Method of treating waste-activated sludge using electroporation

This is a continuation of application Ser. No. 09/612,776, filed Jul. 10, 2000, which is a continuation-in-part application of application Ser. No. 09/468,427, filed on Dec. 21, 1999, which is incorporated by reference herein in its entirety, which is a continuation of application Ser. No. 09/229,279, filed on Jan. 13, 1999, now U.S. Pat. No. 6,030,538, which is a continuation-in-part of application Ser. No. 08/934,548, filed on Sep. 22, 1997, now U.S. Pat. No. 5,893,979, which is a continuation-in-part of application Ser. No. 08/552,226, filed on Nov. 2, 1995, now U.S. Pat. No. 5,695,650.

In parent application Ser. No. 09/468,427, there is disclosed a system and method for dewatering and treating waste-activated sludge (WAS) emanating from municipal waste, or pulp-waste from a paper mill, as well as treating animal and plant waste. In that application, the method for breaking down the WAS is to subject the WAS to electroporation, which incorporates nonarcing, cyclical high voltages in the range of between 15 KV and 100 KV, which break down inter-cellular and intracellular molecular bonds, to thus release inter-cellular and intracellular water, whereby the WAS is rendered inactive and greatly reduced in mass.

In said above-noted copending application, the apparatus and method disclosed therein, while capable in certain circumstances of being a primary municipal-sludge treatment, its intended and main objective was to use it as a secondary treatment to previously-dewatered municipal waste sludge. It is the goal of the present invention to adapt the method and apparatus of said copending application serial No. 09/468,427 into a main, primary treatment of municipal waste sludge.

In a previous (Phase I) project, it has been demonstrated the laboratory feasibility of pulsed electric field (PEF) for disrupting the biomass in waste activated sludge (WAS) derived from municipal wastewater treatment. While there was no significant increase in the solids content of dewatered sludge, the quantity of WAS needing disposal was estimated to be significantly reduced.

Encouraged by the Phase I results, a pilot plant for testing at one or two wastewater treatment plants that generate WAS has been developed. It has been decided that a pulsed electric field (PEF) system that could handle 0.5 to 1.0 pgm WAS feed be designed. This requires an 8 kw power supply capable of generating 30 kV and pulse generator capable of handling 50 amp peak, current, bi-polar pulses, square wave, 10 μs pulse width, and 3000 pulses/second (pps).

It is the primary objective of the present invention to provide a method and apparatus for dewatering municipal waste sludge, paper-pulp waste sludge, animal and plant waste, using electroporation for the primary treatment of the sludge.

It is also a primary objective of the present to provide such a system that will allow flexibility as to the primary and secondary treatment of municipal sludge, paper-pulp sludge, animal and plant waste, whereby the treatment thereof via electroporation may be used either as the primary dewatering treatment, secondary dewatering treatment, direct WAS-treatment, and combinations with other conventional dewatering techniques, in order to provide the municipal treatment plant, or the paper-pulp treatment plant, with the most cost-effective and efficient system as possible.

The invention will be more readily understood with reference to be accompanying drawings, wherein:

FIG. 1 is a schematic showing the electroporation system as used as a secondary dewatering treatment;

FIG. 2 is a schematic showing the electroporation system used in conjunction as a primary dewatering treatment in accordance with the present invention;

FIG. 3 is a schematic showing the electroporation sub-system for use in dewatering municipal, paper-pulp, animal and plant waste sludges; and;

FIG. 4 is a schematic diagram showing the overall apparatus of the present invention incorporating the electroporation sub-system for use as a primary or secondary dewatering treatment;

The original concept for the pulsed-electric field (PEF) effect using electroporation was to dewater the previously-dewatered sludge. However, additional PEF data on a paper plant sludge has indicated that the big PEF effect from electroporation of WAS occurs at higher energy levels (e.g., 100 J/mL; or 400 k Wh/ton (DS) for feed at 6 percent solids), whereby cells are disrupted. The result is inactivation of cells, breakage of cells and release of some intracellular dissolved/organic matter and typically a worsening of flocculation and dewatering. Therefore, a more effective way of using this process is to recycle all of the PEF-treated sludge back to a aerobic bioreactor to utilize the sludge as food; that is, it has been discovered that the PEF-electroporation effect on disrupting the cellular units of the WAS has been to release intracellular dissolved/organic matter. This intracellular dissolved/organic matter is just the type of ideal “food” upon which the aerobic bioreactor flourishes. Thus, returning this released intracellular dissolved/organic matter back to the aerobic bioreactor will increase the BOD load on the bioreactor, and will thus reduce the quantity of WAS by up to about 50 percent. The flowsheet for this scenario is shown in FIG. 2 . Thus, it is now practical to employ the PEF-electroporation system as not only a secondary system for treating previously-dewatered sludge, but also to employ it as a primary system, as described hereinbelow.

