Dr Murat Sarioglu, Associate Consultant & Principal Process Engineer MWH Middle East, jots down the technical undercurrents with regard to wastewater treatment that, in his opinion, may transform into full-blown industry drivers in the not-so-far future.
Last year, I crisscrossed the world attending several water/wastewater industry technical conferences and events. One such event that I attended in October 2010 was the Annual Water Environment Federation Technical Exhibition & Conference (WEFTEC) in the United States. The 2010 edition, the 83rd in the series, took place in New Orleans. I made a presentation on the topic of 'Theoretical and Practical Aspects of SNdN in Membrane Bioreactor Systems' at a workshop titled 'Design and Control Strategy for Simultaneous Nitrification and Denitrification (SNdN) in Wastewater Treatment.'
The concept of SNdN is slowly making its way into the practical operation as well as the design aspects of conventional and membrane bioreactor (MBR) activated sludge systems. SNdN endows a MBR operation with key advantages due to its ability to run on low dissolved oxygen concentrations, lower anoxic mass fractions and lower recirculation rates. The focus being on SNdN design fundamentals that can be used for practical design purposes, I have developed a design procedure for SNdN in MBR. The white paper, titled 'Stoichiometric and Kinetic Evaluation of Simultaneous Nitrification and Denitrification in a Membrane Bioreactor at Steady State' will be published in the Journal of Chemical Technology and Biotechnology in the coming months.
Closer home, I spoke at the Second Public Works Conference, which took place in Kuwait on April 2010. The topic of my presentation was 'A Holistic Approach to Biosolids and Residues Management.' I focused on the benefits of maximising energy recovery through state-of-the-art sludge processing techniques and some of the emerging technologies like enhanced anaerobic digestion, thermophilic anaerobic digestion, co-digestion of MSW and sludge, and energy from waste (EfW). I also touched upon the issue of the importance of dealing with brine (residue) discharged by reverse osmosis (RO) desalination plants, especially with regard to brackish water RO plants located inland.
In July 2010, I was invited to Tripoli, Libya to speak at a workshop titled 'Appropriate Technologies for Wastewater Treatment' organised in the context of the infrastructure development and enhancement plan for Tripoli and other major Libyan cities. The overarching theme of the workshop was the inter-relation between wastewater re-use and energy recovery from biosolids and the need for an integrated master plan to cover both issues. To cover both sides and to explain the interrelation between the liquid and solids stream processes, I presented on the topic of 'Application of wastewater re-use, MBR and sludge management technologies in the Middle East,' which stressed on the importance of selection criteria for the liquid and sludge stream processes and their applicability under Libyan conditions.
I have given below brief summaries of the technical undercurrents that are shaping the future of the wastewater industry. Visits to events like the ones discussed above provided an opportunity to revisit these undercurrents and follow up where they stand today.
• The effluent standards in the US are already very stringent. Now discussions have started on the limit of technology (LOT) of current treatment techniques. Some states require the total nitrogen (TN) limit to be three mgN/L and the total phosphorus (TP) to be 0.018 mg/L but these limits are much lower than the ones in Europe and the Middle East. The EEC Urban wastewater directive requires the lowest total nitrogen limit to be 10 mg/L and total phosphorus to be one mg/L for the member states in Europe. In the Middle East, there are no TN and TP limits because the aim is 100% re-use. However, it is well known that surplus treated sewage effluent (TSE) production tends to get discharged into water bodies like sea, bay or creek, with high nutrient loads. Nutrient limits are now coming into effect in some Gulf countries. The point here is that the downward trend of limits in the US, in terms of ultra low nitrogen and phosphorus, is pushing current technologies to their limits. This is now necessitating further research and development into existing and new technologies.
• I have seen increasing number of pilot to full scale applications of Simultaneous Nitrification & Denitrification (SNdN) in MBR systems to achieve especially low total nitrogen limits. Existing oxidation ditch reactors, based on conventional activated sludge, are also being modified to maximise SNdN in achieving lower nitrogen limits. This can be done with internal modifications within the oxidation ditches. However, the level of SNdN achievable with oxidation ditches is much lower compared to MBRs.
• Moving Bed Biofilm Reactors (MBBR) are gaining traction, especially in the cold regions where issues of low temperature inhibition of nitrification in suspended growth systems can be avoided with the use of attached growth systems. In a MBBR system, the biomass grows on a plastic media where biofilm is formed and in which the biological reactions take place. This moves freely inside the reactor. The MBBR process is being used in the upgrade of existing plants in northern Europe and northern US. Research and applications are centred on achieving very low NOX levels - less than one mg/L - using post denitrifying MBBR systems with external carbon source especially for the upgrade of the existing plants.
• Biological Excess Phosphorus Removal (BEPR) is also entering a new stage where initial compartments are being configured as fermenters with on/off sequence of the mixers to maximise the production of volatile fatty acids (VFA). This modification (settling of some portion of the bacteria) results in higher phosphorus removal and in my view, holds a lot of promise for the future in terms of achieving lower total phosphorus limits. The existing systems can easily be modified by setting up new compartments and with the adjustment of the flow path, specific zones can be operated as fermenters to increase volatile fatty acids and acetate that are essential for the BEPR process. Pilot and full scale trials are underway in parallel to modelling efforts to provide a better of understanding of this mechanism.
