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Professional Water Quality Issues Introduction

Terminologies in Water Analysis
TOD (Total Oxygen Demand)

It refers to the amount of oxygen required when reducing substances in water are burned at high temperatures to become stable oxides.

It reflects the amount of oxygen that needs to be consumed when almost all organic matter in water is burned to turn into CO2, H2O, NOx, SO2, and so on.

COD (Chemical Oxygen Demand)

It is the amount of oxidizer consumed when treating a water sample with a certain strong oxidizer.

It reflects the extent to which water is contaminated by reducing substances and measures the content of all organic matter in the water.

BOD (Biochemical Oxygen Demand)

It is the amount of oxygen required for the stable decomposition of biodegradable organic matter in water at 20°C and under aerobic conditions.

It reflects the content of organic matter that can be biodegraded, chemical oxygen demand (COD) is generally higher than biochemical oxygen demand BOD5, the difference between them can probably reflect the sewage can not be microbial degradation of organic matter content.

TOC (Total Organic Carbon)

It is the amount of carbon, the main element in organic matter, that is used to indicate the amount of organic matter in water.

It reflects the total carbon content of organic matter in wastewater and is a comprehensive indicator that indirectly indicates the amount of organic matter in water.

TSS (Total Suspended Solids)

It is the dry weight of suspended solids filtered into a liquid after filtration.

It reflects the total weight of solids suspended in the water.

TDS (Total Dissolved Solids)

It is the total amount of milligrams of dissolved solids contained in 1 liter of water.

It reflects the amount of dissolved material in the water, through the analysis of the TDS value of the incoming and outgoing water, the reaction system removes the effect of impurities.

SDI (Silt Density Index)

It is the quantity of particles, colloids, and other objects in the water that can block various water purification equipment.

It reflects the degree of contamination of the water.

LSI (Langelier Saturation Index)

It is the value obtained by subtracting the saturated pH value from the measured pH value of the water sample.

It reflects the tendency of reverse osmosis devices to scale. If LSI > 0.3, then the system has a tendency to scale; if LSI < -0.3, then the system has a tendency to corrode; if -0.3 ≤ LSI ≤ 0.3, then the system has no tendency to scale or corrode.

TN (Total Nitrogen)

It is the total amount of various forms of inorganic and organic nitrogen in water.

It reflects the extent to which the water body is contaminated with nutrients.

TP (Total Phosphorus)

It is a general term for any form of phosphorus-containing material.

It is one of the important indicators for assessing the nutrient status of a water body. Phosphorus is an important nutrient element, but excessive total phosphorus will lead to eutrophication of the water body, cause algal blooms, and ultimately lead to hypoxia and death of the water body, which will be harmful to aquatic organisms and the water environment.

DO (Dissolved Oxygen)

It is the content of oxygen dissolved in water.

Dissolved oxygen is an important indicator of good or bad water quality, the higher the concentration of dissolved oxygen water quality is relatively better, and the concentration of low content may be too much pollution impurities that consume oxygen.


It indicates the degree to which suspended and colloidal substances in water impede the transmission of light.


The total amount of calcium ions and magnesium ions in water.


The total amount of substances in water that can be neutralized with strong acids.

Electrical Conductivity

The physical meaning of electrical conductivity is the conductive properties of materials. The greater the electrical conductivity, the stronger the conductivity, and vice versa, the smaller.


The resistivity of water is affected by the purity of water, temperature and other factors.

It reflects the amount of salt content in the water, the higher the purity of water, the lower the salt content, the greater the resistivity of water, the smaller the electrical conductivity.

pH Value

It indicates the degree of acidity and alkalinity of the sewage.

MLSS(Mixed Liquid Suspended Solids Concentration)

It refers to the concentration of mixed liquid suspended solids after mixing sewage and activated sludge in the aeration basin.

Impurity Issues & Removal Processes

Iron ion in groundwater is generally divalent ferrous iron, in the form of Fe(HCO3)2. We first need to oxidize the divalent ferrous iron to trivalent iron to form reddish brown precipitate, Fe(OH)3, which can be removed by filtration. The reaction process is:

Chemical equation for the oxidation of divalent iron ions to trivalent iron ions

The oxidation process is completed by aeration. The aeration device can be fully in contact with water and oxygen to produce natural oxidation; water after aeration is removed iron ions with iron ions and manganese ions removal filters. If most of the iron ions in the water are trivalent iron ions, then it does not need to be aerated and can directly enter the iron ions and manganese ions removal filter to be removed.

