Durch die Klärschlammverordnung (AbfKlärV) wird die Phosphor(P)-Rückgewinnung aus Klärschlämmen bzw. Klärschlammaschen für Klärwerke mit einer genehmigten Ausbaugröße ab 100.000 Einwohnerwerten (EW) ab dem Kalenderjahr 2029 gesetzlich vorgeschrieben. Dies betrifft alle sechs Klärwerke, welche von den Berliner Wasserbetrieben (BWB) betrieben werden. Die vorliegende Studie diskutiert verschiedene Möglichkeiten der Phosphorrückgewinnung mit Blick auf die Vorgaben der AbfKlärV, die Wirtschaftlichkeit der verschiedenen Ansätze und dient den BWB als Konzeptpapier für eine weitergehende Planung Ihrer Strategie zur P-Rückgewinnung.

The sewage sludge ordinance forces wastewater treatment plants to ensure the recovery of phosphate from the produced sewage sludge. In most cases, this obligation is transferred to the company in charge of the sludge disposal. For the recovery process, technology-specific but not product-specific specifications are made. The present article gives an overview of the products of different processes and their possible marketing in two routes: direct marketing and integration into the fertilizer industry. Possibilities, limits, requirements and potential product revenues for selected products are discussed against the background of current world market prices. Finally, the chemical expenditure of certain processes and logistical considerations are addressed. The considerations suggest that wet chemical ash processes should best be integrated in chemical parks.

In this master thesis the redissolution and recovery of phosphorus (P) and other valuable materials from Ruhleben sewage sludge ash (R-SSA) with HCl and H2SO4 was investigated using experimental laboratory tests. The parameters acid amount, solid liquid ratio (s/l ratio) and reaction time were varied and their influence on the redissolution of Ca, P, Al, Fe and SO4 was measured. Results showed that HCl(37%) resolved 91 ± 4 % P with an acid amount of 4 mL on 5 g R-SSA, a s/l ratio of 1:20 (acid concentration 0.46 mol/L) and a reaction time of 60 min. H2SO4(30%) on the other hand resolved 89 ± 3 % P at 6 mL on 5 g R-SSA, a s/l ratio of 1:10 (acid concentration 0.6 mol/L) and a reaction time of 60 min. Ca and SO4 showed very good redissolution of up to 100 % with HCl. The Ca redissolution with H2SO4 is clearly below that of HCl and is 63%. Al resolved to a lesser extent and reaches 65% with H2SO4 and 53% with HCl. Fe shows the lowest redissolution of 14% with HCl and 12% with H2SO4. The H2SO4 leaching experiments also showed that it is possible to separate the gypsum from the R-SSA if the leaching liquid is separated from the R-SSA after short contact times with H2SO4. The contact time, the acid amount and the s/l ratio have a big impact on the precipitation. It was possible to recover 75% of the total amount of gypsum that can be precipitated from R-SSA. In addition to this, the gypsum-free leaching liquid was then added to the R-SSA again which had already been leached, in order to dissolve P. There was no loss of P redissolution when the gypsum was extracted. This approach could not be observed in any other study. Based on the results, a recovery of 2,114 ± 130 t P/a with HCl and 2062 ± 130 t P/a with H2SO4 are possible if an amount of 30,000 t R-SSA/a is treated, which are forecasted for Berlin in the future. Since the redissolution of 91 ± 4% P requires an acid amount of 946 kg HCl(37%)/t R-SSA, this would result in an absolute annual acid requirement of 28,380 t HCl(37%) for 30,000 t R-SSA/a. H2SO4 experiments showed that 725 kg H2SO4(90%)/t R-SSA would be required for 89 ± 3% P redissolution, resulting in an absolute acid requirement of 21,750 t H2SO4(90%). Since H2SO4 has a lower consumption due to the higher concentration, is the cheaper acid of the two and has the possibility of recovering gypsum, H2SO4 would be preferred for leaching R-SSA from an economic point of view. In addition, about 75% of the gypsum can be precipitated with H2SO4, which corresponds to a quantity of 255 kg of gypsum(dry)/t R-SSA. At 30,000 t R-SSA/a, this results in an annual amount of 6630 ± 51 t gypsum that can be recovered.

With the new sewage sludge ordinance from 2017, phosphorus recovery becomes obligatory for large sewage treatment plant operators. Within the last year, the interpretation of this ordinance due to the exact wording has changed. As an example, a process aiming to recover phosphorus within the sewage treatment plant from waste water or sludge before the sludge is legally understood as waste. Therefore, a benchmark of 20 g Phosphorus (P)/kg dry matter (DM) is foreseen. However, this benchmark is an obstacle to increasing energy efficiency and sludge reduction, since carbon and dry matter is transferred into biogas in anaerobic digestion. Normally, raw sludge has a phosphorus content around 20 g P/kg DM, while digested sludge has a phosphorus content of about 35 g P/kg DM. The paper shows estimations of different full-scale combinations targeting phosphorus and advanced energy recovery and the resulting phosphorus content in sewage sludge per kg DM. Furthermore, this paper discusses the legal framework regarding phosphorus recovery from ash based on the sewage sludge ordinance, the national fertilizer regulation, the European Union fertilizing product regulation and the European Union feed/fodder regulation. The author concludes, that the legal framework is not explained properly to sewage treatment plant operators, which leads to confusions. Several questionable paragraphs and their wording should be addressed in future regulation amendments. Finally, there should be a regulatory need to establish phosphorus recovery from demand side (fertilizer industry, farmers) and not only from supplier side (sewage treatment plants). Because otherwise products must be produced, whereby no actual market for these products is established.