton predation (Boonchai and Seo 2015; Hoffmann 1998; Wu et al. 2014). With respect to additional properties, ideal strains should also display (1) low soluble algal product (SAP) secretion (Zhuang et al. 2016) and (2) satisfactory content of value-added components in the harvested biomass to offset the cost of advanced municipal wastewater treatment (Economou et al. 2015; Spolaore et al. 2006). So far, no microalgal strain has been reported to embrace all the aforementioned properties; genetic engineering together with further isolation from natural environments may be future development trends (Radakovits et al. 2010).

Interaction between microalgae and other microorganisms (i.e., non-target microalgae, bacteria, viruses, fungi, zooplankton, invertebrates, etc.) is another intrinsic factor significantly influencing the performance of microalgae-based advanced municipal wastewater treatment systems (see Fig. 6). While predation by zooplankton and invertebrates can destroy a microalgae-based system within just a few days, causing drastically reduced microalgal-growth-related nutrient removal (especially during summer breeding seasons) (Hillebrand and Kahlert 2001; Huang et al. 2014; Kesaano and Sims 2014; Liu et al. 2016), impact of bacteria may vary accordingly. Propagation of bacteria can suppress microalgal growth and growth-related nutrient removal either through competing with microalgae for limited nutrients (Jansson 1988; Zhang and Hong 2014) or through producing extracellular algicidal metabolites (Li et al. 2009). On the other hand, however, coexistence of bacteria with microalgae may have synergetic effects on nutrient removal, since CO2, O2, and pH conditions can be well balanced (Zhang and Hong 2014), and some specific bacterial strains have been observed to improve microalgal growth and nutrient removal by several folds (de Bashan et al. 2004; Lananan et al. 2014). Interactions between bacteria and microalgae, therefore, are complicated for outdoor advanced municipal wastewater treatment and require further investigation (Xu et al. 2016).

Fig. 6
figure 6

Interactions of microalgae and other microorganisms in advanced municipal wastewater treatment system. Solid arrows indicate positive interactions; dashed arrows indicate negative interactions

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Environmental factors

Environmental factors including nutrient concentrations and N:P ratio, pH, CO2 enrichment, lighting, temperature, presence of toxic substances, etc. can all significantly influence nutrient removal in microalgae-based advanced municipal wastewater treatment processes (Aslan and Kapdan 2006). Among them, nutrient concentrations, N:P ratio, and pH values influence N and P removal differently according to stoichiometric flows during microalgal growth and specific nutrient removal mechanisms (Bordel et al. 2009; Decostere et al. 2016; Drexler et al. 2014), while other factors impact both N and P removal in a mutual manner.

Based on the stoichiometric flow of available nutrient substances during microalgal growth (Eqs. 1–4, formula of microalgae biomass according to Chisti 2007), the ideal mass ratio of N to P for microalgal growth and corresponding N removal should be around 5:1; however, such ratio is generally not available for secondary municipal wastewater since P is often limited, rendering the N:P ratio to over 30:1 (Arbib et al. 2013; Boelee et al. 2012). Thus, when treating secondary municipal wastewater, N removal was often fast during the initial stage, then slowed down to negligible levels when P was soon depleted or reduced to un-utilizable concentrations (Boonchai and Seo 2015). Some species of microalgae, however, can maintain growth-related N assimilation even when extracellular P is depleted, using intracellular P storage to support N uptake, rendering very low P content necessary for microalgal cell metabolism (Wu et al. 2015). Estimation of nutrient removal, therefore, should take both extracellular nutrient concentrations and minimal nutrient requirement for microalgae metabolism into consideration, which will be further discussed in this manuscript.

