AND AERATION

TECHNICAL FIELD

The present invention relates to the field of dewatering slurries and in particular, to new apparatuses and methods for increasing the overall efficiency of dewatering.

BACKGROUND OF THE INVENTION

Sludge is a semi-liquid residual material left from water and wastewater treatment processes, among others. This residual is highly charged with organic materials and can sometimes also include toxic products. It is therefore critical not to simply redirect the pollution that originally affected water to other media such as soil and air. As sludge volume increases with population and industrial activity growth, its treatment and disposal is a constant challenge for both public and privately-held wastewater treatment plants (WWTP). In a context of ever-tightening environmental regulations and budgets, WWTP operators need to find new and viable alternatives allowing for efficient water removal, treatment (when possible) and economical sludge disposal or reuse.

In most situations involving the production of large amounts of sludge such as at municipal or industrial wastewater treatment plants, significant efforts are directed toward removing water from the sludge. The main reason is that plants typically pay a weight/based disposal fee for getting rid of the residual and paying to move a substance containing essentially water does not make much sense (commonly in the order of 80-85% water remaining after conventional mechanical dewatering for waste activated sludge). Disposal fees are inversely proportional to landfill capacity and directly proportional to environmental consciousness. Furthermore, in some applications, if additional water can be removed, better quality sludge can be generated and or further treatment is becoming possible, allowing re-use in a sustainable residuals' program, instead of being landfilled. It is thus highly desirable to extract as much water as possible from the sludge. In a typical biological wastewater treatment process, after screening, degreasing and sand removal from the incoming flow, the wastewater is subjected to dissolved air mixing in treatment basins, allowing the fluids to incorporate oxygen, an essential element for proper function of the bacterial component of sludge. This form of aeration is commonly used in association with activated sludge treatment systems (not to be confused with the aeration-drying described hereinafter), and allow for the biological treatment of the fluid. Such fluid, containing inorganic and organic matter, is then allowed to settle. This "heavier" fraction, referred to as liquid sludge, is then removed for further processing.

Free water found in sludge can be relatively easily removed with the mechanical dewatering technologies identified above. Adsorbed water, on the other hand, cannot be effectively removed from sludge by mechanical means due to the strong chemical and physical affinity/binding of water molecules to bacteria and other sludge constituents/particles. Electro-dewatering (EDW) is a process by which water is removed from a substance using an electric current. It is also referred to as electro-osmosis. Electro-osmosis has been used to remove water from clay soils and is gaining wider interest for dewatering and increasing sludge dryness. Indeed, electro-dewatering has many advantages over aeration methods alone in terms of its ability to deliver increased dryness values with low energy expenditure as well as its ability to generate a sludge which satisfies more stringent criteria for reuse in landscaping or for agriculture land application. Applicants have successfully applied EDW to industrial and municipal waste water sludge and have developed several innovative advances in sludge dewatering technologies, apparatuses and processes. Such innovations are hereby incorporated by reference and include methods for electro-dewatering sludge using variable pressure (US Patent 7,828,953), apparatuses for rotary (US Patent No. 7,578,918) and linear electro-dewatering (US Patent Pub. No. 20100163428). Furthermore, in addition to and complementary with the current application, applicants are interested in increasing the efficiency of electro-dewatering by adding electro-dewatering agents (PCT publication No. WO/2010/067340). On the one hand, industrialization and urbanization lead to an ever increasing amount of wastewater sludge being generated. On the other hand, environmental pressures and increasing regulations leave far less alternatives for sludge disposal. It is therefore highly desirable to develop new methods and new apparatuses that increase sludge dewatering performance and/or improve its efficiency by lowering energy consumption and/or equipment footprint.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved methods and apparatuses for sludge dewatering, such methods and apparatuses would avoid the drawbacks of the prior art methods and apparatuses by allowing to reach high dryness levels with lower energy input, shorter duration and/or smaller footprint. It has been discovered that electro-dewatered sludge responds better to aeration than non-electro-dewatered sludge, wherein aeration can be, for example, any one or combination of thermal drying, solar drying (with mixing) and other drying techniques. An additional optional step of mixing the sludge has also been shown to increase efficiency of dewatering by increasing exposed surface area of the sludge. Although pellet type material and very dry matter (60% total solids or higher) cannot be viably achieved with electro-dewatering alone, applicants have discovered that aeration, performed after electro-dewatering, impacts favourably the design and efficiency of methods and apparatuses used. Aeration can be performed with heated or unheated air. Such a combination of methods and apparatuses would allow generating pellet type material and/or very dry matter (60% total solids or higher). Consequently, overall dewatering time, combined equipment footprint and/or energy usage is therefore significantly improved.

