DE102023101146A1 - Procedure for the in situ remediation of a landfill - Google Patents

Abstract

Method for remediating a landfill having a base seal and a surface seal and emitting landfill gas and landfill leachate from a landfill body arranged between the base seal and the surface seal, characterized by the steps: a. removing landfill leachate from the landfill body at a removal point provided on the landfill, b. precipitating salts dissolved in the landfill leachate, c. shifting the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate depleted of salts in favor of the biological oxygen demand (BOD) by gassing the landfill leachate depleted of salts with ozone and/or irradiating the landfill leachate depleted of salts with UV radiation, d. Returning the landfill leachate gassed with ozone and/or irradiated with UV radiation and depleted of salts into the landfill body at at least one first feed point arranged opposite the removal point on the landfill, forming a flow between the removal point and the at least one first feed point, and e. Repeating steps a to d until the chemical oxygen demand (COD), the biological oxygen demand (BOD) and/or the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate removed from the landfill body at the removal point correspond to a predetermined value.

Description

The invention relates to a method for remediation of a landfill in situ. In particular, the invention relates to a method for remediation of a landfill having a base seal and a surface seal and emitting landfill gas and landfill leachate from a landfill body arranged between the base seal and the surface seal.

Thousands of old landfills around the world have already been closed or are about to be closed. They are sealed in such a way that the landfill gas produced can be extracted and recycled. In many cases, these landfills are sealed off from the environment so that no highly contaminated landfill leachate can enter the groundwater surrounding the landfill or the uncontaminated soil surrounding the landfill.

1. 1. Die Deponiegasproduktion verringert sich allmählich. Erschwerend kommt hinzu, dass sich auch die Methangaskonzentration allmählich verringert und bis unterhalb eines Grenzwertes absinkt, der eine elektrische/thermische Verwertung durch ein Blockheizkraftwerk (BHKW) unmöglich macht.
2. 2. Die Abdichtungen halten nicht ewig und müssen früher oder später ausgebessert werden. Gleichzeitig muss jedoch sichergestellt werden, dass das hochbelastete Deponiesickerwasser nicht in das die Deponie umgebende Erdreich gelangt. Irgendwann muss der Deponiekörper wieder mit dem umgebenden Erdreich verbunden werden; spätestens dann muss das Wasser des Deponiekörpers völlig unbelastet sein.

This situation poses two fundamental problems for future management:

1. 1. Landfill gas production is gradually decreasing. To make matters worse, the methane gas concentration is also gradually decreasing and falling below a limit that makes electrical/thermal utilization by a combined heat and power plant (CHP) impossible.
2. 2. The seals do not last forever and must be repaired sooner or later. At the same time, however, it must be ensured that the highly contaminated landfill leachate does not reach the soil surrounding the landfill. At some point, the landfill body must be reconnected to the surrounding soil; at the latest then, the water in the landfill body must be completely uncontaminated.

External landfill body remediation is complex and does not represent an economically viable solution, as it only shifts remediation to other locations, has a very poor energy balance and causes extreme costs.

So-called aerobic remediation processes in situ can only lead to partial solutions because only the aerobically degradable organic matter and some of the nitrogen can be eliminated. In addition, these processes are also very unfavorable in terms of energy. Ventilation elements are installed in the landfill body to ventilate the entire landfill. However, this is associated with immense investment and operating costs and requires massive expenditure on exhaust air purification. In addition, these processes are in direct contradiction to the necessary recycling processes for the recovery of nutrients such as phosphorus (P), nitrogen (N), magnesium (Mg), calcium (Ca) and potassium (K).

Over the course of the landfill periods lasting several decades and thus the periods of anaerobic fermentation, very large quantities of nutrient salts are released, i.e. the salts dissolve. However, they only remain in solution until their combined solubility products are reached. After that, the continued release leads to various nutrient salts immediately precipitating again, especially calcium and magnesium salts, whose concentrations are therefore right at the limit of the lowest solubility product of one of their salts. These too must be recovered after redissolution and completely removed from the landfill body.

The object of the invention is therefore to create a method for landfill remediation by which a complete remediation of the landfill body can be achieved in situ.

This object is achieved according to the invention by the method having the features of claim 1. The dependent claims reflect advantageous embodiments of the invention.

The basic idea of the invention is to repeatedly treat the landfill leachate and, after treatment, to recycle it into the landfill body until the water present in the landfill body can safely come into contact with groundwater. In this process, the salts, organic substances and micro- and nanoplastics present in the landfill body are removed, recovered and recycled or recycled.

• - Rückgewinnung der im Deponiewasser gelösten Nährsalze;
• - Rückgewinnung der im Deponiekörper ausgefällten Salze, insbesondere Struvit;
• - Umwandlung von gelösten und sehr schwer oder gar nicht abbaubaren organischen Verbindungen in anaerob abbaubare Verbindungen;
• - Auswaschung und Entfernung von nicht gelösten organischen Bestandteilen;
• - Entfernung von im Deponiewasser gelöstem und rücklösbarem Nanoplastik und Mikroplastik aus dem Deponiekörper; und
• - Isolierung mindestens eines im Deponiegas enthaltenen Teils an Kohlendioxid (CO₂), um damit die gewünschten Rücklösung der Salze und eine für eine energetische Verwertung genügend hohe Methankonzentration im Deponiegas zu gewährleisten.

In particular, the present invention pursues the following objectives:

• - Recovery of nutrient salts dissolved in landfill water;
• - recovery of salts precipitated in the landfill body, in particular struvite;
• - Conversion of dissolved organic compounds which are very difficult or impossible to degrade into anaerobically degradable compounds;
• - Washing and removal of undissolved organic components;
• - Removal of nanoplastics and microplastics dissolved in the landfill water and resoluble from the landfill body; and
• - Isolation of at least a portion of carbon dioxide (CO ₂ ) contained in the landfill gas in order to achieve the desired redissolution of the salts and a to ensure a sufficiently high methane concentration in the landfill gas.

