PHYSICAL AND CHEMICAL CHARACTERISTICS OF WASTEWATER AND THEIR SOURCES

CHARACTERISTIC SOURCES
Physical properties:

Color Domestic and industrial waste water, natural decay of organic materials
Odor Decomposing waste water, industrial waste water
Solids Domestic water supply, domestic and industrial wastewater, soil erosion, inflow/infiltration
Temperature Domestic and industrial waste water

Temperature affects aquatic life and it affects chemical reactions and reaction rates. An increase in T results to increase in reaction rate resulting to O2 depletion. O2 solubility in water decreases as T increases and the bacterial activity increases.

Chemical constituents:

Organic:
Carbohydrates, fats, oils Domestic, commercial and industrial waste water and grease,proteins, surfactants, volatile organics
Pesticides Agricultural wastewater
Phenols Industrial wastewater
Others Natural decay of organic materials

CHARACTERISTIC SOURCES

Inorganic:
Alkalinity, chlorides Domestic waste water, domestic water supply, groundwater infiltration
Heavy metals Industrial waste water
Nitrogen Domestic and agricultural waste water
pH Domestic, commercial and industrial wastewater
Phosphorus Domestic, commercial and industrial wastewater, natural runoff

Gases:
Hydrogen sulfides, methane Decomposition of urban wastewater
Oxygen Domestic water supply, surface water infiltration

PHYSICAL & CHEMICAL PARAMETERS BY WHICH WASTEWATER IS MEASURED

Biochemical oxygen demand (BOD)
BOD is the most widely used parameter of organic pollution applied to both wastewater and surface water.
It is the amount of dissolved oxygen used by organisms while consuming organic matter in a waste water sample. Many factors can influence this test, such as temperature of incubation, dilution rate, nitrification, toxic substances, nature of bacterial seed and presence of anaerobic organisms.
There are two stages of decomposition in the BOD test:
The carbonaceous stage, or first stage, represents that portion of oxygen demand involved in the conversion of organic carbon to carbon dioxide.
The nitrogenous stage, or second stage, represents a combined carbonaceous plus nitrogeneous demand, when organic nitrogen, ammonia, and nitrite are converted to nitrate.

Nitrogenous oxygen demand generally begins after about 6 days. For some sewage, especially discharge from wastewater treatment plants utilizing biological treatment processes, nitrification can occur in less than 5 days if ammonia, nitrite,
and nitrifying bacteria are present. In this case, a chemical compound that prevents nitrification should be added to the sample if the intent is to measure only the carbonaceous demand. The results are reported as carbonaceous BOD
(CBOD), or as CBOD5 when a nitrification inhibitor is used.

Chemical Oxygen Demand (COD)
The COD test is used to measure the content of organic matter of both wastewater and natural waters. The oxygen equivalent of the organic matter that can be oxidized is measured by using a strong chemical oxidizing agent in acidic medium, which is potassium dichromate.
The test must be performed at an elevated temperature; silver nitrate is required as a catalyst to aid in the oxidation
COD of a waste is in generally higher than BOD because more compounds can be chemically oxidized than can be biologically oxidized.
The test is extensively used because it takes less time (about 3 hours) than other tests such as the BOD5, which takes 5 days. The COD test does not, however, differentiate between biodegradable and non-biodegradable organic matter. Also, COD test is expensive.

Total suspended solids (TSS)
This is the sum of the organic and inorganic solids concentrations and can be subdivided into:
Suspended solids - which represent the solids that are in suspension in the water. Generally comprised of 70%organic and 30% inorganic solids and can be removed by physical or mechanical means.
Organic solids
About 50% of solids present in urban wastewater are derived from the waste products of animal and vegetable life. Sometimes called the combustible fraction or volatile solids as these can be driven off at high temperature.
Inorganic solids - these substances are inert and are not subject to decay. These include sand,gravel and silt.
Settleable solids - this is a subset of suspended solids and represents that fraction of suspended solids that will settle in a given period.
Colloidal suspended solids - these refer to solids that are not truly dissolved and yet do not settle readily. They tend to refer to organic and inorganic solids that rapidly decay.

*Dissolved solids refers to that fraction of solids that pass through a 0.45 μm filter paper.

pH
This expresses the concentration of hydrogen ions in solution and indicates the level of acidity or alkalinity of an aqueous solution.
pH = -log [H+] , where [H+] = concentration of H+ ions in moles per liter
If the pH of the wastewater is outside the range 5-10, there maybe considerable interference with biological processes. Bacterial growth is very low outside the wastewater pH range of 5-10 , slowing down the biological degradation of organic waste.
The pH of aqueous systems can be measured using a pH meter or by a universal indicator
Total phosphorus
Phosphorus is essential to the growth of algae and other biological organisms. But because of noxious algal blooms that occur in surface waters, the amount of phosphorus compounds from wastewater is controlled .
This parameter is normally divided into three fractions, namely:
Orthophosphate - dissolved inorganic phosphate (P04-3 , HPO4-2 , H2PO4-, H3PO4)
Polyphosphates – are complex compounds generally derived from detergents
Organically bound phosphate - dissolved and suspended organic phosphates

Nitrogen
Like phosphorus, nitrogen is essential to the growth of protists and plants, so it is known as biostimulant. Because nitrogen, is an essential building block in the synthesis of protein, nitrogen data is required to evaluate the treatability of wastewater by biological processes. Total nitrogen refers to the to the sum of measurements of total oxidized nitrogen.

Forms of nitrogen
Organic nitrogen
Organic nitrogen maybe in the form of humus or the intermediate products of organic matter decomposition.
Ammonia nitrogen – exists in aqueous solution as either NH4+ or NH3, depending on the pH of the solution, in accordance with the following equilibrium reactions:
NH3 + H2O ↔ NH4+ + OH-
At pH levels above 7, the equilibrium is displaced to the left, at pH levels below 7, the ammonium ion is predominant
Nitrite nitrogen – is determined colorimetrically
- relatively unstable and is easily oxidized to nitrate form
- extremely toxic to most fish and other aquatic species
Nitrate nitrogen – also determined by colorimetric methods
- is the most highly oxidized form of nitrogen found in
wastewaters.
- if present in drinking water above 45 mg/L as NO3- can cause fatal effects on infants.
Heavy metals
Trace quantities of many metals, such as Ni, Mn,Pb, Cr, Cd, Zn, Cu, Fe, Hg are important constituents of most waters. Many of these metals are classified as priority pollutants although some of these metals are necessary for growth of biological life . It is the presence of these metals in excessive amounts will interfere with many beneficial uses of water because of its toxicity.
Gases

Dissolved oxygen – this is required for the respiration of aerobic
microorganisms. However, oxygen is only slightly soluble in water
- The quantity of dissolved gases in water is affected by:
  a. Solubility of the gas
  b. Partial pressure of the gas in the atmosphere
  c. Temperature
- if T is increased, the rate of biological reactions that use oxygen is
  increased, that will decrease the DO in the water. In summer months,
  stream flow is lower that compounds the problem of increased T.
  d. Purity of the water
Hydrogen Sulfide (H2S)
H2S is formed from anaerobic decomposition of organic matter containing sulfur or from the reduction of mineral sulfites and sulfates.
This gas is colorless, inflammable compound with the characteristic odor of rotten eggs. This is the most important gas formed from the standpoint of odors.
3.Methane (CH4)
Methane is the principal by-product of anaerobic decomposition of organic matter in wastewater
It is colorless, odorless, combustible hydrocarbon of high fuel value

Source: Lecture from
Introduction to Wastewater Treatment
Lesson 4