Waste stabilization ponds constitute a simple and robust wastewater treatment process with low operation and maintenance costs and demands. They have the additional advantage that, where treatment includes maturation ponds, they offer greater efficiency in removing both bacteria and parasitic worm eggs than can be expected from almost all other waste treatment processes.
Ponds are classified as “anaerobic” (meaning that the reactions in the ponds take place without oxygen), “facultative” (in which these processes may or may not use oxygen), and “maturation” (in which the pond provides additional treatment in the presence of oxygen and sunlight, to further reduce pollutants before discharge.) These are described in detail below. Note that all these systems could be combined as a series of ponds, e.g. anaerobic ponds are often followed by facultative and/or maturation pond(s).
Facultative and maturation ponds
Constructed wetlands (reedbeds)
All systems are influenced by natural factors, such as wind action, temperature, rainfall, solar radiation and seepage, as well as by such physical factors as surface area, water depth, short-circuiting, pH, toxic materials and oxygen. Site-specific problems could include high water table, flooding, steep topography and undesirable vector breeding such as mosquitoes.
1. Anaerobic Ponds
What is it? Anaerobic waste stabilization ponds are open basins in which wastewater is treated in the absence of oxygen. Solids settle to the bottom of the pond, where they are digested.
When to use it? Use anaerobic ponds as a first stage to treat wastewater prior to secondary treatment in facultative and maturation ponds, or to reduce land use requirements of other systems (e.g. constructed wetlands). Because they are open, anaerobic ponds will give off strong odors and they should therefore not be located close to housing. Some authorities suggest a minimum distance of 1000 meters from the nearest house but a separation of 500 meters and even less may be appropriate.
Advantages:Anaerobic waste stabilization ponds are simple and economical to build and operate. They are relatively inexpensive and can be managed relatively easily by small and medium-sized communities.
Disadvantages: Anaerobic ponds take time to reach full treatment efficiency. These systems require large amounts of land compared to “conventional” activated sludge or other secondary treatment technologies. The restrictions on location near to houses and their size means that they may be more appropriate for use in larger schemes.
Technical features and requirements: Ponds are normally rectangular basins, ideally with a length to width ratio of about 3 to 1. The depth should be at least 3 meters and ideally 4 meters. The sides are normally sloped at 1:2 internally and 1:3 externally, with the inner slope lined with concrete or bricks. Basins must be desludged periodically, and design and management should reflect this. For example, designs should include provision of a ramp, down which a vehicle or animal-drawn cart can be backed once the pond has been emptied of wastewater.
Another option is to provide a float-mounted sludge pump that can periodically pump sludge from the pond into a drying bed. A well-designed pond may achieve up to about 60% BOD/COD removal in warm conditions. Hydraulic retention time should not exceed 2 days and may be one day for temperatures higher than 20 C and BOD of up to 300 mg/l.
2. Facultative and Maturation Ponds
What are they? Facultative ponds are large, shallow basins that hold sewage and allow treatment by a combination of aerobic and anaerobic processes. Treatment takes place through both physical and biological processes, which can be quite complex. Maturation ponds are smaller ponds placed in series after facultative ponds, and can reduce pathogen concentrations to safe levels.
When to use them? Consider using waste stabilization ponds when land is available, there is a need to achieve a good reduction in pathogen levels, and/or there is a probability that the sewage inflow will occasionally include large quantities of stormwater run-off. Land needs can be reduced by about a third through provision of anaerobic ponds ahead of the facultative ponds.
Advantages: Waste stabilization ponds are simple, robust and can deal with fluctuations in wastewater flows. With a retention time of at least 22 days it is the only treatment system considered by WHO to achieve the effluent standard required for unrestricted irrigation
Disadvantages: The major disadvantage of waste stabilization ponds is their large land requirement. This varies depending on sewage strength and temperature but is likely to be in the range 3 – 5m2 per person. Their large land requirement means that they are unlikely to be a viable option where land is either expensive or in short supply.
Technical features and requirements: Facultative ponds are normally between 1.5 and 1.8 meters deep. Most are rectangular in shape although this is not essential, provided that the ratio of length to breadth is greater than about 2:1. The sides normally slope and the base and sides of ponds do not have to be lined, except for about half a meter above and below the pond water level to prevent erosion due to wave action. The inlet and outlet arrangements can be simple although the outlet should incorporate a baffle at top water level to prevent escape of floating solids.
3. Constructed Wetlands (reed beds)
What are they?Constructed wetlands are engineered wetland systems which can treat a variety of waste effluents: domestic wastewater, agricultural runoff, stromwater and even industrial effluents. Treatment takes place through a variety of complex natural chemical, physical and biological processes, including sedimentation, precipitation, adsorption, assimilation from the plants and microbiological activity.
The system utilizes aquatic plants, such as Phragmites reeds, bulrushes and cattails. A cost-effective system is designed to work under gravity, thus minimizing any need for pumps or other electrical devices. The flow may be either horizontal or vertical and, in the case of horizontal flow wetlands, may be either above or below the surface. Most constructed wetlands in developing countries are of the horizontal sub-surface flow type. Above-surface designs are generally avoided because they create breeding sites for mosquitoes.
When to use them?Consider the use of constructed wetlands when there is a need for a better quality effluent than can be achieved by purely anaerobic treatment. Constructed wetlands need land, typically 3 -5 m2 per person if the constructed wetland is to treat full-strength sewage. For this reason, it is often better to use them after primary anaerobic treatment, thus lowering the land requirement.
Advantages:Constructed wetlands are a relatively simple technology, with limited maintenance needs. They can achieve good BOD/COD reduction and can also remove pathogens fairly efficiently. Appropriately designed constructed wetlands are tolerant to fluctuations in both hydraulic and contaminant load, which make them an ideal solution where variable storm water quantity or wastewater quality may be a problem. The system can also provide a variety of indirect benefits, such as wildlife habitat, areas for educational or even recreational purposes. The wetland areas can also be used for growing crops with direct economic value, such as biomass for energy or agricultural purposes.
Disadvantages:The major disadvantage of constructed wetlands is their relatively large land requirement. They are also more complicated than waste stabilization ponds and have greater management requirements. They may also take a year or two to achieve the optimum treatment efficiency.
Technical features and requirements: Horizontal flow beds are normally built using a gravel medium, into which the reeds are planted. Wastewater must be pre-treated to remove gross solids and then flows across the reedbed by gravity. To prevent percolation of wastewater into the ground, the bottom of the filter should be sealed. The gravel should normally be round and uniform, preferably with diameters in the range 8 – 16mm. Filters do become clogged over time and can be partially unclogged by resting them for several months. Such a “resting” requirement implies the need for additional land area.