$$ 100\ {\mathrm{C}\mathrm{O}}_2+84.5\ {\mathrm{H}}_2\mathrm{O}+11\ {\mathrm{N}\mathrm{O}}_3^{-}+13\ {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}\leftrightarrow {\mathrm{C}}_{100}{\mathrm{O}}_{48}{\mathrm{H}}_{183}{\mathrm{N}}_{11}\mathrm{P}+136.75\ {\mathrm{O}}_2 $$
(1)
$$ 100\ {\mathrm{C}\mathrm{O}}_2+73.5{\ \mathrm{H}}_2\mathrm{O}+11\ {\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}\leftrightarrow {\mathrm{C}}_{100}{\mathrm{O}}_{48}{\mathrm{H}}_{183}{\mathrm{N}}_{11}\mathrm{P}+114.75\ {\mathrm{O}}_2+9\ {\mathrm{H}}^{+} $$
(2)
$$ 100\ {\mathrm{H}\mathrm{CO}}_3^{-}+11\ {\mathrm{N}\mathrm{O}}_3^{-}+113\ {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}\leftrightarrow {\mathrm{C}}_{100}{\mathrm{O}}_{48}{\mathrm{H}}_{183}{\mathrm{N}}_{11}\mathrm{P}+136.75{\ \mathrm{O}}_2+15.5\ {\mathrm{H}}_2\mathrm{O} $$
(3)
$$ 100\ {\mathrm{H}\mathrm{CO}}_3^{-}+23.5\ {\mathrm{H}}_2\mathrm{O}+11\ {\mathrm{N}\mathrm{H}}_4^{+}+{\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-}\leftrightarrow {\mathrm{C}}_{100}{\mathrm{O}}_{48}{\mathrm{H}}_{183}{\mathrm{N}}_{11}\mathrm{P}+139.75{\mathrm{O}}_2+9{\mathrm{H}}^{+} $$
(4)

The pH value influences wastewater N removal mostly through NH3 volatilization. With the aid of microalgal photosynthesis, the pH of secondary municipal effluent can easily reach up to 9.0, and according to the equilibrium between NH4 +-N and NH3, nearly 40% of NH4 +-N can be lost through volatilization. Also, in an alkaline environment (pH > 8.5), P can be removed via precipitation with cations (He and Xue 2010; Schumacher and Sekoulov 2002; Sukačová et al. 2015). It has to be noted that, however, much too high pH values may inhibit microalgal metabolism and reduce the amount of both nutrients’ removal via growth-related assimilation, which may not improve overall advanced municipal wastewater treatment performance after all.

Also, as seen from the stoichiometric flow within algal cells (Eqs. 1–4), enrichment of inorganic carbon sources (either bicarbonate or concentrated CO2) can improve nutrient removal via enhanced microalgal growth (Arbib et al. 2014; Chi et al. 2014; Samorì et al. 2013). Utilizing flue gas as an extra inorganic carbon source may save the cost of CO2 enrichment, although problems such as flue gas transportation and toxicity of excessive concentrations of CO2, NO X , and SO X to microalgal cells should be considered (Ho et al. 2011), and reduction of the pH value due to CO2 enrichment may reduce the amount of N removal via NH3 volatilization, and similarly P removal via alkaline precipitation.

For outdoor microalgae-based effluent polishing, treatment systems are generally subjected to both seasonal and diurnal light and temperature changes. It has been reported that, under natural illumination with diurnal light/dark cycles, P removal efficiency and removal rates were respectively reduced to 37–42% and 26–38% of those under continuous artificial lighting (Sukačová et al. 2015). Improved illumination via artificial light sources, however, may largely contribute to the cost of microalgal cultivation (Carvalho et al. 2011) and thus should be cautiously adopted only when drastic improvement of nutrient removal can be expected. Temperature plays an important role in overall microalgal metabolism; excessive heat in summer may cause significantly reduced nutrient removal or even complete collapse of the microalgae-based system (Kosaric et al. 1974), whereas lower temperatures such as 15 °C may reduce nutrient removal rates by half of their 25 °C levels, and much lower temperature of 5 °C was observed with almost no nutrient removal at all (Chevalier et al. 2000). Cost-effective temperature control, therefore, should be carefully conducted for large-scale outdoor microalgae-based advanced municipal wastewater treatment systems.

Operational factors

For operational factors influencing microalgae-based advanced nutrient removal, initial microalgal biomass concentration, pretreatment of secondary municipal wastewater, pretreatment of algal biomass, HRT, system stirring/mixing, operation time, etc. should all be included.

