Agronomic effectiveness of recovered phosphate fertilizer produced from incinerated sewage sludge ash

Phosphorus (P) recovery from incinerated sewage sludge ash (ISSA) has been extensively investigated, but insufficient research has been conducted to evaluate the effect of different kinds of recovered phosphate fertilizers (RPFs) on plant growth with respect to the P and heavy metal contents of RPFs. In this study, three kinds of RPFs, precipitated calcium phosphate fertilizer (CaP), struvite phosphate fertilizer (SP), and P-loaded biochar (BP), produced from ISSA were characterized, and their agronomic effectiveness was verified by hydroponic and soil cultivation. In addition to the three kinds of RPFs, a control group (mono-phosphate fertilizer in hydroponic group/compound fertilizer in soil cultivation group) and a blank control group (BC, with zero P) were tested on choy sum and ryegrass at the same time. SP has the highest P purity (76.0% of struvite) while the BP has the most complex P species (P was co-exist with Fe, Al, and Mg). Plant growth results showed that the RPFs greatly facilitated plant growth and demonstrated superior/comparable effects to those of control group. In hydroponics testing, SP showed the best effect (shoot length of 17.0 cm, chlorophyll content of 2.05 mg/g) due to the Mg involved and the high P purity of SP, while BP performed the best (shoot length of 13.7 cm, chlorophyll content of 2.42 mg/g) in the soil testing system among all of the groups because of the additional nutritional elements and the high P availability of BP. Additionally, the accumulation of heavy metals in the plants under all conditions did not exceed the limits stipulated in the regulations. These results indicate that recovering P from ISSA is an attractive technology to produce P fertilizers, which can alleviate both the scarcity of phosphate resources and the burden of ISSA management. Graphical abstract: [Figure not available: see fulltext.].