How do biogas plants valorize digestate with membrane technology?

The advantages of MBR technology for digestate management

The biogas and biomethane market in Europe

The increase in the number of new plants processing biogas and biomethane in recent years responds to the growing market demand for biogas and biomethane. In fact, according to the European Biogas Association (EBA), in Europe between 2022 and 2023, the number of new plants increased by 30% compared to 2021, reaching a total of 1,322 plants in the first four months of 2023. What’s more, more than 75% of these are connected to transportation or distribution networks. Compared to previous years, the number of plants in Europe has increased significantly: in 2018 (483 plants), in 2020 (729), and 2021 (1,023). The main producers of biomethane are Germany (12,753 GWh), the United Kingdom (6,183 GWh) and Denmark (5,683 GWh).

What are the targets and forecasts in Europe for biogas?

  • The European Commission estimates that 350 TWh, or 33 billion cubic meters (bcm), of biomethane, could be produced annually in 2030;
  • The “European Gas Decarbonization Plan” aims to produce at least 1,000 TWh of biomethane by 2050;
  • According to the European Biogas Association, total emission savings through the use of biogas and biomethane can achieve significant emission reductions compared to fossil fuels contributing to the achievement of climate neutrality goals by 2050.

As demand for biogas grows and the number of plants in operation increases, the volumes of digestate generated are expected to grow dramatically. If not managed properly, digestate can slow down the development of the biogas and biomethane industry and thus also erode the entire circular economy created around this valuable material.

What are the benefits of effective digestate treatment?

The organic waste material from biogas plants, known as digestate, is the main by-product and if effectively enhanced determines the levels of sustainability, efficiency, and profitability of the entire plant.

There are many benefits of using technologies and solutions for the effective treatment of digestate:

  • Cost savings, mainly related to storage, transportation, and handling. The digestate is efficiently managed when landfarming is not possible.
  • Decrease environmental burden, whether the digestate is treated for safe disposal or valorized as fertilizer.
  • Potential for a new source of revenue from the sale of concentrated organic fertilizers, which are competitive with the increasingly expensive chemical-based ones.
  • Contribution to effective resource management: the depletion of the global natural reserves of phosphorous and potassium, together with the population growth and the increasing demand for nutrients, creates a severe imbalance. Therefore, the interest in alternative fertilizers is growing rapidly.

In conclusion, with the rapid increase in the number of biogas plants worldwide, digestate upgrading is expected to become an increasingly important activity for all market players.

The two general approaches for digestate management

  1. Direct filtration: volumetric reduction.
    Direct filtration with tubular UF membranes preserves and concentrates the nutrients for valorization, producing high-value biofertilizers with reduced use of chemicals. However, this approach is not always feasible or cost-effective: certain laws, such as the EU Fertilizing Products Regulation, impede fertilizer production when certain materials are fed to biogas plants. For instance, the organic fraction of municipal waste can be used exclusively when it has been separated at source; if it has been obtained as a separate fraction of the mixed municipal household, the use of digestate for producing fertilizers is forbidden.
  2. Membrane bioreactor for an integrated digestate treatment
    Due to regulatory barriers, competitive constraints, or even location-related restrictions, the biogas plant operator’s main goal may be to achieve maximum volumetric reduction, i.e., the minimum amount generated of concentrated flows. In such cases, the implementation of an advanced biological pretreatment where the reduction of organic, alkalinity, and nitrogen loads is carried on, allows a subsequent consistent and economical super-concentration of the liquid fractions. Digestate is rich in organic matter – part of it refractory or difficult to biodegrade – and nutrients, especially nitrogen: it can be compared to the most complex industrial wastewaters and its treatment demands a high-efficient and robust technology. External tubular MBR copes with the digestate treatment with maximum efficiency: the bioprocess operates stably with increased removal rates, and the highest nitrification rates are achieved. The MBR consists of a biological reactor that combines anoxic and aerobic stages, coupled with external tubular UF for separation of biomass from effluent.

Advantages of MBR technology

Hydrotech Engineering uses the latest technology in the field of semi-permeable membranes to enable the removal and recovery of nitrogen in digestate. MBR technology for digestate treatment has many advantages over conventional biological processes:

  • Highly efficient biological treatment: high MLSS concentration and long SRTs favor the growth of specialized biomass that assimilates the difficult-to-biodegrade organic matter.
  • Nitrogen removal: Long SRT promotes the growth and acclimatization of nitrifying biomass. Even with the unbalanced high N-content of digestate, it is removed highly efficiently by nitrification / denitrification.
  • Superior effluent quality, for discharge or reuse: the UF membranes are a physical barrier for suspended and colloidal matter: a high-quality permeate is obtained, free of solids, turbidity, and pathogens. Direct RO post-treatment is possible when water reuse schemes are of interest (SDI < 3).
  • Robustness and flexibility against the complex composition of digestate. COD and nitrogen removals are high and stable, even with large variability in digestate composition. The filtration conditions are easily adapted to influent fluctuations, providing the most stable operation under any circumstance.
  • Compactness: external MBRs are one of the most compact technologies for digestate treatment, with minimized bioreactor volume and a modular and plug-and-play UF system. Retrofitting and upgrading existing CAS or SBR plants is simple and fast.

Case study tubular MBR of a Hydrotech Engineering plant in Italy

Hydrotech Engineering designs develops, and builds state-of-the-art process water and wastewater treatment plants using semi-permeable membrane technology and biological processes for water recycling. With more than 23 years of experience and 24 biogas plants designed and built, Hydrotech Engineering supports industry operators through automated and high-performance plants. Let’s see below details and process of a biogas plant built by Hydrotech Engineering.

Digestate infographics

What are the management costs of the plant? 

The cost per ton of treated water is equivalent to 10.18 euros (excluding evaporation). Let’s take a look at the table below at costs and energy consumption by departments.

Departments Energy consumption Costs
Biological unit 24.0 kWh/t 4,80 €/t
Ultrafiltration 6.0 kWh/t 1,20 €/t
Reverse osmosis 6.8 kWh/t 1,36 €/t
Super-conc. 4.0 kWh/t 0,80 €/t
Chemicals 0,85 €/t
Consumables 0,67 €/t
General maint 0,50 €/t
Total (without evaporation) 10,18 €/t

Would you like to discover more about the plant, tons treated, water characteristics, and MBR technology?

Download the full case study here

Hydrotech Engineering’s plants are highly efficient, plus high-quality water is obtained and can be reused in industrial processes or discharged by the most stringent environmental regulations. Learn more about biogas plants built by Hydrotech Engineering here.

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