Higher initial biomass inoculation displays a shorter lag phase and higher nutrient removal rates, leading to satisfactory removal performance within a much shorter time (Lau et al. 1995). Pretreatment of secondary municipal wastewater including filtration, UV radiation, and autoclaving can improve microalgal cultivation conditions by eliminating suspended solids, zooplankton, and bacteria (Chan et al. 2014; Cho et al. 2011; Zhang and Hong 2014), although either higher or lower nutrient removal rates compared to systems without pretreatment have been observed, since existence of other microorganisms may also contribute to nutrient removal (Cai et al. 2013; Craggs et al. 1996). Pretreatment of microalgal biomass generally involves starving microalgae of target nutrient before advanced municipal wastewater treatment (Shi et al. 2007); such starved biomass often displays luxurious nutrient uptake once inoculated into wastewater, realizing much faster nutrient removal compared to all-the-way nutrient-sufficient systems (Wang et al. 2015).

HRT is an important operating parameter for microalgae-based advanced municipal wastewater treatment, particularly for traditional suspended systems, for it determines not only the loading rate of nutrients but also the residence time of microalgal biomass. A much-too-long HRT may cause nutrient insufficiency for fast microalgal growth and growth-related nutrient removal (Ruiz et al. 2013a), whereas a much-too-short HRT may lead to dissatisfactory N removal, since the N:P ratio of secondary municipal effluent is generally much higher than the ideal value of 5:1 for microalgal assimilation (Arbib et al. 2013; Boelee et al. 2012). Moreover, biomass washout may occur when HRT is shorter than the microalgal cell multiplication time, leaving not enough microalgal biomass in the treatment system for nutrient removal (Bilad et al. 2014; Boonchai and Seo 2015). Appropriately shortened HRT have been reported to more than double the removal rates of P, obtaining lower than 0.1 mg/L residual TP at the same time (Chan et al. 2014). Utilization of membrane modules could serve as a good solution to separate HRT and microalgae residence time (MRT), achieving satisfactory microalgae-based advanced nutrient removal at HRTs of 1–3 days (Gao et al. 2016b; Gao et al. 2014; Singh and Dhar 2010; Xu et al. 2015; Xu et al. 2016).

Proper mixing/stirring of the microalgae-based treatment system offers better mass transfer, prevents cell settling, and eliminates thermal stratification, which all contribute to improved nutrient removal performances (Abdullah et al. 2016; Santiago et al. 2013). Intensified NH3 volatilization (known as air-stripping effect) often takes place under system agitation, although observations have revealed that the impact of mixing on N removal was much smaller than those of temperature and pH values (Martinez et al. 2000). Mixing has been reported as not effective for improving P removal efficiency (%), but it can enhance the removal rates (mg/L/day) by 25% (Martinez et al. 2000). It has to be noted that, however, shear stress caused by excessive mixing/stirring may inhibit microalgal growth, which would deteriorate nutrient removal performances (Carvalho et al. 2006).

The operation time of a specific microalgae-based advanced municipal wastewater treatment system also influences nutrient removal, since removal performances generally degrade over time, and the system has to be shut down for draining, cleaning, wastewater feeding, and re-inoculating when effluent nutrient concentrations are beyond acceptable levels for water reuse (largely due to nutrient back-release from dead/ruptured microalgal cells) (Martinez et al. 2000). Such procedures do not contribute to nutrient removal; thus, frequent system shut down may result in lower nutrient removal over long-term treatment operation (Wang et al. 2014a).

Kinetics and models of nutrient removal in microalgae-based advanced municipal wastewater treatment

According to the mechanisms of microalgae-based advanced nutrient removal, uptake of N and P for microalgal growth plays a major part in overall removal results. Modeling the kinetics of nutrient removal via microalgal growth, therefore, is crucial for microalgae-based advanced municipal wastewater treatment performance estimation. Among the various influencing factors (intrinsic, environmental, and operational factors), environmental factors including external nutrient concentration, CO2 enrichment, lighting, and temperature, and intrinsic factors represented by internal nutrient storage are most frequently considered when modeling the kinetics of microalgal growth and growth-related nutrient removal (Lee et al. 2015).