In particular, the process designed according to the invention ferments partially oxidized landfill leachate, in which the chemical oxygen demand is shifted in favor of the biological oxygen demand, into biogas in the landfill, which is then used as a kind of biogas plant. The chemical oxygen demand is therefore not reduced by expensive external aerobic biology, but by producing biogas in the landfill itself.

The landfill leachate, in which the NH ₄ ⁺ , PO ₄ ^3- and Mg ²⁺ concentrations have been massively reduced by precipitation and/or ammonia stripping, is returned to the landfill body so that the struvite crystals precipitated in the landfill body are redissolved and washed out in order to be specifically precipitated and recovered by means of the further process.

The circulating process of the method according to the invention is preferably designed in such a way that a plug flow is established, in which short-circuit flows and strong mixing are avoided. This is ensured in particular by the fact that the partial flows to be returned are returned to the feed point located furthest away from the extraction point and have a lower temperature than other partial flows that are returned between these points.

In particular, two different recyclate streams can be established: one partial stream comprises highly pre-cleaned permeate water after cooling in the landfill area far from the extraction site and one partial stream comprises highly concentrated, non-cooled concentrate in an area close to the extraction site. After the landfill has already been remediated far from the extraction site, there is still a strong biogas production close to the extraction site without this area being mixed with the incoming remediation flow.

In particular, the landfill leachate can be washed with raw biogas (with high H ₂ S concentrations), so that this recyclate greatly limits the redissolution of heavy metals. The switch to washing with pure and H ₂ S-free CO ₂ gas takes place in the following steps in order to specifically redissolve and wash out the heavy metals. As a result, the struvite obtained is almost free of heavy metals during the first recursive step; these are then concentrated in the salts precipitated and evaporated in the third step, in which all nano- and microplastics are also present.

Finally, almost fully remediated landfill leachate is returned at a point when practically all salts have been washed out and there is hardly any organic matter left. The last organic nests are washed out and oxidized using O ₂ -supersaturated reverse osmosis permeate. It is important that this final oxidation only takes place after all salts have been washed out, because otherwise they dissolve back much more slowly under O ₂ conditions.

Furthermore, the use of pure methane for precipitation and ammonia stripping, the use of pure _CO2 or raw biogas to acidify the recyclates in order to accelerate the salt dissolution, is planned.

Only in the last stage of the process does the raw, untreated CSBBSB, which is then present in much lower concentrations, get fed to the aerobic biology. As a result, a significant portion of the BSB/COD is respired without prior ozone treatment. Only the refractory residue is treated with ozone, so that ozone can be saved, and is recycled to the biology.

The leaching, recovery and/or elimination steps take place one after the other, with the pre-treated leachate being returned to the landfill body. This guarantees both an increase in landfill gas production and a redissolution of all precipitated salts and organic clusters. The recoveries are exclusively the nutrient salts and the eliminations are the organic carbon compounds, with the aim of maximizing fermentative degradation (= landfill gas production) in the landfill body, i.e. in situ.

The internal circuits (recirculations) are directed as far as possible in order to flush the entire landfill body using a plug flow so that little mixing or short-circuit flows are caused. This is supported by a gradual cooling of the recirculation fractions.

However, there are differences in the morphologies, the proportion of organic waste, the current outgassing rates of the existing landfills and the outgassing rates in different areas within a landfill. The levels of leachate also vary greatly, as does the age of the landfill. Therefore, a customized recirculation strategy must be developed for each landfill in order to align the effectiveness of the plug flow with the local conditions. In this way, optimal recirculation can be achieved from top to bottom, from bottom to top or from one side to the other.

Partially filled landfills are a special case: for these, it is logical that the recirculated waste must be repeatedly trickled down from above through the dry areas in order to dissolve and wash out all precipitated salts and organic matter.

1. a. Entnehmen von Deponiesickerwasser aus dem Deponiekörper an einem an der Deponie vorgesehenen Entnahmeort,
2. b. Ausfällen von im Deponiesickerwasser gelösten Salzen,

• c. Verschieben des Verhältnisses von chemischem Sauerstoffbedarf (CSB) zu biologischem Sauerstoffbedarf (BSB) des um die Salze depletierten Deponiesickerwassers zugunsten des biologischen Sauserstoffbedarfs (BSB) mittels Begasen des um die Salze depletierten Deponiesickerwassers mit Ozon und/oder Bestrahlen des um die Salze depletierten Deponiesickerwassers mit UV-Strahlung,
• d. Rückführen des mit Ozon begasten und/oder mit UV-Strahlung bestrahlten um die Salze depletierten Deponiesickerwassers in den Deponiekörper an wenigstens einem dem Entnahmeort an der Deponie gegenüberliegend angeordneten ersten Zuführort unter Ausbilden einer zwischen dem Entnahmeort und dem wenigstens einen ersten Zuführort entstehenden Strömung, und
• e. Wiederholen der Schritte a bis d bis der chemische Sauerstoffbedarf (CSB), der biologische Sauerstoffbedarf (BSB) und/oder das Verhältnis von chemischem Sauerstoffbedarf (CSB) zu biologischem Sauerstoffbedarf (BSB) des am Entnahmeort aus dem Deponiekörper entnommenen Deponiesickerwassers einem vorbestimmten ersten Wert entsprechen.

