1. Introduction

• Insufficient alignment of the directive with the objectives of the EU policy around the EU Green Deal.

• Insufficient and uneven level of management

2. Nutrient Removal

2.1. Nitrogen Removal Processes

Nitrogen is one of the basic elements of cell mass. However, its excessive release into water bodies can lead to the acceleration of eutrophication in receiving water. Nitrogen removal from wastewater can be carried out using physico-chemical, chemical, bioelectrochemical and biological methods. The authors [20] dealt with operating conditions, advantages, and disadvantages of used processes and influencing factors.

Ammonia stripping with air is the most common pretreatment process. However, it requires large spaces and is time-consuming. Its disadvantage is also the formation of calcium carbonate deposits.

Chemical processes showed the least efficiency around 20-30%, which is dependent on the pH of the process.

Biological treatment processes (Nitrification and Denitification, Anammox,) are among the most effective (in terms of achieved efficiency (> 95%), costs and an environmental origin) for the removal of nitrogenous pollutants from municipal WW.

Due to the complexity of wastewater, a combination of different methods using the advantages of individual systems is recommended for efficient and economical treatment of WW in accordance with the characteristics of WW and the quality requirements.

Ammonium nitrogen can be transferred to nitrite or nitrate in aerobic environment. Inorganic carbon, i.e., carbon dioxide is utilized by chemoautotrophic nitrification bacteria (Nitrosomonas, Nitrobacter) in synthesis of new microorganisms. The energy-yielding two step oxidation of ammonia to nitrate is generally represented as follows [17]:

Nitrosomonas

$$2N{H}_4^{+}+3O_2 \to 2N{O}_2^{-}+4H^{+}+2H_{2}O$$

(1)

Nitrobacter

$$2N{O}_2^{-}+O_2 \to 2N{O}_3^{-}$$

(2)

Total reaction

$$N{H}_4^{+}+2O_2\to N{O}_3^{-}+2H^{+}+H_{2}O$$

(3)

The removal of the products of nitrification can be accomplished in anoxic conditions, i.e., the level of dissolved oxygen is maintained near zero (less than 0,5 mg/l). The reduction of nitrite and nitrate to nitrite to nitric oxide to nitrous oxide to nitrogen occurs. In anoxic conditions, i.e., the level of dissolved oxygen is maintained near zero (less than 0,5 mg/l), the reduction of nitrite and nitrate to nitric oxide to nitrous oxide / nitrogen occurs according to Equation (4):

$$N{O}_3^{-} \to N{O}_2^{-} \to NO \to N_{2}O \to N_2$$

(4)

Nitrate nitrogen is utilized as the terminal electron acceptor, releasing nitrogen gas as the product in the process of denitrification. Organic carbon source is necessary in the process of denitrification.

By combining shortened nitrification, the so-called nitritation, significant savings in both investment and operating costs can be achieved with the anammox process. Nitrifying bacteria (AOB - ammonium oxidizing bacteria) oxidize 57% of the input NH₄⁺ to NO₂^- according to Equation (5), which saves approx. 50-60% of energy for aeration compared to nitrification. The reason is that only part of the ammoniacal nitrogen is oxidized biochemically, while the oxidation is already completed in the nitrite stage. Anammox microorganisms transform the remaining ammoniacal nitrogen and gaseous N₂ according to Equation (6) [21]. As the annammox microorganisms of the process belong, unlike denitrifying bacteria, to chemolithotrophic microorganisms, nitrogen is essentially removed without consumption of organic substrate. At the same time, up to 80% less excess sludge is produced. The anammox process transforms up to 13% of input nitrogen into nitrates. In practice, these nitrates are at least partially denitrified by heterotrophic microorganisms. The advantages of technologies based on the anammox process include high performance (the proposed load can be 0.35–2.3 kg_N/(m³/d) [22], in some cases even 10 kg_N/(m³/d) [23].

$$N{H}_4^{+}+1.5O_2\to N{O}_2^{-}+2H^{+}+H_{2}O$$

(5)

$$\begin{gathered} N{H}_4^{+}+1.32N{O}_2^{-}+0.066HC{O}_3^{-}+0.13H^{+} \\ \to 1.02N_2+0.26N{O}_3^{-}++0.066C{H}_2{O}_{0.5}{N}_{0.15}+2.03H_{2}O \end{gathered}$$