Applicants have discovered that without a prior electro-dewatering step, an aeration step cannot allow to reach levels of dryness comparable to those attained with electro-dewatering with the same efficiency (i.e. time and energy consumption). It is therefore the combination of electro-dewatering and aeration that provides sludge of high dryness according to the invention described herein.

It is an object of the present invention to provide a combined method comprising the step of electro-dewatering to remove water from a substance by electro-osmosis and another step of aeration, to further remove water from the sludge.

Applicants believe that the step of electro-dewatering (i.e. passing a current through layer of sludge causes a physico-chemical change in the sludge such that water molecules may be more easily removed afterwards, by aeration. Electro-osmosis of sludge could cause adsorbed water molecules to become much less tightly bound to sludge particulate (fibres, bacteria, etc). Consequently, size as well as general aeration method and apparatus design can be improved, when electro-dewatered sludge is used as a feed, in lieu of regular sludge.

Furthermore, with the proposed physico-chemical change in the sludge after electro-dewatering, applicants have discovered that a simple aeration technique, incorporating the passage of an airflow (forced or natural air, heated or unheated air) over and/or across the sludge as well as a mixing step can allow evaporation, draining or removal of significantly more water than if no electro-osmosis had been performed. This allows the residual sludge (biosolids) to reach high dryness levels both faster and with lower energy consumption than non-electro-dewatered sludge.

It is an object of the present invention to provide a method and apparatus which achieves high dryness levels without the need for heating means such as with high energy aeration techniques (e.g. thermal dryers, microwave dryers).

It is also an object of the present invention to provide a method of dewatering wastewater sludge by a combination of electric current treatment and simple aeration technique. Such an electric current can perform electro-osmosis dewatering but any other means to pass current through a substance should be understood as being included. The step of aeration can be performed in a closed or partially closed vessel. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by way of the following detailed description of embodiments of the invention with reference to the appended drawings, in which:

Figure 1 is schematic illustration of an apparatus used for increasing the dryness of a substance using a combination of a substance feeding device, an electro- dewatering device and an aeration device.

Figure 2 is a block diagram of a process combining the steps of electro-dewatering and aeration (with an optional step of mixing). Figure 3 is a graph of the absolute increase in sludge dryness using aeration on electro-dewatered and non-electro-dewatered sludge.

Figure 4 is a graph of the energy requirement to reach 90% sludge dryness using aeration (dryer) on electro-dewatered and non-electro-dewatered sludge. Figure 5 is graph of surface area requirements for solar drying in a greenhouse with and without electro-dewatering.

Figure 6 is a graph of sludge dryness as a function of drying time using negative pressure and non-heated air.