According to the invention, a method is proposed for the remediation of a landfill having a base seal and a surface seal and emitting landfill gas and landfill leachate from a landfill body arranged between the base seal and the surface seal, comprising the steps:

1. a. Removal of landfill leachate from the landfill body at a designated removal point at the landfill,
2. b. Precipitation of salts dissolved in landfill leachate,

• c. Shifting the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the salt-depleted landfill leachate in favour of the biological oxygen demand (BOD) by gassing the salt-depleted landfill leachate with ozone and/or irradiating the salt-depleted landfill leachate with UV radiation,
• d. Returning the landfill leachate which has been gassed with ozone and/or irradiated with UV radiation and depleted of salts into the landfill body at at least one first feed point arranged opposite the removal point at the landfill, forming a flow between the removal point and the at least one first feed point, and
• e. Repeating steps a to d until the chemical oxygen demand (COD), the biological oxygen demand (BOD) and/or the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate removed from the landfill body at the removal site correspond to a predetermined first value.

Due to the use of the landfill as a biogas plant according to the invention, it is preferably provided that external liquid biodegradable waste materials are fed to the landfill body in order, on the one hand, to dispose of or degrade the additionally fed external liquid biodegradable waste materials and, on the other hand, to increase biogas production.

Preferably, the precipitation of salts mentioned in step b is carried out by adding magnesium salt and/or stripping the carbon dioxide contained in the landfill leachate to increase the pH value of the landfill leachate and thereby precipitate salts that are insoluble at an increased pH value.

Furthermore, before the return to the landfill body mentioned in step d, the pH value of the landfill leachate that has been gassed with ozone and/or irradiated with UV radiation and depleted of salts is preferably reduced. The pH value is particularly preferably reduced by gassing the landfill leachate that has been gassed with ozone and/or irradiated with UV radiation and depleted of salts using carbon dioxide. The gassing of the landfill leachate that has been gassed with ozone and/or irradiated with UV radiation and depleted of salts is most preferably carried out using carbon dioxide obtained from the landfill gas of the landfill body or using raw landfill gas.

The stripping of the carbon dioxide contained in the landfill leachate in step b is preferably carried out by gassing the landfill leachate taken at the extraction site with methane obtained from the landfill gas of the landfill body.

Furthermore, the stripping of the ammonia contained in the landfill leachate depleted of the salts is provided before the gassing with ozone and/or irradiation with UV radiation mentioned in step c. The stripping of the ammonia (NH ₃ ) contained in the landfill leachate depleted of the salts is particularly preferably carried out by gassing the landfill leachate depleted of the insoluble salts with methane obtained from the landfill gas of the landfill body.

Furthermore, nanofiltration of the landfill leachate depleted of the salts and ammonia and treatment of the nanofiltration concentrate according to step c are provided, wherein the nanofiltration concentrate gassed with ozone and/or irradiated with UV radiation is fed to at least one second feed location arranged on the landfill, which lies between the removal location and the at least one first feed location, and tempering of the nanofiltration permeate to a temperature below the temperature of the nanofiltration concentrate gassed with ozone and/or irradiated with UV radiation and returning the nanofiltration permeate to the landfill body at the at least one first feed location.

According to a further preferred embodiment, the nanofiltration of the landfill leachate taken at the extraction site prior to the precipitation of the dissolved salts, wherein the nanofiltration concentrate is treated according to step b and following and the nanofiltration permeate is fed to an algae culture, the aqueous supernatant of the algae culture is subjected to reverse osmosis and the reverse osmosis permeate is returned to the landfill body at the at least one feed location.

An alternative embodiment provides for the supply of the landfill leachate taken at the extraction site to an algae culture and treatment of the aqueous supernatant by means of reverse osmosis, wherein the reverse osmosis permeate is returned to the landfill body at the at least one supply site.

In both cases, the concentrate obtained from reverse osmosis is preferentially evaporated.

The gases generated during the process according to the invention are preferably processed together with the landfill gas emitted from the landfill body.

• f. Entnehmen von Deponiesickerwasser aus dem Deponiekörper an dem an der Deponie vorgesehenen Entnahmeort,
• g. Ausfällen von im Deponiesickerwasser gelösten Salzen,
• h. Strippen des im um die Salze depletierten Deponiesickerwassers enthaltenen Ammoniaks,
• i. Nanofiltrieren des um die Salze und um Ammoniak depletierten Deponiesickerwassers
• j. Verschieben des Verhältnisses von chemischem Sauerstoffbedarf (CSB) zu biologischem Sauerstoffbedarf (BSB) im Nanofiltrationskonzentrat zugunsten des biologischen Sauserstoffbedarfs (BSB) mittels Begasen des um die Salze depletierten Deponiesickerwassers mit Ozon und/oder Bestrahlen des um die Salze depletierten Deponiesickerwassers mit UV-Strahlung,
• k. Rückführen des mit Ozon begasten und/oder mit UV-Strahlung bestrahlten um die Salze depletierten Deponiesickerwassers in den Deponiekörper an wenigstens einem an der Deponie angeordneten zweiten Zuführort, der zwischen dem Entnahmeort und dem wenigstens einen ersten Zuführort liegt,
• l. Temperieren des Nanofiltrationspermeats auf eine Temperatur unterhalb der Temperatur des mit Ozon begasten und/oder mit UV-Strahlung bestrahlten um die Salze depletierten Deponiesickerwassers und Rückführen des temperierten Nanofiltrationspermeats in den Deponiekörper am ersten Zuführort, und
• m. Wiederholen der Schritte f bis l bis der chemische Sauerstoffbedarf (CSB), der biologische Sauerstoffbedarf (BSB) und/oder das Verhältnis von chemischem Sauerstoffbedarf (CSB) zu biologischem Sauerstoffbedarf (BSB) des am Entnahmeort aus dem Deponiekörper entnommenen Deponiesickerwassers einem vorbestimmten zweiten Wert entsprechen.

