Do you want learn how to apply concepts of sustainable faecal sludge management (FSM) on a city-wide scale? This course starts with an overview of what faecal sludge is and an introduces you to the engineering fundamentals and required information for the design and selection of technologies. Sanitation solutions are prone to failure if an integrated planning approach that includes stakeholder involvement and the development of appropriate institutional, management and financial arrangements is not implemented. The course therefore dedicates a complete week to presenting the full picture, in addition to technology, that needs to be considered for sustainable solutions. It concludes with a focus on current research and innovations in technologies, to provide an understanding of the most up-to-date options. This course is one of four in the series “Sanitation, Water and Solid Waste for Development".
3.4 Planted Drying Beds
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From the course by École Polytechnique Fédérale de Lausanne
Introduction to Faecal Sludge Management
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From the lesson
Treatment technologies for faecal sludge
The third week of this course focuses on engineering aspects of how to size and properly operate treatment technologies.
Meet the Instructors
Dr. Linda StrandeGroup leader
Management of Excreta, Wastewater and Sludge (MEWS), Eawag-Sandec
If you have ever taken care of plants you know that they need to be
watered so that they don't wilt. That's because plants take water up
into their cells and this water pressure helps keep them upright.
But have you ever seen on a really hot day, plants wilting
even though they appear to have enough water?
That's because a plant's respiration includes transpiration
of moisture out of their leaves. On a hot day this transpiration
can occur faster than plants can take up more water
causing them to wilt even when they do have access to water.
Following this module you will be able to explain
treatments processes and operation of planted drying beds,
discuss operation and maintenance requirements
and name important design parameters.
Over the last 20 years Sandec has collaborated with researchers
in Thailand, Cameroon and Senegal to transfer the knowledge of
operating planted drying beds in Europe with waste water and
bio-solid sludge to the treatment of faecal sludge.
The technology has now been successfully proven
but there is still not enough full scale operating experience
to provide precise design and operational parameters
for different contexts and so implementation of this technology
still requires special care and attention.
Planted drying beds are similar to unplanted drying beds.
They consist of a gravel and sand filter bed.
Sludge is loaded onto the top and the leachate percolates
through the bed and is drained away in a perforated pipe.
The differences are in planted drying beds
the filter bed is used for growing plants.
This means that in addition to evaporation
de-watering is occurring through plant transpiration.
Both of these together are called evapotranspiration.
The plant roots also function to help keep the bed from clogging
and provide a more complex environment for the growth of bacteria
within the bed. Planted drying beds are also
operated in continuous mode; not as a batch
so sludge is continually loaded onto the beds
but only removed after a period of years.
One of the most important considerations for the
design and operation of planted drying beds are the plants.
They need to be able to grow in relatively hostile faecal sludge
conditions and be tolerant to fluctuating water levels and salinity.
They need be fast growing, have high transpiration rates,
deep-growing rhizomes and roots and be non-invasive.
In Europe, reeds or Phragmites and cattails or Typha
are the most commonly used plants.
In Africa, Asia and Latin America, indigenous plants are being explored
for adaption such as Antelope Grass or Echinochloa
and papyrus or Cyperus.
An example is this paper identifying 3 new indigenous species in Senegal
that are good candidates for use in planted drying beds.
Echinochloa crus-galli, Paspalidium geminatum
and Pasplaum veginatum.
This is a drying bed following harvesting of plants
also showing the plants starting to grow back.
Plants are harvested by cutting them at the surface of the bed
not by pulling them out which could damage the bed
and prevent regrowth. Plants are harvested when sludge is removed
but can also be harvested more frequently for maintenance or
to sell as fodder. For example,
this paper found that in Cameroon, sale of the plants as fodder
could offset 7% of the annual operating costs.
Another important design and operating parameter
is the aclimazation phase required by plants
prior to growing in full-strength faecal sludge.
The start-up phase takes on average around 6 months
and in arid climates it should start during the rainy season
for best growth. Examples of how to do this
are going gradually from a loading rate of 50 to 200kg total solids
per meter square per year with a feeding frequency of
of at least twice a week. Another way would be
starting out with dilute waste water or for example
effluent from a settling thickening tank
and gradually moving to full-strength faecal sludge.
During this period, visual indicators of plant stress
such as yellowish color or slow growth rates
should be carefully looked out for.
Ponding to retain liquid in the bed can also be used
to prevent wilting in tropical climates
and potentially increase treatment performance
but the bed should then be left to fully drain before reloading.
This is sludge on a bed without loading for 1 week.
Sludge loading rates can be based on hydraulic loadings
or the amount of total solids. Loading frequencies
are also going to depend on the types of plants,
climate and treatment goals.
For example, this paper found that increased loading frequency
can increase plant growth, liquid lost to evapotranspiration,
and the nitrification of leachate.
If you base loading rates on total solids,
to determine the required surface area
you need to know the total volume of sludge
that will be treated on an annual basis.
The average total solids concentration of the sludge;
then we just convert the units here
divided by the loading rate that you're going to use
and this tells you the required surface area.
Desludging frequencies are going to depend on factors
such as loading rate and climate.
Experimental data from Cameroon
suggests that a loading rate of 200kg total solid
per square meter per year had 50 to 70cm per year
sludge accumulation. That means
if you had a free board of 2m, this could result in
3 to 4 years for desludging.
However other studies have had lower accumulation rates
and desludging intervals can even be up to 10 years long.
This is a hole looking down into the sludge layer.
After the last loading, sludge should be then left for a few months
for increased moisture and pathogen reduction.
These long sludge retention times
also mean that following removal, the sludge is much more
stabilized than with the short retention times of
unplanted drying beds.
This table from the Fecal Sludge Management book
summarizes observed treatment performances
with loading rates. Percent removals
meaning what percent stays with the sludge layer versus the leachate
are quite high but what is important to remember with
percent removals is that starting with very high concentrations
that are in faecal sludge, you still end up with
concentrations in the leachate that are higher than
discharged standards. Ways to treat dewatering liquid
from sludge are discussed in Co-treatment
and Leachate Treatment modules.
In summary, important design and operating parameters
are loading, feeding frequency, resting periods,
plant density, plant acclimatization, and plant harvesting.
Conclusions from Sandec's collaborative research
on adapting planted drying beds for the treatment of
faecal sludge include, planted drying beds are viable
for faecal sludge treatment; they're resistant to highly variable
hydraulic loadings; higher loading rates
can be used in tropical climates; plants need to be selected
for the local context; fodder production can offset
some of the operation costs; sludge can be stabilized on the beds
however it needs further treatment for safe use in agriculture.
Desludging periods can be up to 5-10 years long.
Pre-treatment with settling tanks results in better de-watering
on planted drying beds. The leachate requires further
treatment and research as it remains one of the biggest challenges.
In this module we learned about treatment processes and
operation of planted drying beds,
operation and maintenance requirements
and design parameters.
Thanks for joining and see you next time.
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