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Incorporation of rapid thermal conditioning into a wastewater treatment plant
Fuel Processing Technology 56 Ž1998. 183–200
Incorporation of rapid thermal conditioning into a wastewater treatment plant Jianhong Zheng 1, Robert A. Graff ) , John Fillos 2 , Irven Rinard Department of Chemical Engineering, City College of New York, ConÕent AÕenue at 138 Street, New York, NY 10031, USA Received 7 July 1997; revised 17 February 1998; accepted 19 February 1998
Abstract Rapid thermal conditioning is a developing technology recently applied to sludge treatment. Sludge is heated rapidly to reaction temperature Žup to about 2208C. and quenched after 10 to 30 s. This process reduces the amount of biosolids requiring land disposal by increasing its biodegradability and dewaterability. Rapid thermal conditioning may be incorporated into a wastewater treatment plant where, combined with anaerobic digestion, it would reduce the quantity of biosolids requiring disposal, eliminate the need for polymer coagulant, improve dewaterability, increase methane production, and further reduce the concentration of pathogens. Also, the odor problem associated with traditional thermal conditioning processes is largely minimized. Pilot scale equipment was used to assess the process and provide design parameters for scale-up. Introduction of the process into an existing municipal wastewater treatment plant was then evaluated using the Wards Island Water Pollution Control Plant of New York City as an example. Two alternative flow sheets are shown together with preliminary engineering designs. Cost estimates for these alternatives show a substantial advantage in comparison to present plant operation. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Thermal conditioning; Sludge; Dewatering; Wastewater treatment
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C o r r e s p o n d in g a u th o r . T e l.: q 1 - 2 1 2 - 6 5 0 - 8 1 3 6 ; f a x : q 1 - 2 1 2 - 6 5 0 - 6 6 6 0 ; e-mail: [email protected]. 1 Formerly PhD student at CCNY, currently at General Electric, Power System, 1 River Road, Building 40-4S, Schenectady, NY 12345, USA. 2 Department of Civil Engineering, City College of New York, Convent Avenue at 138 Street, New York, NY 10031, USA. 0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 8 . 0 0 0 5 5 - 1
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1. Introduction In recent decades, environmental issues have received increasing attention and ever stricter laws have been enacted relating to the disposal of biosolids. In compliance with the Ocean Dumping Ban Act, passed by Congress in 1988, New York City ceased ocean disposal of digested sludge, a practice it had followed since the 1930s. Since January 1992, digested sludge has been conditioned by polymer coagulants and then dewatered by newly implemented centrifuge facilities, yielding 363 dry metric tons per day of biosolids for land disposal application. These dewatered biosolids are now transported as far as Texas, Arizona, or Colorado, resulting in sharply higher costs. Any process reducing the amount of cake produced without lowering its quality would significantly reduce overall operating cost. More recent regulations impose even more stringent controls on biosolids disposal. For example, the USEPA Sewage Sludge Use and Disposal Regulation set national standards for pathogens and a number of heavy metals. To avoid disposal restrictions and to improve marketability of the sludge end product, treatment plants may decide to upgrade their pathogen reduction process w1x. Under such conditions, there is opportunity for development of a variety of innovative solutions. The rapid thermal conditioning ŽRTC. process is one of them. RTC is a developing technology recently applied to sludge treatment. The sludge is heated rapidly Žin a few seconds or less. to reaction temperature Žup to 2208C. under sufficient pressure to maintain the liquid phase. The reaction is quenched after 10 to 30 s by flashing to atmospheric pressure. This treatment offers several benefits: Ž1. Volatile suspended solids are hydrolysed. This solubilization results in a decrease in the solids output of the waste water treatment plant requiring land disposal. Ž2. The dewaterability characteristics of the sludge are improved, so that a drier cake is obtained by centrifugation even without polymer addition. Ž3. The resulting soluble organic substances stimulate biological activity. This results in increased gas yields in subsequent anaerobic digestion. The amount of solids requiring disposal is further decreased. The methane gas released may be used as fuel for the thermal conditioning process, making the overall process thermally self-sufficient. Ž4. The high temperatures of RTC destroys pathogens, making it possible to meet requirements for Class A sludge as defined in the US EPA Sewage Sludge Use and Disposal Regulations. This allows widespread land application of the residual biosolids. Research on the application of the RTC process to sludge treatment was carried out at The City College of the City University of New York from 1990 to 1993. The experimental results, summarized below, provide a basis for the design of a commercial facility. RTC can be incorporated into either a new or existing plant. The latter case, more relevant to New York City, was studied here. The Wards Island Water Pollution Control Plant ŽWPCP., which is the second largest among the 14 WPCPs in New York City, was chosen as the target facility. It is a typical secondary wastewater treatment facility ŽFig. 1. which produces three types of sludge: waste activated sludge ŽWAS., primary sludge, and partially digested sludge ŽPDS.. During fiscal year 1992 ŽJuly 1991 to June 1992.,
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Fig. 1. Process diagram for Wards Island WPCP.
the plant treated municipal wastewater at an average rate 1 M m3rday and produced approximately 73 dry metric tons of sludge cake daily at 28 to 30% dry solids. Sixty-two percent of the digestion gas produced was exchanged for steam from an off-site source; the remainder was wasted. The feasibility of integrating the RTC process into present plant operation was investigated, and two options emerge as economically attractive. These are described later.
2. Literature review Thermal conditioning as a process for pretreating sludge has a long history. The full scale application of the Porteous process can be traced at least as far back as the 1930s w2x. It has always been an option for sludge treatment and many modifications have been made during past decades. However, the popularity of the Porteous Process is limited because of its sophistication in relation to the treatment processes, operational problems, odor concerns, and higher costs. As to the operating conditions of thermal treatment, temperature mostly ranges from 170 to 2508C w3–5,2x, with some ranging from 60 to 808C w6,7x. Retention time ranges from 15 to 60 min. In some cases additives are employed. Oxygen addition, known as the Zimpro Process, was used to partially oxidize sludge and increase temperature w8x. The addition of acids or alkalis prior to or during thermal conditioning has been used to enhance solubilization w3x. Addition of carbon dioxide would eliminate the formation of struvite which blocks passageways and pipes w7x. The critical factors involved in thermal conditioning were said to be pH, temperature, and retention time w9x. High temperature at a specified retention time is required to produce a significant improvement in solids dewaterability. An increase in specific resistance was observed when retention time was longer than 30 min with a temperature
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higher than about 2008C w4x. Also, high temperature was reported to produce refractory or toxic materials which reverse anaerobic digestibility w5,10,8,11x. Leuschner and Laquidara w11x also suggested not using oxidative hydrolysis of sludge for the purpose of increasing methane production. In recent years, an innovative process, RTC, originally developed to maximize the conversion of corn stover to glucose w12x, was studied for sludge treatment application w13,14x. The most important aspect of the process is its use of rapid heating in order to reduce the retention time to as short as 10 s while at a high conditioning temperature such as 2208C. The experimental results show that short retention time gives a better biodegradability w11x. A plant design was proposed to implement the RTC process w13x. In that design, however, a series of heat exchangers was used to raise the temperature of sludge. This would prolong retention time and would change the nature of conditioning from a rapid to slow process.
3. Laboratory testing 3.1. Rapid thermal conditioning A bench-scale continuous flow RTC apparatus, as described by McPartland et al. w12x, was designed and constructed ŽFig. 2.. Sludge is held in a storage tank and transported to the RTC reactor by a positive displacement pump, while high pressure steam is generated in a boiler. Steam and sludge are continuously injected into a high intensity mixing zone at the base of the first tubular reactor. Condensation of the steam in direct contact with the sludge provides extremely high heating rates. The mixture then passes through one, two, or three reactor sections in series having a residence time of 10 s each. RTC temperatures range from 100 to 2208C. The reaction is quenched when the
Fig. 2. Pilot unit for rapid thermal conditioning of sludge.