The above step e is preferably followed by the following steps:

• f. Removal of landfill leachate from the landfill body at the removal point provided at the landfill,
• g. Precipitation of salts dissolved in landfill leachate,
• h. Stripping of the ammonia contained in the landfill leachate depleted of salts,
• i. Nanofiltration of the landfill leachate depleted of salts and ammonia
• j. Shifting the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) in the nanofiltration concentrate in favour of biological oxygen demand (BOD) by gassing the landfill leachate depleted of salts with ozone and/or irradiating the landfill leachate depleted of salts with UV radiation,
• k. Returning the landfill leachate which has been gassed with ozone and/or irradiated with UV radiation and depleted of salts into the landfill body at at least one second feed point arranged on the landfill, which is located between the removal point and the at least one first feed point,
• l. Tempering the nanofiltration permeate to a temperature below the temperature of the landfill leachate that has been treated with ozone and/or irradiated with UV radiation to remove the salts and returning the tempered nanofiltration permeate to the landfill body at the first feed point, and
• m. Repeating steps f to l until the chemical oxygen demand (COD), the biological oxygen demand (BOD) and/or the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate removed from the landfill body at the removal location correspond to a predetermined second value.

• n. Zuführen des am Entnahmeort entnommenen Deponiesickerwassers zu einer Algenkultur und Behandeln des wässrigen Überstands mittels Umkehrosmose, wobei das Umkehrosmosepermeat an dem wenigstens einen Zuführort in den Deponiekörper rückgeführt wird.

Step m is preferably followed by the step:

• n. feeding the landfill leachate extracted at the extraction site to an algae culture and treating the aqueous supernatant by means of reverse osmosis, wherein the reverse osmosis permeate is returned to the landfill body at the at least one feed site.

The landfill leachate extracted at the extraction site is preferably nanofiltered before step n and the nanofiltration permeate is treated as landfill leachate according to step n.

The invention is explained in more detail below with reference to particularly preferred embodiments shown in the accompanying drawings.

In the following, the invention is explained by way of example using the recirculation strategy for landfills filled with leachate with an exemplary treatment quantity of 240 m ³ /day, in which the leachate is removed from the top of the landfill body and recycled at several points on its underside, so that an almost flat exposure results.

Depending on the proportion of organic waste added, the concentration of dissolved organic ingredients measured by the chemical oxygen demand (COD) varies in different landfills. Only if a minimum limit of around 4,000 mg/l of oxygen is exceeded or not significantly undercut, the first process step explained below comes into full effect - otherwise the second process step is started:

1. Process step

To carry out the method according to the invention, a module for landfill gas treatment is required, in which a minimum fraction of carbon dioxide (= CO ₂ ) and methane (= CH ₄ ) is separated so that the landfill gas has a sufficiently high methane concentration for utilization in a Combined heat and power plant (CHP) is required. In addition, there will be sufficient pure methane (CH ₄ ) for precipitation and ammonia stripping, as well as a daily minimum amount of carbon dioxide (CO ₂ ) for acidification of leachate fractions. For example, it is assumed here that all of the landfill gas is pretreated.

Furthermore, a module for struvite precipitation, a stripping module for ammonia (NH ₃ ) and a module for “oxidizing” COD so that part of this organic fraction is converted to BOD (= biological oxygen demand) are required.

The raw landfill leachate taken from the top of the landfill is first fed to the precipitation module. This water is fed with approximately the stoichiometric amount of magnesium salt, e.g. Mg(OH) ₂ , which ensures extensive precipitation of the phosphate (PO ₄ ^3- ) dissolved in the water. Then, in order to strip significant amounts of CO ₂ , pure methane gas is blown into this water in order to increase the pH of the water to the target value for precipitation. This increase in the pH causes the precipitation of struvite and certain amounts of calcium salts, which are then removed using known methods.

Afterwards (sequentially in the same module or in another module) the water is further gassed with methane (CH ₄ ) in order to largely remove the ammonia (NH ₃ ) from the water. The ammonia is then washed out of the methane gas using known processes. Both of the above modules are operated in a closed circuit, whereby all of the methane gas can be recovered and ultimately used for energy.

As a result, the landfill leachate treated in this way will have only minimal concentrations of magnesium (Mg ²⁺ ), ammonium (NH ₄ ⁺ ) and phosphate (PO ₄ ^3- ) as well as reduced calcium concentrations, while the chemical oxygen demand remains essentially unchanged.

This pre-treated water fraction is then subjected to "pre-oxidation" using known methods, for example with ozone or UV radiation or a combination of both, but only to such a limited extent that only around 30 - 90% of the COD is transformed into biodegradable fractions (so-called BOD = biological oxygen demand), i.e. this sub-quantity is then predominantly fermented anaerobically, thereby generating additional landfill gas. The chemical oxygen demand is therefore reduced overall by around 60%.

This water is then partially enriched with carbon dioxide CO ₂ in a gas-tight CO ₂ gassing module (with pure CO ₂ or with raw biogas before desulfurization, so that an enrichment of hydrogen sulfide (H ₂ S) also takes place in order to prevent the redissolution of heavy metals in this first step) and thereby acidified (pH value reduction) and then recycled into one or more soil areas of the landfill. This water has very low ammonium, magnesium, phosphorus and significantly lower calcium concentrations and also has a significantly lower pH value than the original raw landfill leachate.

Therefore, after recirculation, all of the precipitated struvite adhering to the landfill particles as well as significant amounts of calcium salts will redissolve and gradually be flushed upwards with this recirculation fraction. At the same time, the "oxidized" COD is fermented to landfill gas and the chemical oxygen demand is gradually reduced to approx. 2,000 mg/l.

