Industrial digestates from short-fibre residues, generated in paper recycling mills, are driving interest in resource recovery. This study aims to explore their potential for water recovery. Understanding particle dynamics aids in optimizing dewatering for digestate management. The particle size distribution in this study revealed significant fractions: <0.63 μm (6–20%), 0.63–20 μm (38–52%), and >20 μm (11–16%). Pre-treatment with Na4P2O7 and H2O2 enhances settling and lowers total dissolved solids (TDSs) but results in variation of size distribution. Additionally, this study investigates further water reuse in paper mills, focusing on the quality of ultrafiltration (UF) permeate obtained from the digestate of short fibres. UF permeate analysis reveals deviations from freshwater standards in paper mills. Despite effective TS removal, UF permeate falls short of paper mill water standards due to high TDSs, electrical conductivity, and nutrient concentrations, necessitating further downstream treatment with nanofiltration or reverse osmosis. A substantial reduction of permeate flux from 31 to 5 L/(m2·h) over the time indicated fouling and inefficient membrane wash. The silt density index of the UF membrane at 30 min registered 2.1, suggesting potential fouling. Further investigations on optimizing UF operations to enhance permeate flux and exploring alternative UF membranes are required.

  • Particle size distribution in digestates varies by origin and type of residue.

  • The digestate contains macroplastics with sizes >4,000 μm and microplastics of size <200 μm.

  • Microscopic image analysis reveal that particles of size <200 μm are responsible for poor dewatering efficiency.

  • UF membrane's water permeability lies in the range of 37–46 L/(m²·h·bar).

  • Paper mills as self-water sources with 184 t/d of water availability from digestates.

The paper industry, one of the top five energy-intensive sectors globally, ranks fourth in Germany in the production sector. Germany accounts for about 25.5% of European paper and board production, while having 25% of paper and board consumption in Europe (CEPI 2021). Germany achieved a 99.8% recycling rate for paper and cardboard packaging in 2021 (Cayé et al. 2021). Despite recycling waste paper being more sustainable, it still consumes an average 2.5 kWh of energy per kg of paper. Moreover, the rise in waste paper reuse in production, accounting for 79% in Germany, has been leading to increased waste generation during recycling processes (STMUV 2008; German Federal Environment Agency 2022). In paper recycling processes, various residues are generated, including organic components like fibrous materials, plastics, bio-sludges, and inorganic components such as mineral pigments, calcium carbonate, kaolin, sand, metals, or other foreign substances. The waste residues that contain primarily long fibres are typically recycled back into the product manufacturing process. Short-fibre residues, like deinking reagents, pellets, bio-sludge, and coarse rejects, are mainly organic, while fine rejects contain a mix of inorganic and organic fibrous materials. With increased paper recycling, the production of short-fibre residues has also risen. However, these residues are too short and weak for reuse. Deinking sludge, with the highest ash content, typically contains fillers, fibres, fats, extractives, inks, colours, and adhesive components. In 2013, waste distribution in Germany's paper industry was as follows: 8.0% bark and wood waste, 19% deinking sludge, 13% waste from paper and cardboard pulping, 48% fibre waste, and 3.0% bio-sludge from wastewater treatment. Currently, these residues are primarily incinerated in Germany, with minimal use and even landfilled in other countries (Ouadi et al. 2012; Singh et al. 2018; Faubert et al. 2019).

Biogas and biomethane offer significant reductions in greenhouse gas emissions compared to European Union (EU) fossil fuels – up to 240 and 202%, respectively (Giuntoli et al. 2015). By 2050, the EU plans to achieve carbon neutrality and Germany targets an 80% renewable energy share (EEA 2022). Industries, including the Confederation of European Paper Industry (CEPI), aim to make the paper sector competitive and sustainable for a climate-neutral Europe by 2050, with an 80% reduction in CO2 emissions. Utilizing short-fibre residues for biogas production is vital to achieving this and is sustainably helping to meet the energy demands of paper mills. These wastes are recently recognized as suitable for the anaerobic digestion process (Amare et al. 2020; Di Fraia & Uddin 2022). Steffen et al. 2017 reported a biogas yield of 280 NL/kg of organic dry matter (oDM) from deinking sludge. Amare et al. 2019 investigated various inocula to optimize biogas production from deinking sludge. Bareha et al. 2022 assessed the impact of deinking sludge as a co-substrate on methane production and digestate quality. Continuous biogas production from various recycled paper residues, including short-fibre residues, pellet- and deinking sludge, has been successfully demonstrated at Aachen University of Applied Sciences (FHA), Germany. Their technical feasibility has been validated over the long term (up to four years) (Kuperjans et al. 2019, 2022). The digestion process has been scaled up to 70 L in laboratory-scale anaerobic fermenters by FHA and to a pilot unit of 1,500 L by PlanET Biogastechnik GmbH.

Anaerobic fermentation produces digestate, a residue rich in nutrients such as nitrogen (300–1,000 mg/L) and phosphorus (100–500 mg/L), along with water (70–90%). However, according to the German Fertilizer Ordinance, digestates from paper sludge are not permitted as soil fertilizer due to regulations prohibiting waste paper and plastics in digestates (§7 (3) 19 and §8 (4) 9) (German Federal Office of Justice 2019). This contrasts with conventional digestates of agricultural origin. The incorrect management of conventional digestates can however lead to eutrophication of water bodies, nitrate pollution of aquifers, and ammonia emissions. Therefore, the new German Fertilizer Ordinance 2020 limits soil fertilization to 170 kgN ha−1 year−1. Consequently, digestate from recycling paper cannot compete with conventional digestates as soil fertilizer. Regulations vary among countries and regions regarding digestate use for soil fertility (U.S. Department of agriculture 2017; Stürmer et al. 2020). Industrial contamination and fibrous texture inherent in non-treated digestate from short-fibre residues pose significant challenges and strict limitations on its use.

