operation oaxaca: so much sludge.

how the sludge treatment process works (020)

Carlos Manuel Jarquín Sánchez
7 min readFeb 25, 2024

this is carlos.

the usual is going on:

preparing for lab entry.

yet, i talked with the department head of engineering.

and he brought up how economics always comes first.

hell yes, always.

that’s how i decided the sludge treatment process contains all the valuable plus toxic metals.

go there and remove them.

but first, i need to know the treatment process like the palm of my hand.

so what you waitin’ for?

nose-dive in.


the sludge treatment process.

we first need to divide the treatment process of any wastewater in general.

they consist of:

  • preliminary treatment
  • primary treatment
  • secondary treatment

and during each phase, a different type of sludge is made.

basically, the sludge may have unique properties before and after a specific treatment phase.

preliminary treatment focuses on removing grit, dirt, sand, sticks, etc.

it’s done via wastewater screening;

it removes rags, plastic bags, and large sticks.

grit removal is for large solids, sand, residue, etc.

all preliminary treatment does is prepare the water for the physical/thermal/chemical treatment.

primary treatment focuses on removing smaller solids that settle to the bottom of the water. it can be done via gravity sedimentation to remove:

  • suspended solids in water
  • biodegradable organic material (BOD) in wastewater
  • solids that settle to the bottom of the water

gravity sedimentation → the process of forcing suspended solids in water to sink to the bottom, especially if the solids/particles weigh more than water.

suspended solid → a particle/material that is floating in water but has not sunk to the bottom yet.

and this time, primary treatment reduces the concentration of nutrients, pathogenic organisms, trace elements, and toxic organic compounds.

as for the sludge,

the materials (or constituents) that are removed are contained in primary sludge.

primary sludge is what’s removed from the water in primary wastewater treatment.

next comes the secondary wastewater treatment.

it’s almost always done by using a biological treatment process.

more specifically, by using microorganisms.

this is where we need ‘activated sludge’.

oxygen is pumped into an aeration tank to create a suitable environment for aerobic microorganisms. these microorganisms create a floc. the wastewater flows along with the floc until it meets the microorganisms. they break down the floc material (aka biodegradation). the floc will break down into substances like water, oxygen, and biomass.

the material then settles into a new tank, where the heavy sludge and floc are removed from the filtration system. but smaller chunks remain to continue filtering incoming water, along with the microorganisms. all we have left is the odorless, colorless water. it’s ready for another round of processing.

floc a thick, floating clump of “flocculent material” interacting with wastewater.

flocculent material → a small mass formed by gathering fine suspended particles in a liquid.

then the activated sludge waste and the filtered wastewater are separated in a settling tank.

that activated sludge waste is known as secondary sludge.

then comes tertiary treatment.

this time, we focus on pathogenic microorganisms and viruses, organic compound heavy metals, BOD, nitrogen, and phosphorus.

for heavy metal ions, specifically…

it is typically done via adsorption using activated carbon.

(look into why activated carbon, including why it’s used, costs, and chemicals).

but the residues from tertiary treatment typically become incorporated with sludges from primary and secondary treatment.

so that’s the basics of each filtration step.

but now, onto the sludge treatment process (where the heavy metals lay).

some materials are also removed from wastewater and incorporated into primary and secondary sludges.

and those materials can contain large particles, with a lot of surface exposed to air via the pores.

and that’s our surface area.

but sludge treatment processes are used to reduce sludge volume and alter sludge properties before disposal or application use.

and this is how volume reduction works.

secondary sludge contains 99 kg of water for every kilo of activated sludge.

primary and secondary sludge combined contains about 3% solids by weight. (the rest is water lol)

so to remove the water, we need other methods to extract the water from the waste.

the methods are (for sludge management):

  • sludge thickening
  • dewatering
  • conditioning
  • drying

sludge thickening creates a dense material that still behaves like a liquid.

sludge dewatering creates a solid-like substance from sludge, even though it mostly has water.

the solids content of mechanically dewatered sludge typically ranges from 20 to 45 percent solids by weight.

sludge conditioning doesn’t directly decrease water content but changes sludge solids to help remove water during dewatering.

but here’s why we need sludge conditioning:

the mechanical dewatering techniques discussed in the previous paragraph would not be economical without prior sludge conditioning. (prove this later).

thermal drying is used if water removal is needed beyond the dewatering processes’ capacity.

but now, this is what happens in the treatment plants of the municipality.

this is what’s going on in the background.

now, we will turn our attention to the wastewater treatment plants occurring at the mining sites.

onto the industrial treatment plants.

the mining boss.