(6)

Biological nitrogen removal (BNR) is a critical process in WW treatment. The authors of [24] present new microorganisms that can remove nitrogen through new metabolic pathways. These include ammonia oxidizing archaea (AOA), complete ammonia oxidation bacteria (COMAMMOX), anaerobic ammonium oxidation associated with iron reduction bacteria (FEAMMOX), anaerobic ammonium oxidation bacteria (ANAMMOX) and anaerobic methane oxidation denitrifying microorganisms (DAMO). Compared to current nitrifying or denitrifying bacteria, these new microbial groups have better physico-chemical tolerance and/or lower release of greenhouse gases. They represent promising microbial communities that could provide new technologies for high-performance and energy-saving nitrogen removal from WW.

2.1.1. Single Anoxic System

The development of nitrogen removal technologies gradually progressed from three-sludge to single-sludge systems. In the case of two-sludge systems, it is advantageous to operate the denitrification process in reactors with increased biomass (packed columns, rotating disk reactors, fluidized bed bioreactors). Systems with separated biocenosis are relatively stable and achieve high nitrogen removal efficiencies. However, their disadvantage is relatively high investment and operating costs.

The choice of nitrogen removal technology is marked not only by the characteristics of wastewater, the required quality of treated water and investment options, but also by the stage of decision-making and selection. In the case of the construction of a new sewage treatment plant (WWTP), it is possible to consider more comprehensively all aspects, starting with requirements for the built-up area, ending with guarantees of the quality of the outflow and flexibility in terms of managing individual processes in the event of changes in the amount and composition of wastewater.

Currently, single-sludge systems are most often operated. Under single-sludge systems, the systems in which activated sludge is separated from treated wastewater in only one settling tank is meant. These systems make it possible to use organic wastewater pollution as a source of organic carbon for denitrification, while relatively high nitrogen pollution removal efficiencies are also achieved.

The most widespread arrangements of a single-sludge system regarding the removal of nitrogen compounds from wastewater include upstream denitrification and simultaneous denitrification. In the case of some industrial wastewaters, included denitrification appears to be advantageous.

These systems make it possible to achieve relatively high treatment efficiency also when using municipal WW as a source of organic carbon for denitrification. They are considerably flexible in terms of the possibility of controlling nitrification and denitrification processes. The most used single-sludge system technologies for removing nitrogen compounds from WWV are upstream denitrification and simultaneous denitrification.

Each system with a single anoxic zone (single anoxic system) should be able to achieve for sewage WW average annual output values of total nitrogen (TN) of 8-12 mg/l. [25].

Most of the biological stages of urban WWTPs can be characterized from the point of view of the hydrodynamic regime as systems with a gradual flow. The investigation of the influence of hydrodynamic conditions on the kinetics of the nitrification process Chudoba et al. [26] showed that the gradual flow system is more advantageous for the nitrification process compared to the mixing system [26]. Azimi and Horan [27] also presented the advantages of a gradual flow nitrification system under steady state conditions [27]. Similar results were also published by Horan and Azimi [28] based on the investigation of the responses of nitrification systems with different hydrodynamic conditions to a sudden increase in nitrogen pollution [28]. Both systems showed very similar responses for the monitored forms of nitrogen pollution. Thus, in the mixing system, the effect of dilution in favor of the nitrification process was not observed.

Wuhrmann system

Biological nitrogen removal (BNR) can occur in a two-step process that begins with the use of autotrophs to oxidize NH₄⁺ to NO₃^- (nitrification), followed by the reduction of NO₃^- to N₂ by heterotrophic microorganisms (denitrification). The system was developed by Wuhrmann [29] and in literature is referred according to its developer or post-denitrification system.

In the case of domestic wastewater, the basic disadvantages of this system include the need for an external source of organic carbon as well as the increased cost of oxygen supply in connection with the oxidation of organic wastewater pollution in the aerobic stage. In this system, it is not possible to eliminate the decrease in neutralization capacity due to the nitrification process by hydroxyl ions produced in the denitrification stage. For this system, it is also necessary to include post-aeration before the settling tank.

