operation oaxaca: where we now?

the status quo of metal ion filtration (013)

Carlos Manuel Jarquín Sánchez
8 min readFeb 18, 2024

this is carlos.

we’ll be back with more chemistry.

but i live on earth.

and earth has more constraints than chemistry.

we have this thing called ‘economics’.

economics says this:

if it costs too much for the people u wanna help… bad move.

plus, use what you got.

mother nature is your best mentor.

learn & use from it.

also, i’m not the first one who wants to filter water.

but tech is moving quickly…

and i wonder if the metal ion filtration industry is ahead of the curve,

or is it?

let’s see whaddup.


pass the woota.

i just drank some water that we boil every night.

…just in case if there’s anything in the water.

added some lemon and some salt to it.

that’s all possible to the current method of wastewater filtration in la.

and i know that the filtration method isn’t the same everywhere else.

especially, when i think about the event of flint, michigan.

they had lead in their water because of their old pipes.

they ignored the need to add orthophosphates (a mineral coating that keeps toxic lead to the pipes.)

so how did their wastewater treatment plants remove the heavy metal ions out of their water?

it makes more sense to explain what these wastewater treatment plants actually do.

then we can find its weak points.

this is where you can find a more in-depth answer.

i’ll give the tl;dr version.

the process goes in the order of:

  1. preliminary treatment
  2. primary treatment
  3. secondary treatment
  4. advanced treatment
  5. sludge treatment and disposal

this is a 3 sentence summary of each process.

preliminary treatment deals with extracting large chunks of debris, waste, and sand. these things can range from newspapers, plastic bags, grit, etc.

the larger the things are, it’s better to eliminate them from further filtration processes, as they could clog/block the inflow of potable water. grit, however, could affect the impact microorganisms can have when filtering out sludge in further filtration methods. so get rid of that too.

primary treatment deals with smaller debris & sludge (a “liquid” form of solid). and yes, metal ions can be in sludge too. the velocity of the water is slowed down to allow the sludge to sink to the bottom. that sludge goes through tubes connected to the bottom for their treatment.

secondary treatment deals with chemical/biological breakdown. this includes chlorine, harmful bacteria/viruses, and biomass. a popular method is 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, biomass.

the material then settles to a new tank, where the heavy sludge and floc is 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 “waste” interacting with wastewater.

advanced treatment includes removing the smallest of pathogens, bacteria, viruses, and maybe ions. some techniques include activated carbon for the pathogens to stick to (as activated carbon possesses a large surface area + pores). then UV light can be used to destroy the DNA/RNA of a biological organism (so they can’t reproduce) that surpassed the activated carbon + sand filter.

the ions.


so that’s the overview.

but now, how do we filter the ions during this process?

where do they go?

what’s the desired method?

the unit economics?

environmental consequences?

first, let’s begin by what’s available… or the status quo.

we start with chemical oxidation.

we’ll need an oxidizing agent entering the wastewater. this causes electrons to move from the oxidant to the pollutants (metal ions).

this will create a structural modification of the pollutants and become less destructive. it is used for the pre-treatment of heavy metal wastewater & organic compounds.

next we got chemical precipitation.

ooh, boy this is a good one.

this requires adding a precipitation reagent to the wastewater. this creates a chemical reaction that converts the dissolved metals into solid particles.

these metal ions can be collected into a ball of mass by neutralizing the charges of that mass.

then we can remove by filtration or sedimentation. this is the most common method for removing dissolved heavy metals from wastewater.

it is inexpensive…


the leftover sludge is the problem, especially if the water is highly acidic.

the other one is ion-exchange.

it’s a chemical reaction where heavy metal ions in water are exchanged for a similarly charged ion attached to a solid particle.

that solid particle can be a zeolite.

zeolite → naturally occurring minerals composed of silicon, aluminum, & oxygen… along with water molecules.

good news is that the reusability of the adsorbed metal ion is possible.

the bad news is that it costs to do this. it’s the most expensive choice.

okay, now the economics of each one.

let’s just go with ion-exchange first.

it costs more than $3 USD per ton of wastewater containing 100 parts per million (ppm) of heavy metals.

and for filters like these, filtering 1000 gallons of water using ion-exchange filters cost approximately $0.30 — $0.80 USD.

so definitely no for the mango filter lol.

chemical oxidation is pre-treatment for metal ions, so i will skip.

the big guy is chemical precipitation. so time to dissect him.

chemical precipitation.

using chemistry terminology, this is how chemical precipitation works:


M2+ the metal ions present in water

OH- → precipitant (a substance that creates a solid in the water).