Despite the promising biogas potential of short-fibre residues in the paper industry, addressing downstream digestate management and treatment for industrial-scale application is essential. Treatment through resource recovery for reuse is the most sustainable option. Many treatment options are already available on a large scale (Bernhard et al. 2015; Kovačić et al. 2022). However, there are limited full-scale demonstrations due to bottlenecks in research and development, and technological and infrastructural limitations. The complexity and variability of digestate composition, with each digestate presenting unique characteristics, pose significant challenges for large-scale implementation. The treatment processes for digestate can be categorized into two distinct approaches: (a) dewatering: this method focuses on reducing volume or separating digestate into solid and liquid fractions for easier handling. Techniques such as filtration, centrifugation, electrocoagulation, and chemical coagulation are used. The resulting solid fraction, known as pressed cake, can undergo further treatment, such as thermal drying and thermo-chemical techniques (Wiśniewski et al. 2015). Typically, dewatering serves as the initial stage in digestate treatment, requiring less energy and being more cost-effective compared to (b) purification: this approach involves extracting valuable components from the digestate, while the remaining liquid fraction undergoes purification. Suitable methods include membrane technology, struvite precipitation, ammonia stripping, and vacuum evaporation (Guercini et al. 2014; Alhelal et al. 2022; Zielińska et al. 2022; Pepè Sciarria et al. 2023). This allows for the reuse of valuable components (e.g., nutrients and or organic residues) and the safe discharge of treated wastewater into water bodies. The choice of treatment method for digestate depends on its intended use. Digestate characteristics are primarily influenced by feedstocks and anaerobic digestion conditions, making a universal treatment option impossible. The type of paper mill (kraft or recycling), raw materials used (virgin fibres, wastepaper, or a mix), and internal processes vary widely (Meyer & Edwards 2014). Consequently, the characteristics of waste residues/feedstocks like chemical oxygen demand (COD), total solids, organic matter, and nutrients differ, which affect biogas potential and digestate composition. Digestates from non-agricultural industries, like paper mills, require specific consideration due to their unique processes involving, mineral pigments, chemicals, and acids. Existing studies have primarily focused on determining the biogas potential from short-fibre residues, with none addressing the treatment or utilization of their resulting digestate up to date. Hence, careful selection of digestate treatment methods for short-fibre residues is vital, ensuring compatibility with industrial-scale applications and subjecting them to thorough technological validation studies.

Paper production typically consumes 15 L of water per kg of waste paper. An exemplary paper mill producing 10 million tons of corrugated and test liner paper annually from 100% waste paper generates about 1.4 Mt of process waste per year, requiring approximately 4.7 GL of water annually. In 2022, nearly 80% of the water used in this industry came from surface sources, while 20% was drawn from wells or other underground sources. Paper companies must also pay fees for water withdrawal. Conversely, digestate comprises 70–90% water and significant amounts of nutrients like nitrogen and phosphate. In Germany, paper mills must adhere to stringent standards outlined in the Wastewater Ordinance (Attachment 28), which regulates wastewater discharge for paper production. Consequently, dewatered digestate cannot be released directly into water bodies due to its COD load and must instead be sent to the nearest communal wastewater treatment plant, resulting in additional costs for mill operators. Given these economic and ecological challenges, recovering water and nutrients from digestate is essential. Prioritizing the reuse of these elements can help reduce water demand and costs while ensuring regulatory compliance, thereby enhancing operational efficiency and promoting sustainability in the industry.

This study aims to investigate the particle size distribution in digestate originating from laboratory- and pilot-scale anaerobic fermenters fed with short-fibre residues from paper mills across Germany and Europe. Understanding particle size distribution is crucial for selecting suitable digestate treatment technology. The study also explores treating the liquid fraction of digestate using ultrafiltration (UF) as a preliminary investigation. The quality of permeate from UF is assessed and compared with water quality standards in paper mills as well as with wastewater regulations in Germany. These findings reveal the need for downstream treatment, indicating whether individual or combined membrane filtrations are required for the desired water quality for reuse.

Digestate

For the experimental study in the present work, digestates of paper recycling residues from laboratory- as well as pilot-scale anaerobic fermenters were investigated. A total of eight laboratory-scale anaerobic fermenters, four main and secondary fermenters each, of type CSTR-10S, BPC Instruments AB (10 L, 0–300 rpm, residence time 50 days) are operated at the Institute of NOWUM-Energy, Aachen University of Applied Sciences, Germany. The laboratory fermenters are fed with short-fibre residues such as deinking sludge, short-fibre sludge, and pellet sludge from various German paper mills.

A pilot-scale anaerobic fermenter with a working volume of 1,500 L is operated at PlanET Biogastechnik GmbH. The pilot fermenter is fed with short-fibre residues obtained during the production of deinked pulp from waste paper from a European paper mill. The digestates were stored at room temperature and their compositions are shown in Table 1.

Table 1

Composition of digestates of fermented short-fibre residues from paper recycling mills

ParameterUnitLaboratory-scale digestatePilot-scale digestate
Total nitrogen (TNbmg/L 350–1,340 790–950 
Ammonium-Nitrogen (NH4-N) mg/L 40–450 400–560 
Total phosphor (Ptotmg/L 110–620 80–195 
Ortho-Phosphate (PO4-P) mg/L 80–340 80–180 
Chemical oxygen demand (COD) mg/L 9,000–37,000 22,000 
Total organic carbon (TOC) mg/L 165–2,220  
Total solids (TS) Wt. % 7–16 6–12 
Organic dry matter (oDM) Wt. % 1.8–4.8 1–2 
Suspended solids Wt. % 0.8 – 
Total hardness (hardness) °dH 110–250 125–310 
Magnesium (Mg2+mg/L 70–820 300–880 
Calcium (Ca2+mg/L 430–620 390–760 
pH-value – 7–8.5 
Electrical conductivity (EC) S/m 0.4–1.9 0.9–1.1 
Density kg/m3 2,000 1,700 
ParameterUnitLaboratory-scale digestatePilot-scale digestate
Total nitrogen (TNbmg/L 350–1,340 790–950 
Ammonium-Nitrogen (NH4-N) mg/L 40–450 400–560 
Total phosphor (Ptotmg/L 110–620 80–195 
Ortho-Phosphate (PO4-P) mg/L 80–340 80–180 
Chemical oxygen demand (COD) mg/L 9,000–37,000 22,000 
Total organic carbon (TOC) mg/L 165–2,220  
Total solids (TS) Wt. % 7–16 6–12 
Organic dry matter (oDM) Wt. % 1.8–4.8 1–2 
Suspended solids Wt. % 0.8 – 
Total hardness (hardness) °dH 110–250 125–310 
Magnesium (Mg2+mg/L 70–820 300–880 
Calcium (Ca2+mg/L 430–620 390–760 
pH-value – 7–8.5 
Electrical conductivity (EC) S/m 0.4–1.9 0.9–1.1 
Density kg/m3 2,000 1,700 

