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Membrane Bio Reactor

PC Engineering MBR Introduction:

What are membrane bioreactors?

Membrane filtration has a major role in water and wastewater treatment, which is superior to the conventional water technologies with a proven better performance and more efficient economics. The basic membrane processes are microfiltration (MF), ultrafiltration (UF),  Separation ranges for those membranes are as follows: 100 to 1000 nm for MF, 5 to 100 nm for UF, 1 to 5 nm for NF, and 0.1 to 1 nm for RO.  For more than the last 10 years MBRs have emerged as an effective secondary treatment technology by using membranes in the range of MF and UF.

                  Fig. No. 01 Classification of membranes according to pore size

A Membrane bioreactor (MBR) processes are mainly used for wastewater treatment (WWT) by using microfiltration (MF) or ultrafiltration (UF) and integrating them with a biological process like a suspended growth bioreactor. The membranes are employed as a filter removing the solids which are developed during the biological process, which gives a clear and pathogen free product. A visual example can be found from the following picture of an immersed MBR (iMBR) in following fig. no. 02

Above Fig., The wastewater goes through a fine screen for the removal of big objects that might cause damage to the downstream equipment. Then it enters an Anoxic Zone for the treatment of nitrogenous matter and phosphate following an Aerobic Zone where microorganisms with the help of the oxygen coming out of the FBD will digest the organics matter in the wastewater and clump together as they do so, producing a sludge. This sludge will enter the Immersed Membrane Bioreactor where the membrane will separate  the solids and microorganisms from water.

A membrane bioreactor is essentially a replacement in the conventional activated sludge (CAS) system for the settlement tank for solid/liquid separation. 

The MBR gives to the end user improved process control and much better product water quality.

The MBR process operates over a considerably different range of parameters than the conventional activated sludge process

SRT 5 -20 days for conventional system - 20 -30 days for MBR

F/M 0.05 -1.5 d-1for conventional system - < 0.1 d-1for MBR

MLSS 2,000 mg/L for conventional process - 5,000 -20,000 mg/L for MBR

In general MBRs have three distinct membrane configurations (Fig.3),

1. flat sheet (FS)

2. hollow fibre (HF)

3. multitube (MT)

Fig. No. 03 MBR membrane configurations, A) Hollow Fiber, B) Multitube, C) Flat Sheet

MBR Applications

MBRs are generally a preferred option when,

1. There’s limited space

2. End user requires high quality treated water (e.g. for water reuse)

Increasingly tighter environmental regulations together with a decreasing MBR CAPEX and OPEX has led to an boost in installations and in size all over the world. MBRs have now been implemented in more than 200 countries worldwide with a number of plants over 4,200 m3/d in capacity and their growth rates of up to 15% are regularly reported in various market analyses.

Usually MBR technology is applied to those wastewaters with a readily biodegradable organic carbon content. The latter is especially true when it comes to the food and beverage sector which has made an extensive use of MBR technologies.

Wastewaters markets which are containing sparingly biodegradable content (e.g. landfill leachate and pharmaceutical effluents) have also seen a grow of MBRs due to the long solid retention times (SRT) that allow for an improved biological treatment over the one from the conventional biological processes.

Waters that contain suspended oil (vegetable or mineral) require pretreatment (e.g. plate separation, dissolved air flotation or both) in order to protect the membrane.

Although the MBR global market is mainly dominated by a few major companies, the number of technology suppliers continues to grow, with over 70 MBR membrane module products available on the market today.

In general MBRs have been applied to treat effluent in a number of industrial sectors, like:

1.      food and beverage ? high in organic loading

2.      petroleum industry ? exploration, refining and petrochemical sectors

3.      pharmaceutical  industry – have active pharmaceutical ingredients (APIs)

4.      pulp and paper industry ? high levels of suspended solids, COD and BOD

5.      textile industry effluent ? re-biodegradability, toxicity, FOG content and color

6.      landfill leachate ? wide variety of dissolved and suspended organic and inorganic compounds

7.      ship effluents ? legislative requirements and space restrictions.

8.      Industrial versus municipal treatment

What are the advantages of MBRs?