M(OH)2 → the insoluble metal hydroxide

the biggest condition to obtain success in chemical precipitation is the pH.

the pH of when the metals can morph into a solid is the big factor of the pH. some metals precipitate (morph) at higher levels of pH than others.

and the pH will need to adjust between pH 7 - pH 11.

to get the following metals to form as insoluble particles, we’ll need to use sodium hydroxide and get the pH to:

  • cadmium: pH 11.0
  • copper: pH 8.1
  • chromium: pH 7.5
  • nickel: pH 10.8
  • zinc: pH 10.1

the issue is that sodium hydroxide costs between $1,000 to $1,200 per dry ton.

the environmental consequences is the sludge leftover. that sludge has the metal ions in them.

that will need a good safety measure to prevent a catastrophe.

so we need a better tool than sodium hydroxide.

that’s when we began using lime and limestone.


they’re abundant, cost-effective, and a simpler process.

but first, some chemistry.

the rocky road.

lime’s (Ca(OH2), [calcium hydroxide] solubility decreases when the temperature increases.

0°C = 0.189 gram of calcium hydroxide dissolves in 100 milliliters of water.

20°C = 0.173 gram of calcium hydroxide dissolves in 100 milliliters of water,

100°C = 0.066 gram of calcium hydroxide dissolves in 100 milliliters of water.

aeration → a process where air is circulated, mixed, or dissolved in a liquid or other substances that act as a fluid. (source)

and this is what’s needed to make the metal filtration process with lime work.

lime rock costs about $125.00 USD by the ton.

for lime rock & sodium hydroxide, the pH to remove copper ions from wastewater is 12.

before aeration, the lime consumption was 6000 mg/L with a pH of 10.5.

after aeration, it was 1500 mg/L with a pH of 9.5.

maximum copper efficiency via chemical precipitation is between a pH of 11.5–12.0

but the optimal pH for chromium to precipitate using sodium hydroxide is 8.7… that’s way off from 12.0 pH.

and that will force us to adjust the amount of lime rock already into the water, add/reduce sodium hydroxide…

and adjust pH with either hydrogen ions (H+) (to go acidic), or add sodium hydroxide to go alkaline.

(the hydroxyl group (OH-), will take care of that.)


and the volume of sludge made from the chemical precipitation for 30 liters of waste after mixing & sedimentation was about 2.4 L in the lime rock.

and the volume of sludge made from 30 liters of waste after mixing & sedimentation was about 1.6 L in the sodium hydroxide.

and for a city like big spring, texas… a surface water treatment facility can treat approximately 16 million gallons per day and filter 21 million gallons per day.

so… that is kinda a lot of sodium hydroxide and lime rock.

how a metal precipitation treatment plant works

and the handling of the sludge is unfavorable, economically.

the combined operation of dewatering and hauling vacuum-filtered sludge, the average cost per dry ton was $87 USD (average).

specific example:

an annual cost of $160,520 USD for 970 dry tons of sludge…

which can lead up to $165 USD per dry ton.

and the energy use intensity for an operating wastewater treatment plant is about 50 kBtu/gallons per day.

kBtu → kilo british thermal units

british thermal units → a unit of energy equal to the amount of heat required to raise the temperature of one lb. of water by 1°F.

and it’s over.

where we at?

ye, you can see why this is not even close to our favor.

the present will be devoured by the problems of the past.

and the future was on the horizon.

this is more insight, if u want.

this is why the way out is to use what we got.

and what we have is mother nature.

use her masterpiece, and make it for our cause.

mother nature makes things absorb or adsorb.

and adsorption is the way to go.

we want to use those metals for something.

and the money we gunna save from not buying lime rocks lol.

the only roadblocks left for an adsorption-based material are:

  • low selectivity
  • production of waste products
  • scaling it up into industry-standard

number two is resolved when we use mother nature;

the waste products are biodegradable (the mango peels).

next time, i’ll tackle the “low selectivity” problem.

i’ll bring back that desorption/recycling problem first.

why does vinegar/lemon juice work, economically & chemically?

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