Speciation, mass loadings, and fate of phosphorus in the sewage sludge of China

Spatial and temporal dynamics of sewage sludge phosphorus recovery potential in the cities of Yangtze River Zone in China: Implications for regional recycling policies

Sewage sludge phosphorus (P) recovery presents opportunities to sustainably recycle P from cities to agriculture and alleviate global P scarcity. However, limited research explores sustainable recovery targets considering spatial-temporal variations in sludge generation and implications based on city-level local P demand. This study analyzed sludge production form 2009-2021 across 130 cities in China's Yangtze River Zone, which increased by almost 35 % from 2009 to 2021. Per capita gross domestic product (GDP), influent chemical oxygen demand (COD), and per capita drainage infrastructure were identified as the main significant influencing factors. City-level analysis revealed pronounced spatial-temporal disparities, with yearly sludge generation spanning five orders of magnitude (62-5.4 × 105 t/a). An indicator, "Potential of P recovery to local P demand", was defined, indicating the average city-level P recycle contribution increased from 5.3 % to 18.9 % from 2009-2021. A novel frame paradigm classified cities into six types based on the local P supply-demand characteristics, prioritizing sludge P recovery and implementing strategic management. City-specific dynamics and possibilities of broader "city clusters" to match supply and demand should be considered for policy implement. Recovering P from livestock manure and kitchen waste alongside sludge can further strengthen urban P cycles. This study provides novel city-scale analysis and strategic considerations for regional sludge P recycling policies in China and beyond.

Phosphorus transformation during the carbonaceous skeleton assisted thermal hydrolysis of sludge

On the basis of the carbonaceous skeleton assisted thermal hydrolysis that we proposed to achieve efficient sludge dewatering, this work further explored phosphorus (P) transformation in the process. The results showed that during independent thermal hydrolysis in the temperature range of 120-240 °C, organic-P was first decomposed into soluble-P and particulate-P in liquid, and then combined with Ca, Fe, and Al to form more apatite-P (AP) and less non-apatite inorganic-P (NAIP). When the skeleton assisted the sludge thermal hydrolysis, the turning point of the hydrolysis temperature would reduce from 180 °C to 150 °C, at which the liquid-P began to decrease and the organic-P generally decomposed. Moreover, the increment in the content of AP halved while that of NAIP doubled compared to that in the process without the carbonaceous skeleton. These effects come from the exogenous components introduced by adding the skeleton, which were different from the sludge. Compared with the P-rich compound and metal elements that tend to bond with phosphate introduced by the skeleton, hemicellulose as a main organic component played a leading role in the different P transformations of AP and NAIP. The hemicellulose slightly increased the acidity of sludge products, thereby inhibiting AP production and promoting the production of recyclable NAIP. Overall, the carbonaceous skeleton assisted thermal hydrolysis was beneficial for P recovery with a very low filtrate loss rate.

The Survival of Salmonella Senftenberg, Escherichia coli O157:H7, Listeria monocytogenes, Enterococcus faecalis and Clostridium sporogenes in Sandy and Clay Loam Textured Soils When Applied in Bovine Slurry or Unpasteurised Digestate and the Run-Off Rate for a Test Bacterium, Listeria innocua, When Applied to Grass in Slurry and Digestate

This study investigated the survival of Salmonella Senftenberg, Escherichia coli O157:H7, Listeria monocytogenes, Enterococcus faecalis and Clostridium sporogenes in sandy and clay loam textured soils when applied in bovine slurry or unpasteurised digestate, using laboratory based inoculation studies. The run-off rate for a test bacterium, Listeria innocua, when applied to grass in slurry and digestate, was also examined using field studies. Bovine slurry and digestate were inoculated with the target bacteria to a final concentration of 106 log10 cfu/g or spores/g, thoroughly mixed into soil samples and incubated at 4°C or 14°C. Samples were removed periodically and the surviving cells enumerated using AOAC or equivalent methods. The loss of viability/culturability phase followed first order kinetics and T90 values ranged from 11.9 to 166.7 d at 4°C and from 6.0 to 156 d at 14°C. With the exception of E. coli O157:H7 and E. faecalis in sandy loam textured soil at 14°C (T90 values were significantly (P < 0.05) higher in slurry) the type of soil texture or application material (slurry or digestate) did not affect survival rates. In the field study, 12 grass covered micro-plots were prepared. L. innocua was applied in digestate and bovine slurry and rainfall was simulated at a target rate of ~11 mm per plot per h−1. Rainfall simulation (30 min) took place after 24, 48 h, 14 d and 30 d. Run-off samples were tested for the L. innocua strain using Brilliance Listeria agar supplemented with streptomycin sulphate (1,000 μg/ml) at 37°C for 48 h, as were soil samples after 30, 58, 86 and 112 d. Significantly (P < 0.05) lower counts were obtained in the run-off from digestate after 1, 2 and 30 d as compared to slurry. It was concluded that the type of organic fertiliser does not affect the bacterial survival rates in sandy and clay soils, with the exception of E. coli O157:H7 and E. faecalis in sandy loam textured soil at 14°C. Furthermore, bacteria may be retained better in the soil-digestate matrices during rainfall although additional research is required to further validate and provide the scientific basis for this observation.

