Molecular disruption through acid injection into waste activated sludge – A feasibility study to improve the economics of sludge dewatering

Journal of Cleaner Production xxx (2017) 1e10

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Molecular disruption through acid injection into waste activated sludge e A feasibility study to improve the economics of sludge dewatering Brittany A. MacDonald a, b, Ken D. Oakes a, Michelle Adams a, b, * a b

Verschuren Centre for Sustainability in Energy and the Environment, Cape Breton University, 1250 Grand Lake Rd, Sydney, Nova Scotia, Canada School for Resource and Environmental Studies, Dalhousie University, 6100 University Ave, Halifax, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

Industrial productivity is often judged solely by the primary product's marketability, while opportunities for secondary products derived from process by-products are often overlooked. In paper mills, large volumes of moisture-rich paper mill residuals (cellulose sludge) are produced, for which commercial use is currently difficult. Port Hawkesbury Paper LP, Port Hawkesbury, Nova Scotia, produces over 7 t/hr of waste sludge with a seasonally-dependent dryness ranging from 25 to 38% (w/w). Various chemical or mechanical dewatering options exist; however, knowledge of the unique sludge composition and properties is essential to predicting how the product will react under each method. Sonication, Fournier rotary press technology, freeze/thaw cycling, and gravity drying were among the dewatering opportunities briefly explored outside of the chosen method, acidification. Notably, many industries utilizing dewatering technologies may not be producing value-added by-products, while geographic and climatic placement may limit processes which are possible for others. In the present study examining enhanced end-use value, further dewatering occurred through a comparative in situ experiment contrasting sulfuric acid and ferric sulfate acidification (direct acid injection into sludge). While both proton donors acidified the sludge and decreased moisture content, sulfuric acid was the more cost-effective option, yielding an ~4% increase in dryness, with commensurate economic and environmental benefits. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Acidification Sludge Dewatering Sulfuric acid Ferric sulfate Wastewater treatment

1. Introduction Shifting market and environmental paradigms faced by the pulp and paper sector worldwide forces innovation not only with paper production, but also with energy management, environmental discharges, and fate of waste products. In the paper industry, mill residuals (or sludge) composition varies with the paper product manufactured, but is largely comprised of wood fibres, clay, and secondary treatment bio solids (micro-organisms). Bio-solids are known to have combustion applications as well as land application (NEBRA, 2017). Like many paper mills and waste water treatment facilities, Port Hawkesbury Paper LP (PHP), located on Cape Breton Island, Nova Scotia, Canada, has identified the need to focus on alternative strategies to handle their sludge production and end use/

* Corresponding author. School for Resource and Environmental Studies, Dalhousie University, 6100 University Avenue, Halifax, Canada. E-mail address: [email protected] (M. Adams).

disposal. Currently, a portion of the sludge is transported off-site to be burned as a biomass product, with the balance incorporated into a limited timeframe landfill topping project. Both disposal methods have their inherent challenges. As a biomass product for incineration, high moisture content sludge (25e38% (w/w) dryness, depending upon species and season) requires much of its contained energetic potential to evaporate off moisture as water vapour, thereby dramatically reducing its overall heat value. In some mills, financial losses due to low calorific value are also incurred for delivering sludge with dryness values < 30% to a local combined heat and power (CHP) facility as prescribed under agreement conditions regarding sludge incineration. Alternatively, landfilling options are limited and not considered best practice. On a national scale in Canada, while not all paper mills deposit sludge in landfills, even conservative estimates indicate landfilling requires a large environmental footprint, with long term implications. Acidifying sludge is one mechanism for reducing moisture content by increasing constituent mobility, weakening binding forces, and facilitating chemical and metal releases. Such behaviour

https://doi.org/10.1016/j.jclepro.2017.12.014 0959-6526/© 2017 Elsevier Ltd. All rights reserved.

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is consistent with the theory of biosorption, where sludge often acts as a binding site for heavy metals which can then be released through the addition of acid (Ong et al., 2010). Moisture has the potential to be liberated by adding acid, which acts similarly to thickening chemicals used in typical treatment processes, by bringing zeta potential (reference point for stability within a mixture) of sludge flocs at or close to the point of zero charge (Mahmood and Elliot, 2007). The primary objective of this study was to investigate acidification as a sludge drying method, and to determine the optimal acidifying agent to cost effectively reduce moisture content. Two acid combinations (93% sulfuric acid, and ferric sulfate with 10% sulfuric acid) were contrasted to determine the best dewatering performance that was economically achievable. Analyses include sludge pH change following acid addition, moisture release, and impacts on required polymer and coagulant use. The latter response is an important endpoint as acidification may decrease the need for these chemical inputs to current dewatering processes, resulting in cost savings. Further, the role of pH in modifying sludge Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) has yet to be properly documented, and this alone will be a valuable contribution to the literature. Improving sludge dryness in turn decreases sludge volumes, a beneficial property (both environmentally and economically) when landfilling or transporting is required, improving public perception of the pulp and paper or other sector by demonstrating an innovative approach to operational challenges. Hoffman et al. (2015) noted public perception of industries across sectors directly improved with the integration of more sustainable practices, transparency, and community involvement.

