Main parameters defining the quality of wastewater

Solids

• All the contaminants of water, with the exception of dissolved gases, contribute to the solids load.
• In wastewater treatment, the solids can be classified according to

a. Their size and state,

b. Their chemical characteristics and
c. Their settleability
 Classification by size
• The division of solids by size is above all a practical division.
• Particles of smaller dimensions capable of passing through a filter paper of a specific size
correspond to the dissolved solids
• Particles with larger dimensions and retained by the filter are considered suspended solids.
• To be more precise, the terms filterable =dissolved solids and non-filterable =suspended solids

• Dissolved solids those with a diameter of less than 10−3

µm,
• Colloidal solids those with a diameter between 10−3 and
100 µm
• Suspended solids those with a diameter greater than 100
µm
Classification by chemical characteristics

• If the solids are submitted to a high temperature (550 ◦C), the organic fraction is oxidised
(volatilised), leaving after combustion only the inert fraction (unoxidised).
• The volatile solids represent an estimate of the organic matter in the solids, while the non-
volatile solids (fixed) represent the inorganic or mineral matter.
• In summary:
TSS = (Wpost – Wpre)/V 105o C for 2 h

VSS = (Wpost – Wcombustion)/V 520/550o C for 2

Classification by settleability

• Settleable solids are considered those that are able to settle in a period of 1 hour.
• The volume of solids accumulated in the bottom of a recipient (Imhoff Cone) is measured and
expressed as mL/L.
• The fraction that does not settle represents the non-settleable solids (usually not expressed in the
results of the analysis).
Carbonaceous organic matter
• The organic matter present in sewage is a characteristic of substantial importance, being the
cause of one of the main water pollution problems: consumption of dissolved oxygen by the
microorganisms in their metabolic processes of using and stabilising the organic matter.
• The organic substances present in sewage consist mainly of
• Protein compounds (≈ 40%)
• Carbohydrates (≈ 25 to ≈ 50%)
• Oils and grease (≈ 10%)
• Urea, surfactants, phenols, pesticides and others (lower quantity)
Direct or indirect methods can be adopted for the quantification of organic matter:

• Indirect methods: measurement of oxygen consumption

• Biochemical Oxygen Demand (BOD)
• Ultimate Biochemical Oxygen Demand (BODu)
• Chemical Oxygen Demand (COD)

• Direct methods: measurement of organic carbon

• Total Organic Carbon (TOC)
Biochemical Oxygen Demand (BOD)

• The main ecological effect of organic pollution in a water body is the decrease in the level of dissolved
oxygen.
• Similarly, in sewage treatment using aerobic processes, the adequate supply of oxygen is essential, so
that the metabolic processes of the microorganisms can lead to the stabilisation of the organic
matter.
• “Strength” of the pollution potential of a wastewater is determined by the measurement of the oxygen
consumption by it
• This quantification could be obtained through stoichiometric calculations based on the reactions of
oxidation of the organic matter – called Theoretical Oxygen Demand (TOD).
• If the substrate is glucose (C6 H12 O6 ), the quantity of oxygen required to oxidise the given quantity of

glucose could be calculated through the basic equation of respiration – known as TOD
• BOD represents the quantity of oxygen required to stabilise, through biochemical processes, the
carbonaceous organic matter.
• It is an indirect indication, therefore, of the biodegradable organic carbon.

O2 + Microorganisms + Organic matter  CO2 +

H20
• The determination is undertaken on the 5th day. For typical domestic sewage, the oxygen
consumption on the fifth day can be correlated with the final total consumption (BODu)
• The test is carried out at a temperature of 20◦C, since different temperatures interfere with the
bacteria’s metabolism
• The standard BOD is expressed as BOD205
The main advantages of the BOD test are related to the fact that the test allows:
• an approximate indication of the biodegradable fraction of the wastewater;
• an indication of the degradation rate of the wastewater
• an indication of the oxygen consumption rate as a function of time;
• an approximate determination of the quantity of oxygen required for the biochemical
stabilisation of the organic matter present.
However, the following limitations may be
• low levels of BOD5 can be found in the case that the microorganisms responsible for the

decomposition are not adapted to the waste;

• heavy metals and other toxic substances can kill or inhibit the microorganisms;
• the inhibition of the organisms responsible for the oxidation of ammonia is necessary, to avoid
the interference of the oxygen consumption for nitrification (nitrogenous demand) with the
carbonaceous demand;
• the test takes five days, being not useful for operational control of a WWTP.
The ratio BODu/BOD5 equal to 1.46.

