ABSTRACT
A full-scale struvite crystallization system was operated for the treatment of the centrate obtained from the sludge anaerobic digester in a municipal wastewater treatment plant. Additionally, the feasibility of an industrial grade Mg(OH)2 as a cheap magnesium and alkali source was also investigated. The struvite crystallization plant was operated for two different periods: period I, in which an influent with low phosphate concentration (34.0 mg P·L-1) was fed to the crystallization plant; and period II, in which an influent with higher phosphate concentration (68.0 mg P·L-1) was used. A high efficiency of phosphorus recovery by struvite crystallization was obtained, even when the effluent treated had a high level of alkalinity. Phosphorus recovery percentage was around 77%, with a phosphate concentration in the effluent between 10.0 and 30.0 mg P·L-1. The experiments gained struvite pellets of 0.5-5.0 mm size. Moreover, the consumption of Mg(OH)2 was estimated at 1.5 mol Mg added·mol P recovered-1. Thus, industrial grade Mg(OH)2 can be an economical alternative as magnesium and alkali sources for struvite crystallization at industrial scale.
Subject(s)
Magnesium Hydroxide/chemistry , Phosphates/analysis , Struvite/chemistry , Wastewater/chemistry , Water Pollutants/analysis , Water Purification/methods , Crystallization , Feasibility Studies , Phosphates/chemistry , Sewage/chemistry , Water Pollutants/chemistryABSTRACT
The crystallisation of struvite could be a sustainable and economical alternative for recovering phosphorus from wastewater streams with high phosphate concentrations. Knowledge regarding the kinetics and thermodynamics that are involved in the crystallisation of struvite is the key to determine the optimal conditions for obtaining an efficient process. This study was conducted in a continuous stirred batch reactor. Different sets of experiments were performed in which struvite was either dissolved (undersaturated) or precipitated (oversaturated). These experiments were conducted at different temperatures (25, 30 and 35 °C) and pH values (8.2, 8.5 and 8.8) to determine the kinetics of struvite precipitation and dissolution. Struvite crystallisation was modelled as a reversible reaction. The kinetic rate parameters of struvite precipitation were 1.03·10(-4), 1.25·10(-4) and 1.54·10(-4) mol m(-2) min(-1) at 25, 30 and 35 °C, respectively. Similar kinetic rate parameters were determined for struvite dissolution. Struvite heterogeneous crystallisation can be represented by a first-order kinetic model that fitted well the experimental data.
Subject(s)
Phosphates/isolation & purification , Struvite/chemistry , Waste Disposal, Fluid/methods , Water Pollutants, Chemical/isolation & purification , Water Purification/methods , Crystallization , Kinetics , Thermodynamics , Waste Disposal, Fluid/instrumentation , Water Purification/instrumentationABSTRACT
Struvite crystallization (MgNH(4)PO(4)·6H(2)O, MAP) could be an alternative for the sustainable and economical recovery of phosphorus from concentrated wastewater streams. Struvite precipitation is recommended for those wastewaters which have high orthophosphate concentration. However the presence of a cheap magnesium source is required in order to make the process feasible. For those wastewater treatment plants (WWTP) located near the seashore magnesium could be economically obtained using seawater. However seawater contains calcium ions that could interfere in the process, by promoting the precipitation of amorphous magnesium and calcium phosphates. Precipitates composition was affected by the NH(4)(+)/PO(4)(3-) molar ratio used. Struvite or magnesium and calcium phosphates were obtained when NH(4)(+)/PO(4)(3-) was fixed at 4.7 or 1.0, respectively. This study demonstrates that by manipulating the NH(4)(+)/PO(4)(3-) it is possible to obtain pure struvite crystals, instead of precipitates of amorphous magnesium and calcium phosphates. This was easily performed by using either raw or secondary treated wastewater with different ammonium concentrations.
Subject(s)
Calcium Phosphates/chemistry , Magnesium Compounds/chemistry , Magnesium/chemistry , Phosphates/chemistry , Sodium Chloride/chemistry , Waste Disposal, Fluid/methods , Chemical Precipitation , Microscopy, Electron, Scanning , Seawater , Struvite , Time Factors , Water Purification , X-Ray DiffractionABSTRACT
A two-phase anaerobic digestion process of synthetic domestic wastewater was studied at ambient temperature in mild to cold climates. The hydrolytic stage was carried out in a continuous stirred tank reactor with an effective volume of 1.2 L. The hydrolytic reactor operated at hydraulic retention times (HRTs) in the range of 1.3 to 2.7 h, which allowed for optimum HRT to be obtained in order to achieve a maximum amount of soluble COD. For the methanogenic stage, an up-flow anaerobic filter with a volume of 1.35 L and corrugated plastic rings as biomass immobilization support were used. During the investigation, the ambient temperature ranged between 21 degrees C and 24 degrees C. Synthetic domestic wastewater with a COD of 700 mg/L was used as substrate. The study was performed at total organic loading rates (OLR(T)) of 2.0-4.3 g COD/L. d, with a global HRT (including both hydrolytic and methanogenic stages) of 2.8-5.8 hours. A maximum percentage of organic matter removed of 88% was achieved at a global HRT of 5.8 hours. Under these operating conditions, the production of biogas was 97% higher than that obtained in the one-phase anaerobic digestion process. Additionally, the kinetics involved in the hydrolytic stage was determined using the Contois kinetic model, which adequately predicted the experimental results.
