Principles of Water Purification

FOREWORD

Water purification remains a difficult exercise, depending on the initial water quality, the choice of substances and the quantities to be removed, the type of filtration, the purification medium, its exchange capacity, the required contact time, and the service life of the medium—or its purification cycle duration.

To this must be added potential fluctuations in water quality; the saturation level of the purification medium; the gradual wear of the filtration system; pressure and flow-rate variations; and the volumes of water to be produced. You must also consider issues caused by storage and distribution constraints. Finally, you must take into account the desired final water quality.

DEFINITION

Water purification includes all techniques and methods used to obtain process water from drinking (potable) water. Water is considered potable when it meets a number of characteristics that make it suitable for human consumption. Reference standards in this field differ across time and countries, and depending on the authority responsible for this definition in certain countries. (Wikipedia)

The concept of “potability” varies around the world, shaped by local historical, scientific, and cultural contexts. It determines access to water, because good-quality water is essential to economic and human development. The parameters that may be regulated generally include:

Organoleptic quality (color, turbidity, odor, taste)
Natural physico-chemical parameters (temperature, pH, chlorides: 200 mg/L, sulfates: 250 mg/L, etc.)
So-called undesirable substances (nitrates: 50 mg/L, nitrites, pesticides, etc.)
Toxic substances (arsenic, cadmium, lead, hydrocarbons, etc.)
Microbiological parameters (the water must not contain pathogenic organisms, including fecal coliforms)

Potable does not mean pure! In France, water potability is generally beyond dispute, and water utilities in charge of it do their work rigorously. Samples and analyses are performed every day to ensure potability. Without questioning this potability, it is important to recall what drinking water is: it is water in which we accept that a certain number of pollutants may be present, in certain quantities.

A potable water can be close to pure water (each measured parameter close to 0), or very polluted (each measured parameter close to the maximum). Conversely, a non-potable water can be almost pure (each measured parameter close to 0) and exceed the maximum very slightly on a single measurement—making it, in the eyes of regulations, “non-potable.”

Another issue in the definition of potable water: parameters are expressed either as limits or as quality references (Order of January 11, 2007 relating to limits and quality references for raw waters and waters intended for human consumption, referenced in the French Public Health Code).

Limits determine whether water is potable or not; references are provided for information only. This is notably the case for aluminum, with a quality reference of 200 μg/L.

Quality references:

Microbiological parameters: coliform bacteria; sulfite-reducing bacteria including spores; count of culturable aerobic microorganisms at 22°C and 37°C; physico-chemical parameters; total aluminum; ammonium (NH4+); total organic carbon (TOC); oxidizability by KMnO4; free and total chlorine; chlorites; chlorides; conductivity; color; copper; calco-carbonic balance (aggressiveness); total iron; manganese; odor.

Quality limits:

Microbiological parameters: Escherichia coli (E. coli); enterococci; physico-chemical parameters: acrylamide; antimony; arsenic; barium; benzene; benzo(a)pyrene; boron; bromates; cadmium; vinyl chloride; chromium; copper; total cyanides; 1,2-dichloroethane; epichlorohydrin; fluorides; polycyclic aromatic hydrocarbons; total mercury; microcystins; nickel; nitrates; nitrites; pesticides (individual substance) except: aldrin, heptachlor, dieldrin, heptachlor epoxide (per individual substance); total pesticides (detected and quantified); lead; selenium; tetrachloroethylene and trichloroethylene; total trihalomethanes (THMs); turbidity.

In other words, the notion of potable water does not indicate its purity in any way. Finally, interactions between substances are not taken into account in potability parameters. This is the context in which water purification comes into play: however tiny the amounts may be, water purification should bring them as close as possible to “zero.”

POTABLE FOR WHOM?

The elements above lead us to this question: potable for whom—or for what? Part of the definition of potable water gives the answer: good-quality water is essential to economic and human development.

It is no coincidence that “development” comes before “human.” Users of potable water represent only 24% of overall potable water consumption in France (IFEN data 2006, figures for 2001, representative of recent years), with the rest shared between industry and agriculture, the largest consumers.

