Ανάκτηση αζώτου και φωσφόρου σε εγκατάσταση επεξεργασίας λυμάτων

Abstract

Τις τελευταίες δεκαετίες έχει αυξηθεί η ανησυχία για τους πεπερασμένους πόρους φωσφόρου και τη σπουδαιότητα της ανάκτησής του. Υπολογίζεται ότι οι πόροι του φωσφόρου παγκοσμίως πρόκειται να εξαντληθούν σε 150-200 χρόνια και η παγκόσμια παραγωγή φωσφόρου θα κορυφωθεί το 2033 δημιουργώντας την ανάγκη για ανακάλυψη υλικών που θα αντικαταστήσουν τον ορυκτό P. Η ιλύς των εγκαταστάσεων επεξεργασίας αστικών λυμάτων αποτελεί μία αξιόλογη πηγή φωσφόρου δεδομένου ότι με τις σύγχρονες τεχνολογίες μπορεί να μετατραπεί πάνω από το 90% του φωσφόρου από τα λύματα σε ιλύ. Ο σκοπός της παρούσας εργασίας είναι η μελέτη και η ανάλυση των σύγχρονων μεθόδων ανάκτησης φωσφόρου και αζώτου από την ιλύ των εγκαταστάσεων επεξεργασίας αστικών λυμάτων, τη στάχτη από την καύση της ιλύος και την υγρή φάση μετά την αναερόβια επεξεργασία στην οποία περιέχεται διαλυτός φώσφορος. Οι μέθοδοι ανάκτησης φωσφόρου και αζώτου περιλαμβάνουν την κρυσταλλοποίηση και κατακρήμνιση, τις μεθόδους υγρής επεξεργασίας με χρήση χημικών και τις θερμοχημικές μεθόδους. Κατά την κρυσταλλοποίηση και κατακρήμνιση, ο φώσφορος μετατρέπεται σε στερεά μορφή με προσθήκη χημικών και ρύθμιση του pH. Οι μέθοδοι υγρής επεξεργασίας με χρήση χημικών ανακτούν το δεσμευμένο φώσφορο της ιλύος ή της στάχτης με έκπλυση με οξύ ή βάση και γίνεται ανάκτηση του διαλυτού φωσφόρου με διάφορες μεθόδους, από τις οποίες η πιο κοινή είναι η κατακρήμνιση. Επίσης, η στάχτη που παράγεται με την καύση της ιλύος υφίσταται επεξεργασία με θερμοχημικές μεθόδους προσθέτοντας άλατα χλωρίου και θερμαίνοντας τη στάχτη σε θερμοκρασίες υψηλότερες από το σημείο βρασμού των σχηματισμένων χλωριούχων αλάτων των βαρέων μετάλλων με αποτέλεσμα την εξάτμισή τους. Η παρούσα εργασία μελετά τις τεχνολογίες που βασίζονται στις παραπάνω μεθόδους και έχουν εφαρμοστεί τόσο σε βιομηχανική κλίμακα όσο και σε πειραματικό-πιλοτικό στάδιο. Επίσης, γίνεται οικονομική αξιολόγηση των μεθόδων ανάκτησης φωσφόρου και αζώτου και μελέτη της βιωσιμότητάς τους. Οι περισσότερες από τις βιομηχανικές μεθόδους βασίζονται στην κρυσταλλοποίηση ή την κατακρήμνιση. Η κατακρήμνιση αποτελεί μία μέθοδο που έχει μελετηθεί ευρέως και έχει αποδειχθεί ότι είναι οικονομικά πιο βιώσιμη από τις μεθόδους υγρής επεξεργασίας με χρήση χημικών. Οι μέθοδοι υγρής επεξεργασίας απαιτούν τη χρήση χημικών και παρέχουν περιορισμένα αποτελέσματα από τη χρήση τους σε βιομηχανική κλίμακα. Οι θερμοχημικές μέθοδοι δεν έχουν εφαρμοστεί ακόμα σε μεγάλες εγκαταστάσεις οπότε δεν μπορούν να αξιολογηθούν επαρκώς με τα παρόντα αποτελέσματα.