Particle size distribution analysis

The particle size distribution analysis was carried out using combined wet sieving (for particles >4,000; 2,000; 630; 200; and 63 μm) and sedimentation or pipette analysis (for particles <63 μm) according to DIN ISO 17892-4 and Köhn 1928 in duplicate. Laboratory- and pilot-scale digestates were investigated using their fresh samples as well as pre-treated using two different chemical agents under two different analyzing conditions of particle settling times. Pre-treatment of digestates was performed by the addition of 3.5% v/v sodium pyrophosphate (Na4P2O7) and 0.75% v/v of hydrogen peroxide (H2O2), respectively. Prior to the experiments, the samples were continuously shaken overnight using a rotary apparatus overhead shaker (Model 24/8, Gerhard) for better homogeneity in the samples.

Microscopy image analysis or static image analysis was conducted using the microscope DMI3000, Leica (40× objective). Fresh samples of laboratory- and pilot-scale digestates, diluted into 10-fold, were sedimented overnight prior to the investigation. The different particle sizes presented in the digestates were captured under the microscope (40× objective) in correlation with the settling depth from the surface of the sedimentation unit (1 L of volume). Samples were extracted at the depths of 0, 0.05, 0.1, and 0.4 m from the surface of the sedimented digestates.

Besides microscopic image analysis, jar tests were conducted with and without the addition of 0.75% v/v of H2O2. The jar tests aimed to observe the influence of pre-treatment on the settling of particles by continuously monitoring total dissolved solids (TDSs) and turbidity in the laboratory-scale digestate during sedimentation. TDSs and turbidity were measured using ESP32-based sensors developed by the faculty of Applied Mathematics and Computer Sciences at FH Aachen.

UF experimental set-up

The liquid fraction of pilot-scale digestate was tested into a dead-end UF flat-sheet membrane module using the laboratory-scale plant (Minimem, PS Prozesstechnik GmbH). Figure S1, Supplementary material, shows a schematic representation of the filtration plant, designed for an operating volume of 0.03–0.5 L. A PES membrane (MT Series, Synder®) was employed, having a molecular weight cut-off (MWCO) of 5 kDa and a total filtration area of 0.0117 m2. A total of six UF cycles were performed in duplicate with varying parameters: temperature (20–30 °C), and membrane pressure (1.5 and 4 bar), as in Table 2.

Table 2

Process parameters during the batch tests of UF

UF cycleTested parameterFeedpH [–]Pressure [bar]Temperature [°C]
Preliminary Laboratory-digestate – 20 
1–2 pH Pilot-digestate 7.5 20 
3–4 Temperature Pilot-digestate 7.5 30 
5–6 Pressure Pilot-digestate 7.5 1.5 20 
UF cycleTested parameterFeedpH [–]Pressure [bar]Temperature [°C]
Preliminary Laboratory-digestate – 20 
1–2 pH Pilot-digestate 7.5 20 
3–4 Temperature Pilot-digestate 7.5 30 
5–6 Pressure Pilot-digestate 7.5 1.5 20 

The liquid fraction of both pilot- and laboratory-scale digestates served as the feed in accordance with Table 2. To separate the liquid fraction from the pilot-scale digestate, simultaneous flocculation with 0.05% v/v polydiallyldimethylammoniumchloride (PolyDADMAC) and centrifugation were used.

Fresh feed was prepared for each batch test, and each test was run for 120 min, with the retentate recirculating into the feed container. The membrane underwent physical and chemical cleaning after each batch test to restore the initial permeate flux. Cleaning involved manual removal of the accumulated cake, flushing of the membrane under tap water (5 min), chemical cleaning by pumping 50% ethanol through the membrane module (10 min) at a pressure of 2–4 bar, and final pumping of deionized water (5 min). The filtration unit is not designed for backwash. During the batch tests, permeate samples were measured in triplicate for permeate flow every 30 min as well as after membrane cleaning in order to calculate the flux recovery. Feed at the start and permeate and retentate at the end of each batch test were taken for analysis.

Analytical methods

All the samples generated in this study, including digestates and the liquid fractions from the digestate, i.e., feed, permeate, and retentate, were analyzed at FH Aachen. TNb (LCK 238), NH4-N (LCK 303), Ptot and PO4-P (LCK 350), COD (LCK 380), TOC (LCK 303), hardness, Ca2+ and Mg2+ (LCK 327) were measured using cuvette kits and the DR2800 photometer from Hach Lange®, Düsseldorf. pH and electrical conductivity (EC) were analyzed using electrodes from Greisinger® (DIN EN ISO 10523-C5) and Hanna Instruments® (DIN EN 27888), respectively. The determination of dry matter (DM), organic dry matter (oDM), and suspended solids was carried out according to respective DIN standards. Solid densities were quantified with a pycnometer.

Calculated parameters

Nutrient retention [NR] was defined as:
(1)
where Cf indicates nutrient concentration in feed at the start of the cycle [mg/L]; Cp indicates nutrient concentration in permeate at the end of the cycle [mg/L]; Vf indicates volume of feed at the start of the cycle [L]; Vp indicates volume of permeate at the end of cycle [L].
Permeate flux (J) was defined as:
(2)
where J indicates permeate flow [L/(m²·h)]; Q indicates volume flow of the permeate [L/h]; A indicates membrane surface area [m²].
Slit density index over 30 min (SDI30) for the digestate was calculated using the provided formula:
(3)
where ti is the initial time to collect a 500 mL sample (min); tf is the final time to collect a 500 mL sample after time T has passed (min); T is the time corresponding to tf (min).

The SDI, with a subscript indicating the total elapsed flow time (T) in min, in this study is 30 min.

