WO2017219131A1 - Enhancing dewatering of biosolid slurries using an acid and lignocellulosic materials - Google Patents

Abstract

It is provided a process for dewatering a biosolid slurry comprising adding a lignocellulosic material with reduced particle sizes of about 0.5 to 10 mm to a biosolid combined slurry (CS), composed of a primary slurry (PS) and a secondary slurry (SS), blending the CS and the lignocellulosic material to produce an amended combined slurry (ACS), thickening the ACS; and dewatering the thickened ACS to produce a combined solids cake. An acid is applied either to the PS, SS, CS or ACS but preferably to the SS to achieve the full benefit.

Description

ENHANCING DEWATERING OF BIOSOLID SLURRIES USING AN

ACID AND LIGNOCELLULOSIC MATERIALS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims benefit of U. S. Provisional Application No. 62/353,874 filed June 23, 2016, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] It is provided a process for dewatering a biosolid slurry comprising adding a lignocellulosic material with reduced particle sizes of about 0.5 to 10 mm to a biosolid combined slurry (CS), composed of a primary slurry (PS) and a secondary slurry (SS), blending the CS and the lignocellulosic material to produce an amended combined slurry (ACS), thickening the ACS; and dewatering the thickened ACS to produce a combined solids cake. An acid is applied either to the PS, SS, CS or ACS but preferably to the SS to achieve the full benefit.

BACKGROUND

[0003] Biological processes are commonly used to treat municipal and industrial wastewaters for biochemical oxygen demand (BOD) removal. Typically, an aerobic biological treatment process oxidizes approximately half of the incoming BOD to carbon dioxide, water and other oxidized products while the remaining half converts to excess biological or secondary slurry (also known as wastewater treatment residue, waste activated sludge, biological or secondary sludge) which needs to be disposed. As the generated secondary slurry (SS) is typically at 0.5-3% consistency and occupies a huge volume, it is almost always dewatered before disposal or beneficial use to achieve substantial volume reduction. However, despite using latest dewatering equipment and optimal conditioning chemicals, poor dewatering efficiency (resulting in low solids content of the dewatered cake) continues to be an issue for many dewatering facilities. This is mainly due to the slimy and difficult to dewater nature of the SS.

[0004] The SS dewatering challenges are often partially addressed by combining it with a primary slurry (PS), also called primary clarifier rejects or primary sludge which, depending on its properties, acts as a sweetener in the process. This blend of SS and PS is commonly referred to as combined biosolid slurry or simply combined slurry (CS). This is specifically the case for the pulp and paper industry where the PS is known to contain wood fibre rejected from pulping and papermaking operations. A portion of the fibre discarded in the form of PS is in fact a good material for sale or to make saleable products. As such, motivated by strong financial incentives associated with fibre conservation, pulp and paper mills strive to minimize fiber loses to sewers for maximal product yield from the purchased raw materials. Unfortunately, this is not possible for many mills as they cannot dewater the SS to acceptable levels once a significant portion of the PS fibre is recovered for incorporation into the manufactured products. It is known that when the PS to SS ratio declines below a certain value, the existing dewatering equipment (such as a screw press) becomes very inefficient at dewatering the CS. For this reason, CS dewatering is typically viewed as a limiting step to increase fibre recovery at pulp and paper mills.

[0005] Pulp and paper mills generate primary and secondary slurries. SS is slimy and difficult to dewater whereas the PS is fibrous and relatively much easier to dewater. Typically, the two slurry streams are blended together (CS), conditioned with dewatering chemicals and dewatered using mechanical equipment. Encouraged by incentives associated with fibre recovery from waste lines, many pulp and paper mills are forced to dewater CS with low PS content and are thus facing dewatering challenges. In fact, below a certain PS to SS ratio, the existing dewatering equipment (such as a screw press) ceases to perform acceptably. For this reason, some mills even waste good fibre on purpose to facilitate dewatering resulting in high cost of operation.

[0006] Unlike pulp and paper mill PS, the municipal PS does not contain wood fibre in significant concentrations. This is often compensated by significantly increasing the amount of conditioning (dewatering) chemicals and an energy intensive dewatering technology such as a centrifuge. This increases the overall cost of managing the CS.

