Abstract:
Make-up water derived from an on-shore treatment facility is returned to a slurry processing unit on board a dredge by means of a water supply pipeline floating alongside a slurry delivery pipeline that conveys dredged material to the on-shore treatment facility. After separation from the slurry solids, the return water is pressurized to a transport pressure at the treatment facility with a centrifugal pump and then boosted to the desired working pressure once aboard the dredge. This two-stage pumping and re-cycling process reduces the amount of clear make-up water needed for proper operation of the slurry processing unit by more than 50%. This has resulted in a substantial reduction in the overall cost of remediation by reducing the amount of water that must be cleaned prior to disposal.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to systems for processing waste materials, and more particularly to a system for treating sludge dredged from a waterway with an appropriate amount of water to be pumped in slurry form through a floating pipeline. 
   Much effort has been directed to cleaning up toxic wastes in a drive to restore our natural environment. Some of such toxic waste is in the form of sediment or sludge that lies at the bottom of waterways. These sediments tend to concentrate heavy-metal toxins, halogenated hydrocarbons, pesticides and anaerobic bacteria. Periodically, the sludge is removed by dredging and then transported, either by barge or pumped in slurry form through a floating pipeline, to an upland shore facility for disposal. 
   Conventional dredging processes generally include three operations: digging, transportation and disposal of the sludge. These processes employ either suction (e.g. hopper or cutter-head) or mechanical (e.g. dipper, ladder or clam shell) digging techniques. The dredgers alone, or with the aid of barges, floating pipelines and conveyors, are able to transport the dredged material to an on-shore site. 
   In the case of upland disposal, the sludge may be transported via a floating pipeline as a watery pulp or slurry. The concentration of sludge solids is very low; the ratio by weight between the sludge and water is from about 1/7 to about 1/15, or one part of sludge solids to every 7 to 15 parts of water. At the disposal site, the sludge solids are separated from the water and the water is returned to the waterway as an effluent. Because some contaminated sludge remains in the returned water, pollution of the water environment is very likely. 
   The Hazardous and Solid Waste Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA), United States Public Law P.L. 98-616 and regulations written by the United States Environmental Protection Agency (EPA) include specific provisions restricting the direct land disposal of many hazardous wastes, including contaminated sludge and waste water. These restrictions, commonly referred to as the “Land Ban,” require that many types of waste including sludge and waste water be treated prior to land disposal to reduce the toxicity of the hazardous components. 
   The composition of such sludge is, of course, highly variable. When the sludge is transported via a floating pipeline, a large amount of make-up water must be added to produce a slurry with appropriate pumping characteristics. This make-up water is taken from the surrounding waterway and is further contaminated by treatment chemicals during decontamination and separation of sludge solids at an on-shore treatment facility. The decontamination facility may use chemical agents such as chlorine or sulphur dioxide, chemical coagulants and electrochemical flocculation techniques in various combinations to accomplish sludge decontamination. The process water generated by such chemical treatment must be further treated and made environmentally safe before land disposal or return to the waterway. 
   A slurry processing unit (SPU) as described in U.S. Pat. No. 5,269,635 was designed to deliver an optimum slurry characteristic to the deposition point of a confined disposal facility (CDF) and reduce the total amount of make-up water added to the slurry to the minimum while still maintaining a practical and feasible transportable mixture. 
   Reduction of make-up water added during the transportation phase was originally accomplished by managing the injection of make-up water received from the surrounding water body with an onboard monitoring and control computer. While this optimizes the relationship between the slurry&#39;s geotechnical characteristics, including its viscous properties, it still requires a substantial volume of clear make-up water that must eventually be filtered or treated chemically prior to its release back into the waterway. 
   BRIEF SUMMARY OF THE INVENTION 
   According to the present invention, make-up water for input to a slurry processing unit is supplied by re-circulating process treatment water separated from slurry treated at an on-shore (upland) confined disposal facility (CDF) back to a slurry processing unit (SPU) located on a dredging barge at a dredging site. 
