4.2.3 Lagoons
Ponding or lagooning is effective in treating wastewater and can reduce BOD
and SS to the same levels as mechanical treatment plants (e.g. Activated Sludge
Treatment). In addition because of the longer residence time of wastewater in
the lagoon (days), removal of pathogenic bacteria and viruses by natural die-off
is greater than in an activated sludge treatment plant (residence time usually
several hours). Cysts of parasites and helminth eggs are also usually removed
through sedimentation in the lagoons.
A lagoon is a shallow excavation in the ground (1 to 2 m deep). It is
generally unlined percolation of wastewater into the soil and groundwater takes
place. With time the percolation rate will reduce, because of formation of a
sediment layer. Evaporation loss of water can be significant in arid climate
regions. The soil itself is, however, not involved in the physical and
biochemical wastewater treatment processes taking place in the lagoon. A lagoon
can therefore be lined with a layer of clay or with an impermeable plastic
membrane if protection of groundwater is desired, without affecting the
performance of the lagoon. Wastewater lagoons are also called 'waste
stabilisation lagoons', because the organic substances in the wastewater are
converted to more stable (less degradable) forms.
The following processes take place in a lagoon. As wastewater enters a
lagoon, sedimentation of solids occurs. Because of the long residence time of
the wastewater in the lagoon system, much of the solids in the original
wastewater are removed. Aeration of the water from the atmosphere occurs by a
process of diffusion aided by turbulence caused by wind movement on the surface
of the water. This process is the same as the natural process of aeration of a
lake.
Oxygen is also supplied by algae in the lagoon which thrive on the nutrients
(nitrogen and phosphorus) released by the decomposition of the organic wastes.
The photosynthetic activity of algae, however, only takes place when there is
sunlight. Thus oxygen produced by photosynthesis is only available during this
period. A symbiotic relationship exists between the bacteria and the algae.
Bacteria take up oxygen and release carbon dioxide, while algae take up carbon
dioxide released by the bacteria and produce oxygen for the bacteria (Figure
18).
Depending on the oxygen demand of the bacteria in the lagoon, the following
conditions occur:
| Anaerobic lagoon |
The oxygen demand of the bacteria exceeds
oxygen supply by surface aeration and algal photosynthesis. Biodegradation of
the organic wastes is by anaerobic bacteria. Methane gas is a by-product.
Odorous gases are produced, but impact is reduced when a layer of scum forms at
the water surface. |
| Facultative lagoon |
The oxygen demand of the bacteria is met by
surface aeration and algal photosynthesis, but is not met when the latter is not
active. The water environment is aerobic during the day, but turns anaerobic at
night. Biodegradation of organic wastes is by facultative bacteria, which can
operate under both aerobic and anaerobic conditions. |
| Aerobic lagoon |
The oxygen demand of the bacteria is met by
surface aeration and algal photosynthesis. |
It is common to have a series of lagoons with the first one or two being
anaerobic lagoons, the middle ones facultative lagoons and the last few aerobic
lagoons. The sediment at the bottom of lagoons is anaerobic, and undergoes
anaerobic bacterial decomposition. The first lagoon in a series will eventually
be filled with solids. The sludge produced can be removed and treated for re-use
or disposal (Section 6) or allowed to undergo further biodegradation in the
lagoon prior to re-use. Anaerobic lagoons can be made deeper so that more sludge
can be accommodated and the need to remove sludge made less frequent.
 Lagoon performance is
affected by temperature. At a higher ambient temperature (e.g. in the tropics) a
shorter residence time of wastewater in the lagoon is required to achieve the
same level of treatment compared to when the temperature is lower. Because algae
are present in treatment lagoons, they leave with the treated effluent. One way
of harvesting the algae is through aquaculture (see Section 6).
Oxygen transfer from the atmosphere into lagoons can be increased by
mechanically agitating the surface of the water. This can be done by using a
vertically mounted impeller, and the lagoon becomes more like the aeration tank
of an activated sludge process. The agitation can also be provided using a
horizontally mounted rotor. A configuration that can be used to apply this is a
circular ditch (Figure 19), and the water is continuously circulated around the
ditch so that its movement is like that in a river.
4.2.4 Land based treatment
Land based treatment of wastewater relies on the action of soil bacteria to
degrade the organic wastes in the wastewater. In what is termed 'Soil Aquifer
Treatment' wastewater is applied to unlined basins in cycles of flooding and
drying of approximately one week each (Figure 20). During flooding wastewater
percolates through the soil beneath the basin to the unconfined groundwater
aquifer. Soil bacteria consume organic substances, suspended solids are trapped
at the bottom of the basin, and the percolation rate decreases. During drying
the layer of solids accumulating at the bottom of the basin are degraded by
bacteria and also undergo drying. The percolation capacity for wastewater is
therefore rejuvenated.
