Simultaneous microalgae separation and COD reduction from landfill leachate using electrocoagulation-flotation

Marvin Bruns 1

Alexander Kuss 1

Peter Kern 1

Christian Wolf 1

Himanshu Himanshu 1✉

1 Metabolon Institute, TH Köln - University of Applied Sciences Am Berkebach 1-3 51789 Lindlar Germany

Authors: Marvin Bruns¹; Alexander Kuss¹; Peter Kern¹; Christian Wolf¹; Himanshu Himanshu^1*

^{*Corresponding author}

Affiliations: Metabolon Institute, TH Köln - University of Applied Sciences, Am Berkebach 1–3, 51789 Lindlar, Germany

Corresponding author: Himanshu Himanshu, himanshu.himanshu@th-koeln.de

Abstract (150–200 words)

Microalgae-based wastewater treatment holds potential for nutrient recovery and carbon capture, but efficient biomass harvesting remains energy-intensive. This study evaluates electrocoagulation-flotation (ECF) for simultaneous microalgae separation and chemical oxygen demand (COD) reduction in real landfill leachate. A microalgal consortium was cultivated directly in landfill leachate at low and high biomass densities (OD₆₈₀ 0.3 and 2.2) and treated in a 0.5 L batch ECF reactor operated under a low-duty pulsed-voltage regime (5.0 V for 0.1 s and 1.5 V for 29.9 s, 30 min). ECF achieved 99.7% and 94.7% separation at low and high biomass, corresponding to 0.22–0.40 kWh kg^− 1 total solids and 0.33–0.38 kWh m^− 3. COD for low biomass microalgae decreased from 715 ± 12 mg O₂ L^− 1 (raw leachate) to 564 ± 14 mg O₂ L^− 1 after ECF i.e. 21% relative to raw leachate. These findings demonstrate that ECF effectively integrates microalgae harvesting with partial organic pollutant removal in a challenging wastewater matrix, offering a promising low-energy strategy for combined treatment and resource recovery in landfill leachate management.

Keywords:

Microalgae separation

electrocoagulation

landfill leachate

Circular economy

wastewater

Chlorella

Declarations

Funding -

This contribution was jointly funded by Technische Hochschule Koln’s Initialförderung funding initiative and the project PLan_CV. The project PLan_CV (reference number 03FHP109) is funded by the German Federal Ministry of Education and Research (BMBF) and Joint Science Conference (GWK).

Competing interests –

All authors declare no competing interests

Data Availability

Data supporting the findings of this study are available within the article. Raw data are available on request from the authors.

Author Contribution

Conceptualisation: Marvin Bruns, Himanshu Himanshu; Methodology: Marvin Bruns, Himanshu Himanshu, Alexander Kuss; Investigation: Marvin Bruns, Himanshu Himanshu; Formal analysis: Marvin Bruns, Himanshu Himanshu; Data curation: Marvin Bruns, Himanshu Himanshu; Visualisation: Marvin Bruns, Himanshu Himanshu; Writing – original draft: Marvin Bruns, Himanshu Himanshu; Writing – review and editing: Marvin Bruns, Himanshu Himanshu, Christian Wolf, Peter kern; Supervision: Himanshu Himanshu; Funding acquisition: Christian Wolf, Himanshu Himanshu,

1. Introduction

Microalgae can play a significant role in advancing the circular economy, offering proven potential for wastewater treatment and the recovery of key nutrients such as carbon, nitrogen, and phosphorus (Li et al. 2019; Molitor et al. 2024). In addition to these benefits, microalgae provide valuable ecosystem services, including carbon capture, storage, and utilization through CO₂ absorption, while simultaneously producing value-added products such as biofuels and nutraceuticals (Ezhumalai et al. 2024). However, despite these advantages, the large-scale adoption of microalgae-based processes is limited by challenges in downstream processing. Notably, biomass separation and harvesting can account for up to 30% of process costs and 20–30% of the total process energy input, due to the difficulty of separating small, near-neutrally buoyant microalgae cells (Renuka et al. 2013; Chen et al. 2021; Shaikh et al. 2021).

