Water Treatment Plant Pressurized Dehydrator Application Technology

Namil Cho Vice President Kyungdong ENG ARK CEO Sangheon Hong

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1. Introduction: Water Treatment Plant Pressurized Dehydrators

The water we drink is clean water, taken from a dam or river and treated at a water treatment plant. The water treatment process usually involves mixing and flocculating foreign substances in the water with chemicals to make it flocculate and settle, while the raw water is filtered to remove suspended solids and sterilized to make it safe to drink. The precipitated flocs are called sludge, which is collected by a sludge collector and dewatered in a dehydrator through a concentration process before being landfilled or recycled. The sludge generated from the water purification plant varies depending on the quality of the raw water, but it is about 0.21 TP3T of the raw water, and it is dehydrated to a moisture rate of 60 to 801 TP3T and disposed of in the form of sludge cake, and the Waste Management Act stipulates that it can be recycled if it is dehydrated to a moisture rate of 701 TP3T or less. Most of the sludge cake generated is landfilled or dumped in the ocean, and the cost of sludge cake disposal, including transportation costs, is approaching 20,000 to 30,000 won per ton. Therefore, efforts are being made to reduce the moisture content of sludge cake in order to reduce sludge cake, and dehydrators that are easy for operators to operate and manage are selected after reviewing the economic feasibility in consideration of initial facility costs and maintenance costs. Currently, continuous belt-type dehydrators are the most common sludge dehydrators installed in water purification plants, and recently, pressurized dehydrators, which are expensive in initial facility costs but have low moisture content and low treatment costs, are being applied, so we would like to describe their applicability.

2. Characteristics of water purification sludge

There have been few studies on the sludge behavior of water purification plants, but due to the recent introduction of water purification plant discharge water treatment facilities, the behavior of sludge for performance verification of dehydrators has begun to be investigated, but the behavior of sludge in the case of dams and reservoirs is even more difficult to understand. The water quality standard range for sludge composition defined in the Korea Water Resources Corporation's “Study on Disposal and Utilization of Water Treatment Plant Sludge” report is 251 TP3T for the incoming sludge concentration, 15～301 TP3T for the intense heat reduction, and 1.3～1.7 for the SiO₂/Al₂O₃ ratio, which is used as a performance guarantee condition for dehydrators. Here, the higher the solids concentration in the sludge, the better the dewatering ability, and the specific heat loss is an indicator of organic matter, and the smaller the specific heat loss, the less organic matter content, and the better the dewatering. Since SiO₂ and Al₂O₃ occupy about 501 TP3T in the solids of the sludge, and aluminum hydroxide hydrolyzed by ALUM or PAC as a flocculant shows hydrophilicity, the larger the SiO₂/Al₂O₃ ratio, the better the dewatering ability because the filtration resistance decreases. For reference, the sludge composition of domestic water purification plants is shown in Table 1, and the sludge composition of Japanese water purification plants is shown in Table 2. Table 1 Sludge composition of domestic water purification plants

Water filtration	Water sources	Intense Heat Reduction (%)	SiO₂ (%)	Al₂O₃ (%)	SiO₂/ Al₂O₃
Qingzhou	Primary	46.38	49.46	23.85	2.07
	Secondary		14.98	36.37	0.41
Semi-monthly	Primary	18.83	43.60	27.15	1.61
	Secondary		40.30	35.14	1.15
Stone Castle	Primary	16.40	55.10	19.01	2.90
	Secondary		44.74	24.63	1.82
Bounce	Primary	21.63	43.80	25.36	1.73
Last Name Male	Primary	21.60	32.58	29.71	1.10
Gumi	Primary		40.96	28.14	1.46
Resin		21.43			1.34
and boo		20.72			1.57
Average	Includes sake	23.85	40.61	27.71	1.56
	Excludes sake	20.10	43.01	27.02	1.63
Water Authority	Knowledge Base	15 to 30	35 to 50	20 to 30	1.3 to 1.7
S.Kawa-mura	References	15 to 25	35 to 70	15 to 40

  Table 2 Composition of water treatment plant sludge (Japan)

Samples	Water sources	Intense Heat Reduction (%)	SiO2 (%)	Al2O3 (%)	SiO2/ Al2O3	When to collect
C-1	Dam discharge	19.5	44	25.5	1.73	6/14
C-2		23.4	42.3	21.7	1.95	5/23
C-3		26.6	38.0	24.7	1.54	8/2
E-1	Dam storage water	37.5	20.2	35.4	0.57	9/10
E-2		33.1	24.5	33.4	0.73	6/5
E-3		38.5	22.2	30.6	0.73	8/2
F-1	Dam storage water	22.6	35.4	25.1	1.41	11/27
F-2		29.3	33.1	29.2	1.13	8/5
H	Drifting water	17.5	45.9	23.4	1.96	Early August
I	Groundwater	18.4	19.2	13.1	1.47	Early August
J	Number of lakes	38.3	25.5	25.0	1.02	8/12

