High levels of machine noise result in health issues for those people exposed to the machine for extended periods. Machine noise is becoming a more significant design consideration, often because of legislative requirements. Decanting centrifuges are machines with a rotating bowl. They have a number of noise sources, the most significant being structural noise, vortex/turbulence noise and siren noise. Due to the size, mass and speed of the rotating bowl, the bowl is the main source of structural noise. The structural noise is produced by all surfaces that are directly coupled to the bowl’s bearings. Due to the speed of rotation of the bowl, the turbulence from the various trailing edges generates broad spectrum vortex noise. Siren noise due to air flow through the bowl also generates significant noise especially at the lower harmonics of the bowl rotation frequency. Strategies to reduce decanter machine noise include: • Decoupling the surfaces of the from the main bearings of the rotating bowl and hence reduce the amount of structurally transmitted noise. • Smoothing the surface of the bowl to minimise the edges that produce vortices that are shed and produce vortex/turbulence noise. • As siren noise is produced due to flow through the rotating bowl, which is integral to the function of the decanter, the exit ports should be designed so that the noise is produced within parts of the sound spectrum that have low noise levels.

This project was concerned with the analysis and development of a decanter centrifuge. The aim of the project was to increase the understanding of the operation of the machine, and identify and implement methods of improving the performance. A comprehensive breakdown of the power consumption of a GTech-Bellmor 1456 Centrifuge Decanter was completed. There are four components of the power consumption in a decanter centrifuge: friction during product transport, viscous and kinetic losses during feed acceleration, inefficiencies in power transmission components, and aerodynamic losses, known as windage. A mathematical model was developed to predict the power, torque, and axial force required by product transport. A relationship for the power consumed during feed acceleration was derived from first principles. The power transmission losses are comprised of inefficiencies in the motors, belt drives, gearbox, bearings, and seals; each of these was quantified. The windage has two components: the surface drag on the bowl as it rotates in an annular space and the pressure drag on external protrusions. The windage was predicted empirically and computationally. Methods that were identified for improving the decanter centrifuge were: reduce the mass of bowl and scroll, improve wear resistance, reduce the coefficient of friction of the bowl wall and scroll faces, optimise scroll geometry, redesign the feed accelerator to increase acceleration efficiency, implement control of the bowl speed, differential speed, and pool depth, and recess the bolt heads on the bowl and cover the third phase ports. An analysis of several worn centrifuges revealed that the majority of the wear occurs on the scroll, bowl wall, accelerator, and solids discharge ports. An experiment was developed to recreate the wear conditions inside a centrifuge. A high pressure abrasive film was forced between materials moving relative to each other. Similar results were observed for acetel, UHMWPE, and 316 stainless steel when using a pin-on-disk wear test rig. A new composite bowl was developed for two main reasons, weight reduction, and improved wear and friction characteristics. The full design process was applied to the bowl and several concepts were generated for a new scroll. The design of the bowl included conceptual design, material selection, material testing, constructing scale models, and the manufacture of a full-size bowl for a GTech-Bellmor 1456 Centrifuge Decanter. The potential for using composite materials in decanter centrifuges was demonstrated. The manufacturing method developed during this project was novel and produced parts suitable for use in high-speed rotating machinery. The feed accelerator analysis consisted of three components: theoretical, experimental, and computational analysis. Three feed acceleration mechanisms were identified: viscous dissipation, impulse force, and mass flow induced velocity. An experimental method was developed to examine decanter centrifuge feed accelerator designs. The method allowed for the measurement of efficiency and high-speed photography of the flow between the accelerator and the rotating pool. The order of best to worst performing of the six tested designs was Modified Disk, Disk, Plate, Conical, Drum, and Esbjerg. The feed accelerator was modelled using ANSYS-CFX 14.5 and compared to the experimental results. There was excellent agreement between the flow in the annular space observed using high-speed photography and the paths predicted using the computational model. A parametric study of the Drum and Disk feed accelerator designs was undertaken using the computational model. It was found that increasing the surface area of the port faces of the Drum accelerator and increasing the discharge angle and discharge radius for the Disk accelerator improved the performance.

This study identifies the cause of unstable vibrations that sporadically occur in decanting centrifuges as being caused by a combination of internal bearing clearance, conveyor unbalance and low bearing loads. These centrifuges are different from other rotating equipment common in industry (pumps, fans, compressors, electric motors) in that they are dual rotor systems – one rotor inside the other. Unbalance in either rotor can produce severe vibration of the whole machine when the running speed is close to a mode of vibration – that is, running at or near a critical speed. The external rotor, called the bowl, is subjected to an internal pressure generated by the centrifugal force of the product being separated. The internal rotor is supported from the bowl and is in the form of an auger screw. The main supporting bearings are subjected to forces from both the bowl and the auger - the liquid end bearing also supports the gearbox. Being able to predict critical speeds through numerical or computational analysis is a necessary step in the design process or for troubleshooting vibration problems. As part of the study, the main rolling element bearings were replaced by oil-film journal bearings to assess the viability of their use. Journal bearings are simpler, of lower cost and generate less noise than their rolling element counterparts. However, instability in running above the first critical speed can result due to oil film forces and internal hysteresis of the rotor assembly. The auger is asymmetric so instability in running is possible at around half the first critical speed. This study was undertaken to understand the dynamics of decanting type centrifuges and develop a methodology for identifying their critical speeds and cause of unstable vibration. In the longer term this will assist in the generation of new designs that are quieter, use less energy and have better separation efficiencies.