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mixture flashes through a pressure let-down valve to atmospheric pressure. After further cooling in a plate-and-frame heat exchanger, the product is collected in a holding tank. The pH adjustment, when used, is made in the storage tank prior to RTC. 3.2. Sludges studied Sludges from several New York City WPCPs were tested: PDS, WAS and primary sludge from the Wards Island Plant, and PDS from the Newtown Creek and the Rockaway Plants. Emphasis was placed on PDS. A total of 22 RTC test runs of were made over a range of temperature, retention time, and pH: 1. Thermal treatment temperature: room temperature, 100, 130, 160, 190, 2208C. 2. Retention time: 10 to 30 s. 3. pH: neutral, and 2 to 12. 3.3. Biodegradability studies The anaerobic biodegradability of thermally conditioned sludge was first tested using 160-ml serum bottles as batch reactors for periods exceeding 60 days. The results of these tests were used to select the best conditions for RTC. More extensive anaerobic digestion tests, using sludge treated at the optimal RTC conditions, were made in a 6.5 l continuous stirred tank reactor ŽCSTR. and a hybrid packed bed reactor. The packed bed reactor has an inside diameter of 11 cm and a height of 1.83 m, packed with 1.5 cm diameter pall-ring media. For the CSTR and packed bed tests, an optimum hydraulic retention time was selected. All tests were conducted in an environmental chamber maintained at 358C w15x. 3.4. Dewaterability studies The effect of RTC on the dewaterability of sludge was assessed in centrifuge and filtration tests. For the former, a Marathon 10 K bench-top centrifuge was used. Samples of sludge were centrifuged at 2500 = g for 15 min which approximates plant conditions. The solids content of the resultant cake and supernatant fluid were measured. Filterability was tested in both a low pressure filter press ŽBaroid. and a capillary suction time ŽCST. apparatus.
4. Test results and design criteria 4.1. Solubilization The solubilization of sludge resulting from RTC was monitored using the procedures given in ‘Standard Methods for the Examination of Water and Wastewater’ w16x. According to this method, solids are characterized as volatile suspended solids, volatile dissolved solids, fixed suspended solids, and fixed dissolved solids. The results for PDS treated at 2208C for 30 s are given in Fig. 3. RTC causes dissolution of suspended
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Fig. 3. Cumulative solids distribution in rapid thermally conditioned PDS Ž30 s HRT..
solids, predominantly depleting volatile solids and leaving the fixed solids nearly unchanged. Volatile solids are considered biodegradable and can be further reduced by digestion. For PDS, treated at 2208C for 30 s, approximately 47% of the total solids are solubilized, and in the tested range of solids concentration from 1.4 to 4.9%, the percentage of total solids solubilized is independent of their initial concentration. For WAS at 2208C for 30 s, approximately 55% of total solids are solubilized. 4.2. Dewatering Both filter press and CST tests show that RTC decreases filterability of PDS. However, RTC greatly improves the dewaterability of sludge in a bench-scale centrifuge operated at the same centrifugal force and retention time as used in the full-scale centrifuges of New York City’s WPCPs. Sludge cake with 25% solids is achieved without the addition of polymer conditioners, and the suspended solids recovery rate, in almost all cases, is above 95%. Even higher solids concentration can be expected in full scale commercial centrifuges. Based on these results, it is assumed in the economic analyses below that polymer addition is not required for dewatering of sludge following RTC. This provides major savings. As centrifugation is the only dewatering method used in New York City, RTC becomes highly attractive for deployment at these sites. 4.3. Biodegradation The RTC process increases sludge anaerobic digestibility giving higher yields of methane gas and leaving correspondingly less organics in the residue. RTC not only increases ultimate gas production, but also increases the rate of reaction especially during the first several days of digestion. For WAS in serum bottle tests, RTC at 2208C for 30 s results in a 200% increase in gas production during the first one to two days,
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compared to unconditioned sludge. Over a time period of 20 days, the average gas production improvement is 80%, a value which is maintained even after 60 days. Anaerobic packed bed tests show that 80% VDS in RTC PDS can be removed with 2 days HRT. The significance of this improvement is that shorter retention times can be used in anaerobic digestion w15x. 4.4. Effect of pH Both acid and alkali were added prior to thermal conditioning to test their effectiveness in the process. However, at high operating temperature, the effect of pH adjustment was very slight. Adjustment of pH requires additional equipment, increases the complexity of the process, results in corrosion problems and increases operating cost. Therefore, pH adjustment is not further considered. 4.5. Type of sludge Both WAS and PDS consist mainly of bacterial cells and multiple tests show increased biodegradability and dewaterability of these substrates. In contrast, primary sludges, which contain inorganic and non-living organic matter, show considerably less response to RTC. Consequently, thermal conditioning is considered only for WAS and PDS. 4.6. Odor Sludge itself has odor. However, it was noted during these tests that RTC did not increase the unpleasant odor of sludge. This is a significant improvement over traditional thermal conditioning processes. Evidently, the short retention time avoids formation of new odoriferous compounds.
5. Description of proposed flow sheets Based on the laboratory tests described above, PDS and WAS were selected for RTC at a temperature of 2208C with a retention time of 30 s, the pH being unmodified. Accordingly, two options were devised and are described below. Each option is designed for a sludge influent flow rate projected for the year 2005 in Wards Island WPCP. 5.1. RTC system The RTC system consists, in all cases, of a positive displacement pump, boiler, rapid thermal reactor and flash tank all operating under high temperature and pressure. A positive displacement pump is required to transport sludge into the thermal reactor against its operating pressure of 2500 kPa. A boiler providing saturated steam at 2208C is fueled by methane from digestion or an external source if additional fuel is needed.
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The streams of sludge and steam enter the reactor through a static mixer and reach the reaction temperature of 2208C nearly instantaneously. The liquid retention time in the reactor is 30 s. The maintenance of pressure is achieved by an automated control valve, and the suspension is discharged into a flash tank. Flashing of the treated suspension serves two objectives: First, it quenches thermal reactions. When the hot suspension enters the flash tank, partial vaporization of the water content occurs. The temperature instantaneously drops from 2208C to the saturation temperature of thermally treated sludge which will be about 1008C if the pressure in the flash tank is atmospheric. This sudden drop in temperature results in drastic decrease in reaction rate. A second objective is to separate the water from the sludge in order to reduce the load on both dewatering and digestion equipment which follow. About one fourth of the influent suspension is vaporized. Since vapor contains volatile organics, it is recycled to the aeration section of the plant. Except in the size of its various components, this RTC subsystem is the same in each proposed implementation alternative. The major difference between this design and any of the thermal conditioning processes proposed or used so far, such as Porteous or Zimpro, is that no heat exchanger is used to preheat the sludge flowing into the reactor. This is important to ensure short heating time and rapid thermal processing. There are, nevertheless, several ways to recover the heat from this RTC process: 1. Preheat the boiler feed water to 908C using heat exchangers. 2. Preheat sludge entering the digesters to 458C to compensate heat loss from this equipment. 3. Use the heat content of thermally conditioned sludge to compensate for heat loss from the digester used to treat RTC sludge. However, even with such heat recovery the RTC and digestion system would not be self-sufficient in thermal energy. The digester gas can provide less than 40% of the boiler fuel required for conditioning of PDS. The extra fuel cost alone would then make the RTC process more expensive than current processes to handle and dispose of sludge. To reduce process heat requirements, the proposed flow sheets incorporate thickening of sludge prior to the RTC step. This appears to be a practical and effective way to reduce the energy required without affecting the basic mechanism of solubilizing the volatile suspended solids. Centrifugation thickening is the preferred method because it has a high process capacity and a high suspended solids recovery rate. With polymer addition, the Humboldt centrifuges at the Wards Island WPCP are able to achieve a solids concentration of 30% for dewatered PDS, which is an upper limit for PDS thickening. In laboratory centrifuge tests without polymer addition, 10% is typical for solids concentration and can be considered the lower limit of PDS thickening. This substantial reduction in flow rate decreases the required capacity of the RTC reactor, pump, and digester for conditioning of PDS by about three-fourths, resulting in both lower capital and operating costs. Though a centrifuge thickening process is added, the total number of centrifuges does not necessarily increase beyond what is now used in the current sludge dewatering process. Thickening achieves a lower solids concentration in the cake than does the dewatering process. When a centrifuge is used for thickening, its capacity can be 50% higher than for dewatering. Therefore, as long as the thickening ratio is less than 1r3,
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rearrangement of the centrifuges currently in use may meet the requirements of both thickening and dewatering. Additional units of this costly item might not be required in the proposed process modification. 5.2. System A Table 1 shows the design basis, and Table 2 is a summary of major streams of System A. The flow sheet of System A is shown in Fig. 4. PDS is thickened first using centrifuges, to increase the solids concentration to 10%. The PDS centrate has low BOD and is of little importance for thermal conditioning and digestion. Therefore, it will be returned to the aeration tanks, as is the current practice at the Wards Island Plant. The mass flow rate of thickened PDS is reduced to 27% of that of the feed. ŽAlthough an experimental RTC test with thickened PDS of 10% solids was not done, it is assumed that the fraction solubilized, 47% on a dry basis, is unchanged by thickening.. There are two options for digestion of thermally conditioned sludge. One is that the entire mixture, both suspended and soluble solids, be subjected to anaerobic digestion in a completely mixed suspended growth reactor. The disadvantage of this method is that the hydraulic and solids retention time cannot be controlled independently, requiring a larger reactor. However, a larger volume of gas can be produced and more volatile solids can be destroyed since both soluble and remaining suspended solids will be subjected to biological degradation. This would take full advantage of thermal conditioning on the biodegradability of sludge. An alterative approach is to separate the soluble from the suspended solids and digest only the soluble organics. An attached growth reactor is more appropriate in this case, resulting in high efficiency and short hydraulic retention time. It requires a digester of small volume; an attractive feature in New York City plants where land is scare.