Source: Japan, Water and Wastewater VOL.23 NO.9 (1981)  

3. types of dehydrators

Table 3 Typical comparison of dehydrators

分类	Continuous Belt Type Dehydrator (Belt Press)	Pressurized Dehydrator (Filter Press)	Centrifugal Dehydrator (Centrifugal Decanter)
Structure
How it works	∙Continuous expressions	∙Batch expressions	∙Continuous expressions
Dehydration Methods	∙Belt (filter cloth) crimping	∙Water or air compression	∙Centrifugation
Scaling capacity	∙Increased filtration width and travel speed	∙Increase chamber size or quantity	∙Increase Bowl Diameter
Power required	∙Low power consumption	∙Medium	∙Highly dynamic.
Function Rate	70～80%	50～65%	70～80%
Dehydration rate	100～200 kg.DS/m.hr	1～3.6 kg.DS/㎡.hr	1～90㎥/hr
Injecting medications	Polymer flocculant 0.1～1.0%./kg.DS	Not available without medication infusion	Polymer flocculant 0.2～1.2%./kg.DS
Pros	∙Can be operated continuously. ∙Long supply history and domestic production ∙Low initial facility costs ∙Low power requirement	∙No need for dosing facilities ∙Low cake moisture content ∙Good quality of demineralized water. ∙Excellent response to sludge concentration changes.	∙Easy to operate continuously and remotely. ∙Small footprint ∙Hermetically sealed for low odor generation ∙Low cleaning water requirements
Cons	∙High demand for follicular wash water. ∙Open type, causing odor. ∙Many auxiliary facilities ∙Sensitive to sludge formation. ∙Noisy follicle wash water.	∙High initial facility costs. ∙Open type generates odors. ∙Heavy weight ∙Low performance of water purification plant.	∙Shortened lifespan in case of soil ingress. ∙Requires skilled labor for maintenance ∙Highest power consumption.

Dewatering is essential to reduce the volume of the sludge to facilitate handling, minimize disposal costs, and control the occurrence of secondary salts. The dehydrators used for dehydration of sludge in domestic water treatment plants are continuous belt type dehydrators, pressurized dehydrators, and centrifugal dehydrators. A schematic comparison of the structure, advantages, and disadvantages of currently used dehydrators is shown in Table 3.

4. Features of the pressurized dehydrator

4.1 Facilities overview

The pressurized dehydrator was selected to minimize treatment costs by discharging sludge cake with a relatively low water content compared to other types, and has been used in Korea mainly for sludge dewatering in small-scale sewage and wastewater treatment plants using flocculation chemicals (ferric chloride and slaked lime). In water purification plants, it can be dewatered without using flocculant (polymer) for dewatering, and is being introduced because it is economically advantageous to reuse the dewatered filtrate. A pressurized dehydrator is a facility that supplies sludge to a filter plate covered with a filter cloth and pressurizes and dehydrates it, and is equipped with a dehydrator body, a sludge supply pump, an air compressor facility for pressurization or a pressurized water supply device (squeezing device), a cleaning device for cleaning the filter cloth, a hopper for storing dehydrated cake, a cake transfer conveyor, etc. Early pressurized dehydrators dehydrated sludge by squeezing, but technology has gradually developed to introduce a squeeze dehydration method, which further reduces the moisture content. In the case of membrane (or diaphragm) pressurized dehydrator (Membrane Filter Press), the main body of the dehydrator is composed of hydraulic pump and inlet cylinder, filter plate, filter cloth, pressurized membrane or diaphragm, dripping pan, filter cloth washing device, automatic operation valve, etc. In addition, after dewatering the sludge, a driving device can be installed to move the filter cloth to smoothly remove the sludge cake attached to the filter plate and filter cloth, so that the movement of the filter plate and filter cloth can be automatically performed, and recently, an electro-permeation type that uses the phenomenon of electrophoresis to dewater using an electrode plate installed on the inner surface of the filter plate has been used in foreign countries. The dehydration process using a pressurized dehydrator is shown in Fig. 1.

4.2 How dehydration works

The conceptual diagram of the dehydration process of a pressurized dehydrator is shown in Figure 2 as an example of a mobile filter cloth.

