DE102018111145A1 - Process for the recovery of phosphorus from wastewater - Google Patents

Abstract

The invention relates to a method for recovering phosphorus from wastewater to sewage treatment plants, wherein the method comprises a biological wastewater treatment, wherein the biological wastewater treatment no or no substantial biological phosphate elimination and no or no significant phosphate precipitation takes place, so that the phosphate content the resulting during the biological wastewater treatment sewage sludge under 20 g of phosphorus per kg of dry matter, wherein the biological wastewater treatment is followed by a chemical treatment part in the phosphate precipitated from the wastewater and separated in a sludge with high phosphate concentration. The invention is characterized in that the chemical treatment part comprises the steps of phosphate precipitation, preferably with the aid of lime (first precipitation), sedimentation of the sludge (in the case of phosphate precipitation with the aid of lime: sedimentation of the lime sludge), phosphate precipitation, preferably with metal salt, (second precipitation) and filtration included.

Description

The new Sewage Sludge Ordinance of 2017 requires the recovery of phosphorus from sewage or sewage sludge. The requirement for this requirement is that the sewage sludge contains at least 20 g phosphorus (P) per kg dry matter (TM). The vast majority of municipal sewage sludge contains more than 20 g P per kg DM, so that almost all sewage sludge is subject to this requirement. It is possible to utilize sewage sludge from small and medium-sized sewage treatment plants based on the soil, so that direct P recycling takes place. However, this requires compliance with stricter limits for heavy metals and organic pollutants. In any case, the proportion of soil-related sewage sludge has been falling for years, whereas the share of thermal disposal, usually by incineration, is increasing.

There is the possibility to recover phosphorus from the sewage sludge. The sewage sludge regulation requires in this case a recovery rate of at least 50% or a remaining concentration of a maximum of 20 g P per kg DM. The depleted sewage sludge may and must e.g. co-incinerated in a coal-fired power plant or in the cement industry. It is difficult and expensive to meet this requirement in the prior art.

Sewage sludge with a P-content of more than 20 g P per kg DM must be fed to a mono-combustion or a coal-fired power plant with particularly low-ash coal, so that a phosphate-rich ash is produced. This phosphate-rich ash may be stored until it is processed. During the treatment, at least 80% of the phosphorus must be recovered from the ash. Although there are various methods for recovering phosphorus from ash, these are not yet economical. There is currently not enough capacity for sewage sludge mono-incineration in Germany and it is questionable whether this capacity can be sufficiently increased in the foreseeable future. In any case, mono-combustion with subsequent ash treatment is a very expensive process.

State of the art

Methods are known for removing phosphate from the wastewater by precipitation (see DWA Worksheet A 202). This is done in the prior art almost exclusively by a so-called simultaneous precipitation, in which the biological treatment part (i.e., the biological wastewater treatment) of the treatment plant, this is usually to be activated plants, precipitant are added to precipitate it at the same time phosphate.

As precipitant iron or aluminum salts are used in particular. The precipitated metal phosphate remains in the secondary sludge, usually the excess sludge. Thus, the vast majority of the phosphorus enters the sewage sludge. The same applies to activated sludge plants with increased biological phosphate elimination. It has also been known for decades that phosphate can be precipitated with hydrated lime (slaked lime), e.g. with the Phostrip method, in which the lime precipitation takes place before the biological wastewater treatment, ie in the upstream mechanical cleaning section. Again, the majority of the phosphorus with the primary sludge enters the sewage sludge.

Also, processes for post-precipitation of phosphate (after biological wastewater treatment) are known. Here, the precipitated sludge is separated from the wastewater by sedimentation or filtration (see DWA Worksheet A 202).

Object of the invention

With the present invention, it should be possible to produce sewage sludge having a P-content below 2 g P per kg of TM so that it can be co-combusted, e.g. In coal-fired power plants, waste incineration plants or in cement production, can be disposed of inexpensively.

It is intended to produce nutrient-rich precipitation sludges which are either directly usable for the soil or are suitable as raw material for the fertilizer industry.

Even strict limit values for the phosphorus concentration (P _ges ) should be complied with in the effluent of wastewater treatment plants. Other effluent values, such as the concentrations of filterable substances (AFS), the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) should also be as low as possible in order to reduce the effluent discharge. If possible, remaining residues of total nitrogen (N _ges ) and micropollutants (eg drug residues, hormones, antibiotics and X-ray contrast agents) should also be reduced.

solution

A process is described for the aftertreatment of biologically treated wastewater for the recovery of phosphorus.

