Batch Gravitational Sedimentation of Slurries

Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer

In building and tuning good settling models for secondary clarifiers of wastewater treatment plants, there is a need for measured continuous solids concentration profiles during batch settling. Conventional measuring techniques have difficulties in recording this kind of data, either because they are invasive, or because of the low solids concentration and/or solids density of activated sludge. This paper investigates a novel non-invasive measurement technique borrowed from nuclear medicine, using a solids radiotracer and gamma cameras, to obtain solids concentration profiles during the batch settling of activated sludge, in a pilot-scale column with a height of 1m. The technique does not disturb the settling process, does not alter the settling characteristics, gives profiles every minute and every few millimeters, and is capable of measuring in a range of 0-25 g/l with high accuracy. Dynamic solids concentration profile measurements were performed for sludges of different wastewater treatment plants, and at different initial concentrations. The results show a quantitative representation of the settling process, and reveal hindered and compression settling.

Steady-state humic-acid-containing blanket in upflow suspended bed

We investigated the effects of turbidity and concentration of humic acid on the steady-state behavior of the blanket, which was coagulated using polyaluminum chloride (PACl) as coagulant. The three-dimensional solid-flux plot was constructed. Based on fixed PACl dosage, the iso-humic-acid solid-flux surfaces stacked that enveloped the feasible regime for the blanket bed. The steady-state point moved toward low solid flux and low solid fraction regime with decreasing initial raw water turbidity and/or increasing humic-acid concentration. Low water turbidity and high humic-acid concentration yielded a bulky blanket, with the former producing clean, and the latter turbid effluent. The presence of humic acid was thereby harmful to blanket strength, except for the case of low raw water turbidity. An optimal range of humic acid for blanket strength and clarification efficiency existed at 1 mg l(-1). Low level of humic acid is beneficial to blanket development with low-turbidity raw water.

Blanket dynamics in upflow suspended bed

A one-dimensional model for the spatio-temporal variation of solids fraction in the blanket considering both the hydrodynamic dispersion and the convection effects was proposed in this study. The model equation was a wave equation whose solution presented shock-wave like characteristic. The model output predicted that the distribution of the solids fraction in the blanket would, firstly, evolve to a uniform distribution at a lower solids fraction. Afterward, the blanket would compact itself to another uniform distribution at a higher solids fraction. Experimental works with synthetic raw water coagulated with PACl suspended in a lab-scale suspended bed confirmed the theoretical predictions. The solids flux plot was constructed with underdosed, optimal and overdosed suspensions. Change in coagulant dose would affect the blanket height and the response time. The solids flux curve was shifted accordingly. The "optimal' operational condition for the upflow suspended bed that produced the lowest effluent turbidity was identified and discussed.

Plastic deformation in cake consolidation

Cake filtration has a variety of applications in wastewater treatment by solid/liquid separation processes. In order to obtain the physical properties of the filter cake, a Compression-Permeability Cell (C-P Cell) can be used to examine the constitutive relationships among the solid compressive stresses, porosity and the specific filtration resistance. In this paper, a mathematical model is proposed to study the transient distribution of stresses, strains and void ratios in the confined cake of a C-P Cell under constant axial loads for dewatering. Simulations have been carried out using a plasticity model and the sensitivity analysis has revealed the possible contribution a finite plasticity can bring about the two types of cake examples with either medium or high compressibility.

Gravitational sedimentation of flocculated waste activated sludge

The sedimentation characteristics of flocculated wastewater sludge have not been satisfactorily explored using the non-destructive techniques, partially owing to the rather low solid content (ca. 1-2%) commonly noted in the biological sediments. This paper investigated, for the first time, the spatial-temporal gravitational settling characteristics of original and polyelectrolyte flocculated waste activated sludge using Computerized Axial Tomography Scanner. The waste activated sludge possessed a distinct settling characteristic from the kaolin slurries. The waste activated sludges settled more slowly and reached a lower solid fraction in the final sediment than the latter. Flocculation markedly enhanced the settleability of both sludges. Although the maximum achievable solid contents for the kaolin slurries were reduced, flocculation had little effects on the activated sludge. The purely plastic rheological model by Buscall and White (J Chem Soc Faraday Trans 1(83) (1987) 873) interpreted the consolidating sediment data, while the purely elastic model by Tiller and Leu (J. Chin. Inst. Chem. Eng. 11 (1980) 61) described the final equilibrated sediment. Flocculation produced lower yield stress during transient settling, thereby resulting in the more easily consolidated sludge than the original sample. Meanwhile, the flocculated activated sludge was stiffer in the final sediment than in the original sample. The data reported herein are valuable to the theories development for clarifier design and operation.