(1) Filter plate closure and pressurized dewatering process

It is a process that is performed at the beginning of the pressurized dehydrator operation or after the end of the cycle, in which each filter plate is pressed to one side by a hydraulic pump and a hydraulic cylinder, and a filtration chamber is formed between each filter plate where sludge can be injected. Depending on the thickness of the filter chamber, the dewatering speed and the thickness of the discharged sludge cake vary, and depending on the nature of the sludge, dewatering characteristics, and manufacturer, 20mm to 40mm is used, but 30mm is the most common. The sludge pressed into the filtration chamber by the sludge pump is separated into solids and filtrate by the pressing pressure of the pump, and the solids are retained in the filtration chamber by the filter cloth installed inside the filter plate, and the filtrate is discharged through the filter cloth. If the pressing time is shortened, the thickness of the cake generated in the filtration chamber becomes thin, making it difficult to separate the cake from the filter cloth during the discharge process, and the moisture content may be high, which requires high pressing pressure in the pressed dehydration process. On the contrary, a long pressing time can reduce the moisture content of the cake, but the cycle time is longer and the dehydration rate decreases.  

(2) Squeeze Dehydration(Squeezing)

Compressed air (5～7K) or compressed water is supplied to the membrane or diaphragm installed between each filter plate and filter cloth, and the filter cloth is pressurized to squeeze and dehydrate the sludge in the filtration chamber to reduce the moisture content of the discharged cake. In general, the higher the pressurization pressure, the lower the moisture content of the cake, but the production cost increases as a structure considering the pressurization pressure is required. The membrane installed on the filter plate is usually attached to both sides of the odd numbered plates, but it can also be attached to both sides of the entire filter plate.  

(3) Filter plate opening process(Release-Plate shifting)

When the hydraulic cylinder reverses to create space, the plate shifting mechanism opens each plate and the dripping pan below the plate opens to allow the cake in the filter chamber to fall to the bottom. Depending on how many plates are opened at the same time, the time required varies greatly, which directly affects the overall cycle time, i.e., the speed of dehydration. Depending on the manufacturer, it takes more than 10 minutes if one or two or multiple plates are opened at the same time (fixed filter cloth type), All filter plates are opened at the same time (movable filter cloth), which takes less than a minute, but is more expensive. When each filter plate is opened and the cake in the filter chamber falls to the bottom, the cake must be easily peeled off the filter cloth, and the filter cloth must be cleaned after peeling to ensure maximum filtration efficiency in the next process. According to the operation method of the filter cloth for cake peeling and filter cloth cleaning, the method in which the filter cloth is fixed in place on the filter plate for peeling and cleaning is called fixed filter cloth, and the method in which the filter cloth is moved to the lower part to improve the efficiency of cake peeling and filter cloth cleaning is called movable filter cloth.  

(4) Cleaning filter cloth(Cloth Washing)

The cleaning of the filter cloth is a process to improve the filtration efficiency in the next cycle after the end of the cycle, but it is not performed every cycle and depends on the sludge characteristics and operating characteristics. In the case of fixed filter cloth, 1-2 chambers are cleaned simultaneously by the spray nozzle, and in the case of mobile filter cloth, 5 chambers (adjustable) or more are cleaned simultaneously by the fixed nozzle. In general, the time required for the cake discharge process or filter cloth cleaning process affects the overall cycle time, and the cycle time is the same as the dehydration speed, which is an important factor in determining the performance and specifications of the pressurized dehydrator.  

4.3 Pressurized Dehydrator Classification and Fabrication Vendors

(1) Classification of pressurized dehydrators

The pressurized dehydrator is broadly classified into Chamber Filter Press and Membrane Filter Press according to the type of filter plate, and according to the structure of the filter plate, it is classified into general type, electro-permeable type with electrode plate, and dry type with steam pipe. However, the electro-permeable type and dry type produce cakes with low moisture content and require less cake disposal costs, but as a foreign technology, they have high equipment costs and no domestic experience. In this paper, we mainly reviewed the membrane-based pressurized dehydration method, which is widely used in recent years.

Fig 3 Installation view of pressurized dehydrator at a water purification plant in Japan

 

(2) Membrane pressurized dehydrator comparison

Membrane pressurized dehydrators are available in two types: air squeezing and water squeezing. In addition, depending on the state of the filter cloth when separating the cake or cleaning the filter cloth, it is divided into a fixed filter cloth and a mobile (or traveling) filter cloth, and in the case of a fixed filter cloth, depending on the method of separating the cake, it is divided into a general type without a separate device, a vibrating type that vibrates the filter cloth, and a scraping method that scrapes the cake by a scraper. In addition, according to the method of opening the filter plates (plate shifting), there are two types: one or several filter plates are opened simultaneously and all filter plates are opened simultaneously. These types can be selected according to the processing performance of the dehydrator and the convenience and automation of operation and management, and are summarized in Fig. 4.

