Wastewater Characteristics and Treatment Considerations
Key Components in Wastewater
Sulfur
The sulfate ion is found naturally in most water supplies and in wastewater. Sulfur is required for the synthesis of protein and will be released later in the process of protein degradation. Sulfates are chemically reduced to sulfides and hydrogen sulfide (H2S) under anaerobic bacterial action. H2S released into the atmosphere in sewage systems that do not circulate under pressure tends to accumulate in the key of the pipes. The H2S can suffer cumulative biological oxidation to form sulfuric acid, which is corrosive to sewer pipes. Sulfates are reduced to sulfides in the sludge digesters and can alter the normal development of biological treatment processes if the concentration of sulfur exceeds 200 mg/L. However, these concentrations are rarely achieved. The mixture of gases from the sewer gas (CH4 + CO2) and the H2S that is clear is corrosive to the gas pipes. If subsequently burned in gas engines, the combustion products can cause damage to the engine, causing serious corrosion in the heat recovery circuit exhaust, especially if it could be cooled below the dew point.
Added Organic Compounds
Organic compounds added to wastewater vary widely. According to data obtained, the organic matter in wastewater is basically protein (40-60%), carbohydrates (25-50%), and fats and oils (8-12%), and small amounts of a large number of synthetic organic molecules.
Gases
For the proper operation of wastewater treatment systems, the determination of dissolved gases such as ammonia, carbon dioxide, hydrogen sulfide, and oxygen is very important. Measurements of dissolved oxygen and ammonia are made for the control and monitoring of aerobic biological treatment processes. Furthermore, the presence of hydrogen sulfide is determined not only because it is toxic and has a bad smell, but because its formation causes corrosion in concrete sewers.
Henry’s Law: Pg = H xg; where Pg is the partial pressure of the gas and xg is the mole fraction of the gas.
Biochemical Oxygen Demand (BOD)
BOD is the amount of oxygen used by microorganisms to carry out the reduction of organic matter. In the standard test for BOD, a small sample of wastewater is poured into a bottle (300 mL in volume). This bottle is filled to full volume using water saturated with oxygen and nutrients required for biological growth. Before the bottle stopper measures the concentration of oxygen. The bottle is incubated for 5 days at 20°C, and then the dissolved oxygen concentration is re-measured. The BOD is calculated as the difference in oxygen concentration, expressed in milligrams per liter, divided by the decimal fraction of the sample volume used.
Physical and Chemical Properties of Wastewater
Density
Density allows for the design of sedimentation tanks, constructed wetlands, and the vast majority of water treatment units.
Transmittance
This is defined as the capacity of a liquid to transmit light at a specific wavelength. It is defined as the intensity of the transmitted light after passing through a solution of known thickness and an initial intensity of incident light. (See also: Absorbance)
pH
pH is the expression to measure the concentration of hydrogen ions in a solution. This is defined as the negative logarithm of the hydrogen ion concentration.
Nitrogen (N) and Phosphorus (P)
N and P are the main nutrients of importance in the discharge of treated wastewater. Discharges containing N and P may accelerate the eutrophication of lakes, rivers, and reservoirs, and stimulate the growth of algae and rooted aquatic plants in shallow streams. High concentrations of ammonia in treated effluent may also have other negative effects, like the reduction of the concentration of dissolved oxygen in receiving waters and toxicity to aquatic life.
Alkalinity
This is defined as the ability of water to neutralize acids. In wastewater, alkalinity is due to the presence of hydroxides (OH-), carbonates (CO3 2-), and bicarbonates (HCO3-) of elements such as calcium, magnesium, sodium, potassium, or ammonium ions. These components are the result of the dissolution of minerals in the soil and atmosphere. Phosphates can also be caused by detergents in wastewater discharges and fertilizers and insecticides from agricultural lands. Hydrogen sulfate and ammonium are the product of microbial decomposition of organic material. It is noteworthy that calcium bicarbonate and magnesium bicarbonate are the most common constituents of alkalinity. In large quantities, alkalinity gives a bitter taste to water.
Chlorides
The increase in chloride in water may have different origins. If it is a coastal area, it may be due to the infiltration of seawater. In the case of an arid area, the increase of chlorides in water is due to the washing of soils caused by heavy rains. In the latter case, the increase of chloride may be due to water pollution from sewage. The chloride content of natural waters rarely exceeds 50-60 mg/L. The chloride content is not usually a problem for drinking water. A high content of chloride can damage pipes and metal structures and affect plant growth.