Assessment of three different approaches for integrating phosphorus recovery from sewage sludge and derived products in existing wastewater treatment plants

Three different technological solutions, namely acidogenic fermentation and chemical extraction (alkaline or acidic), followed by precipitation with 1% Ca(OH)2, were investigated in the view of integrating phosphorus recovery into existing wastewater treatment plants. Experiments were conducted at the lab-scale using (i) sludge taken from biologically and chemically promoted phosphorus removal activated sludge processes and (ii) ashes obtained from sludge muffle incineration. Results highlighted the benefits of enhanced biological phosphorus removal (EBPR) systems rather than chemically promoted phosphorus removal in not only phosphorus extraction (up to 40% with EBPR) and recovery directly from secondary sludge (P precipitation between 66 and 92%), but after sludge incineration as well (P extraction up to 96% and precipitation above 96%). Acidogenic fermentation ensured the highest phosphorus release from EBPR sludge (equal to a concentration in solution of 122 mg/L P-PO43-), while the derived ashes had a lower level of metal contamination (particularly Fe and Al content < 2%). The phosphorus-rich product obtained by means of the recovery process showed relevant metal contamination (Cu, Zn, and Ni) under some operating conditions, suggesting the need for further treatments.

Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions

Phosphorus is a non-renewable resource going to be exhausted in the future. Sewage sludge ash is a promising secondary raw material due to its high phosphorus content. In this work, the distribution of 19 elements in bottom and cyclone ashes from pilot-scale grate furnace have been monitored to determine the suitability for the phosphorus acid extraction. Moreover, the influence of some parameters beyond wet chemical leaching conditions were investigated. Experimental results showed that bottom ash presented lower contamination in comparison to cyclone ash and low co-dissolution of heavy metals (especially Cr, Pb and Ni), while high phosphorus extraction efficiencies (76-86%) were achieved. High Al content in the bottom ash (9.4%) negatively affected the phosphorus extraction efficiency as well as loss on ignition, while the particle size reduction was necessary for ensuring a suitable contact surface. The typology of precipitating agents did not strongly affect the phosphorus precipitation, while pH was the key parameter. At pH 3.5-5, phosphorus precipitation efficiencies higher than 90% were achieved, with a mean phosphorus content in the recovered material equal to 16-17%, comparable to commercial fertilizers. Instead, the co-precipitation of Fe and Al had a detrimental effect on the recovered material, indicating the need for additional treatments.

Assessment of a simple and replicable procedure for selective phosphorus recovery from sewage sludge ashes by wet chemical extraction and precipitation

The selective phosphorus recovery by wet chemical extraction and precipitation was assessed at the laboratory scale aiming at identifying a simple and replicable procedure that could be effectively applied to different types of sewage sludge ashes. The experimental work was performed on five samples of sewage sludge ashes, of which three were obtained from muffle-furnace incineration and two from full-scale mono-incineration plants. A single-step extraction procedure has been investigated by applying different operating conditions (type of leaching acid, liquid-to-solid ratio, contact time). Experimental results indicated that phosphorus recovery efficiency varied between 54 and 92% with limited co-dissolution of metals and metalloids, except for arsenic. Operating conditions, sewage sludge ashes characteristics and phosphorus removal processes in the wastewater treatment plant were the main factors affecting phosphorus recovery efficiency. The application of optimal operating conditions (0.2 M sulfuric acid, liquid-to-solid ratio of 20 and contact time of 2 h) resulted in phosphorus recovery from 76 to 92% on four samples. Subsequently, precipitation of phosphorus from acidic leachate was carried out by lime dosing. After filtering and drying, the recovered products presented a P₂O₅ content between 11.5 and 36.7% dry weight, with a fraction of soluble phosphorus between 75 and 91%, a good percentage for application as fertilizer or animal feed. Since few undesired elements (i.e., As, Cu and Zn) exceeded the limits for fertilizer application (exception was represented by Ni and Pb, which were present at low concentration), an additional purification step may be required. Overall, experimental results highlighted the influence of process parameters on phosphorus recovery.

Simultaneous amorphous silica and phosphorus recovery from rice husk poultry litter ash

The livestock sector is one of the most important sectors of the agricultural economy due to an increase in the demand for animal protein. This increase generates serious waste disposal concerns and has negative environmental consequences. Furthermore, the food production chain needs phosphorus (P), which is listed as a critical raw material due to its high demand and limited availability in Europe. Manure contains large amounts of P and other elements that may be recycled, in the frame of circular economy and "zero waste" principles, and reused as a by-product for fertilizer production and other applications. This paper focuses on the extraction and recovery of amorphous silica from rice husk poultry litter ash. Two different extraction procedures are proposed and compared, and the obtained silica is characterized. This work shows that amorphous silica can be recovered as an almost pure material rendering the residual ash free of P. It also addresses the possibility of more specific phosphorous extraction procedures via acid leaching.