Fig 4 Classification of pressurized dehydrators

     

(3) Status of domestic pressurized dehydrator manufacturing suppliers

分类	Fixed filter cloth	Filter cloth mobile (or traveling)	Electropenetrating
Geometry
Features	High dependence on squeeze dewatering due to long sludge pressing time. Pressurized dehydration by diaphragm using the pressurized force of air or water during pressurized filtration	Improved operating efficiency by shortening the sludge pressing time and increasing the number of cycles. Pressurized dehydration by membrane using the pressurized force of air or water	Add electrode plates to the filter cloth mobile ․ Reduce the cake content using electrophoresis. Squeeze dehydration by membrane using air pressure
Dehydration rate	0.1 ～ 1.0kg.DS/㎡.hr	1.0 ～ 3.6 kg.DS/㎡.hr	1.0 ～ 10kg.DS/㎡.hr
End-of-Life Processes	Pressing each filter plate with a hydraulic cylinder	Left	Left
Indentation Process	Supply pressure by sludge supply pump is 4.5～5K	Left	Left
Crimping process	․Pressurized dehydration with compressed air 6～7K or compressed water 15K	Left	Pressurized dehydration with compressed air 6～7K followed by electro-penetration dehydration
Modification Process	․Move 12 filter plates, ․Cake separation without separate device or with vibration device and scraper ․Filter cloth cleaning: Simultaneous cleaning of 1～2 chambers by moving nozzle	․Move all filter plates simultaneously ․Cake separation is simultaneously separated by moving the filter cloth to the bottom ․Filter cloth cleaning: Simultaneous cleaning of more than 5 chambers by fixed nozzle	Left
Function Rate	55 ～ 65%	50 ～ 60%	40 ～ 60%
Cycle time	2hr ～ 4hr	30 ～ 60min	9 ～ 30min
Pros	․Low cost. Less space required Long cycle time and fewer driving cycles, so membranes, etc. have a long lifespan.	High dehydration rate due to short cycle time. Fewer chambers for the same throughput. Excellent cake discharge performance because the follicles are moved.	Shortest cycle time for fast dehydration. Smallest equipment installation area. Excellent cake discharge performance because it is a follicle moving type.
Cons	․Low dehydration rate and high number of chambers. Long closing time for cake discharge and filter cloth cleaning.	There is a risk of short membrane life due to the high number of operations due to the short cycle time. ․The installation area is large because the entire filter plate is opened and closed at the same time.	It is the most expensive and has no domestic installation history. The larger the power required, the lower the cake moisture rate, but it is suitable for large capacity due to large power consumption.
Delivery Performance	Namgang Dam, Suncheon Water Treatment Plant 20 domestic wastewater treatment plants (based on chamber □1,500)	10 locations including wastewater treatment plants (based on chamber □1,500)	Approximately 16 locations in Japan
Vendors	․Sumjin EST (Noritake) Taeyoung Industrial Company ․Union (Ishikaki) Netzsch Korea (Netzsch)	․Union (Ishikaki) ․Dii (Kurita)	․Water environment (Shinko Pantec)

 

Producer (vendor)	Technology Partnerships	Remarks
Nechu Korea, Carbotech	Netzsch (Germany)	Filtration Parameterization
Scattered EST	Noritake (Japan)	Filtration Parameterization
Yucheon Engineering	Self-Production	Filtration Parameterization
Tae Young Industries	Self-Production	Filtration Parameterization
Water Environment Co.	Shinko Pantec (Japan)	Microfiltration (Electrofiltration)
(Union Co.	Ishigaki (Japan)	Removable filter cloth

 

4.4 Domestic water purification plant installation performance(2003,3Month-to-date)

(1) Operational or installed sites

Integer Longevity	Transmission	Sichuan	Hot acid	Singing
Facility capacity (m3/d)	210,000	121,000	221,000	70,000
Dehydrator format	Removable	Filtration Parameterization		Removable
Dehydrator specifications	□1.5 meters	□1.5 meters	□1.5 meters	□1.0 m
Chamber Quantity	50	62(92)	100	32
Dehydrator quantity	2 set	2 sets	3 sets	2 sets
Medications	Unmedicated	Polymer	Unmedicated	Unmedicated
Sludge volume (kg.DS/d)	10,163	4,111	11,990
Sludge Concentration	4%		3%	3%
Dehydration rate (kg.DS/hr.㎡)	1.5	1.6	1.0	1.5
Cycle time	40min	3 hr	2 hr	45 min
Target Function Rate	60% or less	60±5%	60±5%	60%
Facility Year	‘2003	‘2001	‘2001	‘2002
Progress	Driving	Driving	Driving	Driving