Flux recovery rate (FRR) was defined as:
(4)
where JPWM indicates permeate flux of pure water after UF cycle following membrane wash [L/(m2·h)] and Jo indicates initial permeate flux of pure water by membrane before cycle [L/(m2·h)].
The pure water permeability (PWP) was calculated by the ratio of the pure water flux (JPW) to the applied pressure (TMP):
(5)

Particle size distribution in digestate of short-fibre residues

Figure 1 presents the mean volume percentage of particle fractions within specific particle size ranges for both pilot- and laboratory-scale digestate samples, categorized by analyzing conditions and pre-treatment methods. Given the results from the particle size distribution, it was observed that the largest fraction of particles, accounting for up to 20% of the volume, was typically found in the range of 2–0.63 μm in the pilot-scale digestate under all the analyzing conditions in this study. The lowest fraction, approximately <1.8%, was observed above 4,000 μm. However, the pilot-scale digestate contained a significant proportion of the smallest particles <0.63 μm, accounting for maximum of 16% of the volume of digestate. Similarly, in the laboratory-scale digestate, the largest fraction of particles, constituting about 20–29% of the volume, was found in the range of 6.3–2 μm under all analyzing conditions and the lowest fraction, approximately < 0.2%, was observed above 2,000 μm. Nevertheless, a considerable proportion of the smallest particles <0.63 μm corresponded for about 11% of the volume of the laboratory-scale digestate.
Figure 1

Particle size distribution of fractions without (F1) and with (F2) extended settling time for particles, and with and without pre-treatment using Na4P2O7 (P1) and H2O2 (P2) in the digestate (pilot-scale: PSD and laboratory-scale: LSD) of short-fibres from recycling paper mills.

Figure 1

Particle size distribution of fractions without (F1) and with (F2) extended settling time for particles, and with and without pre-treatment using Na4P2O7 (P1) and H2O2 (P2) in the digestate (pilot-scale: PSD and laboratory-scale: LSD) of short-fibres from recycling paper mills.

Close modal

The particle size distribution between laboratory-scale and pilot-scale digestates is comparable, with some variations in the proportions of different size fractions. Notably, laboratory-scale digestate predominantly contained particles sized between 6.3 and 2 μm, while pilot-scale digestate has a higher proportion of particles in the range of 2–0.63 μm. Extended settling times resulted in a higher proportion of larger particles in the sediment, particularly noticeable in both the digestates subjected to standard settling times (5 h) compared to extended ones (days). In the pilot-scale digestate, extending the falling time led to significant increases in larger particles (≥20 μm) in the sediment, such as particles >4,000 μm rising from 0.9 to 1.8% and at 2,000 μm increasing from 2.9 to 3.5%. Intermediate particles (20–2 μm) also showed notable increases, particularly at 20 μm (from 5.8 to 8.4%) and 2.0 μm (from 11.5 to 15.8%). Therefore, there was a substantial decrease and shift of fine particles (<0.63 μm), with the proportion at 0.63 μm decreasing from 20.8 to 8.2%. In contrast, the laboratory-scale digestate exhibited a different response to extended falling time. While the volume of larger particles (>20 μm) remained unaffected under extended falling time, the fine particles (<0.63 μm) reduced the volume from 11 to 2%, shifting the particles to intermediate zone. The intermediate particles (0.63–20 μm) therefore showed an increasing trend of volume from 9.7 to 14.4%, indicating some aggregation into larger sizes from fine to intermediate zone. The behaviours of particle size distribution differences between pilot-scale and laboratory-scale digestate are noticeably not similar under extended falling time. It could be possibly due to their distinct origins and feedstock compositions. Pilot-scale digestate originates from anaerobic fermentation of purified short-fibre residues used in deinked pulp production. In contrast, laboratory-scale digestate derives from fermentation of fibre sludge from a paper mill's stock preparation plant, containing a broader mix of fibrous materials and other contaminants.

Different pre-treatment conditions influenced significantly particle size distribution in both the digestates. Na4P2O7-treated pilot-scale digestate exhibited a 6% higher proportion of particles <0.63 μm under standard settling conditions. Na4P2O7 functions primarily as a dispersing agent that alters the surface charge of particles in the digestate. By releasing pyrophosphate ions in solution, it reduces the electrostatic repulsion between particles. This leads to aggregation and enhances particle settling. The chemical reaction can be simplified as in Equation (6). The presence of these ions helps neutralize the surface charges of suspended solids, allowing for better aggregation and, consequently, improved sedimentation. Conversely, H2O2-treated digestate showed an 8% decrease compared to Na4P2O7. In contrast, pre-treatment of laboratory-scale digestate with Na4P2O7 had minimal impact, while H2O2 treatment resulted in a 3% increase in particles <0.63 μm under extended settling conditions. H₂O₂ acts as an oxidizing agent that facilitates the breakdown of organic matter in the digestate. Upon decomposition, H₂O₂ produces hydroxyl radicals (·OH), which are highly reactive species that can oxidize organic compounds, leading to the reduction of their molecular weight and promoting flocculation:
(1)
(2)
These hydroxyl radicals further react with organic contaminants, breaking them down into simpler and more soluble forms, which can enhance the settling of solids by reducing the dissolved organic solids (Equation (7)). The addition of H2O2 seemed to facilitate particle settling in this study, as evidenced by 50–60% reduction of TDS levels observed in both the pilot-scale (330 mg/L) and laboratory-scale digestates (160 mg/L) compared to samples without the addition of H2O2 (700 mg/L for pilot-scale and 400 mg/L for laboratory-scale digestate), as shown in Figure 2. This suggests that H2O2 may enhance oxidation mechanisms, particularly, it aids the removal of suspended particles as well as dissolved organic solids (Igwegbe & Onukwuli 2019).
Figure 2

Monitoring the (a) turbidity and (b) TDSs of digestates (laboratory-scale: LSD, pilot-scale: PSD) with and without the addition of H2O2.

Figure 2

Monitoring the (a) turbidity and (b) TDSs of digestates (laboratory-scale: LSD, pilot-scale: PSD) with and without the addition of H2O2.