DETAILED DESCRIPTION Figure 1 is schematic illustration of an apparatus used for increasing the dryness of a substance using a combination of a substance feeding device, an electro- dewatering device and an aeration device. An optional mixing device (not shown) would be located after the electro-dewatering device and prior to the aeration device. The mixing device can be included with or separate from any one of these two other devices. The feeding device can be a feeding device according to US Pub. No. 20100129559 which is incorporated by reference or any device that will receive sludge and prepare it for the subsequent electro-dewatering process. The electro-dewatering device will receive "prepared" sludge from the feeding device. The electro-dewatering device can be a device according to US Pub. no. 20100163428 or any other device that causes an electric current to pass through sludge. Preferably, the electro-dewatering device will have at least two electrodes that can apply pressure to said sludge, thereby maintaining constant contact between the electrodes and the sludge, during the electro-dewatering process. An aeration device such as a fan dryer (used in the experiments shown in figure 3), a thermal dryer (used in the experiments shown in figure 4), a solar dryer (used in the experiments shown in figure 5), a negative pressure dryer (figure 6) or any other recognized dryer is used to remove additional water from the sludge, after the electro-dewatering process. Water is removed by allowing forced and/or natural air to flow into and or over the sludge, depending on the type of aeration method and apparatus used. Aeration can be performed with heated or unheated air, again depending of aeration method and apparatus used. Contact of the air with the sludge allows elimination of water molecules by evaporation. The aeration device can be used to introduce air into and/or over the sludge (see Fig. 2, phase 5) in order to increase its exposed surface area. A conveyor such as that included in the electro-dewatering step can be used. The conveyor belt can be of the vibrating dewatering screen type where air can be delivered perpendicularly to the conveyor and sludge. When the sludge is initially prepared (i.e. flattened) by the feeding apparatus, a good surface to volume area is achieved. For this reason, a system could be provided where the aeration is performed concomitantly with said electro-dewatering to minimize the apparatus footprint.

The aeration device could also be created by combining a rotary screen with a commercially available blowing fan. Alternatively, the conveyor belt of an electro- dewatering device can be prolonged such that the first part of the process is for electro-dewatering sludge and the second part of the process is for aerating sludge. Conveyor belts are well adapted and provide a good surface for allowing aeration.

Alternatively, to perform the aeration step, an endless screw, endless screw conveyor or screw press can be provided where aeration (i.e. airflow) follows, or not, the direction of the sludge. It would be advantageous to provide a method or apparatus where electro-dewatered sludge is contained in a moving vessel where aeration allows reaching high sludge dryness levels. The vessel can be a perforated vessel such that air can pass through the vessel.

The aeration step allows removal of the moisture. Aeration is ideally performed with dry air at a temperature that can be above ambient temperature. Air is transferred to a closed or semi-closed vessel where the electro-dewatered sludge resides.

Contact of the air with the sludge allows elimination of « freed » water molecules by evaporation. Ideally, sludge is shredded beforehand to increase exposed surface area of the sludge. Vapours and gases produced during this process can be captured by suction or negative pressures and directed to a treatment system.

The process of the present invention can function as a continuous or batch process. Upon initiation of the process, it will be apparent to those skilled in the art that sludge can be multiply electro-dewatered and that electro-dewatered sludge can be multiply aerated or aerated for longer duration to further increase dryness. Certain sludge types that can be effectively dewatered according to the present invention comprise organic and inorganic sludge such as colloidal, pulp and paper and agro-alimentary sludge, chemical or biological sludge, dairy industry and slaughterhouse sludge, pig manure and of course waste water treatment sludge, sewage sludge, undigested sludge, aerobic and/or anaerobic sludge. It will be appreciated by those skilled in the art that the present invention allows to increase the dryness of any porous substance or slurry.