Table 1 Design basis for System A Parameter and units
Value 3
Flow rate of thickened sludge, m rd TS of thickened sludge, % by weight VS content of thickened sludge, % TS Temperature of thickened sludge,8C VS reduction by digestion, % VS Gas production, m3 rkg VS destroyed Lower heating value of digester gas, kJrm3 TS of thickened PDS, % by weight Digestion temperature,8C RTC temperature,8C RTC time, sec TS solubilization of PDS due to RTC, % TS VDS content, % DS SS recovery rate by centrifuge, % SS Cake solids concentration, % by weight VDS removal rate by anaerobic filter, % VDS
2949 4.1 79 15 40 1.06 22,400 10.0 35 220 30 47 80 96 30 50
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Table 2 Stream summary, System A Stream number
Flowrate ŽMgrday.
State
Temperature ŽC8.
TS Ž%.
DS Ž%.
1 3 5 9 10 12 14 16 18 19 20 30 31
2950 2912 80 354 1157 277 881 152 729 716 3101 42,200 m3 rday 14,400 m3 rday
liquidqsolids liquidqsolids liquidqsolids vapor liquidqsolids vapor liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids gas gas
15 35 35 220 220 100 100 69 45 35 35 35 35
4.10 2.90 10.00 0.00 6.94 0.00 9.12 30.00 4.78 3.01 0.83 0 0
0.15 0.19 0.18 0.00 3.26 0.00 4.29 3.00 4.55 2.50 0.71 0 0
However, the potential of an attached growth system for gas production may be lower than that for a suspended growth system. For this flowsheet, an up-flow anaerobic filter has been selected. In order to provide the filter with soluble feed, the suspended solids are separated and excluded using a dewatering centrifuge preceding digestion. This configuration has the advantage that the high temperature Žclose to 708C. of thermally conditioned PDS would reduce the viscosity of the sludge and improve settleability. As was done previously, the filter effluent is recycled to the aeration tanks. For the dewatering of thermally conditioned thickened PDS, the suspended solids concentration is as high as 6%. Higher concentration of solids in the centrifuge input
Fig. 4. System A, rapid thermal conditioning of thickened PDS.
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Table 3 Design basis for System B Parameter and units
Value
Flow rate of WAS, m3 rday TS of WAS, % by weight VS content of WAS, % TS Temperature of WAS,8C TS of thickened WAS, % by weight RTC temperature,8C RTC time, s TS solubilization of WAS due to RTC, % TS Flow rate of primary sludge, m3 rday TS of primary sludge, % by weight VS content of primary sludge, % TS Temperature of primary sludge,8C VS reduction digestion, % VS Cake solids concentration, % by weight Gas production, m3 rkg VS destroyed Lower heating value of digester gas, kJrm3 Digestion temperature, 8C SS recovery rate by centrifuge, % SS Cake solids concentration, % by weight
4011 2.47 83 15 7.1 220 30 55 500 6 62 15 66 30 1.06 22,400 35 96 30
results in a dewatered cake of higher solids concentration. This is an additional advantage of thickening. 5.3. System B Table 3 shows the design basis, and Table 4 is a summary of the major streams of System B. In System B ŽFig. 5., WAS with a solids concentration of 2.5% is thickened
Table 4 Stream summary, System B Stream number
Flowrate ŽMgrday.
State
Temperature ŽC8.
TS Ž%.
DS Ž%.
3 5 9 10 12 14 17 18 19 20 21 22 25 26
4011 1288 587 1875 423 1452 499 1951 1889 160 1729 4452 69,000 m3 rday 177,000 MJrday
liquidqsolids liquidqsolids vapor liquidqsolids vapor liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids gas gas
15 15 220 220 100 100 69 45 35 35 35 23 35 35
2.47 7.12 0.00 4.89 0.00 6.31 6.00 6.23 3.15 30.00 0.67 0.43 0 0
0.10 0.10 0.00 2.69 0.00 3.47 0.20 2.63 0.55 0.41 0.56 0.28 0 0
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Fig. 5. System B, rapid thermal conditioning of thickened WAS.
by centrifugation to 7.1%. This lower level of thickening reflects the nature of WAS which is more difficult to thicken and dewater than any of the other sludges produced in a WPCP. Moreover, the thickening of WAS requires a different type of centrifuge from that used for dewatering of PDS, and these must be purchased. The centrate will be returned to the influent of the aeration process, and thickened WAS will be pumped to the RTC system. Thermally conditioned WAS is first cooled then mixed with thickened primary sludge. The mixed sludge is at 458C prior to being pumped into the digesters. The solids concentration of thickened primary sludge is assumed to be 6%. Thermal conditioning is limited to WAS because experiments with primary sludge showed negligible improvement in either biodegradability or dewaterability. Digested sludge is dewatered by centrifuge and the centrate returned to the head of aeration. Additional fuel of 177,000 MJrday, 13% of the total, is needed for this flow sheet. However, this requirement would be eliminated if WAS could be thickened to a solids concentration of 8.0%. Compared to current facilities at the Wards Island WPCP, the configuration of System B reduces the size of the gravity thickener by approximately 75% and the number of digesters by approximately forty percent. The digestion is done in one step, making the separate digesters for thermally conditioned PDS, as in System A, unnecessary. Two-thirds of the dewatering centrifuges would have to be replaced by thickening centrifuges specifically for WAS.
6. Economic analysis Existing equipment, which includes digesters, centrifuges, grinders, pumps, and piping, would be used in implementing the RTC system. Of these, digesters and
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centrifuges represent the most expensive items in the RTC-digestion system configuration. Their utilization offers significant saving in capital cost. Their configuration in the system may have to be changed depending on the operating alternative selected. In order to ensure the reliability of rotating machinery, such as pumps and centrifuges, standby units are purchased. Also, two boilers are used for each system. Economics analyses were performed with ASPEN PLUS 8.3-5. This software was used to size the equipment, and estimate cost. The costs of major equipment items, such as centrifuges and boilers were based on prices quoted by suppliers. The design bases of major equipment for Systems A and B are shown in Tables 5 and 6, respectively. The total cost of major new equipment items is estimated at US$2.31 MM for System A, and US$3.74 MM for System B. The itemized costs of the new equipment for each system are shown in Tables 7 and 8. Based on the cost of new equipment, the fixed capital costs, as estimated by using ASPEN PLUS, are US$11.89 MM for System A and US$18.03 MM for System B. As a rough check, it is reasonable for fixed capital cost to be approximately five times the cost of new equipment. Linear depreciation for a life time of 15 years is included in operating costs. Utilities include electricity, fuel, and chemicals to treat boiler water. Six additional operating personnel are estimated to be required for the operation of either System A or B. Operating costs ŽTable 9., ascribable to the additional RTC process equipment is US$2.49 MM per year for System A and US$4.03 MM per year for System B. These operating costs are to be compared with the costs of current Žunmodified. system operation.
Table 5 Major equipment for System A Equipment name
Size
Quantity
Positive displacement pump
7.2 lrs, 30 kW
2
Boiler
5900 kW
2
Reactor
0.4 m3
1
Flash tank Anaerobic filter
0.8 m=1.6 mb 1755 m3
1 1
Heat exchanger 1
45.7 m2
1
Heat exchanger 2
72.1 m2
1
Heat exchanger 3
55.9 m2
1
a b
One unit is standby. Diameter=height.
a
Design basis Flowrate, 4.6 lrs Pressure, 2500 kPa 2208C vapor at 4.08 kgrs Efficiency, 0.8 Retention time, 30 s Pressure, 2500 kPa Retention time, 1 min Retention time, 2 days Flowrate, 727 m3 rday COD, 15,000 mgrl Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 908C Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 458C Heat transfer coefficient, 450 Wrm2 K Hot stream inlet temperature, 708C Hot stream outlet temperature, 408C
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Table 6 Major equipment for System B Equipment name
Size
Quantity
Design basis
Thickening centrifuge Positive displacement pump
2000 m3 rday 7.2 lrs, 30 kW
3a 3a
Boiler
12,500 kW
2
Reactor
0.65 m3
1
Flash tank Heat exchanger 1
1.0 m=2.0 mb 96.7 m2
1 1
Heat exchanger 2
39.7 m2
1
4000 m3 rday Flowrate, 5.6 lrs Pressure, 2500 kPa 2208C vapor at 6.83 kgrs Efficiency, 0.8 Retention time, 30 s Pressure, 2500 kPa Retention time, 1 min Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 908C Heat transfer coefficient, 450 Wrm2 K Hot stream inlet temperature, 698C Hot stream outlet temperature, 538C
a b
One unit is standby. Diameter=height.
