Textile Wastewater Treatment With Membrane Bioreactor (MBR)

Textile wastewater treatment for industrial reuse remains a complicated problem due to several reasons. Among them is 1) Higher levels of Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Dissolved Solids (TDS) content of the wastewater; 2) Non-biodegradable nature of organic dyestuffs present in the effluent. Thus, any adopted treatment system, especially concerning primary treatment, should be able to address these issues. Many technological advancements have evolved to solve these problems, and here we describe one of the most critical improvements, namely, the Membrane Bioreactor (MBR).

 As far as textile wastewater treatment for industrial reuse is concerned, colour removal, reduction of Total Suspended Solids (TSS), BOD and COD are the main problems to be addressed in the primary treatment.

 Most commonly utilized primary treatment processes are

1. Conventional Physico-chemical treatment utilizing lime and ferrous/alum;
2. Traditional biological treatment with aeration to reduce BOD and COD;
3. Chlorination;
4. Ozonation (with/without ultraviolet irradiation).

However, specific problems are associated with these treatment techniques, as described here:

1. Conventional physicochemical treatment with lime and ferrous/alum generates a massive quantity of hazardous waste in the form of sludge, and its safe handling and storage is another problem. Further, conventional physicochemical treatment methods are not highly effective in the removal of colour, TSS, BOD and COD.
2. Though chlorination shall remove colour and result in the reduction of BOD and COD, the chemical reaction of dissolved chlorine with the organics present in the wastewater shall result in the production of chloro-organic compounds. These chloro-organic compounds may find their way into the environment, and they are potentially carcinogenic compounds even under a few parts per million (ppm) concentration. Therefore chlorination may not be an eco-friendly treatment method. Further, chlorine has to be either stored in containers or, produced onsite. In both cases, safety measures are essential since the accidental release of chlorine could severely impact the health of workers.
3. Ozone is highly reactive and potential oxidant than chlorine. Further, the reaction rate of ozone with organics present in the effluent is around 30 times faster than with chlorine. Ultraviolet (UV) irradiation of ozonized wastewater shall further increase the rate of reaction in addition to increasing the oxidation potential by the generation of hydroxyl radical (OH-). Therefore, even non-biodegradable molecules shall be broken down into smaller ones, resulting in better colour removal and reduction of BOD and COD. Besides, dissolved trace elements in the textile wastewater shall be oxidized and precipitated as their respective hydroxides. Such precipitation shall significantly reduce the risk of membrane poisoning in the reverse osmosis system, especially concerning dissolved iron. Though many advantages are there with the use of UV-Ozonation process, the system has to be highly optimized to get the best results as ozone generation efficiency is lower even under ideal conditions. Ozone production and transport to the treatment unit require special techniques as it fastly decomposes into oxygen. Presence of trace elements like Cu, Zn, Fe (which usually exists in the textile wastewater) and higher levels of TSS shall affect the UV-Ozonation process. Thus, UV-Ozonation process design should address these problems.

Conventional biological treatment is an alternative technique. But it suffers from the following problems:

