Wastewater Treatment: Primary and Secondary Processes
Primary Wastewater Treatment
Primary separation of finer suspended solids not separated in pretreatment.
1. pH Control
A preliminary step before precipitation, coagulation-flocculation, biological processes, or the final stage before discharge. The basic principle is: acid + base = salt + water. Different neutralization processes are used for wastewater:
- a) Acidic wastewater: Strong bases (CaO, MgO, NaOH) are added in a stirred tank.
- b) Basic wastewater: Mineral acids are added in a stirred tank.
- c) Multi-stage pH control: In a single stage, pH does not have a linear dependency with reactant concentration, especially around pH 7. pH variations can be very rapid. Wastewater flow rates can experience oscillations, and the volumes of reactants to mix with large flow rates are very small.
2. Coagulation-Flocculation
This process clarifies water by eliminating turbidity caused by suspended or colloidal matter. Colloids are stabilized by superficial electrostatic charges, which create repulsive forces that prevent agglomeration. Coagulation neutralizes the surface charge by adding electrolytes.
Key Wastewater Parameters
- BOD (Biochemical Oxygen Demand): A measure of the amount of oxygen consumed by microorganisms during the degradation of organic matter (OM) under aerobic conditions. This is a slow biological process. Highly pure water has a BOD5 < 3 mg O2, while contaminated water has a BOD5 > 8. Urban wastewater typically has a BOD5 between 100 and 400 mg O2, and some industries, like agri-food, can have values exceeding 10,000 mg O2.
- COD (Chemical Oxygen Demand): The amount of oxygen necessary to degrade organic and inorganic matter by chemical oxidation.
- Biodegradability: A high BOD5/COD ratio (close to 1) indicates high biodegradability, while a low ratio (close to 0) indicates low biodegradability. Urban wastewater typically has a BOD5/COD ratio between 0.4 and 0.8. Industrial wastewater can have ratios below 0.2 or above 0.2, depending on the specific industry and pollutants.
- pH: Values below 7 are acidic, and values above 7 are basic.
Primary Treatment Processes
Objective: Separation of finer solids that have not been separated in pre-treatments.
1. Clarification
Process: Makes water cleaner by removing turbidity caused by suspended matter (MS) and colloidal materials (small size, do not sediment by physical processes, cause turbidity and color).
- A) Coagulation: Agglomeration of colloidal particles by adding coagulants (electrolytes). Factors affecting coagulation include the type of coagulant, amount of coagulant, coagulation time, stirring rate, and temperature. This results in coagulated particles.
- B) Flocculation: Agglomeration of coagulated particles by chemical or mechanical processes. Flocculating agents include non-ionic organic polyelectrolytes, cationic, and anionic agents.
- C) Sedimentation: Separation by gravity of particles with a higher specific weight than water. There are four types of sedimentation, depending on the concentration or interaction between particles: discrete particles, flocculation, delayed or by zones, and compression.
2. Flotation
Small gas bubbles (usually air) are introduced to separate very small and light particles. Particles adhere to the bubbles and accumulate on the liquid surface, where they can be collected by superficial skimming. Bubbles can be added by dissolved air flotation, aeration, or vacuum. Design considers total flux with recirculation.
3. Gas Transfer
Transfer between phases, usually from gas to liquid. Analysis: There are two layers in the gas-liquid interface. Gases passing through phases are limited depending on gas solubility in the liquid.
- Gas Addition: The gas transfer rate is proportional to the difference between the existing concentration and the equilibrium concentration.
- Gas Desorption: Volatile Organic Compounds (VOCs) are present in a dissolved form and can be decreased by volatilization or gas dragging.
Secondary Wastewater Treatment
Objective: Remove, stabilize, or transform biodegradable organic matter in wastewater by sedimentation or biodegradation. Microorganisms are used.
Elimination mechanisms of organic matter:
- Physical adsorption (colloidal and biodegradable matter in suspension)
- Air dragging (VOCs)
- Biodegradation (aerobic, anaerobic) processes.
A) Aerobic Suspended Growth Systems
- A1) Aerated ponds (1-15 days retention time)
- A2) Stabilization tanks (40 days retention time)
- Advantages: Natural process, low-cost infrastructure, stabilized sludge.
- Disadvantages: Requires more area, potential for bad odors, performance dependent on season and weather.
B) Aerobic Fixed-Film Systems
Wastewater passes through a filter or porous material with microorganisms.
- B1) Trickling filters (for biodegradable and stable organic matter).
- B2) Rotating contact reactors: Applied to lower hydraulic loads. The turbulence created during air rotation enhances degradation.