SELECTING NITRIFYING INOCULA ON DIFFERENT AMMONIUM CONCENTRATIONS

摘要

Ammonium (NH4~+) as a pollutant is released from many industries including fish farming. The number of intensive aquaculture systems is continuously increasing. Maintenance of optimal water quality and removal of nitrogen compounds pose challenges insuch systems. In aquaculture water treatment systems, rapid removal of ammonium by ammonia-oxidizing bacteria (AOB) or ammonia-oxidizing archaea (AOA) and nitrite-oxidizing bacteria (NOB) is of great importance. When constructing biofilters its effluentNH4~+ and nitrite (NO_(2-)) concentrations need to be maintained below 0.1 mg N L~(-1), requiring rapid NH4~+ to nitrate (NO_3) with no NO2- accumulation. Under these low NH4~+ and NO_(2-) concentrations no health problems for fishes occur (van Rijn et al, 1990). Commercially available biofilters take several weeks to enable reuse of water in recirculating aquaculture. Start-up of nitrifying moving-bed biofilm reactor system containing plastic carriers take two weeks to achieve a total ammonium nitrogenremoval rate of 35 mg N L~(-1) d~(-1). A rate of 140-175 mg N L~(-1) d~(-1) (with nitrite peaks of 0.2-0.6 mg N L~(-1) appearing) can be achieved after 125 days of operation (Rusten et al, 2006). Bioaug-mentation with a nitrifying inoculum greatly shortens the effective start-up time. Biofilm configurations were preferred over activated sludge systems, since biofilm systems have a better tolerance towards variability in influent concentrations, inhibition and for biofilm systems there is no need for sludge recycling. Additionally, the hydraulic retention time (HRT) in such biofilters is short, requiring the need of attached biomass rather than suspended biomass. It is hence important to determine the optimal conditions for high nitrifying biomass growth, good attachment (suitable carrier material) and high nitrification rate.