Table 1 Initial titration based acid addition trial results comparing ferric sulfate (10% sulfuric acid) and concentrated 97% sulfuric acid in 100 mL volumes of Waste Activated Sludge (WAS). Sulfuric Acid Added (mL)

WAS pH

Ferric Sulfate Added (mL)

WAS pH

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

6.37 6.06 5.72 5.29 4.82 4.32 3.87 3.39 2.99

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2 2.5 3 3.5 4.0 4.1 4.2 4.3 4.4

6.37 6.31 6.25 6.20 6.15 6.10 6.04 5.97 5.90 5.82 5.75 5.25 4.75 4.35 3.89 3.49 3.10 3.07 3.04 3.20 3.00

Sulfuric acid t-based P-value (a ¼ 0.05), t-critical value ¼ 1.86 and P-value ¼ 0.05. Ferric sulfate t-based P-value (a ¼ 0.05), t-critical value ¼ 1.72 and P-value ¼ 0.05.

2. Materials and methods 2.1. Laboratory scale The ability of acids to dewater sludge was first evaluated on a laboratory scale through titration-based acid additions using 60% ferric sulfate and 97% sulfuric acid. Ferric sulfate has a pH typically <2, while concentrated sulfuric acid typically exhibits a pH of 0.3. These acids were selected due to availability, cost feasibility, local regulations and safety objectives, while cognizant a higher hydrogen ion concentration would result in a greater pH change with less product consumed. Differences in concentration from the laboratory to industrial scale-up were negligible, although the laboratory scale experiments were purely to demonstrate capacity of these additions to dewater sludge. Titrants (acids) were diluted by a 1:10 ratio by volume (Table 1) and 100 mL of waste activated sludge (WAS) were used throughout the titration with pH measured with an ATI Orion perpHecT LogR Model 310 Benchtop Meter. 2.2. Preliminary in-situ trial Prior to investing in permanent implementation, a preliminary, short-term manipulation of the secondary treatment process was initiated, and parameters to be measured across the mill process stream were selected (Fig. 1). Fennofloc XP 136H10 (ferric sulfate) was obtained from Kemira; the 93% sulfuric acid was obtained from ChemTrade. To capture treatment effects over the ever-changing conditions of an operational facility, the addition of the two acids was alternated weekly to optimize acid concentration and thickening chemical use during subsequent system modifications. During the preliminary trial, acid was injected into WAS secondary sludge with an initial solids content of 2e3% (w/w) (in contrast, primary sludge

Fig. 1. Flow-based schematic of secondary treatment process monitored during acidification trial. (Adapted from Mitchell, 2015).

is ~4% (w/w) solids content) (Fig. 1). The injection point was prior to mixing with the secondary sludge. The main parameter monitored was pH, which was typically measured at ~ pH 8 within the WAS prior to any acid addition. A final pH of 4 was selected following consultations with Canadian mill experts who indicated much

Please cite this article in press as: MacDonald, B.A., et al., Molecular disruption through acid injection into waste activated sludge e A feasibility study to improve the economics of sludge dewatering, Journal of Cleaner Production (2017), https://doi.org/10.1016/j.jclepro.2017.12.014

