The Dalles Wastewater Treatment Plant (WWTP), Oregon, USA, processes municipal wastewater through a conventional activated sludge system without primary clarification. The plant provides secondary treatment followed by ultraviolet (UV) disinfection, and then discharges to the Columbia River. The SF2000 rotating belt filter (RBF) manufactured by Salsnes Filter was pilot tested as a potential upgrade process for the Dalles WWTP.
RBFs offer solid separation with integrated thickening and dewatering of the sludge. At Dalles WWTP, the technology was tested for its effectiveness at removing suspended organics (carbon) and diverting them directly to anaerobic digesters. Carbon diversion can help reduce organic loading to the secondary process (thereby reducing the aeration requirements) and increase carbon loading to anaerobic digesters resulting in higher production of gas. Therefore, the energy efficiency and resource recovery of the plant can be improved.
Dalles, Oregan - Salsnes filter pilot site.
Materials and methods
The complete pilot unit included an electromagnetic flow meter, SF2000 unit with PLC control, and the associated interconnecting piping and sampling valves. The RBF unit was tested with 350 µm filter mesh (belt) and was operated in automatic mode during the test days. During automatic mode of operation, the belt speed automatically rotates/adjusts to maintain a pre-set water level in the RBF given the varying influent flow rates. The influent and effluent samples were collected after the system achieved steady-state (about 15~30min since the beginning of the test). All samples were analysed by a third-party analytical laboratory.
Results and conclusions
The results showed that the Salsnes Filter RBF could achieve over 60% removal efficiencies for total suspended solids (TSS) (Fig. 2) and volatile suspended solids (VSS). Furthermore, the average removal efficiencies for COD, BOD and FOG were 39%, 20% and 16%, respectively. The highest flow rate for the RBF for this test was 428 gpm. The TSS, VSS and COD removal efficiencies were not significantly affected by the hydraulic fluctuations. According to the results of the grit removal analysis, 82% fixed solid was in the size range of 105 to 1680 µm showing the filtermesh captured grit that was much smaller than the mesh size of 350 µm. It was demonstrated that the RBF was able to remove solids over a wide range, not just the solids larger than the mesh size.
The average dry solid content of the Salsnes Filter sludge (without dewatering) was 7.2%. The average volatile sludge/total sludge (VS/TS) of Salsnes Filter sludge was 0.90 based on the test results of composite daily samples. Normally, the VS/TS of waste activated sludge loading into anaerobic digesters is 0.7. It is known that the sludge with higher VS/TS contains more organics, which is loaded into digesters and produces more biogas. This means the RBF as a primary treatment process could help reduce organic loading to the secondary process and increase carbon loading to anaerobic digesters resulting in higher production of gas. Therefore, the energy efficiency and resource recovery would be improved significantly.
The Salsnes Filter system consistently demonstrated greater than 60% TSS removal efficiency over the duration of the pilot test.
