COMPARING BIOLOGICAL AERATED FILTER (BAF) AND ACTIVATED SLUDGE (AS) OPERATION ON FULL SCALE

摘要

To extend capacity for nitrogen removal at the Wastewater Treatment Plant in Frederikshavn (WWTP) in 1995, for an average daily flow of 4,5 MGD (16,500 m3/24 h), or a load of approx. 130,000 PE, the existing activated sludge plant (AS) was supplemented with a biological aerated filter (BAF) to upgrade treatment for the demands of the Danish Action Plan on the Aquatic Environment (8 mg/l TN, 1,5 mg TP/l). Running two full scale processes in parallel on the same wastewater and treatment objectives enabled a direct comparison in relation to operating performance, costs and experience. After screening, the wastewater is pumped a combined grit and grease chamber and 3 primary settlers. The max. hydraulic load on the pretreatment section as a whole is 10 MGD (1,530 m3/h) . During rain, some of the water can be directed through a stormwater bypass to the BAF. After final bio-treatment, the wastewater is conveyed through a 500 m sea outfall. Approx. 50% of the wastewater comes from the fish processing industry, the remaining being a mixture of other industrial and domestic wastewater. The BAF backwash sludge, generated once every 24 h in dry weather, is returned to the primary settlers, which results in a total load of approx. 1 MGD (3,000 m3/d). Sludge from primary settlers and the AS plant is finally treated in digestion tanks, in which methane gas is produced and used for generation of heat and power. The bio-treatment consists of an AS with recirculation and a BAF, to be operated in parallel in such a way that the AS was given priority over the BAF. This implies that the AS treated the total wastewater amount under a low hydraulic load whereas the BAF was on standby. The operation of the BAF can be combined in several ways, using nitrification/denitrification in all filters, or post-denitrification in 1 filter, or bottom aeration with full nitrification in all filters. Both bioreactors are equipped with multi-channel on-line meters for the parameters NO3, NO2, NH4 and PO4 as well as oxygen. The measurementss are used for control of the oxygen saturation in aeration and for addition of an external carbon source (methanol) to the anoxic tank. Approx. 60% of the sewer system in Frederikshavn is constructed as a combined sewer system with a total of 32 overflow structures, relieving the system of mixed stormwater and wastewater, which is then discharged direct into the receiving waters. To deal with these environmental nuisances in wet weather, the hydraulic capacity was increased by switching all six BAF to full nitrification by applying the bottom aeration, leading to a BAF surface load of 9.6 m3/m2/hour and a total hydraulic capacity of the WWTP during rain of 27 mgd ( 4,330 m3/hour) or 6 times average flow. To optimize the operation and expanding wet weather capacity, hydraulic capacity of the plant during rain, and control strategies for the plants were implemented for high and low flow situations. This paper discusses the experience gained from the plant operation as well as the modifications in control strategies to increase the total hydraulic capacity during rain.