The laundry industry has substantial water consumption. The resulting generation of large volumes of wastewater poses environmental challenges due to the presence of contaminants and chemicals. This study focuses on the integration of hydrodynamic cavitation with anaerobic digestion to improve laundry wastewater treatment. Pretreatment with hydrodynamic cavitation improves biogas production for process wastewater to wash mats, showing a 13% increase after 1 h and a 69% increase after 2 h of cavitation exposure time at a digestion time of 10 days. This indicates improved anaerobic biodegradability and resource recovery through biogas formation. However, the cavitation treatment had a detrimental effect on the towel process wastewater due to the high content of unknown inhibiting constituents which hindered biogas production. This study highlights the importance of innovative approaches to environmentally friendly technologies in the laundry industry. By optimizing the combination, an efficient and sustainable solution can be found for laundry wastewater management while improving resource recovery through biogas production.

  • Hydrodynamic cavitation attack chemicals.

  • Chemical structures modified.

  • Inhibiting effects can be reduced.

  • Biodegradability can improve.

  • Elevated biogas synthesis rate.

The laundry industry consumes a substantial amount of water, around 15 L of fresh water per kilogram of laundry, resulting in large volumes of wastewater (Ciabatti et al. 2009). The quality of laundry wastewater depends on the materials being washed, as well as the cleaning agents used, which can include bleach, surfactants, disinfectants, and water softeners. Typically, laundry wastewater exhibits a high chemical oxygen demand (COD) exceeding 7 g/L, an alkaline pH, and elevated levels of turbidity, solids, and phosphorus compounds (Melián et al. 2023). Consequently, it is imperative to develop ecologically and economically efficient processes for wastewater treatment and pollutant removal (Oturan & Aaron 2014). Over the past few decades, advanced oxidation processes (AOPs) have attracted attention as promising methods for pollutant removal (Garrido-Cardenas et al. 2019). AOPs are chemical treatment procedures capable of removing recalcitrant organic compounds in wastewater (Rekhate & Srivastava 2020). These processes primarily focus on generating powerful oxidizing species, such as hydroxyl radicals (OH•) (Saharan et al. 2011).

The generation of these potent oxidizing species can occur during cavitation processes. Cavitation refers to the formation, growth, and collapse of vapor bubbles resulting from a reduction in the pressure of a liquid at a constant temperature (Gągol et al. 2018). The implosion of the bubbles takes place under high pressure of up to 1,000 bar and high temperature of up to 10,000°K (Benito et al. 2005). In aqueous systems, the extreme pressure and temperature conditions inside the collapsing bubbles lead to the dissociation of water into H• and OH• radicals, which can participate in subsequent oxidation and reduction reactions (Suslick et al. 1997). In the field of wastewater treatment, the radicals generated through cavitation possess high oxidation potential and can effectively oxidize various pollutants (Ozonek 2012).

While AOPs and cavitation hold the potential for complete mineralization of organic matter (Gągol et al. 2018), these processes are known for their high operational costs. Therefore, the integration of these processes with biological processes has emerged as an attractive approach (Andreozzi et al. 1999). Some studies have shown a positive impact on the anaerobic process when hydrodynamic cavitation is used as a pretreatment step, improving biogas yield (Patil et al. 2016; Lanfranchi et al. 2022). The biochemical methane potential (BMP) (Raposo et al. 2011; Da Silva et al. 2018) is used in this study to measure the influence of cavitation on the biodegradability of unknown constituents with high inhibiting potential.

The primary objective of this investigation is to explore the enhancement in biogas yield resulting from cavitation pretreatment at various durations of cavitation exposure times, utilizing two types of laundry wastewater as the substrate. We assess the effects of cavitation time period and laundry wastewater composition on biogas production, ultimately aiming to identify optimal process conditions for maximizing biogas generation through cavitation pretreatment.

Wastewater

The experimental wastewater consisted of two wastewater systems from a large-scale laundry company. Wastewaters from each washing process were used. This included wastewater from mat washing and towel washing (Table 1).

Table 1

Substrate characterization

ParameterMatsTowel
COD (mg/L) 2,649 6,609 
(mg/L) 0.88 7.65 
(mg/L) 0.14 1.58 
Ptot (mg/L) 4.93 19,7 
Conductivity (cm) 2,392 4,142 
TS (g/L) 2.70 – 
VS (g/L) 1.28 – 
Acid capacity KS4.3 609.67 1,227 
TOC (g/L) 1,583 – 
TNb (g/L) 37.24 – 
pH 7.66 11.06 
ParameterMatsTowel
COD (mg/L) 2,649 6,609 
(mg/L) 0.88 7.65 
(mg/L) 0.14 1.58 
Ptot (mg/L) 4.93 19,7 
Conductivity (cm) 2,392 4,142 
TS (g/L) 2.70 – 
VS (g/L) 1.28 – 
Acid capacity KS4.3 609.67 1,227 
TOC (g/L) 1,583 – 
TNb (g/L) 37.24 – 
pH 7.66 11.06 

Inoculum

The inoculum for anaerobic treatment was digested sludge from a local municipal wastewater treatment plant (Table 2).

