Lecture 6: Wastewater Treatment and Reuse

Quiz Two Announcement

Date & Time: Next week, during usual lecture time ( $4:30 \text{ PM} \text{ to } 5:15 \text{ PM}$ , $45$ minutes).
Location: Here.
Arrival: Arrive a little early.
Format: Multiple Choice Questions (MCQs).
Scope: Lectures E4 to E6.
- E4 L was by PSA.
- E5 and E6 were by Rachel.

Materials:
- Bring a calculator for simple calculations (addition, multiplication). If unavailable, calculations can be done manually.
- Bring a pen (paper will be provided).

Attendance: Mandatory.
Answers: Will be announced in the next lecture by Prof SAC.

Introduction to Wastewater

Recap (Last Week): Focused on water treatment (purification of raw water from lakes, rivers, groundwater to drinking water standards).
This Week's Focus: "Season 2" - what happens to water after consumption and flushing, i.e., wastewater.
Comparison to Water Treatment: Structure is similar but with distinct differences in treatment principles and processes.
Lecture Objectives:
- Understand wastewater constituents.
- Identify the impacts of wastewater.
- Learn how to control household wastewater.
- Examine wastewater treatment principles and processes.
- Explore wastewater reuse strategies.

Wastewater Constituents

Wastewater contains a variety of constituents, notably organic matter.
Domestic Wastewater Terminology (frequently used):
- Gray Water: Water not contaminated by fecal or human waste matter.
  - Sources: Washing machines, taps, shower, bathroom tubs.
  - Heavy Gray Water: Contains many impurities like soap, detergents, chemicals (e.g., from washing machines).
  - Light Gray Water: Perceived as more pure (e.g., from showers, bathroom tubs).
- Yellow Water: Urine.
- Brown Water: Urine + feces + water.
- Black Water: The whole thing (all domestic wastewater).
Component Comparison in Different Fractions: (Per liter per person per day, flush included)
- Pathogens:
  - Highest in feces (human organic matter).
  - Low in urine and gray water.
  - Analogy: Kidneys act as an internal wastewater treatment system, filtering out materials and pathogens in urine.
- Organic Matter:
  - High in feces.
  - Can be high in gray water from skin particles (during showering) and detergents.
    - Soaps: Contain alkali salts with long-chain fatty acids (organic matter contributors).
    - Detergents: Contain phosphorus, sodium salts, bleaches (some are organic).
- Phosphorus and Nitrogen:
  - Present in almost every fraction (feces, urine, gray water).
  - Are valuable nutrients, but improper treatment leads to environmental issues (e.g., eutrophication).
- Heavy Metals:
  - Present in feces and gray water.
  - Negligible in urine.
- Organic Toxic Compounds:
  - Mostly present in gray water.
  - Sources: Cosmetics, personal care items, food packaging.
  - Impact: Can cause endocrine disruption.
  - E.g., Chemotherapy drugs (cytotoxic, can damage cells even after metabolism; advisable for cancer patients to use dedicated toilets).
- Pharmaceuticals:
  - More present in feces.

Impact of Wastewater Components

Each component has a distinct impact:
- Pathogens: Cause infections.
- Organic Matter, Phosphorus, Nitrogen:
  - Deplete oxygen.
  - Cause bacterial growth.
  - Lead to eutrophication.
- Heavy Metals: Lead, cadmium, mercury are toxic.
- Organic Toxic Compounds: Toxic.
- Pharmaceuticals: Can be toxic to aquatic life and humans.

Pathogens

Sources:
- Domestic: Feces (major contributor), urine, gray water (can contribute).
- Non-domestic: Industrial sources, public facilities.
  - Industry: Abattoirs (animal farming), food industry.
  - Research Example: Chicken manure from egg farms in Singapore is a significant waste problem due to high organic matter and potential pathogens.
- Environmental: Run-off, animals.
Emerging Pathogens: Newly identified bacteria/viruses contaminating wastewater.
- Zoonoses: Diseases transmitted from animals to humans (e.g., bird flu). COVID-19 is an example, though not typically transmitted via water systems.
- Rotavirus: Common virus causing severe diarrhea, potentially fatal.