Several technologies have been developed for microalgae separation, each with distinct advantages and disadvantages. For example, sedimentation is highly energy efficient 0.05–0.1 kWh m^− 3 and has low capital costs (€0.03 m^− 3), but it achieves only modest concentrations of 0.5-3 g L^− 1 total suspended solids and requires long retention times due to the low intrinsic settling velocity (~ 1 cm h^− 1) of microalgae cells, which have a density similar to water (Vandamme 2017). Chemical flocculation can achieve high separation efficiency (95–99%) with relatively low energy requirements, as energy is primarily needed for mixing. However, it faces significant drawbacks for large-scale application, including high operational costs for organic flocculants (e.g. chitosan 20–50 USD kg^− 1 (Xu et al. 2021), and contamination of the recovered microalgae due to residual flocculants. Inorganic flocculants (alum, ferric chloride) can leave behind residual salts and metal ions, while organic polymers (chitosan, polyacrylamide) may introduce residual polymer chains that complicate downstream biomass utilization (Zhu et al. 2018). Centrifugation offers high separation efficiency (90–98%) with short treatment times (minutes), but requires the highest specific energy (8–20 kWh m^− 3) among common methods and involves significant capital costs (Abu-Shamleh and Najjar 2020; McGrath et al. 2024).

Electrocoagulation flotation (ECF) has emerged as a technology for microalgae separation that offers high separation efficiency, low energy demand, short treatment times, and low operational costs. For example Guldhe et al. (2016) reported 91% separation efficiency of Ankistrodesmus falcatus in 30 minutes, outperforming chitosan at 55% and alum at 86% after 60 minutes. The same study reported ECF energy consumption can be orders of magnitude lower than centrifugation i.e. 1.8 kWh kg^− 1 for ECF versus 65.3 kWh kg^− 1 for centrifugation at similar recovery. Lucakova et al. (2021) reported that adding an electrocoagulation pre-concentration step (using stainless steel cathode and carbon steel anode for Chlorella) before centrifugation reduced total separation energy to 0.136 kWh kg^− 1, energy savings of up to 89% relative to stand-alone centrifugation. In addition Lucakova et al. (2021), using ECF with iron electrodes, demonstrated the harvested Chlorella biomass had the iron content within food/feed safety limits.

Mechanistically, during ECF aluminum and/or iron is oxidized at the anode, releasing Al³⁺ and Fe²⁺/Fe³⁺ ions that produce metallic hydroxides which destabilise negatively charged microalgae cells by charge neutralisation and sweep flocculation. Simultaneously, cathodically produced hydrogen microbubbles attach to the flocs and float them to the surface for easy skimming. Unlike chemical coagulation, ECF does not introduce additional anions into the solution beyond OH⁻ generated at the cathode and the in-situ release of multivalent cations be tightly controlled by controlling the current reducing the risk of overdosing (Visigalli et al. 2021).

Electrocoagulation has also been reported as an effective treatment for wastewaters, including for chemical oxygen demand (COD) reduction. For example Asefaw et al. (2024) reported 98% COD removal from coffee wastewater using electrocoagulation. In landfill leachate Rookesh et al. (2022) reported a reduction of 30% COD and 84% NH₄⁺ using electrocoagulation with Fe–graphite electrodes. The mechanism for COD reduction is similar to that for microalgae separation: Al³⁺ generated at the anode hydrolyses to form amorphous Al(OH)₃, which destabilizes colloids and adsorbs dissolved organics, while H₂ produced at the cathode assists flotation. However, in the case of landfill leachate, COD reduction is primarily driven by adsorption and sweep-flocculation of dissolved organics into Al(OH)₃, and in chloride-rich matrices at higher anode potentials, indirect oxidation via in situ chlorine/hypochlorite may also occur (Jotin et al. 2012).

