Phosphorus Release and Fertiliser Value of Enhanced-Treated and Nutrient-Removal Biosolids

This paper describes a soil-incubation study which was carried out to examine the effects of (a) phosphorus removal during the treatment of domestic sewage and (b) treating sewage sludge to an enhanced status, on the phosphorus fertiliser value of biosolids which are used in agriculture. Phosphorus-enriched and conventional dewatered digested biosolids, thermally-dried pellets and granules, and digested and thermally-hydrolysed liquid sludges were incorporated into two soil types with contrasting physico-chemical properties. Plant-available phosphorus in soil was assessed after incubation by a standard chemical-extraction procedure. Sludge from biological-P removal had the highest phosphorus extract-ability in soil whereas iron dosing slightly reduced or had no effect on phosphorus release from conventionally-treated biosolids, depending upon the type of soil. High-temperature drying significantly reduced the extractable-P content in sludge by 20–60%, compared with dewatered digested cake, and release from thermally-dried biosolids declined further with iron enrichment. Extractable-P recovery was greater from all types of biosolids when mixed with calcareous soil, compared with a loamy sand. A preliminary investigation of phosphorus mineralogy in biosolids, using advanced analytical techniques, is also described.