Eutrophication

Etymology: Greek "eu" (good) + "trophic" (nutrients) = "good nutrients"; ironically, it describes negative effects from excess nutrients.
Mechanism:
1. Excess nutrients (phosphorus, nitrogen) in water bodies.
2. Promotes rapid growth ("blooms") of phytoplankton or algae.
3. Algae form a dense mat on the water surface.
4. The mat blocks sunlight, preventing photosynthesis by submerged aquatic plants.
5. Algae and other organisms respire, consuming large amounts of dissolved oxygen ( $\text{O}_2$ ).
6. When algae die, bacteria decompose the decaying organic matter, further depleting oxygen.
7. Results in anoxic (low oxygen) conditions.
8. Fish and other aquatic life suffocate and die (fish kills).
Real-life Examples:
- Duckweed (a pest-like plant, not true algae) growth.
- Filamentous algae in the US.
- Cyanobacteria (blue-green algae) in Ireland.
Toxins: Some algal blooms produce toxins that directly kill fish.
Global Spread: Fish kills due to algal blooms are a worldwide issue.

General Impacts of Wastewater

Unpleasant odor.
Health hazards (pathogens, toxic compounds).
Environmental pollution (surface water, groundwater) if untreated.
Mosquito breeding (e.g., Aedes mosquito hotspots).

Opportunities from Wastewater

Reduces Water Shortage: Wastewater reuse can significantly address global water scarcity.
Reduces Environmental Degradation: Proper treatment prevents eutrophication and health hazards.
Reclaims Nutrients: Phosphorus and nitrogen can be extracted and potentially used as fertilizers.
Maintains Water Quality: Treated wastewater protects surface and groundwater quality.
Singapore Example: Treats $100\%$ of its generated wastewater, showcasing leadership in this area.

Wastewater Control

Starting at the Source:
- Minimize water use in households, industries, public institutions.
- Reduce contamination by chemicals, oil, particles, fats, and excretes.
Plumbing and Pipe Systems: Leads to pretreatment stages.
- Septic Tanks: Often equipped with grease filters, screens to prevent large particles from reaching later stages.
Treatment: Subsequent processes to purify wastewater.

Control in Households

Minimize Water Use:
- Use as little water as possible (e.g., mimicking showering with a bucket teaches conservation).
- Take quick showers instead of long soaks.
- Mend leaking pipes.
- Use full loads for washing machines.
- Reduce washing under running taps.
Reduce Solid Matter Addition:
- Dispose of leftover food in the bin, not the toilet (complicates treatment).
- Do not flush toothpicks, wet wipes, etc.
Avoid Chemicals and Oils:
- Dispose of cooking oil in the trash bin, not down the drain (creats "grease balls" that float and need removal).
- Use biodegradable soaps where possible.
- Do not flush paint, medicines, or other chemicals down the toilet.
- Use environmentally friendly home products.

Control in Industries

Chemical Substitution: Replace problematic chemicals with more biodegradable alternatives.
- Consider products with fast degradation rates and no harmful effects (balancing efficiency and sustainability).
Product Choices: For example, use non-polluting car wash products.
Regulations: Establish and enforce regulations for industrial discharge.

Wastewater Treatment: Physical Characteristics

Wastewater can be characterized by physical, chemical, and biological properties.
- Physical: Solids, temperature, pH, color, absorbance, odor.
- Chemical: Organic and inorganic compounds.
- Biological: Organisms (bacteria, viruses, protozoa).

Solids

Importance: Critical characteristic; typically, solids content is small ( $0.1\%$ ), but their removal is a main treatment goal.
Types of Solids:
- Total Dissolved Solids (TDS): Materials dissolved in water that pass through a filter.
- Total Suspended Solids (TSS): Particles suspended in water that are retained by a filter (residue).
- Total Solids: Comprises both dissolved and suspended solids.
  - Equation 1: $\text{Total Solids} = \text{TDS} + \text{TSS}$
- Volatile Solids: Organic solids that burn off at $500 \text{ }^\circ\text{C}$ (standard temperature).
- Fixed Solids: Inorganic solids remaining after burning at $500 \text{ }^\circ\text{C}$ .
- Total Solids: Comprises both volatile and fixed solids.
  - Equation 2: $\text{Total Solids} = \text{Volatile Solids} + \text{Fixed Solids}$
- Note: Components can exist in different categories across these two classification systems (e.g., dissolved solids can be volatile or fixed).

