Resource Recovery from Excess Sludge by Subcritical Water Process with Magnesium Ammonium Phosphate Process

Abstract The amount of excess sludge produced in municipal wastewater treatment plants in Japan is increasing every year as the urban population increases. Phosphorus in excess sludge could be a potential phosphorus resource since at present, phosphate rock is being exhausted all over the world. Every year, Japan imports large quantities of phosphorus from abroad but much are discharged as excess sludge. Therefore, solubilization process, one method of recovering phosphorus from sludge, could be a promising solution. In this study, subcritical water process, a new technology that solubilizes sludge under subcritical condition, was applied before the phosphorus in sludge was recovered with magnesium ammonium phosphate (MAP) process. As a result, the solubilization rate of excess sludge achieved approximately 80% and about 94-97% of the phosphorus could be recovered

Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 119 - Resource Recovery from Excess Sludge by Subcritical Water Process with Magnesium Ammonium Phosphate Process Mitsuhiro Arakane 1 ，Tsuyoshi Imai 1 ，Sadaaki Murakami 2 ，Masami Takeuchi 2 Masao Ukita 1 ，Masahiko Sekine 1 and Takaya Higuchi 1 1 Dept. of Civil and Environmental Engineering, Yamaguchi University 2 Dept. of Chemical and Biological Engineering, Ube National College of Technology Abstract The amount of excess sludge produced in municipal wastewater treatment plants in Japan is increasing every year as the urban population increases. Phosphorus in excess sludge could be a potential phosphorus resource since at present, phosphate rock is being exhausted all over the world. Every year, Japan imports large quantities of phosphorus from abroad but much are discharged as excess sludge. Therefore, solubilization process, one method of recovering phosphorus from sludge, could be a promising solution. In this study, subcritical water process, a new technology that solubilizes sludge under subcritical condition, was applied before the phosphorus in sludge was recovered with magnesium ammonium phosphate (MAP) process. As a result, the solubilization rate of excess sludge achieved approximately 80% and about 94-97% of the phosphorus could be recovered. Keywords Subcritical water; Solubilization; Excess sludge; Resources recovery; MAP process Introduction Activated sludge (AS) process is the most commonly used biological treatment process for municipal wastewater treatment plants all over the world. However, one of its major disadvantages is the high production of excess sludge. Every year about 2000 million tons of excess sludge are to be treated in Japan, accounting for approximately 48 % of the total amount of industrial solid waste, and this number is still increasing with the increase in urban population (Environmental white paper, 2005). Although recently, excess sludge is being increasingly used for soil amendment or construction materials, this part takes only about 45 % of the total, and the residual part has to be dewatered, incinerated and then landfilled. Because the area for landfill is becoming less and less available in Japan, new treatment methods and disposal technologies, as well as effective utilization, are strongly expected. In this study, a new method of applying subcritical water to hydrolyze and solubilize excess sludge, as shown in Figure 1, was examined. In addition, UASB process for methane recovery and MAP process for nitrogen and phosphorus recovery from the solubilized excess sludge were also investigated. Subcritical water Figure 2 shows the three-phase (solid, liquid and gas, including critical point) diagram of water and its saturated vapor pressure curve. Subcritical reaction occurs at temperatures and pressures below the critical point, i.e., 374.2 o C and 22.1 MPa. One of the most outstanding characteristics of subcritical reaction is that it has great hydrolysis function (Yamasaki, 1998) which generally takes place in acidic and alkaline catalytic reaction according to the target materials (Daimon, 2001), and therefore can solubilize the solid phase to liquid phase (Shimizu, 2000). Subcritical reaction was applied in this research primarily due to the following reasons: (1) hydrolysis and solubilization of excess sludge using subcritical water can be much easier and more efficient (Imai et al., 2003); and, (2) near the critical point, resource recovery might be impeded because of the gasification and pyrolysis during the transformation from sludge to low molecular weight materials, but this would not happen in the subcritical process (Okuda et al., 2001). Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 120 - Treated water Excess Sludge Wastewater Subcritical water process Solubilized sludge Influent Effluent Methane gas Anaerobic treatment process Aerobic treatment process MAP process MAP recovery Sludge solubilization by subcritical water Wastewater treatment by bacteria Methane recovery by methane fermentation Phosphate recovery on crystallization Figure 1 Flowchart of sludge-reducing wastewater treatment process combined with resource recovery 374 22 Tem perature（℃） Gas Solid Liquid Critical p o int S u p ercritical 　　area P ressure（ MPa） Subcritical area S a tu rated v ap o r pressure curve Figure 2 Three-phase diagram and saturated vapor pressure curve of water Mechanism of sludge solubilization Various organic components of sludge are decomposed (mainly hydrolyzed) and oxidized during subcritical reactions (Goto, 1997; Shimizu, 2000), resulting to low molecular weight degradation products, such as sugars, amino acids, fatty acids, orthophosphoric acid and ammonia nitrogen. Hence, subcritical reaction, with or without the use of oxidants, is capable of decomposing the organic and inorganic solid fractions of sludge and producing highly concentrated liquid of solubilized sludge. Outline of MAP process In this study, MAP (magnesium ammonium phosphate) process was used for the recovery of phosphate as orthophosphoric acid, and ammonia nitrogen. This process is strongly pH- dependent with an optimal pH value of 9 (K.Demeestere, 2001). Next to the pH value, the initial Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 121 - ammonium concentration and the molar ratio NH 4 + /Mg 2 + /PO 4 3- also greatly affect the precipitation efficiency. An advantage of MAP process is that the cost could be reduced drastically by combining NH 4 + and PO 4 3- in liquid phases (Kato, 2003). The chemical reaction in MAP process is as follows. NH 4 + + Mg 2 + + PO 4 3- = NH 4 Mg 2 PO 4 (1) Materials and Methods Preparation of excess sludge The excess sludge used in this study was collected from the laboratory-scale experimental apparatus (10 L) and was thickened to 26,000 mg MLSS/L. Experimental apparatus The experimental apparatus applied in this study is shown in Figure 3. The reactor was preheated for 30 minutes until the temperature reached a certain value. The sludge was then treated with subcritical water for 60 minutes while the temperature was kept constant. After the reaction, the reactor was cooled down to room temperature for 30 minutes, and the solubilized sludge was filtered. The filtrate was then sent to the following MAP process after adjusting its pH to 9 with Mg(OH) 2 . The influence of the phosphate recovery rate on the amount of precipitate formed in MAP process was investigated. Temperature control Ｔimer heating cooling Safty valve Injection pipe Release valve Reactor Pressure gauge Shaker Agitation ball Tank Figure 3 Schematic diagram of the experimental apparatus Moreover, solubilization rate was calculated using equation (2). Solubilization rate (%)=100* (a-b)/a (2) Here, a represents the MLSS content before treatment while b represents the MLSS content after treatment. Phosphorus recovery rate, on the other hand, was calculated using equation (3). Phosphorus recovery rate (%) =100* (d-c)/d (3) Here, c is the concentration of soluble phosphate after MAP process while d is the concentration of soluble phosphate before MAP process. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 122 - Analytical parameters In this study, suspended solid (SS), ammonia nitrogen and orthophosphate were analyzed according to the sewage standard test methods (Standard method, 1992). Results and Discussion Figure 4 shows the variation of solubilization rate at different treatment temperatures. The solubilization rate showed evident increases up to approximately 80% until the treatment temperature achieved 225 o C. After that, the rate was almost constant as the temperature was further raised to 350 o C. These phenomena suggested that the solid organic materials existing in sludge were solubilized into the liquid phase, and when the temperature increased further to 350 o C, part of them was gasified although they were still under subcritical conditions. In Figure 5, it is apparent that the recovery rate of phosphate increased gradually as the temperature increased to 180 o C, kept constant at about 95% from 180 to 240 o C, and dropped as the temperature kept on increasing. This implies that, varieties of orthophosphate were produced as intermediate products during the temperature increment. After treatment, the major component in MLSS was maybe changed to be a char in which orthophosphate was contained. Some other researchers also found that the refractory intermediates were produced in subcritical and supercritical water process (Lee et al., 1990). As a result, MAP process could recover about 95 % of the phosphate from the solubilized excess sludge. 