Formation mechanisms and mechanical properties of anaerobic lagoon scum.

The formation of a floating scum layer on the liquid surface of covered anaerobic lagoons prevents optimal and efficient lagoon operation. Scum can reduce hydraulic retention time, inhibit biogas capture and cause damage to lagoon covers. Managing the negative impact of scum requires understanding what scum is, how it forms and how it consolidates. This paper presents measurements of the physical and mechanical properties of scum and sludge samples from two covered anaerobic lagoons that alternatively treat municipal and abattoir waste. Both scum samples consisted of a large proportion of suspended solids that sank once the sample was diluted, degassed and mixed, indicating that sludge flotation and buoyancy due to biogas generation is a major contributor to scum accumulation. Total and soluble chemical oxygen demand and volatile solids in the scum are approximately 90 % higher than in sludge, which indicates that scum has a large proportion of undigested solids. Fourier-transform infrared spectroscopy demonstrates that scum and sludge have similar organic matter, with both including fats, oils, greases, proteins, and polysaccharides. Scum formation due to gas buoyancy implies that scum accumulation is inevitable and controlling fats, oils, and greases at the source of the wastewater is not enough to stop scum formation. Scum accumulation increases due to buoyancy, which drives scum compaction and increases the strength of the scum, as demonstrated by the measurement of scum compressional rheology. Scum management techniques that disturb the scum layer early enough to release the entrapped gas enable the scum to sink and get digested, thus minimising the impact of scum formation.

Impact of gas injection on physicochemical properties of waste activated sludge: A linear relationship between the change of viscoelastic properties and the change of other physiochemical properties.

Aeration process in the waste activated sludge treatment accounts for 75% of total energy consumption of the treatment plant. The main purpose of the aeration process is to enhance the biodegradation of the liquid waste. Gas bubbles, rising through the liquid, improves mixing, reduces inhomogeneities in the treatment tank and enhances biological reactions. Thus aeration intensity and several physicochemical properties of feed such as viscosity, total suspended solids, and surface charge play a significant role in the biological reaction. This paper examines the impact of the gas injection rate on some physicochemical properties of waste activated sludge namely rheological properties, suspended solids, soluble COD (sCOD), surface tension, and zeta potential. The impact of four different gas flow rates on four different concentrations of waste activated sludge properties was analysed. The results showed that in linear viscoelastic regime the viscous and elastic modulus decreases linearly with an increase in gas flow rate. The amount of stress imposed by gas injection also showed a direct relationship with gas velocity. Gas injection also showed a substantial impact on soluble COD, suspended solids, and zeta potential. Additionally, a linear relationship was observed between the percentage change in the above mentioned physical properties and stress imposed by gas injection. These results confirm that gas injection produces additional shear impacting sludge physicochemical properties and therefore changes its rheological behaviour. The extra stress induced by gas injection can be predicted using a simple model based on sludge concentration and gas velocity.

Influence of gas injection on viscous and viscoelastic properties of Xanthan gum.

Xanthan gum is widely used as a model fluid for sludge to mimic the rheological behaviour under various conditions including impact of gas injection in sludge. However, there is no study to show the influence of gas injection on rheological properties of xanthan gum specifically at the concentrations at which it is used as a model fluid for sludge with solids concentration above 2%. In this paper, the rheological properties of aqueous xanthan gum solutions at different concentrations were measured over a range of gas injection flow rates. The effect of gas injection on both the flow and viscoelastic behaviour of Xanthan gum (using two different methods - a creep test and a time sweep test) was evaluated. The viscosity curve of different solid concentrations of digested sludge and waste activated sludge were compared with different solid concentrations of Xanthan gum and the results showed that Xanthan gum can mimic the flow behaviour of sludge in flow regime. The results in linear viscoelastic regime showed that increasing gas flow rate increases storage modulus (G'), indicating an increase in the intermolecular associations within the material structure leading to an increase in material strength and solid behaviour. Similarly, in creep test an increase in the gas flow rate decreased strain%, signifying that the material has become more resistant to flow. Both observed behaviour is opposite to what occurs in sludge under similar conditions. The results of both the creep test and the time sweep test indicated that choosing Xanthan gum aqueous solution as a transparent model fluid for sludge in viscoelastic regime under similar conditions involving gas injection in a concentration range studied is not feasible. However Xanthan gum can be used as a model material for sludge in flow regime; because it shows a similar behaviour to sludge.

Impact of gas injection on the apparent viscosity and viscoelastic property of waste activated sewage sludge.

Gas injection is known to play a major role on the particle size of the sludge, the oxygen transfer rate, as well as the mixing efficiency of membrane bioreactors and aeration basins in the waste water treatment plants. The rheological characteristics of sludge are closely related to the particle size of the sludge floc. However, particle size of sludge floc depends partly on the shear induced in the sludge and partly on physico-chemical nature of the sludge. The objective of this work is to determine the impact of gas injection on both the apparent viscosity and viscoelastic property of sludge. The apparent viscosity of sludge was investigated by two methods: in-situ and after sparging. Viscosity curves obtained by in-situ measurement showed that the apparent viscosity decreases significantly from 4000 Pa s to 10 Pa s at low shear rate range (below 10 s-1) with an increase in gas flow rate (0.5LPM to 3LPM); however the after sparging flow curve analysis showed that the reduction in apparent viscosity throughout the shear rate range is negligible to be displayed. Torque and displacement data at low shear rate range revealed that the obtained lower apparent viscosity in the in-situ method is not the material characteristics, but the slippage effect due to a preferred location of the bubbles close to the bob, causing an inconsistent decrease of torque and increase of displacement at low shear rate range. In linear viscoelastic regime, the elastic and viscous modulus of sludge was reduced by 33% & 25%, respectively, due to gas injection because of induced shear. The amount of induced shear measured through two different tests (creep and time sweep) were the same. The impact of this induced shear on sludge structure was also verified by microscopic images.