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lower pH values could be reasonably achieved, making pH 4 a safe, achievable starting point. Two acid injection pumps (ProMinent™ Sigma/1 positive displacement) were put in temporary locations within the secondary treatment stream, with only two being utilised at any given time. pH was monitored at two points using ProMinent™ Dulcometer DMT On-site Measurement Transducers, (1) approximately 10 min (250 m) downstream of the initial injection point and (2) prior to mixing with primary sludge, and then again approximately 1 h post-injection, and following blending (Fig. 1). COD was analyzed using the dichromate method (Oxygen Demand, Chemical, n.d.); BOD was analyzed using a 5 d standard test method, CPPA H.2 Dryness values (for WAS and recycled activated sludge [RAS], were determined using consistency pads where 250e500 mL samples were weighed wet, then again after being put in a speed drier for approximately 20 min at 100  C. For cake (final product), 500 g of sludge were placed on an 11  13” pan in an oven at 105þ/- 5  C for 18 h and re-weighed following drying, at this time the cake is accepted as being completely dry, within þ/- 1% dryness. Final dryness values in both cases equal the difference between wet and dry measures. This preliminary full-scale industrial trial took place during the winter months when sludge dewatering is most problematic, to ensure a minimum 30% dryness can be achieved, while providing a ‘worst-case’ chemical cost estimate. 2.3. Permanent process implementation Sulfuric acid was chosen for permanent implementation based on the outcome of the preliminary in situ trial, and was pumped into WAS at the same injection points. Lutz- Jetsco Memdos DX50 Motor-driven Diaphragm Dosing Pumps were used as the relatively low injection volumes of sulfuric acid (in comparison to ferric sulfate) allows these pumps to run at near engineered capacity, reducing mechanical issues. Internal programming meters the amount of acid injected based upon flow rate (m3/hr) and pH of WAS and blend tank sludge. pH probes were retained in situ from the preliminary trial, providing readings at approximately 10 min and 1 h post-injection, and upon mixing in the blend tank. Injection manipulation on this trial was automatic based on secondary sludge flow rate, pH of blend tanks and WAS; these inputs regulate pump speeds, and sulfuric acid input flow rate. Initially, a 1:1 ratio of WAS flow (m3/hr) to acid injected (L/hr) was implemented with a WAS pH objective of 4, before lowering to 3.5 upon system stabilization. 2.4. Notable issues/strategies Pre-existing fittings/points of injection should be evaluated for materials compatibility prior to acid injection as low-grade stainless may not be sufficient; only two days data for the sulfuric acid trial are available in the present study due to corrosion issues. A quill type of injection strategy should also be implemented to reduce further corrosion, as will storage and piping materials of carbon steel (stationary acid) and 316 stainless steel (flowing acid). 3. Theory/calculation 3.1. Purpose Moisture-rich waste sludge produced in agricultural, municipal, and industrial contexts are often stored on-site due to limited alternative uses and/or high energy costs of de-watering. (Resource Converting, LLC, 2017). Consequently, it is critical that the amount of waste produced and its moisture content be reduced such that

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the material can be ultimately integrated into a circular economy setting where the waste of one process (ex. paper production) becomes a feedstock for other processes or products. Drying can be part of the solution by creating additional utilization options for waste products, but there is limited incentive for reuse unless such options are explicitly presented and feasible in each producer's case. Drying the sludge reduces mass and volume of the product, making its storage, transport, packaging and retail easier. For lower value waste, drying it also enables incineration or co-incineration (Flaga, 2005). 3.2. Dewatering strategies Across the various sludge producing sectors, the most successful methods of dewatering are highlighted below. 3.2.1. Freeze/thaw The freeze/thaw method of sludge dewatering usually requires a two-compartment system, allowing for freezing and thawing; this both creates and requires energy. Researchers suggest that this process works best with inorganics if alum (compound attached to (SO4)2$12H2O) is added for conditioning (EPRI, 2002). Crystallization of water occurs during the freezing process allowing for differentiation between water and other sludge constituents due to the binding of crystal structures (EPRI, 2002). Variables to optimize during freeze/thaw cycling are solids concentration as well as freezing rate and duration. One particular technology, a “Biofreeze unit”, was used to determine dewatering abilities of inorganic and biological samples. Good results were obtained with respect to volume reductions observed with inorganic samples, but others sludges with organic composition (i.e., microorganisms, and wood fibers) did not respond as well (EPRI, 2002). Energy required for freezing and thawing is also a concern, but this particular technology also had the unique ability to recover batch energy (EPRI, 2002). 3.2.2. Anaerobic digestion Properly designed anaerobic digestion processes, can reduce sludges to 20% of their initial output volumes, which for some mill scenarios with high disposal costs, may offset the high cost of implementation, while cost-effectively dewatering sludges (Phalakornkule et al., 2017). The anaerobic digestion process typically consists of four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis (Kim et al., 2003). Methane produced from organic compound breakdown can be combusted for an additional heat source to further dry a sludge product. Depending on input sludge composition, anaerobic digesters’ reactive breakdown process can effectively dewater a product relative to input sludge, with final drying accomplished on a drying bed or belt filter press (Chemistry@Elmhurst, 2015), but the key issue is whether it is “dry enough” to warrant the cost. Implementing an anaerobic digestion process, even at a pilot scale is often not economically viable. In the present case study, anaerobic digestion, given the moisture content of the sludge, was not considered an economically viable alternative. 3.2.3. Gravity drying The most inexpensive dewatering strategy, due to its sole requirements being a sufficient expanse of land, turnover equipment, and a warm (and/or dry) climate is gravity drying (Alturkmani, 2012). In our case study, the Nova Scotia climate is less than ideal for gravity drying with high annual precipitation, high humidity, and cooler mean temperatures, although there are limited windows when this process can be implemented (Nordic Waste Water Treatment, 2008).