[0007] In current practice, the dewatering process is facilitated with a range of conditioning chemicals (inorganic chemicals and organic polymers) which modify particle surface chemistry and flocculate fine particles into compact agglomerates to help dewatering. The use of an optimized dewatering chemical program typically helps dewatering but the approach suffers from its associated high cost and inability to achieve a high enough degree of dewatering where the dewatered cake becomes a green fuel with a net positive heating value. [0008] In addition to optimizing the use of dewatering chemicals, other site-specific materials/approaches have been explored for dewatering cost reduction. Inorganic materials such as fly ash, hydrated lime, diatomaceous earth and cement kiln dust are much cheaper than conventional CS conditioning chemicals. The addition of these materials, as skeleton builders, has been found to enhance municipal sludge dewatering albeit compromising the calorific value of the dewatered mass and increasing the dewatered cake mass by 2.5 to 2.7 fold. These limitations would keep the use of these bulky inorganic dewatering aids from wide spread use in the industry.

[0009] The use of organic waste materials such as wood chips and wheat dregs has also been mentioned as dewatering aids. It has been reported that the addition of wood chips and wheat dregs to municipal SS improves its energy content and filtration properties (Ying-Feng et al. , 2001 , Bioresource Technology, 76(2), 161-163). Similarly, the addition of sawdust and hog fuel to the pulp and paper SS has been suggested to increase presscake solids and filtrate flowrate while decreasing suspended solids in the filtrate (Zhao et al. , 2000, Proceedings of the 86th PAPTAC Annual Meeting, Montreal, QC, Book B, p. 127-147). However, these findings were based on lab studies using Buchner filtration and a bench-scale press, respectively. These processes cannot be introduced to the commercial thickening/dewatering equipment. Such processes are known to block the screen holes or damage felt of a belt press and other components of the thickening/dewatering equipment.

[0010] U.S. 5,562,832 describes the use of multiple materials capable of absorbing water to enhance dewatered cake solids. These materials, including dried PS, dust and hot fly ash from boilers, were claimed to absorb water as well as modify the matrix of the material being dewatered. The patent describes a very complicated flow diagram using multiple screw presses, biosolid dryers, dust collectors and conveyors to feed hot fly ash to the biosolid mix tank, which is unlikely to be implemented at municipal or industrial biosolids management facilities.

[0011] U.S. 201 1/0084029 describes a method to dry biosolids to 50-70% solids content using blending (dewatering) agents having a porous structure. The blending agents included woody materials (e.g. , wood shavings, fine wood dust treated with urea formaldehyde, newsprint, milled peat, recycled fibre such as old corrugated cardboard and newsprint), dust collected during the machining of Medium Density Fibreboard (MDF), sander dust, trommel fines or particles collected via trommel screens, wheat, barley, oats, rice and straw. The patent describes a method to further dehydrate beyond the already dewatered cake dryness.

[0012] A serious limitation of the U.S. 201 1/0084029 is that it is based on a two- step approach, i.e. the biosolids are first dewatered using conventional equipment and procedures to a semi-solid state followed by their mixing with a relatively dry blending agent in the second round of dewatering/drying with a compression chamber to attain higher solids content.

[0013] There is thus still a need to be provided with a process for dewatering wastewater treatment SS and CS.

SUMMARY

[0014] It is provided a process for dewatering a combined slurry (CS) comprising providing a CS comprising a mixture of a primary slurry (PS) and a secondary slurry (SS); reducing a lignocellulosic material to a particle size of 0.5 to 10 mm; acidifying the CS; blending the acidified CS and the lignocellulosic material to produce an amended combined slurry (ACS); thickening the ACS; and dewatering the thickened ACS to produce a combined solids cake.

[0015] In an embodiment, the process described herein further comprises the step of blending a conditioning chemical to the ACS or to the acidified CS.

[0016] In another embodiment, the PS and the SS are mixed together directly with the lignocellulosic material forming the ACS.

[0017] In an embodiment, an acid agent is added to the PS, the SS or CS.

[0018] In a further embodiment, the acid agent is added to the SS. Alternatively, the acid agent is added to ACS.

[0019] In another embodiment, the biosolid slurry comprises 0.5-3% solids.