The wastewater is purified in principle with the help of a wastewater treatment plant. A corresponding treatment plant preferably has a mechanical cleaning stage, which may include, for example, a rake and / or a sand trap. The pre-clarified Wastewater subsequently enters a so-called primary clarifier and then an activated sludge tank with aeration. The latter is part of the biological wastewater treatment, the biological wastewater treatment may include a sedimentation tank in addition to the aeration tank. Sewage sludge is produced during biological wastewater treatment.

The removal of phosphorus in the biological treatment section (= biological wastewater treatment) should be as low as possible, so that said sewage sludge contains less than 20 g P per kg DM. Therefore, no or only a slight increase in biological P elimination and preferably also no or only partial phosphate precipitation is carried out in the biological purification section. In the biological purification section, only as much phosphate is removed as the biomass of the activated sludge requires for growth. The majority of the phosphorus contained in the wastewater therefore remains dissolved in the wastewater and is separated by a two-stage replenishment.

The first stage is according to the invention, a precipitation, in particular a lime precipitation, while the lime, preferably lime (slaked lime or Ca (OH) ₂ ) is added to the wastewater. The alkaline lime milk increases the pH to at least 8.5 and precipitates calcium phosphate (as Ca ₃ (PO ₄ ) ₂ or Ca ₅ (PO ₄ ) ₃ OH). Since the equilibrium of the reaction is dependent on the pH, no stoichiometric Ca: P 18 can be given for the written ratio for the dosage. In not too soft sewage calcium is already contained in sufficient concentration for the phosphate precipitation. The greater the buffer capacity in the wastewater, the more milk of lime must be added to increase the pH sufficiently.

In the biological purification part of the ammonium bicarbonate buffer is greatly reduced because ammonium (NH ₄ +) is largely oxidized to nitrate (NO ₃ -) (nitrification) and a large part of the nitrate to elemental nitrogen (N ₂ ) (denitrification) and bicarbonate (HCO ₃ -) is reduced to carbon dioxide (CO ₂ ). These two gases are expelled during the aeration of the activated sludge plant. For a replenishment, therefore, significantly less lime is required than in a pre-precipitation.

The resulting microflakes also contain other substances, for example, washed out from the biological purification part activated sludge flakes and calcium carbonate (Ca (CO ₃ )). If in addition magnesium salt is metered, preferably in front of the milk of lime, magnesium ammonium phosphate or struvite (NH ₄ MgPO ₄ .6H ₂ O) also precipitates, so that the ammonium concentration in the waste water is further reduced.

The microflakes produced during lime precipitation are separated, in particular, by sedimentation, preferably in a lamellar clarifier. In order to improve the sedimentation and make the lamellar clarifier smaller, a flocculant, preferably in the form of a polyelectrolyte, may be added after the precipitation and production of microflakes, thereby producing macro flocs having a higher settling rate and higher shear strength from the microflakes.

In a further embodiment of the invention, the separated lime mud can be recycled based on soil. Its P content is between 10% to 15% of TM. Calcium phosphate is good for plant availability, and acidic soils require the intake of lime anyway, making it a valuable fertilizer and soil improver. Alternatively, the lime mud is a raw material for the commercial production of fertilizers.

The second step is likewise a precipitation, preferably with di- or trivalent metal salts, eg with iron chloride (FeCl ₃ ), aluminum sulfate (Al ₂ (SO ₄ ) ₃ ) or aluminates (eg polyaluminium chloride). In particular, trivalent metal ions precipitate phosphate quantitatively in the pH range between 6.5 and 8.5. However, the pH value should not be too high, because with increasing pH increasingly competing metal hydroxides (Me (OH) ₃ ) precipitate. Other substances are precipitated quantitatively, eg sulfide, which, however, hardly occurs in biologically treated wastewater. The stoichiometric Me: P ratio in the post-precipitation should be chosen between 1.5 and 2, ie a considerable excess of precipitant is required because of side reactions. However, the P concentration after the previous lime precipitation is already low, so that much less precipitant is to be metered compared to the simultaneous precipitation. The mass of the precipitation sludge produced in the second precipitation is therefore comparatively small.

The metal salts are preferably acidic compounds, so that they reduce the high pH after the lime precipitation again. This is advantageous for two reasons. Firstly, the precipitants have a better effect at a lower pH, and secondly, the wastewater treatment plant effluent should only have a maximum pH of 8. Because of the buffering capacity which has been further reduced by the preceding lime precipitation, even a small amount of precipitant dosing suffices for a substantial reduction in the pH. If necessary, some acid may be added for neutralization.