   The slurry processing unit receives process treatment water returned directly from the on-shore CDF via a floating pipeline. The process water at the CDF is captured and pressurized to a level that will transport it back to the SPU on the dredge at an appropriate flow rate that is required by the SPU controller. The pressure of the make-up water pump located at the CDF for initial pressurization is kept at a minimum to reduce the risk of pipeline failure between the CDF and the dredge. 
   Once the return process water reaches the dredge, it is boosted to the level necessary to accommodate the make-up water needs of the SPU. The SPU utilizes the re-cycled process water as needed for transportation of the contaminated sludge sediments in a pumpable slurry to the on-shore CDF. The process water is circulated in a closed loop from the treatment facility to the dredge until the dredging of sediments is complete. 
   An essential step in the process of the present invention involves the re-circulation of the process water from the confined disposal facility (CDF) back to the slurry processing unit (SPU). In the operation of conventional systems, the make-up water used in the injection system for cutting the density and viscosity to the desired parameters was supplied by clear water pumped from the surrounding waterway into the dredge&#39;s sea chest in the barge. While the present invention optionally allows some make-up water to be sourced from overboard, the ability to re-cycle process water from the on-shore disposal facility to meet the make-up water needs of the SPU provides an improved solids-to-water ratio by avoiding the introduction of any new clear water from overboard during the slurry preparation phase. 
   Process treatment water is returned from the on-shore disposal facility to be used as make-up water on the dredge by a supply pipeline that is floated alongside the slurry delivery pipeline that transports slurry from the dredge to the disposal facility. The process treatment water is pressurized by a centrifugal pump located at the at the CDF and then boosted by a high pressure pump to the desired working pressure once aboard the dredge. Some of the process water is discharged as needed directly into the dredge hopper for initial sludge dilution and some of it is injected as make-up water into the slurry at one or more specific gravity measurement stations on board the dredge. 
   Such pump control together with multiple specific gravity measurements and make-up water injection between measurement stations provides an accurate indication of the amount of make-up water to be injected, even though the sludge solids are of varying composition. Inlet slurry flow, inlet make-up water flow, the slurry discharge flow, the make-up water pressure and the pressure in the slurry delivery pipeline can all be controlled using a primary fixed speed inlet pump, a booster pump for the make-up (process treatment) water and a variable speed discharge pump together with throttling valves in the make-up water injection pipes. This minimizes cost and avoids maintenance problems that would be caused by throttling valves in the slurry piping. 
   In the preferred embodiment, the slurry processing unit includes an inlet make-up water booster pump and an inlet slurry pump, with the inlet slurry pump being operated at a constant pumping rate, and with flow of the inlet slurry pump not being throttled; inlet water piping connecting the make-up water pipeline to the dredge hopper and to the inlet of the booster pump; a speed-controlled slurry discharge pump; slurry piping connecting the output of the inlet slurry pump to the input of the discharge pump; discharge piping connected between an outlet of the discharge pump and the floating pipeline; make-up water piping having an input connected to the output of the inlet make-up water pump, and having a make-up water injection pipe connected to an intermediate point of the slurry piping. 
   Generally, the invention is a closed loop slurry processing system for pumping varying compositions of slurry via a floating delivery pipeline to an on-shore treatment facility, a floating return pipeline connecting the on-shore treatment facility for supplying process treatment water as make-up water to the input of a slurry processing unit including an inlet make-up water booster pump and an inlet slurry pump; a discharge pump; slurry piping connecting the output of the inlet slurry pump to the input of the discharge pump; discharge piping connected between an outlet of the discharge pump and the floating slurry pipeline; and a low pressure pump coupled between the on-shore treatment facility and the return pipeline for supplying process treatment water that has been separated from the slurry solids to be used for initial sludge dilution in the dredge hopper and for specific gravity adjustment in one or more make-up water injection stations within the slurry processing unit. 
