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

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., 374.2oC 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)

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

T em perature（℃）

G as

S olid

Liquid

C ritical point

S upercritical 　　area

S ubcritical area

S aturated vapor 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

ammonium concentration and the molar ratio NH4+/Mg2+/PO43- also greatly affect the precipitation efficiency An advantage of MAP process is that the cost could be reduced drastically by combining NH4+ and PO43- in liquid phases (Kato, 2003) The chemical reaction in MAP process is as follows

NH4+ + Mg2+ + PO43- = NH4Mg2PO4 (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

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 oC After that, the rate was almost constant as the temperature was further raised to 350 oC 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

oC, 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 oC, kept constant at about 95% from 180 to 240 oC, 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

Temperature（℃）

Figure 4 Variation of solubilization rate with treatment temperature

0 20 40 60 80 100

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

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 oC, 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 oC, and kept constant at about 0% from 200 to 340 oC In the case of crystallized phosphate, it dropped to 80 % at temperatures higher than 240 oC This might be due

to the orthophosphate decrease caused by accumulation into refractory organics

0 20 40 60 80 100

Effluent phosphate Crystallized phosphate

Temperature（℃）

340

（MAP）

Figure 7 Variation of phosphorus composition with treatment temperature

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 oC to 250 oC 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) Resource recovery from sewage sludge

by subcritical water oxidation process: Environmental Engineering Research, Vol.40, pp.405-414

K.Demeestere, E.Smet, H.Van Langenhove and Z.Galbacs (2001) Optimalisation of Magnesium Ammonium

Phosphate Precipitation and its Applicability to the Removal of Ammonium: Environmental Technology,

Vol.22, pp.1419-1428

Kato F., Oshita K., Takaoka M., Takeda N (2003) Evaluation of Phosphorus recovery from wastewater treatment

system: Environ Sanit Eng Res, Vol.17, No.3, pp.70-75

Lee, D.-S., E F Gloyna and L.Li (1990) Efficiency of H 2 O 2 and O 2 in Supercritical Water Oxidation of 2,4-

Dichlorophenol and Acetic Acid; J Supercritical Fluids, No.3, pp.249-255

Okuda T., Kosaki Y., Murakami S., Kasahara S., Ishikawa M (2001) Study on the volume reduction system of

excess sludge by hydrothermal reaction: Journal of Environmental Systems and Engineering, No.692/

VII-21, pp.21-30

Shimizu Y., SHANABLEH A (2000) 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.