The precipitation sludge of the second precipitation is separated from the wastewater, for example by filtration, for example by filtration General filtration. A spatial filter consists of a filter layer of granules, eg of sand or anthracite. The service life of down-flow two-layer filters, which for example have an upper coarser anthracite layer and a lower, finer sand layer, to the required backwashing, which is required either when exceeding a predetermined hydraulic pressure loss or when solid material breakthrough, is longer than that of single-layer filters. Therefore, two-layer filters are preferably used. Such filters are preferably periodically backwashed by supplying purified wastewater and / or air from below to fluidize and stratify the filter layer as a fluidized bed.

However, it is also possible to use upflowed single-layer filters whose filter layer is more or less continuously backwashed (e.g., a contiflow filter). In such room filters granules is conveyed from below via a compressed air lift up, thereby cleaned and returned to the top of the filter layer. The filter layer moves slowly against the direction of the sewage flow. It is possible to use two or more such filters with different granules in series, e.g. a first sand filter and a second activated carbon filter. But it is also possible to operate such a filter with a granular mixture.

It is particularly advantageous if the spatial filter or one of the spatial filters contains activated carbon. On the huge inner surface of activated carbon, dissolved wastewater components are adsorbed. As a result, the COD can be further reduced. The activated carbon also adsorbs micropollutants and thus separates them from the wastewater. According to this embodiment of the invention, the proposed method after the mechanical and biological wastewater treatment comprises not only a third (chemical) purification part, but even a fourth (adsorptive) purification part. The need to remove micropollutants has been a major issue for several years.

In a further variant of the invention, a surface filter can be used, a so-called microsieve. The filter medium of such filters consists of very fine meshes with a mesh size of the order of, for example, 0.02 mm. A sludge cake builds up on the filter medium, which also retains flakes that are smaller than the mesh size. Microsieves have partially in the wastewater dipped and provided with the filter medium discs or drums. When the pressure loss of the microsieve exceeds a predetermined value, the discs or drums rotate. From emerged from the wastewater filter medium, the filter cake is sprayed and discharged. Submerged in the wastewater filter medium is cleaned. It is therefore a continuously operated filter.

The quality of the filter effluent can be improved by additionally adding flocculant (polyelectrolyte) after the precipitation. The resulting larger and shear-stable flakes are better retained in the filter.

It may be particularly advantageous to dose powdered activated carbon before filtration into the wastewater, instead of using granular activated carbon as a filter layer in a spatial filter. An occasional replacement of loaded granulated activated carbon is then not required. The activated carbon powder retained and loaded in or on the filter is discharged during the filter rinse with the precipitation sludge.

The use of granulated activated carbon or powdered activated carbon after a double precipitation is also particularly advantageous because in the two-stage precipitation COD is eliminated, which competes with the loading of activated carbon with micro-pollutants. This increases the uptake capacity of the micro-charcoal activated carbon, i. that the service life of filters with granular activated carbon is extended or less activated carbon powder must be metered.

Metal phosphates are poorly plant-available and therefore can only be used as fertilizer after treatment. Therefore, it is proposed in a further embodiment of the invention to return the precipitated sludge from the filtration before or in the biological purification part, so that it is disposed of with the sewage sludge. In particular, at a low pH in the biological purification part, metal hydroxide can react to metal phosphate.

If the treatment plant is equipped with a sludge digestion, the metal ions preferably react with the sulfide formed in the digester, so that the sulfur content in the digester gas is reduced.

If the recirculated precipitation sludge contains activated carbon powder, this is further loaded in the biological purification section with dissolved COD and micropollutants. Further advantages of this recycling are that the powdered activated carbon improves the sedimentation of the activated sludge in the secondary clarifier, and that the calorific value of the sewage sludge is increased.

Further advantages of the proposed method are that, firstly, the disinfection of the waste water by means of UV radiation is improved, because after the filtration almost no solids are contained, which hinder the UV radiation, and second, a subsequent treatment with ozone for disinfection and / or for further destruction of micropollutants is improved and cheapened, because the remaining COD is reduced.

The chemical aftertreatment of biologically treated wastewater by chemical precipitation and filtration is known. However, new is the chemical aftertreatment of the wastewater by a combination of precipitation, in particular lime precipitation, and sedimentation with a subsequent second precipitation, preferably with metal salts, and filtration.

Also of particular novelty is the possible integration of processes for the elimination of micropollutants into the chemical aftertreatment of biologically purified wastewater.

