Wastewater Composition and Risks

Wastewater Defined

• Wastewater includes water quality that has been adversely affected by human activities.
• This includes water from domestic, industrial, and agricultural processes.
• Wastewater contains a complex mixture of organic matter like carbohydrates, proteins, and fats.
• It also contains inorganic substances like heavy metals and salts.
• Further components are nutrients like nitrogen and phosphorus.
• It contains microorganisms like bacteria and viruses.
• Untreated wastewater poses risks to public health and the environment.
• Health and environmental risks include waterborne diseases and eutrophication of water bodies.

Composition of Wastewater

• Organic matter is derived from human waste, food, and industrial processes.
• Examples of organic matter are carbohydrates, proteins, fats, and oils.
• Inorganic matter includes heavy metals like lead and mercury.
• It includes salts, and minerals.
• Microorganisms include pathogenic bacteria, such as E. coli.
• Viruses and protozoa can be microorganisms in wastewater.
• Nutrients include nitrogen in the form of ammonia and nitrates.
• Phosphorus as phosphates, can cause eutrophication in water bodies.

Reasons to Treat Wastewater

• Public health is a key reason, as untreated wastewater can spread waterborne diseases.
• Examples of waterborne diseases spread are cholera, typhoid, and dysentery.
• Environmental protection is achieved by preventing eutrophication.
• Eutrophication depletes oxygen in water bodies and harms aquatic life.
• Resource recovery is another reason, as treated water can be reused for irrigation.
• Further, it can be used for industrial processes and even drinking water after advanced treatment.

Stages of Wastewater Treatment

• Primary Treatment: Physical processes like screening and sedimentation remove large solids and suspended particles.
• Secondary Treatment: Biological processes using microorganisms (mainly bacteria) break down organic matter.
• Tertiary Treatment: Advanced processes remove remaining nutrients like nitrogen and phosphorus.
• Tertiary treatment removes pathogens.

Primary Treatment Details

• Screening removes large debris like plastics, rags, and sticks using bar screens.
• Sedimentation allows suspended solids to settle at the bottom of tanks, forming sludge.
• Primary treatment removes about 60% of suspended solids.
• Primary treatments does not address dissolved pollutants.

Secondary Treatment Details

• Secondary treatment is the core of biological wastewater treatment.
• It relies on microorganisms to degrade organic matter.
• Secondary treatment can be aerobic (with oxygen) or anaerobic (without oxygen).
• The process depends on the type of microorganism
• Common methods include the activated sludge process, trickling filters, and anaerobic digesters.

Aerobic Treatment Details

• Aerobic treatment occurs with oxygen and is carried out by aerobic bacteria.
• Examples of aerobic bacteria are Pseudomonas and Bacillus.
• These bacteria oxidize organic matter into carbon dioxide (CO2), water (H2O), and energy (ATP).
• As an example, Glucose (C6H12O6) + 6O2 yields 6CO2 + 6H2O + Energy.
• Pseudomonas degrades a wide range of organic compounds.
• Bacillus breaks down complex organic matter.
• These bacteria form flocs (clumps) that settle out as sludge, which can be further treated.

Nitrification and Denitrification

• Nitrification is a two-step process carried out by aerobic bacteria.
• Ammonia Oxidation converts ammonia (NH3) to nitrite (NO2-) by Nitrosomonas with the formula: NH3 + O2 → NO2- + H2O + Energy.
• Nitrite Oxidation converts nitrite (NO2-) to nitrate (NO3-) by Nitrobacter with the formula: NO2- + O2 → NO3- + Energy.
• Nitrification is essential for removing nitrogen from wastewater.
• Denitrification converts nitrate (NO3-) into nitrogen gas (N2), which is released into the atmosphere.
• This occurs without oxygen and is carried out by facultative anaerobic bacteria like Pseudomonas.
• Denitrification is crucial for preventing nitrate pollution in water bodies.
• The equation: NO3- → NO2- → NO → N2O → N2.

Anaerobic Treatment Details

• Anaerobic treatment occurs in the absence of oxygen and is carried out by anaerobic bacteria.
• One such bacteria is Methanobacterium.
• Anaerobic treatment is used for treating high-strength organic wastewater and sludge.
• The end products include methane (CH4), carbon dioxide (CO2), and stabilized sludge.

