Aerobic Treatment System: Trickling Filters

Definition:

A trickling filter is a type of aerobic treatment system that uses a solid bed of materials (like
crushed stones) to grow a layer of microorganisms, known as biofilm. This biofilm helps treat
wastewater.

Working Principle:

1. Contact and Degradation:

o Wastewater containing organic materials comes into contact with the biofilm on
the filter medium.
o Microorganisms in the biofilm aerobically degrade the organic matter.
2. Distribution and Filtration:
o Settled sewage (partially treated wastewater) is distributed over the surface of the
filter.
o The wastewater trickles down through the filter medium.
3. Processes:
o Sorption: Organic matter is absorbed by the biofilm.
o Biological Oxidation:
Microorganisms break
down the organic
materials into simpler
compounds.

Structure of Trickling Filter:

1. Filter Medium:
o Made of crushed stone or similar materials.
 o These materials are chosen for:
 Hardness: Resistance to breaking.
 Durability: Long-lasting use.
 Chemical Resistance: Withstands wastewater conditions.
2. Size of Medium:
o The size of the stones or materials ranges from 45 mm to 100 mm.

Advantages of Trickling Filters:

1. Simplicity:
o The system is easy to operate and maintain.
2. Low Operating Costs:
o It is cost-effective for wastewater treatment.
3. Small-Scale Use:
o Ideal for small communities in warmer climates.
4. High Efficiency:
o Multistage high-rate filters provide good efficiency in treating wastewater.
5. Control:
o All contributing factors can be well-regulated.

Biotowers: Advanced Trickling Filters

Definition:

Biotowers are deep trickling filters specifically designed to handle high flow rates of
wastewater.

Structure of Biotowers:

1. Media (Filter Material):

o Made of lightweight, modular media like polyvinyl chloride (PVC) sheets
arranged in alternating patterns.
o This arrangement provides:
 Strength and Rigidity without adding much weight.
 Height up to 12 meters to maximize volume in a small space.
2. Materials Used:
o Alternatives to PVC include:
 Wooden sheets in alternating patterns.
  River rock (25–120 mm size).
 Blast-furnace slag (50–125 mm size).
 Plastic with high specific surface area (modern usage).
3. Wastewater Distribution:
o A rotating distributor supplies wastewater, similar to a trickling filter.
o The wastewater flows down through the medium while undergoing treatment.
4. Design Benefits:
o Compact design allows for efficient use of space.

Working of Biotowers:

1. High Flow Rate:

o Biotowers are operated
like high-rate trickling
filters, ensuring all
surfaces are
continuously wetted.
2. Endogenous Respiration:
o Microorganisms in the
biofilm metabolize
waste throughout the
depth of the tower.
3. Nitrification:
o Ammonia is converted
to nitrate when the
carbon content in
wastewater is less than
20 mg/L.
o Produces a nitrified effluent, which is wastewater treated to reduce ammonia
levels.
4. Plastic Modules:
o Less effective than random packing but still functional.

Advantages of Biotowers:

1. High Loading Rate:

o Handles larger amounts of wastewater compared to traditional trickling filters.
2. No Plugging Issues:
o Medium is designed to avoid blockages.
3. Proper Ventilation:
o Ensures minimal odor problems and maintains ideal conditions for treatment.
4. Compact Reactor:
 Efficient space utilization.
o
5. Simple Operation:
o Easy to manage and maintain.

Disadvantages of Biotowers:

1. High Pumping Costs:

o Due to the large amount of water that needs to be recycled, energy costs are
higher.

1. Rotating Biological Contactors (RBC)

Definition:

 RBC is a biological wastewater treatment process that uses rotating disks to grow
microorganisms for breaking down waste.
 It’s an attached growth process where biofilm grows on the disks.

Structure:

1. Disks:
o Circular disks are closely spaced and mounted on a shaft.
o Disks are partially submerged (about 40%) in wastewater.
2. Tank:
Wastewater flows continuously through the tanks where the disks rotate.
o
3. Aeration:
o Achieved as disks rotate out of the wastewater, exposing them to air.
4. Rotation Speed:
o Disks rotate slowly at 1–1.6 revolutions per minute.
5. Air-Driven Units:
o Equipped with cups on the disk edges to enhance aeration as disks rotate.

Working Process:
 1. Biofilm Growth:
o A layer of microorganisms (biofilm) grows on the surface of the rotating disks.
2. Waste Breakdown:
o As the disks rotate, the biofilm alternates between contact with wastewater and
air.
o The microorganisms absorb and degrade organic matter in the wastewater.
3. Clarification:
o Requires primary clarification (to remove large solids) and secondary
clarification (to separate treated water and biomass).