Close modal

However, the influence of H2O2 on turbidity varied between the different digestate samples (Figure 2). The pilot-scale digestate exhibited decreased turbidity from 180 to 140 Arbitrary Units (AU), while the laboratory-scale digestate showed an increase in turbidity from 70 to 110 AU. Turbidity measurements, taken every 20 min using a sensor, can be found in Table S1, Supplementary material. The addition of H2O2 may sometimes result in the formation of oxidized by-products or intermediates that can contribute to turbidity. Improper mixing or dosing of H2O2 in the wastewater may lead to localized increases in turbidity due to incomplete oxidation or uneven distribution of oxidized by-products. In this study, a dosage of approximately 7.5 g/L of H2O2 was utilized. According to Prazeres et al. (2019), non-turbid wastewater can be achieved through the addition of H2O2 with a dosage of up to 7 g/L. For industrial digestates, this dosage needs to be higher. It is crucial to determine the optimal dose for oxidation; insufficient doses may not effectively destabilize the particles, while excess doses can lead to detrimental effects, such as re-stabilization, resulting in increased turbidity levels (Igwegbe & Onukwuli 2019). From the observed results in this study, coagulation and flocculation agents are preferred over oxidizing agents like H2O2 for the dewatering of digestate (Oraeki et al. 2018; Khettaf et al. 2021; Aragaw & Bogale 2023). Oxidizing agents primarily target organic matter through oxidation reactions, while coagulation and flocculation agents work by neutralization of the surface charge of the particles by promoting the aggregation and settling of suspended solids and colloidal particles present in the digestate.

Particle size distribution in correlation with the composition of digestates of short fibres (Table 1) plays a crucial role in selecting suitable digestate treatment technologies, such as dewatering techniques and the post-treatment of the liquid fraction of digestates. Currently, there is limited literature available on the particle size distribution of short fibres or any sludge from paper recycling. However, Deykun et al. (2018) conducted a study on fibre length distribution in various pulps using rapeseed straw as pulping material. Their findings revealed that approximately 90% of the fibres from rapeseed pulp were in the range of 20–1,200 μm, while around 8% were between 1,200 and 2,000 μm, with only about 1% exceeding 2,000 μm in length. Additionally, Migneault et al. (2011) reported fibre lengths of 389, 451, and 1,090 μm in thermomechanical, chemical-thermomechanical, and kraft sludge, respectively, with corresponding volume proportions of 17.3, 14.2, and 2.5%. Kraft sludge is a by-product of the chemical pulping process, also known as the kraft or sulphate process. Microscopic images in this study depicted a wide range of particle sizes in short fibres obtained from recycling paper, regardless of settling depth during sedimentation (Table 3). Some of these particles, identified as microplastics, were present in both laboratory-scale (in blue colour in Table 3) and pilot-scale digestates. The pilot-scale digestate, especially, exhibited a significant amount of microplastics, particularly those screened through a 2,000-μm sieve (as shown in Figure S2, Supplementary material).

Table 3

Microscopic image analysis (40 × objective, image size of 25 μm) – particle size distribution in the laboratory-scale (left) and pilot-scale (right) digestates at various settling depths after 24 h sedimentation

 
 

UF of the liquid fraction of digestate

Permeate quality for reuse

An exemplary paper mill recycling waste paper as its sole raw material produces 500,000 t of paper products such as test liners and cardboard annually. Within this mill, approximately 75,000 t of rejects and short-fibre residues such as deinking sludge, braids, fibre waste, pellet sludge, bio-sludge and paper sludge are generated yearly during the paper recycling processes. They correspond to an energy potential of 6,000 MWh/a through anaerobic fermentation, with biogas potential averaging 400 NL/kgoDM (Cheenakula et al. 2023). This corresponds to a daily production of 200 t of digestate resulting from the anaerobic fermentation of short-fibre residues during biogas production. Based on the determined TS content of 8%, and extrapolating the complete process to a daily mass flow of 200 t/d of digestate, it corresponds to 184 t/d of water available for recovery.

PES membranes, such as the one used in the current study, are widely utilized in UF due to their chemical stability, mechanical strength, and hydrophilic properties. Notable retention efficiencies were observed during the UF of the digestate in this study for key parameters such as TOC – 65%, COD – 62%, TS – 40%, and various nutrients including TNb – 37%, and Ptot – 65%. The treatment efficiency of UF for COD and TS removal from digestate is well known. Zielińska & Mikucka (2021) reported about 60% COD removal using a 5 kDa UF membrane on dewatered digestate from distillery stillage waste, under conditions similar to this study. Gienau et al. (2018a) achieved 70% COD removal with a UF membrane (50 nm) on agricultural digestate. In comparison, Carbonell-Alcaina et al. (2020) noted 51% COD removal when treating fermentation brine. These findings align with the 62% COD removal observed in this study, although Yue et al. (2021) reported up to 90% for swine manure digestate. These observations indicate that COD removal efficiencies by UF vary due to several factors, including the membrane's MWCO, the pre-treatment and dewatering methods applied to the digestate, and the digestate's composition and source.

Ptot up to 65% during UF in the current study could be attributed to its linkage with the particles typically ranging between 0.45 and 10 μm in digestates according to Masse et al. (2005) and Samanta et al. (2022). Masse et al. (2005) found that 20% of the Ptot was soluble, and 50% of the Ptot was associated with particles. Consequently, other authors quantified that more than 70% of the Ptot in digestates such as pig manure is associated with particles or colloids (Christensen et al. 2009). Therefore, it can be stated that the high TS retention during the dewatering of digestate through simultaneous flocculation and centrifugation in the current study led already to the Ptot retention in the earlier stage, where the downstream UF further enhanced it. Since the major part of the TNb in digestate is mainly dissolved NH4-N, the elimination of dissolved N-compounds was rather low for UF membranes. TNb retention efficiencies stayed between 37 and 40% in all the UF cycles, while NH4-N retentions remained lower than TNb but stable at 25% in all the UF cycles. Zielińska & Mikucka (2021) showed TNb retention efficiency of up to 58% when treating the distillery stillage, while Yue et al. (2021) achieved only 20% TNb retention. In contrast, Samanta et al. (2022) achieved higher retention rates of up to 80% for TNb and 70% for NH4-N while treating the pig manure using an NTR7450 membrane (3 kDa). These observations indicate that treatment efficiencies by UF vary due to several factors, including the membrane's MWCO, the pre-treatment and dewatering methods applied to the digestate, and the digestate's composition and source.