Temperature

Importance: Influences biological activity crucial for many wastewater treatment processes.
Optimum Temperature: For biological activity is $25 \text{ to } 35 \text{ }^\circ\text{C}$ .
Effects of Deviations:
- Too High Temperature: Can cause cell lysis (breaking down) and denaturation of proteins in microorganisms, leading to cell death.
- Too Low Temperature: Reduces microbial activity, hindering treatment processes.
- Examples: Aerobic digestion and nitrification (microbial processes) largely stop at $50 \text{ }^\circ\text{C}$ (not $50$ degrees Fahrenheit). Specific bacteria like methanogens, nitrifying bacteria, heterotrophic bacteria are affected at different temperatures.

Color

Indicator of Oxygen Levels:
- Light brownish-gray: Indicates sufficient oxygen.
- Darker colors, eventually black: Indicates low oxygen conditions (septic), due to metallic sulfides formation.
Time Dependence: The color darkens with increasing residence time in the treatment plant.

Wastewater Treatment: Chemical Characteristics

Organic Matter

Definition: Compounds primarily composed of carbon, hydrogen, oxygen, andsometimes nitrogen.
Sources in Wastewater: Similar to nutrients consumed by humans (proteins, carbohydrates, oils, fats), including urea from urine.
Forms:
- Aggregate Organic Matter: Clumped together, visually indistinct.
- Individual Organic Compounds: Specific molecules like Volatile Organic Compounds (VOCs), Disinfection By-Products (DBPs), pesticides, insecticides.
  - DBPs: Formed during chlorination; can be harmful (discussed in water treatment).

Nitrogen

Importance:
- Essential for microorganism growth (proteins).
- Improperly treated nitrogen causes eutrophication.
Forms:
- $\text{Ammonia (NH}_3)$
- $\text{Ammonium (NH}_4^+)$
- $\text{Nitrogen gas (N}_2)$ (not very soluble in water, typically in small amounts).
- $\text{Nitrite (NO}_2^-)$
- $\text{Nitrate (NO}_3^-)$
- Organic Nitrogen
Classification:
- Total Nitrogen: Can be divided into Kjeldahl Nitrogen and Inorganic Nitrogen.
  - $\text{Total Nitrogen} = \text{Kjeldahl Nitrogen (TKN)} + \text{Inorganic Nitrogen}$
  - Kjeldahl Nitrogen (TKN): Represents the sum of organic nitrogen, ammonia ( $\text{NH}<em>3$ ), and ammonium ( $\text{NH}</em>4^+$ ) nitrogen.
  - Inorganic Nitrogen: Comprises nitrite ( $\text{NO}<em>2^-$ ) and nitrate ( $\text{NO}</em>3^-$ ) nitrogen.

Phosphorus

Importance:
- Essential for microorganism growth.
- Improperly controlled phosphorus leads to eutrophication in lakes and reservoirs.
Forms: Organic phosphates, polyphosphates, and orthophosphates.

Wastewater Treatment: Principles and Processes

Utilizes physical, chemical, and biological methods.

Physical Methods (for larger particles)

Filtration: Uses filter media to remove larger particles.
- Partially Unsaturated Flow: Water flows through media with some air spaces.
- Saturated Flow: All spaces in filter media are occupied by water.
Flotation and Sedimentation: Occurs in tanks with baffles to guide flow.
- Sedimentation: Heavier particles settle to the bottom.
- Flotation: Lighter particles (fats, oils, greases) float to the top.
Screening: Removes coarse materials to protect downstream equipment.
- Coarse Screens/Bar Racks: Openings of $6 \text{ to } 150 \text{ mm}$ .
- Fine Screens: Openings smaller than $6 \text{ mm}$ .
- Micro Screens: Openings smaller than $50 \text{ }\mu\text{m}$ (used for effluents).
Membrane Filtration (e.g., Microfiltration): Uses manufactured porous materials/membranes where pressure is applied to force water through, trapping particles.