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Frei et al. (2006) presented a scenario-based protocol for mills to determine if on-site gravity drying is feasible using a set group of parameters (assuming an initial dryness of 26%):  Worst Case Scenario: Long residence time requirement in the reactor and only 45% dryness achieved, low sludge and woodwaste energy content, high wood-waste/sludge mixing ratio of 1:1, low internal drying temperature;  Best Case Scenario: Short residence time in the reactor and 60% dryness, high sludge and wood-waste energy content, low wood-waste/sludge mixing ratio of 1:0.25, high internal drying temperature, short-distance material handling conveyors (Frei et al., 2006). How well a given sludge may perform during gravity drying could be evaluated in a small-scale simple gravity filtration system. Alternatively, vacuum filtration is much less time consuming and produces greater dryness (over a set time period), but could approximate longer-term gravity dryness values. However, depending on time-frame and local climatic conditions, both gravity and vacuum drying may require additional drying steps or heating mechanisms. In small scale trials, the addition of water to a dry sludge sample would predict if cover is required during outdoor gravity drying to prevent moisture incursion through precipitation. If surface moisture inhibits the ability of the sludge to release water without an additional drying force or step, cover placement may be required, offsetting sunlight-augmented drying (Markovic et al., 2014). 3.2.4. Gravity thickening Gravity thickening can be used within a facility's clarifiers where a slow blending process allows the solids to settle out and then be removed by a mechanical rake for further processing. However, this process is limited in its effectiveness as a drying strategy and produces a residual with <5% solids). 3.2.5. Acidification Strong acid addition to secondary sludge breaks down waterfilled pockets or molecules and disrupts cell membranes of micro-organisms; this further releases water and decreases the overall pH of the process waste stream. Forest Product Innovations (FPI) has completed trials at other Canadian mills demonstrating ferric sulfate can increase product dryness and decrease coagulant requirements up to a point where further additions are not economically viable (Talat Mahmood, FP Innovation Research Manager, pers comm). Acidification not only decreases moisture retention, but it can improve the heavy metal bond release from sludge particles as pH decreases (Feng et al., 2008; Ong et al., 2010). Since heavy metals impair performance of both biological waste treatment processes and receiving environment health (Ong et al., 2010), acidification has considerable merit, particularly if the sludge is intended for use external to the paper facility. 3.2.6. Fournier rotary press Fournier rotary presses are engineered to introduce reactant into a flocculent tank (area where clumping can occur) and combine with an optimal amount of polymer additive to aid in sludge aggregation. These presses are also easily cleaned as both internal presses are non-clogging, require minimal electrical inputs, and final cake dryness can be operator controlled (Rotarypress.com, 2015). Fournier Rotary Press technology has been of interest within the pulp and paper sector, but recent laboratory trials conducted by PHP evaluating the benefits were less successful than acidification.

Fig. 2. Overview of the sonication process (Epigentek.com, 2015).

3.2.7. Sonication Rapid vibrations produced during sonication initiate cell lysing, releasing water from cells within sludge (Fig. 2). Laboratory scale sonicators retail from $3500-$8000 depending on wattage, require no continuous acid inputs - a significant human health concern (spillage, burns, corrosion of infrastructure) and ongoing input cost. Although commercial, full-scale sonicators are available for industry, they can be expensive to implement and could not stand alone as a sole strategy for reducing sludge moisture content, but could augment other dewatering processes. Literature suggests sonication can improve solids content from 0 to ~2500 mg/L over 35 min at a power intensity of 125.8 W/cm2 (Zhang et al., 2008). Sonication can increase operational safety through reduced use of chemicals, although hearing protection is required by operators. 3.2.8. Cyclone-based technology Cyclone-based drying is a simple yet extremely successful technology utilizing air blowers to create a centrifuge-like operation which allows material to dry (removing moisture in a separate stream) and presenting an overall dry basis product. The ‘Dryclone’ by Resource Converting, LLC boasts the potential to dewater to more than 85% dryness, which in the case of a moisture rich sludge (25e30% dryness), would greatly increase the overall value (Resource Converting, 2017). 3.2.9. Strategy summary Various sludge dewatering technologies have been successfully employed in diverse treatment and dewatering plants globally. A key element in selecting the best option for an individual sludge is a thorough understanding of each unique sludge composition and properties to better understand how the product will react under each method. Again, many plants employing these dewatering technologies do not produce any value-added by-products, so a different selection criteria would be considered than a mill wishing to recover value from their sludge. Further, climatic and geographical placement may further refine available choices. A comparative overview of dewatering mechanisms is highlighted in Table 2. 3.3. Acidification review The addition of acid to improve properties of waste products has been investigated under various applications. Cellular disruption with acid addition, and commensurate water release will generally decrease sludge volumes, although it has been suggested that a threshold exists where volume increases rather than decreases due to gas bubble formation (Texier, 2008). However, Texier's 2008 study, while relevant in theory, does not appear to be well documented during industrial scale trials in the paper industry, and no supporting evidence of significant volume increases attributable to gas bubble formation was found in the literature. Acidification of municipal and textile sludge were investigated, with the broad range of organic components creating difficulty in obtaining products of high dryness values (Li et al., 2005). A 2005 study also noted an increase in overall volume following acidification,