Figure 2 is a block diagram of a process combining the steps of electro-dewatering and aeration and illustrates one possible embodiment of the process which comprises 4 steps from the sludge inlet arriving through a WAS duct (waste activated sludge) through to a sludge outlet consisting of a container/bin. The first step comprises a standard thickening step found in many wastewater treatment plants. This step concentrates the sludge approximately two to four fold by decantation and flocculent addition. The second step consists of increasing the dryness of sludge by standard mechanical techniques such as centrifugation, filter- press, belt-press or any other mechanical dewatering equipment. This step allows bringing sludge to a dryness of approximately 12-24%. Mechanical dewatering means cannot increase the dryness of many sludge types significantly above 24% due to the fact that the remaining water is relatively tightly adsorbed to the sludge particles. Phase 3 is the electro-dewatering step where electric current passing through the sludge helps convert bound water to free water. Electro-dewatering has been shown by the applicant to increase the dryness of sludge up to approximately 50% dryness. It will be appreciated that the mechanical dewatering step (phase 2) can be advantageously exploited when available but the apparatus of the present invention can allow to completely avoid phase 2 and receive sludge from the thickening phase 1. Upon exiting the electro-dewatering step and depending on the electro-dewatering parameters, sludge temperatures can be between 80 and 120 Celsius. Hot electro-dewatered sludge is then subjected to phase 4 comprising mixing (in this case shredding) the sludge to increase its exposed surface area. The Mixing phase is not essential but helps increase the efficiency of the subsequent aeration step. In some embodiments, the mixing and aeration devices are combined into one device. For example, a rotating drum with inner blades and fan can embody phases 4 and 5 of figure 2. In phase 5, characterized by aeration encompassed into thermal drying (direct or indirect) and other forms of drying such as natural convection drying (drying beds), solar drying and microwave drying allows to remove additional water and achieve a 50-90%+ dryness. As electro- dewatering is a relatively fast and/or much more energy efficient process than most well known aeration methods (at least up to approximately 50% dryness), the design of the aeration apparatus is favourably impacted for a given final dryness sought. As a consequence, the overall efficiency and/or speed of the dewatering, from liquid sludge to final disposal or reuse, as well as the overall footprint of the combined electro-dewatering and aeration solution can be optimized.

The most effective aeration conditions make use of hot air blown over shredded hot sludge in a closed environment. However, ambient temperature air blown over hot sludge is also very effective for the dewatering process. Furthermore, at the other energy consumption end of the spectra, room (ambient) temperature air over room (ambient) temperature sludge in an open environment also helps the dewatering process, but to a lesser degree than the above examples. Phase 5 has been shown by applicants to provide sludge with a dryness of 90% or more. Achieving such dryness levels in a short period could only be envisioned using thermal dryers or incineration. Additionally, non electro-dewatered sludge would never reach such dryness levels because bound water will not easily evaporate.

Figure 3 is a graph showing that, on some sludge types, a significant advantage can be obtained by carrying out electro-dewatering before an aeration step. It can be seen in this figure that dryness of electro-dewatered sludge increased by 32% in 6 hours of aeration whereas the dryness of the same amount (per total weight of sludge) of non-electro-dewatered sludge increased by only 1 1 %.

Figure 4 is a graph of total energy consumed as a function of overall sludge dryness for aeration of electro-dewatered (568 kWh/wet ton of water extracted) as compared to non-electro-dewatered (798 kWh/wet ton of water extracted) sludge. Applicants assessed and compared the energy usage required to reach a given final dryness (90%) from a given initial dryness (13%) using a method and apparatus relying i) solely on aeration and ii) on a combination of electro-dewatering and aeration. Experimental results obtained show improved energy efficiency of the latter in achieving a given final sludge dryness

Figure 5 is a graph showing the surface area required for solar drying a pre- determined amount of electro-dewatered and non-electro-dewatered sludge. It can be concluded that total "equipment footprint", including the electro-dewatering apparatus and the greenhouse for solar drying, is significantly smaller for the electro-dewatered (788 m ² ) vs. the non-electro-dewatered (1250 m ² ) processes. It will be appreciated that the solar drying technique included an aeration component as well as a mixing component. The aeration component consisted of fans (commercially available industrial ventilators) and the mixing was performed by an automated swath turning device which mixed the substance on a regular basis.

Figure 6 is a graph showing sludge dryness as a function of drying time for an experiment lasting 13 hours. Applicants simulated room temperature air drying (with negative pressure) using a chemical fume hood device. The electro-dewatering was performed at lab scale, and the electro-dewatered sludge was then placed inside the fume hood with the draft of air (24°C) going over the sample as a source of aeration. The front air intake (slideable window) of the hood was partially closed so as to limit the air intake while maintaining evacuated air levels from inside the hood, thus creating a negative pressure. Several measures of dryness were taken through time. This trial confirms the feasibility of installing an ambient temperature (non- heated) sludge aeration device after electro-dewatering as dryness levels increased from -45% to -90% in a 13 hour period.

