Coupled 3D hydrodynamic models for submarine outfalls
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This explains the worldwide increasing utilization of submarine outfalls. Positive examples with detailed monitoring programs of the receiving waters have been reported recently, showing no adverse effects of outfalls on the environment: Sydney Australia Philip and Pritchard, , Boston, USA Signell et al. There are six out of seven outfalls along the Sao Paulo coast which operate well Cetesb, , Northern Spain Revilla et al.
This is also important considering the accumulation of non-biodegradable substances, which eventually could have been removed by treatment technologies or source control. Technical limitations of discharge technologies are related to difficult and complex design procedures, which need an interdisciplinary approach, because of the need to understand the interaction of the discharge with the receiving waters.
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Problems with projects handling wastewater discharges have been mainly related to a poor understanding of what measures need to be taken to protect the environmental values of receiving waters. Measures to ensure bathing water quality may be different to those ensuring fishing water quality. Another drawback of discharge technologies is little public acceptance which is directly related to the previously stated little understanding and poor scientific communication.
However, it has been shown, that most of these problems are related to other waste sources often illegal discharges or surface run-off USGS, or Berzin, or uncontrolled industrial sources Braga et al. In polluted regions often very little is discussed regarding the adequate sewage treatment and discharge option that may restore environmental conditions now damaged by uncontrolled wastes discharges.
An example for better public involvement is given in Garber for Victoria, B. Such limitations mainly show the need for a better understanding and control of discharge processes. The additional advantage of studies based on the ecosystem-response on the installation of discharge technologies is their capability to detect the critical pollution sources, thus providing tools for a more efficient wastewater management USGS, The economic damage due to medical treatment and loss of productivity associated with diseases caused by exposure to contaminated food and water are high but difficult to estimate IETC, However, Esrey et al.
The consequent loss in costs is estimated in a few examples: In the US waterborne diseases cost the country between 3 and 22 billion dollars per year in medical bills and lost productivity Bennett et al. Illnesses related to water contaminated by untreated wastewater in China cost the country almost 4 billion dollars per year World Bank, Closed beaches have enormous implications on the local economy.
Costs for proper treatment and discharge installations are small compared to these implications. The balancing approach to decide on an appropriate discharge technology and the compatible treatment technology is analyzed in discharge assessments. Different design alternatives are hereby compared and evaluated regarding environmental impacts, investment costs, and operational demands. These assessments nowadays play an important role not only for the final discharge permit or for permit prolongation, but also especially for investors and politicians.
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Wastewater systems are fundamental infrastructural investments and generally so big, that safe and sustainable financing is essential. Financers are development banks e.
Both financing lines nowadays demand financially safe, thus sustainable, environmentally sound, and socio-economically adopted and economically stimulating investments. Projects hereby have to pass challenging and comprehensive pre-planning investigations. A well-elaborated discharge assessment is substantial for the approval.
Further extensions of such assessments are subsequent steps to the final planning, public communication, and permit approval. Even the regular monitoring can be based on the outcomes of the previous assessment, for example the existing models. The main question in discharge assessments are pollutant plume concentrations, locations, and geometries. A good example of such an assessment for a coastal water quality problem is described in Connolly et al.
This thesis provides solutions to improve the modeling techniques for such outfall designs. As a measure for the concentration reduction generally a reciprocal volume fraction of effluent in a sample defined as dilution S Fischer et al. Diffusers are designed to enhance the naturally slow mixing by strong initialdilutions.
Thus, initial concentration peaks are reduced to comply with ambient standards outside a Regulatory discharge zone around the outfall and a proper siting allows natural processes to further dilute the effluent to comply with with public health standards at the Regulatory preservation zone i. Therefore, it is commonly distinguished between dilutions related to different processes or design steps. Regarding different design steps it is often distinguished between a discharge induced initial or near-field dilution Snf, generally not including decay processes, and an ambient flow induced far-field dilution Sff that occasionally includes bacterial decay, depending on the analyzed parameter.
Permit compliance requires C 2. An application of equation 2. Results indicate that only treatment plant effluents, which passed primary and secondary treatment together with disinfection are allowed to discharge directly in bathing waters, provided that there is no background concentration and no concentration built-up. Discharges from treatment plants with lower treatment levels have to be located a sufficient distance away from the bathing water zone to allow dilution processes to occur before reaching the bathing waters.
Long distances long outfalls and high dilutions i. Required concentration reductions thus can be achieved by several combinations of treatment and discharge techniques. The optimal solution is the cheapest technology combination. Treatment Untreated Primary treatment Primary and secondary treatment Primary and secondary treatment and disinfection Effluent concentration Ce: E.