Despite these advances, several gaps remain in the application of ECF for microalgae separation in real wastewaters. Most studies have focused on single species in synthetic media or clean water, limiting the transferability of results to coloured, saline and highly conductive real-life wastewaters such as landfill leachates (Visigalli et al. 2021). Additionally, energy reporting is often incomplete, as many studies do not log time-resolved voltage and current to accurately integrate the true electrical energy input. Only a limited number of studies have reported both volumetric (kWh m^− 3) and mass-based (kWh kg^− 1 dry biomass) normalisations. Recent guidelines recommends using specific electro-energy consumption per unit dry mass (kWh kg TS^− 1) for comparability and volumetric electro-energy consumption (kWh m^− 3) for reactor-scale context (Visigalli et al. 2021). Finally, to date, there are no reports in the literature on simultaneous microalgae separation using ECF/electrocoagulation and COD reduction in real landfill leachate.

The aim of this study is to evaluate ECF in real landfill leachate to achieve simultaneous microalgae separation and COD reduction, using a mixed microalgal consortium grown in the same leachate. To the author’s knowledge, this provides the first demonstration of simultaneous microalgae separation and COD reduction in real landfill leachate using ECF.

2. Materials and methods

2.1 Matrix and microalgae

Landfill leachate was collected at the :metabolon site (Lindlar, Germany) and inoculated with a working culture consisting of indigenous Chlorella sorokiniana collected from this landfill site and adapted to growth in the same leachate matrix, without any micro- or macronutrient supplementation. The microalgae was identified by nuclear ribosomal ITS sequences determined from three different strains available under GenBank accession numbers PX210920-PX210922 (Kuss et al. 2025). The microalgae biomass was produced at two different optical densities i.e. OD₆₈₀ 0.3 (L1, L2 and L3) and 2.2 (H1, H2 and H3). The microalgae were grown in sealed 1-L Erlenmeyer flasks at 22 ± 2°C and continuously mixed with a magnetic stirrer at 150–200 rpm and illuminated by LED lights at an intensity of 150–200 µmol m^− 2 s^− 1 in a 14/10 h light-dark cycle.

2.2 ECF experimental setup

All ECF batch tests were performed in triplicate in a custom acrylic reactor with internal dimensions 100 × 95 × 65 mm and a working volume of 0.5 L. Unless stated otherwise, values are reported as arithmetic mean ± standard deviation (SD). n denotes the number of independent biological replicates, defined here as independent ECF batch runs performed on independently prepared cultures for each condition (L1L3 for low biomass; H1–H3 for high biomass; n = 3 per condition). The electrochemical cell comprised two parallel aluminium plates mounted in a 3D-printed holder, separated by an inter-electrode gap of 2 mm. The effective anode area was 75 cm² (50 × 75 mm facing surfaces on both sides). UNI-T UDP3305S-E laboratory power supply operated in voltage mode delivered a low-duty pulsed waveform for 30 minutes with each 30 s cycle comprised 5.0 V applied for 0.1 s followed by 1.5 V for 29.9 s (duty 0.33%). For the high-biomass runs (H1–H3), voltage and current were logged at 1 Hz from the power supply. For the low-biomass runs (L1–L3), a logging fault occurred with automatic recorder, so the current and voltage were documented by manual readings taken every 6 minutes. Integrated electrical energy and charge for L1–L3 were computed from the manual reading. The 1 Hz sampling and 0.1 s pulse on-time mean that only a fraction of individual pulses appear in the time series and the peak capture is probabilistic and does not reflect instability. During ECF the microalgae culture was mixed at 250 rpm with a magnetic stirrer. After the 30 minute ECF test the mixing was stopped and the foam/floc layer was allowed to form for 60 minutes. The clarified growth medium was then collected using a pipette from below the foam/floc layer. The residual foam/floc layer was dried at 105°C for 48 hours to determine the dry matter content. The total COD of raw landfill leachate, low biomass density microalgae culture i.e. optical density at 680 nm (OD₆₈₀) 0.3 (after 24 hour of incubation) and the clarified culture medium after ECF was analysed by the dichromate method (ISO 6060/DIN 38409 H41/H44) using Hach-Lange LCK514 (100–2000 mg O₂ L^− 1). pH measured before and after ECF treatment using Si analytics tritroline 6000.