Currently in New York City, dewatered sludge cake with 70% moisture is transported and disposed of by several private companies through composting, land application and reclamation. Charges to New York City vary from US$101.4 to US$261.2 per wet metric ton of sludge, because of differences in subsequent treatment and disposal locations. In order to reach target dewatering levels at the Wards Island WPCP, present practice requires the addition of polymer at the rate of about 10 kgrdry metric ton. The price of this polymer is approximately US$4.4rkg.
Table 7 Purchased cost of major equipment for System A Equipment name
Quantity
Purchased cost
Digester Grinder Thickening centrifuge Positive displacement pump Boiler Reactor Flash tank Dewatering centrifuge Anaerobic filter Heat exchanger 1 Heat exchanger 2 Heat exchanger 3 Unlisted Total
8 2 2 2 2 1 1 1 1 1 1 1 ya ya
existing existing existing US$106,400 US$232,000 US$4700 US$5300 existing US$1,489,600 US$49,100 US$65,600 US$56,000 US$301,000 US$2,310,000
a
Not applicable.
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Table 8 Purchasing costs of major equipment for System B Equipment name
Quantity
Purchasing cost
Digester Grinder Thickening centrifuge Positive displacement pump Boiler Reactor Flash tank Dewatering centrifuge Heat exchanger 1 Heat exchanger 2 Unlisted Total
5 2 3 3 2 1 1 2 1 1 ya ya
existing existing US$2,568,000 US$160,500 US$402,600 US$4700 US$7400 existing US$78,600 US$31,300 US$488,100 US$3,741,200
a
Not applicable.
Savings resulting from the introduction of RTC into the WPCP include: 1. Reducing the amount of sludge cake requiring disposal, 2. Elimination of polymer cost, and 3. Elimination of polymer addition operating costs. If only the first two items are considered, which is conservative, a comparison of annual operating costs among the different processes results in the numbers in Table 9. Only the differences in operating costs resulting from implementation of the RTC processes are shown. For current practice, dewatering PDS and cake disposal, the operating costs are the charges for polymer and for sludge disposal. For plants incorporating the RTC process, the total operating cost will be the charges to handle the sludge cake plus an operating cost attributable to the RTC system. A charge of US$101.4rwet metric ton is the lower limit for solids cake disposal cost and US$261.2rwet metric ton the upper limit. The costs of sludge conditioning and sludge disposal for each system is shown in Table 10. From Table 10, System A has the lowest operating cost. One important reason is that System A makes full use of the existing facility. Thickening the sludge makes the scale of the RTC system small. The scheme does not merely make the plant fuel self-sufficient; it results in a surplus of digestion gas. Table 9 Annual operating cost summary System
A ŽUS$ryear.
B ŽUS$ryear.
Utilities cost Labor O&M supplies General work Laboratory charge Depreciation Annual operating cost
55,000 1,113,000 320,000 174,000 35,000 793,000 2,490,000
652,000 1,428,000 468,000 246,000 35,000 1,202,000 4,031,000
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Table 10 Comparison of sludge conditioning costs and cake disposal costs for existing and RTC modified processes System
Existing
A
B
Cake produced, dry ŽMgrday. Cake produced, wet ŽMgrday.
76.2 254.0
45.5 151.7
49.4 164.8
0
0
Polymer cost ŽUS$MMryear. Cake disposal cost minimum ŽUS$MMryear. maximum ŽUS$MMryear. RTC operating cost
9.40 24.22 0
5.61 14.46 2.49
6.10 15.71 4.03
Total operating cost minimum ŽUS$MMryear. maximum ŽUS$MMryear.
10.63 25.45
8.10 16.95
10.13 19.74
2.53 8.50
0.50 5.71
Savings due to RTC minimum ŽUS$MMryear. maximum ŽUS$MMryear. a
1.23
ya ya
Not applicable.
System B uses less existing equipment. Three of the eight existing digesters cannot be used. The reduction in operating cost due to the shrinkage of the existing digestion facility has not yet been taken into account. System A uses two-step digestion, i.e., digestion both before and after thermal conditioning. System B uses only one step plus thickening prior to digestion. Therefore, System B would have both the lowest capital and operating costs for the digestion part of the plant. System B requires a small amount of additional fuel because WAS, being difficult to thicken, is only dewatered to have a solids concentration of 7.1%, compared with thickened PDS with a solids concentration of 10%. Although System B is not the most economical option to incorporate into Wards Island WPCP, it will be a best option for new plant construction or expansion of the digestion capacity of an existing plant.
7. Conclusions Several important conclusions are drawn from this work on the RTC process. Ž1. The rapid thermal conditioning process is feasible technically. It is both an environmentally and economically beneficial way to handle the sludge. Ž2. Thickening sludge to raise the solids concentration prior to thermal conditioning is crucial to significantly reducing the cost of thermal conditioning and subsequent digestion. Ž3. Integration of System A into the Wards Island WPCP would reduce the cost of handling dewatered sludge by US$2.5 MM to US$8.5 MM per year, which is 24 to 36% of disposal costs by current methods.
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Ž4. For the design of a new WPCP or expansion of the digestion capacity of an existing plant, the configuration of System B is the preferred choice. Several options for further improvement of the RTC process may be suggested. Ž1. Steam produced in the flash tank of the RTC process could be used to dry dewatered sludge to further reduce disposal weight and to produce a sterile product. Ž2. The temperature of sludge digestion could be raised from the present mesophilic range, about 358C, to thermophilic range of 458C. The waste heat from thermal conditioning is sufficient to accomplish this without additional heat. The benefit of such shift would be a high reaction rate and, therefore, high solids destruction, high gas production, and short retention time. A higher digestion temperature will also reduce the size of the heat exchangers needed to cool the thermally conditioned sludge. Ž3. These process schemes for incorporating RTC into WPCPs have the potential for further modification to remove ammonia by making use of the hot streams generated in the RTC system. This will be addressed in a sequent paper. 8. Nomenclature The following abbreviations are used in this paper. BOD s Biological oxygen demand CSTRs Continuous stirred tank reactor CST s Capillary suction time CW s Cooling water DS s Dissolved solids HRT s Hydraulic retention time O & M s Operating and maintenance PDS s Partially digested sludge RTC s Rapid thermal conditioning RAS s Returned activated sludge SS s Suspended solids TS s Total solids VDS s Volatile dissolved solids VS s Volatile solids VSS s Volatile suspended solids WASs Waste activated sludge WPCPs Water pollution control plant Acknowledgements This work was carried out in the Departments of Chemical and Civil Engineering at The City College of the City University of New York with financial support from the New York State Energy Research and Development Authority ŽNo. 1353-ERER-MSW90.. Aspen Technology provided the design software. We thank Mr. Dick Wagner and the Alfa-Laval Thermal Co. for contributing the plate-and-frame heat exchanger used in this work.
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x
w12x w13x w14x w15x w16x
G.W. Foess, R.B. Sleger, WATERrEngineering and Management, June Ž1993. 25–26. C. Lumb, J.P. Barnes, J. Proc., Ins. Sew. Purif. Ž1944. 71–90. J.G. Everett, Water Res. 8 Ž1974. 899–906. R.B. Brook, Water Poll. Control 69 Ž1970. 92–99. R.T. Haug, D.C. Stuckey, J.M. Gossett, P.L. McCarty, J. Water Pollut. Control Fed. 50 Ž1978. 73–85. M. Hiraoka, N. Takeda, S. Sakai, A. Yasuda, Water Sci. Tech. 17 Ž1984. 529–539. B.A. Garelli, B.J. Schwartz, R.W. Dring, Water-Environ. Technol. 4 Ž1992. 6–11. A.A. Friedman, J.E. Smith, J. DeSantis, T. Ptak, R.C. Granley, J. Water Pollut. Control Fed. 60 Ž1988. 1971–1978. G.M. Alsop, R.A. Conway, J. Water Pollut. Control Fed. 54 Ž1982. 146–152. D.C. Stuckey, P.L. McCarty, Water Res. 2 Ž11. Ž1984. 1343–1353. A.P. Leuschner, M.J. Laquidara, Methane from partially digested sewage sludge using a steam injection rapid thermal reactor. Final Report to New York State Energy Research and Develop Authority, 655-RIER-BEA-84, Dynatech Scientific Ž1988.. J.J. McPartland, H.E. Grethlein, A.O. Converse, Solar Energy 50 Ž1982. 55–63. C.E. Garzon-Lopez, Increased methane production from digested sludge through short-time thermochemical pretreatment, PhD thesis, Dartmouth College Ž1987.. J. Zheng, Rapid thermal conditioning of sewage sludge, PhD thesis, The City College of the City University of New York Ž1997.. K. Pierides, Rapid thermal conditioning of sewage sludge to improve digestibility and methane production, PhD thesis, The City College of the City University of New York Ž1993.. Standard Methods for the Examination of Water and Wastewater, 17th edn., Am. Public Health Assoc., Washington, DC Ž1989..
