title: Application of an Anaerobic Membrane Bioreactor for the Treatment of Lipid-rich Sludges
Creators: name: Lutze, Robert
Publisher: name: UNSPECIFIED
Year: 2022

description: Anaerobic membrane bioreactors (AnMBRs) may provide a sustainable technological solution to overcome limitations of lipid-rich sludge digestion in industrial WWTPs. These digesters combine extensive removal of organic matter and sufficient process stability with reduced reactor volumes. However, membrane fouling and a decrease in biomass activity due to ex-posure to high shear forces may limit successful application. This study characterises biological removal kinetics, limitations of anaerobic digestion of lipid-rich sludge, effects of shear stress on biomass activity and membrane performance. The study was carried out using a pilot-scale anaerobic digestion plant equipped with recuperative thickening by ceramic discs installed on a rotating hollow shaft. The membranes had a mean pore size of 0.2 µm and were operated at mean cross-flow velocities between 3.3 - 4.1 m/s. CSTRs were additionally operated for comparison of removal performance. A mixture of WAS from a municipal WWTP and flotation sludge from dairy industry was used as feed. In AnMBR and CSTR, COD removal and methane yields followed substrate first-order kinetics. COD removal of AnMBR corresponded to the performance of CSTR at similar SRT but AnMBR was operated with higher OLRs and reduced HRT. At high COD related shares of flotation sludge over CODflot/CODtotal = 0.85, CSTR could be operated with a maximum OLRCOD,deg = 8.3 kg COD/(m³∙d) at SRT = 15 d while OLRCOD,deg of 11.2 kg COD/(m³∙d) did not lead to process instabilities using an AnMBR with HRT = 10 d and SRT = 15 d. Process limitations, emerging inhibitions and process instabilities were clearly shown by the relationship between SMA measurements and mean COD removal rates. Activity losses of acetoclastic methanogens based on exposure to high shear required for mitigating fouling on the membranes were not observed. A decisive influence on process stability had especially the degradable lipid content in the anaerobic sludge. Inhibition on acetoclastic methanogens started at lipid contents of 50 mg lipid/ g TS while the digestion process remained stable up to 150 mg lipid/ g TS. Lower degradable lipid content in the anaerobic sludge can be achieved with elevated SRT or reduced shares of lipid-rich substrates. Hence, AnMBR can offer a higher process stability compared to CSTR at even HRT when extending the SRT. Common design criteria such as the OLR or the lipid-related sludge load did not assure a stable or efficient digestion process. Owing to the limitations on the applicability of existing design parameters, a generally valid design criterion for lipid-rich digestion – the maximum degradable lipid content in the anaerobic sludge – was elaborated. This criterion refers to the current state of knowledge on LCFA inhibition that inhibition of methanogens is initiated by the LCFA concen-tration related to the TS content in the sludge. In the field of sludge digestion, membrane performance in terms of the critical flux depends in particular on the apparent dynamic viscosity as decisive sludge (characteristic) parameter. A higher apparent viscosity reduces the flux. The characteristic sludge parameters TS, VS and CST did not exhibit a clear correlation to the critical flux but showed correct tendencies. The correlation between apparent viscosity and critical flux can be significantly improved by inte-grating the present shear rate on the membrane surface (R² = 0.9). Filtration performance can be improved by increasing the rotational speed of the membrane and adding Fe salts (here FeCl₂) to the sludge, as they affect shear stress on the membrane disc and apparent dynamic viscosity, respectively. An increase of mean cross-flow velocity from 3.8 to 4.1 m/s improved the critical flux by 20 %. The addition of Ferric salts (70 L FeCl₂ 20 % per t TS) im-proved the critical flux up to 30 %. A maximum critical flux up to 17 L/(m²∙h) was achieved when filtrating sludge with TS ≈ 3.2 %. At TS ≈ 4.0 % and TS ≈ 4.6 %, critical fluxes of around 9 L/(m²∙h) and 7 L/(m²∙h) were observed. In order to realise an economic advantage of AnMBR using rotating disc filters, specific membrane costs need to decrease noticeably. Membrane operation with higher net flux corresponding to less membrane surface required and lower costs results in case of lower TS concentration of the anaerobic sludge. This effect is especially pronounced in applications treating high shares of easily degradable substrates such as flotation sludge. Compared to conventional digesters additional energy demand for membrane rotation was around 1.0-2.5 kWhel/m³ filtrate at TS concentration of 3.5 % but can be compensated by advantages of lower heat loss based on smaller digester volumes.