POTW → public-owned treatment works, used for domestic sewage treatment and owned by the government… (NOT industrial wastewater)

in wastewater treatment, heavy metal partitioning in secondary wastewater treatment plants,

including both surveys of:

  • operating POTWs
  • more controlled pilot-plant studies

and there were seven heavy metals in the wastewater treatment plants (that researchers focused on):

  • cadmium
  • chromium
  • copper
  • lead
  • mercury
  • nickel
  • zinc

these heavy metals are partitioned onto sludges both in primary wastewater treatment such as sedimentation,

and in biological secondary treatment processes such as activated sludge.

5% — 50% of these metals were found to have been removed from wastewater and concentrated into primary sludges.

15% — 80% of these metals were found to have been removed from wastewater and concentrated into secondary biological sludges.

the removal of heavy metals from wastewater into secondary sludge is reported to be (in declining order): mercury, copper, lead, chromium, cadmium, zinc, and nickel.

so yes, it’s true, the heavy metal ions are concentrated in sludge.

so how do we treat the sludge before sending it to the landfill or for land applications?

it starts with pre-treatment of sludge.

POTWs in chicago, illinois and buffalo, new york reported that industrial pre-treatment reduced toxic heavy metal concentrations by 50% — 90%.

the objectives of most municipal sludge treatment processes are to:

  • reduce the water content of sludges
  • to avoid complications from the decomposition of the biologically degradable fraction of sludges
  • to reduce the levels of pathogenic organisms in sludges


economically viable technology for selective removal of trace elements and toxic organic compounds from sludges does not exist.

amounts of these constituents in municipal sludges can currently be controlled only by:

regulating the quality of wastewater entering municipal wastewater collection systems.

the systems.

the activated sludge system removes the metals in two stages.

  1. by primary settling of insoluble metals or metals absorbed by particulates.
  2. by adsorption of dissolved metals or fine particulate metals to the biological floc in the aeration tanks.
the common method for the treatment & disposal of sewage sludge (ss) in united states.

the metals in industrial wastewater are in the dissolved form (mostly),

but when they react with municipal sewage some of the metal is converted into an insoluble state.

this is true for some metals like iron & lead.

but not for manganese and zinc… they do not react with the sewage and enter the wastewater treatment plant in an insoluble form.

so, the removal efficiency of a given metal seems to be related to the dissolved insoluble metal ratio in the raw sewage.

for most metals, the proportion of dissolved to total metal increases as they pass through the system.

this is known as an equilibrium between the insoluble and dissolved states of the metal.


an equilibrium needs a proportionate removal of the dissolved metal.

and it does work for copper, manganese, zinc, nickel…all of the metal in the final effluent is in the dissolved form.

by recommendations, any tertiary/additional treatment process added to the system must be directed at removing dissolved metals from the sludge.

these are the recommendations if i were to use ion-exchange or adsorption techniques:

“if physical methods such as ion exchange, (activated carbon) adsorption, or electrolysis are to be used;

they would best be employed after the primary clarifiers to minimize equipment clogging.

primary clarifiers → (primary wastewater treatment)

this would again inhibit the metals from reaching aerobic microorganisms (i.e. aerobic fermentation, secondary wastewater treatment).”

because all heavy metals metal are concentrated in the sludge.

so some method should be developed to reclaim the metals from the sludge before it is recirculated into the environment by land disposal.

primary clarifiers are part of the primary treatment stage in wastewater treatment plants. they’re used to separate settleable solids from the raw incoming wastewater.

secondary clarifiers are located near the biological treatment facility, typically near aeration basins or filters. they focus on clarification and thickening processes.

this is what makes me question if we need a third round of heavy metal ion removal in the sludge purification process…

before the sludge is ready for landfill, fertilizer, or another application.

© 2024–2100 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.