Long Term Full Scale Comparison of Activated Sludge (AS) with Biological Aerated Filter (BAF)

The Wastewater Treatment Plant (WWTP) of Frederikshavn, Denmark, was extended in the early nineties to increase its average daily flow to 4,5 MGD (16,500 m3/d) and meet new requirements for nutrient removal (8 mg/l TN, 1,5 mg TP/l). A parallel biological aerated filter (BAF) was the selected as the most economical upgrade of the existing activated sludge plant (AS), and started up in 1995. Running two full scale processes in parallel on the same wastewater and treatment objectives for over ten years enabled a direct comparison in relation to operating performance, costs and experience.After screening, a combined grit and grease chamber and 3 primary settlers, the effluent is pumped to the bio-treatment, consisting of AS with recirculation (Modified Lutzack-Ettinger – MLE) and an upflow BAF with floating media. The wastewater is a mixture of industrial and domestic wastewater, with a dominant discharge of fish processing effluent which can amount to 50% of the flow. The maximum hydraulic load on the pretreatment section as a whole is 10 MGD (1,530 m3/h).Approx. 60% of the sewer system is combined with a total of 32 overflow structures. To avoid the direct discharge of combined sewer overflows into the receiving waters, the total hydraulic capacity of the plant is increased during rain to 27 mgd (4,330 m3/hour) or 6 times average flow. During rain, this is achieved by directing some of the raw sewage through a stormwater bypass to the BAF and switching all six BAF to full nitrification. After final biotreatment, the wastewater is conveyed through a 500 m sea outfall.The operation of the BAF can be modified to accommodate various treatment needs:either using simultaneous nitrification/denitrification in all filters with recirculationbottom aeration with full nitrification in all filters for stormwater treatmentor post-denitrification in 1 filter.On-line measurements for the parameters NO3, NO2, NH4 and PO4 as well as dissolved oxygen (DO) are used for control of aeration and for addition of flocculants for P-removal and external carbon source (methanol). The BAF backwash sludge, approx. 0.5 MGD (1.900 m3/d) once every 24 h in dry weather, is redirected to the AS plant. Sludge from primary settlers and the combined biosolids from the AS plant are anaerobically digested, with methane gas being used for generation of heat and power.This paper discusses the experience gained from the plant operation during the last ten years, compiling comparative performance and cost data of the two processes.