Author

Ms. Giulia Pristerà

Treatment Methods for Reducing Nitrogen Content in Landfill Leachate

Abbreviations

Abstract

Leachate is a hazardous by-product of landfills, rich in ammonia and organic material. If it is not properly managed, it can lead to both surface and groundwater contamination. One of the substances which need to be removed from leachate is nitrogen. Nitrogen can be found in several forms, the most toxic of which is ammonia. This paper looks at several available nitrogen removal technologies, highlighting their advantages and disadvantages. The technologies analysed are divided in three categories: biological, physical and chemical methods. The following treatment types are discussed: aerobic/anaerobic treatment, anammox, re- verse osmosis, evaporation, air stripping, chemical precipitation, adsorption, selective ion exchange, breakpoint chlorination, ozone and hydrogen peroxide treatment. Current and future research should focus on bettering the existing technologies and developing new ones. No treatment method emerges as the superior one, as each one has its merits and drawbacks.

2 Introduction

Leachate is a hazardous by-product of landfills, rich in ammonia and organic material. It results from water percolating through the waste and it can lead to both surface and ground- water contamination if it is not properly managed (Abuabdou et al. 2019, Miao et al. 2019, Talalaj 2015b). Its composition can vary significantly depending on the climate conditions and hydrogeologic characteristics of the landfill, as well as its age and the type of waste collected (Díaz et al. 2019). To reduce its polluting potential to acceptable limits, the leachate needs to be collected and treated. Nitrogen is one of the substances the concentration of which is regulated.

Nitrogen can be present in leachate in several forms, namely ammonia (NH₃), ammonium (NH₄⁺), organic nitrogen, nitrate (NO⁻3) and nitrite (NO₂⁻).  The respective concentrations of these components, and particularly ammonia and ammonium, depend mainly on two parameters: pH and temperature. High pH values and temperatures both lead to higher shares of free ammonia (Zhang et al. 2016). Indeed, ammonia is the main nitrogen compound in leachate, with concentrations ranging between 50 and 3500 mg/L (Talalaj 2015b). It is released from the anaerobic degradation of organic material present in landfills (Moestedt et al. 2016). As this is a slow process, ammonia levels in leachate can be high even after a relatively long time: removal treatments are therefore still necessary. Ammonia removal is a particularly important part of leachate treatment, as the compound is toxic to aquatic organisms and therefore needs to be removed for the liquid to be safely discharged (Takeda et al. 2016).

There are several different approaches to removing hazardous nitrogen compounds from landfill leachate. When choosing one method over another, several factors are to be taken into account, such as the initial concentration of the compounds to be removed, as well as their identity and the requirements for the effluent. This paper aims at analysing the most common treatments, in terms of removal efficiency, scale of the technology, costs and target compounds, with a particular focus on ammonia. Potential future developments and innovations are also investigated.

3 Treatments

Due to the high polluting potential of leachate, several treatment methods have been developed to eliminate the hazardous substances it contains, including nitrogen. There is also ongoing research into finding new treatments as well as into making the existing ones more effective and/or more economically viable.

An analysis of the most common methods is available below, followed by a look at new technologies still in the early stages of their development and therefore requiring further study. A combination of different treatments can be and often is used, so as to obtain better results. Table 1 shows an overview of the treatment methods analysed.

Table 1: Overview of the treatment methods analysed in this paper.

3.1 Biological Treatments

Biological treatment of leachate is a category which includes several different processes. They are the most economical methods for total nitrogen (TN) removal; however they have significant energy requirements. The most common biological nitrogen removal (BNR) treatment is the nitrification-denitrification process. Another well-known option is anaerobic ammonium oxidation (anammox) (Miao et al. 2019, Wang et al. 2018, Adam et al. 2019, Karri et al. 2018). Biological processes rely on the ability of microbes to degrade organic matter and remove nutrients, transforming toxic compounds into less polluting ones. These microbes are sensitive to factors such as salinity, temperature, pH and dissolved oxygen (DO); as a consequence, it is important to monitor the conditions of the leachate to ensure that the process works properly (Díaz et al. 2019).

3.1.1 Aerobic-Anaerobic Treatment

The nitrification-denitrification process, or aerobic-anaerobic process is what is known as the conventional biological treatment. Initially, ammonium (NH₄⁺) is reduced to nitrate (NO₃⁻) (nitrification, aerobic conditions), then the nitrate is reduced to N₂ (denitrification, anoxic conditions). During the nitrification process, oxygen is used as electron acceptor; during the denitrification process, nitrite is used instead.