   This invention also provides a method for controlling a slurry processing unit for pumping varying compositions of slurry through a floating delivery pipeline, with the unit having slurry piping connecting the output of an inlet slurry pump to the input of a discharge pump and with discharge piping connected between an outlet of the discharge pump and the floating delivery pipeline. The method includes the steps: pumping slurry through a floating pipeline to an on-shore treatment facility; separating solids from the slurry; supplying make-up water derived from the on-shore treatment facility through a floating return pipeline to a booster pump, injecting make-up water from the booster pump into the slurry processing unit through make-up water piping and supplying make-up water delivered by the return pipeline into the dredge hopper for initial dilution of the sludge. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     This invention can be best understood by reference to the following drawing figures in which: 
       FIG. 1  shows a dredge, barge-mounted slurry processing unit, a floating delivery pipeline to an on-shore slurry separation/treatment facility, and a floating return pipeline for returning process treatment water to the slurry processing unit; 
       FIG. 2  is a plan view of a slurry processing unit mounted on a barge; 
       FIG. 3  shows the barge-mounted slurry processing unit in elevation; and, 
       FIG. 4  shows a schematic slurry piping and process flow diagram. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to FIG.  1 - FIG. 4 , a slurry processing unit  10  is mounted on a dredging barge  12  which transfers slurry through a floating delivery pipeline  14  to an on-shore treatment facility  16 . Toxic sludge  18 , for example creosote sludge, is dredged from the bottom of a river  20 . The toxic sludge  18  is loaded into a sump or hopper  22  by means of a power loader  24 . The hopper  22  is shown in FIG.  2  and FIG.  3 . Make-up water is derived from process treatment water W pumped from the on-shore facility  16  via a floating supply pipeline  25  into the hopper  22  and is mixed with the raw dredge material to produce a pumpable, raw slurry. 
   Referring now to  FIG. 2 , the slurry processing unit  10  includes an inlet slurry pump  26  and slurry piping  28 . Also shown is an inlet make-up water booster pump  30  and a slurry discharge pump  32 . The discharge pump  32  is speed controlled. The slurry piping  28  connects the output  34  of the inlet slurry pump  26  to the input  36  of the discharge pump  32 . In addition, discharge piping  38  connects between the outlet  40  of the discharge pump  32  and the floating delivery pipeline  14  (FIG.  1 ). Specific gravity sensor pairs  42 ,  44  and  46 ,  48  and  50 ,  52  are located at spaced locations along the slurry piping sections  28 A,  28 B and  28 C, respectively. The sensor units of each pair provide analog signals that are combined to produce a specific gravity value that is characteristic of the slurry flowing through the slurry piping section extending between the sensor units of each sensor pair. Also shown are flow sensors  54 ,  56  and  58  connected between the slurry piping sections.  FIG. 3  shows an elevation view of the pumping barge and the relationship between power loader  24  and the hopper  22 . 
     FIG. 4  shows the injection of make-up water W into the slurry piping  28  between the adjoining piping sections  28 A,  28 B and  28 B,  28 C and the terminal section  28 D. In particular, the make-up water piping has a manifold  60  with an inlet port  62  connected to the output  64  of the inlet make-up water pump  30  and has multiple make-up water outputs, including a first make-up water injection pipe  66  connected to the slurry piping section  28 B between the second and third specific gravity sensors  44 ,  46 , and a second make-up water injection pipe  68  connected to the slurry piping section  28 C between the third and fourth specific gravity sensors  48 ,  50 . 
   The first flow sensor  54  is connected in the slurry piping between the first piping section  28 A and second piping section  28 B, and between the second and third specific gravity sensors  44 ,  46 . The second flow sensor  56  is connected in the slurry piping between the fourth specific gravity sensor  48  and the fifth specific gravity sensor  50 . The third flow sensor  58  is connected in the terminal piping section  28 D between the sixth specific gravity sensor  52  and the inlet port  36  of the slurry discharge pump  32 . 
   A controller  70  receives analog specific gravity signals  72 ,  74 ,  76  from the specific gravity sensors  42 ,  44 ,  46 ,  48 ,  50  and  52 , respectively and analog flow signals  78 ,  80  and  82  from the flow sensors  54 ,  56  and  58 , respectively. A first controllable throttle valve  84  is connected in the first make-up water injection pipe  66 . A second controllable throttle valve  86  is in the second make-up water injection pipe  68 . An optional third controllable throttle valve  88  is connected in an optional third make-up water injection pipe  94 , with the third injection pipe  90 , if used, being connected to the terminal piping section  28 D between specific gravity sensor  52  and the discharge pump  32 . 