After up to about 30% of the total landfill leachate has been recycled in this way, the recyclate is cooled slightly (by at least about 3 °C) so that the exchange process progresses more "over the surface", i.e. the recirculate spreads over the entire soil area before/while it slowly rises upwards in the form of a plug flow without mixing strongly with the raw leachate.

• 1 zeigt den Zustand in der Deponie im Schnitt nachdem ca. 50 % des Deponiesickerwassers rezykliert worden ist:

Even before a complete exchange of the landfill leachate has taken place, the Mg ²⁺ and PO ₄ ^3- concentrations will rise sharply (the struvite redissolution then takes effect) and the chemical oxygen demand will slowly fall. When the chemical oxygen demand has reached a value of approx. 3,000 mg/l, the second process step begins.

• 1 shows the average condition of the landfill after about 50% of the landfill leachate has been recycled:

Precipitation takes place in A1, with reductions in Mg and PO ₄ concentrations to approx. 2 mg/l; NH ₃ stripping takes place in B1 and NH ₃ leaching in B2 with a reduction of NH ₃ to approx. 50 mg/l. “An-oxidation” takes place in C and partial saturation with CO ₂ with the desired pH reduction to approx. 6.5 takes place in D1. In the lower area of landfill E, struvite redissolution is shown with significantly higher Mg and PO ₄ concentrations than in the upper area.

2. Process step

The first two stages of the second process step are exactly the same as in the first process step: precipitation and ammonia stripping by means of methane gassing. However, this is not followed by COD "an-oxidation", but rather partial saturation with CO ₂ . This pH reduction causes the concentrations of all dissolved cations and anions to move far away from all their potential solubility products and concentration can take place without precipitation occurring. This concentration is carried out using nanofiltration (via nanofiltration), which retains almost all divalent and higher ions as well as the majority of the COD. The nanofiltration concentration is so extensive that the ion concentrations are close to their solubility products. As an example, it is assumed that this nanofiltration delivers approx. 0.25 parts concentrate (i.e. with a COD of 11,000 mg/l oxygen) and 0.75 parts permeate.

The slightly cooled permeate is recycled directly into the bottom of the landfill without further treatment. All salts still precipitated in this area involving divalent or multivalent ions are gradually completely dissolved again, as the permeate only contains very low concentrations of multivalent ions. At the same time, almost all of the organic particles that are still bound are largely dissolved again.

The concentrate, however, is subjected to a second precipitation by methane injection. This precipitates large amounts of calcium salts in particular. It is then "partially oxidized" using the known methods (ozone or UV or a combination of both), again only limited to such an extent that only approx. 30 - 90% of the COD is transformed into BOD fractions, i.e. only this partial amount is subsequently fermented anaerobically, thereby generating landfill gas. The COD is reduced overall by approx. 30 - 90%.

This concentrate water, which has been treated in this way, is then partially enriched with CO ₂ in a gas-sealed CO ₂ gassing module and thereby acidified (pH value reduction) and then recycled into the upper landfill area opposite the upper extraction point of raw landfill water, so that it is as far away as possible from both the extraction point and the permeate feed points. This ensures that the concentrate fraction has been completely degassed when it returns to the extraction point and its COD concentration is roughly halved.

This concentrate water again has very low NH ₄ ⁺ , Mg ²⁺ and PO ₄ ^3- concentrations and significantly lower calcium concentrations, and also has a significantly lower pH value than the original raw landfill leachate. However, it only causes a slight re-dissolution of struvite, as this has already been largely washed out. Due to the high concentrations of "oxidized" COD, however, greatly increased landfill gas formation rates will occur in the area of the concentrate feed.

After approximately 100% exchange rate with the second process step, the ion concentrations and the chemical oxygen demand begin to drop sharply, as the permeate has then reached the extraction point. The production of digester gas also begins to decrease. Starting at approximately this point, the introduction of the concentrate fractions is gradually moved towards the extraction point; at the same time, the concentration rate of the nanofiltration is gradually increased, as the concentration of multivalent ions and the chemical oxygen demand become increasingly lower.

As soon as the chemical oxygen demand at the extraction point falls below a value of approx. 300 - 1,000 mg/l, the third process step begins. The landfill leachate now only has low or very low concentrations of magnesium, calcium, phosphorus and nitrogen and its biogas potential is too low even after "pre-oxidation" for recirculation after "pre-oxidation" for the purpose of biogas production in the landfill body to be worthwhile.

2 shows the condition of the landfill after about 50% of the landfill leachate has been recycled:

Precipitation takes place in A1, with reductions in the Mg ²⁺ and PO ₄ ^3- concentrations to approx. 2 mg/l; NH ₃ stripping takes place in B1 and B2, with NH ₃ reduction to approx. 20 mg/l; CO ₂ partial saturation takes place in D2, followed by nanofiltration in F, with an increase in the chemical oxygen demand to approx. 11,000 mg/l. The second precipitation takes place in A2, followed by "pre-oxidation" in C, and partial saturation with carbon dioxide in D1, with the desired pH reduction to approx. 6.5. The low concentrations of COD, Mg, PO ₄ and NH ₄ concentrations are listed in the lower area of landfill E, and the high struvite concentrations from the end of step 1 are shown in the upper area.

3. Process step

Alternative 1: The landfill leachate is first subjected to nanofiltration, which as close as possible to the first solubility exceedances.

This is achieved, for example, under suitable circumstances of the composition of the already heavily pretreated leachate with a concentration by a factor of 10, i.e. 10% concentrate (with a maximum of 9,000 mg/l COD, based on 1,000 mg/l in the inflow) and 90% permeate are produced.