It's generally acknowledged that membrane bioreactors have a number of advantages over other wastewater technologies,

I.    Independent control of HRT and SRT

II.   High quality effluent

III.  Small footprint

IV.  Improved bio-treatment.

V.   Independent control of HRT and SRT

VI.  MBR OPEX

I.    Independent control of HRT and SRT:

As the biological solids (mixed liquor or sludge) are completely contained in the bioreactor, this allows for the solids retention time (SRT) to be controlled independently from the hydraulic retention time (HRT). In the CAS process, the flocculant solids (‘flocs’) that are essentially the biomass have to be allowed to grow in size to the point where they can be settled out in the secondary clarifier. So in CAS the HRT and SRT are connected; as the HRT increases, the flocs have to grow which then increases their settleability.

II.   High quality effluent

With the membrane pores of the MBR being of a small size (<0.5), the treated effluent has a very high clarity and significantly reduced pathogen concentration compared with the CAS process. The effluent has enough high quality to be discharged to water bodies or to be used for such applications as urban irrigation, utilities or toilet flushing. It can also be fed directly to a reverse osmosis process in order to get a permeate of even higher water quality.

III.  Small footprint

CAS has a high HRT which leads to a larger plant size required. In MBRs, due to higher concentrations obtained, the same total mass of solids is contained in a smaller volume, so the footprint is smaller.

IV.  Better bio-treatment

MBRs have higher SRT which tends to provide better overall bio-treatment due to encouraging the development of the slower-growing micro-organisms, specifically nitrifiers. This fact makes MBRs very effective at the biological removal of ammonia (‘nitrification’).

V.   What are the disadvantages of MBRs?

The key disadvantages of an MBR are the operational process complexity and the cost which is translated to CAPEX and OPEX.

Both of the latter are highly sensitive to the cost of the membrane. The OPEX is additionally sensitive to,

the membrane life

permeate flux

the membrane air scour rate (air scour energy)

VI.   MBR OPEX

In general, the major elements that affect the MBR OPEX include:

1. membrane cost / m2 membrane surface area

2. membrane life in years

3. net permeate flux (product flow/unit area), taking account of the downtime and the use of the product water for cleaning the membrane

4. membrane specific aeration demand (SADm) in Nm3/m2 membrane area/h. Nm3 is the air volume at a temperature of 20°C and 1 bar pressure.

5. land costs/ m2 land area

6. energy cost/ kWh

7. value of the improved water quality

(7) is not readily quantifiable but it can be translated to the quantitative environmental footprint through life cycle analysis.

Evaluating CAS vs MBR

In order for CAS to produce an effluent with the same water quality to that of the MBR it may need all or some of the following:

1. increased tank sizes (and therefore a large sized land area) for extended HRT

2. increased chemical dosing to achieve high phosphorus (P) concentration

3. post-treatment with either a multi-media filter (MMF) or ultrafiltration/microfiltration (UF/MF) to achieve a comparable treated water 

quality to the MBR with reference to suspended solids (SS) and microorganism concentration.

For existing plants retrofitting them with an MBR is determined by the available area. This allows for improving the treated water quality and /or increasing the flow capacity albeit at higher energy demand.

MBR configurations

MBR membrane filtration has two major configurations; 1) vacuum-driven membranes immersed directly into the bioreactor (iMBR) and 2) pressure-driven filtration in side-stream MBRs (sMBR) (Fig.4).

 

   Fig. No. 4, Main commercial MBR configurations; (A) Immersed MBR, (B) Sidestream MBR

MBRs need a shear force over the membrane surface in order to avoid membrane fouling from the wastewater contents and is critical in maintaining a desired permeate flux.

When an air/ liquid stream flows parallel to the membrane surface it creates a shear force which helps limiting the degree of fouling on the latter (Fig.5).

Fig.No. 5, Using air bubbles from the aeration to scour clean the surface of the MBBR membranes

Immersed processes are using the aeration in the bioreactor for this very reason but sidestream MBRs need to employ pumping, as with most other membrane processes. This difference in energy demands explains the iMBR configuration market dominance.

Also the fouling in the sMBRs is more higher due to the pumping of activated sludge which increases the shear stress to microbial flocs, causing them to break-up, which leads to a decrease in particle size and the release of foulant material.