Since the implementation of the EC Urban Waste Water Treatment Directive (UWWTD) (21 May 1991), a number of fundamental changes in wastewater treatment have occurred. Two of the changes directly impact upon water companies’ treatment of sludge produced from wastewater treatment facilities. - Dumping of sewage sludge at sea is now prohibited - Nitrogen and phosphorus limits have been imposed to reduce the potential of eutrophication of sensitive inland and coastal waters Phosphorus is currently sourced predominantly from phosphate rocks. Phosphorus is an essential nutrient for plants growth and 80% of the phosphorus produced is used to manufacture fertilisers. Based on the reserves left, phosphorus rocks could be depleted at the turn of this century if the increase in the rate of demand is unchanged. Recycling of phosphorus discharged to the environment through various waste streams must be considered seriously to reduce dependence on phosphate rocks and ensure a supply of phosphorous for the future through sustainable development. Recovery of phosphorus and nitrogen from waste streams has potential to recover more than 90% of dissolved phosphorus from digester supernatant as struvite. This recovery is technically feasible and economically beneficial. The payback period of a struvite plant processing 55 000 m3/d of waste stream could be less than five years. Furthermore, it is estimated that there are 7 000 million tons of phosphate rocks as P2O5 remaining in reserves that could be economically mined. The human population consumes 40 million tons of P as P2O5 each year. It is predicted that P demand will increase by 1.5% each year. Estimates are that the resource could be exhausted in as little as 100–250 years. There are 11 000 million tons of phosphate rocks that cannot be processed economically at present. Even if these reserves could be processed economically in the future, they are not a renewable resource and therefore, recovering phosphorus from waste streams is a significant breakthrough technology.

Applications of phosphorus recovery in wastewater technology

Figure 1: Possible locations for phosphorus recovery

In Figure 1 different applications of potential phosphorus and nitrogen recovery processes are illustrated via the scheme of a model wastewater treatment plant. The letters A–C indicate potential locations of phosphorus and nitrogen recovery from the liquid phase, i.e. the effluent of the wastewater treatment plant (A), the supernatant liquor from side-stream treatment (B), and the sludge liquor (C). The numbers 1 to 6 indicate potential applications of phosphorus and nitrogen recovery from sewage sludge, i.e. primary (1), excess (2) and raw sludge (3), stabilized sludge before and after dewatering (4 and 5) and sewage sludge ash (6). As in wastewater treatment plants without phosphorus removal 90–95% of the incoming phosphorus load is contained in the sewage sludge, the theoretical recovery potential is significantly higher than with separation processes from the aqueous phase. The final products of these applications are calcium phosphate, struvite and ash.

The main phosphorus and nitrogen recovery technologies Phosphorus and nitrogen recovery during wastewater treatment (Crystallization/Precipitation) The implementation of phosphorus and nitrogen recovery during wastewater treatment allows for the separation of already dissolved phosphorus and nitrogen applying relatively basic technologies. Phosphorus recovery is particularly successful in combination with biological phosphorus removal in side streams (supernatant liquor of the anaerobic stripper) or from process water during sludge treatment. The phosphorus-rich water is fed into a precipitation/crystallization tank, where by adding calcium or magnesium salts and, where need be, seed crystals, phosphorus and nitrogen are removed as calcium phosphate or magnesium ammonium phosphate. Wet chemical technologies The wet chemical treatment of sewage sludge involves that in a first step phosphorus, bound in sewage sludge is dissolved by adding acid or base, in combination with temperature if necessary. Thereby, in most cases (heavy) metals are re-dissolved as well. After the insoluble compounds have been removed, phosphates can be separated from the phosphorus-rich water, e.g. via precipitation, ion exchange, nanofiltration, or reactive liquid–liquid extraction. The same technologies can be applied to recover phosphorus from sewage sludge ash. The advantage here is that by disintegrating the organic matter – including all organic pollutants–there is an enrichment of phosphorus. In contrast to sewage sludge, solid-liquid separation after alkaline or acidic treatment is significantly easier to realize due to the exclusively inorganic formation of the sewage sludge ash. Thermochemical treatment Via specific thermochemical treatment of sewage sludge ash it is possible to remove heavy metals and, at the same time, improve the plant availability of phosphorus process. Based on the thermochemical approach, ashes are exposed–under suitable conditions–to chlorine containing substances, potassium chloride or magnesium chloride, and treated thermally. With high temperatures, a large percentage (%) of the heavy metals is turned into heavy metal chlorides which vaporize, thus removing them from the ashes. The heavy metals are captured at flue gas treatment.