Concentrate fractions 1 (after nanofiltration): This 10% fraction is then subjected to precipitation (in this case, only calcium salts are precipitated, as only very few MAP ions are present) and NH ₃ stripping by means of methane injection, see the steps above. However, the NH ₃ stripping is only carried out to an ammonium concentration that ensures that the subsequent aerobic biology is not undersupplied with nitrogen.

This fraction is then fed to an aerobic activated sludge biology, designed only for carbon degradation and not for nitrification, since the feed contains only as little nitrogen as the activated sludge biology needs to build up biomass.

The excess sludge, which probably contains quite high amounts of heavy metals, is disposed of. Optionally, the excess sludge can also be digested (e.g. with ultrasound) and then recycled in the upper landfill area - in this case, there is no need to dispose of the excess sludge at all.

The clear water effluent from the biology (optionally after partial saturation with carbon dioxide), from which a large part of its multivalent ions has already been removed during precipitation and whose COD has been roughly halved after aerobic biology, is recycled into the inlet for nanofiltration after a "pre-oxidation" that oxidizes approx. 50% of the COD to BOD. This ongoing recycling means that all multivalent ions from the concentrate and all of the COD are removed or eliminated. Therefore, under favorable circumstances (i.e. favorable composition of multivalent ions) there is no net excess concentrate: all of the water (with all of the monovalent ions that are not part of the precipitated salts from the concentrate) ultimately ends up in the permeate.

Under unfavourable circumstances, a composition of multivalent ions is present which makes nanofiltration increasingly difficult and economically problematic in the long term. Under these circumstances, a corresponding proportion of the concentrate is added to the evaporation of the reverse osmosis concentrate, see below.

However, since the micro/nano plastic particles become increasingly concentrated during the continued nanofiltration concentration, a minimum amount of nanofiltration concentrate must be added to the evaporation of the reverse osmosis concentrate or evaporated separately.

Permeate fraction 1 (after nanofiltration): The permeate contains only very low COD concentrations, but the entire original concentrations of monovalent ions (sodium, potassium, chloride and carbonates) and ammonium concentrations, which are too high for both recycling and direct discharge. In particular, the very high sodium, potassium and chloride concentrations as well as the absence of oxygen preclude mixing with the groundwater.

The permeate is therefore supersaturated with oxygen; this happens either directly or via a mass algae culture system (= MAK) such as in the EP3 065 539 B 1. Outside the photosynthesis phases, additional technical oxygen (O ₂ ) can be added in the required quantities so that the permeate then has a supersaturated oxygen content of approx. 20 - 45 mg/l.

The permeate is then subjected to reverse osmosis (RO), possibly multi-stage reverse osmosis. Since the permeate has very low concentrations of multivalent ions and a very low chemical oxygen demand, reverse osmosis can be operated with only low operating costs and very high permeate yields, up to 90% permeate 2 and 10% concentrate 2, for example.

Concentrate fraction 2 (after reverse osmosis): This concentrate is preferably evaporated. The salt fraction, which consists almost exclusively of sodium and potassium chlorides and carbonates, is disposed of or recycled.

The evaporation condensate with quite high ammonium contents is recycled to the NH ₃ stripping after nanofiltration.

Permeate fraction 2 (after reverse osmosis): The reverse osmosis permeate, which is supersaturated with oxygen and has a chemical composition that allows it to be mixed with groundwater without any problems, is recycled back into the soil area of the landfill after cooling. There, a strong oxygen consumption will take place until the entire Residual organic matter in the landfill body has been metabolized to carbon dioxide. The strong supersaturation with oxygen ensures complete aerobic utilization of even the largest remaining organic nests, as these are continuously flushed from below by these high oxygen concentrations.

Therefore, in the first few meters of the upwardly moving oxygen front, the oxygen is initially completely consumed by respiration of biodegradable residues and by nitrification (oxidation of ammonium to nitrate). As a result, a nitrate front gradually builds up parallel to or slightly ahead of the oxygen front, which is, however, completely denitrified on the way up or eliminated by anaerobic ammonium oxidation (anammox).

Even after a recycling rate well above 100% with the oxygen-supersaturated permeate, the withdrawal water will still not contain any traces of oxygen and the biological oxygen demand, the chemical oxygen demand and the ammonium concentration require continued operation of the aerobic biology and the oxygen enrichment systems.

Alternative 2: If the BOD/COD- nitrogen ratio in the extraction water is so low that complete nitrogen elimination can be achieved by means of nitrification/denitrification in the aerobic biology, the extraction water can also be fed directly to the aerobic biology, with subsequent nanofiltration of the clear water, "oxidation" of the concentrate fraction and return of this fraction to the aerobic biology; in this case, ammonia stripping can be dispensed with entirely. Otherwise, alternative 2 follows alternative 1.

Only when the BOD content in the landfill water is <10-30 mg/l and the salt content is low enough can all parts of the system upstream of the mass algae culture system (apart from perhaps the NH ₃ stripping system) be taken out of operation. In the mass algae culture system, the rest of the BOD is then breathed and the required oxygen enrichment is ensured. The subsequent reverse osmosis will be easier to operate and will have lower operating costs as the COD and ion concentrations become lower and lower. The evaporation condensate will only continue to be fed to the stripping system in alternative 1 until the ammonium concentrations in the water taken are so low that they can only be recovered using the mass algae culture system.

At this point or shortly thereafter, the first traces of oxygen will appear in the extraction water. As soon as the extraction water has oxygen concentrations in the saturation range (9 - 10 mg/l), the remaining parts of the plant, i.e. the mass algae culture plant and the reverse osmosis system, can be taken out of operation: the total remediation of the landfill is complete and a "return" to the surrounding land can take place, as the water quality of the landfill body has now reached groundwater quality.

