Extraction and precipitation of phosphorus from sewage sludge

Abstract: Raw sewage sludge from East Rand Water Care Association (ERWAT) had high phosphorus (P) content, approximately 15.2 % (w/w) P2O5. This indicates a potential resource of the limiting nutrient P. The effect of the acid type, acid concentration, time and solid/liquid ratio on extraction was investigated. Methods of solution purification were also explored namely adsorption and ion exchange. Magnesium oxide and ammonium hydroxide were used as sources of magnesium and nitrogen, respectively, as well as pH regulators to precipitate P as struvite. XRF results showed that 82 % P was recovered from raw sewage sludge after 2 hours leaching using 1 M sulphuric acid and purification using ion exchange. Kinetic studies showed that the leaching of phosphorus follows the Dickinson model. Most graphs show a linear fit up to 100 mins, thereafter the slope changes suggesting a two-step process with different rate parameters. Thus a trend line fitting all the points may not be ideal. A rate of reaction of about 2 × 10-5 s-1 was obtained for the first 100 mins. The rate limiting step is controlled by diffusion. Adsorption of phosphorus using ion exchange provided a better solution for the recovery of this natural resource, which eventually could meet the ever-increasing requirement for phosphorus. The overall results indicated that the raw sewage sludge can be treated with acid to efficiently recover phosphorus and thus can be considered a potentially renewable source of phosphorus. Phosphorus solubility in 2 % critic acid was almost 96 %. Environmental, gram-positive bacillus subtilis were found in the precipitate, which are harmless to the plants since they already exist in the soil conditions where the precipitate would be applied. The precipitate can therefore be used as a fertiliser.M.Tech. (Chemical Engineering