Post-denitrification is advantageous mainly from the point of view of energy (smaller requirements for recirculation) but also from the point of view of the possibility of achieving higher efficiency of the process compared to pre-denitrification. However, to achieve required nitrogen removal efficiency, the supply of an external carbon source that does not contain ammonia or organic nitrogen, or with their small content, is inevitable. Monteith et al. [30], used as an organic carbon source for denitrification 30 different types of industrial wastes and compared the results with denitrification using methanol as an organic carbon source. Among the waste used were various food (brewery, distillery, wine, starch), chemical (organic acids, distillation residues, higher alcohols) and other waste products. In 27 cases, higher denitrification rates were achieved than when using methanol. In addition to liquid industrial waste, it is also possible to use gases, especially methane and natural gas. One of the basic conditions for all carbon sources for denitrification is their good biological degradability and low or no toxicity to the nitrification process [30].

Modified Ludzak-Ettinger system

Ludzack and Ettinger (LE) [31] developed concurrent system by placing anoxic zone ahead of the oxic zone. LE system was improved by Barnard [25,32] providing additional internal recirculation to return mixed liquor back from nitrification to denitrification zone. Such system is in literature referred to as Modified Ludzack-Ettinger (MLE) or pre-denitrification system.

In upstream denitrification in MLE system (Figure 1) WW first flows through an anoxic zone, in which the organically bound nitrogen is hydrolyzed to ammoniacal nitrogen. Ammonia contained in wastewater is consumed in the anoxic zone only for the synthesis of new biomass. In the second, oxic zone, ammoniacal nitrogen is oxidized to nitrites and nitrates. Oxidized forms of nitrogen are transported back to the anoxic zone by internal recirculation (IR) of mixed liquor suspended solids (MLSS) as well as external recycle of return sludge (RS). Heterotrophic microorganisms use organic pollution of WW as a source of carbon, while oxidized forms of nitrogen are reduced to N₂O or nitrogen gas.

The efficiency of the denitrification process depends on the amount of recirculation. On the other hand, with a higher recirculation, there is a danger that a larger amount of oxygen is introduced into the anoxic zone, which inhibits denitrification. It is possible to prevent this effect, e.g., by including a small anoxic zone at the end of the oxic part of the system. Due to the higher specific rates of denitrification compared to nitrification, the volume of the denitrification part is generally smaller than that of the nitrification zone. This volume is also often used to increase the volume of the oxic part of the activation, especially in the winter months due to a significant decrease in the rate of nitrification compared to the summer period [25].

If there is a sufficiently low concentration of dissolved oxygen, microorganisms use organic pollution from wastewater as a source of organic carbon, and oxidized forms of nitrogen are reduced to N₂O or nitrogen gas. The activity of denitrification affects the amount of recirculated liquid R = (RS + IR) according to the Eqn. (7) [27]:

$$E=\frac{R}{R+1}100\%$$

(7)

From Eqn. (7) follows that at the value of the total recirculation ratio R is 4, about 80% of nitrates are removed by denitrification. A further increase in recirculation would be accompanied by a disproportionate increase in operating costs compared to the achieved effect in terms of denitrification efficiency. In addition, at higher values of recirculation, a larger amount of oxygen can be introduced into the anoxic zone, which inhibits the denitrification process. It is possible to prevent this effect, e.g., by including a small anoxic zone at the end of the oxic part of the system [25]. Another disadvantage of this arrangement is the decrease of the concentration gradient in the oxic part due to internal recirculation, which can be reflected in the deteriorated sedimentation properties of the sludge.

A MLE type of approach can be relatively easily retrofitted into an existing wastewater treatment plant through the installation of baffles, mixers, and an internal recycle capacity. ln a diffused air system, the diffusers would have to be relocated to create an anoxic zone and to ensure that there is adequate air for nitrification in the oxic zone. Som diffusers could be left in anoxic zone to provide mixing if the dissolved oxygen concentrations were kept at a low level.

In the MLE system (Figure 1), which is equipped with internal recirculation, the required TN output concentration of 8 mg/l can be achieved. Without internal circulation (the original Ludzack-Ettinger (LE) system) a concentration of 12 mg/l TN could be achieved [27].