Covering one or several of the above influencing factors, various models have been proposed (Arbib et al. 2014; Coppens et al. 2014; Kunikane and Kaneko 1984; Lacerda et al. 2011; Ruiz et al. 2013a; Ruiz et al. 2013b; Schumacher and Sekoulov 2002; Talbot et al. 1991; Travieso et al. 2004; Xu et al. 2015; Yang et al. 2011). When selecting or creating the most appropriate model to estimate microalgal growth and growth-related nutrient removal, the principles below should be taken into serious consideration (Lacerda et al. 2011; Wang et al. 2014b): (1) fitness and accuracy against collected data, (2) relative conciseness, (3) ease of synthesis and utilization, (4) potential physiological significance, and (5) interpretable model parameters. Among these, trade-off between complexity of the model structure and its usability is of overwhelming importance.

Regarding the influence of external nutrient concentration, since concentrations of N, P, and CO2 in secondary municipal effluent and ambient environment are generally limited compared to commonly used microalgae cultivation mediums, kinetics of microalgal growth during advanced municipal wastewater treatment can be expressed with the Monod model (Aslan and Kapdan 2006; Coppens et al. 2014; Decostere et al. 2016; Monod 1949), which considers nutrient limitation conditions:

$$ \mu ={\mu}_{\max}\frac{S_i}{K_{S, i}+{S}_i} $$
(5)

in which

μ is the specific growth rate of microalgae under a nutrient concentration of S i (day−1).

μ maxis the maximal specific growth rate of microalgae with saturated nutrient concentration (day−1).

S i  is the nutrient concentration in secondary municipal wastewater (mg/L).

K S , i  is the Monod half-saturation constant of the nutrient (mg/L).

The nutrient can be N, P, or CO2.

Owing to the limited external nutrient concentrations in secondary municipal wastewater, internal nutrient storage (referring to both N and P, and especially P) can sometimes greatly influence microalgal growth, which can be described with the Droop model (Droop 1968):

$$ \mu ={\mu}_{\max}\left(1-\frac{Q_{i, \min }}{Q_i}\right) $$
(6)

in which

μ is the specific growth rate of microalgae under nutrient content of Q i (day−1).

μ max is the theoretical maximal specific growth rate of microalgae with infinite nutrient content (day−1).

Q i , min is the minimal nutrient content necessary for microalgal cell metabolism (%).

Q i is the nutrient content in microalgae biomass (%).

For outdoor microalgae-based advanced municipal wastewater treatment, light intensity varies widely according to diurnal, weather, and seasonal changes, and the actual illumination received by microalgal cells can be either too low or too high for optimum growth. Both photo-limitation and photo-inhibition, therefore, should be considered for microalgal growth and subsequent nutrient removal, which can be expressed using the Steele model (Steele 1962):

$$ \mu ={\mu}_{\max}\frac{I}{I_{\mathrm{opt}}} e\left(1-\frac{I}{I_{\mathrm{opt}}}\right) $$
(7)

in which

μ is the specific growth rate of microalgae under light intensity of I (day−1).

μ max is the maximal specific growth rate of microalgae with optimal light intensity (day−1).

I is the incident light intensity (μmol photon m−2 s−1).

I opt is the optimal light intensity (μmol photon m−2 s−1).

Also because of diurnal, weather, and seasonal changes, temperature in outdoor advanced municipal wastewater treatment systems normally display significant variations. Since microalgal growth rate depends heavily on temperatures (Talbot et al. 1991), the Arrhenius equation describing the influence of temperature on reaction rates should be adopted when modeling growth of microalgae in outdoor systems (Haario et al. 2009):

$$ \mu ={\mu}_{\max }{\theta}^{T-{T}_{\mathrm{ref}}} $$
(8)

in which

μ is the specific growth rate of microalgae under temperature of T (day−1).

