In 1994 for the first time massive incrustations in the centrifuges treating the digested sludge were detected in some wastewater treatment plants (WWTP’s) of the Berliner Wasserbetriebe, which were, as was found out later, mainly due to the precipitation of white MAP (magnesium ammonium phosphate = struvite) deposits with small portions of different calcium phosphate compounds. The reason for this was the over saturation of the digested sludge and the process water (from the separated centrate of the digested sludge) with carbon dioxide, the spontaneous release of or degassing of carbon dioxide, causing the pH to increase, thus altering the solubility equilibrium, and facilitating crystallisation, resulting in deposits (incrustations) on the walls. Obviously, the wastewater purification process used, with enhanced biological phosphorus removal and concentrated streams of digesting sludge, favours the formation of incrustations during sludge treatment. These incrustations were the cause of a lot of operational problems.
To prevent incrustations, modifications in the process engineering technology have been applied after several studies and trials. Deliberate extraction of the CO2, e.g. by air injection through a fine-bubble surface aeration system at the bottom of the digested sludge storage tank just before the centrifuges, causes the pH to rise to approx. 8 and thus to cause induced struvite precipitation.
This technology enables a precipitation product suitable for recycling to be generated and separated from the anaerobic sludge using a hydro-cyclone. A subsequent second hydro-cyclone stage allowed a further increase in the P-concentration to approx. 40 kg/m3, giving a yield of phosphorus of 20 %. This corresponds with P-load of approx. 316 kg/day and amounts to 18 % of the P inlet load in the WWTP Waßmannsdorf. It was shown that this experimental process is in principle suitable for the separation of the MAP precipitation product in the anaerobic sludge. A complete washing process for the raw MAP is not yet installed, and furthermore, to find the best sizes of nozzles in the hydro-cyclones and most effective pressures tests will have to be carried out soon at pilot level.
A comparison with the minimum nutrients content for different kinds of fertiliser (super-phosphate, dicalcium-phosphate with magnesium, NP-fertiliser) defined by the German Fertiliser Regulations, shows that the precipitated MAP is within the range of other legally certified fertilisers. The MAP showed contaminant levels 5 times or more below the limit values of the German Sewage Sludge Ordinance. The MAP precipitation product is very soluble in different solvents (from ammonium chloride to hydrochloric acid). This implies that the precipitation product can be used as a long term fertilizer. In two test soils having a pH value of 6.6 and 7.1, the MAP precipitation solid was tested using standard ryegrass and showed good phosphorus availability from this material leading to a good grass growth.
Phosphorus Recovery from Excess Sludge with Subcritical Water Process and Magnesium Ammonium Phosphate Process