Thus, discharge assessment is mainly related to the prediction of the dilution. Historically only conservative estimates on initial dilution have been used to design and control outfalls. These worst-case scenarios based for example on the Cederwall equation Wood et al. Advancements have been made regarding additional processes summarized in empirical dilution equations and design procedures, outfall guidelines and books Jirka and Lee, ; Wood et. However, these techniques are still unreliable for outfalls where receiving water conditions depart significantly from the idealized cases Davies, Therefore submarine outfalls generally discharging into highly unsteady, non-uniform coastal waters are still challenging design cases.
Traditional designs are mostly based on measured parameters currents, densities, tracer tests, etc. Such measurements are costly, spatially limited to a few locations and temporally limited to short time periods. Therefore, modeling techniques are needed to receive spatial data over longer time-scales.
Appropriate combination with field studies allow the definition of missing boundary conditions, model calibration and model validation. However, field data extrapolation is limited to relatively small spatial scales and cannot predict changes due to planned installations. Furthermore, interpolation models do not necessarily allow for dynamic representations and understanding.
A waste plume generally cannot be tracked or resolved by a few point measurements and interpolation. Hydraulic laboratory model studies are quite reliable if certain conditions on minimum scales are met as has been demonstrated in the past. Therefore, hydrodynamic mathematical models are needed.
Larsen summarizes that there are excellent computer packages available for the general diffuser design e. Carvalho et al. He recommends that predictions of bathing water quality should have highest priority in the design of an outfall including reasonable safety factors. The question of initial dilution should only have a secondary priority in balance with a reliable engineering design.
Nonetheless the strong interaction of these processes makes a proper model coupling necessary.
A discussion of these issues and an example of predicting dispersion from the Boston Harbor outfall is given by Blumberg et al. This thesis aims for an improved discharge assessment linking simple dilution equations for continuous discharges in steady environments e. Fischer et. These analyses are mainly focused on large scales for long periods, where natural far-field processes dominate with partially overlaying intermediatefield processes. These analyses are focused on smaller scales and shorter periods, where outfall induced near-field processes dominate with partially overlaying intermediate-field processes.
This analysis is based on pipe hydraulics, where manifold processes dominate. The design and the extraordinary construction conditions for submarine installations will not be covered here, though also mainly derived from hydraulic models for the analysis of wave and current resistance loadings and further risks scour, anchor, earthquakes, and tsunamis. The governing equations are generally derived for an infinitesimal control volume in a Cartesian coordinate system and are valid everywhere. For most of the hydrodynamic problems and especially for discharge processes, it is not possible to solve these exact equations.
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However, engineering design does not necessarily demand exact solutions, but safe and reliable approximations. Additional safety factors furthermore account for inaccuracies in constructional techniques there is no exact construction and for uncertainties of the interaction with the environment of the installation, nature cannot be predicted exactly.
Therefore, scale analysis is used to make assumptions and approximations based on characteristic scales occurring in the problem Ferziger and Peric, Results are simplified equations, which can be solved either numerically or, in rare instances analytically. However, the definition or finding of characteristic scales is strongly problem or - 29 - - Governing equations process dependent.
There is no unique characteristic scale for waste discharges, but there are characteristic scales for each hydrodynamic region dominated by distinct processes. Therefore, one could try to find a common denominator heading for a final unique equation set, or one could try to develop as much equation sets, as hydrodynamic regions exist. The result then will not be a unique equation set, but as much equations sets as hydrodynamic regions exist. There is still a scientific and engineering controversy, if a unique equation set based on one common characteristic scaling can be solved with sufficient accuracy or if different coupled equation sets may solve the problem in equal or even better accuracy.
Both approaches will be discussed here.
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Incompressible fluids: Waste discharges occur in hydrodynamic regimes where fluids can be considered as incompressible. This approximation is valid for velocities and pressures occurring in environmental flows, provided that the Mach number is below 0. The only exception is water-hammer in artificial pipe-systems, which will be discussed separately. Newtonian fluids: Water even when heated and with typical seawater or wastewater properties is a Newtonian fluid.
Therefore, approximations for the viscous terms of the momentum equation can be used. An example for a non-Newtonian fluid would be mudflow processes, where other approximations have to be used. If the Rossby number is of the order of one, rotation has to be considered. For example, if a characteristic average velocity of approximately 0. Therefore, in accordance with typical scales of waste discharges Fig. Near-field and intermediate-field equation sets can be simplified neglecting effects of earth rotation. The vertical velocity then results from the continuity equation.
Thus, immediate effects of buoyancy on the vertical flow cannot be considered.