Credit: Judd & Judd Ltd

1. Introduction

The increasing emphasis on energy and resource recovery and the inexorable drive towards net zero has placed ever more focus on anaerobic wastewater treatment technologies generally. Compared to conventional aerobic processes, which consume significant energy for aeration, anaerobic treatment can substantially reduce the energy required for organic carbon removal and offer a means of capturing its latent energy through conversion to methane. It follows that anaerobic treatment can potentially reduce both costs and net carbon emissions.

A number of specific factors associated with the current energy landscape have been driving the adoption of anaerobic wastewater treatment as relating to bioenergy production:

• Unprecedented rise in energy and gas prices: Despite declining from the 2022 peak of >€200/MWh, EU gas prices continue to fluctuate in the €40–50/MWh range – still around twice pre-crisis levels and subject to ongoing market uncertainty.
• Energy dependency amidst global instability: Both the European Commission and the International Energy Agency have intensified efforts to reduce reliance on external energy sources. The REPowerEU plan (launched in 2022 and worth €300 billion) remains a cornerstone, complemented by ongoing measures to diversify gas supply, expand LNG imports, accelerate renewable deployment, and reduce overall energy demand.

• Increasing environmental awareness of fossil fuels impacts: Since fossil fuels are a major contributor to climate change and global warming, growing public and regulatory awareness is driving a shift toward renewable energy and accelerating the transition to cleaner and more sustainable solutions.
• Sustainability as a driver of brand value and trust: Companies are increasingly adopting sustainable practices not only to reduce their environmental impact but also to strengthen corporate social responsibility through demonstrating both pollution abatement and resource recovery. Studies have shown a willingness of consumers to pay a premium for environmentally-responsible companies and their products, providing a further economic incentive.
• Targets for reducing primary energy consumption: Under the European Green Deal, the EU has committed to achieving climate neutrality by 2050. To make this goal legally binding, the European Climate Law sets an ambitious net greenhouse gas emissions reduction target of a minimum 55% by 2030, compared to 1990 levels, demanding implementation of disruptive technologies.

• Economic incentives for green energy production: The REPowerEU plan, which includes the 'Biomethane Action Plan,' aims to stimulate the renewable gas value chain. This initiative seeks to achieve the production of 35 billion m³ of biomethane annually by 2030, promoting sustainable energy solutions such as anaerobic technology whilst reducing dependency on fossil fuels.

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2. Anaerobic treatment

Anaerobic processes, which include anaerobic membrane bioreactor technologies, offer obvious sustainability benefits. These include primarily the energy benefit from the generated biomethane and the reduced waste sludge production. However, they are subject to some constraints when compared with the aerobic counterpart. These include (Table 1):

• a requirement for large bioreactor volumes due to the slow growth rate of methanogenic microorganisms,
• limited nutrient removal, particularly ammonia, and
• complex biokinetics, requiring careful control of pH, temperature and nutrient availability to avoid instabilities.

The energy benefit increases with the COD load, such that anaerobic treatment becomes more attractive generally for high-strength industrial effluents. However, conventional anaerobic processes are subject to biomass washout which then reduces effluent quality; post-treatment is almost always required due to the limited overall COD removal attainable.

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Anaerobic membrane bioreactor (AnMBR) technology addresses these challenges by integrating membrane filtration with the anaerobic biological process. As with the aerobic counterpart, this integration enables the decoupling of hydraulic and solids retention times (HRT and SRT), thereby reducing reactor footprint and enhancing bioprocess efficiency whilst also delivering a higher-quality treated water associated with membrane filtration. This means that the overall footprint of an AnMBR can be considerably smaller than a conventional anaerobic process – in the region of 25% bioreactor volume compared to conventional CSTR (continuous stirred tank reactor) anaerobic reactors. When compared with aerobic conventional activated sludge, bioreactor volume decreases even further, and can reach <20%.

Although first introduced in the 1980s in South Africa (by the company Wier En Vig), AnMBR technology saw significant breakthroughs only in the early 2000s. Today, it is a mature and well-established technology with proven performance across a range of industrial sectors, including dairy, confectionery, distilleries, food processing and bioethanol plants. Its adoption is being driven both by the energy and environmental impact agendas and by the increasing volumes of concentrated industrial effluents requiring treatment.

3. The AnMBR solution

The AnMBR is a compact and highly efficient wastewater treatment system that integrates anaerobic digestion with robust ultrafiltration (UF) membranes. This combination offers substantial advantages over conventional anaerobic technologies. By coupling UF modules with a simple CSTR, it enables stable operation under organic loading rates (OLRs) of up to 8–10 kg COD/m³·day. The UF membranes effectively retain biomass within the digester, supporting elevated mixed liquor suspended solids (MLSS) concentrations, often ranging between 15–25 g/L. This significantly improves the digestion process, making it more robust and increasing biogas production.

