Study Notes on Wastewater Treatment and Physico-Chemical Processes

BSEN 5230/6230: Waste Management & Utilization

Overview

Engineering for Biosystems with a focus on Physico-chemical Processes for Separations in Wastewater Treatment.

Physico-Chemical Treatment Processes

These processes are crucial for the separation of solid-liquid phases in wastewater treatment. The treatment stages include:

• Primary Treatment
  • Focuses on the removal of larger particulates through physical and chemical processes.
• Secondary Treatment
  • Involves biological processes to reduce organic matter.
• Tertiary Treatment
  • Aims for further polishing of effluent and removal of residual contaminants.

Wastewater Treatment Components

• Influent – Incoming wastewater to the treatment plant.
• Effluent – Treated water leaving the treatment plant.
• Solids – Includes grit, debris, and grease & oil.
• BOD (Biochemical Oxygen Demand) – Measures organic matter decomposition capacity.
• COD (Chemical Oxygen Demand) – Measures total organic matter.
• OM (Organic Matter) – A significant component affecting treatment processes.
• Nutrients – Includes Nitrogen (N) and Phosphorus (P) which are essential for biological treatment.

Primary Treatment

Definition

The first step in wastewater treatment that typically includes physical-chemical processes to remove particulates and coarse materials.

Processes Involved

• Screening – Removal of large solids through bar racks or screens.
• Flotation – Separation of solids by introducing gas to form bubbles that attach to particulates.
• Flocculation – Aggregation of small particles into larger ones to enhance removal efficiency.
• Sedimentation – Gravity-based settling of particles in a tank.
• Sand Filtration – Further removes fine solids post-primary treatment.

Typical Physical Unit Processes for Wastewater Treatment

A summary of physical operations and their applications includes:

• Screening (Coarse/Fine) – Removal of debris and solids through varying screen types.
• Comminution – Reducing particle size before treatment.
• Flocculation – Promoting small particle aggregation.
• Sedimentation – Settling of solids in clarifiers.
• Flotation – Removal of oils and finer solids.

Typical Operations

Typically follow a batch reactor approach:

1. Fill
2. Hold & Settle
3. Separation & removal to further processing
4. Clarified effluent is transferred to the next unit process.

Screening (Preliminary Treatment)

Definition

Involves the removal of coarse suspended solids such as branches and plastic from the influent wastewater.

Types of Screens

• Inclined Bar Screen
• Rotary Drum Screen
  These screens typically remove materials larger than 0.7 mm.

Characterization of Screenings

Characteristics

Screenings are removed from wastewater through various screens, characterized by:

• Volume of Screenings
• Moisture Content
• Specific Weight

Example Data

Head Loss in Bar Screens

Head loss (HL) is a critical concern for bar screens and can be calculated using:
HL = rac{1}{C} rac{v_s^2 - v^2}{2g}
Where:

• $C$ = empirical discharge coefficient (0.7 for clean, 0.6 for clogged)
• $v_s$ = flow velocity through bar screen openings (m/s)
• $v$ = approach velocity in upstream channel (m/s)
• $g$ = acceleration due to gravity (9.81 m/s²)

Flotation

Definition

Used to remove oil, grease, and fine/light suspended solids from wastewater.

Process Overview

• Gas (typically air) is introduced into the wastewater, creating bubbles.
• Bubbles attach to particulates, allowing them to aggregate and rise to the surface for removal by mechanical skimmers.

Design Considerations for DAF (Dissolved Air Flotation)

• Conduct empirical tests to identify characteristics of particles.
• Important factors include:
  • Particulates concentration
  • Air quantity used
  • Particle rise velocity
  • Solids loading rate
• DAF performance greatly depends on the air to solids ratio (A/S):
  A/S = rac{air ext{ (mL)}}{solids ext{ (mg)}}
  Where:
• $s_a$ = air solubility in mL/L
• $f$ = fraction of air dissolved at pressure $P$ (usually 0.5)
• The pressure formula used is:
  $rac{(p + 101.35)}{101.35}$
  Where $p$ is the gauge pressure in kPa.

Sedimentation (Settling)

Definition

The process by which suspended solids are removed through gravity settling in a tank or basin called a clarifier.

• Functionality can be used in primary, secondary, or tertiary treatment stages.

Types of Settling Processes

1. Discrete Settling – Settling of dilute suspensions without particle interactions.
2. Flocculent Settling – Particles aggregate into larger flocs, improving their settling.
3. Zone Settling – A dense blanket of flocculated particles and clear water interface forms.
4. Compression Zone Settling – Occurs in clarifiers and sludge thickening zones.

Calculating Settling Velocity

Stokes' Law

The terminal settling velocity $vt$ is given by: vt = rac{g(p_2 - p)d^2}{18ar{
u}}
Where:

• $g$ = acceleration due to gravity
• $p$ = density of the settling particle
• $P$ = density of water
• $d$ = diameter of the particle
• $ar{
  u}$ = dynamic viscosity of the fluid

Critical Velocity

Critical velocity is assessed under the assumptions that particles behave ideally without resuspension. The fate of particles relies on their average velocity relative to the terminal velocity.

Coagulation and Flocculation

Definition

This process destabilizes colloidal dispersions utilizing chemicals or hydrodynamics to enhance particle aggregation.

Steps Involved

1. Hydrodynamic Mixing (Rapid Mixing) – Ensures particle contact and collision.
2. Slow Stirring (Flocculation) – Allows stabilized particles to agglomerate and form larger aggregates for easier removal.

Zeta Potential

• Importance: Indicates stability of colloids; higher values imply lesser interaction and potential to flocculate.

Coagulants Used

• Metal Coagulants: Such as aluminum and iron-based compounds.
• Polymers: Both natural and synthetic varieties can aid in particle bridging and removal efficiencies.

Conclusion

Many technologies and techniques exist for the removal of suspended solids from wastewater, primarily relying on an understanding of flow dynamics and chemical interactions to optimize treatment processes.