0 20 40 60 80 100 0 50 100 150 200 250 300 350 Solubilizati on rate（％） Temperature（℃） Figure 4 Variation of solubilization rate with treatment temperature 0 20 40 60 80 100 0 50 100 150 200 250 300 350 Recovery rate of phosphate（％） Temperature（℃） Figure 5 Variation of phosphate recovery rate with treatment temperature The phosphorus mass balance was determined on the basis of the input and output of MAP Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 123 - process. Basic information needed for the mass balance calculation for MAP process included the following items, as schematically shown in Figure 6: (1) phosphorus contained in the residual solids (residual phosphorus), (2) phosphorus recovered from the MAP process (crystallized phosphate; MAP), and (3) phosphate unrecovered (soluble phosphate in the effluent). Phosphorus recovered from soluble phosphate Subcritical water process Solids-liquid separation by filter MAP process Phosphorus solubilized from sludge Residual phosphorus Crystallized phosphate Soluble phosphate Effluent phosphate (MAP) Phosphate unrecovered from soluble phosphate Phosphorus contained with residual solids Figure 6 Flowchart of phosphorus recovery process Variations of phosphorus composition in MAP process at different temperatures are shown in Figure 7. At temperatures lower than 100 o C, the solubilization reaction was very few so a greater part of the phosphate could not be separated from the sludge. From Figure 7 it can be clearly seen that the effluent phosphate decreased gradually as the treatment temperature increased to 200 o C, and kept constant at about 0% from 200 to 340 o C. In the case of crystallized phosphate, it dropped to 80 % at temperatures higher than 240 o C. This might be due to the orthophosphate decrease caused by accumulation into refractory organics. 0 20 40 60 80 100 100 150 200 250 300 Effluent phosphate Crystallized phosphate Temperature（℃） Rate（ ％） Residual phosphorus 340 （MAP） Figure 7 Variation of phosphorus composition with treatment temperature Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 124 - Conclusions Solubilization of sludge using subcritical water offers versatile and technically viable sludge concept. Subcritical water process can be used to achieve a considerable degree of decomposition of the complex components derived from the sludge. It also offers opportunities for combining efficient solubilization of sludge with the revovery of useful matter, especially phosphate, which can be recycled and used to support the wastewater treatment plants. This study concluded that about 80% of the excess sludge could be solubilized with subcritical water process when the temperature ranged from 200 o C to 250 o C. In addition, MAP process could be a promising process for phosphorus recovery (95 %) from the solubilized excess sludge. Reference Daimon H., Kang K., Sato N., Fujie K. (2001). Development of marine waste recycling technologies using sub- and supercritical water: Journal of Chemical Engineering of Japan, Vol.34, No.9, pp.1091-1096. Environmental white paper 2004 Goto M., Nada T., Kawajiri S., Kodama A., Hirose T. (1997). Decomposition of municipal sludge by supercritical water oxidation: J Chem Eng Jpn, Vol.30, No.5, pp.813-818. Imai T., Fukuda T., Ukita M., Sekine M., Higuchi T., Murakami S. (2003). 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Treatment of sewage sludge using hydrothermal oxidation. Technology application challenges: Water Sci Technol, Vol.41, No.8, pp.85-92. Standard methods for the examination of water and wastewater (1992). 18th edition, American public health association, Washington, D.C. Yamasaki N. (1998). Recycling possibilities of organic waste by hydrothermal process: Sekiyu Gakkaishi, Vol.41, No.3, pp.175-181. . combining NH 4 + and PO 4 3- in liquid phases (Kato, 20 03) . The chemical reaction in MAP process is as follows. NH 4 + + Mg 2 + + PO 4 3- = NH 4 Mg 2 PO 4 (1). oxidation: J Chem Eng Jpn, Vol .30 , No.5, pp.8 13- 818. Imai T., Fukuda T., Ukita M., Sekine M., Higuchi T., Murakami S. (20 03) . Resource recovery from sewage