Membrane Bioreactor MBR
Category / type: suspended biomass, activated sludge, membrane filtration system;
Application: temporary use, containerised plants, isolated settlement as small island;
Population Equivalents (PE) served: from 2 up to 50 (Packaged and/or site assembled) and more than 50 tailor-made.
Normative references and guarantees: EN 12566-3, d.o.p. According to Regulation (UE) n.305/2011.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
The Membrane Biological Reactor (MBR) is a wastewater treatment that combine activated sludge processes and membrane filtration system.
It is a suspended biomass reactor with porous membrane; the final treatment yield depend on membrane porosity.
Membranes are frequently used for medium and large civil settlement mixed with industrial wastewater treatment plant.
MBR systems combine a conventional active sludge biological treatment with a membrane filtration process and provide a high level of suspended solids and bacteria removal without a secondary sedimentation chamber.
Membranes can be immersed in the aerobic biological reactor or placed outside in a technical room. Membrane’s porosity spans from 0.035 up to to 0.4 micron (depending on the manufacturer), which is a typical micro or ultrafiltration value range.
MBR requires small occupied areas and it is a very good solution for plants revamping where new volume is not available.
Membranes are conceptually a two-dimensional material used to separate fluid components through dimension and electrical charge. During filtration process components remain chemical unchanged because process is only physical. Fluid, that passed through pores of the membrane, is called permeate, while mixed liquor remaining in reactor is defined concentrate.
There are different types of membrane available in the market. The most common are: Hollow fibre (HF), spiral-wound, plate-and-frame (i.e. flat sheet FS), pleated filter cartridge and tubular
Blowers are used to supply oxygen to biological reaction, to mix waters and to clean membrane.
Membrane is kept clean by a tangential air flow, that limits membrane occlusion (fouling). A backwashing phase is periodically needed and, it’s often carried out using the permeate in order to save clean water.
How It Works
As well as all activated sludge systems, the main design factor is organic load measured in kg BOD5.
The main parameter for civil wastewater plant (or equivalent) projects is the population equivalent (PE); normally it refers to 60 g BOD5 per day and a flow rate from 150 to 250 litre per day.
Design requires calculation of the nominal daily organic load in terms of BOD 5 and the nominal hydraulic flow (QN) in cubic meters, according to EN 12566-3 and technical procedures.
Population equivalent (PE or sometimes used p.e.) is a conversion value which aims at evaluating non-domestic pollution in reference to domestic pollution fixed by EEC directive (Council Directive 91/271/EEC concerning Urban Waste Water Treatment) at 60 g/day related to BOD5.
European legislation defines Population total (PT) as a sum of population and population equivalent (PE).
A proper wastewater characterization for incoming organic loads a better estimation of nutrients are possible considering the possibility of dosing additives for biological reactions.
Sizing of the oxydation chamber follows activated sludge plants design criteria but, due to membrane efficiency, suspended solid concentration can be increased approximately between 12 and 15 kg/m3 that reduce reaction volume.
A denitrification section may be added according to effluent quality requirement (nitrogen control) and incoming wastewater concentrations.
Membrane sizing is strongly influenced by single commercial module specific available on the market, as reported in the product data sheet provided by the manufacturer.
Due to higher bacteria concentration in the reactor oxygen transfer rate is much lower (0.3 – 0.5) than extended aeration plants, therefore it is very important to calculate aeration system.
Upstream it’s necessary a pretreatment systems as fine screen (2 mm) or a suitable sedimentation basin (e.g mono or two chambers septic tank).
Pretreatment process improves system longevity and efficiency: since the presence of coarse clasts or other abrasive materials can damage membrane. It reduces maintenance costs and exceed sludge volume.
Tertiary treatment, as disinfection or filtration, can be added downstream for specific water reuse.
Sizing should be more accurate than primary treatment; biological process requires more constant water supply (therefore organic loads).
For biological line a small flow variability is possible but it is better to divide it into several lines, typically for medium and large plants.
Equalization tank is always a good solution to ensure a constant and pollutant load flow.
Operation and maintenance
Qualified personnel is required for maintenance operations: especially cleaning operation must be carried out by competent and authorized personnel trained on possible biological hazards, on membranes proper handling and on cleaning procedure.
In order to evaluate treatment performance and to control abnormal pollutants intakes that could completely stop the biological process or allows the development of a unuseful bacterial population (for example, filamentous bacteria) inlet water must be checked.
To control the bacteria process and to improve efficiency and membrane cleaning, nutrient and chemicals are added.
Excess sludge is greater than conventional systems because of better BOD reduction yield: it must be removed and treated.
Maintenance is similar to a normal activated sludge plant except for membranes. They must be periodically removed for physical-chemical cleaning operation.
MBR plant treats continuous flow and it can’t afford long inactivity time. A new start up is needed if the biological reactions don’t work for long time.
This kind of plants treats continuous wastewater flow but membranes typically require a cycling phases (filtration, cleaning and backwashing) so they need multiple line or equalization section.
The European standard EN 12566-3 “specifies requirements, test methods, the marking and factory production control for packaged and/or site assembled domestic wastewater treatment plants (including guest houses and businesses) used for populations up to 50 inhabitants…”
The manufacturer shall state the nominal organic daily load, expressed in kg of BOD5 or BOD7 per day and the nominal hydraulic daily flow, expressed in cubic meters per day.
Plant’s Nominal Designation is the proper daily organic load value (QN) (3.7).
The small wastewater treatment plant must demonstrate compliance with the performance of efficiency… and to operational data declared by the manufacturer: the declaration must be expressed in percentage.
It is also to be considered that the structural resistance and the hydraulic efficiency must be tested only on a representative model of the product line. Representative model for structural resistance test must be the bigger, supposedly as the one of series with the worst characteristic, while the one for hydraulic efficiency test must be the smaller, presuming the one with the worst efficiency.
The standard describes statics and hydraulics methods to determinate resistance and hydraulic efficiency.
Informations to be included in the test report and in the CE mark are also detailed in ZA.3 annex.
For small domestic wastewater treatment plants up to 50 PT (therefore for activated sludge plants), a Declaration of Performance (DoP), is required, as provided by the EU Regulation. 305/2011.
Features and benefits
MBR systems are widely used in industrial civil or domestic wastewater treatment and is also suitable for landfill leachate treatment.
It is one of the best systems for water recycling (irrigation), thanks to good effluent quality.
It is a high-tech system that needs proper planning and skilled and trained personnel.
Mechanical pre-treatment is absolutely necessary to prevent membrane malfunction that worsen the purification process.
- no secondary clarification compartment – simplification;
- no tertiary treatment required;
- footprint reduction;
- good sludge concentration and retention time;
- good effluent quality;
- no bulking problems;
- good BOD and suspended solids reduction;
- possible nutrient reduction;
- automatic and remote controls available.
- high initial investment (membranes);
- complex maintenance operation (chemical cleaning);
- higher operating costs and energy consumption;
- skilled and trained personnel required;
- replacing membranes after useful life (planned maintenance);
- pre-entrainment and equalization required.