Mean current density was obtained by dividing the time-averaged current by the effective anode area of 75 cm². Specific energy demand per unit treated volume (kWh m^− 3) was calculated by dividing the integrated electrical work by the treated volume. Specific energy per unit of biomass separated (kWh kg^− 1 TS) was calculated by dividing the integrated electrical work by the product of capture efficiency, treated volume and initial biomass concentration expressed as total solids. Capture efficiency was determined from the reduction in OD₆₈₀ of the supernatant between the initial and final samples.

Electrode mass loss and residual aluminium in the effluent were not measured in this preliminary study. However, the theoretical aluminium dose was computed using Eq. 1 from Faraday’s law where Q is electric charge in coulomb (C), t is time in seconds, M is molar mass of aluminium (26.98 g mol^− 1), z is the valence of Aluminum ions i.e. 3 and F is the Faraday’s constant (96485 C mol^− 1) and is reported as mg L^− 1 per run (Visigalli et al. 2021).

$\:{m}_{Al}\left(mg{\:L}^{-1}\right)=\frac{QM1000}{zFV\left(L\right)}$

(Eq. 1)

3. Results

ECF achieved a consistently high level of microalgae separation efficiency in both low- (99.7%) and high-biomass (94.75%) microalgae cultures (Table 1). The specific electro-energy consumption per unit dry mass for low- and high-biomass cultures was 0.395 ± 0.014 kWh kg^− 1 TS and 0.222 ± 0.025 kWh kg^− 1 TS, respectively and volumetric electro-energy consumption was 0.332 kWh m^− 3 and 0.383 kWh m^− 3, respectively. The reported figures include only electrical work at the electrodes and exclude supply losses, mixing and controls The mean current density under the imposed waveform was ~ 3 mA cm^− 2 (referenced to the 75 cm² anode area). Figure 1 shows the current profile for high-biomass microalgae culture. The baseline rose during the first 3–5 min then remained near 0.20–0.26 A over 30 min. Single-point peaks appear when the 1 Hz sample coincided with the 0.1 s, 5.0 V pulse. The peak incidence varies with logger-pulse phase and does not indicate instability.

Fig. 1

Current profile during ECF of high-biomass culture in real landfill leachate. Voltage mode: 5.0 V for 0.1 s and 1.5 V for 29.9 s per 30 s cycle. Current sampled at 1 Hz. Baseline remained stable at about 0.20–0.26 A over 30 minutes. Single-point peaks occur when sampling coincides with the 0.1 s pulse. Full raw logs are provided in Supplementary

Theoretical aluminium release was calculated from the time-integrated current using Faraday’s law with aluminium valency of 3. As shown in Table 1 the calculated average aluminium doses were 78.4 ± 2.4 mg L^− 1 and 85.0 ± 12.1 mg L^− 1 for low and biomass microalgae cultures, respectively. The corresponding charges were 420.6 ± 13 C and 456.5 ± 65 C, respectively. Residual dissolved aluminium in the clarified phase and gravimetric electrode mass loss were not measured in this study.

The total COD of the landfill leachate increased from 715.0 ± 12.3 mg O₂ L^− 1 in the raw leachate to 808.5 ± 11.5 mg O₂ L^− 1 after incubation with microalgae for the low biomass microalgae culture. Following the ECF treatment, the COD of the clarified (unfiltered) landfill leachate without microalgae declined to 564.3 ± 13.6 mg O₂ L^− 1 (Fig. 2). This corresponds to a reduction of roughly 30% relative to the culture and 21% relative to the raw landfill leachate. Expressed as specific energy per unit COD removed, the ECF process consumed 1.35 kWh kg^− 1 COD, calculated from the volumetric energy (0.33 kWh m^− 3) divided by the mass-based COD removal (244.2 mg L^− 1 = 0.2442 kg COD m^− 3). The pH of landfill leachate did not change during the ECF treatment and it was 8.3 for both raw and ECF treated leachate.

