Simulations and Laboratory Tests for Assessing Phosphorus Recovery Efficiency from Sewage Sludge

Abstract

Phosphorus is a potential environmental pollutant, which could lead to the eutrophication of water bodies. For this reason, wastewater treatment plants worldwide are often designed and operated to eliminate phosphorous from effluents, at substantial cost. At the same time, phosphorus is an essential nutrient for agriculture and, consequently, human life. Data seem to suggest that the world will run out of phosphorus by around 2300, in the best case scenario, although even shorter estimates exist. This situation evokes the need for more efficient phosphorus recovery technologies, in order to meet current water quality requirements and—at the same time—critical future phosphorous needs. Chemical precipitation is the main process for achieving a phosphorus-containing mineral suitable for reuse as a fertilizer, where Struvite is an example of such a product. In this study chemical equilibrium of struvite precipitation was simulated using US Geological Survey (USGS)’ PHREEQC model, and results were compared to laboratory precipitation tests to evaluate struvite recovery efficiency under various conditions. pH had the most significant effect on the results and P recovery of >90% was achieved at pH = 9.5. Simulations indicated that struvite precipitation is affected by the presence of Amorphous Calcium Phosphate (ACP) and calcite in the final product of the process. The model showed great potential for predicting equilibrium conditions, and could be very helpful for future optimization of the process.

Impact of pH and Ionic Molar Ratios on Phosphorous Forms Precipitation and Recovery from Different Wastewater Sludges

“…A chemical equilibrium model was used to calculate the Saturation Index (SI) values of different solid phases, and therefore, to evaluate the possibility of their precipitation in the specific experimental operating conditions. For this aim, the PHREEQC model (USGS) [35] was applied based on sample characteristics and initial conditions as the input values, and the optimization of PHREEQC's database for accounting of precipitation processes specified in Table 1 were performed as specified in [14].…”

“…In uncontrolled conditions, it is considered as a potential problem causing clogging in pumps, pipes, etc. Nevertheless, under controlled conditions, it can be recovered from different stages of the treatment process [14], and due to its low solubility and high nutrients content (both nitrogen and phosphorus), it can be further reused as a suitable, slow-release fertilizer in agriculture [15,16].…”

Energy Issues in Sustainable Urban Wastewater Management: Use, Demand Reduction and Recovery in the Urban Water Cycle

“…Aerobic digestion has been rated as the greatest consumer of energy out of all sludge stabilization methods, with median energy intensity of 0.52 kWh/m 3 in small plants [38]. While usually adopted in small WWTPs for its simplicity, aerobic stabilization has sometimes been adopted also in large facilities [39].…”

Holistic Approach to Phosphorus Recovery from Urban Wastewater: Enhanced Biological Removal Combined with Precipitation

“…While struvite is perhaps the most "popular" and sought P-precipitated mineral, many other (over a dozen), mineral forms of precipitated P exist, with variable fertilizer recovery value [16]. Different factors affect struvite (as well as other mineral precipitates) crystallization, such as: solution pH, constituent ions (Ca 2+ , Mg 2+ , PO 4 3− and NH 4 + ) molar ratios, agitation rates, process retention time, etc.…”

“…Further tests were subsequently conducted on both liquid media at their original components' molar ratios, with no chemicals addition. Moreover, the expensive NaOH buffer solution was substituted by the much less costly reagent Ca(OH) 2 , that can be purchased at about 1/10 th of the price of the former, to control pH at the values of 8.5 and 9.0 [16].…”