This PhD thesis investigated solid and liquid waste treatment systems for Sureclean, a waste Management company based in the North of Scotland. Sureclean receives a diverse range of waste streams and the increasing need for sustainable development as well as stringent environmental legislation motivated this research to develop an integrated waste treatment system. Waste characterisation was conducted using a range of analytical instrumentation to identify the TPH, COD, heavy metals content, TOC, and particle size of Sureclean waste streams. From there, four treatment systems were investigated utilising Sureclean waste streams: mechanical separation, chemical treatment, electro-coagulation and the advanced oxidation process. Laboratory and field trials were conducted using these different treatment techniques and the analysis was performed to verify the treatment results. The result of these trials led to the development of four modular waste treatment units, that form the outcome of this research: the Sureclean Water Treatment System (SWTS), a filtration based mechanical separation system was shown to reduce the TSS, BOD and TOC content of an oily wastewater; the Sureclean Sludge Separation System (SSSTS), a chemical-enhanced filtration based system was demonstrated to reduce 52.6 % of the sewage sludge volume; the Sureclean Electro-coagulation Water Treatment System (SEWTS), a system that agglomerates colloid particles and demulsifies oil removed 99.9 % of TPH from Sureclean interceptor effluent; and the Sureclean Advanced Water Treatment System (SAWTS), an advanced oxidation process which was demonstrated to reduce the TPH of a contaminated groundwater collected from an ex-gas work. The treated effluent could be discharged to Sureclean interceptor. The four treatment units developed in this research expanded Sureclean waste treatment capabilities and an integrated system was developed to treat different waste streams and to improve the treatment efficiency thus increasing the revenue and future waste stream options for Sureclean.

O tratamento físico-químico da água em ETAs convencionais gera resíduos, denominados de lodo de ETA, cuja destinação constitui um desafio sob os aspectos econômicos, técnicos e ambientais. A disposição deste resíduo pelas ETAs do Brasil tem sido os corpos d’água, portanto, sob a ótica da minimização de impactos ambientais e economia de água, o trabalho avalia a possibilidade de reutilização do lodo de um decantador de alta taxa (DATx), através do procedimento de recirculação. A pesquisa foi desenvolvida com dados da ETA Renato de Freitas, Uberlândia – MG, que apresenta configuração de estação convencional. A caracterização da água de lavagem dos filtros (ALF) foi utilizada como parâmetro norteador para o equacionamento das descargas do lodo do DATx, uma vez que, a recirculação da ALF é uma prática adotada em ETAs. Para isto, foi avaliada a turbidez do lodo do DATx durante a abertura da válvula de descarga para os tempos de operação de 6, 12 e 24 h. Definido o tempo de operação do DATx, para descarga do lodo, foi proposto um pré-tratamento por ensaios de sedimentação, seguido dos ensaios de recirculação do lodo bruto e clarificado, nas taxas de 2 e 4%, em jarras de Jar Test. O equacionamento do DATx mostrou que após um tempo de 6 h de operação o lodo apresenta turbidez (39 UNT) aproximada ao valor encontrado na ALF (45 UNT) e ausência dos protozoários, Giardia spp. e Cryptosporidium spp. Para a taxa de 2%, a recirculação com lodo bruto ou clarificado produziu uma água decantada com valores de turbidez remanescente inferiores a 2,8 UNT. Enquanto, a taxa de 4%, a não clarificação do lodo, conduziu ao maior valor de turbidez remanescente (9 UNT). Os resultados ainda demonstraram ser possível obter redução na dosagem de coagulante de 4 a 13%, mantendo a qualidade da água decantada / The physico-chemical treatment of water in conventional ETAs generates waste, called ETA sludge, whose destination is a challenge under the economic, technical and environmental aspects. The disposal of this residue by Brazilian ETAs has been the water bodies, therefore, from the point of view of the minimization of environmental impacts and water economy, the work evaluates the possibility of reuse of the sludge from a high rate decanter (DATx), through the recirculation procedure. The research was developed with data from ETA Renato de Freitas, Uberlândia - MG, which presents a conventional station configuration the characterization of the filter wash water (ALF) was used as the guiding parameter for the DATx sludge discharges, since recirculation of the ALF is a practice adopted in ETAs. For this, the turbidity of the DATx sludge during the opening of the discharge valve was evaluated for the operating times of 6, 12 and 24 h. Once the DATx operating time was reached, a pretreatment was proposed for sedimentation tests, followed by the crude and clarified sludge recirculation tests, at 2 and 4% rates, in Jar Test jars. The DATx equation showed that after a 6 hour operation the sludge presented turbidity (39UNT) approximately to the value found in ALF (45 UNT) and absence of protozoa, Giardia spp. and Cryptosporidium spp. At the rate of 2%, recirculation with crude or clarified sludge yielded decanted water with remaining turbidity values below 2.8 NTU. Meanwhile, the 4% rate, the non-clarification of the sludge, led to the highest remaining turbidity value (9 UNT). The results showed that it is possible to obtain a reduction in the coagulant dosage of 4 to 13%, maintaining the quality of the water decanted / Dissertação (Mestrado)