The operation of a biological system with complete nitrification is, in the case of sewage WW, accompanied by an approximately 50% increase in the energy required for aeration compared to the removal of organic pollution alone [17]. By appropriate inclusion of the anoxic zone in the technological line, part of the organic pollution is oxidized by anoxic respiration, which will be reflected in the reduction of operating costs associated with the necessary energy for the supply of oxygen. In addition, denitrification can theoretically reuse 62.5% of the oxygen consumed for nitrification. However, due to incomplete denitrification, the real possibilities are around 50%, which roughly covers the costs of mixing the anoxic zone and nitrate recirculation [17].

The authors of [28] present the feasibility of biological nitrogen removal (BNR) using a modified Ludzack Ettinger (MLE) system at a reduced hydraulic retention time (HRT) of 5.5 hours. Complete nitrification was achieved at 75–80% TN removal at a temperature of 12 °C. There was also a reduction in the net observed yield of biomass by 28% compared to the full-scale plant, to 0.31 g_VSS/g_CODr. Compared to the real conventional technology using activated sludge (CAS), the sedimentation properties were also improved (Sludge Volume Index (SVI) value decreased from 202 mlg-1 to 97 ml/g) The values of the heterotrophic biokinetic parameters of the CAS and MLE systems were very similar and consistent with published values for primary runoff. The values of the specific nitrification rates at a temperature of 20 °C in the MLE 0.14 gNH₄-N/(g_VSSd) system were 55% higher than in the CAS [28].

RDN system

Some shortcomings of the system with upstream denitrification are eliminated by the R-D-N system [30,31]. This system is an extension of the upstream denitrification system with a return sludge regeneration zone R. To suppress the excessive growth of filamentous microorganisms, the entrance part of the denitrification zone can be set aside as an anoxic selector (Figure 2).

This system is characterized by a 20-25% reduction in required volume compared to pre-denitrification while achieving the same denitrification efficiency. Simultaneously less internal recirculation is also required. The presence of the regeneration zone ensures the restoration of the accumulation capacity of microorganisms and an increase in the aerobic age (SRT) of the sludge. When controlling aeration in the regeneration zone, increase the share of simultaneous denitrification at the level of micro flakes (micro segregation). Increased phosphorus removal was also observed in this process compared to other nitrification-denitrification systems.

2.1.2. Cyclical Nitrogen Removal

Alternating aerobic and anoxic zones can be achieved in a batch or continuous flow activated sludge system by alternately switching on and off the aerators. This type of intermittent or pulsed aeration in activated sludge facilities is referred to as cyclical nitrogen removal (CNR) [25]. CNR processes can be advantageously applied in existing activated sludge systems for which the permits for nitrogen removal have also been revised. It only requires small process adjustments, e.g., installation of partitions or timers for the cycling of aeration equipment, or pumps to ensure internal recirculation, pipelines, or fittings to ensure the ability of gradual supply). Thus, compared to a single anoxic zone, potential cost savings can be expected when implementing the CNR process.

Simultaneous denitrification

Nitrification and denitrification processes can take place simultaneously in one bioreactor with spatial or time segregation (level of macro segregation level) of oxic and anoxic conditions. Simultaneous denitrification takes place in oxidation trenches and circulation systems with activated sludge in one bioreactor. The creation of oxic and anoxic zones is ensured by the operation of aeration devices. At the same time, the operation of the aerators ensures a sufficient speed of internal recirculation and thus the maintenance of the activated sludge in suspension.

The simultaneous process of nitrification and denitrification (SND) can also take place simultaneously in the aerobic reactor. The conditions for the course of these processes are created inside the structure of activated sludge flocs (micro segregation of oxix/anoxic conditions). The achieved nitrogen removal efficiency without additional carbon is 80 to 96% [5]. The required C:N ratio was 10. The dissolved oxygen (DO) values that need to be ensured are in the range of 0.3 to 0.7 mg/l [33].

The oxidation ditch consists of a ring-shaped channel (Figure) about 1 - 1.5 m deep. Supply of oxygen and maintenance of activated sludge in suspension are provided by aerating rotor consisting of Kessener brush which are placed across the ditch. The mixed liquor circulates at about 0.1 to 0.6 m/s [34]. Arbitrary flow can be expected due to the sequence of the zones with different mixing in the bioreactor and equalization of impact of pollution and hydraulic loads.

Figure 3. Oxidation ditch.

Figure 3. Oxidation ditch.