[0020] In an embodiment, the SS component of the CS is acidified to a pH as low as 2.5 with an acid. In another embodiment, the CS is at a pH between 3.0-3.5.

[0021] In another embodiment, the acid agent is sulphuric acid or a waste acid produced by a chlorine dioxide generator. [0022] In an additional embodiment, the lignocellulosic material is grounded, milled, crushed, refined and/or shredded.

[0023] In a further embodiment, the lignocellulosic material is reduced to a particle size of 0.5 to 10 mm by using a Wiley mill, a thermo-mechanical pulp refiner, in-line shredder, hammer mill, a slushing of newsprint or old corrugated cardboard.

[0024] In an embodiment, the lignocellulosic material is at an amount to dilute the CS at 5-40% mass ratio.

[0025] In another embodiment, the particle size of the lignocellulosic material is of 2 to 4 mm.

[0026] In another embodiment, the lignocellulosic material is hog fuel, wood chips, bark, knots, shives, demolition waste, recycled fibre, sawdust, wood dust, husk, straw, newsprint, old corrugated cardboard, wood, screening rejects or a combination thereof.

[0027] In an embodiment, the CS is from industrial and/or municipal wastewaters.

[0028] In another embodiment, the CS is from municipal PS or SS.

[0029] In an additional embodiment, the conditioning chemical is a proprietary dewatering formulation.

[0030] In an embodiment, the dewatered cake is 30-70% solid.

[0031] In another embodiment, the thickened ACS is dewatered with a screw press, a centrifuge, a vacuum filter or a belt press.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Reference will now be made to the accompanying drawings.

[0033] Fig. 1 illustrates a schematic representation of the effluent treatment and CS dewatering process in accordance to an embodiment.

[0034] Fig. 2 illustrates the effect of milled hog fuel on dewatered cake solids for Mill A (P: primary slurry; H: hog; S: secondary slurry, on dry weight basis).

[0035] Fig. 3 illustrates the effect of milled hog fuel on dewatered cake solids for Mill B (P: primary slurry; H: hog; S: secondary slurry on dry weight basis). [0036] Fig. 4 illustrates plugging of a rotary screen that uses woody residues as dewatering aid without controlling particle size and shape.

[0037] Fig. 5 illustrates the effect of acid and milled hog (lignocellulosic material) addition on CS dewatering efficiency.

[0038] Fig. 6 illustrates the effect of acid and milled or refined hog (lignocellulosic material) on the dewatering efficiency of CS with a fixed flocculant dose of 1 kg/t CS.

[0039] Fig. 7 illustrates the effect of acid and milled or refined hog (lignocellulosic material) on the dewatering efficiency of CS for all flocculant dosages tested. The ratios are reported as P:S: H on dry weight basis.

[0040] Fig. 8 illustrates the synergistic benefit with flocculant dose.

[0041] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0042] In accordance with the present disclosure, there is provided a process for dewatering a combined biosolid slurry (CS).

[0043] It is provided a process for dewatering a CS comprising adding a lignocellulosic material with reduced particle sizes of about 0.5 to 10 mm to a CS, blending the CS and the lignocellulosic material to produce an amended combined slurry (ACS), acidifying the ACS before thickening and dewatering the ACS to produce a combined biosolid cake. In an alternative, the acid can be applied separately to primary slurry (PS) or secondary slurry (SS) or to a blend of them before combining with the lignocellulosic material. Further preferably, the acid is applied separately to the SS before blending them with the PS or other materials.

[0044] The disclosed process makes use of milled or refined lignocellulosic materials such as hog fuel/bark, with the enhancement of acidification, to replace fibre in PS. This will allow the mills to recover saleable fibre from waste lines without compromising the performance of the dewatering equipment. Since majority of mills combust their dewatered cake, the lignocellulosic materials would eventually combust in the power boiler which is its intended use. The current process is applicable to the dewatering of CS from other industries and to the municipal sector in particular. The municipal primary and secondary slurries do not have fibrous materials present in any significant concentrations. So, the current process brings this new idea to the sector and would allow efficient dewatering of SS (where PS is not available) with help from lignocellulosic materials and an acid.