   The controller  70  receives signals from the specific gravity sensors and from the flow sensors and sends first, second and third throttle signals  92 ,  94  and  96  to the first, second and third controllable throttle valves  84 ,  86  and  88 , respectively. The controller  70  also sends a speed control signal  98  to a variable rpm drive  100  to control the output of the discharge pump  32 . 
   Initially, the combination of specific gravity sensor signals, flow sensor signals, and water injection provides rough specific gravity measurements. Then, after a first dilution and after a second dilution (each dilution with a known amount of make-up water), an accurate determination of appropriate total make-up water addition is made. It has further been found that by controlling the flow rate of the discharge pump  32 , that the pressure in the slurry processing unit  10  can be controlled. Since the slurry is not compressible, this pressure control is important in the accurate determination of appropriate total make-up water addition to be made. 
   The specific gravity sensor pairs  42 ,  44 ;  46 ,  48 ; and  50 ,  52  include pressure sensors located at first and second elevations in the substantially vertical sections  28 A,  28 B and  28 C of slurry piping. While the slurry processing unit preferably is constructed as described above, the make-up water piping may be operated without the controllable throttle valve and especially without the third controllable throttle valve  88  and the third make-up water injection pipe  90 . 
   An auxiliary make-up water line  102  connected in the main slurry inlet conduit  104  can also be used to introduce make-up water into the hopper  22  for initial dilution, or can be used to aid in flushing the system, with shutoff valve  106  only opened during cleaning. 
   It will be understood that the slurry processing system  10  can be controlled using a variable speed drive  100  to control the pumping rate of the output pump  32 . In particular, for example, the flow of the inlet slurry pump  26  can be controlled by throttling the make-up water injection and by speed controlling the discharge pump  32  even though the inlet slurry flow is not controlled or throttled. 
   Referring again to FIG.  1  and  FIG. 4 , the on-shore processing facility  16  is connected in a closed loop with the barge-mounted slurry processing unit  12  by the slurry delivery pipeline  14  and the make-up water return pipeline  25 . Pressurized slurry  108  is sprayed into a containment vessel  110  through a delivery manifold  112 . In the containment vessel, the sludge slurry is treated with chemical decontamination reagents C that are sprayed into the containment vessel through a spray nozzle assembly  114 . Additional treatment reagent/additives or any other desired ingredients may also be added through the spray nozzle assembly. 
   In the containment vessel, the sludge solids S are skimmed off from the main compartment of the vessel into an adjoining collector compartment and are introduced for treatment in the next stage. The aqueous phase which includes the make-up water and the treatment chemicals, designated W, is separated and removed through drain lines  116 ,  118  and input into a low pressure pump  120  that has an output connected to the make-up water recovery pipeline  25 . According to this arrangement, the make-up water is derived from the aqueous phase W after it has been separated from the chemically-treated sludge solids at the on-shore facility  16 . 
   The aqueous phase W from which the make-up water is derived is transported via the buoyant pipeline  25  to the inlet of the booster pump  30 , as shown in  FIG. 4 , where it is boosted to the pressure level necessary to accommodate the make-up water needs of the slurry processing unit  12 . Some of the process water W is injected as make-up water through the injection piping  66 , 68 , 90  into the pressurized slurry flow stream at one or more specific gravity measurement stations on board the dredge as previously described. Some of the returned process water W is discharged directly into the dredge hopper  22  through a flow line  122  for initial dilution of the sludge. The amount of dilution flow is regulated by the controller  70  via a throttle signal  124  and throttle valve  126 . 
   The closed loop recycling process described above has reduced the amount of clear make-up water added to the processing system by more than 50%. This has resulted in a substantial reduction in the overall cost of remediation by reducing the amount of treatment water that must be cleaned prior to its release from the contaminated dredging operation. 
   The invention is not to be construed as limited to the particular examples described herein, as these are to be regarded as illustrative rather than restrictive. The invention is intended to cover all embodiments that do not depart from the spirit and scope of the invention as defined by the appended claims.