The oxidation ditch was originally developed for small towns in the Netherlands and usually is operated on the principle of a long-term extended aeration process. The endogenous respiration phase of the growth curve is characteristic for this process. Thus, a relatively low organic load (F/M) and long HRT are maintained. High values of SRT and low excess sludge production are also typical for this mode of operation. Wastewater is usually treated without primary sedimentation. Their main disadvantage is the greater demands on the built-up area.

It enables 90 to 95% BOD₅ removal efficiency. However, in the case of sufficiently long channels (corridors), it enables the creation of oxic and anoxic zones (macro segregation in space). Controlled aeration can achieve an efficiency of total nitrogen removal in oxidation trenches of around 90% [35].

Oxidation ditches can be operated in the range 8-12 mg/l of effluent TN concentration [25]. The performance of a ditch system cannot, be predicted with as much certainty as other activated sludge systems. Acceptable performance will require field monitoring for optimizing the operation, such as determining the DO profile. The process can then be optimized by adjusting the DO level, which is done by turning aerators on or off or by controlling the oxygen transfer rate by varying the aerator submergence and/or horsepower [22].

Intermittently aerated CSTR

For smaller wastewater treatment plants (e.g., oxidation ditch), it is more appropriate to apply systems with time alternation/segregation of oxic and anoxic conditions. Time segregation is an analogy of spatial segregation.

A characteristic feature of systems with time segregation is the high flexibility of changing the length of the cycle, or lengths of the oxic and anoxic periods.

The common of intermittently aerated/operated systems is the possibility of fully automated operation, which creates real conditions for expanding their practical applications.

Intermittent operation regarding oxygen supply can be applied in a batch/semi-continuous mode or a continuous stirred- activated sludge reactor (CSTR). Sedimentation is carried out in a separate sedimentation tank (Figure 4).

In continuously operated systems with time segregation, the activation tank is constantly stirred to prevent sludge settling. Based on a suitable signal (like systems with intermittent operation), air is intermittently supplied to the activated sludge system. During aeration, the concentration of nitrates increases and the concentration of ammonia decreases. In the anoxic period, the course is the opposite [36]. This method of operation is suitable to use e.g., in the intensification of small oxidation trenches for the purpose of nitrogen removal [35], where due to the small lengths of the channels, spatial segregation of oxic and anoxic conditions is not possible. Unlike SBR, this system is close to an ideally mixed system. At low load values, it is necessary to extend the anoxic period to prevent sludge bulking [37].

Intermittent aeration in such a reactor is used intensively for small wastewater treatment plant regarding nitrogen removal [35,37,38].

Sequential Batch Reactors (SBRs)

The predecessor of this bioreactor was a batch bioreactor for BOD removal, known as the fill-and-draw process. The reactor was filled with settled wastewater and aerated long enough to oxidize most of the BOD. The contents of the reactor were then allowed to settle, and the treated supernatant was discharged into a receiving water. Part of the settled sludge was withdrawn and the whole process was repeated. However, this reactor fell out of favor due to the amount of operator control required.

With the advent of microprocessor control, a modification of this process, known as SBR, is gaining more and more popularity. It allows to carry out several time-segregated processes such as nitrification and denitrification in the same reactor. Since, in addition to individual biochemical reactions, all other operations (filling WW, removing supernatant, thickening sludge, removing excess sludge) also take place in the same reactor, two or more reactors are needed for wastewater treatment. SBRs are single reactors that cycle through anaerobic, anoxic, and oxic conditions to achieve biological nutrient / nitrogen removal. This technology is particularly suitable where hydraulic and organic loads are highly variable and skilled operating and maintenance personnel are limited [39].

Due to the existing concentration gradient and the controlled alternation of anoxic and oxic conditions, a sufficient decrease in the concentration of organic pollution is usually achieved before the oxic period. Thus, the anoxic period fulfills the function of a selector zone [17].

The SBRs can be operated with less than 8 mg/l TN in an effluent when optimizing DO level [25]. SBRs are convenient for relatively small flow but with high variability.

Carrousel system

Challenges to the development of the Carrousel process (Figure 5) were the excessive surface area, and therefore also space requirements, the enormous capital expenditures for surface aerators, and the associated energy costs of designing large capacity oxidation trenches. The Carrousel system has been developed to provide sufficient aeration and maintain adequate flow rates while maximizing surface area utilization [25].