Referring to FIG. 1, there is shown the schematic for using the PEF-electroporation system as a secondary treatment for previously-dewatered sludge, as disclosed in Applicant's copending application serial No. 09/468,427. In FIG. 1, the wastewater is delivered to the primary treatment, aerobic-reactor tanks 10 , and to a secondary clarifier 12 . From there, the WAS is delivered to the PEF-electroporation system 14 of the invention for deactivating the WAS to make it a Class “B” biomass for easier disposal. The biomass is then sent to a belt press 16 for further processing and disposal.

Referring now to FIG. 2, there is shown the flow chart of the present invention for employing the PEF-electroporation system as part of the primary treatment. In this system, the biosolids exiting from the PEF-electroporation system 14 are returned to the aeration tanks 10 , since, as explained above, the PEF process causes the release of intracellular, dissolved organic matter, which is used as “food” for the bacteria of the aeration tanks. This “food” not only is further treated in the aeration tanks via aerobic digestion, but actually causes the aerobic digestion process in the aerobic tank itself to be accelerated for the same amount of oxygen supplied.

A practical problem with the system of FIG. 2 is that the PEF throughput needs to be of the same order of magnitude as the WAS disposal rate in order to see a noticeable effect of PEF on WAS reduction. For this reason a 1.8 ton (DS)/day PEF system has been chosen as a pilot plant. With such a system, a WAS reduction of 0.9 ton/day on a dry basis or 7.5 tons/day on a filter press cake (at 12 percent solids) basis may be achieved. In terms of thickened sludge (at 2 percent solids) basis, this translate to elimination of 45 tons/day needing to be flocculated and dewatered. This will require PEF treatment of 15 gpm WAS at 2 percent solids.

One way to reduce the cost of the pilot plant, which is driven by the PEF power supply and pulser cost, is to pre-thicken the WAS. Therefore, a 15 gpm rental centrifuge 18 is used for pilot testing. It is estimated that this will produce a 5 gpm feed for the PEF reactor at a solids content of 6 percent. Such a feed can be handled by a Moyno pump. The feed streams to the centrifuge and the PEF units are represented as Stream Nos. 10 and 11 , respectively in FIG. 2 . However, in practical application such as centrifuge may not be necessary.

PEF Power Supply and Pulser Design

The conceptual design of the power supply and the pulse generator (pulser) for the system of FIG. 2 is shown in FIG. 3 . This figure shows four chambers 20 in series, although two chambers also can be used if the pulse rate is increased. The specifications for the two-chamber design are shown in Table 1. The design requires a 35 kW input power supply 22 (32 kW continuous output) delivering 30 kV. The pulse generator 24 is 200 amp maximum current and a pulse rate of 4,000 hz. (maximum).

The actual sludge handling system and the associated instrumentation is shown in FIG. 4. A detailed list of specifications is provided in Table 2. Tank T 1 holds up to 100 gallons of untreated feed material, delivered through valve V 1 from the centrifuge. A mixer is provided for blending infeed material. A bottom drain allows disposal to sewer at the end of a test run. Valve V 4 is provided for withdrawing a sample for analysis. Material leaves T 1 through V 2 and a strainer to a variable-speed progressing cavity pump, which can flow from 0.5 to 5.0 gallons per minute. The tank, pump mixer and associated valves are mounted to one 42-inch square skid for transport purposes. The feed leaving P 1 passes through quick-connect fittings to a reinforced hose to the reactor.

The PEF-electroporation reactor subsystem includes a power supply, pulse generator and pairs of treatment chambers as described above with reference to FIG. 3 . These would be mounted to a skid, along with associated valves V 5 , 6 and 7 . Quick-connect fittings and hose convey the treated material to valves on the outlet tank skid. Valves V 12 and 13 permit the treated material to be recycled back to T 1 . Valve V 8 permits the treated material to enter tank T 2 , of 100-gallon capacity. As with T 1 , a mixer, a sample port and a bottom drain are provided. Tank Tank T 2 , pump P 2 , mixer M 2 and associated valves are mounted to another skid. Treated material leaving through V 10 leads to transfer pump P 2 . Valve V 15 is a globe style for adjusting the flow rate through V 14 to tank T 1 . Valve V 13 allows treated material from T 2 to return to T 1 , assisted by P 2 , to increase treatment time.

The P 2 pump is used to return the treated sludge to the biotreatment plant, aerobic tanks, when the PEF-electroporation system is used as a primary system, or optionally to filter press, if desired, when the PEF-electroporation system is used as a secondary treatment.

Safety logic has been incorporated as follows. Level control L 1 will close V 1 to prevent overfilling T 1 , with subsequent spillage. Level control L 2 will shut down P 1 and the power supply when the liquid level becomes too low. Level control L 3 will shut down P 1 and the power supply when tank T 2 becomes full, to prevent spillage.

While a specific embodiment of the invention has been shown and described, it is to be understood that numerous changes and modifications may be made therein without departing from the scope and spirit of the invention as set forth in the appended claims.