The bacteria carrying out the reduction of ammonia can be divided into two groups: ammonium- oxidizing bacteria (AOB), which work in aerobic conditions, and nitrite-oxidizing bacteria (NOB), which work in anoxic conditions (Karri et al. 2018, Diaz  et al. 2019).

The nitrification process takes place in two steps: first, AOB convert the ammonium to nitrite, then the nitrite is converted to nitrate. As for the denitrification process, biodegradable organic matter such as methanol is needed, as it is used as electron donor (Miao et al. 2019, Karri et al. 2018). During the nitrification process, the organic matter can be converted into carbon dioxide; as a consequence, it is no longer available for the denitrification process, of- ten leading to the necessity of adding another source of organic material. This tends to cause an increase in costs. It is therefore not advisable to use the conventional aerobic-anaerobic process with leachate characterised by a low COD/nitrogen ratio (Miao et al. 2019).

The efficiency of this treatment is generally in the 70-90% range (Adam et al. 2019).

3.1.2 Anammox

An alternative biological ammonia removal treatment is the anaerobic ammonium oxidation process, or anammox, during which nitrite is used as an electron acceptor to produce nitrogen gas and a limited amount of nitrate (Wang et al. 2018, Miao et al. 2019, Ma et al. 2016). Anammox is the most economical nitrogen removal technique, as it does not require an external source of carbon and it can function with up to 62.5% less aeration than the conventional nitrogen removal process. This is due to the fact that only half of the ammonia in the leachate needs to be oxidised to nitrite instead of nitrate (Miao et al. 2019, Wu et al. 2020, 2018, Ma et al. 2016). The reason no external carbon source is needed for the anammox treatment is that it is an autotrophic process which can occur without an organic substrate being involved. Another factor to take into account is the reduction in costs associated with sludge removal, as this process limits the amount of sludge produced (Miao et al. 2019, Wu et al. 2018). One final advantage of this technique is the decrease in GHG emissions, as no nitrous oxide (N₂O) is produced (Miao et al. 2019). Anammox can occur at temperatures ranging from 6 to 43 ^◦C (Ma et al. 2016). However, for the anammox treatment to work, a long accumulation time and solid retention time are needed; as a consequence, the start-up period is longer than in the conventional nitrification-denitrification process. The reason is that anammox bacteria have low growth rates, with a doubling time of up to 20 days (Miao et al. 2019, Chen et al. 2016). For the process to yield good results, the choice of bacteria is important, just as it is important to ensure a sufficient retention of these bacteria. Several species have been studied and the matter is the subject of ongoing research (Miao et al. 2019, Ma et al. 2016).

The anammox process is particularly promising when it comes to the treatment of wastewater having a low C/N ratio. Large amounts of organic material have been shown to inhibit the process, due to competition between denitrifying and anammox bacteria: it has been calculated that, when COD concentration increases from 100 mg/L to 300 mg/L, TN removal efficiency through anammox decreases from 90% to 69%. (Wu et al. 2018, Miao et al. 2019, Tomar et al. 2015). Moreover the process, while efficient in terms on ammonia removal, is generally less successful as it relates to TN removal. The reason is that, as mentioned above, the chemical reaction leads to the production of 11% of nitrate as a by-product (Wu et al. 2018).

3.2 Physical Treatments

3.2.1 Reverse Osmosis

Reverse osmosis (RO) can be used to treat leachate derived both from young and mature landfills. Its effect is to divide the leachate into two separate streams, permeate (the ”clean” stream) and concentrate (Talalaj 2015b). It works through the application of enough hydraulic pressure to overcome the osmotic pressure of a solution. As a result, water from a contaminated stream (in this case leachate) can cross a semi-permeable membrane, while the contaminants are unable to do the same. The energy requirements for the use of this technology are not particularly high (Volpin et al. 2018). The removal efficiency of the nitro- gen compounds present in the leachate is shown in Table 2. Other studies have calculated the ammonia removal efficiency as having an average value belonging to the range 93-98% (Talalaj 2015b, Brandt et al. 2017). The efficiency is extremely high for ammonia and total nitrogen, but it is significantly lower for nitrates and nitrites. However, these compounds are not as toxic, thus the lower efficiency where they are concerned does not negate the potential usefulness of the technology. The removal efficiency of ammonia appears to be lower in older leachate, as the N-NH₄ content increases and the inorganic carbon concentration ceases to be able to sustain a large enough nitrifying community (Talalaj 2015a).

3.2.2 Evaporation

It is possible to remove ammonia from landfill leachate by way of applying heat, as this causes the compound to be volatilised. Moreover, if the evaporation process is followed by ammonia condensation, there is the additional advantage of being able to use the recovered ammonia, for instance as fertiliser. Tests have been run both at atmospheric pressure and with the application of vacuum pressure, at a temperature of 300 C: the resulting efficiency ranges between 95 and 98% in both cases, thus rendering the vacuum pressure superfluous (Sprovieri et al. 2020).

