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Wastewater Treatment Systems

As wastewater treatment is often costly (Table 1), the maximum allowable concentrations should not be set significantly lower than those that the ecosystem can tolerate without adverse impacts. The costs in Table 1 are valid for treatment of 2000m3 per day, i.e., municipal wastewater for about 10,000 inhabitants. Costs per 100m3 for smaller and larger communities will vary somewhat.

Waste Stabilization Ponds (WSPs)

Traditionally, waste stabilization ponds are built as flow-through systems with the following processes being utilized in the pond system: settling (mainly in the first ponds), anaerobic decomposition of organic matter (mainly in the first ponds), aerobic decomposition of organic matter (mainly in the last ponds, where algae are present and produce oxygen), uptake of phosphorus and nitrogen by algae (maturation ponds), evaporation of ammonia (mainly where pH is high, i.e., in the last ponds), settling of algae, and denitrification (in the anaerobic zones). High removal efficiencies of biological oxygen demand over five days (BOD5), of chemical oxygen demand (COD), of microorganisms, of nitrogen, and of phosphorus may be obtained provided that the guidelines for design and maintenance are followed.

Removal of phosphorus by WSPs can be in the range of 20 to 50% but depends on removal of algae from the effluent. Enhanced removal of phosphorus, which is often required for the discharge of wastewater to lakes and reservoirs, can be achieved by addition of precipitants such as calcium compounds and clay minerals, which often have local sources.

Constructed Wetlands

The transition zones between lakes and terrestrial ecosystems are crucial for protection of lakes against anthropogenic impacts. Transition zones prevent, to a certain extent, entry of undesirable substances into lakes, and, therefore, should be preserved. Hence, construction should not be permitted in a zone 50 to 100 m from shorelines.

Non-point or diffuse pollutants from the environment flows toward lakes, but the transition zone is able to transform or adsorb the pollutants. The most important processes occurring in the transition zone are as follows:

• Nitrate is denitrified by the anaerobic conditions.
• Clay minerals adsorb ammonium.
• Organic matter adsorbs phosphorus compounds
• Biodegradable organic matter is decomposed aerobically or anaerobically by microorganisms.
• Macrophytes store nutrients.
• Soils with a high calcium, magnesium, aluminum or iron content can adsorb phosphorus.
• Suspended matter is removed along with associated nitrogen and phosphorus.

The denitrification potential of wetlands is often high. As much as 2,000 to 3,000 kg of nitrogen in nitrate can be denitrified per hectare of wetlands per year. In addition, denitrification is accompanied by oxidation of organic matter. However, phosphorus, bound in organic matter or adsorbed to the organic matter, may be released.

The siting of artificial wetlands must be carefully planned because their effects are dependent on the hydrology and on the landscape pattern. Non-fertile land of moderate cost should be used. Constructed wetlands can be either surface or subsurface wetlands. Subsurface wetlands are based on flow of water through the soil, and oxygen is provided for biological decomposition via the root network. Subsurface wetlands are usually of higher efficiency than surface wetlands but require more maintenance to avoid clogging. When wetlands with open water are used, mosquitoes should be controlled, for instance, by stocking of insectivorous fish. Harvest of wetland plants will increase the efficiency of nitrogen and phosphorus removal. The harvested plants can be used to feed domestic animals, to produce methane or be composted.

The combination of WSPs and constructed or reclaimed wetlands are attractive for the following reasons:

• Wetlands offer a significant reduction of suspended matter from WSP pond effluents.
• Wetlands buffer the pH of the effluent from WSPs.
• Effluents from WSPs often need a polishing step as a post treatment. Wetlands offer a cost-moderate solution.

Mechanical-Biological Treatment Methods

Mechanical-biological treatment is widely used in industrialized countries and in developing countries where the cost of land is high (see the decision tree in Figure 1). Treatment costs of US$28-48/100m3 are about 2-3 times the cost of treatment based on the combination WSPs and constructed wetlands. However, where the cost of land is high and proper maintenance of the facilities is maintained, mechanical-biological treatment could be preferable.

Mechanical treatment combines the use of a sieve, a grid chamber (with a retention time about 20 minutes during which sand settles and grease is removed) and a sedimentation chamber where finer particles are removed during a retention of 2-6 hours. Biological treatment follows mechanical treatment and uses air to accelerate the decomposition of organic matter. Two alternative processes are an activated sludge treatment or a trickling filter. The trickling filter employs uptake of air from the atmosphere by recycling the water over a large surface, while activated sludge uses input of air from an aerator or compressor. A retention time of 2-6 hours is usually required to obtain 85-95% removal of BOD5. The biological step is followed by a secondary sedimentation where suspended matter resulting from the biological activity is removed. The sludge is partly recycled to ensure a high concentration of active microorganisms in the biological step. The sludge can be used to produce methane, and the stabilized sludge can be applied as soil conditioner, provided that it does not have too high a concentration of toxic substances (for further details see chapter 6 in IETC's Technical Information Series number 11).

A mechanical-biological treatment plant can be modified and expanded to include removal of phosphorus and nitrogen. By addition of a precipitant in the grid chamber, it is possible to achieve a 75-95% removal of phosphorus during the primary sedimentation. Since phosphorus will be present in the stabilized sludge, it improves the quality of the sludge as a soil conditioner. Nitrogen removal by nitrification and denitrification is possible by increasing the retention time considerably in the biological step and by switching between aerobic and anaerobic conditions. Nitrification requires an increased recycling of sludge, as the sludge age has to be increased to about ten days. These modifications lead to an increase in treatment costs by a factor of 2-3 (see Table 1).

Figure 1. Decision tree for the selection of wastewater treatment methods to be used for medium and large-scale facilities.

Figure 2. Decision tree for the selection of wastewater treatment methods

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