WWTP

Wastewater Treatment

• Wastewater is a mixture of human waste, washing water, and food preparation water
• Wastewater treatment aims to remove and break down organic matter under controlled conditions
• The main steps of wastewater treatment are primary, secondary, and tertiary treatment

Nature of Wastewater

• Domestic wastewater combines human waste (feces and urine) and "graywater" (water from washing, bathing, and food preparation)
• Graywater is water used for washing, bathing, and food prep
• The BOD (Biological Oxygen Demand) is a key metric to measure the oxygen consumed by microbes during the organic matter breakdown in the wastewater

Biological Oxygen Demand (BOD)

• BOD is a measure of the oxygen consumed by microbes during the oxidation of organic and inorganic matter in wastewater
• The BOD test measures oxygen consumption by microorganisms in dark conditions over 5 days at room temperature
• The test is performed on a series of wastewater dilutions
• The BOD formula: BOD (mg/L) = (D1 - D5) / P
  • D1 is the initial dissolved oxygen (mg/L)
  • D5 is the final dissolved oxygen (mg/L)
  • P is a decimal volumetric fraction of the sample used
• Typical BOD ranges of wastewater are 110 – 440 mg/L and often reduced to 95%

Wastewater Treatment Process

• The main goal of the wastewater treatment process is to remove and break down organic matter
• Three main steps include: primary treatment, secondary treatment, and tertiary treatment

Primary Treatment

• Primary treatment physically separates solids from wastewater
• Removes large solids (metal grating, bar screens, grit tanks)
• Approximately 50% of suspended organic matter settles out as primary sludge in settling tanks

Secondary Treatment

• Secondary treatment involves the biological degradation of organic matter by microorganisms
• Other benefits include reduced pathogen numbers.
• Performed in aeration tanks, trickling filter beds, or sewage lagoons.
• Pathogens are removed through microbial competition, or adsorption
• Detention time for aeration tanks ranges from 4-8 hours

Trickling Filters

• Trickling filters use plastic or stone units for older facilities and are a form of secondary treatment.
• Effluent is pumped over a filter bed.
• Microorganisms within the biofilm intercept and degrade organic matter via aerobic decomposition
• Biofilm maintenance allows organic matter to be converted to biomass.
• BOD removal efficiency is about 85%

Activated Sludge

• Activated sludge is a biological process involving effluent mixing with bacterial-rich slurry and adding air to encourage aerobic decomposition
• Secondary sludge is removed, combined with primary sludge, and anaerobically digested
• Pathogen removal occurs through competition and adsorption
• Detention time for aeration tanks is 4-8 hours

Aeration Tank Contents

• Mixed liquor suspended solids (MLSS) contains organic matter, including volatile suspended solids (MLVSS).
• Proper food-to-microorganism ratio is important (F/M)
• MLSS must maintain proper load with substrate (organic matter) and microbes to allow adequate F/M
  • F/M is often 0.2-0.5 lb BOD5/day/lb MLSS
• A lower F/M is desirable, enabling microbes to efficiently break down wastewater

More on Activated Sludge

• Treatment processes can modify for nitrogen and phosphorus removal
• Nitrogen removal often requires encouraging nitrification, followed by denitrification to encourage nitrifying bacteria.
• Sludge ages need to be determined to encourage growth of nitrifying bacteria over heterotrophic bacteria for optimum denitrification
• Phosphorous modifications can include A/O process

Tertiary Treatment

• Tertiary treatment further reduces organics, turbidity, nitrogen, phosphorus, metals, and pathogens
• Common physiochemical methods are coagulation, filtration, activated carbon adsorption, reverse osmosis, and disinfection

Pathogen Removal

• Processes before tertiary treatment remove most pathogens
• Activated sludge removes 90% of enteric bacteria, up to 99% of enteroviruses, and rotaviruses, up to 90% of Giardia, and Cryptosporidium
• Pathogens are difficult to remove, so multiple steps might be needed
• "Crypto" is a genus of eukaryotic parasites causing respiratory issues

Septic Tanks

• Wastewater enters tank and oils rise to the top, with solids sinking to the bottom
• Anaerobic decomposition results in septic tank sludge
• Wastewater stays in tanks for 24-72 hours
• Residual sludge may need to be pumped
• Significant contaminants can be present and possibly pollute groundwater

Wetlands

• Constructed wetlands offer secondary effluent treatment
• Vegetation increases surface area for microbes, filtering out contaminants
• Two types: free water surface (FWS) and subsurface flow (SFS) systems

Drinking Water Treatment

• Past treatment was basic, primarily slow sand filtration.
• Germ theory was developed in the 1870s, demonstrating chlorine's effectiveness in killing bacteria (1880s)
• Drinking water disinfection reduced typhoid fever outbreaks

Process Train

• Simple treatment is chlorination to disinfect water
• Filtration using sand or coal may be included to reduce turbidity
• In-line filtration involves a coagulant to flocculate suspended particles, improving filtration efficiency

Coagulation

• Chemicals are added to remove dissolved and suspended solids via sedimentation and filtration
• Common chemicals include hydrolyzing metal salts ("alum") or organic molecules (polyelectrolytes)

Flocculation

• Flocculation is a physical process of stirring the water
• Encourages particle collision, promotes larger particles
• Particles come together, and this process makes filtration easier
• Microorganisms in the water are trapped in the "flocs" and removed

Sedimentation and Filtration

• Sedimentation allows particles to settle out
• Filtration filters out particles
• Combined, these processes remove 99.99% of microorganisms, including viruses

Filtration

• Filtration is important in removing protozoan parasites.
• Cysts and oocysts are resistant to disinfection, requiring additional procedures to prevent illness

Drinking Water Distribution Systems

• Treated water is transported to consumers over long distances
• Quality can degrade due to factors like reduced disinfectant residual, biofilm buildup, pipe breaks, and microbial regrowth

Microbial Growth

• Bacterial concentrations in distribution systems range from low to high
• Pipe biofilms can contain high concentrations of microorganisms
• Biofilms can tolerate chlorine residuals
• E. coli is 2400x more resilient when attached to a biofilm

Legionella in Distribution Systems

• Legionella is more resistant to chlorine compared to E. coli
• Legionella thrives in hot water tanks and hospitals
• Thrives at 50°C and can grow at 42°C

Organic Matter

• Organic matter growth is influenced by retention time and temperature
• Examples of organic matter affecting wastewater are Pseudomonas, and amino acids (acetate, lactate)
• Amount of biodegradable organic matter analysis is complex.
• Assimilable organic carbon (AOC) is a significant factor, about 0.1 – 9% of total organic carbon

Course Information (Monday)

• Quiz on the week's lecture material
• Review of previous exam materials (Exam 2)
• Topic review on Organic Pollutants and emerging topics