Chemical Methods (for smaller particles)

Coagulation: Destabilizes colloids (floating particles) by adding positively charged chemicals.
- Mechanism: Adds polyvalent metal salts (e.g., aluminum, ferric trivalent salts like $\text{Al}^{3+}$ and $\text{Fe}^{3+}$ ) to neutralize negatively charged particles.
- Steps:
  1. Coagulant Addition: Chemicals added and mixed well.
  2. Flocculation: Destabilized particles clump together to form larger, heavier aggregates called "floc."
  3. Sedimentation: Floc settles to the bottom.
Chemical Precipitation: Similar to coagulation, but aims for higher removal percentages.
- Purpose: Removes Total Suspended Solids (TSS), phosphorus, heavy metals, Biological Oxygen Demand (BOD), and bacteria.
  - Effectiveness: Achieves $80-90\%$ TSS removal (vs. $50-70\%$ without it).
  - Heavy Metals: Arsenic, barium, cadmium, copper, mercury, etc. (Can cause severe health issues like chronic kidney disease; requires careful monitoring in lab settings).
- pH Importance: Critical for effective precipitation.
Disinfection: Kills or inactivates microorganisms and maintains a residual to prevent regrowth in distribution.
- Chlorination:
  - Advantages: Low cost, highly effective (historically reduced typhoid deaths).
  - Disadvantages: Forms harmful Disinfection By-Products (DBPs).
- UV Light Radiation:
  - Mechanism: UV photons damage the genetic structure (DNA) of bacteria, viruses, and other pathogens, preventing reproduction.
  - Advantages: No chemicals, natural water taste, no DBPs.
  - Disadvantages: Hard to maintain UV lamps, costly.
- Ozonation:
  - Mechanism: Oxidizes pathogenic microorganisms.
  - Advantages: Safer than chlorination, fewer by-products.
  - Disadvantages: Costly.

Biological Methods (mainly for organic treatment)

Objective:
- Transform dissolved and particulate organic matter into acceptable end products.
- Capture suspended and colloidal materials into flocs or biofilms.
- Remove nitrogen and phosphorus, preventing eutrophication.
Mechanism:
- "Good" microorganisms (bacteria) use organic matter and other nutrients (with oxygen) for metabolism.
- They reproduce, forming new cells (biomass), and produce carbon dioxide ( $\text{CO}2$ ) and water ( $\text{H}2\text{O}$ ).
- Biomass has a higher density than water, allowing it to settle out and be separated from treated wastewater.
Sludge & Scum:
- Sludge: Heavier particles that settle at the bottom during biological treatment.
  - Singapore Sludge Treatment: Incinerated, and ash sent to Semakau landfill (sustainability challenge).
  - Research Avenues: Gasification for energy generation, fertilizer production.
- Scum: Lighter particles that float to the top.
Types of Biological Processes:
- Aerobic Processes (with air/oxygen): Microorganisms thrive in oxygen-rich environments, breaking down organic matter into $\text{CO}_2$ and water.
- Anaerobic Processes (without air/oxygen): Microorganisms survive without oxygen, using other compounds for metabolism, often producing biogas (e.g., methane) which can be used for energy.
Anaerobic Reactors:
- Upflow Anaerobic Sludge Blanket (UASB) Reactor: Cylindrical reactors where wastewater flows upwards through a sludge blanket, generating biogas.
- Anaerobic Baffled Reactor (ABR): Tanks with internal baffles that increase wastewater residence time and create multiple reaction chambers for anaerobic treatment.
Aerobic Processes: Conventional Systems:
- Activated Sludge Process:
  - Mechanism: Involves a reactor where activated sludge (rich in diverse microbes) is mixed with wastewater and aerated (oxygen added). Microorganisms use organics to multiply, forming biomass floc.
  - Components: Influent wastewater, return activated sludge (from system), aeration, secondary clarifier.
  - Products: Waste activated sludge, treated effluent.
  - Problem: High energy consumption due to aeration.
  - Research Example: Using microalgae to reduce oxygen demand.
- Constructed Wetlands: Natural-based solution (e.g., Nanyang Lake, Yunnan Garden).
  - Mechanism: Plants grow on a porous media (sand, soil, gravel). Plants and soil trap contaminants and uptake pollutants.
  - Type: Horizontal subsurface flow wetlands.