Aeration basin site photo

Biological denitrification utilizes the principle of the nitrogen cycle in nature. First, in an anaerobic environment, through ammonification, organic nitrogen will be converted into ammonia nitrogen. The ammonification process is easy to carry out, and in general, treatment facilities can be completed; then in an aerobic environment, through nitrification, ammonia nitrogen will be converted into nitrate nitrogen; then in an anoxic environment, through denitrification, nitrate nitrogen will be converted into ammonia, and escape from the water. The reaction can be carried out mainly by A2O, SBR, MBBR, and other processes.


Phosphorus ions often exist in the form of phosphate, polyphosphate, and organic phosphorus in wastewater. Biological phosphorus ion removal is the use of polyphosphorus bacteria in the body of the ATP to hydrolyze and release PO43- under anaerobic conditions; polyphosphorus bacteria ingest phosphorus from external environments under aerobic conditions, and store phosphorus in the form of polyphosphate in the cell to form high-phosphorus sludge, which will be discharged from the system to achieve the effect of phosphorus removal from wastewater.

Flow chart of the AAO process
Residual Chlorine

Chlorine gas as a disinfectant protects against pathogens, but excessive concentrations of chlorine gas can be absolutely harmful to the human body. To reduce the residual chlorine concentration, the use of SO2 or NaHSO3 reducing agent for the reduction of residual chlorine or the use of an activated carbon filter can also adsorb chlorine gas.

Bacteria & Viruses

Bacteria and viruses over a certain range can cause great harm to human health, the general use of bactericides to kill bactericides, and RO membrane can also retain a portion of the bacteria and viruses. And UV germicidal lamps can be installed after the RO membrane for final interception.

One set of dosing equipment
Calcium & Magnesium Ions

Excessive water hardness will form scale to block the RO membrane or pipeline. People generally use ion exchange resins or antiscalants to prevent scale generation.

Metal Ions, Carbonates, Sulfates, Chlorides & Other Inorganic Salts

They are usually filtered by ion exchange resin replacement or reverse osmosis device.

Heavy Metals

There are three main common industrial heavy metal wastewater treatment methods: chemical, physical, and biological. Chemical method involves adding chemicals to remove heavy metal ions by generating hydroxide and sulfide precipitates through chemical precipitation reactions; physical methods usually involve replacing heavy metal ions with ion exchange resins, and physical methods can also be used to achieve purification through membrane separation technology; biological methods utilize the chemical structure and compositional properties of biological materials to adsorb heavy metal ions in water bodies.

Dissolved Oxygen

If the dissolved oxygen content in the water is low, it may indicate the existence of certain pollutants or excessive organic matter in the water body. These pollutants will consume oxygen, resulting in a lack of oxygen in the water body; the higher the dissolved oxygen content, the better the water quality. When the oxygen concentration in the water is low, the aeration and oxygenation process needs to be carried out in the water, and when the concentration is too high, a reducing agent must be added to restore oxygen through the dosing system.

Silt, Sediment & Colloid

They can be removed by physical methods such as flocculation and sedimentation or retained and removed by multi-media filters.

Common Water Sources

Natural water resources available are surface water and groundwater. Surface water is the general term for dynamic and static water on the land surface, including all kinds of liquid and solid water bodies, mainly rivers, lakes, swamps, glaciers, ice sheets, etc. Groundwater refers to water in rock voids below the ground surface.

Surface water has low salinity, lower hardness, high turbidity, and high bacterial content compared to groundwater, which is susceptible to the influence and pollution of the external environment and has a higher chance of being polluted.

Groundwater has stable water temperature and quality, less suspended impurities, low turbidity, less organic matter and bacteria, high salinity, and hardness, and is less susceptible to the influence and pollution of the external environment.

A topographic and physiognomic drawing
Common Water Sources

In all cases, drinking water supplied to residential users must be treated to meet drinking water standards, and any water quality indicator that exceeds the limits set by drinking water standards must be treated.

For each water quality indicator, the World Health Organization (WHO) has provided recommended values in the "Guidelines for Drinking Water Quality", which can be adjusted by each country according to their people's health and economic levels. The U.S. Environmental Protection Agency (EPA) has established the Safe Drinking Water Act (SDWA) to protect public health. China is currently implementing GB/T 5749 Standards for Drinking Water Quality. Different countries and regions have set strict drinking water standards. These standards or regulations may vary to some extent, but basically, all use the WHO guidelines as a reference and are revised according to the specific situations and conditions of their respective countries or regions.