Fig. 2

Total COD for raw leachate, leachate after 24 h microalgae (low biomass concentration) growth and post ECF landfill leachate. Bars show mean ± SD, n = 3

Table 1

Charge, energy input and theoretical aluminium dose during ECF. Values are from three independent biological replicates per condition (L1–L3 and H1–H3; n = 3). Mean (SD) is reported in parentheses. TS = total solids. Theoretical Al dose calculated from Faraday’s law; separation efficiency from OD₆₈₀ reduction
ID	Biomass	Charge Q (C)	Energy input (mWh)	Theoretical Al dose (mg L^− 1)	Specific energy (kWh m^− 3)	Specific energy (kWh kg^− 1 TS)	Separation efficiency (%)
L1	Low	423.0	176.3	78.9	0.353	0.404	99.7
L2	Low	432.0	180.0	80.5	0.360	0.380	99.4
L3	Low	406.8	141.3	75.8	0.283	0.380	99.9
Mean (SD)		420.6 (13)	165.9 (21.4)	78.4 (2.4)	0.332 (0.04)	0.395 (0.01)	99.7 (0.2)
H1	High	477.0	198.5	88.9	0.397	0.233	96.6
H2	High	508.2	215.7	94.7	0.431	0.240	94.9
H3	High	383.0	159.6	71.4	0.319	0.194	92.7
Mean (SD)		456.5 (65)	191.3 (28.7)	85.0 (12.1)	0.383 (0.06)	0.222 (0.03)	94.7 (1.9)

4. Discussion

This study tested whether ECF, in real landfill leachate, can simultaneously deliver microalgal separation and COD reduction. ECF achieved high microalgae separation efficiencies for both biomass concentrations, with a slightly lower efficiency observed at higher biomass. This reduction is likely due to increased floc loading and carry-through in the clarified layer, rather than ineffective destabilization. The high separation efficiencies are comparable to the high recoveries for ECF reported in literature for the conductive media and optimised ECF processes by Vandamme et al. (2011), Visigalli et al. (2021), and de Morais et al. (2023). The presence of various constituents e.g. dissolved organics, salts etc in the landfill leachate did not prevent efficient microalgae aggregation and flotation while these constituents have been reported to sometimes inhibit flocculation (Vandamme et al. 2013).

The separation efficiency of the present study is comparable to state-of-the-art high-separation-efficiency methods, such as chemical flocculation, but without the need for chemical flocculants. For example, chitosan flocculation of Chlorella sorokiniana achieved > 99% clarification efficiency (Xu et al. 2021), while the present study achieved 94.7–99.7% separation efficiency. The separation efficiency of present study compared to some of the established mechanical microalgae separation methods e.g. centrifugation are very similar but the energy requirement for ECF is one to two order of magnitude lower than centrifugation (Danquah et al. 2009; Guldhe et al. 2016). For comparison among ECF methods for microalgae separation, the specific energy demand for both high and low biomass concentration cultures in this study is an order of magnitude lower than typical energy consumptions reported for freshwater media. e.g. Vandamme et al. (2011) reported a specific energy requirement of 2 kWh kg^− 1 for Chlorella vulgaris in fresh culture. However, the specific energy requirements observed here are comparable to those in saline conditions, where Vandamme et al. (2011) reported 0.3 kWh kg^− 1 for Phaeodactylum tricornutum separation in seawater. The mass-based specific energy decreased at higher biomass because more dry solids were removed per unit electrical work, while the volumetric energy remained similar since the batch electrical work was comparable (Table 1).