[0045] Physical and biotreatment of industrial and municipal wastewaters results in the generation of slurries at 0.5-3% consistency which need to be dewatered for volume reduction before disposal or useful applications. The present disclosure relates to mechanical dewatering of primary, secondary, deinking or combined slurries produced by industrial or municipal wastewater treatment plants. It is disclosed that the addition of processed (milled, refined, shredded, ground, powdered or crushed) lignocellulosic materials such as hog fuel, bark, demolition waste, fine wood residues could serve as sweetener in the SS or the CS dewatering processes used by industrial and municipal facilities. This provides an improvement over the existing art of dewatering and would result in reduced conditioning chemicals cost, enhanced dewatered SS or CS cake solids and increased calorific value of the dewatered cake converting dewatered SS or CS to fuel. Also, the process encompassed herein renders the less efficient commercial dewatering equipment efficient once again. Specifically, for the pulp and paper industry, the process described herein allows saleable fibre recovery from process waste lines.

[0046] Practical implications of using uncontrolled particles size at commercial dewatering facilities have not been appreciated before the present disclosure which is critically important to the use of the process described herein. Particles of uncontrolled size range, as reported in Ying-Feng et al. (2001 , Bioresource Technology, 76(2): 161— 163) and Zhao et al. (2000, Proceedings of the 86th PAPTAC Annual Meeting, Montreal, QC, Book B, p. 127-147), cannot be introduced to the commercial thickening/dewatering equipment. Such practice is known to block the screen holes or damage felt of a belt press and other components of the SS or CS thickening/dewatering equipment.

[0047] On the contrary, the current process is a single-step dewatering approach using the existing commercial equipment and starting with a SS or CS at 0.5-3% solids. Contrary to the process described herein, U. S. 201 1/0084029 ignores to take into account a crucially important dewatering parameter, i.e. plugging of the thickening/dewatering equipment screen holes or the filter felt. U.S. Patent 201 1/0084029 mentions that the coarser blending materials should be ground to particles size of less than 500 μηι which would be energy intensive and not recommended in the current process. The intent of the methodology described in U.S. 201 1/0084029 is to attain dewatered solids at very high solids content (50-70%). Attaining dewatered CS cake at 70% solids content is made possible in the process described herein with the use of a blending agent which is required to be dryer than the already dewatered semi-solid CS cake. It is simply not possible to dewater CS to such high solids content (approaching 70%) with mechanical dewatering means.

[0048] The present disclosure enhances the efficiency of existing CS dewatering plants. This is achieved by blending with low consistency (0.5-3% solids) CS a sweetener (mainly originating from cellulosic materials) refined to a certain size fraction. The sweetening material, which does not need to be drier than the CS, must be milled, refined or shredded to avoid plugging of the screen holes or felts of the dewatering equipment. Technologies for particle size reduction include Wiley mill, thermo-mechanical pulp refiner, in-line shredder, hammer mill and slushing of newsprint, and other lignocellulosic products).

[0049] The sweetener could be added to dilute CS at 5-40% mass ratio depending on the required degree of improvement. In addition to improved dewatering, the addition of the sweetening material increases the organic content and therefore the calorific value of the dewatered material. Thus, the process described herein renders CS with net negative energy value to a valuable green fuel. This will help divert CS away from a landfill.

[0050] The screw press technology, which is frequently used to dewater low consistency materials, is very sensitive to the nature of the material being dewatered. In the absence of a skeleton (network) building material, its performance declines sharply. The use of the process described herein allows the use of existing presses without compromising their performance.

[0051] Wastewater treatment generates large quantities of primary and secondary residue slurries at low consistencies of 0.5-3%. Because of their dilute nature, these dilute slurries occupy large volumes making it difficult to handle, manage and dispose them. This is resolved by mechanically dewatering them. However, in the state-of-the- art, the degree of dewatering achieved with the mechanical dewatering equipment remains below the value where the dewatered cake is considered a valuable product, an economically transportable fertilizer or a green fuel source for example. [0052] Generally, the primary and secondary slurries are blended together for dewatering in common practice. This is done to address dewatering challenges associated with the dewatering of SS due to its slimy and difficult to dewater nature. The process described herein uses processed lignocellulosic materials, as defined herein, to facilitate the dewatering process.