• Anaerobic MBRs have started to emerge out of the labs. Moving forward from bench-scale to pilot-scale, they are finding applicability in full-scale installations. The applicability of this system to highly concentrated industrial wastewater seems to be promising; however, issues with membrane fouling and membrane lifetime need further research and attention. Membrane flux and hydraulic permeability seem to be the limiting factor for this type of reactors. I've come across many R&D efforts and papers on anaerobic membrane bioreactors last year.
• Research and advances in the MBR technology are focussed on minimising fouling and at the same time reducing energy costs by applying different aeration patterns for scouring of the membrane surface. Some of the major suppliers have significantly reduced the time needed for air scouring over a 24 hour period with their proprietary aeration and permeate extraction patterns. Ceramic membranes are also being applied in MBR systems, their biggest advantage being their ability to withstand much higher liquid temperatures when compared to polymeric membranes. This is especially significant for applications in the Middle East. I will not be surprised to see ceramic membranes penetrating the MBR market and finding wider applicability. Production of lower membrane cut- off sizes seems to be another apparent trend, especially in flatsheet membranes.
• Green House Gas (GHG) emissions from wastewater treatment plants are a primary topic of• research in the US, Europe and Australia. This is in relation to the movement to reduce carbon footprint of these regions and quantifying the emissions from various treatment processes for carbon trade-off incentives.
• Enhanced anaerobic digestion using thermal hydrolysis and pressure is also gaining increasing interest and application especially in regions/countries where there are incentives for maximising the use of renewable energy. Enhanced anaerobic digestion brings the advantage of increased volatile suspended solids (VSS) destruction rates with decreased amount of dry solids to be discharged, increased gas production hence thermal energy, and fully pasteurised sludge with no pathogens and compliance with US EPA Class A Biosolids Standards. This technique is finding increasing interest in the UK, US and Australia and the number of full-scale applications is already on the rise. Majority of the applications have been installed in the existing plants as part of upgrade of anaerobic digestion systems to boost gas production and energy recovery.
• Co-digestion of municipal solid waste and sewage sludge is also gaining interest, and is currently being applied in Europe. Significant amount of renewable energy is generated from this process.
• Beneficial use of brine enriched with valuable salts is another area where research & development activities are increasing. The ability to recover salts from the brine using emerging technologies such as membrane distillation, membrane crystallisation, thermal and chemical crystallisation, also referred to as zero liquid discharge technologies, are especially important in the Middle East where there are large number of RO applications. With these technologies, fractionation of salts is also possible yielding high grade sodium chloride that has a market value. The down side of these technologies is that they are energy intensive and require rigorous pre-treatment.
• Sometimes, high side-stream nutrient load in plants with anaerobic digestion creates problems in the main liquid stream of the plant. The combined Sharon and Anammox process reduces the nitrogen load to the main plant, thereby mitigating the difficulties experienced in removing high nitrogen loads. The principle of this process is that the ammonia in the digester sludge liquor is oxidised to nitrite (NO2-N) in the Sharon reactor with only 50% efficiency and the remaining 50% ammonia - 50% nitrite mixture is anaerobically converted to nitrogen gas in the Anammox reactor, leaving only five per cent of the nitrate in the effluent to the treatment plant inlet. This has been proved to be a very promising technique to reduce nitrogen loads to the sewage treatment plant inlet without the use of any additional chemicals and the number of applications is increasing especially in large plants.
Lastly, I would like to share my two pence worth regarding innovations on the operation side, as follows:
• Operation & Maintenance (O&M) is an integral part of the success of a sewage treatment plant in meeting the required standards. Even though the trend is to increase automation and control in treatment plants to reduce manpower, discussions continue to be centred on the dependability of highly sensitive instruments and related control systems. In highly automated systems, reliable instruments are a must. Therefore, suppliers are now increasing their research efforts in providing more reliable instruments which are self calibrating. For example, luminescent type dissolved oxygen probes have proven to be very robust and reliable which do not need calibration up to a year with accuracy levels at 99%. In my view, this is a breakthrough in the instrumentation technology which can be used develop probes for other monitoring purposes involving different parameters. These will, in turn, have a positive effect in depending on fully automated systems.
• Real time control is also gaining popularity in biological sewage treatment plants. It utilises feed forward control coupled with a predictive modelling software ie. calibrated activated sludge models. Real time data is fed to the Programmable Logic Controller (PLC) which has embedded modelling software. The program predicts the optimum set-points for the various equipments such as the blower, recirculation pumps, and waste sludge pumps with respect to predetermined set-points. These set-points correspond to common loops such as control of sludge age, control of oxygenation, control of methanol dosing. Inline ammonia, nitrate, MLSS probes and instruments are used to implement real time control. Significant reductions in recirculation rates and energy have been reported in some full scale plants in the UK but the technique still needs to undergo rigorous testing before it can find a wider application.
• Integration of equipment diagnostics and asset management systems for the sewage treatment plants enhance facility operations and substantially improve maintenance. These systems are embedded into the operational control system via diagnostic screens where the operator will be able to follow the condition of the assets and translate them into actions to be taken in advance. Considering that the life time of a sewage treatment plant is in the range of 30-50 years, this becomes extremely important and is finding wider application in various treatment plants around the world.
© H2O 2011
