One of the main factors of the increased eutrophication level of surface waters is the high anthropogenic loads of biogenic substances discharged into water bodies. Municipal wastewaters, containing large amounts of nitrogen and phosphorus play one of the key roles in the acceleration of eutrophication intensity. The main direction in the prevention of eutrophication caused by wastewater discharge has become the reduction of nutrient loads introduced to wastewater receivers in accordance with strict legal requirements achievable only in advanced technologies. The treated wastewater quality standards are actually developed for total nitrogen and total phosphorus content, disregarding the fact that eutrophication potential of treated wastewater is determined by the content of non-organic nutrient forms directly bioavailable for water vegetation. That is why the currently used energy-consuming and expensive technologies do not always guarantee effective protection against eutrophication and its consequences. The goal of the study was to analyze the most widely used wastewater treatment technologies for enhanced biological nutrients removal in treated wastewater eutrophication potential. For this purpose, an analysis of the operation of 18 wastewater treatment plants based on different technologies in Finland, Canada, Poland, Russia and the United States was realized. The analysis concluded that the eutrophication potential of treated wastewater to a large extent is conditioned by the applied technology. The results of the research concluded that the eutrophication potential can serve an important criterion for decision-making regarding the proper selection of wastewater treatment technologies aimed at eutrophication mitigation.
The effluent quality standards elaborated to prevent eutrophication are developed mostly for total nutrient content, disregarding their bioavailability for water vegetation.
Broad knowledge analysis was carried out to develop a definition of the eutrophication potential of treated wastewater (EPTW), which is conditioned by the share of bioavailable nutrient forms.
EPTW reduction efficiency was analyzed for 18 wastewater treatment plants.
The analysis identified the enhanced biological nutrient removal (EBNR) technologies that allow low EPTW to be achieved.
The authors’ approach may form the methodological basis for ecologically and economically justified wastewater treatment technology selection.
The intensified development of anthropogenic eutrophication in recent decades has reached a global scale due to the negative impact of this process on surface water ecological state and water quality in many countries all over the world (Jin et al. 2019). This has led to the disturbance of water ecosystems equilibrium and secondary water pollution, which limits the possibilities of their use, threatening the total loss of their economic and biospheric functions (Cai et al. 2013). One of the main factors of the increased eutrophication development is the high content of nutrients of anthropogenic origin in water bodies (Howarth & Marino 2006; Choi et al. 2011). Municipal wastewater, containing large amounts of nitrogen (N) and phosphorus (P) compounds plays one of the main roles in the acceleration of eutrophication process intensity in surface waters (Bhagowati & Ahamad 2019). The main direction in eutrophication prevention has become the reduction of nutrient loads introduced to wastewater receivers, legally required in different counties e.g. by the Council Directive 91/271/EEC concerning urban wastewater treatment. Modern enhanced biological nutrient removal (EBNR) technologies can meet the appropriate legal requirements only if based on complicated and expensive treatment schemes (Panepinto et al. 2016; Henriques & Catarino 2017). But even such technologies do not always provide effective protection against eutrophication and its consequences (Svendsen et al. 2015; Smol et al. 2020). Such paradox largely appears due to the fact that currently the treated wastewater quality standards are mainly developed for such parameters as total nitrogen (TN) and total phosphorus (TP). However, the fact that eutrophication potential of treated wastewater (EPTW) is determined by the content of non-organic nutrients directly bioavailable for water vegetation is ignored in most legal documents setting the wastewater quality standards (Kobayashi et al. 2008; Mengel 2008; Zhang et al. 2017).
Moreover, considering this basic eutrophication mechanism it needs to be highlighted that appropriate wastewater treatment technology for prevention of eutrophication should be targeted not only at the efficient reduction of TN and TP loads in treated wastewater in accordance with strict modern requirements, but also at achieving their minimal eutrophication potential. When developing the quality standards for treated wastewater, which are the base for further decision-making about the proper selection of treatment technology, the knowledge about eutrophication mechanisms and the content of bioavailable forms of nutrients must be taken into account. Without such consideration, it is very difficult to ensure the effective protection of wastewater receivers against eutrophication, even when using extremely expensive technologies. Furthermore, taking into account the EPTW as a criterion of conditions for effluent discharge will allow for a proper environmentally and economically justified decision-making concerning EBNR technologies.