(2) Sites under design or construction

Integer Longevity	Uiwangchenggye	South Jeolla Province	Bamboo grains	Commodity
Facility Capacity (㎥/d)	40,000	210,000	250,000	40,000
Dehydrator format	Removable	Stationary	Removable	Removable
Dehydrator specifications	□1.5 meters	□1.25 m	□1.5 meters	□1.0 m
Chamber Quantity	18	90	50	26
Dehydrator quantity	2 sets	2 sets	2 sets	2 sets
Medications	Unmedicated	Unmedicated	Unmedicated	Unmedicated
Sludge volume (kg.DS/d)	2,061	10,163	14,838	1,992
Sludge Concentration	3%	4%	3%	3%
Dehydration rate (kg.DS/hr.㎡)	1.3	1.6	1.5	1.5
Cycle time	50 min	3.5 hr	45 min	45 min
Target Function Rate	60% or less	60% or less	60% or less	60% or less
Design Year	‘2003	‘2000	‘2002	‘2002
Progress	Designed	Under Construction	Designed	Designed

 

5. Considerations for applying a pressurized dehydrator

5.1 Determine capacity

In general, products that adopt Japanese technology determine the capacity of the dehydrator by the dehydration rate (kg.DS/hr.㎡) in consideration of the amount of sludge and the operating time, but some European and domestic products calculate the capacity of the dehydrator by converting the volume of the chamber to the amount of sludge and convert it to the dehydration rate. To determine the capacity, the filtration rate, like other types of dehydrators, must be sufficient to handle the sludge generated, ensure the planned moisture content, and allow unattended automated operation according to the latest design concepts. Fig 5 Characteristics of a pressurized dehydrator

Fig 5 Characteristics of a pressurized dehydrator

 

5.2 Dehydration rate(Filtration Rate or Throughput)

(1) General

The treatment capacity of a dehydrator, like other dehydrators, is determined by the dehydration rate, which is usually expressed as the amount of dry sludge treated per unit area of filter cloth in a unit time (dry solid -kg.DS/hr.㎡). The throughput per cycle is sometimes used, but it should be converted to hourly because the cycle time varies depending on the manufacturer and dehydrator model (from 30min to 24hr depending on the model), In addition, the number of cycles per day should be considered in light of the planned daily operating time and reviewed when selecting a dehydrator. Factors that affect the dehydration capacity of the pressurized dehydrator include the following items.

• 1) Sludge concentration
• 2) Sludge composition (SiO₂, thermal reduction, Al₂O₃)
• 3) Sludge Characterization (Particle Size Distribution)
• 4) Water temperature
• 5) Raw water turbidity

Fig 6 Relationship between sludge consistency C and filtration rate F

 

(2) Dewatering rate according to sludge concentration and composition The following relationships are summarized for dewatering rate according to sludge concentration and composition, referring to the technical data of Ishigaki's filter cloth mobile pressurized dehydrator.

1) Influence of sludge concentration The relationship between sludge concentration and filtration rate is shown in Figure 6. In this graph, the higher the concentration, the better the filtration rate. Therefore, we can see that the dewatering pretreatment (concentration in the concentrator) is very important for the dewatering capacity.   2) Impact of sludge composition Heat Reduction In recent years, the quality of raw water has been deteriorating despite various efforts to preserve water quality. In particular, the content of organic substances (thermal reduction) is increasing because the amount of domestic wastewater continues to increase. The high level of thermal decay increases the compressibility of the sludge and reduces its filterability. Figure 7 shows the filtration rate as a function of thermal reduction. In addition, eutrophication is occurring in the water source, causing green algae to grow, which causes odor. In order to remove odors during the water purification process, powdered activated carbon is increasingly added, and the discharge sludge often contains activated carbon. Due to this, the dehydration performance of electrophoresis-based dehydrators is severely reduced due to the small amount of ionizing substances such as Mn++ and Fe++ in powdered activated carbon, but in mechanical dehydrators, the performance is almost unchanged, and in some cases, the performance is slightly improved when powdered activated carbon is added.  

Fig 7 Relationship between heat loss and filtration rate F

    Aluminum salts are hydrolyzed to aluminum hydroxide by the injection of flocculants such as silica/aluminum non-PAC, which is contained in the sludge. Aluminum hydroxide is hydrophilic and forms a hydrated floc with high compressibility, so as this component increases, compressibility and dewatering become worse. Higher raw water turbidity increases silica, which reduces the aluminum component and thus improves dewatering. This silica/aluminum ratio is often the reason why summer sludge has good dewaterability and degrades in winter. The relationship between silica/aluminum ratio and filtration speed is shown in Fig. 8.  

Fig 8 Silica-aluminum ratio versus filtration rate F

  In recent years, due to the deterioration of raw water quality at water sources across the country, the sludge thickening effect has been reduced, and dewatering performance has been declining every year. Especially in winter, there is a risk of further deterioration due to the decrease in raw water turbidity and sludge temperature and the decrease in silica/aluminum ratio. Several methods have been recently tested to improve dewatering capacity. Electro-penetration and heated dehydration are representative of these methods, which use electricity or heat energy to improve filtration speed and reduce cake moisture content while using a chemical-free dehydration method as a base. This method requires careful consideration because it increases energy consumption and increases equipment costs.  