Permeate quality analysis in this study is critical to the realization of water reuse in paper mills. To ensure the safety and reliability of reused water in the frame of this study, the UF permeate quality obtained in this study was compared to three kinds of guidelines and standards. Table 4 shows the UF permeate quality with a comparison to fresh water standards in paper mills, German standards on discharge limits of wastewater, irrigation- and drinking water for water reuse (German Federal Environment Agency 2016; German Federal Ministry of Justice 2004, 2023).

Table 4

Upper limit of UF permeate quality compared to freshwater standards in paper mills, German standards on discharge limits of wastewater, irrigation water, and drinking water for water reuse

ParameterUnitUF permeate in this studyFreshwater standards in an exemplary paper millDischarge limits of wastewaterIrrigation water regulationDrinking water ordinance
TNb mg/L <330 – ≤20 – – 
NH4-N mg/L <321 – ≤10 ≤0.5 ≤0.5 
Ptot mg/L <8.4 ≤0.1 ≤2 – – 
PO4-P mg/L <7.6 – – – – 
Ca2+ mg/L <106 ≤75 – – – 
Mg2+ mg/L <34 ≤15 – – – 
Chlorine mg/L – ≤45 – ≤500 – 
Sulphate mg/L – ≤250 ≤600 ≤300 ≤250 
COD mg/L <847 ≤30 ≤200 – ≤40 
TOC mg/L <210 ≤9 ≤70 <20 – 
TS Wt.% 0.3  ≤0.5 – – 
TDS mg/L <5,778 ≤450 – – – 
Colour436nm 1/m – ≤0.5 – – ≤0.5 
Turbidity NTU – ≤5 –  
pH – 7.8–8.4 6.5–8 6–9 5–9.5 6.5–9.5 
Hardness °dH <23 ≤14 – 30–60 8.3–8.4 
ECa S/m <1.149 ≤0.9 – ≤0.3 0.0002 
Bacterio-logyb CFU/100 mL – ≤100 – ≤400–2,000 
ParameterUnitUF permeate in this studyFreshwater standards in an exemplary paper millDischarge limits of wastewaterIrrigation water regulationDrinking water ordinance
TNb mg/L <330 – ≤20 – – 
NH4-N mg/L <321 – ≤10 ≤0.5 ≤0.5 
Ptot mg/L <8.4 ≤0.1 ≤2 – – 
PO4-P mg/L <7.6 – – – – 
Ca2+ mg/L <106 ≤75 – – – 
Mg2+ mg/L <34 ≤15 – – – 
Chlorine mg/L – ≤45 – ≤500 – 
Sulphate mg/L – ≤250 ≤600 ≤300 ≤250 
COD mg/L <847 ≤30 ≤200 – ≤40 
TOC mg/L <210 ≤9 ≤70 <20 – 
TS Wt.% 0.3  ≤0.5 – – 
TDS mg/L <5,778 ≤450 – – – 
Colour436nm 1/m – ≤0.5 – – ≤0.5 
Turbidity NTU – ≤5 –  
pH – 7.8–8.4 6.5–8 6–9 5–9.5 6.5–9.5 
Hardness °dH <23 ≤14 – 30–60 8.3–8.4 
ECa S/m <1.149 ≤0.9 – ≤0.3 0.0002 
Bacterio-logyb CFU/100 mL – ≤100 – ≤400–2,000 

aElectrical conductivity.

bE. coli, Enterococci, anaerobic sulphito-reducers, specific F-RNA bacteriophages.

None of the parameters of UF permeate quality meet water standards for immediate reuse in all four applications except for the TS concentration. The current study primarily aims to reuse water from short-fibre digestate in paper mills. Since transport of short-fibre residues from paper mills to external biogas plants impacts the carbon footprint and associated factors, an on-site biogas plant integrated with digestate processing and water recovery in paper mills is beneficial. Therefore, the study compares UF permeate mainly targeting paper mills. The use of water in paper operations is integral to the process, as pulp and paper production would be impossible without it. However, since water is not part of the final product, it is often considered a utility and receives attention only when it interferes with production. Fresh water for paper mills typically comes from one of two sources: surface water sources such as rivers and lakes, or groundwater sources such as underground wells (Zaman Shakil & Mostafa 2021). Each of these sources has its characteristics, requiring different strategies to overcome various obstacles. Surface water contains low TDS, high TSS, high biological activity, and a temperature range of 0.5–27 °C, with significant seasonal variation. In contrast, groundwater has varying TDS (depending on the region and geological characteristics of the area), low TSS, low biological activity, and a temperature range of 10–13 °C, with minimal seasonal variation.