Soil aquifer treatment is also known as rapid-rate land application. It works
well when the soil permeability is high (> 1 m/day), and the highest groundwater
table is at least 2 m below the bottom of the basin. Upon reaching the
groundwater the SS and BOD of the water is generally low. Furthermore if the
soil beneath the basin contains clay minerals, pollutants like heavy metals may
be adsorbed by the clay minerals. The groundwater aquifer acts as storage for
the treated wastewater, which is usually withdrawn for reuse.
In what is termed 'slow-rate land application system' wastewater is applied
to land through channels in the upper part of the gradient and treated
wastewater is collected in channels in the lower part of the gradient of a
slightly inclined ground (Figure 21). The application is intermittent and its
rate is dependent on the permeability of the soil and the loss of water due to
evaporation. The organic substances in the wastewater are biodegraded by soil
bacteria at the surface of the soil and during percolation through the soil.
Vegetation is usually part of the treatment process. It takes up nutrients
(nitrogen and phosphorus) released from the degradation of the organic
substances. The vegetation (usually grasses) is harvested by grazing animals
(cattle or sheep). In New Zealand, treated wastewater is successfully disposed
by spray irrigated into forests and crops. The trees and crops take up the
disposed nutrients and use then to promote growth. This is mainly for disposal
purposes and not for re-use. Crops (usually grass) are harvested as silage and
then fed to live stock. This disposal system is referred to as “cut and carry”
as the livestock do not graze the irrigated paddocks. The silage is of good
quality and there is a demand for it. Sub-surface irrigation disposal of
wastewater for silage is also being promoted.
When the soil is saturated with water (e.g. during the rainy season),
'overland flow' or 'grass filtration' mode of operation is used. In this case
wastewater flows over the soil surface and bacteria attached to the vegetation
and soil surface remove the organic substances (Figure 22).
Raw wastewater can be used in any of the above land based treatment system
provided that the application rate is small. Settled wastewater needs to be used
for higher rates of application. Land application treatment systems work well in
arid or semi-arid regions, where the soil is generally not saturated with water
over much of the year, and reuse of wastewater for agriculture is attractive.
Particular attention has to be given to public health requirements.
4.2.5 Constructed wetlands
Constructed wetlands lie in-between lagoons (4.2.3) and land based treatment
systems (4.2.4). They are based on natural wetlands, which act as a water filter
and purifier. A constructed wetland consists of a gravel bed in which wetland
species, such as reeds, are planted (Figure 23). Wastewater, usually after the
settling of solids, passes through the gravel bed. The flow of wastewater can
either be surface or sub-surface. The bacteria that are attached to the surfaces
of the bed and plant roots degrade organic substances. The reed beds remove N
and P as or more effectively than conventional wastewater treatment plants. They
are suitable for treating domestic sewage as well as other forms of wastewater
such as contaminated groundwater and agricultural and animal waste. Wetland
plants take up nutrients (nitrogen and phosphorus) when water residence time is
long. Long-term nutrient removal requires harvesting of the plants. Constructed
wetlands need to be designed to minimise problems with insects (mosquitoes and
midges).
Constructed wetlands are particularly of interest in low-income areas as they
are simple to construct, operate and maintain, usually by trained local people.
This keeps the both the capital and operating costs low.
4.2.6 Anaerobic treatment of wastewater
 Anaerobic treatment is more
suited to wastewater high in BOD. It is used to treat the sludge from an
activated sludge treatment or biological filtration process (see Section 5). In
households where there is cottage industry (such as food processing to supply
restaurants or food market) the wastewater may be high in BOD. Wastewater high
in BOD may also be generated when water conservation measures result in less
water being used. A simple method to treat blackwater and kitchen waste is shown
in Figure 24. The biogas produced can be combusted for use in cooking.
In the Upflow Anaerobic Sludge Blanket (UASB) process settled wastewater is
passed upward through a sludge blanket. The sludge blanket consists of anaerobic
bacteria, which have developed into granules. Because of the high settling
velocity of the granules, the granules are not carried over in the upflowing
wastewater. A high concentration of bacteria is therefore retained in the tank.
The tank itself has no internal moving parts (Figure 25). If wastewater is
distributed evenly at the base of the tank, mixing between the wastewater and
the granules of bacteria is promoted by the carbon dioxide and methane gases
produced by the anaerobic treatment process and the upward moving flow of the
wastewater.
 Although the reactor itself
has a simple configuration with no moving parts, pumping of the feed is still
required. Methane gas is produced which needs special handling procedures to
prevent leakage and explosion. Wastewater treated anaerobically requires further
aerobic treatment to reduce its BOD and odour. Excess granules need to be
treated prior to reuse or disposal, although currently there is a demand for the
granules to start up UASB reactors. The mixture of methane and carbon dioxide
(termed 'biogas') can be combusted and used for heating the content of the
anaerobic reactor or for other purposes.
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