Activated Sludge Concentration Research Articles

Low-strength wastewater treatment by a UASB reactor

This study describes the feasibility of anaerobic treatment of a low-strength (500 mg-COD/l) synthetic wastewater using a semi-pilot-scale upflow anaerobic sludge blanket (UASB) reactor. Under ambient temperature conditions (20–35°C) with a hydraulic retention time (HRT) of 3 h and corresponding organic loading of 4 kg-COD/m 3-day, 90–92% COD and 94–96% BOD reductions were achieved. Methane production was found to be about 141 l/kg-COD removed at standard temperature and pressure (STP). Because of the presence of a high concentration of active granular sludge in the lower portion of the reactor, 80% reduction of COD occurred within the bed level. Morphological examination of the sludge showed that the granules contained diverse groups of microorganisms, where rod-type, Methanothrix-like, cells were dominant on the surface.

A simple empirical model for hindered settling in activated sludge clarifier

The determination of activated sludge concentration at the bottom of secondary settler tanks is necessary for proper design and control of wastewater treatment plants. To model its time dependence and the effect of plant operation parameters, sludge thickening should be taken into account. Most of the models proposed in litterature require large computational efforts or the knowledge of parameters that require delicate measurements. This study gives a new equation for activated sludge thickening, Xi = a * (ti + Δt)b, determined from 23 different experiments, with formulas to obtain all the model parameters (a, b, and Δt). They have been fitted using settling data obtained on biomass sampled on different activated sludge treatment plants (industrial [brewery], municipal, and laboratory pilot plant). Model predictions are compared to experimental data obtained on a full‐scale secondary clarifier under unsteady‐state conditions.

Stock and Concentration Dynamics of Activated Sludge Processes

A new model is introduced for description of the activated sludge concentration dynamics in the aeration tank and stock dynamics in the settler. The model is obtained by decomposition of a real settling process into the sequence of simple batch-settling processes. The model is tested in the simulation and pilot-plant experiment, it is used for study of the control influence on stock and concentration processes in the normal and low stock cases.

Denitrification in secondary clarifiers

Denitrification in secondary clarifiers is primarily due to hydrolysis of particulate degradable COD and decay of biomass. The reduction factor of these processes under anoxic as compared to aerobic conditions was investigated in the treatment plants of the City of Zurich. The Zürich-Glatt plant has circular clarifiers with central inlet. The treatment plant Zürich-Werdhölzli has rectangular clarifiers with cross flow and scrapers with vacuum removal of the sludge. At Zürich-Glatt denitrification capacity in the clarifiers was 30% of total denitrification. whereas at Zürich-Werdhölzli only 15 % was observed. The sludge mass in the secondary clarifier depends on inlet flow, return sludge flow, activated sludge concentration, sedimentation properties of the sludge and scraper intervals. During dry weather and due to good sedimentation properties the activated sludge blanket in the thickening zone of the rectangular clarifiers at Zörich-Werdhölzli is almost completely removed after passage of the scraper. Denitrification volume in the secondary clarifier can therefore be modelled in relation to the ratio of inlet to return sludge flow and scraper interval. With the aid of control of return sludge flow and scraper intervals denitrification capacity in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a variable sludge volume in the clarifier related to scraper interval under the assumption of constant sludge mass in the entire system.

Denitrification in secondary clarifiers

Denitrification in secondary clarifiers is primarily due to hydrolysis of particulate degradable COD and decay of biomass. The reduction factor of these processes under anoxic as compared to aerobic conditions was investigated in the treatment plants of the City of Zurich. The Zürich-Glatt plant has circular clarifiers with central inlet. The treatment plant Zürich-Werdhölzli has rectangular clarifiers with cross flow and scrapers with vacuum removal of the sludge. At Zürich Glatt denitrification capacity in the clarifiers was 30% of total denitrification, whereas at Zürich-Werdhölzli only 15 % was observed. The sludge mass in the secondary clarifier depends on inlet flow, return sludge flow, activated sludge concentration, sedimentation properties of the sludge and scraper intervals. During dry weather and due to good sedimentation properties the activated sludge blanket in the thickening zone of the rectangular clarifiers at Zürich-Werdhölzli is almost completely removed after passage of the scraper. Denitrification volume in the secondary clarifier can therefore be modelled in relation to the ratio of inlet to return sludge flow and scraper interval. With the aid of control of return sludge flow and scraper intervals denitrification capacity in the secondary clarifiers may be improved. In order to control denitrification in the secondary clarifier the model must include a variable sludge volume in the clarifier related to scraper interval under the assumption of constant sludge mass in the entire system.

