Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | AUGUST 2014 | 41 In the know Technically speaking: anaerobic technology in organics removal 2 . Recent work at Newcastle Univer- sity supports such ndings, with the viability of the anaerobic granules used maintained down as low as 4°C. Whilst initially this appears at odds with the previous nding surround- ing 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 (<0.1 LCH3/gCOD). In fact the liquid phase can be super- saturated with pseudo stable methane bubbles such that the liquid phase methane concentration can exceed theoretical limits by more than seven times 3 . The consequence of this is that even at 16°C around 50% of the produced methane can exit in the liq- uid 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 a– ords a large contact surface area for liquid to gas exchange of methane under the in— u- ence of very gentle negative pressures and/or sweep gas — ow. The impact of its inclusion has been shown to add around 0.14 kWhe/m3 of treated — ow su˜ cient to render the overall energy balance of an anaerobic — owsheet The process slightly positive 3 . 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 with- in the overall — owsheet (see Figure). Variations and confi gurations Perhaps the greatest variation is seen with respect to the in— uent source including the use of forti cation (COD supplementation from sludge hydroly- sis) down to settled sewage (with the sludge going to AD). Whilst total gas production mirrors load and is hence improved when crude or forti ed feed is used, the ability to meet organic dis- charge 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 t going forward. The next most common variation is the con guration of reactor with — oc- culent, attached and granular reactors considered. Direct comparison reveals no di– erence in treatment e˜ cacy such the decision becomes focused on the energy demand associated with operating the membrane. Attempts to run traditional style MBRs as anaero- bic (— occulent) demonstrate a very high energy demand associated with gas sparging which renders the overall energy demand negative. In contrast, use of granular reac- tors limits the solids load onto the membrane to supernatant colloids that are easily managed with low frequency gas sparging. Consequently — uxes around 30 LMH are achievable with gas sparge frequencies as low as 10% of the operating sequence 4,2 . This transforms the viability of the technology and o– ers 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 o– the air. For further reading on anaerobic technology visit wwtonline.co.uk Trials on settled sewage for an anaerobic MBR with downstream contact media 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. Settled infl uent 93.3 265.9 134.3 76.1 Effl uent 7.7 18.2 18.2 0.0 Total removal (%) 91.7 93.2 86.4 100.0 BOD 5 (mg/L) TCOD (mg/L) SCOD (mg/L) SS (mg/L)

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