Hiroshi Imai

Magnesium requirement for biological removal of phosphate by activated sludge

Abstract The effects of magnesium on excess uptake of phosphate in an aerobic-anaerobic activated sludge process were examined by the fill and draw procedure. The alternation of anaerobic and aerobic conditions in one cycle of fill and draw process was varied many ways. The presence of sufficient magnesium was necessary for uptake of excess phosphate. When sludge contained more phosphorus than the upper limit of phosphorus content in the usual aerobic activated sludge, 2.5% by weight, magnesium was also contained in more than an ordinary amount (0.5%). Their contents in the sludge at the end of each cycle of the process were correlated with each other by a linear equation with the correlation coefficient of 0.99. When magnesium concentration was insufficient for the uptake of excess phosphate, its concentration in the treated water was of the order of 0.1 mg/l. In the first anaerobic period both phosphate and magnesium were released, and in a successive aerobic period they were taken up again. The weight ratio of differential amounts of phosphorus and magnesium released or taken up changed with time in one cycle. Dynamic behaviors of phosphate and magnesium removal against the step change of feed magnesium concentration also showed a stoichiometric relationship supporting the correlation equation above mentioned.

Simulation of citric acid production by rotating disk contactor

A simple model was presented to describe the time courses of citric acid production by a rotating disc contactor (RDC) using Aspergillus niger. The model is expressed by Monod-type cell growth, Luedeking-Piret-type citric acid production rate equations, and the diffusion equation for oxygen in the biofilm. The model contains five parameters which were determined by the nonlinear least squares method by fitting the numerical solution to the experimental data. In solving the equations, the cell density of the biofilm was estimated from the value of cellular mass per unit of biofilm area using an empirical equation. The experimental time courses in citric acid production period were well simulated with this model. The relation between the specific biofilm surface area and the rate of citric acid production was also explained by the simulation using the average values of five parameters of twelve runs. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 689-696, 1997.

Citric acid production by surface culture using Aspergillus niger: Kinetics and simulation

Abstract The citric acid production rate for batch surface culture using Aspergillus niger Yang no. 2 was examined experimentally in terms of rate coefficients and the effect of various depths of medium. The citric acid production rate was affected by the biofilm mass expressed as mg-Cell/cm2-film. The biofilm mass increased in parallel with medium depth. The citric acid production rate divided by the biomass weight at the end of cultivation was constant when the biofilm mass was less than 40 mg/cm2, but decreased above this critical value. The citric acid fermentation time course was expressed by the Luedeking-Piret type rate expression. The time course of the surface cultivation was simulated well with the expression when biofilm mass was less than 40 mg/cm2. In this range, rate coefficients were obtained by the nonlinear least squares method. Comparison in regard to the citric acid productivity between surface and other types of cultures in the literatures was made.

Effect of operational conditions on the rate of citric acid production by rotating disk contactor using Aspergillus niger

Abstract The effect of operational conditions such as the specific biofilm area, oxygen concentration, and rotation speed on citric acid production by a rotating disk contactor (RDC) was examined. The overall productivity for a repeated batch culture was also compared with that for a non-repeated batch culture. The citric acid production rate per unit biofilm area (P) was almost constant regardless of the value of the specific film area. The rotation speed of the disks had no effect on P in the range of 5–20 rpm. The apparent saturation constant of dissolved oxygen concentration was 2.05 mg/l. Overall citric acid productivity for a 3-times-repeated batch culture was 1.7 times higher than that for the non-repeated batch culture.

Effect of oxygen tension on citric acid production by surface culture

Abstract The effect of oxygen tension on citric acid production by a surface culture of Aspergillus niger was examined. Both yield of citric acid and biomass were almost constant in the oxygen tension range of 21–74 v/v%, and decreased with an increase in the oxygen tension in the range of 74–100 v/v%. The citric acid production rate showed a Michaelis-Menten type dependence on the oxygen tension. Between the kinetic parameters and the oxygen tension, the following relationships were identified. The rate coefficient of citric acid production proportional to the biomass concentration increased with an increase in the oxygen tension. The rate coefficient of citric acid production proportional to growth rate was almost constant regardless of the oxygen tension.

Potassium removal accompanied by enhanced biological phosphate removal.

Abstract The effects of potassium on excess uptake of phosphate in an aerobic-anaerobic activated sludge process were examined by the fill-and-draw procedure. The presence of sufficient potassium was necessary for excess uptake to occur. The contents of potassium and phosphate in the sludge at the end of each cycle of the process were correlated with each other by a non-linear equation, with some scattering. However, the sum of the molar ion valences of magnesium ( Mg ) and potassium ( K ), 2 Mg + K , was well correlated by a linear equation with the moles of P expressed as mmol/g-VSS, with a correlation coefficient of 0.993. When the potassium concentration was insufficient for the enhanced uptake of phosphate, its concentration in treated water was of the order of 0.1 mg/ l . In the first anaerobic period phosphate was released into the liquid phase, but potassium was released after initial instantaneous removal, and in the successive aerobic period they were both taken up again.

Aeration method and phosphate release behavior in phosphate removal by sea water activated sludge

Abstract The upper limit of the length of the anaerobic period necessary to achieve an excess uptake of phosphate (P), and the relationship between P release and removal were examined by the fill-and-draw anaerobic-aerobic sea water activated sludge process. An anaerobic period of less than 8 h was favorable to achieve an uptake of excess P. The dimensionless amount of released P normalized by BOD fed in one cycle was smaller for sea water sludge than that for fresh water sludge when both sludges realized the same dimensionless amount of P removal normalized by BOD fed in the cycle. The orthophosphate concentration in cold acid extract was 4.49% of total P in the sludge solid. Thus P was mainly taken up biologically by sea water activated sludge.

Parametric Analysis of Effect of Oxygen Tension on Citric Acid Production by Surface Culture

Citric acid has been produced using Aspergillus niger in an aerobic fermentation. Much oxygen has to be supplied for the fermentation. Raise of oxygen tension is an easy method to supply much oxygen to the fermentor. The purpose of the present research is to examine the effect of oxygen tension on citric acid production using Aspergillus niger by surface culture, and to obtain the relationships between the oxygen tension and the kinetic parameters.