3.1 Process configurations

As with the aerobic counterpart, anaerobic MBR technologies are available both as the immersed and sidestream configurations, with the latter pre-dating the immersed configuration by more than 20 years. Whilst the immersed configuration was first implemented in 2008, full-scale references remain relatively limited. In contrast, the sidestream configuration has gained broader market acceptance and established a stronger track record of successful full-scale operation.

While the immersed configuration can potentially offer reduced energy demand associated with lower-energy membrane permeation, this can only be achieved by scouring the immersed membrane with biogas at a sufficient rate to promote a reasonable flux. Harnessing the biogas in this manner is challenging and, at <10 LMH, reported fluxes generated are only around 20% of the values attained for the sidestream configuration. This is because shear forces generated by the crossflow velocity (CFV) in the sidestream configuration are significantly greater than the corresponding forces achievable in the immersed system. The high-shear conditions effectively mitigate membrane fouling, compensating for the inherently lower filterability of anaerobic biomass relative to aerobic biomass.

3.2 The sidestream tubular AnMBR

The AnMBR sidestream (AnsMBR) comprises an anaerobic reactor with the mixed liquor pumped through a series of tubular membrane modules. The (UF) modules are arranged in a serpentine manner (Fig. 1) to extend the flow path and increase the conversion and so the permeation energetic efficiency.

Credit: Berghof Membranes

The AnsMBR offers a number of practical process attributes which combine to provide a higher return on investment than alternative treatment technologies for high-strength industrial effluents:

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• Superior and stable effluent quality: The AnsMBR consistently delivers solids-free effluent and achieves 95–99% COD removal efficiency (Table 2), significantly exceeding the 75–90% COD removal efficiency typical of conventional anaerobic systems. Consequently, post-treatment is often not required; the UF permeate can be discharged directly into municipal sewer systems, reused, or further polished by reverse osmosis (RO) to meet high-end water reuse requirements. Recorded feed and effluent COD concentrations indicate good tolerance to shock organic loads (Fig. 2).

• Reducing or eliminating pretreatment: the system's high tolerance to total suspended solids (TSS) and fats, oils, and grease (FOG) simplifies pre-treatment, with the DAF (dissolved air flotation) pretreatment step – often implemented in conventional treatment – no longer being required in an AnsMBR-based treatment scheme.

• Maximized biogas production: The complete retention of organic solids and the accompanying extended sludge retention time (SRT) significantly enhances the removal of COD and BOD. The AnsMBR technology can increase biogas production by over 20% compared to conventional anaerobic treatment systems, and production is further increased by the elimination of the DAF pretreatment since the organic load to the anaerobic digestor is increased.

• Robustness and reliability: UF membranes prevent biomass washout, enabling rapid start-up, fostering of specialized microbial communities, and providing resilience to organic or toxic shocks.

• Reduced sludge production: By eliminating the DAF pre-treatment, chemical consumption is significantly lowered, and the generation of large volumes of unconditioned sludge is avoided. This not only reduces the need for on-site sludge handling and treatment but also the logistical and environmental burden of off-site sludge disposal.

Credit: Berghof Membranes

In addition, the tubular membrane separation component of the sidestream process configuration offers a number of benefits:

• Stable filtration: Tubular membranes operating in crossflow mode are well-suited to handling anaerobic sludge, known for its low filterability. The high shears generated mitigate membrane fouling, allowing for prolonged operation between clean-in-place (CIP) cycles. The CIP process can be fully automated, minimizing operational downtime and ensuring consistent performance.

• Compact and space-efficient design: The modular and compact UF skids, high organic loads in the digester, and minimized pre- and post-treatment result in a significantly reduced overall plant footprint.

• Reduced maintenance and ‘out-of-tank’ service: The membrane modules are designed for straightforward, clean and easy access, ensuring fast inspection and simplifying maintenance.

• Long-lasting, durable membranes: Engineered for strength and reliability, tubular membranes are resistant to mechanical stress and breakage. With a proven operational lifespan of 6–8 years or longer, they offer long-term performance with minimal replacement needs – contributing to lower lifecycle costs.
  

4. AnMBR vs. other anaerobic technologies

The attributes of the aerobic and anaerobic treatment processes (Table 1), and specifically the impact on energy demand, mean that anaerobic treatment is generally more suitable for high-strength wastewaters (COD>10,000 mg/L) whereas aerobic treatment is preferred at lower influent concentrations (COD<1000 mg/L). For high-strength wastewaters key competitors to AnMBR technology are the granular technologies. The success of both process types relies on the retention of the slow-growing methanogenic bacteria within the reactor, for which efficient decoupling of hydraulic and solids retention times (HRT and SRT, respectively) is required. In the case of MBRs, retention is through rejection by a membrane, and for anaerobic filters by adsorption onto a filter media. For granular technologies, retention relies on accelerated particle settling through forming high-density granular biomass.

