Treatment processes occur by different mechanisms.

Some treatment relies on physical forces, such as filtration,

evaporation, and gravity.

Some treatment relies on the metabolism of living organisms

such as microorganisms, invertebrate animals, and insects such as

these black soldier fly larvae. And other treatments rely on

chemical reactions, such as redox reactions and altering

chemical bonds.

Following this module you'll know the difference between physical,

chemical and biological treatment mechanisms. And be familiar with

mechanisms for solid liquid separation, stabilization,

nutrient management and pathogen reduction.

What are the most important treatment processes in fecal

sludge management is dewatering. Dewatering is based on physical

processes such as evaporation, evapotranspiration, filtration,

gravity, surface charge attractions, centrifugal force and pressure.

Gravity is probably the most commonly employed method

of liquid solid separation, for the separation of suspended particles

and unbound water.

Particles that are heavier than water set decreasing conditions

at rates based on size of particles, suspended solids concentrations,

and flocculation

These are the basic fundamentals used in the design of settling

thickening tanks and grit and sand chambers.

Filtration is also a commonly applied mechanism for dewatering.

Many different types of filtration media are used for water,

waste water and treated sludge or biosolids processing.

The most common for the treatment of fecal sludge are drying beds,

and planted drying beds. Drying beds use sand and gravel filter media to

to trap solids on the surface of the filter bed while the liquid

percolates through the filter bed and is collected in a drain or

evaporates.

Water that can be separated through gravity or filtration is free water

that is more easily separated from solids. This is analogous to water

that can be wrung out of a towel. If you need more liquid removed from

the sludge, then you also need drying. For example, by the sun

through evaporation on the surface of drying beds.

Evaporation occurs when water is released into the air as a vapor.

Transpiration occurs when plants release vapor to the air as part of

their metabolic processes. Evapotranspiration is a combination

of these two processes. Evaporation can also occur from other sources of

heat, such as thermal drying or even somewhat from other physical

processes, such as centrifugation.

Ok, so next on to biological mechanisms.

Biology is essential in a transformation of organic matter

and nutrients. Biology is also essential for understanding

pathogens and pathogenic activation.

Biological treatment harnesses the metabolism and growth rate of

microorganisms. And even invertebrates' in naturally occurring

processes. And employs them in controlled situations to optimize

the desired outcomes. Treatment systems usually rely on complex

populations of microorganisms. As the microbes grow, they are

dynamically altering the system by modifying forms of organic matter

and releasing and binding up nutrients. They also release gasses

and other byproducts that can affect the environment.

Here we see viruses, bacteria, protozoa and invertebrates.

The biodegradable organic matter in fecal sludge varies depending on

the source. But usually needs to be stabilized prior to final end use or

disposal. Stabilization involves the degradation of readily degradable

material, leaving behind more stable less degradable organics.

This is important in order to reduce the oxygen demand, produce stable

and predictable characteristics, reduce odors, and allow for easy

storage and manipulation.

Stabilized organic matter does not have an exact agreed upon scientific

definition. But in general, it refers to resistance to further

biodegradation. Stabilized sludge consists of particles like cellulose

lignin, inorganic matter, and the cellular material of microorganisms

that consume the readily degradable organics. Whereas, unstabilized

sludge contains easily degradable compounds, such as carbohydrates,

proteins, and sugars. Stabilization can be achieved through multiple

types of aerobic and anaerobic treatments. For example, composting.

Nutrient management can also be achieved thru biological treatment.

For example, through the mineralization of organic matter

as it is degraded releasing nutrients into the environment.

Or, through the immobilization of nutrients as they are taken up

during growth into new cellular material.

Further information on the role of biology and Nitrogen and Phosphorus

cycling can be found in the book Fecal Sludge Management.

Biology is also very important in managing pathogens.

As these disease causing organisms are also biological.

It's important to have an understanding of all of the

interrelated ways to reduce pathogens during treatment.

As they can also affect the performance of treatment technologies.

The main mechanisms of inactivation that we're concerned with are:

Predation from other organisms. Starvation as they run out of food.

Temperature, for example in treatment processes like composting

thermophilic anaerobic digestion or even Lime treatment.

UV in cases where light rays can penetrate, such as the surface of

waste stabilization ponds. But not in tanks and ponds where solids at

the surface are high enough to block significant transmission of rays.

pH, or other chemical changes, as most microorganisms thrive only

within a range of 2-3 pH units.

Time is important in processes that have long retention times.

Like planted drying beds of up to 10 years.

Desiccation or dehydration. For example, in processes like

pelletizing that can get to 90% dryness and could be combined with

time as storage or temperature as external heat to achieve safe levels

of pathogen reduction.

Pathogens can also be reduced through sorption. For example, when

helminth eggs sorbed solids, then settle out in settling thickening tanks.

or remain with the sludge layer on a drying bed.

OK, and finally chemical mechanisms. Chemicals can be mixed with fecal

sludge to improve the performance of other physical mechanisms.

Or to inactivate pathogens and stabilize fecal sludge.

Examples include addition of polymers or other conditioners to

increase the flocculation and settling efficiency

Ammonia or Lime to increase pathogene reduction.

Both types of chemical reactions are covered in more detail in other

modules.

So, in summary, in this module you learned about physical,

biological and chemical mechanisms that occur in treatment processes.

And about treatment mechanisms that are occurring during these

four treatment objectives for fecal sludge. Solid-liquid separation,

stabilization, nutrient management and pathogen reduction.

Thanks for joining! See you next time!

2.2 Overview of Treatment Mechanisms

Introduction to Faecal Sludge Management

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