Settling Characteristics of Activated Sludge in Danish Treatment Plants with Biological Nutrient Removal

In 1989-91, a study was performed to investigate the settling characteristics of activated sludge in Danish treatment plants with biological nutrient removal. The study included three screening series on 38 treatment plants. Furthermore, the study included investigations during one year on seasonal variations in sludge settling characteristics at three treatment plants. The screening investigations were performed in November 1989 and May and September, 1990. Results showed that in the May-screening, 35-45% of the plants had a filament index of 2-2.5 or above, corresponding to a sludge volume index above 150 ml/g. When comparing data for diluted and non-diluted sludge volume indices, a SVI value of 150 ml/g seemed parallel to a DSVI of 110 ml/g. In the November- and September-screenings, some 30% of the plants had activated sludge showing a filament index in or above the critical area. Dominating filamentous microorganisms were found to be (in decreasing order): Microthrix parvicella, Type 0041, Type 021N, Type 0092, Type 0914, and Type 1851. A distinct variation over the year in sludge settling characteristics was found for the three plants. Sludge settling characteristics improved during summer, and deteriorated during winter. For activated sludge with a high content of filamentous microorganisms, the best parameter to follow the variations in sludge settling properties was the filament number. If the activated sludge concentration, the MLSS, varied significantly, the specific filament number was to be applied.

Activated sludge concentration dynamics

The relationship between activated sludge concentration in the aeration and settling tank is determined on the basis of settling process characteristics. The relationship is used for description of the activated sludge concentration and stock dynamics in the aeration and settling tank. A limit value for maximization of activated sludge concentration is determined.

Assessment of biodegradability of polycaprolactone by MITI test method

The biodegradation of polycaprolactone (PCL) and PCL-diol was studied by MITI test method varying the particle size and molecular weight of the test substance and the activated sludge concentration. The biodegradation began after an induction period. The induction period for the biodegradation of PCL was longer with the larger particle size of the test substance and it was dependent on the molecular weight. The biodegradation rate was faster and consequently the biodegradability after 28 days was higher with (i) smaller particle size of the test substance, (ii) lower molecular weight and (iii) higher activated sludge concentration. Especially the particle size of the test substance strongly affected the rate of the biodegradation of PCL. These results were different to those of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHB/PHV), the biodegradation of which was not dependent on the particle size and the activated sludge concentration. The cause of the difference between both plastics and the mechanism of the biodegradation are discussed. From this study, it was concluded that the particle size of the test substance is one of the factors affecting the biodegradability of plastics and that it should be taken into account when the biodegradability of different kinds of plastics is compared.

Biodegradability of poly (3-hydroxybutyrate-co-3-hydroxyvalerate) in activated sludge

The biodegradability of microbial copolyester of 3-hydroxybutyrate and 3-hydroxyvalerate, P(3HB-co-3HV), in activated sludge was studied by MITI test method to examine the applicability of the test to biodegradable plastics. The biodegradability after 28 days was 88% under standard condition and did not vary with the particle size of test substance in the range of 56 to 335 μm. The biodegradabilities were nearly the same at 30 and 100 mgMLSS/l of the activated sludge concentration, but slightly smaller at 10 mgMLSS/l. During whole period of the test, the concentration of the watersoluble product in the test solution was less than 2.9 mgC/l. It was concluded that MITI test method is applicable to the evaluation of the biodegradability of P(3HB-co-3HV). The biodegradability of P(3HB-co-3HV) was independent on 3HV content in the range of 7 to 22%.

Effect of copper on nitrification in activated sludge

The concentration of free copper in activated sludge with copper added is strongly influenced by pH. For example, at pH 6.5 with 9.13 × 10 −5 mol Cu l −1, the free copper concentration is 4.0 × 10 −7 mol l −1 (pCu = 6.4) and at pH 8.4 this concentration is 10 −8 mol l −1 (pCu = 8.0). In both cases the activated sludge concentration is 0.7 g MLSS l −1. The free copper concentration is also affected by the concentration of mixed liquor suspended solids (MLSS). In batch experiments with constant pH, the effect of copper on the nitrification rate was not regulated by total copper concentration but by copper/sludge ratio or by free copper concentrations. Experiments at different pH showed a linear correlation between nitrification capacity and free copper concentration, suggesting that the pH effect on nitrification below 8.3 is in fact a copper effect. Activated sludge with copper did not become acclimatized to the copper in a period of three days. Addition of nitrilotriacetic acid (NTA) within one day did cancel the copper inhibition. The results were compared with the effect of copper on acetate removal by heterotrophic micro-organisms. The nitrifiers proved to be no more susceptible to copper than heterotrophic micro-organisms.

Absorption method of determining the concentration of active sludge in the biochemical treatment of the effluent of the viscose rayon sector

Absorption method of determining the concentration of active sludge in the biochemical treatment of the effluent of the viscose rayon sector