Species, fractions, and characterization of phosphorus in sewage sludge: A critical review from the perspective of recovery

Phosphorus recovery from municipal sewage sludge is a promising way to alleviate the shortage of phosphorus resources. However, the recovery efficiency and cost depend greatly on phosphorus species and fractions in different sewage sludges, i.e., waste activated sludge and chemically enhanced primary sludge. In this review, the phosphorous (sub-)species and fractions in waste activated sludge and chemically enhanced primary sludge are systematically overviewed and compared. The factors affecting phosphorus fractions, including wastewater treatment process, as well as sludge treatment methods and conditions are summarized and discussed; it is found that phosphorus removal method and sludge treatment process are the dominant factors. The characterization methods of phosphorus species and fractions in sewage sludge are reviewed; non-destructive extraction of poly-P and microscopic IP characterization need more attention. Anaerobic fermentation is the preferable solution to achieve advanced phosphorus release both from waste activated sludge and chemically enhanced primary sludge, because it can make phosphorus species and fractions more suitable for recovery. A post low strength acid extraction after anaerobic fermentation is recommended to facilitate phosphorous release and improve the total recovery rate.

Investigation of the polyphosphate-accumulating organism population in the full-scale simultaneous chemical phosphorus removal system

The simultaneous chemical phosphorus removal (SCPR) process has been widely applied in wastewater treatment plants (WWTPs) due to the high phosphorus removal efficiency through the synergy of biological and chemical phosphorus removal (BPR and CPR). However, phosphorus removal reagents could affect the bacterial community structure in the SCPR system and further affect the BPR process. The BPR phenotypes and community structures in the SCPR system, especially the population of polyphosphate-accumulating organisms (PAOs), are not completely clear. In order to clarify these problems, the phosphorus removal performance and the PAO population in a full-scale SCPR system were investigated. Results showed that diverse PAOs still existed in the SCPR system though the BPR phenotypes were not observed. However, the relative abundances of Accumulibacter and Tetrasphaera, the two most important genera of PAOs, were only 0.59% and 0.20%, respectively, while the relative abundances of Competibacter and Defluviicoccus, two genera of glycogen-accumulating organisms (GAOs), were as high as 5.77% and 1.28%, respectively. Batch tests showed that PAOs in the SCPR system still had a certain polyphosphate accumulating metabolic activity, which could gradually recover after stopping the addition of chemical reagents. This study provided a microbiological basis for the SCPR system to recover the enhanced biological phosphorus removal (EBPR) performance under suitable conditions, which could reduce the dosage of chemical reagents and the operational cost.

Occurrence and transformation of phosphonates in textile dyeing wastewater along full-scale combined treatment processes

Phosphonates discharged from wastewater treatment plants (WWTPs) have attracted increasing concerns because of their potential impact on eutrophication and potential risks to aquatic ecosystems. However, very few studies are available on their occurrence and transformation in WWTPs, partly due to the lack of sensitive methods for phosphonate analysis in complex matrices. Herein, based on our recent progress in phosphonate analysis, the occurrence and transformation of phosphonates along the full-scale wastewater treatment processes of two textile dyeing WWTPs were revealed. A set of typical phosphonates, including six phosphonate chelators (PCs) and four potential degradation products of PCs (DP-PCs) were quantified in different units and the final dewatered sludge. Three PCs (2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), 1-hydroxyethane 1,1-diphosphonic acid (HEDP) and nitrilotris(methylene phosphonic acid) (NTMP)) at upmost mg/L and a considerable amount of four DP-PCs (9.12-608 μg/L) were detected in the influents of both WWTPs. In the subsequent treatment, NTMP could be removed more efficiently than PBTC and HEDP, especially in the coagulation unit, and the dissolved phosphonates were eliminated more readily than other dissolved organic phosphorus fractions. Of particular note, the toxicologically critical DP-PC (i.e., aminomethylphosphonic acid) was produced during the coagulation and biological treatment units. The final precipitation unit seemed essential to ensure satisfactory removal of PCs and DP-PCs. In addition, a significant accumulation of phosphonates in dewatered sludge (up to 7.81 g/kg) and the widespread occurrence of harmful DP-PCs also reminded us to pay more concerns on their potential risks during further sludge disposal in future.

Anaerobic digestion, mixing, environmental fate, and transport

This section covers research published during the calendar year 2018 on mixing and transport processes. The review covers mixing of anaerobic digesters, mixing of heat transfer, and environmental fate and transport.

Transformation of phosphorus and stabilization of heavy metals during sewage sludge incineration: the effect of suitable additives and temperatures

Phosphorus (P), an irreplaceable nutrient for all living organisms, is facing scarcity via phosphate resources. In this research, the effect of suitable additives and temperature on P and heavy metals speciation during sewage sludge (SS) thermochemical treatment was investigated. The results demonstrated that additives (CaO and MgO) could promote the conversion of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). X-ray diffraction measurements indicated that the phosphorus mineral phase in sewage sludge ash (SSA) mainly was AP, with addition of MgO and CaO. Moreover, orthogonal testing revealed that the optimal molar ratio of Mg:Ca:P for P recovery as AP was 1:3.5:1 at 750 °C. Risk index results implied that the heavy metals in the phosphorus-enriched SSA have low potential ecological risk. Thermodynamic equilibrium calculations revealed that P reacted with the other metal ions was in the following order: Ca²⁺ > Mg²⁺ > Al³⁺ > Fe³⁺ > Zn²⁺ > K⁺. Graphical Abstract.