Overview of Wastewater Treatment Processes (General Flow)

Preliminary Treatment: Screens, grit chambers, settling/flotation to remove large particles (rags, grit, grease balls).
Primary Treatment: Clarifiers reduce flow speed to allow suspended solids and organic matter to settle (primary sludge) or float (scum). Not a complete removal.
- Sludge handling facilities pumped from primary clarifiers.
- Can sometimes be bypassed, with secondary treatment handling all organic removal.
Secondary Treatment: Focuses on removing organic materials using biological methods (aerobic or anaerobic).
Advanced or Tertiary Treatment: Removes hard-to-treat materials (e.g., nasty organic compounds).
Disinfection: Kills remaining microorganisms.
Product Water:

Order Rationale: Disinfection is at the end because large particles and high organic load at the beginning would hinder disinfectant effectiveness and increase chemical demand.

Wastewater Reuse

Motivation:
- Water Shortage: Crucial for water-stressed nations (like Singapore).
- Global Context: Globally, $10\%$ of the world's population may consume wastewater-irrigated foods; millions of hectares are irrigated with raw or partially treated wastewater.
Advantages:
- Reduces wastewater treatment costs.
- Improves agricultural production, reducing reliance on chemical fertilizers.
Drawbacks of Direct Reuse (untreated/partially treated):
- Pathogens (health hazards).
- Heavy metals.
- Requires comprehensive hazard identification, dose-response assessment (e.g., $1 \text{ mg/L}$ heavy metal response), exposure assessment, risk characterization, and risk management.
Purification Steps for Reuse: Highly complex, often involving advanced processes like reverse osmosis, air stripping, chlorination, and UV radiation.
Applications of Reused Wastewater:
- Agriculture and Landscape Irrigation (treated wastewater).
- Industrial Recycling and Reuse: Cooling water, boiler feed.
- Groundwater Recharge: Replenishing aquifers faster than natural replenishment.
- Recreational and Environmental Uses: Lakes, ponds, snowmaking.
- Non-Potable Urban Uses: Fire protection, toilet flushing, air conditioning.
- Potable Use: Indirectly, by blending into raw water reservoirs which then undergo full water treatment again.

Singapore's NEWATER

Success Story: Exemplifies turning water challenges into opportunities, a pillar of Singapore's water sustainability.
Description: Ultra-clean, high-grade, weather-resilient water source.
Primary Uses: Industrial and air cooling (especially for wafer fabrication plants), demonstrating high quality and reliability.
Indirect Potable Use: A small amount is used to top up raw water reservoirs during dry periods.
Quality Assurance: Has passed over $130,000$ scientific tests and meets World Health Organization (WHO) guidelines.
Three-Stage Purification Process:
1. Microfiltration: Removes suspended solids, minute particles, bacteria, and viruses.
2. Reverse Osmosis: Uses a semi-permeable membrane to allow only water molecules to pass through, filtering out dissolved salts and contaminants.
3. Ultraviolet (UV) Disinfection: Provides an additional barrier to ensure product water safety.
Public Acceptance: PUB launched a public engagement campaign:
- Educated the public on stringent production processes.
- Engaged stakeholders (grassroot leaders, experts, businesses, schools).
- Established the NEWATER Visitor Centre for interactive tours and workshops.
- $98\%$ of respondents in a $2002$ poll indicated they would drink NEWATER, showing overwhelming acceptance.
Terminological Note: In Singapore, "wastewater" is often referred to as "used water" to promote a mindset of resource recovery rather than waste disposal.

Summary

Vocabulary: Used water vs. wastewater.
Environmental Impact: Nitrogen and phosphorus lead to eutrophication (excess plant/algae growth).
Control: Households and industries must minimize wastewater discharge.
Characteristics: Wastewater is defined by its physical, chemical, and biological properties.
Treatment Stages: Primary and