Table 2: Removal Efficiency of nitrogen compounds in leachate through RO. TON: Total Organic Nitrogen; TIN: Total Inorganic Nitrogen.

3.2.3 Air Stripping

Air stripping is one of the most commonly used techniques for ammonia removal, as it does not require complicated equipment and it is easily implemented. As a consequence, it is often used on the industrial level (Adam et al. 2019, Raboni & Viotti 2017). However, it can lead to environmental issues due to the ammonia being released into the atmosphere (Sprovieri et al. 2020). For the process to work, lime (or a similar substance) needs to be inserted into the water, so as to increase the pH up to the range 10.8-11.5. This makes it possible for NH₄⁺ to be converted into free ammonia gas. A stripping tower is then used to remove the gas from falling droplets of water (Karri et al. 2018, Adam et al. 2019). The ammonia removal efficiency can vary significantly, but it is generally included in the range 50-90% (Adam et al. 2019, Hanira et al. 2017).

3.3 Chemical Treatments

3.3.1 Chemical Precipitation

Chemical Precipitation Chemicals are added to the leachate and they react with the ammonia to create an insoluble compound which can easily be removed from the water stream. This approach has as a consequence the formation of sludge which needs to be regularly removed, thus increasing the operating costs. Furthermore, while the equipment in itself is inexpensive, the chemicals needed for the process to take place are not (Karri et al. 2018, Adam et al. 2019). The most commonly used version of this process is also known as magnesium ammonium phosphate (MAP) precipitation, or struvite precipitation. Using this particular chemical minimises the amount of sludge generated during the process, while guaranteeing an elevated ammonia removal efficiency (around 98%). Moreover, MAP can be applied on a large variety of scales (Zhang et al. 2015, Adam et al. 2019, Karri et al. 2018). The recovered MAP can also potentially be used as fertiliser, thus creating an added incentive for the use of this method (Sprovieri et al. 2020).

3.3.2 Adsorption

Adsorption is a physico-chemical process based on using a substance (adsorbent) to the surface of which the contaminant (in this case ammonia) can adhere. Table 3 shows the removal efficiency values obtained in several studies by making use of different adsorbent materials. The efficiency varies substantially based on the chosen material. Zeolites and activated carbon (AC) are the most common adsorbents; zeolites have the advantage of having a polar surface, which facilitates the adsorption of the ammonium cation and results in higher removal efficiencies. This system can function for long periods of time, making it reliable. Currently the focus is on the search for adsorbents which are both effective and inexpensive (Karri et al. 2018, Xue et al. 2018).

Table 3: Removal efficiency of ammonia using different adsorbents (%) (Karri et al. 2018, Couto et al. 2017).

3.3.3 Selective Ion Exchange

An ion exchanger is used to remove polluting ions from the leachate; this process is therefore effective to reduce the ammonia content and it works for a large ammonia concentration range. Specifically, the exchanger’s cation is substituted by the ammonium cation. The process is reversible and the material can be regenerated and reused. However, this comports non-insignificant costs (Karri et al. 2018, Adam et al. 2019). The ion exchanger can be either natural (of mineral origin) or synthetic (polymers). One of the most common types of natural exchanger is the zeolite resin, due to its three-dimensional structure (Adam et al. 2019, Kasmuri et al. 2018).

3.3.4 Breakpoint Chlorination

Breakpoint chlorination can be used for the purpose of ammonia removal. However, it does not have an impact on organic nitrogen. To limit the formation of disinfection by-products (DBP), breakpoint chlorination should be preceded by a DBP-precursor removal treatment (Brandt et al. 2017, Xue et al. 2018). The treatment ensures that ammonia is completely oxidised to nitrogen, leaving behind residual free chlorine. Initially, either chlorine gas or sodium hypochlorite is added to the leachate, leading to an increase in combined chlorine residual (i.e. a combination of chlorine and ammonia). Afterwards, both the ammonia concentration and the residual decrease until the latter reaches its breakpoint and starts increasing again, finally achieving an almost complete removal of NH₃ (Adam et al. 2019, Qi 2018). This treatment method is effective even at relatively low ammonia concentrations, but it has high operation costs and it generates acidity in the leachate (Qi 2018).