3 shows the average condition of the landfill after approximately 70% of the landfill leachate has been recycled under conditions of Alternative 1:

First, nanofiltration takes place in F with a COD concentration to approx. 9,000 mg/l. In the following A1, the nanofiltration concentrate is precipitated with subsequent NH ₃ stripping in B1 and B2. The nanofiltration permeate 1 is conveyed directly to the mass algae culture plant (G) with subsequent final treatment in reverse osmosis (H) and recirculation of the reverse osmosis permeate after cooling to the landfill E. The reverse osmosis concentrate is evaporated in I.

The nanofiltration concentrate 1 after NH ₃ stripping B is fed to an aerobic activated sludge biology J and the clear water effluent after a "pre-oxidation" in C1 and optional partial enrichment with carbon dioxide is recycled into the inlet to the nanofiltration. A small partial flow of the nanofiltration concentrate is fed into the reverse osmosis evaporation I.

4 shows the average condition of the landfill after approximately 120% of the landfill leachate has been recycled and the biological oxygen demand in the landfill extraction is less than approximately 10-30 mg/l oxygen.

Now only the mass algae culture system, the reverse osmosis and the evaporation are still in operation: First, the BOD elimination and oxygen supersaturation take place in the mass algae culture system (G). Then the residual salts and the residual COD are removed in the reverse osmosis (H) and the reverse osmosis concentrate is evaporated in module I.

Finally, an overview of the overall balance is given:

1. Biogas

At the beginning of the treatment phase, the landfill leachate had COD concentrations of approx. 4,000 mg/l. These are carbon compounds that are only very heavy or no longer anaerobically fermentable. After the landfill is closed, even decades later, only a fraction of this organic load (perhaps around 20%) would be fermented into biogas. In addition, the methane concentration would continuously decrease, making energy recovery impossible or very difficult.

In process steps 1 and 2, approximately 75 - 85% of the dissolved landfill COD is converted into fermentable BOD, the end product of which is biogas. Approximately 5% is converted directly into _CO2 through the "pre-oxidation" and only approximately 5% has to be respired in the aerobic biology. The remaining approximately 10% is disposed of with the excess sludge and the evaporation products.

In addition, biogas production can generally be increased by adding liquid biodegradable waste materials, e.g. fats, to the landfill body. This design means that the landfill is used for biogas production to dispose of external liquid biodegradable waste materials that are foreign to the landfill, i.e. do not come from the landfill itself, which would otherwise be fed into conventional biogas plants.

2. Excess sludge

The amount of excess sludge produced and to be disposed of is extremely low compared to other processes, at only about 5% of the total landfill COD. This is of course because the majority of the COD has been fermented into biogas.

However, the excess sludge (from aerobic biology and mass algae culture) can also be digested and recycled in the landfill - in this case disposal is completely eliminated.

3. Nitrogen

Practically the entire nitrogen content of the landfill in the form of dissolved, organically bound nitrogen and nitrogen precipitated in salts, at least over 90%, is recovered by stripping as highly concentrated ammonia water and is used, for example, as fertilizer or for the Power to X technology. Approximately 5% of the nitrogen is recovered as a component of the struvite and only around 2% is found in the excess sludge and in the evaporation concentrates. However, only the ammonia is recovered by stripping - the organically bound nitrogen is recycled into the landfill body and there it is largely broken down enzymatically, which releases ammonia and also enters the ammonia water or the struvite.

4. Phosphorus and magnesium

Well over 90% of the total phosphorus and magnesium is recovered as struvite during the first two process steps in the precipitation, with only very small amounts of calcium salts. The struvite salt contains only the monophosphate PO ₄ ^3- .

All non-monophosphate components of phosphorus are recycled into the landfill body during the first two process steps. There they are gradually mineralized enzymatically by phosphatases to monophosphate, which is recovered as struvite during the second process phase. Only about 5% of the phosphorus is recovered using the excess sludge and the evaporation concentrates.

The precipitated struvite is contaminated with only very low concentrations of heavy metals (see below) and can therefore be directly marketed at a high price as the most valuable fertilizer of all.

5. Calcium

During the first two process steps, part of the calcium is precipitated together with the struvite as phosphate, carbonate or hydroxide. This product is therefore not just a pure magnesium-phosphorus fertilizer, but a magnesium-phosphorus-calcium fertilizer.

Another part of the calcium precipitates with small amounts of struvite in the third process step, while at the same time a high proportion of heavy metals precipitates, so that, depending on the type of landfill, immediate use as fertilizer is not possible. If necessary, the salt load from the third process step must therefore be processed, for example in sewage sludge incineration plants.

Less than 5% of the total calcium is found in the excess sludge and about 5% of the total calcium is found in the evaporation concentrates.

6. Heavy metals

Since raw landfill gas contains quite high hydrogen sulphide (H ₂ S) concentrations, the sulphide (S ^2- ) and hydrogen sulphide (HS ^- ) concentrations in landfill leachate are also so high that practically all heavy metals are almost completely removed as Sulfides are precipitated and are not in the water phase. Therefore, the salt precipitates during the first two process steps are only contaminated with heavy metals to a very small extent.

However, this situation begins to change with the start of the second process step, when nanofiltration permeate is recycled to the landfill, and with the start of recycling of the reverse osmosis permeate, the remaining heavy metals are dissolved back. Therefore, the heavy metal concentration in the extraction leachate increases from the start of the third process step, so that the salts extracted by precipitation in the third process step have significantly higher heavy metal concentrations. The rest of the heavy metals are disposed of with the excess sludge and the reverse osmosis evaporation concentrate.