Paper mills demand fresh water with TDS <450 mg/L and EC below 0.09 S/m (personal communication with a German paper mill). However, the permeate quality from UF in this study falls short of these requirements, containing a high number of ions and thus higher TDS (<5,000 mg/L) and EC (<1.1 S/m). Specifically, water hardness, maintained at 14°dH by paper mills, is a challenging factor. Hard water, rich in minerals like Ca2+ and Mg2+ ions, can affect the effectiveness of chemicals and additives used in the recycling process. Excessive hardness can lead to reduced efficiency of deinking agents, hindered pulp dispersion, and increased equipment maintenance costs due to scaling and build-up (Walter 1971). Furthermore, it can affect the quality of the recycled paper product, leading to issues like decreased paper strength and increased susceptibility to tearing. The UF permeate, with a hardness of 23°dH in this study, contains high levels of dissolved solids, nutrients like NH4-N (<321 mg/L) and PO4-P (<7.6 mg/L), salts, minerals like Ca2+ (<106 mg/L) and Mg2+ (<34 mg/L), as well as other organic matter and pollutants, resulting in high COD (<850 mg/L) and TOC (<210 mg/L) concentrations. According to Yu et al. (2018), UF systems can effectively retain microorganisms, including E. coli, Enterococci, spores of anaerobic sulphito-reducers, and bacteriophages, from municipal wastewater. While fresh water for paper mills should maintain bacteriological contamination below 100 CFU/100 mL, the UF permeate in this study should also be analyzed for bacteriological contamination. Nevertheless, the UF permeate is free from solids. However, its chemical contamination renders it unsuitable for immediate reuse as process water to replace fresh water in paper mills. Several factors likely contributed to this outcome, including membrane fouling, the effectiveness of the chemical pre-treatment, and operational conditions. While PES membranes are known for their chemical resistance, they are still prone to fouling by organic compounds, colloids, and dissolved solids, which can impact permeate quality. Although a detailed analysis of the membrane cake (Figure S3, Supplementary material) was not performed to identify the specific fouling mechanisms, several hypotheses can be drawn based on the observed data. The digestate contains a complex mixture of organic materials, including proteins, polysaccharides, and microbial biomass, which can adsorb onto the membrane surface. The high concentration of NH₄-N and PO₄-P in the permeate indicates that a portion of the nutrients may have remained in the solution due to membrane selectivity limits. The permeate quality analysis revealed elevated levels of Ca2+ and Mg2+ ions, contributing to water hardness. The presence of these ions can lead to scaling and fouling on the membrane surface, further hindering treating performance. The potential for calcium carbonate precipitation under varying pH conditions in the digestate must be considered as this can significantly affect membrane integrity and efficiency. Colloidal materials and fine particles, even after flocculation (solid–liquid separation of digestate), may still contribute to fouling. In this study, the UF process faced challenges to effectively reduce TS, with the feed having a concentration of 0.46% and the permeate retaining 0.3%. This suggests that the UF system was not fully effective in removing all solids from the digestate, indicating potential issues with colloidal fouling. The chemical pre-treatment using polyDADMAC flocculant may have introduced residual flocculant into the permeate, influencing TDS and conductivity levels. Notably, after polyDADMAC flocculation, the COD was significantly reduced from 22,000 to 2,233 mg/L. However, the subsequent UF process yielded a permeate with a COD of 847 mg/L, indicating that UF did not fully remove organic contaminants. This implies that while chemical pre-treatment improved CSB elimination, it may not have adequately addressed smaller molecules or colloidal matter, limiting the effectiveness of the UF process in meeting water reuse quality standards.

The water needed to produce 1 t of paper from recycled paper varies based on factors like recycling methods, paper type, and water efficiency. On average, 1 t of paper requires about 7 t of water. With 184 t/d of water recovered from short-fibre digestate, it covers the demand for 26 t/d of paper production. Therefore, UF permeate requires further treatment. All organic and dissolved components must be transferred to the retentate of downstream nanofiltration (NF) or reverse osmosis (RO). Alternatively, the liquid phase of dewatered digestate can be directly treated with NF or RO, bypassing the UF step.

This approach is dependent on the distribution of total solids during the solid–liquid separation in the digestate. The results of NF and RO will be presented in the upcoming article. In either scenario, the final permeate must meet water quality standards for reuse in paper mills.

If the water recovered from digestate derived from short-fibre residues meets the quality requirements for reuse in paper mills, it could yield significant environmental and economic benefits. A rough estimation: by reusing approximately 184 t of water daily, mills could save approximately 0.2 million € annually, based on a typical water price of 3€/m3. Additionally, this approach would help avoid costs associated with transporting and incinerating digestate, enhancing the material value as well as the overall economic viability of the process chain. Utilizing treated digestate water would also reduce energy demands for treatment and transportation of fresh water from water bodies, thereby lowering the carbon footprint of paper production. Furthermore, decreased wastewater discharge would alleviate the burden on municipal treatment facilities and minimize pollution risks. Reduced chemical use in water treatment would lead to additional cost savings and environmental benefits. However, investments in comprehensive digestate management systems, including membrane filtration plants, need to be factored into future calculations. A detailed life cycle assessment analysis for the water recovered from the digestate should be conducted to evaluate these factors, with findings presented in an upcoming article.

Membrane performance and stability

UF typically amounts to more than 50% of the total operation costs of the treatment chain (Gienau et al. 2018a). Therefore, UF performance was constantly monitored in this study. For this purpose, permeate flux of the digestate (JD) and Slit density index at 30 min (SDI30) were measured for all the UF cycles (Figure 3). Moreover, the membrane performance concerning permeate flux of pure water (JPW) after membrane wash following every cycle, the FRR of pure water (FRR) and permeability of pure water (PWP) were calculated (Figure 4). During batch tests, 20–30 °C of process temperature was applied (Table 2), assuming a heat loss of 10–20 °C during the transport and dewatering of digestate from the mesophilic fermenter conditions (40 °C). The operating pressure of 1.5 and 4 bar was tested in a series of UF batch cycles.
Figure 3

Permeate flux and the slit density index of dewatered digestates of short-fibre residues during UF treatment.

Figure 3

Permeate flux and the slit density index of dewatered digestates of short-fibre residues during UF treatment.

Close modal
Figure 4

Permeate flux after membrane wash, flux recovery rate, and permeability of pure water during the UF treatment of dewatered digestates of short-fibre residues.

Figure 4

Permeate flux after membrane wash, flux recovery rate, and permeability of pure water during the UF treatment of dewatered digestates of short-fibre residues.