This type of activated sludge process represents an innovation of an oxidation ditch with respect to a specific kind of aeration. Vertically mounted mechanical aerators are used to introduce oxygen into this system. At the same time, they provide sufficient horizontal velocity for the activated sludge. Thus, they prevent solids from settling in the reactor channels [40].

Technologically it is an extended activated sludge process, for which high solid retention time (SRT) and hydraulic retention time (HRT), low food to microorganism ratio values and low excess sludge production are representative. From the point of view of the hydrodynamic regime, the Carrousel system corresponds to a series of completely mixing sections with different dissolved oxygen concentrations.

The application of this bioreactor for domestic wastewater treatment (fed with pre-settled sewage and a sludge load ≥ 0.1 kg BOD/kg MLSS.d) relates to severe bulking problems (Microthrix parvicella) independently of the type of selector (plug flow or completely mixed) used [41]. This gives another evidence about completely mixed flow in the bioreactor.

The nominal capacity of this bioreactor is significantly higher in comparison to an oxidation ditch. The other advantage of this type of activated sludge process relates to the aerobic stabilization of the activated sludge and the possibility of the exclusion of a primary sedimentation unit in WWTP. High control flexibility is typical for this type of bioreactor by changing the number of aerators in use, the rotating velocity of aerators and/or the depth of submerging [42].

2.1.3. Multi-Anoxic Zones Systems

The current approach should be towards the use of systems with low oxygen and carbon content, which are beneficial for saving energy and preventing environmental pollution.

The four-stage Bardenpho process (Figure 6) has demonstrated its viability in the reconstruction of existing WWTP in Chesapeake, for which the permit was revised with nutrient removal requirements [20].

If sufficient volume of the activation tank is available, its modification to the Bardenpho system may only require the installation of baffles and devices for internal MLSS recirculation. In the case of activation devices in which nitrification was not operated, it will also be necessary to increase the aeration capacity [25,43].

The multi-anoxic zone with step feed arrangement (Figure 7) makes it possible to eliminate some of the disadvantages of pre-denitrification. Recirculation with return sludge is usually sufficient. The efficiency (E, %) of denitrification depends on the number of anoxic-oxic sections n (n = 2 - 4) of the according to the relationship (8):

$$E=1-\frac{1}{n\left(RS+1\right)}100\%$$

(8)

Figure 6 shows a diagram of the four-stage Bardenpho system.

It is obvious from relation (8) that with the return sludge recirculation ratio RS = 1, a 75% denitrification efficiency can be achieved in the four-stage Bardenpho system (Figure 5), resp. in multi-anoxic zone with step feed (Figure 7) 83% efficiency.

For comparison, in the case of a system with upstream denitrification (pre-denitrification MLE, it is necessary to ensure the internal recirculation ratio IR = 2, resp. IR = 5 to achieve the same efficiencies. For higher denitrification requirements, internal recirculation can also be included in 6 stage systems (Figure 7).

In addition to reducing the necessary recirculation, these modifications lead to a gradual equalization of changes in the acid neutralization capacity (total alkalinity) and thus also the elimination of the possible negative impact of decreasing pH on the course of the nitrification process. This is especially advantageous in the case of wastewater with a high nitrogen content, or wastewater with low buffering capacity. Another advantage is the elimination of the influence of internal recirculation on the intensity of axial mixing in individual sections. Due to the cascade arrangement, this system shows a certain concentration gradient. A significant advantage is the smaller total required volume compared to upstream denitrification.

With the required output concentrations of 6 to 8 mg/l TN, it is advantageous to use systems with dual anoxic zones. For example, the Bardenpho system makes it possible to achieve output concentrations of <3 mg/l TN in combination with filtration. For the area where effluent concentrations must be in a range of 6-8 mg/l TN, the application of the Bardenpho system with two anoxic zones without effluent filtration [25] is sufficient.

In systems with separated sludge (separated-sludge denitrification), concentration limits of <3 mg/l TN can also be achieved. If the strict limit for TSS is <10 mg/l, separate-stage denitrification using a downflow filter is required. If effluent filtration is not required, an up flow packed-bed denitrification bioreactor can be used. During the warmer period, it is possible to use cyclic aeration of the nitrification stage to achieve the conditions for denitrification and thereby also reduce the consumption of supplied organic carbon in the denitrification bioreactor, saving operating costs [25].

