0:13
Now we will look at the aerobic technologies,
which can be used as stand-alone systems
or to polish the effluent of anaerobic units.
Aerobic systems are biological systems which rely on the action of bacteria
needing oxygen to live, like us.
0:52
Some of these are more engineered,
such as the "trickling filter" or the "activated sludge".
Let's start with the waste deposition ponds, or lagoons.
It is a system that consists in a succession of ponds
with different functions.
First one anaerobic pond, then one or two facultative ponds,
and finally, one or two maturation ponds.
1:17
Let's look in more detail at the differences
between these three types of ponds.
The anaerobic pond is the deepest, with a depth of two to five meters,
and also the smallest.
It is highly loaded because it receives the rawest water.
The anaerobic pond has a hydraulic retention time from one to three days,
and its main function is sedimentation
and anaerobic stabilization of sludge, thus, settling.
1:44
The facultative pond or facultative ponds
if they are several, are shallower.
Usually, less than 1.5 meters,
but large.
The idea here is to maximize the oxygen supply
either through algae, wind, or artificial aeration.
The hydraulic retention time there
is longer: from ten to twenty days.
2:07
Here, the main function is the aerobic degradation
of suspended and dissolved matter,
thus, degradation.
Finally, the maturation ponds are even shallower,
usually less than one meter, but large.
2:35
Waste stabilization ponds can treat high strength wastewater
to a high quality effluent.
They are generally reliable and good functioning
and inexpensive, compared to either centralized options.
However, they require a lot of space and may generate bad odors,
especially the anaerobic pond, if poorly designed.
It requires expert design and supervision,
especially to avoid short-circuiting,
in which case, the effluent
goes from the inlet to the outlet without mixing properly,
that is, not staying through the intended retention time.
Besides, if they work at best in warm climates,
waste stabilization ponds are not always appropriate
for colder climates.
This picture shows large waste stabilization ponds
for the city of Cuenca, in Ecuador.
A way to increase the performance of waste stabilization ponds
is to provide artificial aeration,
which is done in Cuenca in the first pond,
upstream of this picture.
It is also a good way to upgrade the ponds
when then reach design capacity,
especially extension, is not always easy.
In aerated ponds, mechanical aerators provide oxygen
and keep the aerobic organisms suspended
to achieve a high rate of organic degradation.
4:05
The depth may be from two to five meters
and a retention time from three to twenty days.
In some cases, it can even replace the anaerobic pond,
thus avoiding the bad odors that anaerobic ponds may generate.
4:29
Interruption of electricity service may cause the pond to turn anaerobic,
thus changing completely the inner biological processes.
We will now have a look at another family
of treatment technologies: the "constructed wetlands".
4:51
We distinguish three types:
the "free water surface constructed wetland",
the "horizontal subsurface flow constructed wetland",
and the "vertical flow constructed wetland".
Let's start with the "free water surface constructed wetland".
5:07
In that case, the water slowly flows through the wetland,
particles settle, and pathogens are eliminated
through the combined action of sun, settling, adsorption,
and predation from higher organisms.
The organisms and plants utilize the nutrients.
5:53
Good operation and maintenance is needed
for it not to turn into a mosquito breeding area,
and to avoid short-circuiting.
Let's look now at the "horizontal subsurface flow constructed wetland".
6:06
Its performance regarding reduction of BOD,
suspended solids, and pathogens
is higher than in the freewater surface constructed wetlands,
and it does not have the mosquito problem
as the water flows only under the surface,
6:51
It acts both as a filter and as a fixed surface
upon which bacteria can attach, forming large biofilms.
The plant roots play an important role
in maintaining permeability in the filter.
It is very important to have a good primary treatment
before the wetland.
Otherwise, the risk of clogging will be very high.
7:34
This picture illustrates the last type of constructed wetland,
the "vertical flow constructed wetland",
which is, of course, difficult to differentiate from outside.
This is the most sophisticated and performance type of wetland
By injecting the wastewater from above the whole surface,
the distribution is greatly improved.
8:11
The wastewater is applied intermittently,
four to ten times a day.
Thus, the filter goes through stages of being saturated and unsaturated,
and accordingly, different phases of aerobic and anaerobic conditions.
With this regime, there is less plugging risk
than in horizontal subsurface flow wetland.
Other than that,
the treatment processes are the same as in the horizontal flow wetland.
8:51
Constructed wetlands can reach a good performance
with processes close to nature.
However, one has to be aware of the risk of clogging
and the management of the plants,
especially the startup and maintenance,
and beware high ammonia levels
which may prevent the plants from growing properly.
In all cases, a good primary treatment is crucial.
9:16
Before finishing this module,
we will look at two higher end technologies
which we may call <i>Conventional</i>:
the "trickling filter", and the "activated sludge".
9:26
These systems are really performant in developed countries,
but imply high capital costs,
operation and maintenance by skilled personnel,
and a constant source of energy.
9:57
Organics are degraded by the biofilm covering the filter material.
The filter is usually from one to 2.5 meter high,
but some filters may reach a height up to 12 meters.
Adequate airflow is important to ensure sufficient treatment performance
and prevent odors.
The air can circulate vertically through the filter.
10:31
The "activated sludge system"
is the best known and most widespread in Western countries
under different forms, such as sequencing batch reactor,
oxidation ditches, moving beds, or membrane bioreactors.
It makes use of highly concentrated micro-organisms
to degrade organics and remove nutrients,
leading to a high quality effluent.
To maintain aerobic conditions
and to keep the activated sludge suspended,
a continuous and well-timed supply of oxygen is required.
Aeration and mixing can be provided by pumping air or oxygen into the tank
or by surface aerators.
Agglomerations of sludge particles called <i>flux