The MBR configurations have three principal membrane configurations currently employed inpractice,

1. flat sheet (FS)

2. hollow fiber (HF)

3. multi-tube or multi-channel (MT/MC)

Although sMBRs are more energy intensive than iMBRs, they offer a number of advantages:

a.   reduced membrane area requirement, from higher flux operation

b.  operational flexibility for operation & cleaning cycle; unlike iMBR in-situ chemical cleaning of the membranes can be performed without any chemical risk to the biomass

c.  maintenance and plant downtime costs, particularly for membrane module replacement, are generally slightly lower; the modules are readily accessible and so can be replaced in a much shorter time than for the immersed membranes

d.  the membrane modules can be brought on- and off-line according to hydraulic loading

e.  operation at higher solids concentrations is possible

f.  operation at a lower energy demand is possible if the pressure and flow rates are reduced or if the membranes are configured for air-lift operation, though this then demands more membrane area

For small flows of hard to treat effluents, the sMBR is often predominant due to its simple operation, smaller footprint and simpler maintenance especially when it comes to membrane replacement. For very large plants, the iMBR is always selected with the HF membranes since the OPEX is usually lower.

For intermediate flows, we usually choose an HF or FS iMBR. The FS is more simple to operate but both the membrane cost/ m2 area and the energy demand are slightly higher than the HF configuration.

Finally, although most of the sMBRs use the classical pump configuration, if the membranes are arranged in series of a serpentine pattern, they can operate in air-lift mode which has an energy demand almost the same like the iMBRs and so can be considered for municipal wastewater applications 

with the advantage of lower footprint and effective rag removal. The membranes though are usually more costly than the immersed ones.

MBR operation and maintenance – fouling, clogging and cleaning:

Permeability decline in MBRs is caused mainly by membrane fouling and membrane channel clogging:

1. By ‘fouling’ we mean the coating of the membrane surface or the plugging of the membrane pores with dissolved, colloidal or fine solids. It is normally removed by the physical and chemical cleaning cycles.

2. By ‘clogging’ we mean the agglomeration of gross solids within or at the entrance to the membrane channels. Clogging within the channels is sometimes referred to as ‘sludging’.

In municipal WWT, membranes may sometimes also become clogged with ‘rags’ (or ‘braids’) formed from aggregated filamentous matter (specifically textile fibres such as cotton wool). We refer to this as 'ragging' or 'braiding'.

 

                                       Fig. No. 6. Permeability decline in MBRs

Membrane cleaning can be either physical or chemical. Physical cleaning removes gross solids attached to the membrane surface (reversible fouling). Chemical cleaning removes more tenacious material (irreversible fouling).
 
1. Physical cleaning is normally performed by ‘backflushing’ (reversing the flow back through the membrane) which may be enhanced by combination with air, or ‘relaxation’ (ceasing permeation while continuing to scour the membrane with air bubbles). These two techniques may be used in combination
 
2. Chemical cleaning usually uses sodium hypochlorite (NaOCl), an oxidative chemical, in combination with mineral or organic acids (most often citric  acid, C6H8O7) without removing the membrane from the tank or skid (cleaning in place or CIP). If chemical cleaning is combined with backflushing this is normally referred to as a ‘chemically-enhanced backflush’ (CEB). CEBs are routinely carried out on a weekly/monthly basis for HF iMBRs.
 
Physical cleaning is 1) generally rapid, 2) demands no chemicals, 3) generates no chemical waste and 4) is less likely to degrade the membrane. However chemical cleaning is always required at some points since there remains a residual resistance which can be defined as ‘irrecoverable fouling’ which may build up over an amount of time and decrease the membrane life.
 
Chemical cleaning which aim is to recover permeability requires higher reagent concentrations and longer contact times.
 
Neither the routine physical clean nor the chemical clean can be expected to counter clogging, which demands manual intervention (removal of the immersed  membrane from the tanks) to remove the agglomerated material.  

 

Fig.No. 7, MBR membrane cleaning physical and chemical cleaning procedures

                            Fig. No.8, MBR Unit Process Diagramm