A newly developed process applying a technique of microorganism immobilization to biological nitrogen removal is reported. Nitrifiers immobilized in 2 to 3 mm diameter polyethylene-glycol resin (Nitrifying pellets) are mixed with activated sludge in the nitrification tanks to promote quick nitrification. Due to the high nitrifying activity of the pellets, nitrification of municipal wastewaters is completed within a few hours. This paper introduces the synthetic polymer and immobilization method suited to nitrifiers, the conditions for pellet fluidization, the activity of nitrifying pellets and its treatment performance as a nitrogen removal process.

Thermal sludge conditioning liquor was successfully treated by a pilot‐scale upflow anaerobic hybrid system with an effective volume of 10.4 m3. The reactor was similar to the HYAN reactor in Lakeview but without a recirculation system. A gas‐solid separator was installed above a filter packed with polypropylene pall rings. The organic loading rate was increased stepwise to 30 kg chemical oxygen demand (COD)/m3 · d. The hydraulic retention time was only 6 hours at the target loading rate.
More than 70% of soluble COD in the influent was removed, even at an organic loading as high as 30 kg COD/m3 · d. Soluble 5‐day biochemical oxygen demand (BOD5) removal efficiency at the loading rate exceeded 95%. Volatile fatty acids concentration in the effluent was consistently less than 60 mg/L after the day when stable treatment had been obtained. The methane production fluctuated between 0.3 and 0.4 L/g COD removed. Solids production including effluent suspended solids (SS) was between 0.16 and 0.24 kg SS/kg COD removed.
The inside of the reactor consisted of two zones, a sludge blanket zone with less than 1% solids concentration and a sludge bed zone with 47% solids concentration. Although the reactor was seeded with an anaerobically digested sludge, granular sludge was produced in the reactor. The ratio of granules to the solids remaining in the reactor increased to approximately 70% at the end of the experiments. Most of the granules had diameters of less than 1 mm and their settling velocity was 0.6 cm/s or more. The granules were composed of bacteria like Methanothrix according to SEM observations. The methane production rate was between 0.8 and 1.1 kg methane as COD/kg SS · d in an acetate solution.
According to substrates distribution in the reactor and tracer tests, the bed zone in the reactor worked similarly to a continuously stirred tank reactor (CSTR). Evaluation of soluble COD removal rates in the reactor indicated that the rate could be simulated by a Monod type reaction.

The oxidation ditch process has become increasingly important as a secondary treatment process for small communities, because of its ability to remove nitrogen by providing aerobic and anoxic areas in a ditch. Through surveys at the three oxidation ditches in service in Japan, it has been proved that removals of BOD and suspended solids could be consistently achieved irrespective of seasonal, daily and hourly fluctuations of inflow. It has also turned out that providing an anoxic area in the oxidation ditch is effective for removing nitrogen and recovering alkalinity as well as saving energy. The removal efficiency of total nitrogen is expected to be more than 70%.

Thermal sludge conditioning liquor from a municipal wastewater treatment plant was successfully treated with a pilot-scale upflow anaerobic hybrid reactor with an effective volume of 10.4 cu.m. More than 70% of soluble COD in the influent was removed even at an organic loading as high as 30 kgCOD/cu.m/day. Though the reactor was seeded with an anaerobically digested sludge, granular sludge was produced in the reactor. The ratio of granular sludge to the solids in the reactor increased up to about 70% at the end of experiment. Biological activity of the granular sludge was measured as a methane producing rate using an acetate solution. Physical characteristics of the sludge, such as diameter distribution, composition and settling velocity, were also measured. It was found through tracer tests using lithium chloride that the reactor worked as a CSTR.

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