Jan Baeyens

An investigation into slugging fluidized beds

Abstract A study of slugging fluidized beds has been carried out in four tubes ranging in size from 5–30 cm i.d. using particles having mean sizes 55–3380 μm and densities 0·85–2·8 gm cm−3. For all but the smallest column neither particle size nor size distribution had any effect on slugging. Equations are presented which enable us to calculate: 1. (a) The height and gas velocity at which slugging commences; 2. (b) The distance above the distributor at which further slug coalescence ceases; and 3. (c) The numerical value of the stable slugging frequency which is thereafter attained.

The analysis of volatile siloxanes in waste activated sludge

The increasing presence of siloxanes in waste activated sludge (WAS) considerably hampers the energy use of the biogas obtained during the anaerobic digestion of the sludge when concentrations exceed critical limits. To prevent the occurrence of unacceptable operating conditions, it is hence necessary to have a reliable analysis method for determining the siloxane content of the sludge. This paper describes and validates such a method, consisting of the extraction of the siloxanes using n-hexane and a subsequent analysis of the extract using GC-FID. The validation procedure confirms the excellent recovery and repeatability of the proposed method.

The Use of Ultrasonics in the Treatment of Waste Activated Sludge

Abstract The present paper reports on extensive investigations using an ultrasonic treatment of WAS, to study its potential to meet one or all of four objectives: (1) reduce WAS quantities; (2) achieve a better dewaterability; (3) provoke a release of COD from the biosolids, preferably transformed into biodegradable organics and (4) possibly destroy the filamentous micro-organisms responsible for sludge bulking. The experiments are carried out in a batch reactor of volume up to 2.3L. The ultrasonic apparatus consisted of a generator, a converter and a sonotrode, supplied by Alpha Ultrasonics under the brand name of Telsonic. Three different sludge kinds were tested, at approximate concentration (DS/WAS) between 3.5 and 20g·L −1 . The release of COD from the WAS-phase into the filtrate phase is a function of the specific energy-input SE with yields of about 30% achievable at SE-values of 30000 kJ·kg −1 . A major fraction of the COD is transformed into biodegradable organics (BOD). The reduction of DS-fraction of the sludge is equivalent to the COD-release rates. Although the dry solids content (DS) is reduced, the dewaterability of the sludge is not improved. This reflects itself in a slightly decreased dryness of the filter cake using vacuum filtration, and in increased values of the capillary suction time (CST). This more difficult dewaterability is the result of considerably reduced floc sizes, offering an extended surface area. More surface water is bound (CST increases) and the filterability decreases due to clogging of the cake. To reach the same dryness as the untreated cake, the required dosage of poly-electrolyte increases proportionally with the level of ultrasound energy supplied. The ultrasonic reduction of filamentous WAS organisms is not conclusive and very little effect is seen at low intensities and short treatment durations. Microscopic analysis of the WAS identified the dominant presence of Actynomyces. Especially the release of COD and its transformation into BOD certainly merit further research.

Effect of operating temperature on minimum fluidization velocity

We present experimental and literature data on the minimum fluidization velocity for operation at temperatures above ambient values. By regression analysis on the combined and transformed data, a new empirical correlation was developed. We investigate the effect of temperature on the minimum fluidization velocity of beds of different materials, such as limestone, lime and sand. These powders were fluidized at temperatures ranging from 20 to 400 o C (up to 900 o C for line)

Ultrasonically enhanced anaerobic digestion of waste activated sludge

Anaerobic digestion is increasingly applied to stabilise sludge and to reduce operating costs of the wastewater treatment plant by generating biogas, considered as a sustainable energy source. The process suffers from many drawbacks such as slow and incomplete degradation rates, the result of the slow and rate‐limiting sludge hydrolysis step, due to the low biodegradability of the cell walls and the presence of extra‐cellular biopolymers. Methods to enhance the biogas formation should hence make the substrate more accessible to the anaerobic micro‐organisms. Research on increasing the biogas production is extensive, with low‐frequency ultrasound treatment emerging as the technical and economic most appropriate technology. Although ultrasound is now widely applied in practice, underlying phenomena and optimum operating conditions are still open to debate. Laboratory ultrasound treatment was coupled with digestion experiments (at 37°C) using 12 parallel digesters, two of them being used as control digesters with untreated sludge. The present paper demonstrates that ultrasound treatment can – within specific ranges of energy‐input – achieve (i) an increased disintegration of the sludge, as witnessed by an increasing soluble COD‐fraction accompanied by an increasing presence of BOD; (ii) an increased release of volatile fatty acids as a result of the oxidising radicals being formed through cavitation; and (iii) a slightly reduced dewaterability of the residual sludge, needing an increased dosage of poly‐electrolyte to obtain a high degree of cake dryness. As pre‐treatment to digestion, ultrasound enhances the biogas production by more than 40% at low specific energy‐inputs (SE), and approximately 15% at higher SE‐values.

Towards safety, hygiene and environmental (SHE) management in African small and medium companies

Although Safety, Health and Environment (SHE) principles are adhered to in companies of developed countries, the application in developing countries is at its infant stage, as shown by the present article where the authors surveyed SHE Practice in 242 companies of 8 West-African countries. The survey demonstrated that (i) the overall performances are poor, although significant improvement can be achieved with a minimum of goodwill and management support; (ii) despite financial difficulties faced by small and medium enterprises (SME), SHE management is a must and should start with a pollution prevention program that will vary with individual needs but some conscious planning effort is always necessary; (iii) it is necessary to prepare for appropriate actions for pollution abatement, adapted to the existing and future production facilities. Although the recorded results are rather poor, there are obviously many possibilities of improvement, with a little goodwill of the SME managers and supervisors. Reaching a standard level of the environmental management is a long but an important process.