Wastewater Treatment and Environmental Engineering Study Guide

Introduction to Wastewater Treatment

This material is provided by Engr. Elisa G. Eleazar from the School of Chemical, Biological, and Materials Engineering and Sciences. It serves as a comprehensive guide to wastewater infrastructure, constituents, and the multifarious stages of treatment.

Wastewater Infrastructure and Purpose

Domestic wastewater is a complex mixture whose components are categorized based on their source:

• Wastewater sources: These include domestic, commercial, and industrial users.
• Stormwater runoff: Water resulting from precipitation that enters the system.
• Infiltration: Groundwater that enters the wastewater system through leaks in pipes or joints.
• Inflow: Water that enters the system through direct connections such as manhole covers or drains.

Purpose of Wastewater Treatment:

1. Protect Human Health: By removing pathogens and toxic substances.
2. Prevent Pollution: Mitigating the contamination of surface water and groundwater resources.

Constituents of Wastewater

The composition of wastewater determines the degree of treatment required. Key constituents include:

• Organic Matter: If left untreated, it can significantly deplete the oxygen content of receiving waters, leading to hypoxia.
• Suspended Solids (SS): These cause turbidity in water and can serve as carriers for organic matter, other pollutants, or pathogens.
• Pathogens: Microorganisms (bacteria, viruses, etc.) that cause diseases.
• Nutrients: Primarily Nitrogen and Phosphorus. These accelerate eutrophication and can contribute to Nitrogenous Biochemical Oxygen Demand (NBOD).
• Toxic Chemicals: This category includes heavy metals and various organic chemicals.
• Emerging Pollutants: These are modern contaminants including pharmaceuticals, surfactants, personal care products, and endocrine-disrupting chemicals.

Preliminary Treatment

The primary objective of preliminary treatment is to prepare the wastewater for subsequent treatment stages, protect mechanical equipment from damage, and balance the loading of the plant.

• Screening: The removal of large materials to prevent clogging and damage to downstream pumps and sensors.
• Comminution: A process of grinding solids into smaller pieces, typically approximately $0.3\,cm$ in size.
• Grit Removal: This step is essential to prevent the abrasion of piping and various mechanical equipment components.
• Flotation: This process utilizes buoyancy to separate and remove fats, oils, and grease (FOG).

Flow Equalization

Flow equalization is employed to dampen the flow and organic loading rate, overcoming problems associated with large variations in flow over a $24$-hour period.

• In-Line Equalization: All of the influent flow passes directly through the equalization basin.
• Off-Line Equalization: Only a portion of the flow (the excess during peak times) is diverted to the basin.

Primary Treatment

Primary treatment focuses on the physical removal of solids by gravity settling.

Removal Efficiencies:

• $60\%$ of Suspended Solids (SS)
• $30\%$ of Biochemical Oxygen Demand (BOD)
• $20\%$ of Phosphorus ($P$)

Sample Problem 01 (SP 01)

Scenario: A municipal wastewater treatment plant treats an average flow of $12,000\,m^3/day$ and a peak hourly flow of $30,000\,m^3/day$. Two circular clarifiers are designed with a depth of $4\,m$ and an overflow rate of $40\,m^3/m^2\text{-day}$.

Required: Calculate the area, diameter, volume, and detention time for each clarifier.

Secondary Treatment: Biological Processes

The fundamental purpose of secondary treatment is the removal of organic matter using biological reactors. These are generally classified into two systems:

1. Attached Growth (Fixed Film) Reactors: Microorganisms are attached to a surface (e.g., trickling filters).
2. Suspended Growth Reactors: Microorganisms are maintained in suspension within the liquid.

Bacterial Growth Requirements

For effective biological treatment, bacteria require several key inputs:

• Electron Acceptor: (e.g., oxygen for aerobic processes).
• Macronutrients: Carbon ($C$), Nitrogen ($N$), and Phosphorus ($P$).
• Micronutrients: Trace metals and vitamins.
• Environment: Adequate moisture, specific temperature ranges, and stable $pH$.

Kinetics of Suspended Growth (Monod Kinetics)

In the exponential growth phase, bacterial growth is governed by nutrient availability and predator-prey relationships.

Growth Equation:$\frac{dX}{dt} = \mu X$

• $X$: Concentration of biomass.
• $\mu$: Instantaneous growth rate.
• $t$: Time.

Specific Growth Rate ($\mu$):$\mu = \frac{\hat{\mu} S}{K_s + S}$

• $\hat{\mu}$: Maximum growth rate constant.
• $S$: Concentration of limiting food (substrate).
• $K_s$: Half saturation constant (concentration when $\mu = 0.5\hat{\mu}$).

Yield Coefficient ($Y$):$Y = \frac{\text{mass microorganisms produced}}{\text{mass substrate consumed}}$$\frac{dX}{dt} = Y \frac{dS}{dt}$$\frac{dS}{dt} = \frac{X}{Y} \frac{\hat{\mu} S}{K_s + S}$

Reactor Design and Operational Parameters

Suspended Growth Reactor without Recycle

• Liquid Retention Time ($\bar{t}$):$\bar{t} = \frac{V}{Q}$

• $V$: Reactor volume.