Incorporation of rapid thermal conditioning into a wastewater treatment plant Jianhong Zheng 1, Robert A. Graff ) , John Fillos 2 , Irven Rinard Department of Chemical Engineering, City College of New York, ConÕent AÕenue at 138 Street, New York, NY 10031, USA Received 7 July 1997; revised 17 February 1998; accepted 19 February 1998
Abstract Rapid thermal conditioning is a developing technology recently applied to sludge treatment. Sludge is heated rapidly to reaction temperature Žup to about 2208C. and quenched after 10 to 30 s. This process reduces the amount of biosolids requiring land disposal by increasing its biodegradability and dewaterability. Rapid thermal conditioning may be incorporated into a wastewater treatment plant where, combined with anaerobic digestion, it would reduce the quantity of biosolids requiring disposal, eliminate the need for polymer coagulant, improve dewaterability, increase methane production, and further reduce the concentration of pathogens. Also, the odor problem associated with traditional thermal conditioning processes is largely minimized. Pilot scale equipment was used to assess the process and provide design parameters for scale-up. Introduction of the process into an existing municipal wastewater treatment plant was then evaluated using the Wards Island Water Pollution Control Plant of New York City as an example. Two alternative flow sheets are shown together with preliminary engineering designs. Cost estimates for these alternatives show a substantial advantage in comparison to present plant operation. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Thermal conditioning; Sludge; Dewatering; Wastewater treatment
)
C o r r e s p o n d in g a u th o r . T e l.: q 1 - 2 1 2 - 6 5 0 - 8 1 3 6 ; f a x : q 1 - 2 1 2 - 6 5 0 - 6 6 6 0 ; e-mail: [email protected]. 1 Formerly PhD student at CCNY, currently at General Electric, Power System, 1 River Road, Building 40-4S, Schenectady, NY 12345, USA. 2 Department of Civil Engineering, City College of New York, Convent Avenue at 138 Street, New York, NY 10031, USA. 0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 8 . 0 0 0 5 5 - 1
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1. Introduction In recent decades, environmental issues have received increasing attention and ever stricter laws have been enacted relating to the disposal of biosolids. In compliance with the Ocean Dumping Ban Act, passed by Congress in 1988, New York City ceased ocean disposal of digested sludge, a practice it had followed since the 1930s. Since January 1992, digested sludge has been conditioned by polymer coagulants and then dewatered by newly implemented centrifuge facilities, yielding 363 dry metric tons per day of biosolids for land disposal application. These dewatered biosolids are now transported as far as Texas, Arizona, or Colorado, resulting in sharply higher costs. Any process reducing the amount of cake produced without lowering its quality would significantly reduce overall operating cost. More recent regulations impose even more stringent controls on biosolids disposal. For example, the USEPA Sewage Sludge Use and Disposal Regulation set national standards for pathogens and a number of heavy metals. To avoid disposal restrictions and to improve marketability of the sludge end product, treatment plants may decide to upgrade their pathogen reduction process w1x. Under such conditions, there is opportunity for development of a variety of innovative solutions. The rapid thermal conditioning ŽRTC. process is one of them. RTC is a developing technology recently applied to sludge treatment. The sludge is heated rapidly Žin a few seconds or less. to reaction temperature Žup to 2208C. under sufficient pressure to maintain the liquid phase. The reaction is quenched after 10 to 30 s by flashing to atmospheric pressure. This treatment offers several benefits: Ž1. Volatile suspended solids are hydrolysed. This solubilization results in a decrease in the solids output of the waste water treatment plant requiring land disposal. Ž2. The dewaterability characteristics of the sludge are improved, so that a drier cake is obtained by centrifugation even without polymer addition. Ž3. The resulting soluble organic substances stimulate biological activity. This results in increased gas yields in subsequent anaerobic digestion. The amount of solids requiring disposal is further decreased. The methane gas released may be used as fuel for the thermal conditioning process, making the overall process thermally self-sufficient. Ž4. The high temperatures of RTC destroys pathogens, making it possible to meet requirements for Class A sludge as defined in the US EPA Sewage Sludge Use and Disposal Regulations. This allows widespread land application of the residual biosolids. Research on the application of the RTC process to sludge treatment was carried out at The City College of the City University of New York from 1990 to 1993. The experimental results, summarized below, provide a basis for the design of a commercial facility. RTC can be incorporated into either a new or existing plant. The latter case, more relevant to New York City, was studied here. The Wards Island Water Pollution Control Plant ŽWPCP., which is the second largest among the 14 WPCPs in New York City, was chosen as the target facility. It is a typical secondary wastewater treatment facility ŽFig. 1. which produces three types of sludge: waste activated sludge ŽWAS., primary sludge, and partially digested sludge ŽPDS.. During fiscal year 1992 ŽJuly 1991 to June 1992.,
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Fig. 1. Process diagram for Wards Island WPCP.
the plant treated municipal wastewater at an average rate 1 M m3rday and produced approximately 73 dry metric tons of sludge cake daily at 28 to 30% dry solids. Sixty-two percent of the digestion gas produced was exchanged for steam from an off-site source; the remainder was wasted. The feasibility of integrating the RTC process into present plant operation was investigated, and two options emerge as economically attractive. These are described later.
2. Literature review Thermal conditioning as a process for pretreating sludge has a long history. The full scale application of the Porteous process can be traced at least as far back as the 1930s w2x. It has always been an option for sludge treatment and many modifications have been made during past decades. However, the popularity of the Porteous Process is limited because of its sophistication in relation to the treatment processes, operational problems, odor concerns, and higher costs. As to the operating conditions of thermal treatment, temperature mostly ranges from 170 to 2508C w3–5,2x, with some ranging from 60 to 808C w6,7x. Retention time ranges from 15 to 60 min. In some cases additives are employed. Oxygen addition, known as the Zimpro Process, was used to partially oxidize sludge and increase temperature w8x. The addition of acids or alkalis prior to or during thermal conditioning has been used to enhance solubilization w3x. Addition of carbon dioxide would eliminate the formation of struvite which blocks passageways and pipes w7x. The critical factors involved in thermal conditioning were said to be pH, temperature, and retention time w9x. High temperature at a specified retention time is required to produce a significant improvement in solids dewaterability. An increase in specific resistance was observed when retention time was longer than 30 min with a temperature
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higher than about 2008C w4x. Also, high temperature was reported to produce refractory or toxic materials which reverse anaerobic digestibility w5,10,8,11x. Leuschner and Laquidara w11x also suggested not using oxidative hydrolysis of sludge for the purpose of increasing methane production. In recent years, an innovative process, RTC, originally developed to maximize the conversion of corn stover to glucose w12x, was studied for sludge treatment application w13,14x. The most important aspect of the process is its use of rapid heating in order to reduce the retention time to as short as 10 s while at a high conditioning temperature such as 2208C. The experimental results show that short retention time gives a better biodegradability w11x. A plant design was proposed to implement the RTC process w13x. In that design, however, a series of heat exchangers was used to raise the temperature of sludge. This would prolong retention time and would change the nature of conditioning from a rapid to slow process.
3. Laboratory testing 3.1. Rapid thermal conditioning A bench-scale continuous flow RTC apparatus, as described by McPartland et al. w12x, was designed and constructed ŽFig. 2.. Sludge is held in a storage tank and transported to the RTC reactor by a positive displacement pump, while high pressure steam is generated in a boiler. Steam and sludge are continuously injected into a high intensity mixing zone at the base of the first tubular reactor. Condensation of the steam in direct contact with the sludge provides extremely high heating rates. The mixture then passes through one, two, or three reactor sections in series having a residence time of 10 s each. RTC temperatures range from 100 to 2208C. The reaction is quenched when the
Fig. 2. Pilot unit for rapid thermal conditioning of sludge.
