*********** +++++++++++++++++++++ 072595B.ENG + Source: ONR Asia + *********** +++++++++++++++++++++ Contributory Categories: ENV Country: South Africa From: Marine Sciences and Technology in South Africa Foundation for Research Development 1990 KEYWORDS: South Africa; Marine Science, Marine Technology, Coastal Engineering, Ocean Engineering +++++ MARINE SCIENCE AND TECHNOLOGY IN SOUTH AFRICA Part XIV/XXV 1 Item EFFLUENT DISPOSAL G Toms formerly of CSIR, Stellenbosch "Dump it in the sea-out of sight out of mind!" This attitude is fast disappearing and marine disposal has become a specialist field in today's conservation-conscious society. All over the world, disposal of effluent, or liquid wastes, in the sea is an alternative that . is often chosen because it appears to be economical. If approached cautiously and thoroughly studied, marine disposal schemes can be designed to be safe-not only for man but also for the marine environment. If approached hastily and superficially such schemes can fail badly, rendering coastal waters unpleasant and often unhealthy for man and causing ecological damage, sometimes drastic but often subtle. Recent trends in marine disposal practice in South Africa show a growing tendency for caution and thoroughness in design ensuring the necessary protection for coastal waters. Methods of disposal In South Africa, coastal towns and industries discharge a daily total of about 850 million litres of effluent to sea! This is less than 10 M3 per second (the mean flow of the Orange River) and originates from over 3 000 km of coastline. Figure 1 shows the location of marine discharges in South Africa. Disposal to sea can occur as dumping from barges, discharge to shallow surf zone water by canal or short pipelines or as discharge to deeper water beyond the surf zone by long deepsea pipelines often termed ocean outfalls. In South Africa, dumping from barges is practised only for dredge spoil, which normally consists of dredged silt or sand being transported away from a harbour mouth or coastal construc- tion and dumped out at sea. Dumping is then done in such a way and at such a location that the spoil will not be transported by currents or waves back to the dredge site. Such dumping takes place at most South African harbours and is done subject to the approval of the Sea Fisheries Research Institute. In the case of the smaller discharges, treated or untreated domestic or industrial effluent is discharged at the water's edge into the waves by pipe or canal. As it is released, it is diluted by sea water taken from the surrounding area. For most of the smaller discharges (below 1 M3/s) entrainment of uncontaminated seawater is rapid and the effects of pollution are very localized. Entrainment is much less efficient for larger shoreline discharges (more than 1 M3/S) which typically originate from a combination of effluent and stormwater run-off. The poor quality of urban stormwater and its higher flow rate and shoreline discharge tempo are a cause for concern, particularly where discharges occur near popular bathing beaches, poorly flushed areas or sensitive estuarine waters. In these discharges the surf zone area immediately surrounding the outlet becomes contaminated with effluent and entrainment of uncontaminated seawater, so necessary for efficient dilution, is inhibited. Furthermore, the effluent is often held within the wave breaking or surf zone instead of escaping to deeper "cleaner" water, thereby exposing the shoreline to low dilution levels. Worldwide the largest flows of shoreline-discharged effluent to sea occur from power stations, such as South Africa's nuclear power station at Koeberg, where seawater taken in and used as coolant undergoes a rise in temperature before it is returned to the surf water. Normally such discharges are contaminated only by excess heat and chlorination and the effects of this on the nearshore environment are carefully assessed. Typically, however, such discharges are large enough (50-100 M3/S) to move through the surf zone to deeper water and therefore undergo greater dilution by entrainment of cooler offshore water. Discharge pipelines to deeper water beyond the surf zone constitute a more efficient and controllable method of achieving rapid dilution of effluent, thereby decreasing contamination levels close to the discharge, and especially pre- venting contamination of the shoreline, where activities such as recreation or collection of shellfish may otherwise be threatened. In South Africa, effluent flow rates for such ocean outfalls range from 0,03 to 1,5 M3 /s and depths of discharge range from 23 to 60 m. At present there are eight ocean outfalls in South Africa which discharge to water depths well beyond the surf zone. Types of effluent Sewage or domestic effluent is discharged to sea only through shoreline or deepwater outfalls, none being discharged by dumping from barges. The effluent is discharged either raw or partially treated, but shoreline discharges of raw sewage effluents are rapidly being phased out. The Durban outfalls are the largest of the South African deepwater sewage outfalls. Industrial effluent is also discharged either at the shoreline or in deep water, depending on the quantity and quality. These discharges occur mainly on the Natal coast due to the presence of large pulp mills and chemical plants. At present the largest industrial effluent outfall is at Richards Bay. Fish-processing effluent discharges are restricted to the south and, more particularly, the west Cape coast where the commercial fishing industry is centred. These are generally small, localized, periodic (seasonal) discharges in shallow water adjacent to (or within) Yhe fishing harbours. Recent improvements in processing (e.g. dry off-loading) have, however, significantly reduced pollution problems. The total amount of effluent discharged from fish process- ing plants around the coast is not more than about 1,5 M3/S. Design Considerations Shoreline Outfalls The degree of dilution (or mixing) in shoreline discharges is difficult to predict, because it is influenced by many environmental factors such as winds, waves, nearshore currents, bed slope and nearshore topography, which are themselves difficult to measure or predict. The effluent density and, in particular, flow rate also has a significant effect on the behaviour of the effluent and its dilution rate. In view of these complications extensive field work may be required to understand local conditions influencing effluent dilution before making design recommendations, but even then predictions can be notoriously inaccurate. Ocean Outfalls Deepwater outfalls or ocean outfalls (well beyond the surf zone, deeper than 10 m) may be used to discharge effluent which is either buoyant, neutral or dense in