The use of the method according to the invention considerably increases the investment in sewage treatment plants, in particular because lamellar clarifiers and filters are additionally required. However, in the future, filters will also be required for smaller and medium-sized sewage treatment plants to effectively separate micropollutants.

As a result of the improved by wastewater filtration wastewater quality wastewater discharge is lower. Grants for investment and offsetting of wastewater discharge are likely.

It has recently been discovered that bathing waters contain multidrug-resistant bacteria. These develop when bathing waters contain antibiotics. The requirement for the elimination of micropollutants, including antibiotics, is thereby exacerbated. It also supports the demand for wastewater disinfection, which has been state-of-the-art in the USA for decades. The proposed method allows or facilitates the installation of the procedures necessary to eliminate micropollutants and disinfection.

The increase in the cost of sewage sludge disposal expected by the new Sewage Sludge Ordinance will be avoided and the disposal safety of sewage sludge will be significantly improved.

The biological purification part is relieved that an anaerobic basin is no longer needed for increased biological P-elimination and can be used elsewhere.

The costs of the precipitants are of the same order of magnitude as those for the simultaneous precipitation according to the prior art.

The problem of precipitation of struvite in the digester, a consequence of increased biological P elimination, is avoided because the sewage sludge contains only little phosphate. Because of its low phosphate concentration, the drainage of sewage sludge is improved, which saves further costs.

It is expected that the phosphate- and lime-rich precipitation sludge can be sold because calcium phosphate is well plant-available and many soils require a supply of lime. All forecasts indicate that phosphorus, which contains little cadmium and uranium, is more expensive for the production of fertilizers. After prolonged storage, the lime-rich precipitated sludge is sanitized because of its high pH, i. largely free of pathogenic microorganisms.

The costs of the process according to the invention must not be compared with the processes of the prior art because the process according to the invention offers added value, which improves the quality of the effluent effluent and substantially facilitates the completion of processes for the removal of micropollutants and for disinfection becomes.

Claims (10)

A method for recovering phosphorus from wastewater to sewage treatment plants, the method comprising a biological wastewater treatment, wherein the biological wastewater treatment no or no significant biological phosphate elimination and no or no significant phosphate precipitation takes place, so that the phosphate content during the biological Sewage sludge produced under wastewater treatment is below 20 g of phosphorus per kg of dry matter, wherein the biological wastewater treatment is followed by a chemical treatment part in which phosphate is precipitated from the wastewater and separated in a sludge with high phosphate concentration, characterized in that the chemical treatment part at least the Stages of phosphate precipitation (first precipitation), preferably with the aid of lime, sedimentation of the sludge (in the case of phosphate precipitation with the aid of lime: sedimentation of the lime sludge), another phosphate precipitation (second precipitation), preferably with Hil Fe of metal salt, and a subsequent filtration involves. Method according to Claim 1 , characterized in that the first precipitation is carried out by metering of burnt or slaked lime and the precipitated lime sludge sediments in a lamella. Method according to one of the preceding claims, characterized in that the second precipitation is carried out by metering of 2- or 3-valent iron or aluminum salts, for example with the precipitants FeCl ₃ , Al ₂ (SO ₄ ) ₃ or aluminate, wherein the precipitated solids preferably be filtered off. Method according to one of the preceding claims, characterized in that the filtration is a spatial filtration in which the wastewater flows through a filter layer consisting of a granulate. Method according to one of the preceding claims, characterized in that the granules of sand, anthracite and / or activated carbon or comprises one or more of said substances, and / or that two-layer filter, in particular with activated carbon as the upper layer and sand as the lower layer, for Use come. Method according to one of the preceding claims, characterized in that the filtration is a surface filtration, in which wastewater flows through a sieve, preferably a microsieve. Method according to one of the preceding claims, characterized in that prior to filtration activated carbon, preferably powdered activated carbon, is metered into the wastewater to remove micro-pollutants. Method according to one of the preceding claims, characterized in that in addition to the precipitating agents polyelectrolytes are added as flocculants in order to increase the size and strength of the microflakes resulting from the precipitation. Method according to one of the preceding claims, characterized in that the wastewater additionally magnesium is added to precipitate magnesium ammonium phosphate. Method according to Claim 1 , characterized in that the precipitation sludge separated by sedimentation of the first precipitation is utilized as fertilizer or raw material for the production of fertilizers and the precipitation sludge separated by filtration of the second precipitation with iron or aluminum salt recycled before or in the biological purification part and disposed of with the sewage sludge becomes.