The research methodology includes a summary of the current state of knowledge in the area of nutrient removal efficiency in terms of EPTW, which was proceeded by a comprehensive literature analysis focused on EBNR technologies and nutrients bioavailability for the aquatic vegetation. Based on the present knowledge state review the authors' definition of EPTW was formulated. In order to analyze the common wastewater technologies in terms of EPTW assessment a primary statistical analysis of wastewater quality parameters, obtained directly from wastewater treatment plant (WWTP) operators and other documents and information in open access, was carried out. This analysis was based on the results of long-term monitoring of wastewater treated in 18 WWTPs in Finland, Canada, Poland, Russia and the United States, shown in Table 1. The selection of the facilities for this study was dictated by limited access to reliable data on the bioavailable nutrient compounds content in many WWTPs. This can be explained by the fact that in many countries there is no obligation for specific N and P compounds monitoring. In the EU Member States, the Council Directive 91/271/EEC concerning urban wastewater treatment imposed an obligation of the monitoring of only total N and P concentrations, omitting the bioavailability context of various nutrient compounds. Therefore, due to the selected WWTPs, which were located in similar climates, external factors such as temperature and the occurrence of summer and winter season were not included in this analysis.
The database consisted of a systematized and properly prepared data bank of raw and treated wastewater quality parameters treated in different technologies: conventional activated sludge (CAS), anoxic–oxic (AO), 3-step Bardenpho, Johannesburg (JHB) and University of Cape Town (UCT) systems with optional chemical precipitation. The capacity of the examined WWTPs ranged from 20,000 to 2,200,000 population equivalent (PE).
The study involved the analysis of the content of different forms of biogenic matter in raw and treated wastewater: TN, Kjeldahl nitrogen (NKj), organic nitrogen (Norg), ammonium nitrogen (N–NH4), nitrate nitrogen (N–NO3) nitrite nitrogen (N–NO2), TP and orthophosphates (P–PO4). As a result of the analysis, the examined technological systems were evaluated in terms of their eutrophication prevention capacity estimated on the base of nutrient removal efficiency and EPTW level.
Eutrophication potential of treated wastewater
Nutrients are the key elements needed for the development of all plant organisms, including the growth of aquatic vegetation. The occurrence of specific N and P forms is often considered as a limiting factor for eutrophication development according to the Redfield ratio (C:N:P = 106:16:1), which provides the optimal metabolism of phytoplankton (Redfield 1958; Zhou et al. 2018).
It is a well-known fact that predominantly dissolved inorganic forms of nutrients are directly available to aquatic vegetation (Berge & Kallquist 1995; Gao et al. 2010; Gu et al. 2011; U.S. EPA 2015). However, in rare cases, it can be also some organic compounds (Granéli et al. 1990). Among the N compounds, nitrates and ammonium are considered to be the most available forms (Priha 1994; Sova 1996; Nakajima et al. 2006; Beiras 2018). The only directly available P source for planktonic algae and bacteria are orthophosphates (H2PO4−, HPO42− or PO43−). However, the content of their individual forms in water depends on the pH value: at a water pH of 3–7 P is mainly in the form of H2PO4− and at pH of 8–12 HPO42− forms predominate (Warwick et al. 2013; Venkiteshwaran et al. 2018). Some hardly available P forms can be transformed into available forms in naturally occurring physical (e.g. desorption), chemical (e.g. dissolution) and biological processes (e.g. enzymatic degradation) (Boström et al. 1988). The share of bioavailable P compounds in treated wastewater is varied and depends to a large degree on the technologies used for their treatment (Brett & Li 2015). In most cases, over 50% of P in treated wastewater is not available for aquatic vegetation (Adamczyk & Jachimowski 2013). The bioavailability of different N and P compounds for aquatic vegetation is presented in Figure 1.
