Study on the existing forms and recovery efficiencies of phosphorus in biochar from excess sludge.

With the background of carbon neutrality, the resource and energy utilization of excess sludge (ES) have become the focus of future research. The pyrolysis of ES can produce biochar and enrich phosphorus (P). In this paper, the existing forms and recovery efficiencies of P in biochar from ES (BCES) were investigated. The results showed that the total phosphorus (TP) content of BCES at 850 °C was 65.1 mg/g, and the inorganic phosphorus (IP) content was 64.2 mg/g. The TP content of BCES was two times heavier than that of ES. The main ingredient of ES was quartz (SiO2), while the main phases of BCES were quartz (SiO2) and aluminum phosphate (AlPO4) at 650 -850 °C, and P mainly existed in the form of AlPO4. When the pyrolysis temperature was 800 and 850 °C, two new minerals appeared: Ca5(PO4)3OH and CaZn2(PO4)2·2H2O. Based on the conditions of a leaching time of 150 min, a H2SO4 concentration of 0.2 mol/L, a stirring rate of 220 rpm and a liquid-solid ratio of 50 mL/g, the leaching efficiency of P in BCES was found to be 100%. The pyrolysis temperature had no effect on leaching efficiencies of P; however, a higher pyrolysis temperature promoted metal leaching content.

[Optimization and comparison of nitrogen and phosphorus removal by different aeration modes in oxidation ditch].

The oxidation ditch operation mode was simulated by sequencing batch reactor (SBR) system with alternate stirring and aeration. The nitrogen and phosphorus removal efficiencies were investigated in two different aeration modes: point aeration and step aeration. Experimental results show that oxygen is dissolved more efficiently in point aeration mode with a longer aerobic region in the same air supply capacity, but dissolved oxygen (DO) utilization efficiency for nitrogen and phosphorus removal is high in step aeration mode. Nitrification abilities of the two modes are equal with ammonia-nitrogen (NH4(+) -N) removal efficiency of 96.68% and 97.03%, respectively. Nitrifier activities are 4.65 and 4.66 mg x (g x h)(-1) respectively. When the ratio of anoxic zones and the aerobic zones were 1, the total nitrogen (TN) removal efficiency of point aeration mode in 2, 4 or 7 partitions was respectively 60.14%, 47.93% and 33.7%. The total phosphorus (TP) removal efficiency was respectively 28.96%, 23.75% and 24.31%. The less the partitions, the higher the nitrogen and phosphorus removal efficiencies, but it is in more favor of TN removal. As for step aeration mode with only one partitioning zone, the TN and TP removal efficiencies are respectively 64.21% and 49.09%, which is better than in point aeration mode, but more conducive to the improvement of TP removal efficiency. Under the condition of sufficient nitrification in step aeration mode, the nitrogen and phosphorus removal is better with the increase of anoxic zone. The removal efficiencies of TN and TP respectively rose to 73.94% and 54.18% when the ratio of anoxic zones and the aerobic zones was increased from 1 : 1 to 1. 8 : 1. As the proportion of anoxic zones was enlarged further, nitrification and operation stability were weakened so as to affect the nitrogen and phosphorus removal efficiencies.