Stages of Anaerobic Digestion

• Hydrolysis: Complex organic matter is broken down into simpler compounds.
• Hydrolysis involves breaking down proteins and fats into amino acids and fatty acids.
• Acidogenesis: Acid-forming bacteria convert compounds into volatile fatty acids (VFAs).
• Acidogenesis uses Clostridium to convert compounds into VFAs, alcohols, and gases (CO2, H2).
• Methanogenesis: Methanogens convert VFAs and gases into methane.
• Methanogenesis includes using Methanobacterium to convert VFAs and gases into methane (CH4) and CO2.

Tertiary Treatment Details

• Tertiary treatment is the final stage of wastewater treatment.
• It removes remaining nutrients (nitrogen, phosphorus) and pathogens.
• It includes processes like phosphorus removal, nitrogen removal, and disinfection.
• Treated water from this stage can be safely discharged or reused.

Phosphorus Removal Details

• Achieved through Enhanced Biological Phosphorus Removal (EBPR).
• Bacteria like Acinetobacter accumulate phosphorus in their cells.
• Phosphorus is accumulated in the form of polyphosphate.
• The phosphorus-rich sludge is then removed from the system.

Disinfection Methods

• Chlorination is where Chlorine is added to kill pathogens, although it can produce harmful byproducts.
• In UV Treatment, ultraviolet light disrupts the DNA of microorganisms, rendering them harmless.
• With Ozonation, ozone (O3) acts as a strong oxidant that kills pathogens without leaving residues.

Sludge Handling

• Sludge is the semi-solid material left after wastewater treatment.
• Sludge contains organic matter, microorganisms, and inorganic compounds.
• Sludge treatment is essential to reduce its volume and stabilize its organic content.
• It also makes sludge safe for disposal or reuse.
• Dewatering reduces the volume of sludge by removing water.
• Common dewatering methods include centrifugation, filtration, and drying beds.
• Dewatered sludge is easier to handle and transport for disposal or reuse.
• Dewatered sludge can be used as fertilizer in agriculture, incinerated for energy recovery, or landfilled.
• In some cases, sludge is composted to produce a nutrient-rich soil amendment.
• Nutrient-rich sludge can be used directly as fertilizer.
• Sludge is burned for incineration to recover energy.
• Landfilling provides safe disposal of non-reusable sludge.

Key Bacteria in Wastewater Treatment

• Aerobic Bacteria:
  • Pseudomonas degrades organic matter and denitrifies.
  • Nitrosomonas and Nitrobacter nitrify ammonia to nitrate.
• Anaerobic Bacteria:
  • Methanobacterium produces methane during anaerobic digestion.
• Phosphorus-Accumulating Bacteria:
  • Acinetobacter removes phosphorus through EBPR.

Enzymes in Wastewater Treatment

• Amylase is used to break down carbohydrates into simpler sugars.
• Protease is used to break down proteins into amino acids.
• Lipase is used to break down fats into fatty acids and glycerol.
• Nitrate Reductase converts nitrate to nitrogen gas during denitrification.

Applications of Wastewater Treatment

• Biogas from anaerobic digestion is a renewable energy source.
• It reduces reliance on fossil fuels and lowers greenhouse gas emissions.
• For example, biogas plants in India and Germany produce electricity and heat for communities.
• Phosphorus and nitrogen recovered from sludge are used as fertilizers.
• This reduces the need for chemical fertilizers and promotes sustainable agriculture.
• Struvite (magnesium ammonium phosphate) is recovered from sludge and used as a slow-release fertilizer.
• Proper treatment ensures safe water for reuse or discharge.
• It prevents waterborne diseases like cholera and typhoid.
• Treated wastewater is reused for irrigation in arid regions as an example.

Challenges in Wastewater Treatment

• High Energy Consumption: Aerobic treatment requires significant energy for aeration.
• Toxic Sludge: Heavy metals and persistent organic pollutants in sludge pose disposal challenges.
• Emerging Contaminants: Microplastics, pharmaceuticals, and personal care products are difficult to remove.