Close modal

The mean JD initially corresponded to 31 L/(m2·h) at the start of UF operation during the preliminary tests, with an eventual decrease of up to 59% over the time at Cycle 0. Through membrane wash, an average of 55% of the JD which is 18 L/(m2·h) was recovered at an operating pressure of 4 bar (cycles 1–2) in comparison to its initial performance. With increasing temperature from 20 to 30 °C, 65% of the JD which is 21 L/(m2·h) was recovered at an operating pressure of 4 bar (cycle 3–4). With operating pressure decreasing up to 1.5 bar (cycle 5–6), the JD has dropped too largely down to 3 L/(m2·h). These results indicate that operating temperature and pressure impact the permeate flux of the membrane significantly. Gienau et al. (2018a) attained a permeate flux of 25 L/(m2·h) on pilot-scale cross-flow UF under operational conditions of 42 °C and 4 bar, treating the liquid phase of agricultural digestate. Waeger et al. (2010) reported membrane fluxes ranging from 20 to 50 L/(m2·h) for digestate from an organic waste biogas plant at an operating pressure of 1 bar with total permeate recirculation and cross-flow operation. The differences in flux rates can be explained by several factors according to the literature. The ability to maintain membrane flux not only depends on the pore size and nature of the membrane material but also operating parameters such as temperature and pressure, as well as feed characteristics (particle size distribution, organic matter concentration). Generally, organic waste biogas plants tend to exhibit higher flux rates (Gienau et al. 2018b). The permeate flux in this study is lower than that reported in the literature, largely due to the disadvantages of dead-end filtration over cross-flow filtration mode. The continuous flow and shear forces in cross-flow UF help to minimize fouling by continuously removing particles from the membrane surface, resulting in lower fouling compared to dead-end filtration. While the specific composition of foulants was not analyzed in this study, the decline in permeate flux can be attributed to organic and colloidal fouling, as indicated by the feed composition with respect to COD and TS concentrations. Yue et al. (2021) suggests that foulants in UF systems treating complex wastewater are often dominated by organic matter, proteins, and polysaccharides. These contribute to pore blocking and cake layer formation, increasing resistance to permeate flow. Furthermore, membrane surface modifications can mitigate such fouling effects. Organic foulants can alter membrane surface properties, leading to irreversible fouling and degradation of long-term membrane performance. Thus, while specific foulant characterization was not conducted, it is reasonable to assume similar mechanisms are affecting our UF process, highlighting the need for future studies to analyze these foulants and their impact on membrane performance. Additionally, the UF in this study has a smaller pore size, corresponding to a 5 kDa MWCO, which makes filtration more challenging, a similar finding also reported by Wang & Wang (2006) and Maaz et al. (2019).

The SDI30 of UF in this study initially corresponded to 0.8 but increased significantly to 2.3 over the filtration time. SDI30 indirectly measures the amount of suspended solids in water and serves as an indicator of membrane fouling potential. However, it does not quantify the specific matter due to variations in size and shape (Baker 2012). The target SDI after filtration is typically 3–5 or less. Surface or seawater may have an SDI of up to 200, necessitating flocculation, coagulation, and deep-bed multimedia filtration before RO treatment. The interpretations of SDI values are as follows: SDI < 1 indicate minimal risk of colloidal fouling over several years; SDI < 3 suggest potential for fouling within several months, requiring occasional cleaning; SDI 3-5 indicates likelihood of particulate fouling, necessitating frequent cleaning; SDI > 5 are considered unacceptable, requiring additional pre-treatment.

Similar to the JD, the JPW decreased over time from 185 to 154 L/(m2·h) at an operating pressure of 4 bar (Figure 4) and further to 56 L/(m2·h) with pressure dropping to 1.5 bar. Likewise, the FRR of pure water followed a decreasing trend from 94 to 79% at 4 bar during cycles 0, 1–2, and 3–4, indicating both the complexity of the feed characteristics and inefficient membrane washing. Moreover, the PWP declined from 46 to 37 L/(m2·h·bar). The decrease in JPW was primarily due to a combination of reversible and irreversible fouling, while the decline in PWP after filtration cycles was mainly associated with irreversible fouling. The low normalized fluxes in cycles with these membranes are likely attributed to concentration polarization. Winter et al. (2017) observed that higher concentration polarization played a significant role in lower MWCO membranes when filtering natural organic matter.

This study utilized batch experiments to evaluate membrane performance, limiting the assessment of long-term stability and degradation after repeated cleaning and multiple uses. The FRR for the liquid fraction of digestate ranged from 55 to 69% (Figure 3) following tap water and chemical cleaning using 50% ethanol. These results align with the findings of Yue et al. (2021) who reported that tap water cleaning typically yield flux recovery rates below 70% for UF membranes with PES material. Additionally, Yue et al. (2021) found that NaOH and NaCl could achieve up to 80% flux recovery. Principally, commercial cleaning agents were not used in the current study, as the focus is on investigating the potential of digestate from short-fibre residues for water recovery and assessing the suitability of membrane filtration application. Future research will examine effective cleaning protocols to enhance membrane performance and reduce fouling.

The particle size distribution of digestates of short fibres from paper recycling processes showed extended settling times with increased proportion of larger particles in the sediment, while pre-treatment methods influenced size distribution. Furthermore, UF technology in this study aimed to separate nutrients and organic loads for potential reuse in the paper industry. Challenges were identified in meeting water quality standards for immediate reuse in paper mills, necessitating further treatment, either through downstream processes or by replacing UF with NF or RO.

This study emphasizes the importance of sustainable water management in the paper industry by analyzing water demand and quality requirements for reuse. It introduces the innovative use of digestate from short-fibre residues as a potential water source, being one of the first to explore this in paper recycling. Notably, this research is among the first to investigate digestate derived from short-fibre residues, marking a significant innovation in the field. These findings contribute to optimizing the treatment process, potentially reducing costs and enhancing the feasibility of water reuse in the paper industry. Ultimately, this work aims to minimize the environmental impact of paper production, advance resource efficiency, and support broader sustainability goals within the industry.

The authors would like to sincerely thank the paper companies and their representatives for their long-term cooperation, for sharing essential data on water quality standards, and providing short-fibre residues for the research at FH Aachen. The authors would like to extend thanks to Dr Simone Krafft, Svea Ziegner, and Arno Firmenich from the working group ‘Bioenergy and bioresource management’ of the institute for their consistent supervision of the anaerobic fermenters.

This study was part of the doctoral research project. The project is supported by the Bauhaus-Universität Weimar with a scholarship within the framework of ‘Thuringian graduate promotion 2023/24’. The research work was conducted in cooperation with the FH Aachen University of Applied Sciences.

D.C. and M.G. conceptualized the study; D.C., J.P., and M.G performed the methodology; D.C., R.H., I.S., J.P. did formal analysis and investigation; D.C. wrote and prepared the original draft; D.C., J.P., M.G., S.B. wrote, reviewed, and edited the article; funds were acquired from Bauhaus University Weimar; resources were collected from FH Aachen; J.P., I.K., M.G., S.B. supervised the article. All authors have read and agreed to the published version of the manuscript.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.

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