Acta Petrologica et Mineralogica

In the process of treating landfill leachate using the upflow anaerobic sludge blanket (UASB), microbial mineralization products can deposit and block the circulation and effluent pipes. Repeatedly replacing pipes to remove blockages is a labor-intensive process and exerts a negative impact on the stable operation of the system. The mineralogy and microscopic structural features of scaling in UASB system pipelines were investigated by mineralogical characterization methods in this study. The mechanism of scaling formation was explored. It provides a theoretical foundation for the prevention, inhibition, and cleaning of scaling in UASB reactor pipelines. The results indicated that the pipe scaling constituted a complex mixture of microbial mineralization products and organic matter, primarily consisting of acid-soluble materials. The main crystalline phases were monohydrocalcite and struvite. Struvite presented columnar crystals with diameters ranging from 0.2 nm to 0.5 mm, and arranged in bundles or radiating patterns. Monohydrocalcite appeared as spherical particles with diameters ranging from 50 nm to 350 nm. The anaerobic process producing methane production of landfill leachate consumed H⁺ and generated a large amount of CO₂, which increased the carbonate alkalinity of the anaerobic digestate. Meanwhile, organic nitrogen was ammonified, and organic phosphorus was converted into phosphate. In the digestion, phosphate, magnesium, and ammonium ions combine to crystallize into struvite, while calcium ions and carbonate ions combine to crystallize into calcite. Although the ion concentrations in the anaerobic digestate were supersaturated for hydroxyapatite and calcite, the presence of magnesium ions and high concentrations of organic matter in the anaerobic digestate may inhibit their nucleation and crystal growth. In the UASB system, struvite undergoes heterogeneous nucleation on the pipeline surface, growing into large columnar crystals. Monohydrocalcite filled the gaps between struvite crystals, incorporating a small amount of organic matter. Then, struvite and calcium carbonate were cemented together, forming pipe scaling with a certain level of strength resisting to fluid flushing. Considering the use of an in-situ acid dissolution method with 5% acetic acid to remove pipe scale, the residual acidity of the acid wash solution will keep as low as possible without altering the pH of the liquid inside the large-scale UASB reactor. Additionally, Low concentration acetic acid can serve as a substrate for methane-producing bacteria to enhance methane production.

pipe scaling; struvite; monohydrocalcite; landfill leachate; anaerobic fermentation