Moreover, according to the French Directorate General for Health, only 7% of potable water is used for food-related purposes, and only 1% for drinking (DGS information file, September 7, 2005). In other words, drinking water represents only 0.24% of total consumption, and water for food use 1.68%. The water distributed today is essentially intended for non-food needs.

However, certain pollutants can also be harmful through skin contact or inhalation.

Potable: tailor-made standards?

Finally, drinking-water standards are also linked to the natural quality of available water in a given country. Given consumption types and their proportions, it seems obvious that public authorities cannot risk raising potability criteria beyond what available water reserves can provide.

That would amount to inflicting a shortage of potable water on ourselves, considering that only 1.68% is truly intended for food use in France. This is why many specialists call for stricter standards without ever being heard. It also explains disparities in standards across countries. These are what we call “tailor-made standards.”

FILTRATION

Filtration methods: As with many things, there are many possible approaches. For the sake of simplicity, I will retain only two: membrane filtration and selective filtration.

Membrane filtration, also called “physical” filtration, includes all filtration methods based on filtration size. This is also called “blind filtration” (as opposed to selective filtration). It includes reverse osmosis, ultrafiltration, nanofiltration, microfiltration, ceramic systems, etc.

Principle: Water is forced against a semi-permeable barrier, most often made of plastic, and sometimes ceramic or cotton.
Results: Results vary depending on the permeability of the initial membrane and its resistance to aging or wear. Limescale content also impacts performance, disturbing proper membrane function.
Advantages: Simple and easy to implement. Low cost. Compact footprint.
Note: The finer the filtration, the more “stripped” the water becomes—resulting, for example in reverse osmosis, in what the author calls “dead” water, considered of little interest in the context of living systems.

Selective filtration, also called “chemical” filtration, includes all filtration methods based on the sequestration (capture) of substances on a medium. This includes activated carbon filters, resins, etc.

Principle: As water passes through these filters, it deposits (retains) the targeted substances.
Results: Variable depending on the amount of material to be retained, the quality of the medium, the volume already filtered, contact time, and flow speed.
Advantages: High purification volume. Flow rate preserved. Direct production. Water structure respected. Often long-lasting filtration (several years).
Disadvantages: Cost; possible “release” phenomena if saturated; complex to implement.

SUBSTANCES TO FILTER

Not all components of water are pollutants: some are useful, some have no effect. Here is a commonly used list of pollutants to remove from water:

Chlorine, tastes and odors
Heavy metals (arsenic, cadmium, chromium, mercury, nickel, lead, selenium, and zinc)
Aluminum
Ferrous and ferric iron
Post-filtration bacteria
Nitrates and sulfates
Limescale
Chemical products
Organic products
Pharmaceutical residues
Petroleum products

This list is not intended to be exhaustive. In contrast to these pollutants, there remain minerals—some say they are useful to our bodies, others claim the opposite—and trace elements, which seem to have broad agreement regarding their beneficial effects. Finally, there remains the water molecule itself, also carrying its “history” and its energy—provided the filtration system has respected them.

KNOWING YOUR WATER: A DECISIVE STEP

Before taking any purification action, there is one indispensable step: knowing your water.

How can a result be guaranteed to the user without first understanding the issues affecting the water they have and their significance? Yet thousands of filter jugs and other filters are sold commercially, touting the benefits of finally purified water—without even knowing the local characteristics! How could that same jug or cartridge filter “x” liters or filter for “x” days, when the water can be diametrically different? Added to this are issues that vary more or less depending on seasons, weather events, or population changes.

Designing a water purification system

Based on the elements above, designing an effective filtration system remains a complex operation. It requires taking into account:

Local issues related to the source water
User requirements regarding the desired quality
Expected consumption (instantaneous flow rate and daily consumption)
Filter element capacities and their operating conditions
Cost constraints (investment and maintenance)
Technical constraints (location, manometric head, pressure loss)

Principles of Water Purification

FOREWORD