Impact of volatile solids destruction on the shear and solid-liquid separation behaviour of anaerobic digested sludge.

Systematic and comprehensive characterisation of shear and solid-liquid separation properties of sludge across a wide range of solids concentration and volatile solids destruction (VSD) is critical for design and optimization of the anaerobic digestion process. In addition, there is a need for studies at the psychrophilic temperature range as many unheated anaerobic digestion processes are operated under ambient conditions with minimal self-heating. In this study, two digesters were operated at different combinations of operating temperature (15-25 °C) and hydraulic retention time (16-32 d) to ensure a wide range of VSD in the range of 0.42-0.7 was obtained. For shear rheology, the viscosity increased 1.3 to 3.3 times with the increase of VSD from 43 % to 70 %, while other parameters (temperature, VS fraction) having a negligible impact. Analysis of a hypothetical digester indicated that there is an optimum VSD range 65-80 % where increase in viscosity due to the higher VSD is balanced by the decrease in solids concentration. For solid-liquid separation, a thickener model and a filtration model were used. No significant impact of VSD on the solids flux, underflow solids concentrations or specific solids throughput was observed in the thickener and filtration model. However, there was an increase in average cake solids concentration from 21 % to 31 % with increase of VSD from 55 % to 76 %, indicating better dewatering behaviour.

Shear and solid-liquid separation behaviour of anaerobic digested sludge across a broad range of solids concentrations.

Due to the non-homogeneous and multiphase nature of anaerobic lagoon constituents, CFD modelling for process optimisation requires continuous functions for shear and solid-liquid separation properties across a large range of solids concentrations. Unfortunately, measurement of existing material properties of anaerobic sludges is limited to only shear or solid-liquid separation, or to a limited solids concentration. In this work, the shear properties of an anaerobic sludge were measured from 0.4 to 12.5 vol%, which corresponds to the solids concentrations seen in lagoons. The sludge showed Newtonian behaviour at 0.4 vol% and Herschel-Bulkley yield stress fluid behaviour for higher concentrations ranging from 0.5 to 12 vol%. We compared multiple approaches to determine relationships between the model fitting parameters of consistency, k, flow index, n, and shear yield stress, τy with solids volume fraction ϕ.The solid-liquid separation properties were measured from sedimentation and filtration experiments to obtain compressibility and permeability properties across all the above-mentioned concentrations, enabling development of hindered velocity sedimentation curves. Comparison to full-scale anaerobic digestate identified that the pilot lagoon sludge had faster sedimentation at a given solids concentration in comparison to the digestate. This is the first study on simultaneous rheological characterisation and solid-liquid separation behaviour of an anaerobic sludge across a wide range of concentrations, thus enabling CFD modelling of the hydrodynamics and performance of anaerobic lagoons.

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.

Characterising sedimentation velocity of primary waste water solids and effluents.

Sedimentation in waste water is a heavily studied topic, but mainly focused on hindered and compression settling in secondary sludge, a largely monodispersed solids, where bulk sedimentation velocity is effectively described by functions such as double Vesilind (Takacs). However, many waste water solids, including primary sludge and anaerobic digester effluent are polydispersed, for which application of velocity functions is not well understood. These systems are also subject to large concentration gradients, and poor availability of settling velocity functions has limited design and computational fluid dynamic (CFD) analysis of these units. In this work, we assess the use of various sedimentation functions in single and multi-dimensional domains, comparing model results against multiple batch settling tests at a range of high and low concentrations. Both solids concentration and sludge bed height (interface) over time are measured and compared. The method incorporates uncertainty analysis using Monte Carlo regression, DIRECT (dividing rectangles), and Newton optimisation. It was identified that a double Vesilind (Takacs) model was most effective in the dilute regime (<1%v/v), but could not effectively fit high solids concentrations (>1%v/v) without a substantial (50%) decrease in effective maximum sedimentation velocity (V0). Other parameters (Rh, Rp) did not change. A power law velocity model (Diehl) was significantly less predictive at low concentrations, and not significantly better at higher concentrations. The optimised model (with reduction in V0) was tested vs a standard (optimised) double Vesilind velocity model in a simple primary sedimentation unit, and resulted in deviation from -12% to +18% in solids capture prediction from underload to overload (washout) conditions, indicating that the effect is important in CFD based analysis of these systems.

Rheological characterisation of synthetic and fresh faeces to inform on solids management strategies for non-sewered sanitation systems.

In order to obviate the economic issues associated with pit latrine emptying and transport such as high water additions and rheologically difficult sludge properties, the implications of prompt solid/liquid separation were investigated. This was achieved through rheological characterisation of fresh human faeces and synthetic faeces, and comparison with aged faecal sludges. Shear yield stress, thixotropy and post-shear structural recovery were characterised for a total solids (TS) concentration range of 5-35% total solids (TS) and stickiness yield stress was determined for concentrations up to 100% TS. Fresh faeces rheology proved to be favourable when compared to aged matrices, evidenced by a lower shear yield stress and higher gel point solids concentration, suggesting that aging could alter the physico-chemical properties of faecal sludge. Fresh and synthetic faeces exhibited similar shear thinning, thixotropic behaviour with the majority of structural breakdown occurring at a low shear rate of 10 s-1, and the extent increasing with higher solids concentrations. At 32% TS, fresh faeces shear yield stress was permanently reduced by 80%, suggesting that low shear pumping could reduce the energy demand required for faeces transport. The sticky phase, which represents the region to avoid faecal transport and mechanical drying processes, was identified to range from 30 to 50% TS, with 25% TS as ideal to commence dewatering processes. This also coincides with the average solids concentration of faeces, which is achievable by source separation. This study has identified that handling of fresh faeces as opposed to aged faecal sludges would result in economic and environmental benefits, with energy, water and labour savings.