3.3.5 Ozone and hydrogen peroxide treatment

Advanced oxidation processes (AOPs) can be used to remove contaminants from wastewater. They are processes taking place at near ambient temperature and pressure which involve the formation of non-selective hydroxyl radicals in a sufficient amount to lead to water purification (Gautam et al. 2019). The process is used to remove ammonia from leachate by way of oxidising it to nitrate. This treatment can be carried out with ozone or with a mixture of ozone and hydrogen peroxide. The latter is known as peroxone oxidation. The disadvantage of using only pure ozone is that it does not have a long permanence time in the water. This method is mostly effective at low concentrations of ammonia (Capodaglio et al. 2015).

4 New Developments and Areas of Research

There are two large areas of research, when it comes to nitrogen removal methods: the improvement of currently existing technologies and the discovery and application of new technologies. As it relates to the former, research is needed, and indeed ongoing, on different anammox bacteria and how they interact with each other and with the surrounding conditions. Moreover, there is also ongoing research related to finding new bacteria suitable for this process (Miao et al. 2019). When it comes to the conventional biological treatment, studies have been carried out comparing the use of sulfate as electron acceptor in anaerobic conditions rather than nitrite to produce N₂. This process is known as sulfammox. Iron (Fe) can also be used as electron acceptor, in a process called Feammox. However, this last technique currently does not appear entirely feasible when it comes to leachate, as the ideal pH value would be below 6, as opposed to the 7.0 – 8.5 typical of landfill leachate. Conversely, sulfammox has potential, due to the generally high sulfate concentrations in leachate (Wu et al. 2020).

One of the main issues, when it comes to nitrogen removal treatments, is the cost. While different processes incur in different expenses, energy-related costs are generally relevant. For this reason, studies have been conducted focusing on the possibility of generating energy from the leachate itself and then reusing it to treat it, so as to make the process at least partially self-sustainable. An example is a coupled redox fuel cell reactor, which can be used to remove nitrogen from leachate while simultaneously producing and storing electricity. Fuel cells convert the chemical energy in a fuel to electricity; in this set-up, ammonia is used as a fuel and it undergoes oxidation in the anode, while nitrate reduction takes place in the cathode (Zhang et al. 2016). The main challenge is the identification of suitable catalysts to be used as electrodes in the process; research has shown that there is the potential to use materials which do not contain precious metals, thus decreasing costs and increasing the feasibility of the technology on a larger scale (Zhang et al. 2020). The results obtained are promising, but not yet ready to be used at a large scale, as ammonia removal and cell performance need to be improved. Further research on the subject, however, could make it possible in the future to have a self-powered ammonia removal system.

Fuel cell use in the context of ammonia removal would present a further advantage. Indeed, when selecting the preferred treatment method, one aspect to consider is whether it allows for the ammonia to be stored and later reused. One possible use for the compound is as hydrogen carrier in fuel cells, which, when paired with the use of a suitable catalyst, leads to a performance as good as, and in certain designs better than, that of conventional hydrogen fuel cells. The added advantage in using ammonia in this context is that it is three times cheaper than hydrogen (Sprovieri et al. 2020, Mathew & Thaker 2015).

Overall, the area where the most research is focused is how to combine different treatment methods so as to increase efficiency and reduce costs. For instance, the possibility of combining air stripping and biological ammonia removal has been looked into, with promising results at the experimental scale (Jurczyk et al. 2020). Another TN and ammonia removal approach that’s been studied is SNAD, or simultaneous partial nitrification, Anammox and denitrification (Zhang et al. 2017). The possibility of coupling nitrogen and sulfate anammox has also been analysed, with promising results (Wu et al. 2020).

5 Conclusions

While there are several nitrogen compounds which can be found in landfill leachate, the most polluting one, and therefore the one most treatments aim to eliminate, is ammonia. As the removal of nitrogen from wastewater is a well-known issue, there is no shortage of methods available to carry it out. The choice of one treatment over another depends on a series of factors, such as costs, available technology, whether there is a need to store the ammonia rather than just eliminate it, climate change impacts and age of the landfill. There is no treatment method which can be considered universally suitable, as it is always necessary to take into account the factors mentioned above.

In general, biological methods have the advantage of being inexpensive, when compared to many of the other options. However, they have high energy requirements. Among the approaches most commonly used there is also air stripping. However, there are environmental concerns related to the release of gaseous ammonia into the atmosphere. Chemical treatment method are often rather expensive, due to the chemicals needed for them to function. Therefore, when selecting a technology, it is necessary to make compromises, focus on what the main goal is in a specific context and consider advantages and disadvantages of each treatment method.

While the existing treatment methods can achieve elevated nitrogen removal efficiencies, there is constant ongoing research into bettering them and developing new ones, with a particular focus on coupling different techniques to achieve the best possible results.

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