Exploring membrane technology
Naston's Derek Rodman analyses the pros and cons of membranes' performance and operation in biological wastwater treatment
The use of membrane technology for biomass separation in the activated sludge processes has been an important advance in biological wastewater treatment technology. The advantages of membrane separation bioreactors over conventional activated sludge processes are well known – compact treatment due to a high biomass, greater resistance to shock loads and a superior, more reliable
The primary role of the membrane in an activated sludge system is to retain solids in the process tank without the membrane fouling. Unfortunately, there are a number of early membrane references that display the tendency for biological fouling. It is these earlier problems that led to the perception in some quarters that membranes are unreliable and expensive.
As a result of the earlier experiences, much has been learnt, giving rise to better process designs and the latest generation of reliable membrane processes. Net present value (NPV) evaluation, combining the capital and operational costs, can show potential savings when tighter effluent discharge consents are needed. There are a number of membrane references for submerged systems, immersed membranes bio reactors (IMBR). The main IMBR systems used to-date are dead-end filtration using hollow fibre and cross-flow using flat-plate configuration. Both systems have advantages and disadvantages relating to performance and operation. The degree of fouling of the membrane systems, increase in the trans-membrane pressure, and the method of cleaning is the main distinguishing factor between the processes. The dead-end system uses a frequent back flush like conventional filtration, whereas flat-sheet membranes rely on an effective cross flow of combined air and sewage to remove solids. Fouling is a general term and can be attributed to inorganic deposition (iron and hardness as examples) and natural organic material (NOM) such as colloidal colour, by-products from other biological processes and/or bio fouling specifically related to the interaction between active bio solids and the membrane.
Generally, bio fouling with domestic sewage is the
main contributor in the
short-term, while the others are normally long-term.
During operation the mixed liquor and soluble components are driven towards the membrane where they accumulate and introduce the opportunity for the right conditions for fouling the membrane surface. A flat sheet IMBR unit has the advantage of not needing the back pulse to clean the membranes but relies upon an air scouring to remove solids. A number of factors may affect flat-sheet membrane bio fouling, but two specifically – hydrodynamic conditions and the physiochemical properties of the sludge interaction with the membrane.
The hydrodynamic conditions are frequently discussed by researchers (air scouring effect), but they rarely look at the relation to the physiochemical properties of the sludge in relation to the membrane material. Most membranes have a hydrophobic nature that will repel well-formed healthy floc. Encouraging hydrophobic bacterial species, like filamentous bacteria, will cause very short filter runs. Research work (sponsored by Naston) has shown the interaction between certain materials and biomass growth.
When the nature/composition of the bacteria in the biomass is examined, the results are different. In Figure 2 the level of nitrifiers on the same sheets of plastic shows the selective growth on the different plastics. The plastics have different charges, as do the bacterial species, and to reduce bio fouling on membranes the conditions need to be right to reduce the level of interaction.
On flat-sheet membrane systems the effect of sludge age, sludge retention time (SRT) on the flux rate is well-documented – an example from Naston’s IMBR work is shown in Figure 3. At a certain sludge age the hydrophilic nature of an environmentally controlled biomass will increase resulting in a sustainable flux without the need to change the scour rate. Operational results have also clearly shown membrane flux rates vary depending on the position of the membrane and state of growth of the biomass (Figure 4).
Our research has shown the younger the biomass the greater affinity to foul the membrane, not one of concentration gradient between the membranes or module design. Pre-contact time for biomass growth is therefore important to reduce fouling. A well-formed biological floc is an important factor in IMBR operation, which is common to all membrane designs whether dead end or flat sheet.
Floc shear is detrimental to flux rate formation, as shown in Figure 5 where the submersible pumps were changed to a lower shear design. Also, cross-flow rates, high shear with air scouring, will affect the floc structure and increase fouling. A properly controlled environment for a biomass will generate a measurable SVI and a sustained flux rate.
Therefore, controlling the biomass environment is similar for IMBR systems as it is for conventional activated sludge systems to suppress unwanted bacterial species. The RAS rate and mixing is known to be important for a healthy biological growth to achieve good settlement in conventional activated sludge processes and IMBR. Some of the earlier membrane designs rely upon the formation of a dynamic biomass layer on a membrane to achieve an effective pore size. However, this will be sensitive to air scouring.
New membranes (manufactured by Toray) have already a pore size smaller than 0.1µm and so do not rely on the effect of a biomass dynamic filter layer. Scouring becomes less sensitive for process performance and only becomes an issue to prevent fouling.
The hydrophobic nature of the Toray membrane also ensures lower fouling with a healthy biomass. Optimisation of the biological and hydrodynamic performance with respect to bio fouling is necessary through better process design and control.
Relying only upon trying to achieve a balance between operation flux (level of fouling), air scouring and aeration requirements will have process limitations due to lack of operational flexibility and control. The two requirements, scouring and aeration, should be possible in any design with separate controls to ensure optimum operation to give confidence in the success of the process. Domestic sewage is never fed to a works at a fixed concentration so biological demand will change – industrial loads can also be better controlled.
The future for IMBR systems is promising. The flat-sheet membranes are simple to operate with less operating equipment and can be fully automated. The infrequent membrane cleaning can and has been automated to reduce operator involvement. With better designs to ensure a healthy biomass, understanding of the biological interaction and the newer generation of flat-sheet membranes (Toray), the IMBR process may be considered for all applications rather than specific sites. It is becoming a more cost effective wastewater treatment option.