4.1 Advantages over conventional granular technologies

For industrial wastewater treatment, granular sludge reactors have been widely applied in their different process configurations, namely upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB), and internal circulation (IC). Granulation, the process of forming dense microbial aggregates, is influenced by several factors including hydraulic conditions, wastewater composition, and various physico-chemical parameters. While granular technology is feasible, well-stablished and efficient in many scenarios, it also presents certain limitations and operational challenges that must be carefully managed:

• Effluent quality concerns: Degranulation and subsequent biomass washout can deteriorate effluent quality.
• Incomplete COD removal: Partial or incomplete removal of COD can reduce biogas production and lower overall treatment efficiency (Fig. 3).
• Prolonged start-up time: Start-up of the process is long and complex, requiring seeding with mature granular sludge which may not be easily sourced.
• More complex reactor design: Compared to simpler systems such as CSTR-based AnMBRs, granular sludge reactors feature more intricate internal configurations, increasing construction complexity and maintenance requirements.
• Lower resilience to challenging feedwaters: Sidestream AnMBRs are more tolerant of high salinities, high TSS concentrations (e.g. 5 %), FOG (e.g. 0.5 %) and the presence of toxic substances that otherwise can hinder granulation (Table 3).

Credit: Berghof Membranes

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5. Membrane permeability and process energy consumption

Membrane permeability is sustained by crossflow velocity, backwash (when applied, based on system design), flushing and chemical cleaning (i.e. CIP). CIP cleaning is typically applied once every 1–2 months, depending on sludge characteristics and operating conditions. An alkaline/oxidant cleaning step, e.g. NaOH at pH 11 combined with approximately 250 mg/L as Cl₂, is generally applied to remove organic fouling. This is usually followed by an acid cleaning stage to help remove inorganic scaling. The acid clean step, typically performed with citric acid at 2–3 g/L, is particularly important for feedwaters more prone to form scales such as struvite.

As a rule of thumb, anaerobic sludge characteristically delivers approximately half the steady-state permeate flux (or membrane permeability in flux per unit transmembrane pressure) achieved with aerobic sludge under comparable operating conditions of crossflow filtration. The lower filterability of anaerobic sludge is mainly attributed to higher concentrations of soluble microbial products (SMP), a larger colloidal fraction, poorer floc structure and higher viscosity – all of which increase filtration resistance.

At crossflow velocities (CFVs) of 2–4 m/s, typical permeate fluxes range from 25 to 55 LMH. Corresponding energy consumption varies between 2 and 5 kWh/m³, depending on the system design and operating mode (standard crossflow operation or low-energy configuration). Based on reasonable assumptions for the various energy components, there is a positive net energy for process operation which increases with increasing COD load (Figure 3).

Credit: Berghof Membranes

6. Summing up

Anaerobic treatment of industrial effluent streams is becoming increasingly accepted, particularly for high-strength effluents where the amounts of biogas generated can potentially produce a positive net energy balance. The inclusion of membrane separation for retaining the biomass permits a TSS-free permeate and COD removals of 95–99%.

The robustness of anaerobic sidestream MBR technology (AnsMBR) to high loads means that DAF pretreatment can be avoided, minimizing chemicals use and maximizing conversion of the organic matter (including suspended particles and the normally problematic FOG component) into biogas. The high biomass concentrations and high loading rates permit smaller bioreactor volumes than those required for conventional processes.

OPEX is lower than for aerobic processes because no aeration is required, either for biological treatment or for membrane scouring, and biogas utilization can further offset energy demand. Sludge volumes generated are lower than for the aerobic counterpart. The sidestream configuration permits regulation of the shear, through adjusting the crossflow velocity, and easy membrane access.

Tubular UF membrane-based AnsMBRs are ideal for treating high-strength (high COD, FOG and TSS) industrial effluents such as food and beverage wastewaters from dairies, distilleries and confectionery production where viscous, highly-concentrated mixed liquors are generated. The superior fouling control provided by the high crossflows attainable for this configuration offers a significant advantage over immersed anMBR systems.

Given the trend towards more highly concentrated industrial effluents, to some extent promoted by process water minimisation, anaerobic treatment of effluents is likely to become increasingly viable. AnMBRs become particularly attractive where a solids-free treated wastewater is required – particularly for potential reuse – with an associated resilient process generating low levels of waste and permitting energy recovery.