Table 2

Inoculum characterization

ParameterInoculum
COD (mg/L) 11,791 
Conductivity (μS/cm) 8,381 
TS (g/L) 21.5 
VS (g/L) 12.9 
Acid capacity KS4.3 3,474.8 
pH 8.0 
ParameterInoculum
COD (mg/L) 11,791 
Conductivity (μS/cm) 8,381 
TS (g/L) 21.5 
VS (g/L) 12.9 
Acid capacity KS4.3 3,474.8 
pH 8.0 

Experimental setup

The laundry wastewater was first treated by cavitation and then transferred to anaerobic digestion, to investigate anaerobic biodegradability.

Pretreatment by hydrodynamic cavitation

Cavitation was achieved using a plug-flow-tubular-cavitation reactor, as shown in Figure 1. The reservoir was filled with the wastewater and the temperature was stabilized at 20 °C by a heat exchanger. A centrifugal pump with a flow rate of 30 L/min and a pressure of 2 bar feed the venturi (GRUNDFOS CME 5-4 A-R-A-E-AQQE U-A-D-N). Cavitation takes place in the plug-flow-tubular-cavitation reactor, when the liquid passes through a venturi. The kinetic energy of the liquid increases at the expense of the pressure. The pressure at the throat of the reactor drops to the vapor pressure of the liquid, consequently that part of the liquid is vaporized, creating vapor cavities. The induced cavitation bubbles collapse afterwards and generate radicals with high oxidation potential. Depending upon the ambient pressure conditions downstream of the venturi, new cavities are generated, increasing cavitation bubble rates and therefore the chemical impact on wastewater constituents. The smallest diameter of the venturi was 5 mm, followed by a cylindrical pipe with a diameter of 12 mm and a length of 300 mm. The cavitation number at the smallest diameter of the venturi was 0.58. Bagal & Gogate (2014) claimed that cavitation occurs when the cavitation number drops to about 1, and that the best cavitation performance is obtained at a cavitation number ranging from 0.1 to 1.0. Samples of 10 mL were taken every 10 min and the UV–Vis Spectrum was measured with a HACH® DR6000 UV–VIS spectrophotometer after filtration (0.45 μm). Additional samples of 500 mL were taken at 0, 60, and 120 min, to be used as substrates in the Biogas Potential Test (BPT).
Figure 1

Laboratory scale plant for pretreatment.

Figure 1

Laboratory scale plant for pretreatment.

Close modal

The process parameters to investigate the effect on BMP are shown in Table 3.

Table 3

Experimental setup

Experiment Nr.1234
Wastewater Mat Mat Towel Towel 
ᴓ Cavitation reactor (mm) 12 12 12 12 
Pressure (bar) 
Flow (L/min) 25.9 30.8 31.3 30.3 
Cavitation time (min) 60/120 60/120 60/120 60/120 
Digestion time (days) 10 15 24 26 
COD of the wastewater (mg/L) 1,592 2,412 5,166 6,634 
COD after digestion (mg/L) 159 724 1,033 1,327 
Experiment Nr.1234
Wastewater Mat Mat Towel Towel 
ᴓ Cavitation reactor (mm) 12 12 12 12 
Pressure (bar) 
Flow (L/min) 25.9 30.8 31.3 30.3 
Cavitation time (min) 60/120 60/120 60/120 60/120 
Digestion time (days) 10 15 24 26 
COD of the wastewater (mg/L) 1,592 2,412 5,166 6,634 
COD after digestion (mg/L) 159 724 1,033 1,327 

BPT analysis

Batch experiments using the liquid displacement method were carried out to evaluate the anaerobic biodegradability of the wastewater after cavitation using the BPT test method (Figure 2). One-liter bottles were used with 800 mL of working volume. In all the experiments, 300 mL of inoculum was added, the amount of substrate that would contain between 1,000 and 1,200 mg COD was calculated and added, and the remaining volume was filled with tap water. At the same time, bottles containing only inoculum were tested for endogenous activity and cellulose was used as a control. Conductivity and pH were measured before and after the experiment. If the pH was above 7.6, hydrochloric acid was added until a target value between 6.5 and 7.6 was reached. The bottles were closed with an adapted cap that had a hose connected to a measuring cylinder. When the biogas is produced, due to a change of pressure it is transported through the hose, creating a displacement of water that corresponds to the volume of gas produced. For each treated sample, three simultaneous fermentation tests were carried out. A discussion of outliers was carried out using a comparison with cellulose as a substrate within the same experimental setup.
Figure 2

BPT analysis.