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mixture flashes through a pressure let-down valve to atmospheric pressure. After further cooling in a plate-and-frame heat exchanger, the product is collected in a holding tank. The pH adjustment, when used, is made in the storage tank prior to RTC. 3.2. Sludges studied Sludges from several New York City WPCPs were tested: PDS, WAS and primary sludge from the Wards Island Plant, and PDS from the Newtown Creek and the Rockaway Plants. Emphasis was placed on PDS. A total of 22 RTC test runs of were made over a range of temperature, retention time, and pH: 1. Thermal treatment temperature: room temperature, 100, 130, 160, 190, 2208C. 2. Retention time: 10 to 30 s. 3. pH: neutral, and 2 to 12. 3.3. Biodegradability studies The anaerobic biodegradability of thermally conditioned sludge was first tested using 160-ml serum bottles as batch reactors for periods exceeding 60 days. The results of these tests were used to select the best conditions for RTC. More extensive anaerobic digestion tests, using sludge treated at the optimal RTC conditions, were made in a 6.5 l continuous stirred tank reactor ŽCSTR. and a hybrid packed bed reactor. The packed bed reactor has an inside diameter of 11 cm and a height of 1.83 m, packed with 1.5 cm diameter pall-ring media. For the CSTR and packed bed tests, an optimum hydraulic retention time was selected. All tests were conducted in an environmental chamber maintained at 358C w15x. 3.4. Dewaterability studies The effect of RTC on the dewaterability of sludge was assessed in centrifuge and filtration tests. For the former, a Marathon 10 K bench-top centrifuge was used. Samples of sludge were centrifuged at 2500 = g for 15 min which approximates plant conditions. The solids content of the resultant cake and supernatant fluid were measured. Filterability was tested in both a low pressure filter press ŽBaroid. and a capillary suction time ŽCST. apparatus.
4. Test results and design criteria 4.1. Solubilization The solubilization of sludge resulting from RTC was monitored using the procedures given in ‘Standard Methods for the Examination of Water and Wastewater’ w16x. According to this method, solids are characterized as volatile suspended solids, volatile dissolved solids, fixed suspended solids, and fixed dissolved solids. The results for PDS treated at 2208C for 30 s are given in Fig. 3. RTC causes dissolution of suspended
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Fig. 3. Cumulative solids distribution in rapid thermally conditioned PDS Ž30 s HRT..
solids, predominantly depleting volatile solids and leaving the fixed solids nearly unchanged. Volatile solids are considered biodegradable and can be further reduced by digestion. For PDS, treated at 2208C for 30 s, approximately 47% of the total solids are solubilized, and in the tested range of solids concentration from 1.4 to 4.9%, the percentage of total solids solubilized is independent of their initial concentration. For WAS at 2208C for 30 s, approximately 55% of total solids are solubilized. 4.2. Dewatering Both filter press and CST tests show that RTC decreases filterability of PDS. However, RTC greatly improves the dewaterability of sludge in a bench-scale centrifuge operated at the same centrifugal force and retention time as used in the full-scale centrifuges of New York City’s WPCPs. Sludge cake with 25% solids is achieved without the addition of polymer conditioners, and the suspended solids recovery rate, in almost all cases, is above 95%. Even higher solids concentration can be expected in full scale commercial centrifuges. Based on these results, it is assumed in the economic analyses below that polymer addition is not required for dewatering of sludge following RTC. This provides major savings. As centrifugation is the only dewatering method used in New York City, RTC becomes highly attractive for deployment at these sites. 4.3. Biodegradation The RTC process increases sludge anaerobic digestibility giving higher yields of methane gas and leaving correspondingly less organics in the residue. RTC not only increases ultimate gas production, but also increases the rate of reaction especially during the first several days of digestion. For WAS in serum bottle tests, RTC at 2208C for 30 s results in a 200% increase in gas production during the first one to two days,
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compared to unconditioned sludge. Over a time period of 20 days, the average gas production improvement is 80%, a value which is maintained even after 60 days. Anaerobic packed bed tests show that 80% VDS in RTC PDS can be removed with 2 days HRT. The significance of this improvement is that shorter retention times can be used in anaerobic digestion w15x. 4.4. Effect of pH Both acid and alkali were added prior to thermal conditioning to test their effectiveness in the process. However, at high operating temperature, the effect of pH adjustment was very slight. Adjustment of pH requires additional equipment, increases the complexity of the process, results in corrosion problems and increases operating cost. Therefore, pH adjustment is not further considered. 4.5. Type of sludge Both WAS and PDS consist mainly of bacterial cells and multiple tests show increased biodegradability and dewaterability of these substrates. In contrast, primary sludges, which contain inorganic and non-living organic matter, show considerably less response to RTC. Consequently, thermal conditioning is considered only for WAS and PDS. 4.6. Odor Sludge itself has odor. However, it was noted during these tests that RTC did not increase the unpleasant odor of sludge. This is a significant improvement over traditional thermal conditioning processes. Evidently, the short retention time avoids formation of new odoriferous compounds.
5. Description of proposed flow sheets Based on the laboratory tests described above, PDS and WAS were selected for RTC at a temperature of 2208C with a retention time of 30 s, the pH being unmodified. Accordingly, two options were devised and are described below. Each option is designed for a sludge influent flow rate projected for the year 2005 in Wards Island WPCP. 5.1. RTC system The RTC system consists, in all cases, of a positive displacement pump, boiler, rapid thermal reactor and flash tank all operating under high temperature and pressure. A positive displacement pump is required to transport sludge into the thermal reactor against its operating pressure of 2500 kPa. A boiler providing saturated steam at 2208C is fueled by methane from digestion or an external source if additional fuel is needed.
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The streams of sludge and steam enter the reactor through a static mixer and reach the reaction temperature of 2208C nearly instantaneously. The liquid retention time in the reactor is 30 s. The maintenance of pressure is achieved by an automated control valve, and the suspension is discharged into a flash tank. Flashing of the treated suspension serves two objectives: First, it quenches thermal reactions. When the hot suspension enters the flash tank, partial vaporization of the water content occurs. The temperature instantaneously drops from 2208C to the saturation temperature of thermally treated sludge which will be about 1008C if the pressure in the flash tank is atmospheric. This sudden drop in temperature results in drastic decrease in reaction rate. A second objective is to separate the water from the sludge in order to reduce the load on both dewatering and digestion equipment which follow. About one fourth of the influent suspension is vaporized. Since vapor contains volatile organics, it is recycled to the aeration section of the plant. Except in the size of its various components, this RTC subsystem is the same in each proposed implementation alternative. The major difference between this design and any of the thermal conditioning processes proposed or used so far, such as Porteous or Zimpro, is that no heat exchanger is used to preheat the sludge flowing into the reactor. This is important to ensure short heating time and rapid thermal processing. There are, nevertheless, several ways to recover the heat from this RTC process: 1. Preheat the boiler feed water to 908C using heat exchangers. 2. Preheat sludge entering the digesters to 458C to compensate heat loss from this equipment. 3. Use the heat content of thermally conditioned sludge to compensate for heat loss from the digester used to treat RTC sludge. However, even with such heat recovery the RTC and digestion system would not be self-sufficient in thermal energy. The digester gas can provide less than 40% of the boiler fuel required for conditioning of PDS. The extra fuel cost alone would then make the RTC process more expensive than current processes to handle and dispose of sludge. To reduce process heat requirements, the proposed flow sheets incorporate thickening of sludge prior to the RTC step. This appears to be a practical and effective way to reduce the energy required without affecting the basic mechanism of solubilizing the volatile suspended solids. Centrifugation thickening is the preferred method because it has a high process capacity and a high suspended solids recovery rate. With polymer addition, the Humboldt centrifuges at the Wards Island WPCP are able to achieve a solids concentration of 30% for dewatered PDS, which is an upper limit for PDS thickening. In laboratory centrifuge tests without polymer addition, 10% is typical for solids concentration and can be considered the lower limit of PDS thickening. This substantial reduction in flow rate decreases the required capacity of the RTC reactor, pump, and digester for conditioning of PDS by about three-fourths, resulting in both lower capital and operating costs. Though a centrifuge thickening process is added, the total number of centrifuges does not necessarily increase beyond what is now used in the current sludge dewatering process. Thickening achieves a lower solids concentration in the cake than does the dewatering process. When a centrifuge is used for thickening, its capacity can be 50% higher than for dewatering. Therefore, as long as the thickening ratio is less than 1r3,
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rearrangement of the centrifuges currently in use may meet the requirements of both thickening and dewatering. Additional units of this costly item might not be required in the proposed process modification. 5.2. System A Table 1 shows the design basis, and Table 2 is a summary of major streams of System A. The flow sheet of System A is shown in Fig. 4. PDS is thickened first using centrifuges, to increase the solids concentration to 10%. The PDS centrate has low BOD and is of little importance for thermal conditioning and digestion. Therefore, it will be returned to the aeration tanks, as is the current practice at the Wards Island Plant. The mass flow rate of thickened PDS is reduced to 27% of that of the feed. ŽAlthough an experimental RTC test with thickened PDS of 10% solids was not done, it is assumed that the fraction solubilized, 47% on a dry basis, is unchanged by thickening.. There are two options for digestion of thermally conditioned sludge. One is that the entire mixture, both suspended and soluble solids, be subjected to anaerobic digestion in a completely mixed suspended growth reactor. The disadvantage of this method is that the hydraulic and solids retention time cannot be controlled independently, requiring a larger reactor. However, a larger volume of gas can be produced and more volatile solids can be destroyed since both soluble and remaining suspended solids will be subjected to biological degradation. This would take full advantage of thermal conditioning on the biodegradability of sludge. An alterative approach is to separate the soluble from the suspended solids and digest only the soluble organics. An attached growth reactor is more appropriate in this case, resulting in high efficiency and short hydraulic retention time. It requires a digester of small volume; an attractive feature in New York City plants where land is scare.