μ max is the maximal specific growth rate of microalgae under the reference temperature (day−1).

θ is the temperature coefficient for microalgal growth.

T is temperature (°C).

T ref is the reference temperature (20 °C).

As the above factors all have a great impact on microalgal growth, comprehensive considerations should be given on their joint influences on microalgal growth and growth-related nutrient removal from secondary municipal wastewater. Based on the principle of multiplicative models (Lee et al. 2015), the growth rate of microalgae in secondary municipal wastewater can be expressed as

$$ \mu ={\mu}_{\max } f\left({x}_1\right) f\left({x}_2\right) f\left({x}_3\right)\cdot \cdot \cdot f\left({x}_i\right) $$
(9)

in which

μ is the specific growth rate of microalgae (day−1).

μ max is the overall maximal specific growth rate of microalgae (day−1).

f(x i ) is the influence of factor i.

While a multiplicative model considering both external concentration and internal storage of N and P can be much too complicated and not convenient for parameter determination (Kunikane and Kaneko 1984), more frequently only the influences of external N concentration and internal P storage were considered, since the influence of each of the above factors was dominant for its corresponding nutrient (Wu et al. 2013a). Thus, the growth rate of microalgae in secondary municipal wastewater can be estimated as

$$ \mu ={\mu}_{\max}\left(\frac{S_{\mathrm{N}}}{K_{\mathrm{S},\mathrm{N}}+{S}_{\mathrm{N}}}\right)\left(\frac{S_{{\mathrm{CO}}_2}}{K_{\mathrm{S},{\mathrm{CO}}_2}+{S}_{{\mathrm{CO}}_2}}\right)\left(1-\frac{Q_{\mathrm{P}, \min }}{Q_{\mathrm{P}}}\right)\left\{\frac{I}{I_{\mathrm{opt}}} e\left(1-\frac{I}{I_{\mathrm{opt}}}\right)\right\}\left({\theta}^{T-{T}_{\mathrm{ref}}}\right) $$
(10)

in which

μ is the specific growth rate of microalgae (day−1).

μ max is the overall maximal specific growth rate of microalgae (day−1).

S N is the nitrogen concentration in secondary municipal wastewater (mg/L).

K S , N is the Monod half-saturation constant of nitrogen (mg/L).

\( {S}_{{\mathrm{CO}}_2} \) is the CO2 concentration in secondary municipal wastewater (mg/L).

\( {\mathrm{K}}_{\mathrm{S},{\mathrm{CO}}_2} \) is the Monod half-saturation constant of CO2 (mg/L).

Q P , min is the minimal phosphorus content necessary for microalgal cell metabolism (%).

Q P is the phosphorus content in microalgae biomass (%).

I is the incident light intensity (μmol photon m−2 s−1).

I opt is the optimal light intensity (μmol photon m−2 s−1).

θ is the temperature coefficient for microalgal growth.

T is the temperature (°C).

T ref is the reference temperature (20 °C).

Microalgal nutrient removal rates can be therefore estimated by the following equation:

$$ {R}_i=\mu \times X\times {Q}_i $$
(11)

in which

R i is the microalgal nutrient removal rate (g/L/day).

μ is the specific growth rate of microalgae (day−1).

X is the microalgal biomass concentration (g/L).

Q i is the nutrient content in microalgae biomass (%).

It should be noted that the estimated nutrient removal from the above model is only the fraction assimilated via microalgal growth, and values of the parameters in such a model may vary according to different microalgal species, physiological growth stages, quality of wastewater streams, and illumination attenuation. For instance, Q P , min and Q P vary with species and physiological stages (Ruiz et al. 2013b), K S , N vary with types of N source (NH4 +-N or NO3 −-N) (Lee et al. 2015) and physiological stages (Silva et al. 2015), and light attenuation happens both because of water depth increase and mutual shading by microalgal cells (Kunikane and Kaneko 1984). Thus, it is important to consider the applicable ranges of parameter values and also the conditions of model development. On the other hand, however, determination of parameters for a model considering multiple factors may not be easy, since co-limitation conditions are difficult to simulate (Lee et al. 2015). But even if the above kinetic model has its inevitable limitations, it can still offer important information on the potential relationship between advanced municipal wastewater treatment and microalgae production (Drexler et al. 2014).

