Water Disinfection Byproducts

17
Oct

Water Disinfection Byproducts (DBP)

The use of some disinfectants, such as chlorine, results in the formation of byproducts, many of them are considered carcinogenic.

Disinfection byproducts are formed as a result of a reaction of the disinfectant with several compounds in the water.

The type of disinfection byproducts that is formed depends on:

  • The disinfectant that is being used
  • The dose of the disinfectant
  • The type of residue formed by the disinfectant
  • A precursor must exist in the water

DPB precursors can be natural organic matter (MON) or bromide. In order for a disinfection byproduct to be formed, it is required that both the precursor and the disinfectant be simultaneously present in the water.

The disinfectants that can form DBPs are chlorine, chloramines, chlorine dioxide and ozone.

 

FORMATION OF DISINFECTION BYPRODUCTS

Disinfection byproducts can be formed in various reactions. For example:

  • The reaction between natural organic matter and chlorine or chloramines forms trihalomethanes (THM) and haloacetic acids (HAA5).
  • The reaction between bromide + ozone forms bromate:
    Br- + O3 → BrO – 3
  • Chlorine dioxide is reduced to a chlorite:
    ClO2 → ClO

The most predominant disinfection byproduct  is trihalomethane (THM). Its removal is difficult and requires adsorption by activated carbon or separation by air.

The level of THM is higher in chlorinated surface waters than in chlorinated groundwater.

Typical concentrations of TTHM (Total Trehalmomethanes): Surface water: 0.03 – 0.15 mg/L,

Groundwater: 0.001 – 0.01 mg/L.

The concentration of TTHM in the water increases with the temperature, the pH and the chlorine dosage applied.

 

FACTORS THAT AFFECT THE AMOUNT OF DBPs FORMED

Several factors in the amount of DBP formed:

  • The concentration and the type of precursor.
  • The temperature – the concentration of DBPs increases with temperature.
  • The pH – concentration of THM increases and that of HAA5 decreases with the pH.

Contact time – the concentration of DBPs increases with the contact time between the SPD precursor and the disinfectant.

The concentration of bromide – higher concentration of results in greater amount of DBPs.

The dose of the disinfectant – higher dose  results in a greater amount of DBPS.

 

CONTROL PARAMETERS AND MEASURES

The best measure to avoid or minimize the formation of disinfection byproducts is the elimination of the precursors.

Improved coagulation is one of these measures, and it is a regulatory requirement in many countries. It involves the application of higher doses of coagulants, at lower pH values. This process results in a better elimination of natural organic matter (NOM).

The parameters used to monitor and control the level of disinfection byproducts are MRDL, which is the maximum permissible level of the disinfectant in water and MCL, which is the maximum permissible level of the DBP.

Examples of MRDL and MCL (EPA) values:

Chlorine – 4.0 mg/L as Cl2

Chloramines – 4.0 mg/L as Cl2

Chlorine dioxide – 0.8 mg/L as ClO2

TTHM – 0.08 mg/L

HAA5 – 0.06 mg/L

Bromate – 0.01 mg/L

Chlorite – 1.0 mg/L

 

REGULATED DISINFECTION BYPRODUCTS

Several DBPs are subject to regulation. These are:

 

  • 4 Trihalomethanes – Bromotrichloromethane, bromoform, dibromochloromethane and chloroform.
  • 5 Haloacetic acids – monobromoacetic acid, dibromoacetic acid, monochloroacetic acid and trichloroacetic acid.
  • Bromate (BrO3)
  • Chlorite (ClO)

 

EMERGING DISINFECTION BYPRODUCTS (NOT REGULATED)

Due to the use of alternative disinfectants to chlorine, other types, of disinfection byproducts, that are not yet regulated, emerged. Some of them are more dangerous to human health than the regulated DBPs!

These include:

  • Iodine-acids
  • Bromonitromethanes
  • Nitroso-dimethyl amines (NDMA)

 

This topic requires further investigation.

 

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