2.1.4. Biofilm-Based Processes

Technologies such as Submerged Aerobic Fixed Film Reactor (SAFF), trickling filters, and biofilters are commonly known as fixed-bed processes in wastewater treatment. A study of nitrogen removal from wastewater plant using a submerged attached growth bioreactor showed 85% average total nitrogen removal [44].

A recent study showed that nitrogen removal of 52–54% was achieved with partial nitritation in sponge trickling filters (STFs) [45]. The report also shows increased nitrogen removal of more than 60% in currently designed trickling plants.

2.1.5. Hybrid Systems

In single-reactor hybrid systems with combined suspended and attached growth biomass, a block filling installed in the supporting structure above the aeration elements can be used as a biomass carrier. Another option is the use of submerged modules made of plastic mesh or submerged rotating discs. The growing biomass in the activation tank can also be fixed in suitably shaped plastic particles that move freely in the activation mixture. It is advantageous to use a mesh made of plastic or foam plastic, as the biomass is captured not only on the surface of the carrier particles, but also inside the porous structure. This significantly increased the amount of fixed biomass compared to carriers with a solid surface.

In Germany, the Linpor system was developed for the purpose of intensification of overloaded treatment plants [46]. Polyurethane foam cubes are used as a biomass carrier. The main function of the carrier is to concentrate the biomass in the tank without increasing the material load of the settling tank. The amount of carrier can vary from 10 to 40% depending on the required biomass concentration.

As a result of the operational verification of the Linpor system at the WWTP, where the activated sludge system was fed by WW from the brewery and dairy, a more than two-fold increase in the biomass concentration was determined. Significant decrease in the sludge volume index (SVI) values from 500 ml/g to 85 ml/g was also measured. These changes occurred 30 days after the addition of polyurethane cubes (20% of the tank volume). Nitrification also has started. The nitrogen balance showed that 63% of oxidized forms of nitrogen (created by nitrification or present in WW) were removed by simultaneous denitrification in the anoxic cores of the carrier cubes.

Another system is the Captor system, developed by the University of Manchester [47]. The bioreactor is operated on the principle of a pseudo-fluidized layer. The fluidized state of the biomass carrier made of polyester foam is achieved by compressed air distributed by a system of nozzles at the bottom of the reactor. Part of the carrier is regularly removed from the fluidized layer by a mammoth. After straining, the polyester foam particles are mechanically wrung out. The biomass obtained in this way contains up to 6% dry matter. Cleaned particles of the biomass carrier are returned to the reactor by the mammoth. The average concentration of biomass in the reactor is 8 to 12 kg.m^-3. For municipal wastewater, the retention time in the system is up to 60 minutes.

Intensification of the processes of nitrification and denitrification can also be achieved by applying a hybrid system by combining growth and suspended biomass [4]. The use of growth biomass carriers can achieve a two- to three-fold increase in biomass concentration in the system [46] and, subsequently, a proportional reduction in sludge loading. In this context, it should be emphasized that the above-mentioned increase in biomass concentration will not be manifested by an increase in the loading of the secondary sedimentation tank. The topicality of the application of these systems is also increasing in connection with the need for increased removal of nutrients. Especially with some industrial wastewaters, a negative to toxic effect of higher concentrations of organic pollution on the nitrification process can be expected. On the other hand, by using growth biomass carriers, two biocenoses with different sludge ages are cultivated in the system, which enables the simultaneous course of slower biochemical processes (nitrification, removal of slowly decomposable organic substances) as well as faster processes (removal of easily decomposable organic substances). In addition, depending on the carrier used, there is a certain degree of so-called meso-segregation and subsequently creating conditions for simultaneous nitrification and denitrification processes in the growing biomass.

Drtil et al. [48] measured in the system with 1.5 cm polyurethane cubes a maximum concentration of 8 mg/l O₂ at which simultaneous denitrification was still taking place. At the same time, the use of a growth biomass carrier reduces the denitrification volume and thereby also shortens the residence time of nitrifying bacteria outside of oxic conditions. In these systems, an improvement in the sedimentation properties of the sludge compared to the suspended biomass itself is usually observed because of the fixation of fibrous microorganisms in the growing biomass.

2.1.6. Advanced Biological Nitrogen Removal Systems

Biological processes Anammox [49], Nitrification and Denitrification [50], Simultaneous Denitrification [51] are among the most effective for the removal of nitrogenous substances from municipal WW, in terms of achieved efficiency (≥ 95%), costs and an environmental friendly nature.