For large-scale outdoor continuous microalgae-based advanced municipal wastewater treatment systems, influences of bacterial contamination should be included in kinetic model development as well, since either reduced nutrient removal or synergism-related improved performance may take place (Lee et al. 2015). Operational factors such as hydraulic retention time and dilution rate should also be considered (Lacerda et al. 2011), and other more macroscopic factors such as geographical location and climatic changes need also to be taken into account in the kinetic models of nutrients and possibly organic substance removal (Heaven et al. 2012).

Limitations and prospects

Two major challenges of microalgae-based advanced municipal wastewater treatment are as follows: (1) how to shorten HRT to the uttermost extent while obtaining satisfactory nutrient removal? and (2) how to realize efficient and easy-operating separation of microalgae from the treated effluent to prevent violation of SS and BOD regulations and avoid visual and/or olfactory unpleasantness? Although use of membrane modules can achieve both separated HRT and MRT, and water-algae separation, such techniques are mostly reported only in lab-scale studies (Gao et al. 2016a; Kumar et al. 2010; Xu et al. 2015), with concerns of increased costs and severe membrane fouling limiting their large-scale application (Bilad et al. 2014; Drexler and Yeh 2014).

Another approach to overcome the above two challenges is transforming the microalgae cultivation mode from suspended cultivation to attached cultivation. With provision of substratum, attached microalgal biofilms can be developed, rendering retained microalgal biomass in the treatment system with even shorter HRT; meanwhile, effective separation of algae and treated effluent can be obtained (Boelee et al. 2013; Schumacher and Sekoulov 2002). Although several pilot-scale systems have been reported with good nutrient removal performances (Adey et al. 1993; Boelee et al. 2013; Christenson and Sims 2012; Shi et al. 2014), full-scale application of attached microalgae systems for advanced municipal wastewater treatment is still missing. Besides, as so far the most commonly used method for attached microalgae harvesting, mechanical scraping is far too complex and difficult to operate in large-scale microalgae-based systems (Lee et al. 2014; Naumann et al. 2013). Other limitations such as unpredictable shedding of microalgal biofilm and inadequate durability of substratum for attached microalgal growth need also to be resolved to fulfill the application of large-scale microalgae-based advanced municipal wastewater treatment (Christenson and Sims 2012; Gross et al. 2015; Naumann et al. 2013).

Conclusions

Reuse of secondarily treated municipal wastewater in water bodies can be an effective approach to alleviate the ever-increasing shortage of freshwater resources; however, effective removal of nutrients from secondary municipal effluents is required to prevent possible eutrophication. Among the various methods for advanced nutrient removal, microalgae-based processes are the most advantageous, displaying outstanding virtues of simultaneous N and P removal, extra-chemical-free treatment, generation of O2, sequestration of CO2, reduction of metal ions, and production of value-added compounds from the harvested biomass. Performances and mechanisms of microalgae-based advanced nutrient removal are summarized, although results of existing studies are not really comparable, owing to lack of detailed information on various evaluating parameters such as specific nutrient forms, initial and final nutrient concentrations, nutrient removal efficiencies and removal rates, HRT, and modes of microalgae cultivation and system operation for each study. Influences of intrinsic, environmental, and operational factors on microalgae-based advanced municipal wastewater treatment are discussed, and a kinetic model based mainly on environmental and intrinsic factors for evaluating microalgal-growth-related nutrient removal is also integrated. For future large-scale microalgae-based advanced municipal wastewater treatment, maximum shortening of HRT while maintaining satisfactory nutrient removal performances and efficient microalgae-water separation after wastewater treatment are two major limitations to be resolved. Among the two approaches to overcoming the above limitations, utilization of attached microalgae for advanced nutrient removal is more promising, although great breakthroughs are required to transform the currently available pilot-scale systems into full-scale ones.