Key Points to Remember:

 Origin: First used in Germany (1960).

 Unique: Combines wastewater treatment and aeration in a single rotating system.
 Pretreatment: Needs clarification steps before and after treatment.

2. Fluidized Bed Bioreactors

(FBBR)

Definition:

 FBBR combines attached

growth (like RBC) and
suspended growth systems.
 It uses a bed of small
particles (sand or activated
carbon) to support biofilm
growth.

Structure:

1. Support Medium:
o Small particles (0.4–0.5
mm) made of sand or
activated carbon.
2. Bed Depth:
o The bed is 4–5 meters
deep.
3. Flow System:
 o Wastewater is fed upward through the bed.
4. Upflow Velocities:
o Liquid moves at a speed of 35–40 meters per hour to keep the particles
suspended.

Working Process:

1. Biofilm Formation:
o Microorganisms grow on the surface of the sand or carbon particles.
2. Waste Treatment:
o As wastewater flows upward, the biofilm degrades the organic matter.
3. Suspension:
o The upward flow keeps the particles and biofilm in constant motion, enhancing
contact between microorganisms and wastewater.

Key Points to Remember:

 Introduced: 1980.
 Process: Combines attached and suspended growth methods.
 Efficiency: The upward flow keeps particles and biofilm active and effective.

1. What is a UASB Reactor?

 Definition:
UASB is an anaerobic wastewater treatment technology where microorganisms
(anaerobes) break down pollutants without oxygen. It operates in a column reactor
without packing material.
 Developed By:
Invented by Lettinga and his team in 1980 in the Netherlands.
 Main Feature:
A sludge blanket of microorganisms forms at the bottom, which is key to pollutant
removal.

2. How Does It Work?

 1. Wastewater Entry:
Wastewater enters from the bottom of the reactor via a liquid distributor.
2. Sludge Blanket:
o A thick layer of anaerobic microorganism granules (called the sludge blanket)
forms above the distributor.
o These granules break down organic pollutants into simpler substances.
3. Upflow Velocity:
o Wastewater flows upward through the sludge blanket, ensuring effective contact
between pollutants and microorganisms.
4. Gas and Effluent Separation:
o At the top, the reactor is designed to separate treated wastewater, gas (biogas),
and solids effectively.
5. Biogas Collection:
o Biogas (methane + carbon dioxide) is collected at the top as a byproduct of the
anaerobic digestion process.

3. Design Considerations

1. Wastewater Characteristics:
o Wastewater rich in proteins or fats may cause problems like foaming or poor
sludge granulation.
o The ratio of particulate vs. soluble COD affects design.
2. Volumetric Organic Loading:
o Typical COD (Chemical Oxygen Demand) loading: 12–25 kg COD/m³/day.
o Removal efficiency: up to 95% COD removal at temperatures of 28–35°C.
3. Upflow Velocity:
o Velocity depends on the strength of wastewater:
 Soluble wastewater: 6 m/h.
 Stronger wastewater: Velocity depends on COD loading.
4. Physical Features:
o Includes systems for
effluent withdrawal, gas
collection, and feed inlet.

4. Advantages of UASB Reactors

1. High Efficiency:
o Removes 70% or more
COD.
o Produces biogas (0.85
m³/kg COD removed).
2. Compact Design:
 No need for packing material, saving space.
o
3. Low Operational Costs:
o Efficient energy usage due to anaerobic digestion.
4. Wide Applications:
o Treats various industrial wastes like food processing, dairy, and paper mill waste.

5. Limitations of UASB Reactors

1. Disturbances in Sludge Blanket:

o High gas production or effluent flow can disrupt the sludge blanket.
2. Not Effective for Particulate Waste:
o Works better for soluble COD than particulate waste.
3. Poor Granulation:
o Wastewater with high insoluble COD can lead to poor sludge granulation.

6. Applications

 Industries:
Treats food, dairy, paper mill, and citric acid wastes.
 Denitrification:
Used in experiments for nitrogen removal in Japan.
 Biogas Production:
Generates methane-rich biogas as a renewable energy source.
 Full-Scale Usage:
Successfully treats sugar beet waste with 90% COD removal at a rate of 18 kg
COD/m³/day.

Key Points to Remember

Aspect Details

Invented 1980, Netherlands

Main Process Anaerobic digestion without oxygen

Key Feature Thick sludge blanket of microorganisms

Efficiency 70%+ COD removal, 0.85 m³ biogas/kg COD removed

Retention Time 3–8 hours

Applications Food processing, dairy, paper mill, sugar beet waste

Limitations Ineffective for particulate waste, sludge blanket disturbances