Wastewater Treatment: Principles, Contaminants, and Primary Treatment Processes

Introduction to Wastewater Treatment

Objectives of Wastewater Treatment

• Understand the goals of wastewater treatment and its individual processes, including the removal of contaminants and protection of public health and the environment.

• Comprehend the operation of pretreatment and primary treatment stages in municipal and industrial wastewater management.

• Recognize methods for sludge treatment and disposal as a critical aspect of wastewater management.

Wastewater Contaminants

Types and Effects of Contaminants

• Suspended solids: Can cause sludge deposits and anaerobic conditions in receiving waters.

• Biodegradable organics: Lead to oxygen depletion and anaerobic conditions.

• Pathogens: Microorganisms that transmit diseases.

• Nutrients (e.g., nitrogen, phosphorus): Cause eutrophication, leading to excessive algal growth.

• Heavy metals: Toxic to aquatic life and humans.

• Refractory organics: Resistant to biodegradation; toxic to biota and humans.

• Dissolved solids: Interfere with water reuse and industrial processes.

Domestic Wastewater Composition

Typical Characteristics of Untreated Domestic Wastewater

The following table summarizes the typical composition of untreated domestic wastewater, categorized by strength:

Constituent	Weak	Medium	Strong
Alkalinity (as CaCO3)	50	100	200
BOD5 (as O2)	100	200	300
Chloride	35	50	100
COD (as O2)	250	500	1000
Suspended solids (SS)	100	200	350
Settleable solids (in mL·L-1)	5	10	20
Total dissolved solids (TDS)	200	500	1000
Total Kjeldahl nitrogen (TKN as N)	20	40	85
Total organic carbon (TOC as C)	75	100	300
Total phosphorus (as P)	3	7	12

Additional info: BOD5 (Biochemical Oxygen Demand over 5 days) is a key indicator of organic pollution.

Municipal Wastewater Treatment Processes

Stages of Treatment

• Pretreatment: Removes large debris and materials that can cause operational problems. Equalization may be used to balance flow rates.

• Primary treatment: Removes approximately 60% of solids and 35% of BOD.

• Secondary treatment: Removes about 85% of BOD and solids, typically using biological processes.

• Advanced (tertiary) treatment: Achieves 95%+ removal of BOD, solids, nitrogen, and phosphorus as needed.

Example: A typical municipal wastewater treatment plant includes screening, grit removal, primary sedimentation, biological treatment, and disinfection.

Industrial Pretreatment

Purpose and Regulation

• Industrial wastewaters must be pretreated before discharge to municipal systems to remove materials not handled by municipal treatment.

• Local authorities monitor and regulate industrial discharges.

• Pretreatment requirements are set by the U.S. EPA to protect municipal systems and the environment.

Pretreatment Equipment

Bar Racks and Grinders

• Purpose: Remove and grind large objects from influent wastewater.

• Collected solids are stored and sent to landfill.

• Cleaning can be mechanical or manual.

Grit Chambers

• Purpose: Remove inert, dense materials such as sand, silt, and glass to prevent abrasion of pumps and equipment.

• Material removed is termed "grit".

• Types include aerated and vortex grit chambers.

Physical Principles: Discrete Settling

Settling of Particles in Water

• Particles are characterized by density ($\rho_p$), volume ($V_p$), mass ($m_p$), and cross-sectional area ($A$).

• Settling velocity ($v_s$) is determined by balancing gravitational, buoyant, and drag forces.

• For laminar flow (Re < 1), the settling velocity is given by:

$v_s = \frac{g (\rho_p - \rho_w) d_p^2}{18 \mu}$

• where $\mu$ is the dynamic viscosity of water, $\rho_w$ is the density of water, $d_p$ is particle diameter, and $g$ is acceleration due to gravity.

Example: Grit Chamber Calculation

• Given: Particle radius = 0.04 mm, specific gravity = 2.65, chamber length = 13.5 m, flow = 0.15 m3/s, width = 0.56 m, velocity = 0.25 m/s, temperature = 22°C.

• Density of water = 997.774 kg/m3, dynamic viscosity = 0.955 mPa·s.

• Use the settling velocity equation to determine if the particle will be captured.

Example: Grit Chamber Design

• Design for sand particles ($\rho_p = 2650$ kg/m3), diameter = 0.21 mm, flow = 10,000 m3/d, velocity = 0.3 m/s, depth = 1.5 × width.

• Calculate settling velocity:

$v_s = \frac{g (\rho_p - \rho_w) d^2}{18 \mu}$

$v_s = 0.039 \frac{m}{s}$

• Calculate cross-sectional area:

$A_x = \frac{Q}{v}$

$A_x = 0.39 \ m^2$

• Calculate width and depth:

$A_x = W \times D = W \times 1.5W = 1.5W^2$

$W = \sqrt{\frac{A_x}{1.5}} = 0.51 \ m$

$D = W \times 1.5 = 0.76 \ m$

• Calculate detention time and required length:

$t_d = \frac{D}{v_s} = 19.4 \ s$

$L = t_d \times v = 5.8 \ m$

• Final tank dimensions: W = 0.51 m, D = 0.76 m, L = 5.8 m.

Primary Settling Basins

Design and Operation

• Primary settling basins (sedimentation tanks) remove settleable solids and reduce BOD.

• Key parameters: detention time, overflow rate, weir loading rate.

Example: Primary Sedimentation Tank Sizing

• Given: Flow = 0.150 m3/s, Length = 40 m, Width = 10 m, Depth = 2 m, Weir length = 75 m.

• Detention time:

$t_o = \frac{V}{Q} = \frac{40.0 \ m \times 10.0 \ m \times 2.0 \ m}{0.150 \ m^3/s} = 5333 \ s \approx 1.5 \ h$

• Overflow rate:

$v_o = \frac{Q}{A} = \frac{0.150 \ m^3/s \times 86,400 \ s/d}{40.0 \ m \times 10.0 \ m} = 32 \ m^3/(m^2 \cdot d)$

• Weir loading rate:

$WLR = \frac{Q \times 86,400 \ s/d}{75.0 \ m} = 173 \ m^3/(m \cdot d)$

• All values are within acceptable design ranges.

Summary of Key Learning Outcomes

• List BOD values for strong, medium, and weak municipal waste.

• Explain differences among pretreatment, primary, secondary, and tertiary treatment.

• Determine grit particle capture in velocity-controlled chambers.

• Evaluate or size sedimentation tanks for detention time, overflow rate, and weir loading.

Additional info: These principles are foundational for environmental and wastewater engineering, and are closely related to aqueous solution chemistry, stoichiometry, and physical chemistry concepts in General Chemistry.