• $Q$: Flow rate.

• Solids Retention Time ($\theta_c$) (Sludge Age/Mean Cell Detention Time):$\theta_c = \frac{\text{mass microorganisms in the system}}{\text{mass solids wasted per unit time}}$$V = \frac{V X}{Y \frac{dS}{dt}}$$\frac{1}{\theta_c} = \frac{\hat{\mu} S}{K_s + S} = \mu$$S = \frac{K_s}{\hat{\mu} \theta_{c} - 1}$

Sample Problem 02 (SP 02)

Scenario: A biological reactor with no solids recycle operates to reduce influent BOD from $600\,mg/L$ to $10\,mg/L$. Kinetic constants: $K_s = 500\,mg/L$ and $\hat{\mu} = 4/d$. Flow is $3\,m^3/day$.

Required: Calculate the required volume of the reactor.

The Activated Sludge System

This system involves the recycling of biological solids. Key terms include:

• Mixed Liquor: The mixture of wastewater and biological mass in the aeration basin.
• MLSS: Mixed Liquor Suspended Solids.
• WAS: Waste Activated Sludge (removed from the system).
• RAS: Return Activated Sludge (recycled to the aeration basin).

Standard Operating Assumptions:

• Influent biomass concentration ($X_0$) is $0$.
• Steady-state conditions.
• Perfect mixing in the reactor.
• No substrate removal occurs in the settling tank.
• The settling tank has no volume.

Sample Problem 03 (SP 03)

Scenario: An activated sludge system uses an $8\,million\text{-}L$ aeration basin. $\theta_c = 12\,days$. MLSS = $3,100\,mg/L$. RAS = $11,000\,mg/L$.

Required: Find the WAS rate if sludge is wasted from (a) the aeration basin and (b) the recycle line.

Efficiency and Loading Factors

• Substrate Removal Velocity ($q$):$q = \frac{\text{mass substrate removed per unit time}}{\text{mass microorganisms under aeration}}$$q = \frac{S_0 - S}{X \bar{t}}$$q = \frac{\mu}{Y} = \frac{\hat{\mu} S}{Y(K_s + S)} = \frac{1}{\theta_c Y}$

• Food-to-Microorganism Ratio ($F/M$):$F/M = \frac{Q S_0}{V X} = \frac{S_0}{\bar{t} X}$

Sample Problem 04 (SP 04)

Scenario: Flow = $400\,m^3/d$. Influent BOD ($S_0$) = $300\,mg/L$. Kinetic constants: $Y = 0.5\,kg\,SS/kg\,BOD$, $K_s = 200\,mg/L$, $\hat{\mu} = 2/d$. Aeration tank MLSS ($X$) = $4,000\,mg/L$. Effluent BOD ($S$) = $30\,mg/L$.

Required: Determine the volume of the aeration tank, the sludge age ($\theta_c$), and the $F/M$ ratio.

Tertiary Treatment

Tertiary treatment targets constituents remaining after secondary treatment, specifically nutrients and remaining solids.

Nitrogen Removal (Nitrification and Denitrification)

1. Nitrification (Aerobic):
   • Step 1 (Nitrosomonas): $2NH_4^{+} + 3O_2 \rightarrow 2NO_2^{-} + 2H_2O + 4H^{+}$
   • Step 2 (Nitrobacter): $2NO_2^{-} + O_2 \rightarrow 2NO_3^{-}$
2. Denitrification (Anaerobic):
   • Accomplished by Pseudomonas: $2NO_3^{-} + \text{organic matter} \rightarrow N_2 + CO_2 + H_2O$

The Modified Ludzak–Ettinger (MLE) Process is a common configuration used for this purpose.

Phosphorus Removal

Phosphorus is removed through chemical precipitation: $Al^{3+} + PO_4^{3-} \rightarrow AlPO_4$ This can also be integrated into the activated sludge process using anaerobic and aerobic reactor stages.

Further Solids and Organic Removal

• Oxidation Ponds: Utilize a symbiotic relationship between algae (producing $O_2$ via sunlight/$CO_2$) and aerobic bacteria (using $O_2$ to break down organics into $CO_2$ and $H_2O$). Anaerobic bacteria at the bottom break down organics into $CH_4$, $NH_3$, and $CO_2$.
• Wetlands: Natural or engineered systems used to filter water through soil and plant roots.

Sludge Treatment and Disposal

Raw sludge is odoriferous, water-rich, and pathogenic. Treatment is essential for stabilization before disposal.

Stages of Sludge Treatment

1. Stabilization: Objective is to remove odors and pathogens.
   • Lime addition: Increases $pH$ to kill pathogens.
   • Anaerobic digestion: Breaks down organic matter in the absence of oxygen.
2. Dewatering: Reducing water content to transform liquid sludge into a solid "cake."
   • Sand Beds: Natural drying via drainage (through sand and gravel) and evaporation.
   • Belt Filter: Mechanical compression using belts and polymers.
   • Centrifuge: Using centrifugal force to separate solids (cake) from liquids (centrate).
3. Disposal: The final step to remove the stabilized and dewatered sludge from the site.