Table 1 Design basis for System A Parameter and units
Value 3
Flow rate of thickened sludge, m rd TS of thickened sludge, % by weight VS content of thickened sludge, % TS Temperature of thickened sludge,8C VS reduction by digestion, % VS Gas production, m3 rkg VS destroyed Lower heating value of digester gas, kJrm3 TS of thickened PDS, % by weight Digestion temperature,8C RTC temperature,8C RTC time, sec TS solubilization of PDS due to RTC, % TS VDS content, % DS SS recovery rate by centrifuge, % SS Cake solids concentration, % by weight VDS removal rate by anaerobic filter, % VDS
2949 4.1 79 15 40 1.06 22,400 10.0 35 220 30 47 80 96 30 50
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Table 2 Stream summary, System A Stream number
Flowrate ŽMgrday.
State
Temperature ŽC8.
TS Ž%.
DS Ž%.
1 3 5 9 10 12 14 16 18 19 20 30 31
2950 2912 80 354 1157 277 881 152 729 716 3101 42,200 m3 rday 14,400 m3 rday
liquidqsolids liquidqsolids liquidqsolids vapor liquidqsolids vapor liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids gas gas
15 35 35 220 220 100 100 69 45 35 35 35 35
4.10 2.90 10.00 0.00 6.94 0.00 9.12 30.00 4.78 3.01 0.83 0 0
0.15 0.19 0.18 0.00 3.26 0.00 4.29 3.00 4.55 2.50 0.71 0 0
However, the potential of an attached growth system for gas production may be lower than that for a suspended growth system. For this flowsheet, an up-flow anaerobic filter has been selected. In order to provide the filter with soluble feed, the suspended solids are separated and excluded using a dewatering centrifuge preceding digestion. This configuration has the advantage that the high temperature Žclose to 708C. of thermally conditioned PDS would reduce the viscosity of the sludge and improve settleability. As was done previously, the filter effluent is recycled to the aeration tanks. For the dewatering of thermally conditioned thickened PDS, the suspended solids concentration is as high as 6%. Higher concentration of solids in the centrifuge input
Fig. 4. System A, rapid thermal conditioning of thickened PDS.
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Table 3 Design basis for System B Parameter and units
Value
Flow rate of WAS, m3 rday TS of WAS, % by weight VS content of WAS, % TS Temperature of WAS,8C TS of thickened WAS, % by weight RTC temperature,8C RTC time, s TS solubilization of WAS due to RTC, % TS Flow rate of primary sludge, m3 rday TS of primary sludge, % by weight VS content of primary sludge, % TS Temperature of primary sludge,8C VS reduction digestion, % VS Cake solids concentration, % by weight Gas production, m3 rkg VS destroyed Lower heating value of digester gas, kJrm3 Digestion temperature, 8C SS recovery rate by centrifuge, % SS Cake solids concentration, % by weight
4011 2.47 83 15 7.1 220 30 55 500 6 62 15 66 30 1.06 22,400 35 96 30
results in a dewatered cake of higher solids concentration. This is an additional advantage of thickening. 5.3. System B Table 3 shows the design basis, and Table 4 is a summary of the major streams of System B. In System B ŽFig. 5., WAS with a solids concentration of 2.5% is thickened
Table 4 Stream summary, System B Stream number
Flowrate ŽMgrday.
State
Temperature ŽC8.
TS Ž%.
DS Ž%.
3 5 9 10 12 14 17 18 19 20 21 22 25 26
4011 1288 587 1875 423 1452 499 1951 1889 160 1729 4452 69,000 m3 rday 177,000 MJrday
liquidqsolids liquidqsolids vapor liquidqsolids vapor liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids liquidqsolids gas gas
15 15 220 220 100 100 69 45 35 35 35 23 35 35
2.47 7.12 0.00 4.89 0.00 6.31 6.00 6.23 3.15 30.00 0.67 0.43 0 0
0.10 0.10 0.00 2.69 0.00 3.47 0.20 2.63 0.55 0.41 0.56 0.28 0 0
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Fig. 5. System B, rapid thermal conditioning of thickened WAS.
by centrifugation to 7.1%. This lower level of thickening reflects the nature of WAS which is more difficult to thicken and dewater than any of the other sludges produced in a WPCP. Moreover, the thickening of WAS requires a different type of centrifuge from that used for dewatering of PDS, and these must be purchased. The centrate will be returned to the influent of the aeration process, and thickened WAS will be pumped to the RTC system. Thermally conditioned WAS is first cooled then mixed with thickened primary sludge. The mixed sludge is at 458C prior to being pumped into the digesters. The solids concentration of thickened primary sludge is assumed to be 6%. Thermal conditioning is limited to WAS because experiments with primary sludge showed negligible improvement in either biodegradability or dewaterability. Digested sludge is dewatered by centrifuge and the centrate returned to the head of aeration. Additional fuel of 177,000 MJrday, 13% of the total, is needed for this flow sheet. However, this requirement would be eliminated if WAS could be thickened to a solids concentration of 8.0%. Compared to current facilities at the Wards Island WPCP, the configuration of System B reduces the size of the gravity thickener by approximately 75% and the number of digesters by approximately forty percent. The digestion is done in one step, making the separate digesters for thermally conditioned PDS, as in System A, unnecessary. Two-thirds of the dewatering centrifuges would have to be replaced by thickening centrifuges specifically for WAS.
6. Economic analysis Existing equipment, which includes digesters, centrifuges, grinders, pumps, and piping, would be used in implementing the RTC system. Of these, digesters and
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centrifuges represent the most expensive items in the RTC-digestion system configuration. Their utilization offers significant saving in capital cost. Their configuration in the system may have to be changed depending on the operating alternative selected. In order to ensure the reliability of rotating machinery, such as pumps and centrifuges, standby units are purchased. Also, two boilers are used for each system. Economics analyses were performed with ASPEN PLUS 8.3-5. This software was used to size the equipment, and estimate cost. The costs of major equipment items, such as centrifuges and boilers were based on prices quoted by suppliers. The design bases of major equipment for Systems A and B are shown in Tables 5 and 6, respectively. The total cost of major new equipment items is estimated at US$2.31 MM for System A, and US$3.74 MM for System B. The itemized costs of the new equipment for each system are shown in Tables 7 and 8. Based on the cost of new equipment, the fixed capital costs, as estimated by using ASPEN PLUS, are US$11.89 MM for System A and US$18.03 MM for System B. As a rough check, it is reasonable for fixed capital cost to be approximately five times the cost of new equipment. Linear depreciation for a life time of 15 years is included in operating costs. Utilities include electricity, fuel, and chemicals to treat boiler water. Six additional operating personnel are estimated to be required for the operation of either System A or B. Operating costs ŽTable 9., ascribable to the additional RTC process equipment is US$2.49 MM per year for System A and US$4.03 MM per year for System B. These operating costs are to be compared with the costs of current Žunmodified. system operation.