Hydrodynamic cavitation was generated using a venturi with an internal diameter of 5 mm, followed by a cylindrical pipe with a diameter of 12 mm and a length of 300 mm. The changes in composition were examined using UV–Vis spectroscopy for in-situ analysis. Changes in absorbance were observed (Figure 3(a) and 3(b)), indicating variations in the molecular composition of the wastewater constituents by cavitation impact.
Figure 3

UV–Vis spectrum of cavitation experiments for two different mat wastewater samples.

Figure 3

UV–Vis spectrum of cavitation experiments for two different mat wastewater samples.

Close modal
The results for anaerobic degradation of mat wastewater after pretreatment with hydrodynamic cavitation revealed an initial adaptation step of the microorganisms to the modified substrates (Figure 4(b)). However, after 2 days, the microorganisms are adapted and can degrade the constituents. Figure 4(a) shows a significant increase (+69%) in biogas production after 10 days digestion time for wastewater treated with cavitation for 2 h. The wastewater treated with 1 h of cavitation pretreatment achieved a 13% increase in biogas formation.
Figure 4

Specific biogas production (BPT) after pretreatment with hydrodynamic cavitation: (a) mat wastewater sample 1 and (b) mat wastewater sample 2.

Figure 4

Specific biogas production (BPT) after pretreatment with hydrodynamic cavitation: (a) mat wastewater sample 1 and (b) mat wastewater sample 2.

Close modal

The repetitions of the experiment (Figure 4(b)) resulted in a similar improvement in biogas yield due to hydrodynamic cavitation. The highest biogas yields were observed with an exposure time of 1 h (+37%) while 2 h treatment led only to minor changes in specific biogas production (+8%).

The treatment of towel wastewater with cavitation showed no positive effect (Figure 5(b)). In Figure 5(a), even a slight decrease in specific gas production can be observed. It was found that the microbial community was severely impaired. Only after about 10 days, the degradation stabilized. This behavior was confirmed by further experiments. The wastewater composition with unknown inhibiting constituents is too challenging for anaerobic degradation, possibly due to transformation products produced by cavitation. Cesaro & Belgiorno (2013) confirmed that the positive effect of pretreatment can be reversed by very high concentrations of radicals. This ultimately leads to a deterioration of the biogas yield, which can be justified by a toxic milieu for the anaerobic microorganisms.
Figure 5

Specific gas production (BPT) after pretreatment with hydrodynamic cavitation: (a) towel wastewater sample 3 and (b) towel wastewater sample 4.

Figure 5

Specific gas production (BPT) after pretreatment with hydrodynamic cavitation: (a) towel wastewater sample 3 and (b) towel wastewater sample 4.

Close modal

Pretreatment with hydrodynamic cavitation can improve anaerobic biodegradability. The generated hydroxyl radicals as well as the mechanical stress and thermal reactions (supercritical conditions close to the imploding cavities) of the cavitation process can break down long-chain molecules, e.g. recalcitrants, making them more easily bioavailable.

However, anaerobic bacteria need time to adapt. An improvement in biogas production usually takes place after 3–5 days. Pretreatment with hydrodynamic cavitation improves biogas production for mat process wastewater, showing a 13% increase after 1 h of cavitation and a 69% increase after 2 h of cavitation exposure time. Longer exposure times do not lead to a significant improvement of biogas synthesis, possibly due to the synthesis of inhibiting substances by radical reactions. The formation of radicals, especially in combination with a high concentration of other bio-toxic compounds as in the case of the wastewater from towel washing, can reverse this effect. The deterioration of biodegradation might be caused by an increase of H2O2 synthesis by possible rebound of OH•-radicals. Due to the very complex wastewater composition with high contents of inhibiting substances, the limits and optimal conditions of the system have not yet been defined. This might be a starting point for future research.

The research highlights the potential of hydrodynamic-induced cavitation as a pretreatment method for laundry wastewater before anaerobic digestion, with the aim of improving biogas production. The influence of cavitation on anaerobic biodegradability showed experiment-dependent outcomes, in some cases demonstrating improved efficiency and in others displaying reduced performance. The choice of cavitation time is critical to the success of cavitation as a pretreatment method. The relationship between hydrodynamically induced cavitation and related chemical effects is still unknown. Further investigations into the specific characteristics of the cavitation number, dimension of cavitation reactor, cavitation flow regime, and their influence on chemistry and microbial activity within the wastewater are recommended to optimize biogas production.

Financial funding by the German Federal Ministry of Education and Research BMBF. Funding number 02WV1568F, as part of the WavE II project.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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