Phosphorus speciation in sewage sludge from three municipal wastewater treatment plants in Sweden and their ashes after incineration

Phosphorus speciation in sewage sludge and their ashes after incineration as a function of treatment processes

Phosphorus (P) is a key component in agricultural fertilizers, but it is also a scarce resource, why its recycling has been thoroughly investigated and one promising resources is sewage sludge. Because of stricter regulations in terms of sludge disposal, thermal treatment (e.g. incineration) has become an attractive option. The incineration process alters the chemical speciation of P in favour to calcium-associated (apatite, apatite phosphorus (AP)) species, which is preferred for P recovery. In order to achieve qualitatively transformation, it is important to identify limiting or promoting factors. This study reports on the impact of iron, aluminium and calcium on the transformation of iron- and aluminium-phosphate (NAIP) to AP species, assessed by studying sludge and ash from 10 municipal wastewater treatment plants in Sweden. The effect of iron and aluminium added in the treatment processes was also evaluated. The obtained results show that high calcium concentration favours formation of AP species in both sludge and ashes, whereas high concentration of iron and aluminium favours formation of NAIP species in the sludge. The transformation from NAIP to AP species is hampered by aluminium, irrespectively of its origin, whereas no such correlations could be seen for iron. Therefore, in order to enable efficient P recovery from sewage sludge ash, the amount of aluminium added in the treatment process, as well as its concentration in influent streams to the treatment plants, must be limited.

Bioavailability transition path of phosphorus species during the sewage sludge incineration process

Sewage sludge incineration ash (SSIA) is rich in phosphorus (P), thus being considered as a reliable source of phosphorus recovery. Different P species behaved significant bioavailability. Based on this, a comprehensive investigation into the bioavailability transition path of P species during sewage sludge (SS) incineration was conducted. P predominantly existed in the form of inorganic phosphorus (IP) in SS with a higher concentration of non-apatite inorganic phosphorus (NAIP) and less concentration of apatite inorganic phosphorus (AP). During the SS incineration process, OP existed in the flocs and cell structures of SS underwent destruction, the released P then combined with metal elements such as Ca, Mg, Fe, and Al to form AP species (Ca/Mg-P) and NAIP species (Fe/Al/Mn-P), and the NAIP decomposition to release into gas phase. This was the initial step for enhancing the bioavailability of P species. As temperature increased and the incineration process progressed, the low-temperature-resistant NAIP dissociated, and the metal-binding sites of Al, Fe and Mn in NAIP species were gradually replaced by the Ca and Mg thus forming thermal stability AP species (Ca/Mg-P, such as CaHPO4, Ca2PO4Cl, and Mg3(PO4)2 et al.). This step was crucial for the bioavailability improvement of P species during the incineration process. Therefore, the IP proportions in TP were extremely high (＞98%), and this value gradually increased as incineration temperature raised. The higher incineration temperature, the lower NAIP concentration and higher AP concentration. Besides, additives such as coal/rice husk/eggshell played a significant affect. Additives wither higher Ca content were inclined to react with P to form Ca/Mg-P (AP), while the presence of SO2 would react with Ca metals to form CaSO4 thus inhibiting the formation of AP species (such as CaHPO4 and CaPO4Cl). This results could provide theoretical support for the efficient and directional migration of P during sewage sludge incineration.

A comprehensive investigation to the fate of phosphorus in full-scale wastewater treatment plants using aluminum salts for enhanced phosphorus removal

This study investigated the fate of phosphorus (P) in 8 full-scale municipal wastewater treatment plants (WWTPs) in Shanghai, China, in which both biological nutrient removal and aluminum-based chemical phosphorus removal were used. The results showed that 83.8-98.9 % P was transferred to the sludge in the 8 WWTPs by both chemical and biological reactions. P speciation analysis indicated that chemical P precipitates accounted for 84.3 % in the activated sludge, of which crystalline AlPO4 and amorphous iron‑phosphorus compounds (FePs) were the main components. Sludge with more water-soluble and weakly adsorbed P was generated in the anaerobic-anoxic-oxic (A/A/O) process than in other processes. Among the 8 WWTPs, the one with the largest flow rate and relatively short sludge retention time (SRT) had the best potential to release P from all types of sludge. The recovery potential of P from thickened sludge can be improved by separately thickening the sludge produced in the high-efficiency sedimentation tank or feeding it into the dewatering process directly. Different P removal chemicals and dosing points changed the amount of chemical precipitate formed but had little effect on the composition of P accumulating organisms (PAOs) at the genus level. Although aluminum-based coagulants were applied in the investigated WWTPs, Fe in wastewater had the most positive effect on the proliferation of PAOs. The synthesis of polyphosphate was also related to the metabolism of PAOs as it affected transmembrane inorganic phosphate (Pi) transport and polyhydroxybutyrate (PHB) synthesis. The in-depth understanding of the fate of P is beneficial to improve P recovery efficiency in WWTPs.

Phosphorus recovery from sewage sludge ash (SSA): An integrated technical, environmental and economic assessment of wet-chemical and thermochemical methods

Incineration is a promising disposal method for sewage sludge (SS), enriching more than 90% of phosphorus (P) in the influent into the powdered product, sewage sludge ash (SSA), which is convenient for further P recovery. Due to insufficient bioavailable P and enriched heavy metals (HMs) in SSA, it is limited to be used directly as fertilizer. Hence, this paper provides an overview of P transformation in SS incineration, characterization of SSA components, and wet-chemical and thermochemical processes for P recovery with a comprehensive technical, economic, and environmental assessment. P extraction and purification is an important technical step to achieve P recovery from SSA, where the key to all technologies is how to achieve efficient separation of P and HMs at a low economic and environmental cost. It can be clear seen from the review that the economics of P recovery from SSA are often weak due to many factors. For example, the cost of wet-chemical methods is approximately 5∼6 €/kg P, while the cost of recovering P by thermochemical methods is about 2∼3 €/kg P, which is slightly higher than the current P fertilizer (1 €/kg P). So, for now, legislation is significant for promoting P recovery from SSA. In this regard, the relevant experience in Europe is worth learning from countries that have not yet carried out P recovery from SSA, and to develop appropriate policies and legislation according to their own national conditions.