(3) Dehydration rate by type

The dewatering rate is applied based on the results of pilot test with similar water quality or applying products with operating data because there are few operating data in domestic water purification plants. The dewatering rate and water content range according to raw water turbidity and sludge concentration from the experience data of Ishigaki in Japan are shown in Table 4, and the dewatering rate and water content range from the experience data of Shinko Pantec are shown in Table 5. Table 4 Dewatering rate of filter cloth movable type

NTU	Sludge Concentration (%)	Dehydration rate (kg.DS/㎡.hr)	Cake function rate (%)
20	3	1.0	65±5
40	4	1.5	60±5
80	5	2.0	55±5
120	6	2.7	55±5

  Table 5 Dehydration rates for electrofiltration

NTU	Sludge Concentration (%)	Dehydration rate (kg.DS/㎡.hr)	Cake function rate (%)
20	3	1.5	60±5
40	4	2.2	55±5
80	5	2.8	50±5
120	6	3.4	50±5

The dehydration rate to maintain a cake moisture content of 60～65 % is categorized according to the cycle time in the following Table 6. Table 6 Dehydration rate according to cycle time

Separation	Long form	Short Time	Electropenetrating
Filtration	Stationary	Removable	Removable
Cycle time	2～24 hr	1-2hr	9～30 min
Dehydration rate (kg.DS/hr.㎡)	0.1 to 0.5	0.5 to 3	1 to 10
cake function rate	60～65%	60～65%	60～65%

    The dewatering rate is inversely proportional to the cycle time, so the sludge impregnation time, which is a large part of the cycle time, determines the dewatering rate. In the case of the fixed filter cloth type, it takes a lot of time to open and close the filter plates, so the dewatering rate decreases as the number of cycles increases, so to shorten the opening and closing time of the filter plates, a movable filter cloth type that can open and close all the filter plates at the same time was developed. Therefore, even if the number of cycles is large, the filter cloth movable type has a short miscellaneous time and a short pressing time, so the filtration efficiency is high and the dehydration speed is fast. Since the size of the device varies depending on the dewatering speed, the size of the device should be calculated by applying the dewatering speed according to the amount of sludge for each type. In general, during the high turbidity period predicted in the design, it can be operated for up to 24 hours without a preliminary stage and has good dewatering efficiency, so there is no problem with the processing speed because it has high dewatering efficiency and plenty of operating time. Therefore, it is important to plan to satisfy the throughput of sludge generated during the average turbidity period with low dewatering efficiency such as winter. Table 7 Pilot test results of a domestic water treatment plant pressurized dehydrator

Format	Water filtration	Water source	Experimental period	Sludge Concentration (%)	Dehydration rate (kg.DS/㎡.hr)	Cake Function Rate (%)	Remarks
A (removable)	Cheongju	Dacheng Dam	‘98.5	1.62	1.36 to 1.67	54.0 to 56.8	15K Sq.
	Diamond	Diamond	‘98.6	2.45	1.94 to 2.27	56.5 to 59.3	15K Sq.
	Resin	Faldang Dam	‘99.7	3.15	1.48 to 2.69	56.2 to 60.5	15K Sq.
	Wabu	Faldang Dam	‘98.7 ‘98.7 ‘98.7 '99.7	2.86 0.68 5.1	1.74～2.4 0.96～1.0 1.69～2.01	42.7～45.4 54.0～54.7 58.9～59.6	15K Sq.
	Magog	Nakdong River	‘02.8	6.3	1.52 to 3.95	41.9 to 45.5	15K Sq.
	Alpine	Unmoon Dam	‘02.8	4.62	1.04 to 2.78	47.8 to 52.1	15K Sq.
	Singing	Gacha Dam	‘02.8	2.0	1.13 to 1.73	53.1 to 56.3	15K Sq.
B (fixed)	Hot acid	Nakdong River	‘99.6～9	2.53	0.7 to 1.32	46.1 to 56	7K Sq.
	Singing	Gacha Dam	‘00.8	6.2	1.0	57.3	15K Sq.
	Hwasun	Hwasun Dam	‘00.7	1.47	0.3 to 10.5	65.7 to 67.5	15K Sq.
	Bounce	Nakdong River	‘00.6～8	2.88	0.69 to 0.92	61.7～70 36.5～50	7K Sq. 15K Sq.
	Bamboo grains	Nakdong River	‘02.9	5.4	1.09 to 1.3	42.7 to 43.6	15K Sq.
C (fixed)	Bounce	Nakdong River	‘02.1	3.39	0.39 to 1.0	55.5 to 63.2	15K Sq.
D (fixed)	Resin	Faldang Dam	‘00.7	4.0	1.75 to 1.87	58.1 to 60.0	15K Sq.
	Bamboo grains	Nakdong River	‘02.9	4.1	0.67	53.5	15K Sq.