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Table 2 Comparative overview of Dewatering Technologies (Adapted from WERF - Innovations in Dewatering Sludges, 2008). Method

Mechanism

Advantages

Disadvantages

Belt-Press filter

- Pressure exerted between two belts - Water draining can occur prior to pressure exertion

- Good flocculation/mixing vital - Often operated with high hydraulic loads or low residence times

Filter press

- Filtering caused by pressure differential between feed and discharge

Solid-bowl centrifuge

- Centrifuge motion pulls solids to walls of bowl and allows it to move up the sides and discharge while liquids move in the opposite direction (Klima et al., 2011) - Low pressure without shearing - Differential pressure produced on filtrate side (Rotary drum vacuum filter for small-scale industrial processes or pilot trials, 2004) - High pressure without shearing

- Simplistic - Can produce 12e35% solids from 1 to 4% initial solids (Belt Filter Press dewatering, 2017) - Simplistic - Can be used for removing residual solids from a stream - Retention time can be modified based on chosen speeds

Vacuum drum filter

Hyperbaric filters Screw Press

Electrodewatering filter press Thermal filter press Centridry centrifuge V-fold belt-press filter Impulse dewatering

- High pressure without shearing - Screw-like motion prompts water-solids separation - Electric field promoting electro-osmosis and heating for moisture removal - High pressure without shearing - Heat and vacuum promoting moisture removal - High pressure without shearing - Combined thermal drying and dewatering - Pressure and shearing

- Contact with hot surface compounded with pressure (Zawadzki et al., n.d.)

- Easily customized to desired solids size

-

-

Emission reduction Small footprint (Andritz, 2013) Low capital costs Comparable solids-wise to centrifuge (Moss, 2012) Small footprint Potential for pathogen and odor reduction (Moss, 2012) Potential for reuse of energy and adaptation to season requirements (energy reduction, etc.) (Lee, 2013) Automatic (Euroby, 2014) Simplistic Tolerates poor flocculation Continuous Potential for combination with current belt press technologies (Zawadzki et al., n.d.)

attributing the increase to repulsion of particles (Li et al., 2005).Sulfuric acid and ferric sulfate are not the only additives capable of injection, acidic compounds such as hydrochloric acid have also worked similarly (Devlin et al., 2011), the key component being hydrogen ions for donation. Perhaps the most relevant literature is that of Mahmood and Elliott (2007) investigating acidification of paper mill sludge, comparing sulfuric, phosphoric, and acetic acid additions, and their effect, in turn, on required thickening chemicals, particularly polymer. Consistency or dryness of the final sludge cake was optimal using sulfuric acid, with the small gain in solid consistencies at higher contact times attributed to hydrogen ions diffusing into the sludge flocs and/or conformational changes in sludge constituents (Mahmood and Elliott, 2007).

- Semi-continuous (but automated) - May need to stop process to discharge - Often operated with high hydraulic loads or low residence times - Noise disruption and vibration associated with high operating speeds (Moss, 2012) - Low throughput or low cake solids contents - Large energy requirement for vacuum operation Low throughput or low cake solids contents - Low throughput or low cake solids contents - Prefers high solids contents - Electrical costs-but offset by high solids (needs 10e20% solids) (Moss, 2012) Semi-continuous (but automated) - Electrical costs-but offset by high solids - Semi-continuous (but automated) - Energy costs-but offset by high solids - Additional flowsheet unit operations vital - Low throughput

Development stalled due to low throughput

where HCV is in kJ/kg; specific heat of water is 4.18 J/g  C; 78.4 and Carbon, Hydrogen, Oxygen, and Sulfur represent elemental percentages of the carbon, hydrogen, oxygen and sulfur found in the samples (Nhizou et al., 2014). Using wet wood as a benchmark (due to the ever-changing composition of pulp mill sludge), Equation (3) provides a comparative result based solely upon dryness to provide a lower calorific value (LCV).