One of the crucial factors in the biological removal of nitrogen from WW is the presence of a sufficient amount of biodegradable organic carbon. However, an external carbon source must be added to increase the effectiveness of the processes [52,53,54]. If the organic content of wastewater is too low compared to the needed COD/N ratio (analogous to the COD/P ratio for biological enhanced phosphorus removal). In general, the addition of an external carbon source (e.g., methanol, ethanol, acetic acid, glucose), necessary to increase the kinetics of these processes [53,55], together with the costs of disposal of the produced excess sludge, represent the most significant costs in the operation of a WWTP [56,57]. However, as a source of necessary carbon, e.g., WW from the product processes of food, confectionary, milk, and dairy products, thanks to the high content of organic carbon, which is biodegradable can be used [58,59]. while cost savings will be achieved and a lower impact on the environment will be assured. A serious disadvantage could be the insecurity of the qualitative and quantitative characteristics of these WWs, which may be related to the diversity of product cycles [55].

Collivignarelli et al. [60] present results of an innovative biological process applied in sludge line. One of the points of their work was the possibility of recovering the acceptable carbon residue (supernatant) in denitrification processes, replacing bought external carbon sources. Tests on full scale Thermophilic Alternate Membrane Biological Reactor (ThAlMBR) were carried out for 12 months. The ThAlMBR is an advanced biological membrane system used to lysate and oxidize the redundant biological sludge produced by WWTP through thermophilic bacteria, under controlled conditions of temperature and aeration. The ThAlMBR was applied both on thickened (TBSS) and digested sewage sludge (DBSS) with intermittent aeration conditions. Using the respirometric tests, an excellent biological treatability of the supernatant by the mesophilic biomass was observed and the denitrification kinetics reached with the supernatant (4.0 mgN-NO₃⁻/(g_VSS h)) and was set up similar to those of methanol (4.4 mgN-NO₃−/(g_VSS h)). Thanks to the analogous results attained on TBSS and DBSS, ThAlMBR proved to be compatible with different sludge line points, maintaining important minimization of sludge production in both cases.

Kim et al. [61], investigated using organic wastes as an alternative to commercial carbon sources. A food waste - recycling wastewater (FWR) was estimated as a convenient alternative to commercial carbon source for natural denitrification. Process stabilization was linked to the acclimatization and function of bacterial populations to the change of carbon source. Stable denitrification performance over a period of seven months was achieved in a full-scale WWTP using FWR as indispensable carbon source. The denitrification performance was stable with a mean nitrate junking effectiveness of 97.2%. These results demonstrate that FWR can be an effective external carbon source that could be beneficial by reducing costs and environmental impacts.

Nitritation - denitrition

The process of nitritation - denitritation is also referred to in the literature as a “nitrite shunt”, which limits the oxidation of nitrites to nitrites and enables the reduction of the produced nitrite to N₂ by heterotrophic denitrification. Compared to denitrification, oxygen consumption is 25% lower, organic carbon consumption is 40% lower, and excess sludge production is also lower [62]. The authors of [44] state that by combining the control of the length of aeration and the concentration of dissolved oxygen at a temperature of 15°C, a nitrogen removal efficiency of 95% was achieved in SBR with aerobic granular biomass.

ANAMMOX system

A new technology has been developed for the treatment of wastewater with a high nitrogen content, which uses the process of anaerobic ammonium oxidation (ANAMMOX). This process is performed by anaerobic ammonia-oxidizing bacteria (AOB), which oxidize half of the ammonia to nitrite. ANAMMOX bacteria (Candidatus Brocadia fulgida) use nitrogen nitrite as the final electron acceptor to oxidize residual ammonia to N₂ [48]. In 2014, the authors of [45] from Delft University published on simultaneous partial nitritation and the ANAMMOX process in a suspended biomass system.

Advanced biological nitrogen removal processes (BNR), e.g., nitritation-denitritation and ANAMMOX, therefore, compared to current BNR processes, show a significant reduction in the need for oxygen and organic substrate. The ANAMMOX process further reduces the oxygen demand by 60% without the need for an organic substrate [62].

3. Nutrient Recovery

5. Conclusion

Abbreviations

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