It will also be appreciated that the process and apparatus of the present invention can improve residuals quality. Residuals are understood as being treated/ dewatered sludge. Sludge, residuals and biosolids are used interchangeably in this document but it is understood that biosolids are typically obtained after treating/dewatering sewage sludge, for example. Using methods and apparatuses of the present invention, it is possible to provide a sanitized sludge with a high dryness level. Reuse of such "sanitized and concentrated" sludge in agriculture would have many advantages, namely to decrease transportation costs and increase its potential market for reuse. Indeed, electro-dewatering the sludge according to the present invention can generate Class A sludge, including Vector attraction reduction, for use in agriculture. Interestingly, the process of the present invention allows to reach dryness levels of over 90% solids, which, in addition to favouring an important reduction in bacteria, virus, ova and other micro-organisms, allows to reduce Vector attraction of the resulting sludge/biosolids/residuals, as described in Chapter 8 of Control of Pathogens and Vector Attraction in Sewage Sludge. Environmental Regulations and technology. United States Environmental Protection Agency. EPA/625/R-92/013. Revised July 2003.

The process of the present invention can function as a continuous or batch process. Upon initiation of the process, it will be apparent to those skilled in the art that sludge can be multiply electro-dewatered and that electro-dewatered sludge can be multiply aerated. Final dryness levels required can determine the duration of aeration treatment. Electro-dewatering has been shown by Applicants to provide a bactericide effect on the sludge.

Various substances can benefit from dewatering according to the present invention. Amongst these substances, organic and inorganic sludge such as colloidal, pulp and paper and agro-alimentary sludge, chemical or biological sludge, dairy industry and slaughterhouse sludge, pig manure and of course waste water treatment sludge, sewage sludge, undigested sludge, aerobic and/or anaerobic sludge can be advantageously dewatered. It will nevertheless be appreciated that the present invention allows to increase the dryness of any porous substance or slurry.

The aeration step or device can, if performed in a closed area, comprise an airflow of 100 to 2000 CFM (cubic feet per minute) but -1000 CFM. If heated air is used, heat can be recovered from the inside of a housing of the electro-dewatering rectifiers or the air can be heated by an electrical resistance or any other means for heating air.

Upon initiation of aeration of the electro-dewatered sludge, air temperature can be between room temperature and 120 Celsius. A sludge mixing device can be any device such as a shredder or rotary shredder that cuts sludge into smaller pieces of the desired size, preferably a small size in order to maximise the exposed surface area.

Aeration should be understood, in the present invention, as comprising any means that will help introduce air into a substance to favour evaporation processes and ensuring removal of "moist air". Mixing sludge with a mixer, such a swath mixer for example, will help introduce air into the sludge. Any means to introduce air into the sludge or to increase the exposed surface area hastens the evaporation process. Flattening the sludge using a feeding apparatus for electro-dewatering will also contribute to increasing the exposed surface area to volume ratio. It will also be understood by those skilled in the art that the combination of aeration and mixing, which both contribute positively to the evaporation process, are additive in nature. Although aeration and mixing can independently (but to different degrees) contribute to high sludge dryness levels according to the present invention, their combination offers the quickest method to achieve a desired dryness level. A ventilated rotary drum comprising inner blades for mixing (and shredding) is one possible embodiment that will efficiently combine aeration and mixing.

It will be appreciated by those skilled in the art that mixing includes moving, shredding, shaking, rolling, vibrating, stirring, blending, and flattening. Indeed, anything that creates movement of the sludge will allow increase the efficiency of dewatering after electro-osmosis. Aeration should be understood as comprising blowing wind over an immobile substance as well as circulating air through a substance as it is being mixed. Circulating air through a substance can be efficiently achieved while moving the substance such as while the substance is being transported on a vehicle from a first location to a second location.

The term aeration should be understood as including air drying, passing air over sludge as well as circulating air through sludge as it is mechanically mixed. The air can be heated air or non-heated air. A fan dryer should be understood as standard ventilator that can push ambient temperature air in a predetermined direction