7. Nano/microplastics

After completion of the remediation, the water phase of the landfill contains almost no nano- or microplastics, which are almost completely channeled into the reverse osmosis evaporation concentrate and are also found to a small extent in the excess sludge.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• EP3065539 [0069]

Claims (17)

Method for remediating a landfill having a base seal and a surface seal and emitting landfill gas and landfill leachate from a landfill body arranged between the base seal and the surface seal, characterized by the steps: a. removing landfill leachate from the landfill body at a removal point provided on the landfill, b. precipitating salts dissolved in the landfill leachate, c. shifting the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate depleted of salts in favor of the biological oxygen demand (BOD) by gassing the landfill leachate depleted of salts with ozone and/or irradiating the landfill leachate depleted of salts with UV radiation, d. Returning the landfill leachate gassed with ozone and/or irradiated with UV radiation and depleted of salts into the landfill body at at least one first feed point arranged opposite the removal point on the landfill, forming a flow between the removal point and the at least one first feed point, and e. Repeating steps a to d until the chemical oxygen demand (COD), the biological oxygen demand (BOD) and/or the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate removed from the landfill body at the removal point correspond to a predetermined first value. Procedure according to Claim 1 , characterized by supplying external liquid biodegradable waste materials to the landfill body. Procedure according to Claim 1 , characterized in that the precipitation of salts mentioned in step b is carried out by adding magnesium salt and/or stripping the carbon dioxide contained in the landfill leachate to increase the pH value of the landfill leachate and thereby precipitate salts which are insoluble at an increased pH value. Method according to one of the preceding claims, characterized by reducing the pH value of the landfill leachate which has been gassed with ozone and/or irradiated with UV radiation and depleted of salts before the return to the landfill body mentioned in step d. Procedure according to Claim 4 , characterized in that the pH value is reduced by gassing the landfill leachate which has been gassed with ozone and/or irradiated with UV radiation and depleted of salts using carbon dioxide. Procedure according to Claim 5 , characterized in that the landfill leachate which has been gassed with ozone and/or irradiated with UV radiation and depleted of salts is fumigated with landfill gas or with carbon dioxide obtained from the landfill gas of the landfill body. Method according to one of the preceding claims, characterized in that the stripping of the carbon dioxide contained in the landfill leachate in step b is carried out by gassing the landfill leachate taken at the extraction site with methane obtained from the landfill gas of the landfill body. Process according to one of the preceding claims, characterized by stripping the ammonia contained in the landfill leachate depleted of the salts before the gassing with ozone and/or irradiation with UV radiation mentioned in step c. Procedure according to Claim 8 , characterized in that the stripping of the ammonia (NH ₃ ) contained in the landfill leachate depleted of the salts is carried out by gassing the landfill leachate depleted of the insoluble salts with methane obtained from the landfill gas of the landfill body. Procedure according to Claim 9 , characterized by nanofiltration of the landfill leachate depleted of the salts and ammonia, treating the nanofiltration concentrate according to step c, wherein the nanofiltration concentrate gassed with ozone and/or irradiated with UV radiation is fed to at least one second feed location arranged on the landfill, which lies between the removal location and the at least one first feed location, and tempering the nanofiltration permeate to a temperature below the temperature of the nanofiltration concentrate gassed with ozone and/or irradiated with UV radiation and returning the nanofiltration permeate to the landfill body at the at least one first feed location. Method according to one of the preceding claims, characterized by nanofiltration of the landfill leachate taken at the extraction site before the precipitation of salts dissolved in the landfill leachate mentioned in step b, wherein the nanofiltration concentrate according to step b and following and the nanofiltration permeate is fed to an algae culture, the aqueous supernatant of the algae culture is subjected to reverse osmosis and the reverse osmosis permeate is returned to the landfill body at the at least one feed location. Procedure according to one of the Claims 1 until 10 , characterized by supplying the landfill leachate extracted at the extraction site to an algae culture and treating the aqueous supernatant by means of reverse osmosis, wherein the reverse osmosis permeate is returned to the landfill body at the at least one supply site. Procedure according to one of the Claims 11 and 12 , characterized in that the concentrate obtained from reverse osmosis is evaporated. Method according to one of the preceding claims, characterized by processing the gases produced according to the method together with the landfill gas emitted from the landfill body. Procedure according to Claim 1 , characterized by the steps following step e after the predetermined first value mentioned in step e has been reached: f. removing landfill leachate from the landfill body at the removal point provided at the landfill, g. precipitating salts dissolved in the landfill leachate, h. stripping the ammonia contained in the landfill leachate depleted of salts, i. nanofiltration of the landfill leachate depleted of salts and ammonia, j. shifting the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) in the nanofiltration concentrate in favor of the biological oxygen demand (BOD) by gassing the landfill leachate depleted of salts with ozone and/or irradiating the landfill leachate depleted of salts with UV radiation, k. Returning the landfill leachate that has been gassed with ozone and/or irradiated with UV radiation and depleted of salts into the landfill body at at least one second feed point arranged on the landfill, which lies between the removal point and the at least one first feed point, l. Tempering the nanofiltration permeate to a temperature below the temperature of the landfill leachate that has been gassed with ozone and/or irradiated with UV radiation and depleted of salts and returning the tempered nanofiltration permeate to the landfill body at the first feed point, and m. Repeating steps f to l until the chemical oxygen demand (COD), the biological oxygen demand (BOD) and/or the ratio of chemical oxygen demand (COD) to biological oxygen demand (BOD) of the landfill leachate removed from the landfill body at the removal point correspond to a predetermined second value. Procedure according to Claim 15 , characterized by the step following step m after reaching the predetermined second value mentioned in step m: n. by feeding the landfill leachate taken at the removal location to an algae culture and treating the aqueous supernatant by means of reverse osmosis, wherein the reverse osmosis permeate is returned to the landfill body at the at least one feed location. Procedure according to Claim 16 , characterized in that the landfill leachate extracted at the extraction site is nanofiltered before step n and the nanofiltration permeate is treated according to step n.

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