Baseline current in the logged runs for high biomass microalgae culture remained stable over 30 minutes, with occasional single-point peaks when sampling coincided with the 0.1 s pulse (Fig. 1). The lack of current drift suggests sustained electrode activity, likely due to reduced buildup of insulating oxide on the aluminium anode, which can occur during constant DC voltage application (Jotin et al. 2012). Another advantage of pulsed voltage application is that the interrupted current promotes rapid nucleation and detachment of small bubbles, which can improve attachment to fine microalgae flocs. Smaller bubbles provide a larger total surface area, thereby improving flotation efficacy (Khosla et al. 1991).

The total COD of the treated landfill leachate decreased 30% relative to the microalgae culture and 21% relative to the raw landfill leachate. This decrease likely combines removal of microalgal particulates formed during incubation with adsorption and sweep-flocculation of dissolved organics by in situ Al(OH)₃ formed at the anode, aided by H₂ flotation at the cathode. These mechanisms and their pH-dependent speciation are well established for EC in leachate, with COD removal generally maximised between pH 4 and 8 when amorphous Al(OH)₃ dominates the speciation (Jotin et al. 2012). The COD reduction is comparable to EC studies using iron electrode e.g. 32.4% COD reduction by Rookesh et al. (2022) and 35% COD reduction by Ilhan et al. (2008) in landfill leachate at near-neutral pH. However, the present COD reduction is lower than the EC studies which have used Al electrodes e.g. 74% COD reduction by Jotin et al. (2012) and 70% COD reduction by Dia et al. (2017). The lower COD reduction efficiency is probably because the present study didn’t optimise the pH, ECF treatment time and voltage and current input.

Although the COD removal was modest (21%), the specific energy requirement to achieve it was 1.35 kWh kg^− 1 COD which is lower than the specific energy reported for some other landfill leachate treatment technologies. For example, electro-Fenton achieved 91.90-93.35% COD removal at 3.32–6.24 kWh kg^− 1 COD (Li et al. 2022), electrochemical ceramic membrane filtration achieved 70.8% removal at 49.3 kWh kg^− 1 and combined bipolar flocc-oxidation with electrobioreactor systems achieved 94.7% removals at 32.02 kWh kg^− 1 COD (He et al. 2023). Although these comparative technologies target higher COD removals, the low specific energy requirement of ECF for moderate COD reduction suggests it may be a low-energy method for simultaneous microalgae separation and wastewater treatment.

Aged landfill leachate contains high levels of recalcitrant humic and fulvic substances that impart COD and colour. Traditional biological treatment often struggles with these compounds, as indicated by BOD₅/COD ratios that decline with landfill age from 0.5-1.0 (young) to 0.1 (aged) (Foo and Hameed 2009), reflecting the shift toward non-biodegradable organic matter. Studies comparing native and non-native biomass demonstrate that only indigenous organisms adapted to leachate can achieve > 75% COD removal, while conventional biomass achieves only ~ 40% due to inhibitory effects of humic substances (Corsino et al. 2020). ECF can aid in reduction of this recalcitrant COD through adsorption and sweep flocculation mechanisms.

5. Conclusions

This study shows that ECF can recover microalgae from real landfill leachate at low electrical energy, achieving replicated recovery of 94.7–99.7% at 0.22–0.40 kWh kg^− 1 TS, while delivering a measurable reduction in COD. Performance was demonstrated in a demanding matrix using time-resolved voltage–current logging and dual normalisation, providing a replicable benchmark. The combined microalgae separation and COD reduction support integration of microalgal processes into landfill leachate treatment for simultaneous waste treatment and resource recovery enabling circular economy at landfill sites.

Acknowledgement

The authors acknowledge the technical and logistical assistance provided by the landfill operator (Bergischer Abfallwirtschaftsverband, Engelskirchen).

Electronic Supplementary Material

Below is the link to the electronic supplementary material

Supplementary Material 1

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Yes