This means that, in the case of having a BOD5 of 300 mg/L,

the BODu is assumed to be = 1.46 × 300 = 438 mg/L

Chemical Oxygen Demand (COD)
• The COD test measures the consumption of oxygen occurring as a result of the chemical oxidation
of the organic matter.
• The value obtained is, therefore, an indirect indication of the level of organic matter present.
• The main difference with the BOD test is clearly found in the nomenclature of both tests. The BOD
relates itself with the biochemical oxidation of the organic matter, undertaken entirely by
microorganisms. The COD corresponds to the chemical oxidation of the organic matter, obtained
through a strong oxidant (potassium dichromate) in an acid medium.
The main advantages of the COD test are:
• the test takes only two to three hours;
• because of the quick response, the test can be used for operational control;
• the test results give an indication of the oxygen required for the stabilization of the organic matter;
• the test allows establishment of stoichiometric relationships with oxygen;
• the test is not affected by nitrification, giving an indication of the oxidation of the carbonaceous
organic matter only (and not of the nitrogenous oxygen demand)
The main limitations of the COD test are:
• in the COD test, both the biodegradable and the inert fractions of organic matter are oxidised.
Therefore, the test may overestimate the oxygen to be consumed in the biological treatment of
the wastewater;
• the test does not supply information about the consumption rate of the organic matter along the
time;
• For raw domestic sewage, the ratio COD/BOD5 varies between 1.7 and 2.4. For industrial wastewater,
• certain reduced inorganic constituents could be oxidised and interfere with the result.
however, this ratio can vary widely.
• Depending on the value of the ratio, conclusions can be drawn about the biodegradability of the
wastewater
and the treatment process to be employed
Total Organic Carbon (TOC)

• In this test the organic carbon is directly measured, in an instrumental test

• TOC test measures all the carbon released in the form of CO2.

• To guarantee that the carbon being measured is really organic carbon, the inorganic forms of
carbon (like CO2 , HCO− 3 etc) must be removed before the analysis or be corrected when

calculated

• The TOC test has been mostly used so far in research or in detailed evaluations of the
characteristics of the liquid
Nitrogen

Nitrogen is a component of great importance in terms of generation and control of the water pollution,
principally for the following aspects:

Water pollution
• Nitrogen is an essential nutrient for algae leading, under certain conditions, to the phenomenon of
eutrophication of lakes and reservoirs;
• Nitrogen can lead to dissolved oxygen consumption in the receiving water body due to the processes
of the conversion of ammonia to nitrite and this nitrite to nitrate;
• Nitrogen in the form of free ammonia is directly toxic to fish;
• Nitrogen in the form of nitrate is associated with illnesses such as methaemoglobinaemia
Sewage treatment
• Nitrogen is an essential nutrient for the microorganisms responsible for sewage treatment;
•
Nitrogen, in the processes of the conversion of ammonia to nitrite and nitrite to nitrate
(nitrification), which can occur in a WWTP, leads to oxygen and alkalinity consumption;
• Nitrogen in the process of the conversion of nitrate to nitrogen gas (denitrification), which can take
place in a WWTP, leads to
(a) the economy of oxygen and alkalinity (when occurring in a controlled form) or
(b) the deterioration in the settleability of the sludge (when not controlled)

Distribution between the forms of ammonia

• pH < 8 Practically all the ammonia is in the form of NH4+

• pH = 9.5 Approximately 50% NH3 and 50% NH4+

• pH > 11 Practically all the ammonia in the form of NH3

Phosphorus
Total phosphorus in domestic sewage is present in the form of phosphates:
• Inorganic (polyphosphates and orthophosphates) – main source from detergents and other household chemical
products
• Organic (bound to organic compounds) – physiological origin

• Phosphorus in detergents is present, in raw sewage, in the form of soluble polyphosphates or, after hydrolysis, as
orthophosphates.
• Orthophosphates are directly available for biological metabolism without requiring conversion to simpler forms.
• In typical domestic sewage the prevailing form is HPO4−2.

• Polyphosphates are more complex molecules, with two or more phosphorus atoms.
• Polyphosphates are converted into orthophosphates by hydrolysis, which is a slow process, even though it takes
place in the sewerage collection system itself.
 Another way of fractionating phosphorus in wastewater is with respect to its form as solids (IAWQ,
1995):
• Soluble phosphorus (predominantly inorganic) – mainly polyphosphates and orthophosphates
(inorganic phosphorus), together with a small fraction corresponding to the phosphorus bound to
the soluble organic matter in the wastewater
• Particulate phosphorus (all organic) – bound to particulate organic matter in the wastewater
The importance of phosphorus is associated with the following aspects:
• Phosphorus is an essential nutrient for the growth of the microorganisms responsible for the
stabilisation of organic matter. Usually domestic sewage has sufficient levels of phosphorus, but a
lack may occur in some industrial wastewaters;
• Phosphorus is an essential nutrient for the growth of algae, eventually leading, under certain
conditions, to the eutrofication of lakes and reservoirs