Introduction
Coastal towns have specific positions due to the possible utilization of sea and attraction tourists. Finding useful methods of sewage disposal at lower costs and higher efficiency and also expandable in terms of the capacity of coastal towns is very important. Releasing residential sewage of Chalous city into the Caspian Sea makes marine environment a source of raw wastewater and the receiving water body affected by entering these contaminants.
The contaminants cause to pollute fishes, sediments and coastal waters and then will transmitted to humans through the consumption of contaminated marine organisms and pathogens or by swimming.
Many of researchers and experts believe that secondary treatment of wastewater, needs a lot of costs and energy and sludge disposal and production should be continuously monitored. But with proper planning of drainage system for the sea, we can discharge large amounts of sewage into sea with no need to treatment maintenance. Oceans, seas and Great Lakes have high self-purification capacity to absorb sewer of coastal towns.
There are many types of wastewater discharging into the sea include directly discharges onto the beach, discharges with a short outfall with likely contamination of recreational waters and discharge with an effective outfall designed so that the sewage is efficiently diluted and dispersed and does not pollute recreational areas.
Dilution of the wastewater into the sea have three-phase including near-field, far-field and long-term flushing. Generally, the near and far field dilution mechanism strictly disagree that they act separately as a independent treatment service. The outfall system typically consists of tunnel or pipeline, diffuser and ports on the diffuser; in this system the basis is the dilution of wastewater without any problem for the marine environment.  Prediction of near field mixing requires understanding of the dynamics of jets, plumes and buoyant jets; a jet is a flow driven by the source momentum flux only, a plume is driven by the source buoyancy flux only and a buoyant jet is driven by both momentum and buoyancy fluxes. Discharges from an outfall diffuser have both momentum and buoyancy and are therefore buoyant jets, but the buoyancy flux is usually dominant and ocean outfall discharges can often be approximated as plumes.
In this study, the use of outfall as sewage disposing system in the city of Chalous that is located near the southern coast of the Caspian sea, has been assessed in term of environmental.
Materials and methods
Mathematical models are now widely used to predict the fate and transport of ocean discharges. This is not possible to simulate these types of discharges with one overall omnibus model, because of the very wide range of lengths and times scales for various mixing processes. Therefore linked sub-models of the various phases are usually used.
In general, assumptions used in modeling the dispersion and dilution of waste water include the incompressibility of flow, the pressure is hydrostatic and molecular scattering is ignored.
The theory of mixing zone and near-field models are different by far-field models and includes the length scale, entrainment and CFD models. For far-field the hydrodynamic models have been developed in past few decades and are being used increasingly to predict the fate and transport of marine discharges.
There are three basic equations for the discharge of sewage into the sea through an outfall include: mass flux, momentum flux and buoyancy flux that using dimensional analysis leads to considerable analytical simplification and to useful equations for predicting plume behavior and dilution.
One of the common near-field models is CORMIX that is provided for analyzing the discharge of various types of discharge to marine waters. CORMIX is a comprehensive software system for the analysis, prediction, and design of outfall mixing zones resulting from discharge of aqueous pollutants into diverse water bodies. It contains mathematical models of point source discharge mixing within an intelligent computer-aided-design (CAD) interface. Its focus is environmental impact assessment and regulatory management. CORMIX has been developed under several cooperative funding agreements between U.S. EPA, U.S. Bureau of Reclamation, Cornell University, Oregon Graduate Institute (OGI), University of Karlsruhe, Portland State University, and MixZon Inc. during the period of 1985-2007. The major emphasis of the CORMIX system is on the geometry and dilution characteristics of the initial mixing zone including compliance with regulatory constraints as well as predicting the behavior of the discharge plume at larger distances. However, this system also has capabilities to predict the wastewater over long distances in the far-field. CORMIX contains three core hydrodynamic simulation models includes: Simulation models for single port discharges (CORMIX1), Simulation models for submerged multiport diffusers (CORMIX2) and Simulation models for buoyant surface discharges (CORMIX3). In this paper the CORMIX2 has been used for predicting and modeling the discharges from an outfall diffuser; the following items must be respected in the design of a multi-port diffuser: port and nozzle diameters (not too large to cause the sea water to enter diffuser and not too small that waste energy and increase pumping costs), nozzles angle to the seabed in a way to flow horizontally into the water body because of optimal dilution and the outlet port shape of nozzles be such that not to cause friction in the direction of wastewater disposal and preferably be circular.