In Figure 1 the bioavailable P was defined as the sum of immediately available P and the potentially bioavailable P that can be transformed into an available form by naturally occurring processes (Boström et al. 1988). Whether the potentially bioavailable particulate P actually becomes available in receiving waters depends on such factors as the receiving water dissolved reactive phosphorus concentration and the position (location) of the particle in the water (Sonzogni et al. 1982). In terms of N compounds, the potential bioavailability of dissolved organic nitrogen (DON) may be dependent on the water ecosystem trophic state, while in waters with low biogenic compounds content (oligotrophic waters) due to limited primary production DON can also accelerate eutrophication (Bronk et al. 2006).
The knowledge analysis on the nutrients forms bioavailability stated that the majority of anthropogenic loads of these substances are classified as unavailable for direct consumption by aquatic organisms (Ekholm & Krogerus 2003; Shen et al. 2011; Anwar & Tao 2016).
Based on the analysis of the knowledge obtained from numerous literature sources on the bioavailability of various nutrients forms (Stepanauskas et al. 1999; Neverova-Dziopak 2010; Glibert et al. 2014; Wang et al. 2018), a definition of the EPTW was formulated: EPTW is the share of bioavailable (inorganic) forms of N and P in treated wastewater introduced into the receiver, which determines the intensity of aquatic vegetation development. Otherwise, the EPTW is the degree of the impact of wastewater discharged to the receiver on the development of eutrophication processes, conditioned by the content of bioavailable forms of nutrients.
Consideration of the contribution of bioavailable forms of N and P in the effluent would allow for the selection of effective nutrient removal technologies, justified in ecological and economic terms. Currently, there is insufficient information on the impact of different treatment technologies on the level of EPTW and their ability to intensify eutrophication processes.
The selection and design of wastewater treatment technologies are presently based on the restrictive legal requirements, which are also the main tool for water protection actions (EC 1991). Unfortunately, most legal requirements specifying the conditions of wastewater discharge into surface waters are developed without consideration of the specificity of mineral nutrition of aquatic vegetation. Therefore, in the water legislation of most countries, the permissible content of N and P in treated wastewater is set only for TN and TP without consideration of the EPTW (Neverova-Dziopak & Preisner 2015).
The above considerations constituted a reason for attempting to study the impact of different technologies on the level of EPTW and to indicate the importance of this criterion when selecting appropriate technologies to ensure water protection against eutrophication. By analyzing the EPTW in selected WWTPs it is possible to identify the technologies ensuring the lowest EPTW level and efficient prevention against eutrophication caused by municipal wastewater.
Analysis of the EPTW
The composed dataset was analyzed to calculate the share of bioavailable forms of N and P in raw and treated wastewater and to assess their EPTW. The basic characteristics of the examined objects are presented in Table 1.
CBAN – share of BAN in TN content [%],
CBAP – share of BAP in TP content [%],
Nmin – inorganic N as the sum of N–NH4; N–NO2; N–NO3 [mg/l],
Pmin – inorganic P as P–PO4 [mg/l].
ΔCBAN – change in the share of bioavailable N in TN content in raw and treated wastewater [%],
ΔCBAP – change in the share of bioavailable P in TP content in raw and treated wastewater [%],
CBAN(RW) – share of bioavailable N in TN content in raw wastewater [%],
CBAP(RW) – share of bioavailable P in TP content in raw wastewater [%],
CBAN(TW) – share of bioavailable N in TN content in treated wastewater [%],
CBAP(TW) – share of bioavailable P in TP content in treated wastewater [%].
RESULTS AND DISCUSSION
The values of the analyzed parameters, including the average influent concentration (in dark grey) and average effluent concentration (in light grey) of all analyzed parameters, are shown in Figure 2. Because of the wide spectrum between influent and effluent values, an inverted logarithmic scale was used.
