Gravitational sedimentation of flocculated waste activated sludge

Dewatering in biological wastewater treatment: A review

Biological wastewater treatment removes organic materials, nitrogen, and phosphorus from wastewater using microbial biomass (activated sludge, biofilm, granules) which is separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess sludge) is transported to digesters for bioenergy production and then dewatered, it is dewatered directly, often by using belt filters or decanter centrifuges before further handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to dewater, but great variations in dewaterability are observed for sludges from different wastewater treatment plants as a consequence of differences in plant design and physical-chemical factors. This review gives an overview of key parameters affecting sludge dewatering, i.e. filtration and consolidation. The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances. Polyvalent ions such as calcium ions improve floc strength and dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry) reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability dramatically decreases at high pH due to floc disintegration. Storage under anaerobic conditions lowers dewaterability. High shear levels destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.

Performance of a Novel Decentralised Sewage Treatment Reactor

A novel decentralised sewage treatment reactor (DSTR) for treating domestic sewage in rural areas was designed and investigated. The reactor was started aerobically after inoculation with biomass; the amount of initial mixed liquid suspended solids was 1.5 g/L. Oxygen was supplied to the reactor and the dissolved oxygen concentration was maintained at 1.3 ± 0.2 mg/L. The pollutant removal performance was investigated, and the average removal efficiencies of total chemical oxygen demand (TCOD),NH4+-N, suspended solids, and turbidity were 76%, 77.15%, 84.17%, and 83.93%, respectively. The DSTR exhibited good performance compared with the traditional activated sludge (AS) reactor. The resistance to impact load of the DSTR was superior to that of the AS reactor during surface load experiments. During the 120 days of operation, no sludge bulking was observed. The DSTR effluentNH4+-Nand TCOD levels were a little higher than those for the AS reactor, but the disparity was not major.

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.

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.