[0053] The municipal dewatering facilities predominantly use centrifugation and belt press technologies to dewater residues. Industries use site specific equipment to fulfill their dewatering needs. For example, the pulp and paper industry does not commonly use the centrifugation technology. The industry rather uses screw press and belt press technologies. When the thickened CS going to a screw press does not contain sufficient fibre, it ceases to dewater effectively. The same could be said about a belt press, to a lesser extent though. This will leave the industry with the option of either accepting the low dewatering efficiency or wasting good fibre on purpose to facilitate dewatering using the existing equipment. Another option is to replace the existing equipment with a more suitable one at a huge capital cost. The process described herein provides a solution to pulp and paper and other industrial sectors to continue using the existing equipment to achieve acceptable dewatering. It also helps municipal sector in dewatering wastewater treatment residues to higher cake solids with or without using PS. This would open up new avenues for the beneficial utilization of dewatered residues.

[0054] Fig. 1 provides a simplified flow diagram of the effluent treatment and CS dewatering processes encompassed herein. Mill effluent (1 ) enters the primary clarifier (101 ) where a large portion of the settleable solids sinks to the bottom and is removed as primary solids in the form of dilute PS (3). The clarified effluent (2) is directed to the aeration basin ( 102) where the organics are decomposed by mixed liquor suspended solids (MLSS) (4). The MLSS (4) in the aeration basin overflow into the secondary clarifier (103) which produces biotreated effluent (5) for discharge to receiving waters. The secondary or biological solids settle to the bottom of the clarifier and are removed from the clarifier as a dilute SS (6). A portion of the SS is returned to the aeration basin (7) for process activation while a portion is discarded from the process (8). The primary (3) and discarded secondary slurries (8) are combined in a blend tank (104) forming a combined slurry (CS).

[0055] A suitable source of lignocellulosic material (such as hog fuel, wood chips, bark, knots, shives, screening rejects, demolition waste, recycled fibre, wood dust, old corrugated cardboard, newsprint, husk or straw (10) is processed for size reduction (105) using an appropriate technology, such as a hammer mill, Wiley mill, shredder, woodgrinder or a pulp refiner to produce particles in the size range 0.5-10 mm (1 1 ). The processed lignocellulosic material (12a) is added to the blend tank (104) which also receives the conditioning chemical(s) (9). The ACS mixture (13) is thickened (106) for substantial volume reduction. The thickened ACS (15) is then dewatered using appropriate equipment ( 107) to produce combined solids cake (17) for beneficial use or disposal. In some cases it would be preferable to add the processed lignocellulosic material to the thickened CS (12b). The filtrate produced from thickening (14) and dewatering (16) operations is recycled to the aeration basin ( 102).

[0056] A suitable acidic stream or an acid is provided for acidification (18). Acid is applied either to PS (18a), SS (18b) or CS (18c) in the blend tank (104) which might contain the lignocellulosic material. The preferred point of application is the SS. The pH of the SS (8) would target between 2-4 while the CS or ACS (13) pH being around 5, or less in some cases.

[0057] Fig. 2 shows representative experimental data generated at FPInnovations using milled hog fuel to represent lignocellulosic materials, alone without acidification, as the dewatering aid. These data were generated using PS and SS from an integrated newsprint mill ( Mill A). The hog (H) fuel sample was hammer milled in the size range 2- 4 mm before use. The three materials (P, H and S) were blended in different ratios based on dry weight, conditioned with an organic polymer at different dosages and dewatered mechanically with a laboratory press. The dewatered cake solids data in Fig. 2 show clearly that the dewatering efficiency (shown by dewatered cake solids) improved dramatically as the processed hog was added to the CS containing equal proportions of PS and SS (40:40 and 50:50).

[0058] Fig. 2 also compares the sweetening effect of milled hog fuel with pulp and paper PS at a constant SS ratio of 50%. The blend which contained 10% milled hog fuel and 40% PS exhibited superior dewatering properties than the blend using 50% PS. This delineates clearly that hammer milled hog fuel performed superior to the pulp and paper PS. This will specifically allow pulp and paper mills (and other relevant industrial plants) to save useable fibre from wasting as well as enhance CS dewatering.