  Table 8 Average Dehydration Rate by Concentration

Producer	Format	Water filtration	Average dewatering rate by sludge concentration (kg.DS/㎡.hr)			Remarks
			2%	3%	5%
A	Removable filter cloth	Cheongju/Goshan/Gachang	1.0 to 1.4	1.4 to 1.7	1.7 to 2.0	Dam source water
		Geum/Megok	1.1 to 1.6	1.7 to 2.2	2.2 to 2.6	Drifting water
		Resin/Widow	1.2 to 1.5	1.6 to 1.9	1.9 to 2.1	Drifting water
B	Filtration Parameterization	Chorus/chord order	0.3 to 0.5	0.4 to 0.6	0.6 to 0.8	Dam source water
		Bounce/Hot Acid/Cooked Grain	0.4 to 0.7	0.6 to 0.9	0.9 to 1.2	Drifting water
C	Filtration Parameterization	Bounce	0.3 to 0.5	0.4 to 0.6	0.6 to 0.9	Drifting water
D	Filtration Parameterization	Resin/Straw	0.23 to 1.36	0.35 to 1.6	0.5 to 1.8	Drifting water

  The pilot test results of each type without chemical injection at a domestic water treatment plant using dam water or drift water as the water source are shown in Table 7. For the design application according to the pilot test results, the concentration of sludge flowing from the thickening tank was converted to the average dewatering rate at 2%, 3%, and 5% and shown in Table 8. The water temperature is based on 5℃～25℃, and the filtration chamber thickness is based on 30mm except for 20mm for B, which is based on 30mm.

Fig 9 Indentation Dehydration Characteristics by Format

    ** < Pilot Test Conversion of results > > vc = Qc × (100-Wc)/100 ÷ Af vc : Dehydration rate per cycle (kg.DS/㎡.cycle) Qc: Weight of the cake (kg) Wc : Cake moisture content (%) Af: Filtration area (㎡) vh = vc × 60/tc vh : Dehydration rate per hour (kg.DS/㎡.hr) tc : cycle time(min) TC = TF(pressing dehydration time)+TS(squeezing dehydration time)+TD(cake separation time)+ tw(cleaning time) + to(other time) ․Dewatering rate per cycle : VC’ = Reference chamber thickness/Test chamber thickness ․Conversion pressurization time: tf’ = (test sludge concentration/design concentration)×(reference chamber thickness/test chamber thickness)2×(sludge viscosity at design temperature/sludge viscosity at test temperature) ․Conversion cycle time : tc’ = tf’ + ts + td+ tw + to ․Converted hourly dehydration rate : vh’ = vc’× 60/tc’  

5.3 Dehydrator specifications

Dehydrators are sized by the length of one side of a square filter plate, and the larger the size, the larger the filtration area, but even within the same size, there is some variation in filtration area between manufacturers. In general, medium and large filter plates of 1,000, 1,500, and 2,000 are most commonly used, and there are several sizes as shown in the following Table 9. The number of chambers (number of filter plates) should be determined by considering the amount of sludge, operation time, cycle time, and installation area, but in the case of fixed filter cloth, the more filter plates, the longer the release process takes, which reduces the dewatering speed, so it should be considered when determining the capacity.   < Selecting filtration area > > Qs×1/To×1/vh×1/Ac×1/Nf = Chamber Quantity Qs : Sludge volume of average or design turbidity (kg.DS/㎡.hr) To : Operating hours (hr/day) vh : Filtration rate (kg.DS/㎡.hr) Ac : Filtration area per chamber (㎡) Nf : Number of dehydrator driving wheels (units)  

Plate size (mm)	Filter Area(㎡) /chamber	No. of Chamber	Remarks
250	0.085	10	Company S
320	0.072	3 to 10	Company T
400	0.18	3 to 40	T, Y
470(500)	0.23 to 0.264	10 to 40	S, Y
630	0.43 to 0.46	10 to 50	T, Y, N threads
800	0.86 to 0.977	20 to 60	S, Y, N threads
930	1.16	30 to 100	Company T
1,000	1.5 to 1.6	8 to 100	S,U,Y,J,N companies
1,200	2.1 to 2.2	40 to 100	S, T, Y, N
1,250(1,300)	2.0 to 2.7	40 to 100	T, Y
1,500	3.5 to 3.6	18 to 100	S,U,Y,J,N companies
2,000	6.4 to 6.95	32 to 100	U, Y, N threads

 

5.4 CakeFunctionRate

The moisture content of dewatered cake in a pressurized dehydrator varies depending on the sludge composition and the type of dehydrator, but it is common to be in the range of 50 to 651 TP3T, and within certain limits, the longer the pressurization time and pressurization dehydration time, the lower the moisture content can be maintained. It is also known that the higher the solids concentration of the incoming sludge, the greater the dewatering rate and the lower the cake content. In general, in the case of fixed filter cloth type, the sludge pressing time and pressing time are kept long in order to maximize the dewatering speed and reduce the cake content rate because the release process takes time and it is difficult to peel off the cake if the cake thickness is thin, and in the case of moving filter cloth type, the pressing and pressing time is shortened to increase the dewatering speed because the opening process takes less time and the cake is easy to peel off as the filter cloth moves.  

