Nitrogen and phosphorus are beneficial nutrients to many ecosystems in small amounts. In excessive, however, they cause a type of pollution called eutrophication (process causing reduction of oxygen concentration in water bodies due to significant growth of algae). The wastewaters are characterized by a high level of ammonia and phosphorus contents [1] , therefore the effluents must be treated before discharging into water bodies [2] [3] . Conventional nitrogen removal from wastewater is carried out by biological nitrification and denitrification [4] , or other methods as ion exchange [5] , microwave irradiation [6] and struvite precipitation [7] . Phosphorus can be removed from wastewaters by incorporation of phosphate into Total suspended solids (TSS) and the subsequent removal from these solids. Alternative method for P and N recovery is struvite precipitation. The product could be used as a slow release fertilizer [8] [9] . This is the reason why struvite precipitation is to be widely investigated. Struvite (MgNH₄PO₄・6H₂O, magnesium ammonium phosphate hexahydrate―MAP) usually precipitates as a white orthorhombic crystals in a molar ratio Mg:MH₄:PO₄ = 1:1:1. MAP precipitation is a function of pH and molar ratios among ammonia, magnesium ions and phosphorus [10] . The solubility of the product can be defined by Ksp (solubility product constant), and struvite formation occurs when the concentration product exceeds struvite’s solubility product (supersaturation). The constant can be described by following equation [11] :

Controlled MAP precipitation has been reported in treatment of digested sludge as source of phosphorus and ammonia. pH is a crucial variable that needs to be controlled in order to maximize the product production. Many authors have reported that the range of pH for MAP precipitation is from 8 to 11 [12] . According to the diagram showing concentration of species depending on pH (Figure 1), the optimal value of pH is about 9.6 [13] .

Chemically pH can be controlled by using alkaline solutions as sodium hydroxide. High phosphorus and ammonia removal level were achieved at increased pH [14] [15] . NaOH is generally used for pH adjustment, but its addition rapidly increases pH value. Also the addition of NaOH will sharply increase the saturations of other magnesium precipitations, i.e. bobierrite and magnesite [16] . Alternative of addition of NaOH is the CO₂ stripping process. Willams (1999) has used CO₂ stripping for pH elevation [17] . Advantage is that pH increases slowly, which gives the optimum conditions for struvite crystal growth and crystallization process [18] . CO₂ stripping is non-reagent process for increasing of pH by dissolved carbon dioxide release using aeration of liquid [19] . It is a technique where dissolved CO₂ produced from an aerobic digestion process is removed from wastewater resulting in reduction of the total carbonate carbon concentration. The high concentration of CO₂ in wastewater is due to the relatively low specific exchange rates which do not allow the removal of substantial quantities of CO₂. Therefore, an effective CO₂ control requires aeration for stripping process. When atmospheric air is placed in contact with wastewater, there is a tendency for a dissolved gas to come to equilibrium at saturation: undersaturated gas such as oxygen is transferred from the air to the wastewater, and supersaturated gas such as CO₂ is transferred to the air [20] .

The gas―liquid equilibrium influences the transfer of CO₂ between air and wastewater. The carbonate species, , , and, are involved in instantaneous equilibrium among each other, as shown in Figure 1. In summary, dissolved CO₂ exists in the wastewater as part of the carbonate acid―base system. Therefore, the concentration of total dissolved CO₂ can be altered by change of the total amount of carbonate carbon in the solution. The intensity of this process is characterized by the volumetric liquid-side mass transfer

coefficient,. If we estimate the volumetric liquid-side mass transfer coefficient, then we can predict pH solution by the following equation [21] :

where, , , and are the CO₂ transfer reaction rate, total carbonates concentration in the liquid phase, equilibrium concentration of total carbonates in the liquid phase, H₂CO₃ fraction of the total carbonates in the liquid phase and equilibrium concentration of H₂CO₃ fraction of the total carbonates in the liquid phase, respectively.

In gas-liquid reactors, mass transfer from the gas phase to the liquid phase is a key parameter of the process. The value in gas-liquid contacting equipment has mostly been determined by the oxygen physical absorption or desorption technique (classical method). In the actual large-scale aeration system, the application of the existing method for determination can be limited by various factors such as absorption rate from air, complicated operating conditions, measuring equipment quality and cost, and also operator skills. Therefore, a simple theoretical way to predict the volumetric liquid-side mass transfer coefficient would be appreciated. These values can be predicted theoretically if we know how to estimate the liquid-side mass transfer coefficient and the specific interfacial area, separately [22] -[25] . In order to improve the accuracy of the theoretical model for calculating the volumetric mass transfer coefficient for pH adjustment using air stripping, experimental data were needed for comparison.