It needs to be underlined that unfortunately the P–PO4 concentration in treated wastewater was not monitored in four WWTPs in Katowice (Podlesie, Gigablok, Panewniki, Dąbrówka Mała), while in Włocławek and Chicago-Lemont WWTPs the P–PO4 content monitoring were excluded in both influent and effluent.
The TP reduction rate was the lowest in WWTPs in Chicago based on CAS technology and ranged from 28% in Chicago-Calumet to 80% in Chicago-Kirie. The highest reduction of TP was obtained in Katowice-Dąbrówka Mała WWTP (99%) in which the highest TP influent concentration was observed (>34 mg/l). City of Penticton and Helsinki-Viikinmaki, Kraków-Płaszów, Petersburg (South-West WWTP) and Włocławek WWTPs also achieved over 95% TP reduction efficiency.
In terms of TN, five of the six CAS-based WWTPs presented the lowest reduction rates (32% in Chicago-Lemont, 45% in Chicago-Egan, 47% in Chicago-Hanover Park 48% in Chicago-Calumet and 53% in Chicago-North Side), while in Chicago-Kirie WWTP TN reduction was 66%. The highest TN removal efficiency was observed in Katowice-Dąbrówka Mała, Katowice-Gigablok, Helsinki-Viikinmaki, City of Penticton and Włocławek where it reached over 90%.
However, the value of TN and TP in treated wastewater is not informative in terms of the EPTW value. Therefore, the next stage of analysis was the evaluation of the BAP and BAN compounds share in total wastewater nutrient content. The results of the analysis are presented in Tables 2 and 3 as a ranking of WWTPs according to the ascending order of ΔCBAP and ΔCBAN values, respectively.
The results presented in Tables 2 and 3 show the potential of nutrient removal efficiency in different EBNR technologies used in the analyzed municipal WWTPs in the context of the content of their bioavailable forms. The general regularity observed for all WWTPs was the lower share of bioavailable N and P in raw wastewater than in treated ones. Exception accounted for wastewater treated in Helsinki-Viikimaki and Katowice-Podlesie WWTPs (where 8% and 10% reduction of Nmin share in TN was observed, respectively) and City of Pentiction, Helsinki-Viikinmaki and Kraków-Kujawy WWTPs (where 4%, 2% and 2% reduction of P–PO4 share in TP was observed, respectively). On the other hand, the highest increase of the Nmin share in TN compared to the influent value was observed in the effluent of the City of Penticton (41%) and in all analyzed WWTPs in Chicago (21–33%). A similar situation concerned the share of Pmin in TP, with the highest increase observed also in Chicago WWTP effluents (32–48%) compared to influent values.
The analysis of data on the share of bioavailable phosphorus compounds (Table 2) in treated wastewater leads to the conclusion that the lowest EPTW was achieved in the advanced technologies with optional chemical P precipitation (as key eutrophication limiting factor) such as UCT (City of Penticton WWTP: ΔCBAP = −4% and BAP(TW) = 0.04 mg/l), AO with post-filtration (Helsinki-Viikinmaki WWTP: ΔCBAP = −2% and BAP(TW) = 0.10 mg/l), and modified 3-step Bardenpho (Kraków-Kujawy WWTP: ΔCBAP = −2% and BAP(TW) = 0.20 mg/l). Such technologies would be most appropriate in the case of wastewater discharge to the receivers with P-limited eutrophication. Moreover, the concentration of BAP in the effluents from the above-mentioned WWTPs was relatively low and so the eutrophication risk would be minimal.