Factors of water contamination are nutrients, bacteria and pathogens, toxic, chemical, organic, heavy metals and suspended solids. Pathogens are the most important factor to design the outfall and can be controlled by initial mixing, diffusion and destruction by sunlight.
California ocean plan is an instruction to a successful discharge of sewage to marine waters without complications for the environment. This plan limits the level of water quality such as bacterial, chemical and physical properties and also considers some limitation for sewage.
The most important indicator bacterium in sea water and natatorium is Enterococcus. According to the California Ocean Plan, USEPA limited levels of coliform bacteria Enterococcus for the 30-day geometric mean to 35 units per 100 ml. Also maximum of coliform units in the discharged wastewater to the sea is limited to 1000 units per 100 ml.
Discharges near environmentally sensitive areas such as coral reefs or shell fishing beds should be avoided. If the diffuser cause rapid dilution and dispersion of the effluent and is positioned so the waste field transport to critical areas is minimized, only the preliminary wastewater treatment such as milliscreening may be needed. To choose a convenient location for an outfall, the various parameters are effective includes water depth and the slope of the seabed, type and direction of currents, swim areas, regional water quality, concentration of population and entry of rivers water into the sea; So given the above parameters, three routes along the coast of the Chalous city proposed and investigated and the proper route and discharge depth chosen during a process of trial and error.
The worst dilution conditions such as no currents and strongest density stratification must be considered for designing an outfall. Therefore to obtain the worst dilution case, the model is run for different seasons. Thus, outfall designing takes place based on specifications of the summer season that currents are minimal and the density difference is the highest value.
Results
Diffuser geometrical design is plotted using CorSpy that is one of the CORMIX post processing tools. The diffuser has the length of 50 meters with eleven ports that each port has a T-shaped nozzle with 25 cm height and 50 cm from the seabed. Due to strong density stratification of sea in the summer, the discharged sewage doesn’t reach to surface water and becomes stuck in the terminal layers. The distance between the source of discharge and where the plum meets bank is about 6500 meters and due to average currents that is 0.058 meters per seconds, the time of reaching to bank is about 31 hours that is more than 24 hours considered and there is enough time to kill bacteria. According to the results of CORMIX the bacteria reach certain standards within regulatory mixing zone and there are no worries about natatorium area that is the thousands of meters away.
Because of poor density stratification of sea in winter, the discharged sewage reaches to surface water and therefore the 100 value of dilution must be checked at that water level; By CORMIX modeling the distance between discharging sewage and where it reaches to surface level is about 120 meters and the dilution value of is 250.
According to the results of modeling, discharged wastewater meets the bank with a distance about 10 to 25 kilometers from the source with different ambient currents that is adjacent to residential and swimming areas but due to its arrival time is more than 24 hours, so there is enough time to kill bacteria.
The model also was tested for the reversal ambient current that is usually occurs on cold days and the plum meets the bank with a distance about 10 to 20 kilometers from the discharging source with different ambient currents. The results of the modeling in terms of average monthly current in summer and winter show that the value of plum dilution when it reaches to water surface in winter and the time of meeting plum to bank from the discharging source in summer, are two main parameters for designing of Chalous city outfall.
Discussion of Conclusions
The project goal was to design an efficient outfall that have features such as proper drainage location, sufficient diffuser length, proper discharge depth, safety purposes, economic, and trustworthiness of the effects of biological, chemical and physical. The outfall was designed in the worst environmental case modeling with almost no currents and maximum density stratification that occurs in summer and to ensure the proper functioning of the outfall, other conditions were also modeled.
Desired location for installing outfall is chosen according to the parameters like beach slope for decreasing outfall pipe length, flows and wastewater fate in the far field so as not to reach swimming and tourist areas and pollution from streams and existing municipal waste.
The results of the model show that in the first movement, sewage discharge is diluted more than 100 times in a few minutes and within 10 meters of the discharge location, all pollutants comply with environmental conditions and adherences to the strictest standards. This amount of dilution is more than a lot of secondary treatments ability.
The major environmental advantages of the project are protect groundwater from contamination by sewage, eliminating the direct discharge of wastewater into the Caspian Sea and thus improve the quality of sea water, eliminating the harmful effects caused by sewage sludge dumping and reduce energy consumption and thus produce less heat and pollution.
The results of this study indicated that the use of marine outfall in the city of Chalous with some considerations such as preliminary treatment prior to discharge is desired option. In addition, the results indicated that outfall not only eliminates the environmental problems of wastewater disposing, but it is the most economical method of sewage disposal.