LCV ¼ 19:2  ð0:2164*MCÞ

(3)

where MC is the moisture content in percent of total weight (COFORD, 2006) and LCV is in GJ/tonne, and 19.2 represents a typical wood calorific value. 4. Results and discussion

3.4. Estimated values

4.1. Laboratory scale

Regarding calorific value, in the absence of bomb calorimetry data, the drier the material, the greater the ease, efficiency, and net energy released during burning, especially in wood based products such as the sludge produced by the mill in this case study. The Dulong and Vandralek equations (Equations (1) and (2)) have been previously compared for higher calorific value (HCV) determination of waste products (Nhizou et al., 2014), allowing a plausible comparison of sludges. These formulas demonstrate the multiplicative value in elemental component increase seen in a sample which may, in weight be equivalent; however, in a dry sense are vastly different.

HCV ¼ 4:18*ð78:4*C þ 241:3*H þ 22:1*SÞ

(1)

HCV ¼ 4:18*ð85*C þ 270*H þ 26*ðS  OÞÞ

(2)

Results revealed that a lesser amount of 97% sulfuric acid is required to trigger a large decrease in sludge pH. Ferric sulfate has the capacity to create the same change pH change, but requires larger volumes of acid. It was estimated that the addition of each acid is related by a 1:5 ratio, meaning that to create a common result, five times as much ferric sulfate as sulfuric acid would be required to compensate for the concentration of hydrogen ions present (with sulfuric acid being most concentrated) (Fig. 3). A clear differentiation between supernatant and sludge volumes after acid addition was expected if cell lysis/sludge densification and commensurate water liberation was a significant outcome of acidification (Fig. 4). The right-most sample, with 0.152 mL sulfuric acid addition, clearly yields the greatest water liberation. Even in the case of the pure sludge sample, a water layer is formed due to gravity settling. Overall, the water layer was clearly visible in all

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Fig. 5. Dryness and Ratio vs. Time for the trial duration.

Fig. 3. Results of a 1:10 titrant dilution (v/v) utilizing 93% sulfuric acid and ferric sulfate (10% sulfuric acid) on the bench-scale.

Fig. 4. Sludge samples taken from Mill B with the addition of 93% sulfuric acid in varying volumes during a settling period of 1 h.

samples, and could be decanted, siphoned, or removed through drainers and presses on an industrial scale. Acid was added initially in 0.152 mL aliquots with this value being representative of a trial performed at a second Canadian paper mill (Fig. 4). This value represents a comparison of the pH limits (set by the Canadian mill's operators), in comparison to data obtained from the initial test site's in-situ trial with 0.152 being the estimated value for this mill based off of PHP's data. The volume of water that separated from the sludge following a 30 min gravity settling period is presented in Table 3. The addition of 0.152 mL was not repeated; the limits for the Canadian Mill's pH parameters had already been exceeded. 4.2. Industrial trial scale The ratio of primary to secondary sludge (lower value representing a decrease in secondary sludge) and dryness of the output sludge over time are shown in Fig. 5. High points of the ratio line, 1:2, represents a high/‘worst case scenario’/optimized value. This refers to a situation where there is a greater amount of secondary sludge, which is more difficult to dewater, in comparison to primary sludge. The ratio changes over time, not due to acid injection, but due to presence of filamentous algae of clarifiers due to seasonal and process conditions. Dewaterability decreases as the ratio increases, due to secondary sludge excess. Through the addition of Table 3 Dewaterability via bench scale acidification of Canadian Paper Mill sludge utilizing 93% sulfuric acid. Volume Added (mL)

Water Level (mL)

0.00 0.05 0.10 0.152

12 14.5 þ/- 0.05 20 þ/-2 23

t-based p-value (a ¼ 0.05), t-critical value ¼ 2.01 and p < 0.05. Uncertainties u(Volume Added) ¼ 0.01 mL, u(Water Level) ¼ 1 mL.

Fig. 6. Press speed (%) vs. sludge load leaving to the boilers (%).

acid at a high ratio value dryness was increased by ~4% throughout the trial. Waste treatment/sludge producing facilities have varied methods of water expulsion, and some are not overly concerned with the final moisture content of their sludges; however, most are concerned as transportation of sludge in some form is variably required from the site of production, and reducing the moisture content will reduce transportation costs. There are numerous ways of assessing the impact of acid addition on moisture content, but a unique means that may be relevant to many sludge-producing applications is screw press speed. Ideally, both press speed and load (expressed as a percentage of operating capacity) would decrease with acid addition, which was the case during the latter portion of the trial (Fig. 6). Reductions in both press speed and load are due to decreased moisture content reducing the overall amount of sludge and difficulty to dewater. Weightometer readings represent sludge output per hour; the addition of acid resulted in more than 1 t/h less sludge produced (Fig. 7). The deviation between the first baseline and post-trial baseline are due to process parameter changes throughout those periods (due to dynamic and ongoing adjustments to mill processes), leaving the lowest points, and post-trial baseline, not representative of typical conditions. Reducing the output volume of sludge, reflecting a reduced moisture content, is an important process parameter, from both a disposal and transportation perspective, but also that of alternative usages. Sludge volumes were successfully decreased, potentially since measures were evaluated on sludges collected downstream of rotary drainer and screw press processes, which may reduce effects of any gas or repulsive forces occurring. Calorific value, a qualitative sludge fuel value measurement, increased following acid injection as dryness increased (Fig. 8). The