seawater (i.e. lighter than, of equal density or heavier than seawater). Dense effluents are not common, and only two of the eight ocean outfars discharge dense effluents to sea. The discharge of dense effluents requires a design technique very different from that in discharging buoyant effluents. AR effluents discharged by such ocean outfalls are released from the pipe at the sea-bed. Dense effluents will simply remain at the sea-bed and will not rise away from it or entrain clean seawater unless discharged at high velocity. This is normally achieved by limiting the number of openings or ports in the pipe through which effluent can escape and also by keeping the diameter of the ports as small as possible. In the case of dense gypsum effluent at Richards Bay on the Natal north coast it was necessary also to apply an additional pumping head to jet effluent out of the ports at about 15 m'/s! This causes the effluent to rise up into the water column of about 10 to 12 m and, because the ports discharge at an angle of 60' to the horizontal, the effluent undergoes a trajectory path similar to that shown in Figure 2, ensuring very rapid initial dilution up to the jet apex and slower dilution beyond the apex. Currents present in the seawater around the jet cause a significant increase in entrainment into the jet, and deform the jet in such a way that it actually follows a longer path, allowing more time for entrainment of seawater and therefore enhancing dilution of the effluent. Buoyant effluents are far more commonly encountered, as these are normally freshwaterbased. Sewage effluent, for example, has a density very close to that of freshwater (- 1,0 g/cm'). Similarly, where fresh water is used for industrial processes and for the washing of contaminated plants, the resulting effluents normally have a density close to l'o g/CM3. Industrial effluents are also often warm, up to 50'C in the case of some pulp mius, and this heat increases the buoyancy by reducing the density of the effluent. In the case of ocean outfalls, optimum use is made of the buoyancy of the effluent by releasing it in deep water. The lower density of the effluent causes it to rise and thus entrain sea water. Typically, in a calm sea of uniform density the plume will continue to rise and very efficiently entrain seawater until it reaches the water sur- face, where it will spread horizontally. The process is illustrated in Figure 2. This mechanism is very well understood and initial dilution levels can be predicted accurately. The following procedures will lead to increased dilution: o increasing the depth of the discharge; o discharging through a series of holes or ports (i.e. through a diffuser) rather than one open end; o decreasing the diameter of the ports and increasing the number of ports without diminishing the efficiency of the diffuser; o positioning the diffuser in such a way that the plumes of effluent are exposed to the strongest possible currents. The last point above is important as currents cause a dramatic increase in the rate of initial dilution and, while the strength and direction of the current is not under the control of the designer, the placing and orientation of the diffuser is. A similar factor not under the direct control of the designer is that of density stratification, where the density of the seawater varies with depth. For example, factors such as surface warining, convection, density currents or upwelling of colder denser water from offshore can cause the bottom water to be colder, and therefore more dense, than the water at the surface. This causes the plume of effluent to stop rising and spread horizontally before it can reach the water surface, and therefore reduces the path length for entrainment and dilution. This process is also shown in Figure 2 [NOT GIVEN HERE]. A great deal of field measurement is required to understand, describe and quantitatively predict uncontrollable environmental factors characteristic of a specific disposal site. Such measurements typically include measurements at sea of waves, winds, currents, sea temperatures and salinities, sea-bed characteristics and the seasonal variation of all of these. Measurement programmes are therefore normally longer than one year in duration and, while costly, are vitally necessary for a cautious and thorough design. Another important factor to take into account in the design of outfalls is the degree of decay of constituents, such as the decrease of bacteria and viruses due to die-off during dilution. Understanding the mechanism and rate of decay is important and many studies have been and are being conducted in this regard. To design effluent dischargers not only physical mixing but also concepts like decay, effluent pathways, deposi- tion rates, solubility rates, and rates of other chemical changes such as those which deplete oxygen or satisfy the oxygen demand of the effluent have, therefore, to be taken into account and understood. Regulatory Measures Legislation affecting marine effluent disposal in South Africa is under the control of the Department of Water Affairs, which is responsible for setting discharge standards and the issue of permits to enforce these. In each permit application submitted to the Department the method and particulars of discharge have to be specified. Permits are judged and standards and controls for each discharge are determined on an individual basis by considering factors such as the mixing characteristics of the region into which discharge is to be made, the accessibility and use of the region and the nature of the effluent. In order to provide a guide to the licensing authority in the assessment of proposed discharges and the establishment of controls, the South African National Committee for Oceanographic Research published a document Water Quality Criteria for the South African Coastal Zone in 1984. Permits expire after a number of years, and this enables the Department to enforce tighter control on existing discharges if this is shown to be necessary by research. In particular, discharge practices of untreated effluent to shallow surf water are being critically reviewed at present. +++++ End Part XIV/XXV +++++ CMR Disclaimer================================================== This document could contain information all or part of which is or may be copyrighted in a number of countries. Therefore, commercial copying and/or further dissemination of this text is expressly prohibited without obtaining the permission of the copyright owner(s) except in the United States and other countries for certain personal and educational uses as prescribed by the "fair copy" provisions of that countries Copyright Statues. ================================================================ ************** END Msg. B.ENG **************