Functional stability of temperature-compensated nitrification in domestic wastewater treatment obtained with PVA-SbQ/alginate gel entrapment

Gel beads (3 mm) of PVA-SbQ were cast by entrapment in Ca-alginate beads prior to photo-induced crosslinking of the synthetic polymer. Such beads have maintained their physical integrity, being continuously exposed to domestic wastewater for more than 2 years. Gel strength measurements indicated that the alginate gel network had been degraded and lost. Gel entrapment of homogenized nitrifying sludge gave an immediate 30% loss of activity compared to that of the same sludge kept in suspension, but it reached the same level within 3 days in a sequenced batch reactor. A continuous 4-litre lab-scale reactor containing 25% (v/v) gel-entrapped nitrifying sludge was run successfully fed with ammonium-enriched domestic wastewater for a period of 300 days. At 30°C, the system reached a steady-state activity of 15 mgN pr litre and hour (/Lh) after a month. After reducing the temperature successively to 20 and 10°C, at least 75% of this activity was maintained. Even at 10°C, activities could be increased to 25 mgN/Lh by improving oxygen transfer by increased aeration or stirring, verifying that oxygen transfer should be considered the rate-limiting factor of the system. Despite an organic load of 10–50 mgCOD/Lh during the whole period, heterothropic overgrowth or grazing protozoa could not be detected attached to the beads, even though such organisms were abundant in the sludge leaving the reactor. The system presented may be suitable for specially designed full-scale reactors with short hydraulic retention times, as well as an additive to existing biofilm or activated sludge nitrification plants.