Industrial Scale Processes The most important processes concerning recovering P and N that have been applied on industrial scale are Crystalactor (crystallization of calcium phosphate), Air Prex (precipitation of struvite), Pearl Process (precipitation of struvite), Phosnix (precipitation of struvite) and Seaborn (wet chemical technology). Crystalactor An advanced alternative is to apply crystallization instead of precipitation. The Crystalactor, a fluid-bed type of crystallizer, has been developed for this purpose. Instead of bulky sludge, this process generates high purity phosphate crystal pellets that can be re-used in many ways. Recovery of phosphate becomes more and more important since it is a sustainable solution to the environmental problems related to the mining and processing of natural phosphate resources. The Crystalactor enables phosphate removal and recovery by means of several process routes. The most important routes are: • Crystallization as calcium phosphate (CP) • Crystallization as magnesium phosphate (MP) • Crystallization as magnesium ammonium phosphate (MAP) • Crystallization as potassium magnesium phosphate (KMP) The chemistry of the process is comparable to the conventional precipitation. By dosing a suitable reagent to the water (e.g. lime, calcium chloride, soda, caustic soda), the solubility of the target component is exceeded and subsequently it is transformed from the aqueous solution into solid crystal material. The primary difference with conventional precipitation is, that in the crystallization process the transformation is controlled accurately and that pellets with a typical size of approx. 1 mm are produced instead of fine dispersed, microscopic sludge particles. The Air Prex procedure Spontaneous precipitation of struvite was a problem at Waßmannsdorf WWTP causing incrustations to the sludge treatment equipment. The problem was solved by developing a method for controlled precipitation of struvite in cooperation with Technical University of Berlin. The principle in the AirPrex is the same as in the old method: airstripping of CO2 to adjust pH and a dosage of MgCl2 to induce the struvite precipitation. The main difference is the possibility to remove struvite continuously from the bottom of the reactor. In addition, it provides more efficient phosphorus recovery: whereas the effluent from the old precipitation tanks contained 50 mg/l PO4P, the new reactor can reach concentrations as low as 5 mg/l PO4P showing a decrease of 98% in the PO4P content. The Pearl Process The Pearl® process was developed in University of British Columbia(Canada), and it holds an U.S. Patent 7622047 B2. It has a fluidized bed reactor recovering nutrients from sludge liquor as struvite. The influent is sludge liquor from dewatering process typically containing 100 – 900 mg/l PO4P if the WWTP has biological phosphorus removal. Typically, the process removes 85 % of phosphorus and 10 – 15% of ammonium. The chemical used for precipitation and the pH adjustment are MgCl2 and NaOH, respectively. Struvite production rate is 500 kg/d; the final product contains 10% of magnesium. The Phosnix process The Phosnix process was developed in Japan by Unitika Ltd Environmental and Engineering Div. The process is a side stream process that can treat water from a number of processes including digester, industrial and biological nutrient removal systems. The inflow to the reactor is liquid phase from sludge dewatering, containing approximately 120 mg/l PO4P. The reactor is an airagitated column with chemical dosing equipment. The sludge liquor is pumped to the bottom of the reactor and the chemicals, sodium hydroxide and Mg(OH)2 are added for precipitation and pH adjustment to pH. Crystals grow, and sink to the bottom of the column where they are removed periodically.