5.5 Conclusion

(1) Design filtration rate by type

It is common for dewatering performance of any model to vary somewhat depending on the water treatment process, such as changes in sludge composition due to seasonal fluctuations in raw water quality or flocculant usage. In the case of river water or lake water, filtration resistance increases due to high viscosity at low water temperature, and if algae occurs and enters the sludge, it is contained as fine particles, so it is analytically classified as organic matter and becomes difficult to filter. In winter, when the amount of stormwater is low and the water volume is reduced, the incoming particles become smaller and the proportion of organic matter increases. In the case of pond water, the amount of organic matter tends to be higher than that of river water, and the solids concentration of sludge tends to be lower, resulting in poor dewatering performance. As such, the dewatering performance varies depending on the water quality conditions of the sludge source water, but since the pressurized dehydrator is in its early stages of introduction, there is a lack of data on the treatment performance of each water quality, so it is accurate to apply the sludge of the applied water quality by conducting experiments in advance. Considering that the design concentration of concentrated sludge in domestic water purification plants averages from 3 to 51 TP3T, the lowest standard of 31 TP3T is used as the design condition, and based on the above experimental results, an example of the appropriate dehydration speed design is shown in the following Table 10 by pressurized dehydrator type. In the case of dam source water, it is difficult to set a standard because the performance of each type is not sufficient to date, but there are periods when the dewatering rate decreases, such as low turbidity in winter, so it would be economical to adopt a dewatering rate in a low range and consider some leeway when selecting a standard, or to adopt measures such as extending the operating time or utilizing a spare machine.

分类	Design condition : concentration 3%, chamber thick.30mm
	Filtration Parameterization	Removable filter cloth
Dam source water, river water	0.5～0.7 kg.DS/㎡.hr	1.0～1.5 kg.DS/㎡.hr

  In summer high turbidity, the dewatering performance increases by up to 2 times due to the improved dewatering performance, and the operation time is sufficient to operate within 24 hours using a reserve, so it is important to select a device specification with a dewatering speed that satisfies the average turbidity of the design conditions.  

(2) Review your behavior for design function rate assurance

In the process of squeeze dewatering by applying pressure to the membrane after squeeze filtration of sludge, it is common for the water content of the sludge cake to be proportional to the squeezing force within a certain range. According to the following Table 11 and Table 12, in order to maintain the design cake moisture content of 60%, it is necessary to squeeze dewatering with a water pressure of 15 kg/㎠, except for electrofiltration with a separate dewatering device, according to Japanese technical data and the results of pilot tests in Korea.

Sludge Concentration (%)	Cake moisture content
	7 kg/㎠ (air pressure) compression	15 kg/㎠ (hydrostatic) crimping
2	73%	60%
3	70%	56%
4	67%	54%
5	64%	51%
6	60%	48%

 

Test	Average cake moisture content
	7 kg/㎠ (air pressure) compression	15 kg/㎠ (hydrostatic) crimping
Fixed filter cloth	59%～65%	57%～60%
Removable filter cloth	-.	55%～60%

 

(3) Reviewing the cake for unattended automation

After sludge dewatering, the cake closely adhering to the filter cloth is separated from the filter cloth and discharged to run the next cycle, but if the cake is not separated smoothly, it must be separated by manpower, so unmanned automatic operation is not possible. In general, when dehydrating by injecting flocculating chemicals (mainly slaked lime and ferric chloride), the cake is thick and separated relatively well, but when no chemicals are injected, the cake is thin and sticks to the filter cloth and does not separate well. In the case of a mobile filter cloth, the filter cloth itself moves from the bottom to the top, so the separation performance is good even if the cake thickness is thin. In the case of the fixed filter cloth type, the thicker the cake thickness, the better the separation by self-gravity, but if the cake thickness is thin, the separation is not good, so a separate automatic separation device (filter cloth vibration device, scraper, etc.) is required for separation.       References

• Integrated Design of Water Treatment Facilities (S.Kawamura)
• Sewerage Facilities Standards. 1998 Korea Water Works Association
• Water Supply Facilities Standards. 1997 Korea Water Works Association
• Ishigaki Company LTD. of Japan Knowledge Base
• Shinko Pantec Knowledge Base, Japan
• Korea Water Resources Corporation Research Report