Is it time to switch the air off on activated sludge?
Future use of anaerobic processes will go beyond treatment of solids to encompass the entire flow and the latest pilot work highlights the advantages. Professor Bruce Jefferson explains.
One hundred years on and whilst the significance and impact of the activated sludge process is beyond doubt it is perhaps timely to think about what next. The challenge going forward is how to upgrade works to meet tighter discharge consents without a commensurate increase in energy, chemicals and ultimately costs.
To date this has mainly focused on the last two areas with widespread use of forced aeration to upgrade existing treatment facilities. This occurs across all scales of operation as demonstrated by the recent uptake of aerated constructed wetlands on small sewage works1.
The process efficacy of the approach appears clear, and its cost and chemical use are attractive on sites with existing horizontal flow wetlands. However, the approach raises questions linked to its place within the overall philosophy of small works (low energy, low maintenance). For instance, a recent energy audit on an active site showed the energy demand to be equivalent to that of an activated sludge process on a population equivalent basis (approximately 2.5 Wh/PE). Accordingly alternatives are required at all scales or a continuing trend of increasing energy use will be unavoidable.
Is anaerobic the answer?
For many people the answer lies in anaerobic technology. Anaerobic processes unlock the true resource potential of sewage by converting the carbon content into methane whilst decoupling the nutrient removal processes, enabling recovery and the use of innovative treatment technologies.
Future consideration for the use of anaerobic processes goes beyond the use of anaerobic digestion of solids to encompass the entire flow. Such thinking is not new but has traditionally been accompanied with negative perceptions due to the relatively high half saturation constant of anaerobic communities, leading to low methane yields.
However, recent work is now challenging the validity of such perceptions and assessing the potential of modern bioreactors such as anaerobic membrane bioreactors (MBRs) and high-rate expanded granular sludge blanket reactors (EGSB). For instance, recent pilot-scale work performed by Cranfield on a sewage works in the north of England has shown an anaerobic MBR treating settled sewage to be effective down to 6°C without loss in organics removal2.
Recent work at Newcastle University supports such findings, with the viability of the anaerobic granules used maintained down as low as 4°C. Whilst initially this appears at odds with the previous finding surrounding low methane yields, the fate of the produced methane needs to be considered.
Methane is a fairly soluble gas (Henry’s law constant is 0.033 at 25°C) and the combination low organic strength, high hydraulic loading rates and low temperatures means that substantial quantities of the produced methane remain in the liquid phase and hence gas space yields can appear very low (3/gCOD).
In fact the liquid phase can be supersaturated with pseudo stable methane bubbles such that the liquid phase methane concentration can exceed theoretical limits by more than seven times3. The consequence of this is that even at 16°C around 50% of the produced methane can exit in the liquid phase. At lower temperatures the partitioning increases such that at 6°C this can reach around 80% of the total methane produced from the treated sewage. Recovery is thus required as both the available energy production does not want to be lost and methane is a potent greenhouse gas with a global warming potential 21 times that of carbon dioxide.
The recovery question
A number of solutions exist but perhaps the most viable is the use of membrane degassing technology whereby the membrane affords a large contact surface area for liquid to gas exchange of methane under the influence of very gentle negative pressures and/or sweep gas flow. The impact of its inclusion has been shown to add around 0.14 kWhe/m3 of treated flow sufficient to render the overall energy balance of an anaerobic flowsheet slightly positive3.
Much of the work conducted around the world at the moment is based around anaerobic membrane bioreactors as they negate concerns about washout of the slow growing anaerobic organisms. Beyond that, large variations have emerged as to how the technology is integrated within the overall flowsheet.
Variations and configurations
Perhaps the greatest variation is seen with respect to the influent source including the use of fortification (COD supplementation from sludge hydrolysis) down to settled sewage (with the sludge going to AD). Whilst total gas production mirrors load and is hence improved when crude or fortified feed is used, the ability to meet organic discharge levels is easily achieved when treating settled sewage (see Table). Overall this must be balanced against use of AD to process solids and so a range of solutions are likely to fit going forward.
The next most common variation is the configuration of reactor with flocculent, attached and granular reactors considered. Direct comparison reveals no difference in treatment efficacy such the decision becomes focused on the energy demand associated with operating the membrane. Attempts to run traditional style MBRs as anaerobic (flocculent) demonstrate a very high energy demand associated with gas sparging which renders the overall energy demand negative.
In contrast, use of granular reactors limits the solids load onto the membrane to supernatant colloids that are easily managed with low frequency gas sparging. Consequently fluxes around 30 LMH are achievable with gas sparge frequencies as low as 10% of the operating sequence4,2. This transforms the viability of the technology and offers the potential of an economically viable option going forward. Whilst developments are ongoing the future looks increasingly anaerobic and perhaps it is time to start switching off the air.
Professor Bruce Jefferson, Professor of water engineering, Cranfield University
1. Butterworth et al, 2012. Ecological Engineering, 54, 236-244.
2. Soares et al, 2012. Ectoechnologies for Wastewater Treatment, 25-27th June 2012, Santiago de Compostela, Spain.
3. Cookney et al, 2012. Water Science and Technology, 65, 604-610.
4. Martin et al, 2013. Water Research, 47, 4853-4860.
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