[0059] Some facilities use mechanical dewatering equipment that is not good to handle CS high in SS, such as a screw press. The dewatering efficiency of such equipment could be increased with the addition of suitable fibrous materials to the blend being dewatered along with acidification and a much smaller dose of the dewatering polymer, as stated in this invention.

[0060] The positive impact of using hog fuel without the benefit of acidification on dewatering was demonstrated, for another mill, using PS and SS from Mill B. The CS was dewatered at a challenging PS: SS ratio of 40:60 (no hog added). The blend was then modified by adding hammer milled hog to it. The resulting dewatering data in Fig. 3 delineate clearly that the hog addition dramatically improved dewatering. The PS and SS originated from the mill did not respond well to the dewatering polymer. However, the addition of milled hog substantially improved dewatering (Fig. 3). Although a particle size range 0.5-10 mm could be effective, the data in Fig. 3 were generated with hog particles 2-4 mm in size.

[0061] The data presented in Figs. 2 and 3 suggest that CS dewatering performance is enhanced and/or dewatering polymer consumption is reduced if a sufficient quantity of milled hog fuel is added to the blend tank. The use of conditioning polymer could be essential for most applications for highly effective dewatering even when using milled hog fuel as dewatering aid. One of the benefits of the process described herein is the dryer dewatered cake solids which improves its combustibility and heating value.

[0062] Hog fuel is one form of the lignocellulosic materials. Other lignocellulosic materials (e.g. , bark, wood chips, demolition waste, fine wood residues, screening rejects, re-slushed old corrugated cardboard and newsprint) when processed to ensure appropriate size range can also enhance sludge dewatering. The effectiveness of using hog as dewatering aid is evident at all polymer dosages (Figs. 2 and 3).

[0063] With reference to the pulp and paper industry, if the quality or the quantity of the produced PS is compromised (even eliminated) due to enhanced fibre recovery by a mill, the use of milled, crushed, powdered, ground or refined hog fuel, bark, wood or other lignocellulosic materials could sustain, or even enhance, the dewatering process.

[0064] Some dewatering plants have attempted to use wood waste (sawdust, fine wood chips) as dewatering aid. With some of these plants, the implementing sites ran into issues with respect to plugging of holes of the sludge thickening and dewatering equipment. Fig. 4 shows plugged holes of a rotary screen thickener with elongated wood particles (also called pin chips) contained in sawdust. This delineates clearly that the use of lignocellulosic materials with uncontrolled size distribution (especially elongated particles) is a grave concern towards the plugging of thickening/dewatering equipment. The operation of the dewatering facilities would not be sustainable without addressing this aspect. The current process addresses this by choosing particle size in the right range.

[0065] The full benefit of process described herein occurs when acidification is applied in addition to the introduction of lignocellulosic materials. Fig. 5 shows that when low pH (acidification) is used for dewatering, the CS and the milled lignocellulosic material (hog in this case) mixture (ACS) dewatered more efficiently. A great improvement in the dewatering efficiency of the P: H:S mixture can be seen in Fig. 5 when the second aspect of the process is applied.

[0066] When combining both acid treatment and milled hog addition technologies for the dewatering of wastewater CS synergistic effects are seen.

[0067] Fig. 6 shows that when a 50:50 mixture of PS and SS was combined with a dose of 1 kg/t of flocculant the final dewatered cake reached a solid consistency of 1 1.1 %. When the CS mixture is acidified the dewatered solids content increased to 14.2%. When a portion of the PS is replaced with milled hog, without acidification, a similar cake dryness was found (14.9%). If the dewatering enhancement from acidification and hog addition was merely additive the calculated value would be 18% (the light shaded bar in Fig. 6). However, the actual measured value when technologies were combined was found to be 27.1 % which represents a 50% synergistic benefit, which one would not expect.