Please cite this article in press as: MacDonald, B.A., et al., Molecular disruption through acid injection into waste activated sludge e A feasibility study to improve the economics of sludge dewatering, Journal of Cleaner Production (2017), https://doi.org/10.1016/j.jclepro.2017.12.014

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Fig. 7. Weightometer values of sludge output to boiler and/or site storage.

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Fig. 9. Secondary and primary sludge flow rates throughout duration of trial.

Target

Fig. 8. Estimated calorific value of sludge based upon experimental dryness values achieved throughout trial. Fig. 10. Mixed Liquor Suspended Solids readings throughout duration of trial.

however, the trend with the final ferric treatment indicates the drop in MLSS was ongoing, signifying that with continued acid injection MLSS would continue to decrease. MLSS values correspond to COD and BOD (Figs. 11 and 12), which are difficult to regulate during the winter months (Fig. 10) and can vary greatly. Average values given relate to all seasons, and in the context of the acidification trial period the acid was added in the winter months which represent a higher secondary sludge production.

50

BOD (%)

numerical values are purely a relative metric against that typical of dry wood (COFORD, 2006). The biomass line represents an ideal, typical biomass value and would be the goal upon future permanent implementation. However, any calorific value increase is potentially advantageous to the consumer if sludge is used for burning. Sludge flow rates (m3/hr) were held constant throughout the trial, with outliers seen in the final ferric sulfate trials; however, this variation is negligible relative to the consistency of positive results throughout the trial (Fig. 9). Not reflected in this graph are the various parameters affecting the operational ability to maintain a steady primary and secondary sludge flow, reflecting fluctuations in the paper making process; this variation would be present in any large scale operation. Awareness of these fluctuations influenced the experimental methodology, in that the switching of acid types throughout the duration of the trial helped amortize variation across daily and weekly process changes to maintain overall consistency of operating conditions during experimental introduction of both acids. Micro-organisms are recognized as a significant component of paper mill sludges, and are quantified as mixed liquor suspended solids (MLSS) which are critical for successful treatment of an organic waste stream. A desirable operational MLSS is between 2000 and 4000 mg/L (MLSS, 2017); with the addition of acid, overall MLSS can be lowered (as observed in the case study) to within the desirable range (Fig. 10). MLSS fluctuations are to be expected as the loads and characteristics in a waste stream change,

40 30 20 10 0

5

10

15

20

25

30

Time (d) Baseline Post Trial

Ferric Sulfate Final Ferric

Sulfuric Acid Average

Fig 1 - Biochemical oxygen demand values throughout duration of trial. Fig. 11. Chemical oxygen demand values.

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WAS pH

6.5 6 5.5 5 4.5 4 3.5 0

5

10

15

20

Time (d) Ferric Sulfate

Sulfuric Acid

Post Trial

Final Ferric

Fig 1 - WAS pH changes throughout the trial Fig. 12. Biochemical oxygen demand values throughout duration of trial.

Fig. 13. WAS pH changes throughout the trial.

COD and BOD values reflect both sulfuric acid and ferric sulfates’ impact on these critical wastewater parameters values during the