On the compressional rheology of fresh faeces: Evidence for improving community scale sanitation through localised dewatering.

Non-sewered sanitation is currently dependant upon pit latrine emptying, the safety of which is compromised by the high costs of faecal sludge transport to centralised treatment facilities. Transport in turn is hindered by the complex rheology of pit latrine sludge. This study therefore characterised the compressional rheology of fresh faeces and modelled the implications for passive (gravity) or mechanical (forced) solid/liquid separation. This informs on the viability of decentralising dewatering for more efficient volume reduction and improving the economics of transportation. The gel point (ϕg) is the solids concentration where the material has a networked structure and signifies the point when mechanical intervention is required for further solid-liquid separation. For fresh faeces, ϕg ranged between 6.3 and 15.6% total solids (TS) concentration. This is significantly higher than the ϕg observed for wastewater sludge at centralised facilities, and it implies that passive gravity driven processes can suffice to improve localised dewatering. The kinetics of passive sedimentation of faecal material were modelled and illustrate thickening from 3 to 10% TS concentration in <0.5 h. This highlights that early intervention to thicken faeces while fresh can improve solid/liquid separation efficiency. Filtration of fresh faeces was characterised by lengthy cake filtration times and comparably short compression times, more similar to mineral slurries than to wastewater sludge. Consequently, fresh faeces presented improved dewatering characteristics, supporting higher final cake solids concentrations and improved dewatering kinetics. By complementing thickening with 300 kPa filtration, a 1.4 cm thick 25% TS product could be achieved in <24 h. Investigation of matrix properties highlighted that increased conductivity (e.g. exposure to urine) negatively influenced dewaterability, an effect which could be mitigated by introducing solid-liquid separation earlier. The thermodynamically favourable compressional rheology of fresh faeces has identified that focussing on localised dewatering could radically improve the economics of faecal sludge management, primarily through reducing transport costs.

Compartmentalization of extracellular polymeric substances (EPS) solubilization and cake microstructure in relation to wastewater sludge dewatering behavior assisted by horizontal electric field: Effect of operating conditions.

Sludge treatment and disposal have become important environmental issues in China. Mechanical dewatering is widely used to reduce the amount of sludge to be disposed and relieve the rapid growth pressure of waste sludge. In comparison to traditional sludge dewatering processes, pressure electro-osmotic dewatering has many advantages on sludge dewatering efficiency, low conditioner dosage and concentrated cake are both beneficial to further recycling of waste sludge. In general, complex electrochemical effects (eg. electrochemical oxidation, ohmic heating and pH gradient effect) are accompanied by the pressure electro-osmotic dewatering process. These electrochemical effects will inevitably cause solubilization and/or degradation of key constituents of wastewater sludge - extracellular polymeric substances (EPS). In this study, the effects of voltage, pH and ionic strength on sludge electro-osmotic dewatering performance and electrochemical effects were investigated. The solubilization and degradation of EPS were analyzed by examining the variation of dissolved organic matter (DOM) in the filtrate, and the relationships between microstructural properties of sludge cake and DOM and electro-osmosis dewatering performance in electro-dewatering process was examined. It was found that electro-dewatering properties were improved by raising the operating voltage or decreasing the pH value, while dewatering rate initially increased at low ionic strength it decreases with increased ionic strength. In addition, the porous structure of cathodic cake was more plentiful than that at the anode. At the cathode, the EPS dissolution was mainly related to alkalization, while the oxidation and acidification were responsible for release of EPS at the anode. Meanwhile, electrophoresis effect was able to promote migration of EPS toward the anode. The average electro-osmotic dewatering rate at the anode (R2.>0.79, p < 0.02) and at the cathode (R2.>0.87, p < 0.03) strongly correlated with the volume of pore of sludge cake. There was no correlation between the total content of anodic DOM (R2<0.31, p>0.08) and electro-osmotic dewatering rate at the anode, however, the content of cathodic DOM (R2 > 0.62, p < 0.09) negatively correlated with average electro-osmosis dewatering rate of cathode. Since cathode is the main water-permeable side in sludge electro-dewatering, and the sticky biopolymers (proteins and humic subtances) could not be converted into small molecules, higher EPS release was associated with worse sludge filterability. As for the anode, the biopolymers were degraded into small molecules due to electrochemical oxidation, which greatly reduced the impact of DOM on dewatering effect. Therefore, the operating conditions (voltage, pH and ionic strength) caused changes in electrochemical effects, which played a crucial role in compartmentalization of sludge EPS dissolution and consequently sludge electro-dewatering behavior.

Quantification of wastewater sludge dewatering.

Quantification and comparison of the dewatering characteristics of fifteen sewage sludges from a range of digestion scenarios are described. The method proposed uses laboratory dewatering measurements and integrity analysis of the extracted material properties. These properties were used as inputs into a model of filtration, the output of which provides the dewatering comparison. This method is shown to be necessary for quantification and comparison of dewaterability as the permeability and compressibility of the sludges varies by up to ten orders of magnitude in the range of solids concentration of interest to industry. This causes a high sensitivity of the dewaterability comparison to the starting concentration of laboratory tests, thus simple dewaterability comparison based on parameters such as the specific resistance to filtration is difficult. The new approach is demonstrated to be robust relative to traditional methods such as specific resistance to filtration analysis and has an in-built integrity check. Comparison of the quantified dewaterability of the fifteen sludges to the relative volatile solids content showed a very strong correlation in the volatile solids range from 40 to 80%. The data indicate that the volatile solids parameter is a strong indicator of the dewatering behaviour of sewage sludges.