1. Most of the dyestuffs are non-biodegradable. Thus, conventional biological treatment systems are less efficient [Here the term non-biodegradable implies the organic molecules that are resistant to biological degradation as well as those which take disproportionately longer time or, special conditions for complete degradation].
2. High TDS content of textile wastewater, typically in the range of 8,000 – 10,000 mg/L, retards microbiological growth. Because under high concentration of solutes the energy requirement for transfer of solvent (in this case water) across the cell membrane is more due to higher osmotic pressure of the wastewater. If biological growth is retarded, then the efficiency of the treatment system to bring down the BOD is affected. Under highly saline conditions, filamentous growth shall occur, resulting in poor settleability of suspended solids from the treated effluent in the clarifier. That poses a severe problem for the secondary treatment system.
3. The conventional biological treatment system shall not function correctly under low F/M ratio (due to low levels of BOD in the raw effluent) typical of textile wastewater. Sludge settleability in the clarifier is affected by the F/M, and its maintenance is of paramount importance for achieving best results.
4. In conventional biological treatment, the MLSS concentration does not exceed 4,000 mg/L; thus, the biological activity is less. That results in large aeration tank size, low BOD throughput, higher detention time and increased operating costs.
5. Since most of the dyeing houses use different dyestuffs, their biodegradability shall be changed. Depending on the dyestuff utilized and the shade required, it may be necessary to vary the concentration of dye bath for efficient dyeing to take place. As a result, the discharged combined raw effluent (dye bath + wash water) shall have widely fluctuating BOD and COD levels. However, the conventional biological treatment system could not withstand shocks, i.e., sudden changes in the raw effluent chemical characteristics. That is because the traditional biological treatment system does not have recirculation or, higher biological activity to dampen these variations. Also, due to substrate specificity, the composition of the microbial population shall be different depending on the chemical composition of the effluent. At a given moment, the microbial community in the aeration tank of the conventional biological system is specific to the chemical composition of the effluent, especially concerning the organics present in it. Due to low MLSS concentration and ineffective detention of microbial population in the aeration tank, the microbial community shall take some time to respond to the changes in the chemical composition of the textile wastewater. Thus, the conventional biological system is unable to yield stable treated water output.
6. Compared with MBR sludge production is higher in conventional biological treatment. That shall require more area for secured landfill.
7. Nutrient removal from the raw effluent is inefficient, and that shall encourage the growth of microbial organisms on the reverse osmosis membrane utilized in the secondary treatment of the effluent. Antifouling agents containing chlorine are applied to prevent biofouling of the reverse osmosis membrane. Since free chlorine shall damage the reverse osmosis membrane, the residual chlorine needs elimination with the addition of sodium metabisulphite (SMBS). Addition of chemicals invariably increases the operating cost and treatment overheads.

Membrane Bioreactor in Textile Waste Water Treatment

Though MBR technology got developed over a decade ago, its applicability for textile wastewater treatment remained elusive due to the involvement of substantial capital investment and the requirement of specific skills for its operation and maintenance. However, as technology evolved, the costs have been brought down. With process automation, now MBR has found its way into the textile wastewater treatment in many parts of the world, including India.

Recirculation of wastewater in the MBR

Membrane Bioreactor is an advancement over the conventional activated sludge process. It uses ultrafiltration or microfiltration membrane to maintain higher levels of Mixed Liquor Suspended Solids (MLSS). High MLSS concentration ensures good quality of treated water. Utilization of membrane filtration results in the retention of active microorganisms, extracellular enzymes generated by these microorganisms for degradation of the organics present in the effluent, organics resulting from cell-lysis, and other heavy molecular weight organics typical of textile effluent. Since some microorganisms, especially nitrifiers, are slowly growing one, their loss shall reduce the efficiency of the treatment system and nutrient removal. Under conventional biological treatment, these microorganisms might escape from the aeration tank and the weirs of the clarifier. In the MBR, these organisms get retained, and that results in better treatment. Besides, the retention of active enzymes secreted by microorganisms taking part in the metabolization of organics present in textile wastewater is an essential aspect of MBR technology. Maintenance of higher concentration of these enzymes shall result in rapid and better degradation of complex organic molecules present in the textile wastewater. Thus, the overall efficiency of BOD and COD removal is improved, detention time required to achieve specific BOD and COD get reduced, and the footprint of the treatment plant becomes less.

Though MBRs have various configurations, in general, submerged MBRs are well suited for textile wastewater treatment. Submerged MBR design incorporates two zones viz., i) anoxic; ii) aerobic[1]. The membranes may either be placed inside the aerobic chamber or, kept separately in another compartment. Maintenance of a separate compartment reduces membrane fouling.

Some bacteria can use oxygen as an electron acceptor when it is available. In the absence of oxygen, the same bacteria can switch the respiration mode to utilize nitrate/nitrite as electron acceptors. This kind of bacteria is called facultative bacteria. These bacterias can survive both in anoxic as well as in aerobic conditions unlike other types of bacteria that can survive only in anaerobic or aerobic conditions. The MBR has anoxic, aerobic, and membrane compartments with a recirculation from the membrane zone to the anoxic zone. The anoxic zone shall have deficient levels of dissolved oxygen (the condition chemically referred to as suboxic) brought back by the recirculating effluent. Thus, facultative bacteria dominate the microbial population in the MBR.

The bacteria growing under anoxic condition can break down recalcitrant macromolecules, which is then digested by the aerobic bacterial population persisting in the aerobic zone. In this way, a significant portion of the dyestuff and other organics could be broken down and oxidized. Thus, anoxic biological degradation is an essential step if we consider MBR treatment for textile wastewater.