Table 5 Major equipment for System A Equipment name
Size
Quantity
Positive displacement pump
7.2 lrs, 30 kW
2
Boiler
5900 kW
2
Reactor
0.4 m3
1
Flash tank Anaerobic filter
0.8 m=1.6 mb 1755 m3
1 1
Heat exchanger 1
45.7 m2
1
Heat exchanger 2
72.1 m2
1
Heat exchanger 3
55.9 m2
1
a b
One unit is standby. Diameter=height.
a
Design basis Flowrate, 4.6 lrs Pressure, 2500 kPa 2208C vapor at 4.08 kgrs Efficiency, 0.8 Retention time, 30 s Pressure, 2500 kPa Retention time, 1 min Retention time, 2 days Flowrate, 727 m3 rday COD, 15,000 mgrl Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 908C Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 458C Heat transfer coefficient, 450 Wrm2 K Hot stream inlet temperature, 708C Hot stream outlet temperature, 408C
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Table 6 Major equipment for System B Equipment name
Size
Quantity
Design basis
Thickening centrifuge Positive displacement pump
2000 m3 rday 7.2 lrs, 30 kW
3a 3a
Boiler
12,500 kW
2
Reactor
0.65 m3
1
Flash tank Heat exchanger 1
1.0 m=2.0 mb 96.7 m2
1 1
Heat exchanger 2
39.7 m2
1
4000 m3 rday Flowrate, 5.6 lrs Pressure, 2500 kPa 2208C vapor at 6.83 kgrs Efficiency, 0.8 Retention time, 30 s Pressure, 2500 kPa Retention time, 1 min Heat transfer coefficient, 450 Wrm2 K Cold stream inlet temperature, 158C Cold stream outlet temperature, 908C Heat transfer coefficient, 450 Wrm2 K Hot stream inlet temperature, 698C Hot stream outlet temperature, 538C
a b
One unit is standby. Diameter=height.
Currently in New York City, dewatered sludge cake with 70% moisture is transported and disposed of by several private companies through composting, land application and reclamation. Charges to New York City vary from US$101.4 to US$261.2 per wet metric ton of sludge, because of differences in subsequent treatment and disposal locations. In order to reach target dewatering levels at the Wards Island WPCP, present practice requires the addition of polymer at the rate of about 10 kgrdry metric ton. The price of this polymer is approximately US$4.4rkg.
Table 7 Purchased cost of major equipment for System A Equipment name
Quantity
Purchased cost
Digester Grinder Thickening centrifuge Positive displacement pump Boiler Reactor Flash tank Dewatering centrifuge Anaerobic filter Heat exchanger 1 Heat exchanger 2 Heat exchanger 3 Unlisted Total
8 2 2 2 2 1 1 1 1 1 1 1 ya ya
existing existing existing US$106,400 US$232,000 US$4700 US$5300 existing US$1,489,600 US$49,100 US$65,600 US$56,000 US$301,000 US$2,310,000
a
Not applicable.
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Table 8 Purchasing costs of major equipment for System B Equipment name
Quantity
Purchasing cost
Digester Grinder Thickening centrifuge Positive displacement pump Boiler Reactor Flash tank Dewatering centrifuge Heat exchanger 1 Heat exchanger 2 Unlisted Total
5 2 3 3 2 1 1 2 1 1 ya ya
existing existing US$2,568,000 US$160,500 US$402,600 US$4700 US$7400 existing US$78,600 US$31,300 US$488,100 US$3,741,200
a
Not applicable.
Savings resulting from the introduction of RTC into the WPCP include: 1. Reducing the amount of sludge cake requiring disposal, 2. Elimination of polymer cost, and 3. Elimination of polymer addition operating costs. If only the first two items are considered, which is conservative, a comparison of annual operating costs among the different processes results in the numbers in Table 9. Only the differences in operating costs resulting from implementation of the RTC processes are shown. For current practice, dewatering PDS and cake disposal, the operating costs are the charges for polymer and for sludge disposal. For plants incorporating the RTC process, the total operating cost will be the charges to handle the sludge cake plus an operating cost attributable to the RTC system. A charge of US$101.4rwet metric ton is the lower limit for solids cake disposal cost and US$261.2rwet metric ton the upper limit. The costs of sludge conditioning and sludge disposal for each system is shown in Table 10. From Table 10, System A has the lowest operating cost. One important reason is that System A makes full use of the existing facility. Thickening the sludge makes the scale of the RTC system small. The scheme does not merely make the plant fuel self-sufficient; it results in a surplus of digestion gas. Table 9 Annual operating cost summary System
A ŽUS$ryear.
B ŽUS$ryear.
Utilities cost Labor O&M supplies General work Laboratory charge Depreciation Annual operating cost
55,000 1,113,000 320,000 174,000 35,000 793,000 2,490,000
652,000 1,428,000 468,000 246,000 35,000 1,202,000 4,031,000
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Table 10 Comparison of sludge conditioning costs and cake disposal costs for existing and RTC modified processes System
Existing
A
B
Cake produced, dry ŽMgrday. Cake produced, wet ŽMgrday.
76.2 254.0
45.5 151.7
49.4 164.8
0
0
Polymer cost ŽUS$MMryear. Cake disposal cost minimum ŽUS$MMryear. maximum ŽUS$MMryear. RTC operating cost
9.40 24.22 0
5.61 14.46 2.49
6.10 15.71 4.03
Total operating cost minimum ŽUS$MMryear. maximum ŽUS$MMryear.
10.63 25.45
8.10 16.95
10.13 19.74
2.53 8.50
0.50 5.71
Savings due to RTC minimum ŽUS$MMryear. maximum ŽUS$MMryear. a
1.23
ya ya
Not applicable.
System B uses less existing equipment. Three of the eight existing digesters cannot be used. The reduction in operating cost due to the shrinkage of the existing digestion facility has not yet been taken into account. System A uses two-step digestion, i.e., digestion both before and after thermal conditioning. System B uses only one step plus thickening prior to digestion. Therefore, System B would have both the lowest capital and operating costs for the digestion part of the plant. System B requires a small amount of additional fuel because WAS, being difficult to thicken, is only dewatered to have a solids concentration of 7.1%, compared with thickened PDS with a solids concentration of 10%. Although System B is not the most economical option to incorporate into Wards Island WPCP, it will be a best option for new plant construction or expansion of the digestion capacity of an existing plant.
7. Conclusions Several important conclusions are drawn from this work on the RTC process. Ž1. The rapid thermal conditioning process is feasible technically. It is both an environmentally and economically beneficial way to handle the sludge. Ž2. Thickening sludge to raise the solids concentration prior to thermal conditioning is crucial to significantly reducing the cost of thermal conditioning and subsequent digestion. Ž3. Integration of System A into the Wards Island WPCP would reduce the cost of handling dewatered sludge by US$2.5 MM to US$8.5 MM per year, which is 24 to 36% of disposal costs by current methods.
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Ž4. For the design of a new WPCP or expansion of the digestion capacity of an existing plant, the configuration of System B is the preferred choice. Several options for further improvement of the RTC process may be suggested. Ž1. Steam produced in the flash tank of the RTC process could be used to dry dewatered sludge to further reduce disposal weight and to produce a sterile product. Ž2. The temperature of sludge digestion could be raised from the present mesophilic range, about 358C, to thermophilic range of 458C. The waste heat from thermal conditioning is sufficient to accomplish this without additional heat. The benefit of such shift would be a high reaction rate and, therefore, high solids destruction, high gas production, and short retention time. A higher digestion temperature will also reduce the size of the heat exchangers needed to cool the thermally conditioned sludge. Ž3. These process schemes for incorporating RTC into WPCPs have the potential for further modification to remove ammonia by making use of the hot streams generated in the RTC system. This will be addressed in a sequent paper. 8. Nomenclature The following abbreviations are used in this paper. BOD s Biological oxygen demand CSTRs Continuous stirred tank reactor CST s Capillary suction time CW s Cooling water DS s Dissolved solids HRT s Hydraulic retention time O & M s Operating and maintenance PDS s Partially digested sludge RTC s Rapid thermal conditioning RAS s Returned activated sludge SS s Suspended solids TS s Total solids VDS s Volatile dissolved solids VS s Volatile solids VSS s Volatile suspended solids WASs Waste activated sludge WPCPs Water pollution control plant Acknowledgements This work was carried out in the Departments of Chemical and Civil Engineering at The City College of the City University of New York with financial support from the New York State Energy Research and Development Authority ŽNo. 1353-ERER-MSW90.. Aspen Technology provided the design software. We thank Mr. Dick Wagner and the Alfa-Laval Thermal Co. for contributing the plate-and-frame heat exchanger used in this work.
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