The digested sludge was taken from one of the digesters of wastewater treatment plant (WWTP) (Pomorie, Bulgaria). The sludge contains 9310 mg P0₄-P/kg solids and dry matter of 26 g/l as TSS. Principle scheme of WWTP-Pomorie and sampling point (digested sludge after digester) is shown in Figure 2. Al₂(SO₄)₃ is dosed before aeration tank.

The sludge was centrifuged in lab scale and the obtained supernatant was aerated in an up-flow reactor aiming pH elevation. Analytical measurement for chemical oxygen demand (COD, mgO₂/L), NH₄-N (mg/L) and PO₄ (mg/L) were determined by Spectrophotometer (HACH LANGE DR 3900) using cuvette test (Cuvette test-

range 5 - 90 mg/L PO₄; range 100 - 2000 mgO₂/L COD; range 2.5 - 60 mg/L NH₄-N) and the obtained data is listed in Table 1. The samples were tested for magnesium and calcium content by the Ethylenediaminetetraacetic acid (EDTA) complexometric method [26] . The preliminary obtained information related to the concentrations of ammonia and phosphorous (o-PO₄) shows that the liquid phase following the dewatering of sludge from conventional WWTP contains significant concentration of these constituents for MAP precipitation. The concentrations of the dissolved oxygen and solution pH were measured by Multi-Parameter Meter (HQ40d Portable).

Plastic cylindrical column with diameter of 0.05 m was used as reactor for the CO₂ stripping process (Figure 3). Plastic perforated plate was installed at the bottom of the reactor and served as a gas sparger. Supernatant of 1000 ml was placed in the reactor. Air compressor was run and air flow was passing through the plastic perforated plate and the supernatant was aerated. The first series of experiments were carried out at different air volumetric flow rates (0.067 m/s; 0.134 m/s; 0.201 m/s) using large specific area yield by plate with number of orifices 132 and a diameter of each orifice 2 mm. For the second series of experiments was used a plate with a single orifice and airflow rate of 0.067 m/s. Dissolved oxygen and pH level were measured in both sets of experiments. Dissolved oxygen and pH level were measured in both sets of experiments aiming determination influence of organic matter on pH elevation. For this purpose the supernatant was diluted 2, 3 and 4 times using distilled water and COD levels in the diluted samples were 650 mgO₂/L, 400 mgO₂/L, and 320 mgO₂/L, respectively. The measurement of oxygen concentration was needed for the experimental calculation of the volumetric mass transfer coefficient,. Sodium sulfite (Na₂SO₃) was added in order to deoxygenate the supernatant to an essentially zero concentration of dissolved oxygen. To increase the rate of the reaction copper sulfate (CuSO₄) was used as catalyst. After deoxygenation, the supernatant was aerated and the increasing concentration of the oxygen was recorded in order to calculate the mass transfer rate by the following equation:

where:, , a and V are the saturation concentration of oxygen, concentration of oxygen in the body of the liquid, the specific interfacial area and the volume of the liquid, respectively.

was obtained after integration of the above equation:

For the theoretical prediction of the volumetric liquid-side mass transfer coefficient, we need to calculate some hydrodynamic (, ,) and mass transfer parameters [27] -[29] . Once we have calculated the liquid-side mass transfer coefficient and the specific interfacial area a, some correction is needed due to the ellipsoidal shape of the bubble. Miller has introduced the following correction factor [30] :

The values should be multiplied by this correction factor for optimal prediction [31] [32] :

CO₂ stripping was applied as an alternative method for pH elevation of supernatant taken from digested sludge. It was found that the rate of increase of pH depended strongly on the sparging area of the air distribution system while the air flow rate did not influence considerably DO level which governed the CO₂ stripping process. Based on the data obtained, relationships of pH/k_La (mass transfer coefficient) were developed. These correlations served as a tool for prediction of pH during the struvite precipitation process.

Relationships among pH, w [m/s] and K_La [s⁻¹] were determined for the specific case of struvite precipitation using supernatant of digested sludge.

The support of the Project MIS ETC 2614, Scientific Network for Earthquakes, Landslide, and Flood hazard

prevention, funded under JOP “Black Sea Basin 2007-2013”.

DiyanRadev,GerganaPeeva,ValentinNenov, (2015) pH Control during the Struvite Precipitation Process of Wastewaters. Journal of Water Resource and Protection,07,1399-1408. doi: 10.4236/jwarp.2015.716113