[0068] Subsequent tests were performed using different flocculant dosages (Fig. 7). A linear relationship with flocculant dose was found with the CS alone (50:00:50; P: H:S) to 25.5% dryness at a 5 kg/t dose. The optimal polymer dose for the acidified CS was achieved at 3 kg/t resulting in a cake consistency of 29%. As well, a 30% dry cake was obtained with the substitution of the PS with milled hog (30:20:50; pH 3.5). In the absence of sludge acidification (scenarios 50:00:50 and 30:20:50) the dewatered cake consistency increased roughly linearly with increase in flocculant dose from 1 -5 kg/t CS with a marginal increase in cake consistency at lower floculant doses (1 -2 kg/t CS). On the other hand, the use of acid and milled hog resulted in optimal cake consistency at 3 kg/t CS and 2 kg/t CS for the scenarios with no hog (50:00:50 pH 3.5) and with 20% hog (30:20:50 pH 3.5), respectively. [0069] The synergistic benefit of simultaneously using acidification and hog addition at different polymer doses is calculated from data in Fig. 7, and plotted in Fig. 8. This plot (Fig. 8) shows clearly that the synergistic benefit of combining the two approaches simultaneously is much greater at low polymer dosages than that at higher polymer dosages. In detail, a nine percentage point improvement in the measured cake dryness, as compared to calculated additive value was found at the lowest flocculant dose of 1 kg/t. Accordingly, the process described herein allows saving cost while lowering the polymer dose required to achieve a higher (actually the highest) dewatering efficiency (at the lowest polymer dose of 1 kg/t used in Figure 7).

[0070] While the present disclosure has been described with particular reference to the illustrated embodiment, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative and not in a limiting sense. It will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations, including such departures from the present disclosure as come within known or customary practice within the art to which and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for dewatering a combined slurry (CS) comprising: providing a CS comprising a mixture of a primary slurry (PS) and a secondary slurry (SS); reducing a lignocellulosic material to a particle size of 0.5 to 10 mm; acidifying the CS; blending the acidified CS and the lignocellulosic material to produce an amended combined slurry (ACS); thickening the ACS; and dewatering the thickened ACS to produce a combined solids cake.

2. The process of claim 1 , further comprising the step of blending a conditioning chemical to the ACS or to the acidified CS.

3. The process of claim 1 or 2, wherein the PS and the SS are mixed together directly with the lignocellulosic material forming the ACS.

4. The process of any one of claims 1 -3, wherein an acid agent is added to the PS, the SS or CS.

5. The process of any one of claims 1 -4, wherein an acid agent is added to the SS.

6. The process of claim 4, wherein the acid agent is added to the ACS.

7. The process of any one of claims 1 -5, wherein the biosolid slurry comprises 0.5-3% solids.

8. The process of any one of claims 1 -6, wherein the SS component of the CS is acidified to a pH as low as 2.5.

9. The process of any one of claims 1 -6, wherein the CS is at a pH between 3.0-3.5.

10. The process of any one of claims 4-6, wherein the acid agent is sulphuric acid or a waste acid produced by a chlorine dioxide generator.

1 1. The process of any one of claims 1-10, wherein the lignocellulosic material is grounded, milled, crushed, refined and/or shredded.

12. The process of any one of claims 1-1 1 , wherein the lignocellulosic material is reduced to a particle size of 0.5 to 10 mm by using a Wiley mill, a thermo-mechanical pulp refiner, in-line shredder, hammer mill, a slushing of newsprint or old corrugated cardboard.

13. The process of any one of claims 1 -12, wherein the lignocellulosic material is at an amount to dilute the CS at 5-40% mass ratio.

14. The process of any one of claims 1 -13, wherein the particle size of the lignocellulosic material is of 2 to 4 mm.

15. The process of any one of claims 1 -14, wherein the lignocellulosic material is hog fuel, wood chips, bark, knots, shives, demolition waste, recycled fibre, sawdust, wood dust, husk, straw, newsprint, old corrugated cardboard, wood, screening rejects or a combination thereof.

16. The process of any one of claims 1 -15, wherein the CS is from industrial and/or municipal wastewaters.

17. The process of any one of claims 1 -15, wherein the CS is from municipal PS or SS.

18. The process of claim 2, wherein the conditioning chemical is a proprietary dewatering formulation.

19. The process of any one of claims 1 -18, wherein the dewatered cake is 30-70% solid.

20. The process of any one of claims 1 -19, wherein the thickened dewatered with a screw press, a centrifuge, a vacuum filter or a belt press.