The Seaborn Process The first fullscale implementation is located at Gifhorn WWTP in Germany. The process consists of three main steps: acid leaching, removal of heavy metals and the precipitation of struvite. In addition to these steps, the process contains stripping unit for ammonium recovery. Table 1 presents the estimates of price for recovered phosphorus including the values for crystallization processes Crystalactor® and AirPrex, and the wet chemical process Seaborne. Table 1: The price of recovered phosphorus (€/t)

Pilot and Laboratory Scale Processes However, there are processes that have been used on a pilot and laboratory scale. These are the P-Roc process (Crystallization/Precipitation) The P-RoC (Phosphorus Recovery from wastewater by Crystallization of calcium phosphate compounds) is a crystallization process using calcium silicate hydrate (CSH) as seed material to produce calcium phosphates from concentrated side stream. The maximum recovery potential is estimated 45% of total phosphorus. the PRISA process (Crystallization/precipitation) The PRISA process (Phosphorus Recovery by ISA) process aims at phosphorus and ammonium recovery precipitating struvite from supernatant liquor separated from the sludge in the thickeners before and after anaerobic digestion. the Rem Nut® ion exchange process (Crystallization/precipitation) The principal of the Rem Nut® (Removal of Nutrients) process is to remove nutrients from secondary treatment effluent with ion exchange columns using cationic resin for ammonium ions and basic resin for phosphate ions. The regeneration of the resins produces nutrient containing eluents that are mixed with magnesium salt precipitating the nutrients as struvite. the Sephos and advanced Sephos process (wet chemical) The SEPHOS process (Sequential Precipitation of Phosphorus) was developed in the Technical University of Darmstadt (Germany), at the Institute of Water Technology (WAR). The SEPHOS Process is based on the wet chemical approach of phosphorus recovery from sewage sludge ash. Further processing with the advanced SEPHOS process recovers calcium phosphate that can be used in agriculture. the PASH process (wet chemical) Developed in the Institute of Applied Polymer Science (IAP) at Aachen University (Germany), the PASH (Phosphorus recovery from Ash) process recovers phosphorus as calcium phosphate from incinerated sewage sludge ash. the Biocon process (wet chemical) The BioCon process recovers phosphorus as phosphoric acid from sewage sludge ashes. The process was developed in Denmark and consists of three parts: sludge drying, sludge incineration, and recovery unit with ion exchangers. the Aqua Reci process (wet chemical) A pilot plant is located in Karlskoga, Sweden. The process uses supercritical water oxidation (SCWO) with conditions of p > 221 bar and T > 374 °C. Under these conditions, pure oxygen is added for complete oxidation of sludge constituents. the ASH DEC process (thermochemical) The ASH DEC process treats monoincinerated sewage sludge ashes by a chloride dosage and thermal treatment in order to remove heavy metals making the product suitable for agricultural use. the Mephrec process (thermochemical) The Mephrec (Metallurgical Phosphorus Recovery) process was developed by the German company Ingitec. The process recovers phosphorus and energy from dried sludge.

Conclusion To sum up, phosphorus recovery has received increasing attention as the phosphate rock resources deplete and the need for finding a replacing source of phosphorus becomes even more important. At the same time, a great amount of sewage sludge is disposed off by using methods that do not ensure the sustainable recycling of the nutrients bound in the sludge. Furthermore, as the most economical way to dispose the sludge, the agricultural use, appears to be increasingly restricted, new technologies are needed to recover phosphorus from sludge and to process it into a suitable form for fertilizing purposes. It is necessary to recognize the consequences of the depletion of the phosphorus resources nationally on the governmental level as well as globally. The developments of the recovery methods conserve the remaining phosphorus resources, and ensure that reliable technology will be available, when the secondary raw materials become increasingly important source of phosphorus.