In the case of N-limited eutrophication in the wastewater receiver the recommended technologies would be the following: 3-step Bardenpho (Katowice-Dąbrówka Mała WWTP: ΔCBAN = −10% and BAN(TW) = 5.57 mg/l), AO with post-filtration (Helsinki-Viikinamki WWTP: ΔCBAN = −8% and BAN(TW) = 2.93 mg/l) and modified 3-step Bardenpho (Włocławek WWTP: ΔCBAN = 1% and BAN(TW) = 6.05 mg/l). Additionally, the foregoing technologies are capable of ensuring relatively low concentration of BAN in the WWTP effluents.
Unfortunately, conventional and relatively inexpensive technologies, such as CAS, which are used among others in Chicago WWTPs, are characterized by high EPTW in terms of N and P and can directly result in accelerating the eutrophication process in the receiving water body. Furthermore, the treated wastewater in WWTPs based on CAS have usually higher BAN and BAP concentrations (e.g. Chicago-Lemont in terms of BAN) than the more advanced systems.
In European countries, CAS technology can only be recommended when supported by chemical precipitation (Szabó et al. 2008) due to its very high efficiency in P removal (Aboulhassan et al. 2006). Research from Norway (Kallqvist et al. 2002) shows that the average effluent concentration of TP in effluents from 99 chemical treatment plants was 0.42 mg/l, when in WWTPs with biological P removal it was approx. 2.93 ± 1.68 mg/l (Ødegaard & Skrøvseth 1997). A different study confirms that larger WWTPs with chemical precipitation achieves an average effluent P concentration of 0.26 mg/l (Ødegaard 1992); however, the higher the coagulant amounts used for chemical P removal the higher the costs of metal and sludge separation for future sewage sludge recovery (Rossi et al. 2018). In light of limited P rock raw deposits (Shaddel et al. 2019), the world's attention is brought to secondary P sources, such as sewage sludge (Smol et al. 2016) obtained in different wastewater treatment technologies. This direction gives hope that the current and future EBNR technologies will provide not only effluents with low EPTW but also a P-rich sewage sludge with possibilities for economically reasonable P extraction (Bashar et al. 2018).
The results of the present study suggest that the highest efficiency of P elimination from wastewater affecting the high P content in the sludge is possible to achieve in chemically supported EBNR technologies. However, further research should be conducted to indicate an optimal type of applied coagulant, its optimal doses and the points of its addition to providing an opportunity for further P recovery.
The presented analysis of selected WWTPs operating in various technologies stated that the EBNR technologies are able to efficiently reduce the total load of biogenic substances in treated wastewater. However, the majority of the analyzed WWTPs demonstrate an increase of bioavailable nutrients share in treated wastewaters, which means an increase of their EPTW which, in turn, can stimulate the intensive development of aquatic vegetation in wastewater receiver.
Undoubtedly, the high content of BAN and BAP in the effluent has a significant role in shaping the unfavourable surface waters trophic state; however, we must consider that along with the enhanced removal of both bioavailable N and P the costs of wastewater treatment increase exponentially. Thus, this study presents an alternative approach to the EPTW by highlighting the ability of some wastewater treatment systems to reduce the share of bioavailable nutrient forms, which can rapidly enter the biological and biotic circle and, under favourable conditions, can accelerate eutrophication.
It should be stipulated that the conclusions drawn are based on the results of wastewater quality analysis from several selected WWTPs. The authors are aware that EPTW is also influenced by a number of other factors not taken into account in these studies. The analysis carried out determined an approach based on the assessment of EPTW as a valuable tool supporting decision-making in the selection of EBNR technologies, aimed at mitigating eutrophication and its negative effects. However, the additional effect of preventing the intensification of eutrophication can be obtained by taking into account the knowledge about the limiting factors of eutrophication in individual receivers and other key factors that will enable the more ecologically sound decision-making in the field of wastewater treatment technology.
The study was developed under the project ‘Monitoring of water and sewage management in the context of the implementation of the circular economy assumptions’ (MonGOS), no. PPI/APM/2019/1/00015/U/00001/ZU/00002 (2020–2021), which is financed by the Polish National Agency for Academic Exchange (NAWA) under the International Academic Partnerships Programme.