Action Group “Anaerobic Membrane Bioreactor for Recovery of Energy and Resources to achieve Sustainable Water Reuse (ID AG036)”: review of results.

Action Group “Anaerobic Membrane Bioreactor for Recovery of Energy and Resources to achieve Sustainable Water Reuse (ID AG036)”: review of results.

The Action Group “Anaerobic Membrane Bioreactor (AnMBR) for Recovery of Energy and Resources to achieve Sustainable Water Reuse” started its activities in 2013. The objective of this AG, formed by multidisciplinary scientific and industrial members that work in the areas related to wastewater treatment and MBR technology, is to achieve a new design for a sustainable and economic wastewater treatment based on this technology. The final aim will be to offer new market opportunities over conventional wastewater treatment technologies. The double experience both in the public and private sector will improve the technological transference of the activities developed, enhancing the professionals perspectives and employability in this sector.

Among these 3 years, the AG has incorporated new members up to 14 within the consortium, and has turned into a representative group of the water value chain.

The work developed by the AG was divided into several activities, which address the main challenges that the AnMBR technology for wastewater treatment has to deal with. In the following lines, the main results obtained after these 3 years are presented:  

Hollow-fibre (HF) membrane technology can be considered a promising, competitive technology for the anaerobic treatment of urban wastewater. AnMBR fitted with industrial-scale hollow-fiber membranes for urban wastewater treatment reveals that the energy demand for membrane scouring by gas sparging represents the main factor contributing to power requirements in anaerobic membrane bioreactors.

When long term operation is considered, the formation of cake or gel layers may also reduce the influence of membrane properties over filtration performance, since the membrane is not anymore in direct contact with the suspension. Dynamic membrane (DM) technology may be a promising approach to resolve fouling problems. Advanced control techniques to improve membrane operating mode and minimise chemical membrane cleaning have demonstrated an optimal filtration performance under minimum membrane fouling.

Fouling control strategies should be focussed on increasing the net sustainable membrane flux for practical engineering application of AnMBRs. One key challenge for sustainable full-scale AnMBR operation consists in achieving proper membrane performances under minimum operating cost whilst minimising membrane fouling, particularly irrecoverable/permanent fouling that cannot be removed by chemical cleaning. The extent of irrecoverable/permanent fouling is what ultimately determines the membrane lifespan. It is therefore necessary to optimise filtration whilst minimising not only capital expenditure (CAPEX) but also operating and maintenance expenditure (OPEX). Gas sparging intensity, usually measured as the specific gas demand per permeate volume (SGDP) or as the specific gas demand per membrane area (SGDm), is considered a key operating parameter to maximise energy savings in AnMBRs. Thus, new on-line controllers aimed at optimising gas sparging intensity for membrane scouring and membrane operating modes are of high interest.

The most sustainable and economical alternative for this technology is mainly given by: optimising SRT for any operating temperature to maximise methane production and both minimise and stabilise the produced sludge; optimising the design volume or the operating strategy for reducing the increase in MLTS due to increasing SRT; increasing the COD/SO4–S ratio of the wastewater when necessary by incorporating, for instance, the organic fraction of different solid wastes; and recovering the methane dissolved in the effluent.

Most of micropollutants are degraded aerobically (e.g. Alkylphenols, APs). Overall, AnMBR technology offers the possibility of complete degradation of some micropollutants (APEOs and APs) when combining with oxidation processes, such as CAS, microalgae cultivation or aerobic MBR. The use of microalgae as aerobic process is especially relevant, due to microalgae culture can produce dissolved oxygen without energy input.

Among the different schemes that can be found in literature, AnMBR based-technology could be proposed as itself or with primary settling and further anaerobic digestion of the wasted sludge (McCarty et al., 2011). When ambient temperature is not so high, including a previous settling step and anaerobic digestion in the AnMBR based-scheme could reduce the reactor volume required to achieve the same methane production. Due to the high COD in primary and wasted sludge, anaerobic digestion can be operated at 35ºC using the biogas produced. Therefore, the SRT required in the anaerobic digestion will be lower than in the AnMBR system. assessed the economic impact of including in AnMBR based scheme, a primary settling stage and further anaerobic digestion of the wasted sludge. AnMBR without primary settling and anaerobic digestion was identified as the most economic option for an AnMBR-based WWTP treating low-sulphate urban wastewater (maximum surplus energy of 0.1 kWh per m3), whilst AnMBR with primary settling and anaerobic digestion was the optimum option when treating sulphate-rich urban wastewater with maximum surplus energy of 0.09 kWh per m3, (Pretel et al, 2015).

Microalgae cultures have been used successfully to treat artificial and real wastewater (Ruiz-Marín et al., 2010) and to eliminate nutrients from samples taken at different points in a wastewater treatment plant, as well as in tertiary treatment (Wang et al., 2010; McGinn et al., 2012), or after an anaerobic digestion (Sahu et al., 2013). Others, e.g. Li et al. (2011), used centrate for microalgal growth. Also, Ruiz-Martinez et al. (2012) studied microalgae cultivation for nutrient removal from an AnMBR effluent. The satisfactory percentages of nutrients removed in some cases confirm the possibility of combining wastewater treatment with microalgal biomass cultivation. However, long-term data of microalgae cultivation for nutrient removal is scarce and to a lesser extent for AnMBR effluents.

A design methodology that holistically considers the key operating factors that affect both biology and filtration is still necessary in order to lay the foundations for the optimum design of full-scale AnMBRs for urban wastewater treatment. Ferrer et al., (2015) proposed a design methodology on the basis of simulation and experimental results from an AnMBR plant featuring industrial-scale hollow-fibre membranes. The proposed methodology aims to minimise total annual costs, which are defined as the sum of capital and operating expenses (CAPEX/OPEX). OPEX take into account energy requirements, methane production and capture, sludge handling and disposal, and membrane maintenance and replacement. Based on this methodology, an AnMBR WWTP operating at 15 and 30 ºC with both sulphate-rich (5.7 mg COD•mg-1 SO4-S) and low-sulphate (57 mg COD•mg-1 SO4-S) urban wastewater was designed. Results showed that the net energy consumption was 0.14 and -0.07 kWh per m3, respectively. On the basis of these results, from an energy perspective, AnMBR is a promising sustainable system compared to other existing

In any case, improving the feasibility of AnMBR technology for wastewater treatment still must address some issues: recovering efficiently the methane dissolved in the effluent; treating low-loaded wastewaters at low temperatures, treating sulphate-rich wastewaters which reduces the energy production due to the important fraction of COD that is consumed by sulphate-reducing bacteria (SRB) (this is significantly important when treating low-loaded wastewaters), discharging AnMBR effluent which may contribute to eutrophication as a function of the nutrient content of this stream; and most of micropollutants are not anaerobically degraded.

It is also worth to highlight the importance of establishing an EU framework for:

  • Quality effluent requirements for irrigation purposes to exploit the nutrient content in the AnMBR effluent.
  • Quality sludge requirements for farmland purposes to exploit the sludge produced in the AnMBRs (giving an added value to this stream).