problematic winter season; however, again it must be noted that the acid injection took place during the winter months relating to higher secondary sludge production and in turn higher BOD and COD. pH of the WAS was monitored to ensure it was maintaining at a pH of less than 4 from an initial value of ~7. Early data readings reflected initial fluctuations in pH due to an unexpected waiting period prior to sufficient mixing of acid and sludge to produce a notable change on the pH meters (Fig. 13). Following this period of adjustment the addition rate was determined and applied appropriately. Findings indicate that small decreases were initially due to cautious addition to gain understanding of the effects seen by the acid addition. Overall, addition rate fluctuated throughout the initial portions of the trial. By the end of the trial the optimal pH of <3.8 was achieved using the ferric sulfate addition. The mixed pH reflects the process stream pH approximately an hour after injection (Fig. 14). The first four columns represent the final pH in the blend tank, demonstrating either ferric sulfate (10% sulfuric acid) or 93% sulfuric acid can achieve the same pH decrease, although a larger volume of ferric sulfate is required. From a cost savings perspective, sulfuric acid became the acidifying compound of choice, although technically, both acids were effective at reducing WAS pH. Initially, increasing dryness of the sludge product was the primary objective of the experiment. However, it was observed that reduced volumes of costly thickening chemicals were required with acid injection, to the extent that it was postulated that polymer and coagulant may no longer be needed in excess. Mahmood and Elliott (2007) reported that there is great potential for cost reduction when thickening or preconditioning chemicals are replaced by an acid alternative. Focusing on a ‘worst case’ ratio value of 1.2:1, The lowest overall cost is seen around the 8e9th of February, during the sulfuric acid portion of the trial (Fig. 15). Looking towards the end of the chart, the last bar, as in the previous figure, represents a predicted value of sulfuric acid using the ratio of the two acids and the data obtained from the trial. The estimated savings and expenditures associated with the implementation of acidification on a full-time basis are presented in Fig. 16. Focusing on each set of columns, it is notable that in coagulant and polymer costs, and penalties, savings would likely be the same with each acid. However, there is clear variation in the acid cost column set, where the cost of ferric sulfate in both instances exceeds that of sulfuric acid.

Cost per Day ($)

1.20 1.15 1.10 1.05 1.00 0.95

Ratio (sec/prim)

1.25

01-Dec 02-Dec 03-Dec 04-Dec 05-Dec 06-Dec 01-Feb 02-Feb 03-Feb 04-Feb 05-Feb 06-Feb 07-Feb 08-Feb 09-Feb 10-Feb 11-Feb 12-Feb 13-Feb 14-Feb 15-Feb 16-Feb 04-Apr 05-Apr 06-Apr 07-Apr Predicted Sulfuric

0.90

Coagulant

Polymer

Acid

Ratio

Fig. 14. Mixed WAS, primary, and thickening chemical pH values.

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replacement for thickening chemicals (polymer and coagulant) commonly used in waste treatment facilities as these chemicals act create flocculation, acidification acts to breakdown microorganisms and release water The composition of the final sludge is negligibly affected as acid is removed as a filtrate and treated prior to outfall. Acknowledgements

Fig. 15. Cost savings analysis of thickening chemicals throughout the duration of the trial.

The Mitacs Accelerate program and Port Hawkesbury Paper LP have provided funding while Kemira facilitated the use of equipment and troubleshooting assistance throughout the trial, specifically through representative Brad MacLean. FPInnovations worked collaboratively for acid preconditioning laboratory trials. Also, thank you to Ken Mitchell, Marc Dube, Bevan Lock, Clayton Carmichael, Bill Coady, Joe Allen, Glenn MacDonald, Derrick Cameron, Jason Spears, Krista Young, Floyd Fougere, Darren MacPherson, Bruce Embree, Phoebe Timmons, Jamie Smith, Devon Clark, John Campbell, Kevin Lee, Bill Campbell and all Port Hawkesbury Paper LP employees for the tremendous guidance and support throughout this project. References

Fig. 16. Overall cost estimate and savings upon theoretical installation with the inclusion of penalty relief regarding sludge burned in biomass burners.

5. Conclusion In conclusion, the addition of both ferric sulfate and sulfuric acid creates an opportunity to decrease the moisture content of the resulting sludge by 4%, while decreasing the need for thickening chemical, and decreasing sludge pH. The differentiating factor between ferric sulfate and sulfuric acid is the relative cost, resulting in the selection of sulfuric acid as an obvious choice for acidifying agent on a permanent basis. With a savings estimated at ~$360,000 CAD per year, this system can become more beneficial with further optimization, as additional decreases in the use of coagulant and polymer are expected. The environmental benefits are also promising, as the drier sludge output has a higher end use value for burning, and dramatically limits landfilling requirements. In addition, this waste material has an array of potential end uses whether being fertilizer, incineration, or pelletization. With regards to transfer of process, it is suggested that short-term industrial trials be implemented prior to full scale adoption as processes vary across mills, but that a prior inventory of areas available for alteration be considered in light of project goals. The goal was to provide the test facility with 30% dryness within the final sludge product, regardless of seasonal influences. As a tangential benefit, the acid injection produced cost-saving alterations in chemical additions, including the reduction of thickeners. As follow-up, further efforts are required once a permanent installation is completed, as well as experimentation with other methodologies for dewatering such as sonication which could work as an additional or replacement process. Future work will determine the threshold beyond which incremental acid injection no longer aids in dewatering or reducing thickening chemical requirements. Environmental benefits stemming from acidification are also broad; with a decrease in output waste volume the test facility can justify a management strategy that eliminates the expansion of landfilling projects for the sludge while also cutting back on the energy to burn through increased calorific value. Acidification overall acts as both a supplement and

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