Due to a high concentration of active microorganisms (MLSS is typically in the range of 10,000 – 15,000 mg/L), the MBR treated water has a uniform physicochemical characteristic, and the colour gets reduced, making this suitable for further treatment by the reverse osmosis system.

The quality of treated effluent output from Membrane Bioreactor is more stable compared to other treatment techniques, enabling the optimal functioning of the secondary treatment system. This stabilized output water quality is due to the prevalence of a steady-state condition inside the MBR compartments due to the maintenance of high microbial activity.

Also, the stable output from MBR is due to recirculation of wastewater from the membrane zone into the anaerobic and aerobic compartments and the maintenance of high MLSS concentration. If X is the inflow, and therefore outflow, anaerobic to aerobic and, aerobic to membrane flow is 4X, with a recirculation from membrane compartment to anaerobic being 3X to balance. Whenever the feed quality changes, its characteristics are dampened by this recirculation. The feed wastewater gets diluted by the recirculating fluid, and any oscillations in effluent parameters get dampened. The dampening effect shall be linear for all parameters having a linear relationship with concentration viz., BOD, COD, TDS, etc. For parameters having a logarithmic relationship with their strength, for example, pH this relationship shall be more complicated – but still, the changes in such parameters due to variations in effluent quality shall get dampened.

Similarly, due to the maintenance of higher MLSS concentration, spikes in BOD and COD in the influent shall be dampened within a short residence time due to high level of MLSS resulting in intense biological activity inside the MBR. Since high biomass concentration with different species of biologically active microbial organisms prevails in the MBR, sudden changes in the chemical composition of the feed shall not affect the performance of the MBR. Thus, the MBR treated wastewater shall have more stabilized chemical characteristics.

Availability of food (biologically digestible matter) for the microorganisms growing in the MBR is another crucial aspect to be considered. Food limitation is mainly dependent on the amount of food available (which is given by Flow x BOD = organic loading) and the amount of biomass available in the digestion chamber. Thus, for a given BOD level in the effluent, if the number of microorganisms per unit volume is kept high by recycling the sludge, better BOD reduction is achieved.

MBRs can be operated effectively under low food/microorganisms (F/M) ratio [2]. When the F/M ratio is low, the settleability of the sludge is increased [3]. Because under the low F/M, the microorganisms are under food-limited condition. Once the food is limiting, the rate of metabolism rapidly declines until the microorganisms are in the endogenous respiration phase with cell lysis and re-synthesis taking place [4]. Since microbial activity remains high due to their retention by the membrane, low food/biomass is maintained, leading to better BOD removal and good sludge settleability. Even if the sludge fails to settle down, the MBR membrane blocks the suspended flocs (biomass) from leaving the digester. However, in the case of conventional biological treatment, sludge retention is mainly dependant on its settleability in the clarifier.

Thus the advantages of using MBR can be summarized as follows:

1. Membrane Bioreactors have proven to be quite effective in removing both organic and inorganic contaminants as well as biological entities from wastewater. The removal of organic, inorganic, and microbiological organisms and suspended solids present an excellent output from these systems. Whereby the biofouling and chemical scaling of the reverse osmosis system get drastically minimized. It also reduces the use of cleaning chemicals in secondary treatment.
2. Since suspended particles are not lost, total separation and control of the solids retention time (SRT) and hydraulic retention time (HRT) are possible, enabling optimum control of the microbial population and flexibility in operation.
3. With MBR, the required HRT is lower (8-10 hrs) than conventional biological treatment process (15-28 hrs). That results in reduced tank volume.
4. MBRs operates at low F/M ratio and long SRT and generates less sludge. That reduces costs for sludge disposal and hassles associated with it. Further, sludge produced in MBR eliminates sludge bulking.
5. Since MBR operates under a low F/M, it minimizes oxygen consumption since microbes are in the endogenous respiration phase and not in the growth phase.
6. Under conventional biological treatment the nominal MLSS concentration is from 1,500 to 3,000 mg/L, and it never exceeds 4,000 mg/L. Whereas, with MBR high MLSS concentration (10,000-15,000 mg/L) [5] could be easily achieved, which allows more BOD throughput than conventional biological treatment system.
7. Compared to conventional biological treatment, MBR requires a smaller footprint per unit BOD loading or per unit feed flow rate. MBR is ideal for expansion of existing facilities without an increase in the footprint of the treatment plant. Usually, the footprint required for MBR is about half or less compared to the conventional biological treatment process.
8. Due to membrane separation, use of clarifier is unwarranted. At the same time, due to total retention of microbes, slow-growing species (nitrifying bacteria and bacteria capable of degrading complex organic compounds) are allowed to persist in the system, improving nitrification and biological degradation. The membrane retains not only microbes but also extracellular enzymes and soluble oxidants synthesized by these organisms, thereby creating a more conducive environment capable of degrading a more extensive range of organic compounds.
9. Better removal of phosphorus and suspended solids (bacteria and colloids) occurs in the MBR.
10. High molecular weight organic compounds, which are not readily biodegradable in conventional systems, are retained in MBR. Thus, their residence time is prolonged, and the possibility of biodegradation is improved.
11. Further, MBR eliminates the problems associated with settling, which is the most troublesome part of wastewater treatment. The potential to operate MBR at very high SRT without having the obstacle of settling allows high biomass concentrations in the bioreactor. Consequently, treatment of high strength wastewater and realization of lower biomass yields becomes possible. That also results in a more compact system, significantly reducing plant footprint and making it desirable for water recycling applications.
12. The system is also able to handle fluctuations in nutrient concentration due to extensive biological assimilation and retention of decaying biomass. If some portion of complex organics is not digestible by MBR, they shall be retained by the membrane within the system, and let out as sludge. Membrane Bioreactor shall reject total suspended solids (TSS) in addition to reducing BOD and COD to the desired levels, making the effluent more suitable for direct treatment with the reverse osmosis system for desalination. Post treatments, such as sand-filtration, is not necessary. Membranes provide a final barrier for pathogens and suspended solids.
13. Process control is more comfortable and amenable to automation - no more clarifier upsets or, Total Suspended Solids carry-over.
14. Due to efficient retention and recycling of the activated sludge, the MBR system minimizes the energy needed to reduce the BOD and COD to the desired levels when compared with simple aeration or, other primary treatment schemes. The MBR employs fine-bubble diffuser and achieves high oxygen transfer efficiency that reduces power consumption. Though conventional biological systems can also use the fine-bubble diffuser, increased HRT results in high operating cost.

Thus, Membrane Bioreactor is a suitable alternative for textile wastewater treatment as it reduces sludge production, requires less footprint, and addresses most of the problems associated with other treatment systems described above.

Notes:

[1] Any redox reaction involves an electron donor - electron acceptor pair. The electron donor is oxidized while electron acceptor gets reduced. Generally, the terms anaerobic and anoxic are used interchangeably to represent the absence of oxygen in the wastewater. However, there is a significant difference, as outlined here. Under biological oxidation/reduction processes, the terms anaerobic and anoxic have different notions. Anaerobic refers to the general condition where dissolved oxygen is absent in the effluent, and anyone or, more of electron acceptors present in the wastewater, such as nitrate, nitrite, and sulphate can act as electron acceptors. The term anoxic refers to the condition where nitrate/nitrite are the only electron acceptors taking part in the biological oxidation/reduction processes.

[2] Food to microorganism ratio (F/M) is an important parameter to be maintained in the activated sludge process so that optimal TSS removal could be achieved while maintaining a necessary level of MLSS concentration. Changes in the F/M shall affect the settleability of sludge in the clarifier. In the MBR, by the retention of microorganisms by the membrane, the F/M ratio can be lowered without concern on settleability. Due to higher SRT and high levels of MLSS, endogenous respiration takes place, resulting in lesser sludge production. Also, the secondary clarifier becomes unnecessary. That is one of the main advantages of MBR over the conventional activated sludge process. 

[3] Gray, N.F. (1999) Water Technology: An Introduction for Environmental Scientists and Engineers, First Indian Edition, Viva Books Private Limited, New Delhi, pp.548.

[4] Endogenous respiration shall reduce sludge generation as most of the refractory substances get degraded due to increased SRT.

[5] The MLSS concentration in the MBR can get increased to 20,000 mg/L. However, this shall impose operational problems. Maintenance of 15,000 mg/L of MLSS in the MBR is usual.

Authored by S.Eswaramoorthi, K.Dhanapal and D.S.Chauhan, Environment With People's Involvement & Coordination in India, Coimbatore.

For inquiries, please contact: info@ecpconsulting.in